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J Virol. 2013 January; 87(1): 2–15.
PMCID: PMC3536369

The Tale of the Long Tail: the Cytoplasmic Domain of HIV-1 gp41

Abstract

Envelope glycoproteins (Env) of lentiviruses typically possess unusually long cytoplasmic domains, often 150 amino acids or longer. It is becoming increasingly clear that these sequences contribute a diverse array of functional activities to the life cycle of their viruses. The cytoplasmic domain of gp41 (gp41CD) is required for replication of human immunodeficiency virus type 1 (HIV-1) in most but not all cell types, whereas it is largely dispensable for replication of simian immunodeficiency virus (SIV). Functionally, gp41CD has been shown to regulate rapid clathrin-mediated endocytosis of Env. The resultant low levels of Env expression at the cell surface likely serve as an immune avoidance mechanism to limit accessibility to the humoral immune response. Intracellular trafficking of Env is also regulated by gp41CD through interactions with a variety of cellular proteins. Furthermore, gp41CD has been implicated in the incorporation of Env into virions through an interaction with the virally encoded matrix protein. Most recently, the gp41CDs of HIV-1 and SIV were shown to activate the key cellular-transcription factor NF-κB via the serine/threonine kinase TAK1. Less well understood are the cytotoxicity- and apoptosis-inducing activities of gp41CD as well as potential roles in modulating the actin cytoskeleton and overcoming host cell restrictions. In this review, we summarize what is currently known about the cytoplasmic domains of HIV-1 and SIV and attempt to integrate the wealth of information in terms of defined functional activities.

BACKGROUND: BIOSYNTHESIS AND PROCESSING OF ENVELOPE

The target cell tropism of human immunodeficiency virus (HIV) and simian immunodeficiency virus (SIV) is determined by the virally encoded envelope glycoprotein (Env). Env consists of two components, the surface subunit (SU) gp120 and the transmembrane subunit (TM) gp41, which are produced from the precursor protein gp160 (13). In the plasma membranes of virions and infected cells, the mature Env complex is a trimer formed by three pairs of gp120 and gp41 (412). gp120, which is located entirely extracellularly, mediates sequential recognition of and binding to CD4 and the coreceptor (1315); gp41 anchors the Env complex in the plasma membrane and is responsible for the fusion of viral and target cell membranes (1618). The standard model of Env topology describes gp41 as a type I transmembrane protein, with one extracellular domain, one membrane-spanning domain, and one cytoplasmic domain (19, 20).

The precursor protein gp160 is translated from env mRNA at the rough endoplasmic reticulum (ER) and is modified cotranslationally by the addition of multiple N-linked carbohydrates (2125) as well as O-linked carbohydrates (26, 27). While still within the ER, gp160 oligomerizes into homotrimers. Subsequently, trimeric gp160 is transported to the Golgi apparatus, where it is cleaved proteolytically by cellular furin or a cellular furin-like protease into gp120 and gp41 (2832). gp120 and gp41 remain noncovalently associated after the cleavage (18), thus forming the mature trimeric Env complex. These Env complexes are then transported to the plasma membrane where they can be incorporated into budding virions (33, 34). Env trimers can be observed as “spikes” on the surface of virus particles by electron microscopy (12, 35, 36).

COMPARISON OF THE CYTOPLASMIC DOMAINS OF RETROVIRAL ENVELOPE PROTEINS

Arguably the most unusual feature of the lentiviral Env protein is the extraordinary length of its cytoplasmic domain. Env of most lentiviruses has a cytoplasmic domain longer than 130 amino acids (aa), the only exceptions being the closely related feline immunodeficiency virus (FIV; 50 aa) and puma lentivirus (PLV; 76 aa). In contrast, the analogous region of viruses from other retrovirus genera is typically shorter than 60 aa (Fig. 1).

Fig 1
Comparison of Env cytoplasmic-domain lengths in lentiviruses and other retroviruses. Cytoplasmic-domain lengths are based on the UniProt database. BIV, bovine immunodeficiency virus; CAEV, caprine arthritis encephalitis virus; EIAV, equine infectious ...

The genomes of lentiviruses are highly adaptable; genetic information that conveys no advantage for viral fitness is lost quickly (39, 40). This implies that the unusually long cytoplasmic domain of lentiviruses is not a mere evolutionary artifact but rather contributes functional activities important for the virus. Indeed, extensive scientific investigation has identified a considerable variety of cellular interaction partners and functional activities of the cytoplasmic domain of HIV and SIV gp41 (gp41CD).

STRUCTURAL FEATURES AND TOPOLOGY

gp41CD can be divided into distinct domains based on biophysical properties (Fig. 2): the membrane-spanning domain is followed by a region of high hydrophilicity (20), which contains a highly immunogenic region (HIR) that frequently elicits high levels of antibodies in HIV-infected individuals and SIV-infected rhesus macaques (4146). The C-terminal half of gp41CD contains two amphipathic segments believed to form α-helices, designated lentivirus lytic peptide 2 (LLP-2) and LLP-1 (47, 48). Additionally, a leucine zipper motif located between LLP-2 and LLP-1 has been identified and designated LLP-3 (49).

Fig 2
Schematic representation of gp41CD regions. (A) HIV-1. Amino acid sequences and numbering are based on full-length Env of strain NL4-3. (B) SIVmac. Amino acid sequences and numbering are based on full-length Env of strain SIVmac239. Black lines indicate ...

The high prevalence of antibodies against the HIR in sera from HIV-infected individuals has led to the proposal of an alternative model for gp41 topology in which sequences C-terminal to the membrane-spanning domain are not entirely contained within the cell or virion but instead form an extracellular loop which exposes the HIR to the extracellular medium (5053). Recent studies have provided substantial evidence against this alternative model for Env of both HIV-1 and SIV (45, 54). While there is little proof to convincingly support the alternate topology, a transient (52) or low-level existence of such a structure cannot be rigorously excluded at this time.

Synthetic peptides corresponding in sequence to LLP-1, -2, and -3 interact with lipid bilayers in vitro (49, 55, 56), and gp41CD as a whole has been shown to weakly associate with lipid bilayers (19, 57). Based on this indirect evidence, a model has been proposed in which the LLPs form an association with the intracellular or intravirionic leaflet of the membrane (interaction partners of gp41CD are summarized in Table 1), with the hydrophobic face of each LLP buried in the membrane, while the hydrophilic faces are exposed to the cytoplasm (49, 55, 80). This model was recently corroborated by a study using iodonaphthyl azide labeling to detect membrane immersion of gp41CD (83).

Table 1
Interaction partners of HIV-1 gp41CDa

It has been suggested that gp41CD may help to stabilize the Env trimer in its prefusion “spring-loaded” conformation, as truncation of gp41CD has been documented to enhance the fusogenicity of surface-expressed Env (8489). LLP-2 appears to play a particularly important role for this stabilization (90, 91). The precise mechanism by which gp41CD consolidates the metastable prefusion state of Env is unknown, but it has been proposed to be due to (i) an interaction with the matrix (MA) domain of the viral Gag precursor protein (see below) (9294) and/or (ii) a stabilizing effect of the association of gp41CD with the plasma membrane (91).

REQUIREMENT OF THE CYTOPLASMIC DOMAIN OF GP41 FOR REPLICATION

One of the most enigmatic phenomena regarding gp41CD is its cell-type-dependent requirement for HIV-1 replication. Truncation of gp41CD blocks replication of HIV-1 in most cell types, including peripheral-blood mononuclear cells (PBMC) and monocyte-derived macrophages (MDM). However, there are at least two cell lines in which HIV-1 with truncated Env is able to establish productive infection (Table 2). Truncation does not prevent proper processing of Env, nor does it inhibit the fusion process (88, 9698, 100). When HIV-1 virions with truncated Env are produced in a permissive cell line, their single-round infectivity in both permissive and nonpermissive cells is almost as efficient as the single-round infectivity of wild-type virions (96, 97), implying that there is no structural defect associated with Env truncation that prevents entry into nonpermissive cell lines. However, cell types that are not permissive for replication of HIV-1 with truncated Env show a marked decrease in incorporation of truncated Env into virions, whereas permissive cell lines incorporate truncated Env at only slightly reduced levels into nascent virions (88, 96100). This selective incorporation defect is likely the underlying cause for the cell type dependence of replication of HIV-1 with truncated Env. It is unknown why some cell lines incorporate truncated Env much more efficiently than others, at least in part because the process of Env incorporation itself is still incompletely understood (33). Regardless of the precise mechanism, it seems clear that the cytoplasmic domain of HIV-1 gp41 is critical for orchestrating the incorporation of Env into virions in most cell types.

Table 2
Summary of cell lines tested for permissiveness of HIV-1 replication and incorporation of Env with truncated gp41CDa

There are only three known instances in which HIV-1 evolved truncated gp41CD variants. In each case, these adaptations were the result of extensive passaging in tissue culture and were accompanied by major compensatory changes (101104). Conversely, SIV routinely evolves truncations of gp41CD when passaged in human cells, including cells that are not permissive for replication of HIV-1 with truncated Env (105108). Indeed, it has been shown that truncation of SIV gp41CD increases Env incorporation into virions and consequently increases the levels of infectivity and fusogenicity (89, 109, 110). This offers a satisfying explanation for the phenomenon that SIV with truncated Env replicates more efficiently in human cells and thus quickly replaces viral species encoding full-length Env in culture (105108). Intriguingly, however, truncation of Env appears to confer this selective advantage only in human cells, as gp41CD truncations do not evolve when SIV is cultured in rhesus PBMC (106108). Consistent with this, SIV variants with a premature stop codon in gp41CD replicate less efficiently in rhesus PBMC than parental virus with full-length Env (107, 111), and reversion of gp41CD-truncated mutants to a full-length phenotype has been documented after prolonged passage in rhesus PBMC (106108). It remains unknown why the selective advantage of SIV gp41CD truncations observed in human cells does not apply to SIV replication in rhesus-derived cells. As there are only very few rhesus cell lines, most tissue culture experiments examining the effects of SIV gp41CD truncation have been performed with human cell lines. Consequently, no information is available about any resultant influences of SIV gp41CD truncation on Env incorporation, infectivity, or fusogenicity in rhesus cells. A possible explanation for the negative effect of SIV gp41CD truncation on replication in rhesus, but not human, PBMC could be the presence of a hitherto-unrecognized host restriction factor. Such a restriction factor might inhibit the incorporation of SIV Env but be counteracted by full-length gp41CD. The human orthologue of this factor may not be effective against SIV, allowing SIV with truncated gp41CD to replicate freely in human cell lines, even in those not permissive for replication of HIV-1 with truncated gp41CD. The existence of this hypothetical factor would also offer an explanation for the selective incorporation defect of truncated HIV-1 Env discussed above. Indeed, the phenotype of this incorporation defect bears the hallmarks of a species-specific, differentially expressed restriction factor. In human cells permissive for replication of HIV-1 with truncated gp41CD, expression levels of this factor may be significantly lower than in nonpermissive cells. If such a putative restriction factor does exist, comparing the expression profiles of permissive and nonpermissive cell lines might provide a means to identify this factor.

Consistent with the observation that SIV with truncated gp41CD shows reduced viral fitness in rhesus PBMC during tissue culture experiments, gp41CD-truncated SIV mutants quickly revert to a full-length Env phenotype during in vivo infection of rhesus macaques (106, 108, 111). Moreover, a cohort of rhesus macaques infected with a rhesus macaque SIV (SIVmac) mutant that had gp41CD truncated through multiple redundant mutations which prevented reversion to a full-length phenotype, had markedly reduced viral loads and did not show clinical signs of progression to AIDS (111). Different selective pressures, such as the ability of the virus to evade the host immune response, are likely to influence the sequence evolution of gp41CD during infection in vivo. As discussed in further detail in the following section, HIV and SIV limit the levels of Env on the surface of cells and virions through gp41CD; this could be another selective force working against Env truncation during infection of rhesus macaques.

ENDOCYTOSIS

After transport to the cell surface, Env trimers of HIV-1 and SIV are rapidly internalized by clathrin-mediated endocytosis, resulting in low steady-state levels of Env surface expression (60, 64, 65, 112). This process is regulated by at least two separate domains of gp41CD.

The first established endocytosis motif conforms to the consensus sequence YXXΦ (where X is any and Φ a hydrophobic amino acid) and is located close to the membrane-spanning domain (Fig. 2, amino acids 709GYSPL713 in HIV-1 NL4-3 and 720GYRPV724 in SIVmac239) (64, 65, 112). The cellular interaction partner mediating endocytosis through this membrane-proximal YXXΦ motif is the μ2 subunit of the clathrin-associated adaptor protein complex AP-2 (Table 1) (5860, 62). Any effects resulting from mutation of this YXXΦ motif on viral infectivity and fusogenicity in tissue culture-based experiments may vary with the virus strain, the cell type, and the nature of the assays used (58, 110, 113115). However, disruption of this motif in SIV gp41CD results in markedly lower viral loads and strongly reduced pathogenic potential during infection of rhesus macaques (116).

The membrane-proximal YXXΦ motif is extremely well conserved among isolates of HIV-1 groups M, N, and O and of chimpanzee SIV (SIVcpz) (present in 175 of 176 sequences examined), as well as of HIV-2, SIVmac, pigtailed macaque SIV (SIVmne), sooty mangabey SIV (SIVsmm), and stump-tailed macaque SIV (SIVstm) (68 of 68 sequences) (117). gp41CD of HIV-1 has a second, similarly well-conserved YXXΦ motif located closer to the C terminus of HIV-1 gp41CD (172 of 176 sequences; amino acids 766YHRL769 in HIV-1 NL4-3) (117), which does not mediate significant endocytosis but induces NF-κB activation (see below) (62, 65, 82). SIV gp41CD features two additional YXXΦ motifs, one highly conserved in HIV-2, SIVmac, SIVmne, SIVsmm, and SIVstm (68 of 68 sequences; amino acids 768YIHF771 in SIVmac239), the other conserved only within SIVmac, SIVmne, SIVsmm, and SIVstm (40 of 44 sequences; amino acids 795YQIL798 in SIVmac239) and not at all in HIV-2. Neither of these secondary YXXΦ motifs mediates detectable levels of endocytosis (59).

The second functional endocytosis motif identified in HIV-1 gp41CD is a dileucine motif at the very C terminus (Fig. 2), which also utilizes the AP-2 complex for internalization (63). Disruption of this dileucine motif by itself only leads to a small increase in cell surface expression of Env but results in a strong synergistic effect when mutated in combination with the membrane-proximal YXXΦ motif (63). Typically, endocytosis-mediating dileucine motifs recruit the σ2 subunit of the AP-2 complex (118), but such an interaction between gp41CD and σ2 has yet to be formally demonstrated. Surprisingly, this dileucine motif (amino acids 853LL854 of sequence 849LERILL854 in HIV-1 NL4-3) does not fully conform to the consensus dileucine motif for clathrin-mediated endocytosis ([D/E]XXXL[L/I/M]) (118), and it is not pervasively conserved within HIV-1 and SIVcpz (135 of 176 sequences) (117). There is strong evidence that gp41CD of SIV also contains a second endocytosis motif close to the C terminus, as truncations C-terminal to the membrane-proximal YXXΦ motif have been shown to increase cell surface expression of SIV Env, and disruption of the membrane-proximal YXXΦ motif reduces Env endocytosis more strongly when part of gp41CD is removed (58, 59, 110). Consistent with this observation, the C-terminal dileucine motif of HIV-1 gp41CD is highly conserved in gp41CD of HIV-2, SIVmac, SIVmne, SIVsmm, and SIVstm (67 of 68 sequences; amino acids 878LL879 in SIVmac239) (117) (Fig. 2).

In addition to the C-terminal L[L/I] motif, gp41CDs of HIV-1 and SIV have multiple L[L/I] motifs (six in HIV-1, four in SIVmac), with various degrees of conservation among isolates. None of these motifs conform to the consensus motif for endocytosis [D/E]XXXL[L/I/M]. Studies in which these additional endocytosis motifs were mutated did not detect major effects on cell surface expression of Env (59, 113). Nonetheless, there is some evidence that the gp41CD, at least of SIV, contains additional signals that contribute to endocytosis (59). It is not clear whether residual endocytosis in the absence of the established endocytosis motifs of gp41CD is mediated by signals that do not conform to the known consensus motifs or whether multiple redundant endocytosis motifs mask the contribution of individually mutated motifs. Further studies will be required to comprehensively elucidate the complexities of Env endocytosis.

Interestingly, Egan et al. reported that the presence of HIV-1 Gag was sufficient to counteract the endocytosis of Env, presumably because binding of the Gag domain matrix (MA) to gp41CD (see following section) blocks the interaction of gp41CD with AP-2 (119). In order to be efficiently incorporated into the virion, sufficient numbers of Env molecules need to be present on the cell surface, and consequently it has been suggested that Gag prevents endocytosis of Env molecules that are about to be incorporated, while redundant Env is removed from the cell surface (119). Contrary to this, it has been demonstrated that CEMx174 cells chronically infected with SIVmac239 rapidly endocytose SIV Env from their surfaces despite the presence of SIV Gag (59, 64). This disparity could possibly reflect differences between HIV-1 and SIV in the interactions of their respective Env and Gag proteins. To fully understand the relevance of these observations, further studies will be needed.

The rapid endocytosis of Env from the cell surface translates into a low number of Env trimers incorporated per virion. The number of spikes per virion has been estimated to range from 7 to 16, as calculated based on the molar ratio of Env to Gag by high-performance liquid chromatography (HPLC) (120) and Western blotting (110), as well as visual detection by electron tomography (12). All of these different methods performed by independent laboratories yielded remarkably consistent results. The same studies also found that SIV variants with truncated gp41CD invariably show much higher levels of Env incorporation, ranging from ca. 10- to 40-fold more Env trimers per virion than SIV with full-length gp41CD. Yuste et al. compared several SIV gp41CD mutants with premature stop codons or a mutation of the membrane-proximal YXXΦ motif and observed that the level of Env surface expression was closely mirrored by the level of Env incorporation into virions (110).

Why would HIV-1 and SIV evolve mechanisms to minimize the level of Env on the surfaces of host cells and virions? The most likely explanation is that it serves as a mechanism for immune evasion. The presence of Env on the cell or virion surface exposes viral epitopes to antibody binding, antibody-dependent cell-mediated cytotoxicity (ADCC), and complement-mediated cytotoxicity. Consequently, limiting the amount of Env on the cell surface could reasonably provide HIV and SIV with some degree of protection against the humoral branch of the immune system, although this protection must necessarily be incomplete to allow for Env incorporation into virions and viral entry. This theory is supported by the observation that a hard-to-revert mutation which increased the cell surface expression of SIV Env by disrupting the membrane-proximal endocytosis motif of gp41CD resulted in severely attenuated pathogenicity of SIV infection in rhesus macaques, consistent with better control of viral infection by the host's immune system (116).

ENVELOPE INCORPORATION INTO VIRIONS

The matrix (MA) component of the Gag precursor protein has long been known to be an essential regulator of Env incorporation into HIV and SIV virions (115, 121124). MA tethers the Gag polyprotein to the inner leaflet of the membrane through a myristoylated, membrane-binding domain (125129). Targeting of Gag to the budding site at the plasma membrane is also directed by MA (130133) and involves direct binding of MA to phosphatidylinositol (4,5)bisphosphate on the inner leaflet (136139). At the membrane, MA and Env interact through gp41CD, but the exact nature of this interaction is not entirely clear (Table 1). A small deletion in the LLP-2/LLP-3 region of gp41CD that blocks Env incorporation into HIV-1 virions was shown to be counteracted by mutations in MA (140); conversely, substitutions of single amino acids in MA can block incorporation of Env into virions (95, 141). Surprisingly, mutations in MA that block Env incorporation can be overcome by truncation of the gp41CD N-terminal of LLP-2 (85, 95, 141, 142). These data suggest that an interaction between Env and MA may not be required for Env incorporation per se but may be necessary to structurally accommodate the presence of the long cytoplasmic domain of gp41. In this context, it is interesting to note that MA, like gp41, may assemble into trimers or hexamers of trimers (126, 143), although the definitive in vitro structure is not yet known. Alternatively, the higher levels of Env at the cell surface resulting from truncation of gp41CD (see above) may simply allow for sufficient Env to become incorporated into virions despite the MA mutation.

How exactly gp41CD and MA interact physically is controversial. An early publication by Cosson reported a direct interaction of purified, bacterially expressed Env and MA that mapped to the C-terminal half of gp41CD (75). In direct contradiction to these findings, Lopez-Vergès et al. later described the intracellular trafficking protein perilipin-3, more commonly known as tail-interacting protein of 47 kDa (TIP47), as an essential connector between gp41CD and MA which mediates their interaction as part of a heterotrimeric complex that is indispensable for Env incorporation into HIV-1 virions in CD4+ T cells and HeLa cells (Table 1) (79). The same group recently reported similar findings for HIV-1 infection of primary macrophages (144). A different study confirmed the interaction between perilipin-3 and HIV-1 gp41CD, but the presence or absence of perilipin-3 did not seem to affect in vitro binding of gp41CD to MA (76). The same publication also demonstrated a direct interaction between SIV MA and SIV gp41CD and, interestingly, between SIV MA and HIV-1 gp41CD (76). Further studies are needed to resolve the specifics of gp41CD/MA interaction. Perilipin-3 has also been suggested to be involved in intracellular trafficking of Env, as described in more detail below.

INTRACELLULAR TRAFFICKING

gp41CD regulates intracellular trafficking of Env through a variety of interaction partners. The membrane-proximal YXXΦ motif in the gp41CDs of HIV-1 and SIV and the C-terminal dileucine motif of HIV-1, which mediate endocytosis (see above), have been shown to bind the μ1 subunit of the AP-1 complex in addition to μ2 of the AP-2 complex (Table 1) (5862, 65). Unlike the AP-2 complex, whose principal function is clathrin-mediated endocytosis from the plasma membrane, the AP-1 complex is primarily involved in clathrin-mediated trafficking between the trans-Golgi network (TGN) and endosomes (145). In polarized epithelial cells, the AP-1 complex is able to effect targeting to the basolateral membrane (145). This last property is consistent with the observation that the membrane-proximal YXXΦ motif of gp41CD directs the location of HIV-1 budding in Madin-Darby canine kidney (MDCK) cells (146) and Jurkat T cells (147). In the case of Jurkat T cells, this polarized budding was shown to be relevant for cell-to-cell viral transmission (147). Specifically, AP-1-mediated trafficking of Env might play an important role in targeting Env to the virological synapse during T-cell-to-T-cell transmission (148).

Another interaction partner of gp41CD that is involved in intracellular trafficking is the cellular protein perilipin-3 (Table 1) (78). The interaction between gp41CD and perilipin-3 requires the presence of a diaromatic YW motif contained within the LLP-3 region (800YW801 in Env of HIV-1 NL4-3). Perilipin-3 has been reported to be essential for the retrograde transport of mannose-6-phosphate receptors from endosomes to the TGN (149). Interestingly, perilipin-3 is also a member of the PAT family (thus named for the member proteins perilipin, adipophilin, and TIP47) of lipid droplet-associated proteins and as such is involved in lipid metabolism (150). The physiological relevance of perilipin-3 binding to gp41CD is not entirely clear, but disruption of this association altered the subcellular localization of Env (78, 144). It is worth noting that a cellular interaction partner of perilipin-3, the GTPase Rab9 (151), was independently shown to be required for efficient replication of HIV-1 (152).

Interestingly, another intracellular-trafficking protein reported to interact with gp41CD of both HIV-1 and SIV, the prenylated Rab acceptor 1 (PRA1) (Table 1) (38), also interacts functionally with Rab9 (153). PRA1 was identified as an interaction partner of SIV gp41 in a yeast two-hybrid screen (38). Binding could not be verified by coimmunoprecipitation or glutathione S-transferase (GST) pulldown but was detectable with modest efficiency in a mammalian two-hybrid assay. Based on the same assay, an interaction of PRA1 with HIV-1 gp41CD was also identified. The interaction between SIV gp41CD and PRA1 mapped to the LLP-2 region (38). However, a subsequent study found that neither overexpression of PRA1 nor its downregulation by RNA interference (RNAi) had any measurable effect on Env incorporation, infectivity, or virus production (154). The observation that depletion of PRA1 had no effect on various aspects of the life cycle of HIV-1 and SIV raises the question of whether PRA1 is truly a relevant interaction partner of gp41CD.

In addition to these protein interactions, gp41CD harbors determinants that have been reported to target Env to detergent-resistant membrane fractions, also known as lipid rafts. A highly conserved cysteine located in the vicinity of the LLP-2 region of HIV-1 and SIV gp41CD is subject to palmitoylation, which might serve as a membrane anchor and stabilize the association of gp41CD with the plasma membrane (80). A second cysteine located closer to the C terminus has also been shown to be palmitoylated but is less conserved among HIV-1 isolates and is absent in SIV isolates (117). The palmitoylation of gp41CD cysteines (or their replacement with bulky hydrophobic amino acids) has been suggested to target Env to lipid rafts (155, 156). Bhattacharya et al. reported that targeting of Env to lipid rafts enhances Env incorporation into virions and infectivity but is not required for either (155). Chen and colleagues have questioned the role of palmitoylation and have suggested instead that the LLP regions are principally responsible for targeting of Env to lipid rafts (157, 158).

CELLULAR ACTIVATION

Recently, the gp41CDs of HIV-1 and SIV were shown to induce activation of NF-κB, providing the first description of a role for gp41CD in the regulation of cellular activation (82). NF-κB (for nuclear factor κB) describes a heterogeneous family of transcription factors that are essential regulators of diverse biological processes, including cellular proliferation, survival, and differentiation, as well as innate and adaptive immunity (159). Importantly, NF-κB also serves as a major regulator for the transcription of the HIV-1 and SIV proviral genomes. The long terminal repeat (LTR) region of HIV-1 typically has two NF-κB-binding sequences; clade C isolates have at least three (160163). The LTR of SIVmac has one NF-κB-binding site (164). As a result of considerable redundancy of cis-acting elements within the LTR, these NF-κB-binding sites are not absolutely required for replication of HIV-1 or SIV (165168). However, activation of NF-κB can drastically enhance LTR-dependent transcription (161, 169, 170), and stimulation of HIV-1-infected cells with NF-κB-activating cytokines can strongly increase viral replication (134, 171, 172). gp41CD of HIV-1 and SIV induces NF-κB activation through the canonical pathway, and this process requires activation of the serine/threonine kinase transforming growth factor β (TGF-β)-activated kinase 1 (TAK1) (82). TAK1 is a central regulator of NF-κB activation upon which several branches of the canonical pathway converge. TAK1 phosphorylates and thereby activates the inhibitor of κB kinase (IKK) complex, which in turn phosphorylates inhibitor of κB (IκB) proteins and thus marks them for proteasomal degradation. Removal of IκB proteins allows cytoplasmic NF-κB dimers to translocate into the nucleus and act as regulators of transcription (173, 174). gp41CD of HIV-1 interacts physically with TAK1, and this interaction requires the same region of the cytoplasmic domain that is required for NF-κB activation (Fig. 2 and Table 1) (82), implying a causal relationship between TAK1 binding and induction of NF-κB activation by HIV-1 gp41CD. This region of HIV-1 gp41CD partly overlaps with LLP-2 and includes the second, distal YXXΦ motif (see above). SIV gp41CD does not interact detectably with TAK1 but appears to intersect the canonical NF-κB pathway through a signaling protein upstream of TAK1. This signaling protein remains to be identified. A mutant of HIV-1 NL4-3 deficient for gp41CD-mediated NF-κB activation did not display impaired infectivity or replication in fully activated cells but was unable to replicate efficiently in suboptimally activated host cells in which wild-type virus replicated successfully. Importantly, this defective phenotype of the gp41CD mutant could be rescued by exogenous stimulation of the host cells (82). These observations imply that the capacity of gp41CD to induce activation of NF-κB is important for the virus's ability to replicate under conditions of limited cellular activation and thus may be particularly relevant for replication in vivo, where the virus is likely to continuously encounter non- or suboptimally activated lymphocytes.

CYTOTOXICITY AND APOPTOSIS

Shortly after the isolation of HIV-1, it was suggested that gp41CD is responsible, at least in part, for the strong cytopathic effects observed during infection in cell culture (175, 176). Consistent with this notion, synthetic peptides corresponding in sequence to the LLP regions have the ability to lyse prokaryotic and eukaryotic cells in vitro, leading to their designation as lentivirus lytic peptides (55, 56, 177, 178). It is not clear what significance the cytotoxic potential of the individual LLP regions has in the context of the whole Env protein, much less during infection, and how much they contribute to the cytopathicity of infection by HIV or SIV. Costin et al. have suggested that the LLP regions might form a membrane pore in the context of the complete Env trimer and thus cause cytopathicity through uncontrolled ion flux between the cytosol and the extracellular medium (179, 180); however, there is currently limited evidence to support this hypothesis.

In addition to the cytopathic potential inherent in the LLP regions, the gp41CDs of HIV-1 and SIV have also been shown to interact with a regulator of apoptosis, calmodulin (CaM) (Table 1) (6671). CaM is a major sensor and regulator of intracellular Ca2+ concentration and, in addition to apoptosis regulation (181), is involved in a plethora of different cellular processes, including lymphocyte activation via the transcription factor family NF-AT (182, 183). Synthetic peptides corresponding to LLP-1 and LLP-2 sequences of HIV-1 and SIV have been shown to interact with CaM (6871), but binding in the context of full-length gp41CD appears to be confined largely to LLP-1, at least in the case of HIV-1 (66, 67). HIV-1 gp41CD induces an increase in the intracellular concentration of Ca2+, leads to spontaneous apoptosis, and enhances FAS-mediated apoptosis; these effects can be blocked by CaM inhibitors and by mutations that abrogate the interaction between CaM and gp41CD (66, 67, 184186). Mutant virus that is unable to bind to CaM through gp41CD was shown to replicate with kinetics indistinguishable from that of wild-type virus in tissue culture (184).

What benefit—if any—might the virus derive from inducing necrosis and/or apoptosis in the infected host cell? It is certainly conceivable that these cytotoxic effects are an unavoidable consequence of maximizing virus production and infectivity. Other possible explanations for the cytotoxicity ascribed to the LLP regions and the observed interaction between gp41CD and CaM include the following.

(i) The cytotoxicity of gp41CD might be exaggerated under conditions of experimental Env overexpression. Cytotoxic expression levels may not be reached during infection in vivo.

(ii) The cytotoxic sequences might have beneficial properties that outweigh any negative consequences of killing the host cell. For instance, the LLP regions are believed to form amphipathic helices, providing the biophysical basis for the postulated interaction of gp41CD with the membrane, which may be important for targeting of Env to lipid rafts and for stabilizing the Env trimer in the prefusion conformation (see above). The ability to interact with both hydrophobic and hydrophilic surfaces brings with it an inherent potential for membrane disruption, making it difficult to separate the ability to associate with the membrane from the cytotoxic potential. Similarly, CaM is a highly versatile regulatory protein that is involved in activities other than apoptosis (182, 183). It is possible that the interaction between gp41CD and CaM has evolved to exploit another function of CaM, for instance, its role in lymphocyte activation (182, 183). As optimal replication of HIV and SIV requires fully activated host cells (187190), using CaM to enhance the state of cellular activation would offer an obvious advantage to viral replication that may outweigh the concomitant disadvantage of inducing apoptosis.

(iii) Env is expressed late in the viral life cycle. It is possible that Env does not accumulate to cytotoxic levels before the host cell has already released large amounts of viral progeny, at which point the death of the host cell may be of limited consequence to the virus.

INTERACTIONS WITH REGULATORS OF THE CYTOSKELETON

Virtually all known vertebrate viruses, including HIV, manipulate the actin cytoskeleton during their replication cycles. The cortical actin fibers form a densely polymerized meshwork adjacent to the cell membrane and constitute a considerable physical obstacle to a virus (191). Manipulation of the actin cytoskeleton may allow the virus to penetrate the cortical actin network upon entry and egress (192, 193). While direct manipulation of actin filaments by gp41CD has not been experimentally established, gp41CD has been shown to interact with at least two proteins that are involved in the regulation of the actin cytoskeleton.

Specifically, HIV-1 gp41CD interacts with p115-RhoGEF, a guanine nucleotide exchange factor and activator of the RhoA GTPase (Table 1) (77). RhoA is a central regulator of actin stress fibers and focal adhesions (194) and is the target of manipulation by several other viruses (193). Additionally, RhoA is involved in the regulation of cell cycle progression and the transcription factors serum response factor (SRF) and NF-κB (195). Importantly, activation of RhoA by p115-RhoGEF has been shown to inhibit replication of HIV-1 (196). The mechanism of this inhibition has never been resolved, but it surprisingly appears to be unrelated to actin reorganization. Wang et al. were able to show that inhibition of HIV-1 transcription by RhoA activation was independent of the known RhoA functions of stress fiber formation, SRF activation, and transformation and therefore suggested RhoA might inhibit HIV-1 transcription through a novel, hitherto-unknown pathway (196). The interaction between gp41CD and p115-RhoGEF required the presence of LLP-2, LLP-3, and LLP-1 and could be abrogated by single-amino-acid substitutions in LLP-3, e.g., L798R (77). Intriguingly, the L798R mutation blocked viral replication in three different cell lines. Conversely, the presence of wild-type gp41CD inhibited p115-RhoGEF signaling, including RhoA activation (77). Although it was never formally demonstrated, it seems likely that the inhibition of p115-RhoGEF by gp41CD binding has evolved to inhibit activation of RhoA and thus to facilitate viral replication. gp41CD of SIV did not detectably interact with p115-RhoGEF (38).

Interestingly, RhoA interacts functionally and physically with the cadherin complex, which includes another interaction partner of gp41CD, α-catenin (Table 1) (72, 73, 197). α-Catenin is primarily known for its pivotal role in cell-cell adhesion as part of the α-catenin/β-catenin/cadherin complex. During the formation of intercellular cadherin interactions, α-catenin is believed to induce the reorganization of the actin cytoskeleton by binding to actin filaments (198). Recently, additional activities have been attributed to α-catenin, including a role in the regulation of cell proliferation and hedgehog signaling (199). Kim et al. reported that HIV-1 gp41CD and α-catenin interact directly through the LLP-3 region (72, 73). SIV gp41CD also interacted with α-catenin in yeast two-hybrid assays, but the group did not confirm this interaction with a more rigorous assay (72). The interaction with HIV-1 gp41CD mapped to the C-terminal region of α-catenin, which also harbors the actin-binding domain. β-Catenin did not interact directly with gp41CD (73). As cadherin complexes mediate cell-cell adhesion, it has been suggested that gp41CD binding to α-catenin might be involved in cell-to-cell transmission of HIV-1 (73). However, to our knowledge there is currently no report that implicates cadherins and/or α-catenin in the formation of cell contacts between T cells. Alternatively, α-catenin might serve as an adaptor between gp41CD and the actin cytoskeleton, serving either to target Env to the site of viral budding or to reorganize the actin cytoskeleton to facilitate egress. Attractive hypotheses notwithstanding, a physiological role for the interaction of α-catenin with gp41CD remains to be demonstrated.

Of note, α-catenin has been reported to interact with gp41CD in the same region as p115-RhoGEF, inviting speculation that the separately published interactions of α-catenin and p115-RhoGEF with gp41CD might indeed be related phenomena.

OVERCOMING CELLULAR RESTRICTION

Two recently identified interaction partners of gp41CD, luman (74) and prohibitin 1/2 (81) (Table 1), indicate that gp41CD might also act to overcome certain aspects of cellular restriction of HIV replication.

Luman (also known as CREB3 or LZIP) is a poorly characterized member of the OASIS family of transcription factors (200). Luman appears to be associated with the ER stress response, or unfolded protein response (UPR) (200). Both known target genes of luman are involved in ER-associated degradation (ERAD) of unfolded protein (200). Blot et al. found that the ER membrane-bound, inactive, full-length form of luman interacted with gp41CD of HIV-1, SIV, and HIV-2 in yeast two-hybrid assays (74). For HIV-1 gp41CD, this interaction was confirmed by coprecipitation and mapped to a region N-terminal of LLP-2. Expression of gp41CD led to accelerated degradation of full-length luman, implying that the interaction between gp41CD and luman has evolved to reduce levels of luman in the infected cell. Interestingly, a cleaved, constitutively active form of luman reduced proviral transcription (74). The mechanism by which activated luman represses proviral transcription and what role these interactions play during infection remain to be elucidated; however, gp41CD may have evolved to counteract the antiviral effects of luman.

Emerson et al. have identified an interaction between HIV-1 gp41CD and a (presumably heterodimeric) complex of the proteins prohibitin 1 and prohibitin 2 (81). Prohibitin 1/2 are involved in a multitude of cellular processes, including signal transduction, mitochondrial function, apoptosis, and cell proliferation (201, 202). They are localized primarily to mitochondria but have also been detected at the plasma membrane and in the nucleus (202). The interaction between gp41CD and prohibitin 1/2 mapped to a region in the center of LLP-3 (81). Mutations of gp41CD that abrogated binding to prohibitin 1/2 produced a replication phenotype reminiscent of complete gp41CD truncations; i.e., mutant HIV-1 was able to replicate in a cell line permissive for gp41CD truncation but not in a nonpermissive cell line (see above). This prompted the authors to speculate that prohibitin 1/2 binding by gp41CD may be essential for HIV-1 replication in nonpermissive cell lines. It is not clear how prohibitin 1/2 might influence viral replication, as Env incorporation into virions was not affected in the absence of prohibitin 1/2 binding to gp41CD, and no other mechanism has been proposed (81).

Serra-Moreno et al. recently described a particularly interesting example of gp41CD adaptation to counteract a cellular restriction factor (203). They reported that an SIV variant from a rhesus macaque that had been infected with a form of SIVmac with a nef deletion had acquired the ability to bind to and counteract the cellular restriction factor bone marrow stromal antigen 2 (BST-2; also known as CD317, HM1.24 antigen, or tetherin) via its gp41CD (203). BST-2 is an interferon-inducible factor that is able to inhibit the spread of enveloped viruses by “tethering” them to the cell surface after egress and mediating their endocytosis (204206). HIV-1 typically uses its Vpu protein to bind to and counteract BST-2, while HIV-2 uses the extracellular domain of its Env protein. Most SIV strains counteract BST-2 through Nef (204). Infection of rhesus macaques with nef-deleted SIVmac (SIVmac Δnef) typically has a considerably attenuated phenotype (207). However, the infection persists at low levels, and in some cases, the virus can acquire compensatory mutations that restore its pathogenic potential to various degrees (135, 208). Serra-Moreno et al. studied one such variant of SIVmac Δnef with increased pathogenic potential and found that it had accumulated mutations in its LLP-3 and LLP-1 regions of gp41CD that allowed it to bind to and counteract BST-2 (203). Interestingly, inhibition of BST-2 activity, but not binding to BST-2, required the membrane-proximal YXXΦ motif of gp41CD (203). This example illustrates the impressive functional versatility and adaptability of gp41CD.

CONCLUDING REMARKS

It now seems quite clear that the cytoplasmic domain of the HIV-1 and SIV gp41 glycoproteins contributes a variety of different functional activities to the viral life cycle. In particular, these include endocytosis to regulate levels of Env surface expression, incorporation of Env into virions, intracellular trafficking of Env, and activation of the host cell. Some of the cellular interaction partners mediating these functions have been reasonably well characterized. However, several other interaction partners of gp41CD whose functional relevance is less clear have been described. These interactions seem to implicate gp41CD in an even wider range of functions, including apoptosis induction, actin modulation, and countering aspects of cellular restriction. Assessment of the significance of these interaction partners is currently incomplete and will require further investigation. Considering that α-catenin, p115-RhoGEF, prohibitin 1/2, and perilipin-3 all have been reported to interact with gp41CD in the same region (Table 1), use of a mutational approach to gauge relative importance and phenotypic manifestations will be a daunting challenge.

In addition to these relatively new questions, the long-standing puzzle of the cell-type-dependent requirement of gp41CD for viral replication remains an exciting area of research. The complex phenotype observed with truncated gp41CD of HIV-1 and SIV bears the hallmarks of a species-specific and differentially expressed restriction factor that can be counteracted by gp41CD; however, whether such a restriction factor does indeed exist and, if so, the identity of this factor remain to be elucidated. Another intriguing question is whether difficult-to-revert mutations that reduce Env endocytosis result in more effective immunological control of viral replication. Answering this question will undoubtedly require creative experimental design using the rhesus macaque model of SIV infection.

Biographies

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Thomas S. Postler performed his Ph.D. research in the laboratory of Dr. Ronald Desrosiers at the New England Primate Research Center (NEPRC) of Harvard Medical School. For his thesis, he investigated topological and functional properties of the cytoplasmic domain of the HIV and SIV envelope glycoprotein. Dr. Postler's research interests are centered around host-pathogen interactions at a molecular level, with a particular focus on the immune system and how pathogens manipulate signal transduction events to evade or usurp it. He is currently a postdoctoral research scientist in the laboratory of Dr. Sankar Ghosh at Columbia University.

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Ronald C. Desrosiers is Professor of Microbiology and Immunobiology at the New England Primate Research Center (NEPRC) of Harvard Medical School. He received a Ph.D. in Biochemistry from Michigan State University and did postdoctoral work with Dr. Peter Lengyel at Yale University before joining NEPRC. Since his publication of the discovery of SIV in 1985, he has contributed to a better understanding of the relative importance and functional contribution of the so-called “nonessential” genes, of envelope protein structure/function relationships, and of antibody-mediated virus neutralization. He has studied a variety of approaches toward a preventive vaccine against HIV/AIDS and continues to be a strong advocate for the importance of discovery research toward this goal. His interest in the cytoplasmic domain of gp41 dates back to the original observation of its truncation when SIV is cultured in human cells.

Footnotes

Published ahead of print 17 October 2012

REFERENCES

1. Allan JS, Coligan JE, Barin F, McLane MF, Sodroski JG, Rosen A, Haseltine WA, Lee TH, Essex M. 1985. Major glycoprotein antigens that induce antibodies in AIDS patients are encoded by HTLV-III. Science 228:1091–1094. [PubMed]
2. Veronese FD, Joseph B, Copeland TD, Oroszlan S, Gallo RC, Sarngadharan MG. 1989. Identification of simian immunodeficiency virus SIVMAC env gene products. J. Virol. 63:1416–1419. [PMC free article] [PubMed]
3. Veronese FD, DeVico AL, Copeland TD, Oroszlan S, Gallo RC, Sarngadharan MG. 1985. Characterization of gp41 as the transmembrane protein coded by the HTLV-III/LAV envelope gene. Science 229:1402–1405. [PubMed]
4. Blacklow SC, Lu M, Kim PS. 1995. A trimeric subdomain of the simian immunodeficiency virus envelope glycoprotein. Biochemistry 34:14955–14962. [PubMed]
5. Chan DC, Fass D, Berger JM, Kim PS. 1997. Core structure of gp41 from the HIV envelope glycoprotein. Cell 89:263–273. [PubMed]
6. Gelderblom H, Özel M, Hausmann E, Winkel T, Pauli G, Koch MA. 1988. Fine structure of human immunodeficiency virus (HIV), immunolocalization of structural proteins and virus-cell relation. Micron Microsc. Acta 19:41–60.
7. Grief C, Hockley DJ, Fromholc CE, Kitchin PA. 1989. The morphology of simian immunodeficiency virus as shown by negative staining electron microscopy. J. Gen. Virol. 70:2215–2219. [PubMed]
8. Lu M, Blacklow SC, Kim PS. 1995. A trimeric structural domain of the HIV-1 transmembrane glycoprotein. Nat. Struct. Biol. 2:1075–1082. [PubMed]
9. Pinter A, Honnen WJ, Tilley SA, Bona C, Zaghouani H, Gorny MK, Zolla-Pazner S. 1989. Oligomeric structure of gp41, the transmembrane protein of human immunodeficiency virus type 1. J. Virol. 63:2674–2679. [PMC free article] [PubMed]
10. Weiss CD, Levy JA, White JM. 1990. Oligomeric organization of gp120 on infectious human immunodeficiency virus type 1 particles. J. Virol. 64:5674–5677. [PMC free article] [PubMed]
11. Weissenhorn W, Dessen A, Harrison SC, Skehel JJ, Wiley DC. 1997. Atomic structure of the ectodomain from HIV-1 gp41. Nature 387:426–430. [PubMed]
12. Zhu P, Chertova E, Bess JJ, Lifson JD, Arthur LO, Liu J, Taylor KA, Roux KH. 2003. Electron tomography analysis of envelope glycoprotein trimers on HIV and simian immunodeficiency virus virions. Proc. Natl. Acad. Sci. U. S. A. 100:15812–15817. [PubMed]
13. McDougal JS, Kennedy MS, Sligh JM, Cort SP, Mawle A, Nicholson JK. 1986. Binding of HTLV-III/LAV to T4+ T cells by a complex of the 110K viral protein and the T4 molecule. Science 231:382–385. [PubMed]
14. Trkola A, Dragic T, Arthos J, Binley JM, Olson WC, Allaway GP, Cheng-Mayer C, Robinson J, Maddon PJ, Moore JP. 1996. CD4-dependent, antibody-sensitive interactions between HIV-1 and its co-receptor CCR-5. Nature 384:184–187. [PubMed]
15. Wu L, Gerard NP, Wyatt R, Choe H, Parolin C, Ruffing N, Borsetti A, Cardoso AA, Desjardin E, Newman W, Gerard C, Sodroski J. 1996. CD4-induced interaction of primary HIV-1 gp120 glycoproteins with the chemokine receptor CCR-5. Nature 384:179–183. [PubMed]
16. Bosch ML, Earl PL, Fargnoli K, Picciafuoco S, Giombini F, Wong-Staal F, Franchini G. 1989. Identification of the fusion peptide of primate immunodeficiency viruses. Science 244:694–697. [PubMed]
17. Freed EO, Myers DJ, Risser R. 1990. Characterization of the fusion domain of the human immunodeficiency virus type 1 envelope glycoprotein gp41. Proc. Natl. Acad. Sci. U. S. A. 87:4650–4654. [PubMed]
18. Kowalski M, Potz J, Basiripour L, Dorfman T, Goh WC, Terwilliger E, Dayton A, Rosen C, Haseltine W, Sodroski J. 1987. Functional regions of the envelope glycoprotein of human immunodeficiency virus type 1. Science 237:1351–1355. [PubMed]
19. Haffar OK, Dowbenko DJ, Berman PW. 1988. Topogenic analysis of the human immunodeficiency virus type 1 envelope glycoprotein, gp160, in microsomal membranes. J. Cell Biol. 107:1677–1687. [PMC free article] [PubMed]
20. Muesing MA, Smith DH, Cabradilla CD, Benton CV, Lasky LA, Capon DJ. 1985. Nucleic acid structure and expression of the human AIDS/lymphadenopathy retrovirus. Nature 313:450–458. [PubMed]
21. Kitchen LW, Barin F, Sullivan JL, McLane MF, Brettler DB, Levine PH, Essex M. 1984. Aetiology of AIDS—antibodies to human T-cell leukaemia virus (type III) in haemophiliacs. Nature 312:367–369. [PubMed]
22. Montagnier L, Clavel F, Krust B, Chamaret S, Rey F, Barré-Sinoussi F, Chermann JC. 1985. Identification and antigenicity of the major envelope glycoprotein of lymphadenopathy-associated virus. Virology 144:283–289. [PubMed]
23. Montefiori DC, Robinson WE, Mitchell WM. 1988. Role of protein N-glycosylation in pathogenesis of human immunodeficiency virus type 1. Proc. Natl. Acad. Sci. U. S. A. 85:9248–9252. [PubMed]
24. Stansell E, Desrosiers RC. 2010. Fundamental difference in the content of high-mannose carbohydrate in the HIV-1 and HIV-2 lineages. J. Virol. 84:8998–9009. [PMC free article] [PubMed]
25. Zhang M, Gaschen B, Blay W, Foley B, Haigwood N, Kuiken C, Korber B. 2004. Tracking global patterns of N-linked glycosylation site variation in highly variable viral glycoproteins: HIV, SIV, and HCV envelopes and influenza hemagglutinin. Glycobiology 14:1229–1246. [PubMed]
26. Bernstein HB, Tucker SP, Hunter E, Schutzbach JS, Compans RW. 1994. Human immunodeficiency virus type 1 envelope glycoprotein is modified by O-linked oligosaccharides. J. Virol. 68:463–468. [PMC free article] [PubMed]
27. Stansell E, Canis K, Haslam SM, Dell A, Desrosiers RC. 2011. Simian immunodeficiency virus from the sooty mangabey and rhesus macaque is modified with O-linked carbohydrate. J. Virol. 85:582–595. [PMC free article] [PubMed]
28. Dewar RL, Vasudevachari MB, Natarajan V, Salzman NP. 1989. Biosynthesis and processing of human immunodeficiency virus type 1 envelope glycoproteins: effects of monensin on glycosylation and transport. J. Virol. 63:2452–2456. [PMC free article] [PubMed]
29. Hallenberger S, Bosch V, Angliker H, Shaw E, Klenk HD, Garten W. 1992. Inhibition of furin-mediated cleavage activation of HIV-1 glycoprotein gp160. Nature 360:358–361. [PubMed]
30. Hallenberger S, Moulard M, Sordel M, Klenk HD, Garten W. 1997. The role of eukaryotic subtilisin-like endoproteases for the activation of human immunodeficiency virus glycoproteins in natural host cells. J. Virol. 71:1036–1045. [PMC free article] [PubMed]
31. Moulard M, Decroly E. 2000. Maturation of HIV envelope glycoprotein precursors by cellular endoproteases. Biochim. Biophys. Acta 1469:121–132. [PubMed]
32. Stein BS, Engleman EG. 1990. Intracellular processing of the gp160 HIV-1 envelope precursor: endoproteolytic cleavage occurs in a cis or medial compartment of the Golgi complex. J. Biol. Chem. 265:2640–2649. [PubMed]
33. Checkley MA, Luttge BG, Freed EO. 2011. HIV-1 envelope glycoprotein biosynthesis, trafficking, and incorporation. J. Mol. Biol. 410:582–608. [PMC free article] [PubMed]
34. Wyatt R, Sodroski J. 1998. The HIV-1 envelope glycoproteins: fusogens, antigens, and immunogens. Science 280:1884–1888. [PubMed]
35. White TA, Bartesaghi A, Borgnia MJ, Meyerson JR, de la Cruz MJ, Bess JW, Nandwani R, Hoxie JA, Lifson JD, Milne JLS, Subramaniam S. 2010. Molecular architectures of trimeric SIV and HIV-1 envelope glycoproteins on intact viruses: strain-dependent variation in quaternary structure. PLoS Pathog. 6:e1001249 doi:10.1371/journal.ppat.1001249. [PMC free article] [PubMed]
36. Zhu P, Liu J, Bess JJ, Chertova E, Lifson JD, Grise H, Ofek GA, Taylor KA, Roux KH. 2006. Distribution and three-dimensional structure of AIDS virus envelope spikes. Nature 441:847–852. [PubMed]
37. Krogh A, Larsson B, von Heijne G, Sonnhammer GEL. 2001. Predicting transmembrane protein topology with a hidden Markov model: application to complete genomes. J. Mol. Biol. 305:567–580. [PubMed]
38. Evans DT, Tillman KC, Desrosiers RC. 2002. Envelope glycoprotein cytoplasmic domains from diverse lentiviruses interact with the prenylated Rab acceptor. J. Virol. 76:327–337. [PMC free article] [PubMed]
39. Kirchhoff F, Greenough TC, Brettler DB, Sullivan JL, Desrosiers RC. 1995. Brief report: absence of intact nef sequences in a long-term survivor with nonprogressive HIV-1 infection. N. Engl. J. Med. 332:228–232. [PubMed]
40. Kirchhoff F, Kestler HW, Desrosiers RC. 1994. Upstream U3 sequences in simian immunodeficiency virus are selectively deleted in vivo in the absence of an intact nef gene. J. Virol. 68:2031–2037. [PMC free article] [PubMed]
41. Broliden PA, von Gegerfelt A, Clapham P, Rosen J, Fenyö EM, Wahren B, Broliden K. 1992. Identification of human neutralization-inducing regions of the human immunodeficiency virus type 1 envelope glycoproteins. Proc. Natl. Acad. Sci. U. S. A. 89:461–465. [PubMed]
42. Cheingsong-Popov R, Callow D, Beddows S, Shaunak S, Wasi C, Kaleebu P, Gilks C, Petrascu IV, Garaev MM, Watts DM. 1992. Geographic diversity of human immunodeficiency virus type 1: serologic reactivity to env epitopes and relationship to neutralization. J. Infect. Dis. 165:256–261. [PubMed]
43. Goudsmit J, Meloen RH, Brasseur R. 1990. Map of sequential B cell epitopes of the HIV-1 transmembrane protein using human antibodies as probe. Intervirology 31:327–338. [PubMed]
44. Kennedy RC, Henkel RD, Pauletti D, Allan JS, Lee TH, Essex M, Dreesman GR. 1986. Antiserum to a synthetic peptide recognizes the HTLV-III envelope glycoprotein. Science 231:1556–1559. [PubMed]
45. Postler TS, Martinez-Navio JM, Yuste E, Desrosiers RC. 2012. Evidence against extracellular exposure of a highly immunogenic region in the C-terminal domain of the simian immunodeficiency virus gp41 transmembrane protein. J. Virol. 86:1145–1157. [PMC free article] [PubMed]
46. Vella C, Minor PD, Weller IV, Jenkins O, Evans D, Almond J. 1991. Recognition of poliovirus/HIV chimaeras by antisera from individuals with HIV infection. AIDS 5:425–430. [PubMed]
47. Eisenberg D, Wesson M. 1990. The most highly amphiphilic alpha-helices include two amino acid segments in human immunodeficiency virus glycoprotein 41. Biopolymers 29:171–177. [PubMed]
48. Venable RM, Pastor RW, Brooks BR, Carson FW. 1989. Theoretically determined three-dimensional structures for amphipathic segments of the HIV-1 gp41 envelope protein. AIDS Res. Hum. Retroviruses 5:7–22. [PubMed]
49. Kliger Y, Shai Y. 1997. A leucine zipper-like sequence from the cytoplasmic tail of the HIV-1 envelope glycoprotein binds and perturbs lipid bilayers. Biochemistry 36:5157–5169. [PubMed]
50. Cleveland SM, McLain L, Cheung L, Jones TD, Hollier M, Dimmock NJ. 2003. A region of the C-terminal tail of the gp41 envelope glycoprotein of human immunodeficiency virus type 1 contains a neutralizing epitope: evidence for its exposure on the surface of the virion. J. Gen. Virol. 84:591–602. [PubMed]
51. Dimmock NJ. 2005. The complex antigenicity of a small external region of the C-terminal tail of the HIV-1 gp41 envelope protein: a lesson in epitope analysis. Rev. Med. Virol. 15:365–381. [PubMed]
52. Lu L, Zhu Y, Huang J, Chen X, Yang H, Jiang S, Chen YH. 2008. Surface exposure of the HIV-1 Env cytoplasmic tail LLP2 domain during the membrane fusion process: interaction with gp41 fusion core. J. Biol. Chem. 283:16723–16731. [PubMed]
53. Steckbeck JD, Sun C, Sturgeon TJ, Montelaro RC. 2010. Topology of the C-terminal tail of HIV-1 gp41: differential exposure of the Kennedy epitope on cell and viral membranes. PLoS One 5:e15261 doi:10.1371/journal.pone.0015261. [PMC free article] [PubMed]
54. Liu S, Kondo N, Long Y, Xiao D, Iwamoto A, Matsuda Z. 2010. Membrane topology analysis of HIV-1 envelope glycoprotein gp41. Retrovirology 7:100 doi:10.1186/1742-4690-7-100. [PMC free article] [PubMed]
55. Moreno MR, Pérez-Berná AJ, Guillén J, Villalaín J. 2008. Biophysical characterization and membrane interaction of the most membranotropic region of the HIV-1 gp41 endodomain. Biochim. Biophys. Acta 1778:1298–1307. [PubMed]
56. Srinivas SK, Srinivas RV, Anantharamaiah GM, Segrest JP, Compans RW. 1992. Membrane interactions of synthetic peptides corresponding to amphipathic helical segments of the human immunodeficiency virus type-1 envelope glycoprotein. J. Biol. Chem. 267:7121–7127. [PubMed]
57. Haffar OK, Dowbenko DJ, Berman PW. 1991. The cytoplasmic tail of HIV-1 gp160 contains regions that associate with cellular membranes. Virology 180:439–441. [PubMed]
58. Berlioz-Torrent C, Shacklett BL, Erdtmann L, Delamarre L, Bouchaert I, Sonigo P, Dokhelar MC, Benarous R. 1999. Interactions of the cytoplasmic domains of human and simian retroviral transmembrane proteins with components of the clathrin adaptor complexes modulate intracellular and cell surface expression of envelope glycoproteins. J. Virol. 73:1350–1361. [PMC free article] [PubMed]
59. Bowers K, Pelchen-Matthews A, Hoening S, Vance PJ, Creary L, Haggarty BS, Romano J, Ballensiefen W, Hoxie JA, Marsh M. 2000. The simian immunodeficiency virus envelope glycoprotein contains multiple signals that regulate its cell surface expression and endocytosis. Traffic 1:661–674. [PubMed]
60. Ohno H, Aguilar RC, Fournier Hennecke M-CS, Cosson P, Bonifacino JS. 1997. Interaction of endocytic signals from the HIV-1 envelope glycoprotein complex with members of the adaptor medium chain family. Virology 238:305–315. [PubMed]
61. Wyss S, Berlioz-Torrent C, Boge M, Blot G, Hoening S, Benarous R, Thali M. 2001. The highly conserved C-terminal dileucine motif in the cytosolic domain of the human immunodeficiency virus type 1 envelope glycoprotein is critical for its association with the AP-1 clathrin adaptor. J. Virol. 75:2982–2992. [PMC free article] [PubMed]
62. Boge M, Wyss S, Bonifacino JS, Thali M. 1998. A membrane-proximal tyrosine-based signal mediates internalization of the HIV-1 envelope glycoprotein via interaction with the AP-2 clathrin adaptor. J. Biol. Chem. 273:15773–15778. [PubMed]
63. Byland R, Vance PJ, Hoxie JA, Marsh M. 2007. A conserved dileucine motif mediates clathrin and AP-2-dependent endocytosis of the HIV-1 envelope protein. Mol. Biol. Cell 18:414–425. [PMC free article] [PubMed]
64. LaBranche CC, Sauter MM, Haggarty BS, Vance PJ, Romano J, Hart TK, Bugelski PJ, Marsh M, Hoxie JA. 1995. A single amino acid change in the cytoplasmic domain of the simian immunodeficiency virus transmembrane molecule increases envelope glycoprotein expression on infected cells. J. Virol. 69:5217–5227. [PMC free article] [PubMed]
65. Rowell JF, Stanhope PE, Siliciano RF. 1995. Endocytosis of endogenously synthesized HIV-1 envelope protein. J. Immunol. 155:473–488. [PubMed]
66. Ishikawa H, Sasaki M, Noda S, Koga Y. 1998. Apoptosis induction by the binding of the carboxyl terminus of human immunodeficiency virus type 1 gp160 to calmodulin. J. Virol. 72:6574–6580. [PMC free article] [PubMed]
67. Micoli KJ, Pan G, Wu Y, Williams JP, Cook WJ, McDonald JM. 2000. Requirement of calmodulin binding by HIV-1 gp160 for enhanced FAS-mediated apoptosis. J. Biol. Chem. 275:1233–1240. [PubMed]
68. Miller MA, Mietzner TA, Cloyd MW, Robey WG, Montelaro RC. 1993. Identification of a calmodulin-binding and inhibitory peptide domain in the HIV-1 transmembrane glycoprotein. AIDS Res. Hum. Retroviruses 9:1057–1066. [PubMed]
69. Sham SWS, McDonald JM, Micoli KJ, Krishna NR. 2008. Solution structure of a calmodulin-binding domain in the carboxy-terminal region of HIV type 1 gp160. AIDS Res. Hum. Retroviruses 24:607–616. [PubMed]
70. Srinivas SK, Srinivas RV, Anantharamaiah GM, Compans RW, Segrest JP. 1993. Cytosolic domain of the human immunodeficiency virus envelope glycoproteins binds to calmodulin and inhibits calmodulin-regulated proteins. J. Biol. Chem. 268:22895–22899. [PubMed]
71. Yuan T, Mietzner TA, Montelaro RC, Vogel HJ. 1995. Characterization of the calmodulin binding domain of SIV transmembrane glycoprotein by NMR and CD spectroscopy. Biochemistry 34:10690–10696. [PubMed]
72. Kim EM, Lee KH, Kim JW. 1999. The cytoplasmic domain of HIV-1 gp41 interacts with the carboxy-terminal region of alpha-catenin. Mol. Cells 9:281–285. [PubMed]
73. Kim JT, Kim EM, Lee KH, Choi J-E, Jhun BH, Kim JW. 2002. Leucine zipper domain of HIV-1 gp41 interacted specifically with alpha-catenin. Biochem. Biophys. Res. Commun. 291:1239–1244. [PubMed]
74. Blot G, Lopez-Verges S, Treand C, Kubat NJ, Delcroix-Genete D, Emiliani S, Benarous R, Berlioz-Torrent C. 2006. Luman, a new partner of HIV-1 TMgp41, interferes with Tat-mediated transcription of the HIV-1 LTR. J. Mol. Biol. 364:1034–1047. [PubMed]
75. Cosson P. 1996. Direct interaction between the envelope and matrix proteins of HIV-1. EMBO J. 15:5783–5788. [PubMed]
76. Manrique JM, Affranchino JL, Gonzalez SA. 2008. In vitro binding of simian immunodeficiency virus matrix protein to the cytoplasmic domain of the envelope glycoprotein. Virology 374:273–279. [PubMed]
77. Zhang H, Wang L, Kao S, Whitehead IP, Hart MJ, Liu B, Duus K, Burridge K, Der CJ, Su L. 1999. Functional interaction between the cytoplasmic leucine-zipper domain of HIV-1 gp41 and p115-RhoGEF. Curr. Biol. 9:1271–1274. [PubMed]
78. Blot G, Janvier K, Le Panse S, Benarous R, Berlioz-Torrent C. 2003. Targeting of the human immunodeficiency virus type 1 envelope to the trans-Golgi network through binding to TIP47 is required for Env incorporation into virions and infectivity. J. Virol. 77:6931–6945. [PMC free article] [PubMed]
79. Lopez-Vergès S, Camus G, Blot G, Beauvoir R, Benarous R, Berlioz-Torrent C. 2006. Tail-interacting protein TIP47 is a connector between Gag and Env and is required for Env incorporation into HIV-1 virions. Proc. Natl. Acad. Sci. U. S. A. 103:14947–14952. [PubMed]
80. Yang C, Spies CP, Compans RW. 1995. The human and simian immunodeficiency virus envelope glycoprotein transmembrane subunits are palmitoylated. Proc. Natl. Acad. Sci. U. S. A. 92:9871–9875. [PubMed]
81. Emerson V, Holtkotte D, Pfeiffer T, Wang Schnolzer I-HM, Kempf T, Bosch V. 2010. Identification of the cellular prohibitin 1/prohibitin 2 heterodimer as an interaction partner of the C-terminal cytoplasmic domain of the HIV-1 glycoprotein. J. Virol. 84:1355–1365. [PMC free article] [PubMed]
82. Postler TS, Desrosiers RC. 2012. The cytoplasmic domain of the HIV-1 glycoprotein gp41 induces NF-κB activation through TGF-β-activated kinase 1. Cell Host Microbe 11:181–193. [PMC free article] [PubMed]
83. Viard M, Ablan SD, Zhou M, Veenstra TD, Freed EO, Raviv Y, Blumenthal R. 2008. Photoinduced reactivity of the HIV-1 envelope glycoprotein with a membrane-embedded probe reveals insertion of portions of the HIV-1 Gp41 cytoplasmic tail into the viral membrane. Biochemistry 47:1977–1983. [PMC free article] [PubMed]
84. Abrahamyan LG, Mkrtchyan SR, Binley J, Lu M, Melikyan GB, Cohen FS. 2005. The cytoplasmic tail slows the folding of human immunodeficiency virus type 1 Env from a late prebundle configuration into the six-helix bundle. J. Virol. 79:106–115. [PMC free article] [PubMed]
85. Freed EO, Martin MA. 1996. Domains of the human immunodeficiency virus type 1 matrix and gp41 cytoplasmic tail required for envelope incorporation into virions. J. Virol. 70:341–351. [PMC free article] [PubMed]
86. Mulligan MJ, Yamshchikov GV, Ritter GD, Gao F, Jin MJ, Nail CD, Spies CP, Hahn BH, Compans RW. 1992. Cytoplasmic domain truncation enhances fusion activity by the exterior glycoprotein complex of human immunodeficiency virus type 2 in selected cell types. J. Virol. 66:3971–3975. [PMC free article] [PubMed]
87. Ritter GDJ, Mulligan MJ, Lydy SL, Compans RW. 1993. Cell fusion activity of the simian immunodeficiency virus envelope protein is modulated by the intracytoplasmic domain. Virology 197:255–264. [PubMed]
88. Wilk T, Pfeiffer T, Bosch V. 1992. Retained in vitro infectivity and cytopathogenicity of HIV-1 despite truncation of the C-terminal tail of the env gene product. Virology 189:167–177. [PubMed]
89. Zingler K, Littman DR. 1993. Truncation of the cytoplasmic domain of the simian immunodeficiency virus envelope glycoprotein increases Env incorporation into particles and fusogenicity and infectivity. J. Virol. 67:2824–2831. [PMC free article] [PubMed]
90. Kalia V, Sarkar S, Gupta P, Montelaro RC. 2003. Rational site-directed mutations of the LLP-1 and LLP-2 lentivirus lytic peptide domains in the intracytoplasmic tail of human immunodeficiency virus type 1 gp41 indicate common functions in cell-cell fusion but distinct roles in virion envelope incorporation. J. Virol. 77:3634–3646. [PMC free article] [PubMed]
91. Wyss S, Dimitrov AS, Baribaud F, Edwards TG, Blumenthal R, Hoxie JA. 2005. Regulation of human immunodeficiency virus type 1 envelope glycoprotein fusion by a membrane-interactive domain in the gp41 cytoplasmic tail. J. Virol. 79:12231–12241. [PMC free article] [PubMed]
92. Jiang J, Aiken C. 2007. Maturation-dependent human immunodeficiency virus type 1 particle fusion requires a carboxyl-terminal region of the gp41 cytoplasmic tail. J. Virol. 81:9999–10008. [PMC free article] [PubMed]
93. Murakami T, Ablan S, Freed EO, Tanaka Y. 2004. Regulation of human immunodeficiency virus type 1 Env-mediated membrane fusion by viral protease activity. J. Virol. 78:1026–1031. [PMC free article] [PubMed]
94. Wyma DJ, Jiang J, Shi J, Zhou J, Lineberger JE, Miller MD, Aiken C. 2004. Coupling of human immunodeficiency virus type 1 fusion to virion maturation: a novel role of the gp41 cytoplasmic tail. J. Virol. 78:3429–3435. [PMC free article] [PubMed]
95. Mammano F, Kondo E, Sodroski J, Bukovsky A, Göttlinger HG. 1995. Rescue of human immunodeficiency virus type 1 matrix protein mutants by envelope glycoproteins with short cytoplasmic domains. J. Virol. 69:3824–3830. [PMC free article] [PubMed]
96. Murakami T, Freed EO. 2000. The long cytoplasmic tail of gp41 is required in a cell type-dependent manner for HIV-1 envelope glycoprotein incorporation into virions. Proc. Natl. Acad. Sci. U. S. A. 97:343–348. [PubMed]
97. Akari H, Fukumori T, Adachi A. 2000. Cell-dependent requirement of human immunodeficiency virus type 1 gp41 cytoplasmic tail for Env incorporation into virions. J. Virol. 74:4891–4893. [PMC free article] [PubMed]
98. Dubay JW, Roberts SJ, Hahn BH, Hunter E. 1992. Truncation of the human immunodeficiency virus type 1 transmembrane glycoprotein cytoplasmic domain blocks virus infectivity. J. Virol. 66:6616–6625. [PMC free article] [PubMed]
99. Yu X, Yuan X, McLane MF, Lee TH, Essex M. 1993. Mutations in the cytoplasmic domain of human immunodeficiency virus type 1 transmembrane protein impair the incorporation of Env proteins into mature virions. J. Virol. 67:213–221. [PMC free article] [PubMed]
100. Gabuzda DH, Lever A, Terwilliger E, Sodroski J. 1992. Effects of deletions in the cytoplasmic domain on biological functions of human immunodeficiency virus type 1 envelope glycoproteins. J. Virol. 66:3306–3315. [PMC free article] [PubMed]
101. Jones DR, Suzuki K, Piller SC. 2002. A 100-amino acid truncation in the cytoplasmic tail of glycoprotein 41 in the reference HIV type 1 strain RF. AIDS Res. Hum. Retroviruses 18:513–517. [PubMed]
102. LaBranche CC, Hoffman TL, Romano J, Haggarty BS, Edwards TG, Matthews TJ, Doms RW, Hoxie JA. 1999. Determinants of CD4 independence for a human immunodeficiency virus type 1 variant map outside regions required for coreceptor specificity. J. Virol. 73:10310–10319. [PMC free article] [PubMed]
103. Shimizu H, Hasebe F, Tsuchie H, Morikawa S, Ushijima H, Kitamura T. 1992. Analysis of a human immunodeficiency virus type 1 isolate carrying a truncated transmembrane glycoprotein. Virology 189:534–546. [PubMed]
104. Shimizu H, Morikawa S, Yamaguchi K, Tsuchie H, Hachimori K, Ushijima H, Kitamura T. 1990. Shorter size of transmembrane glycoprotein of an HIV-1 isolate. AIDS 4:575–576. [PubMed]
105. Chakrabarti L, Emerman M, Tiollais P, Sonigo P. 1989. The cytoplasmic domain of simian immunodeficiency virus transmembrane protein modulates infectivity. J. Virol. 63:4395–4403. [PMC free article] [PubMed]
106. Hirsch VM, Edmondson P, Murphey-Corb M, Arbeille B, Johnson PR, Mullins JI. 1989. SIV adaptation to human cells. Nature 341:573–574. [PubMed]
107. Kodama T, Burns DP, Kestler HW, Daniel MD, Desrosiers RC. 1990. Molecular changes associated with replication of simian immunodeficiency virus in human cells. J. Med. Primatol. 19:431–437. [PubMed]
108. Kodama T, Wooley DP, Naidu YM, Kestler HW, III, Daniel MD, Li Y, Desrosiers RC. 1989. Significance of premature stop codons in env of simian immunodeficiency virus. J. Virol. 63:4709–4714. [PMC free article] [PubMed]
109. Yuste E, Johnson W, Pavlakis GN, Desrosiers RC. 2005. Virion envelope content, infectivity, and neutralization sensitivity of simian immunodeficiency virus. J. Virol. 79:12455–12463. [PMC free article] [PubMed]
110. Yuste E, Reeves JD, Doms RW, Desrosiers RC. 2004. Modulation of Env content in virions of simian immunodeficiency virus: correlation with cell surface expression and virion infectivity. J. Virol. 78:6775–6785. [PMC free article] [PubMed]
111. Shacklett BL, Weber CJ, Shaw KE, Keddie EM, Gardner MB, Sonigo P, Luciw PA. 2000. The intracytoplasmic domain of the Env transmembrane protein is a locus for attenuation of simian immunodeficiency virus SIVmac in rhesus macaques. J. Virol. 74:5836–5844. [PMC free article] [PubMed]
112. Sauter MM, Pelchen-Matthews A, Bron R, Marsh M, LaBranche CC, Vance PJ, Romano J, Haggarty BS, Hart TK, Lee WM, Hoxie JA. 1996. An internalization signal in the simian immunodeficiency virus transmembrane protein cytoplasmic domain modulates expression of envelope glycoproteins on the cell surface. J. Cell Biol. 132:795–811. [PMC free article] [PubMed]
113. Bhakta SJ, Shang L, Prince JL, Claiborne DT, Hunter E. 2011. Mutagenesis of tyrosine and di-leucine motifs in the HIV-1 envelope cytoplasmic domain results in a loss of Env-mediated fusion and infectivity. Retrovirology 8:37 doi:10.1186/1742-4690-8-37. [PMC free article] [PubMed]
114. Day JR, Muenk C, Guatelli JC. 2004. The membrane-proximal tyrosine-based sorting signal of human immunodeficiency virus type 1 gp41 is required for optimal viral infectivity. J. Virol. 78:1069–1079. [PMC free article] [PubMed]
115. West JT, Weldon SK, Wyss S, Lin X, Yu Q, Thali M, Hunter E. 2002. Mutation of the dominant endocytosis motif in human immunodeficiency virus type 1 gp41 can complement matrix mutations without increasing Env incorporation. J. Virol. 76:3338–3349. [PMC free article] [PubMed]
116. Fultz PN, Vance PJ, Endres MJ, Tao B, Dvorin JD, Davis IC, Lifson JD, Montefiori DC, Marsh M, Malim MH, Hoxie JA. 2001. In vivo attenuation of simian immunodeficiency virus by disruption of a tyrosine-dependent sorting signal in the envelope glycoprotein cytoplasmic tail. J. Virol. 75:278–291. [PMC free article] [PubMed]
117. Kuiken C, Leitner T, Foley B, Hahn B, Marx P, McCutchan F, Wolinsky S, Korber B, editors. (ed). 2009. HIV sequence compendium 2009. Report LA-UR 09–03280. Theoretical Biology and Biophysics Group, Los Alamos National Laboratory, Los Alamos, NM.
118. Traub LM. 2009. Tickets to ride: selecting cargo for clathrin-regulated internalization. Nat. Rev. Mol. Cell Biol. 10:583–596. [PubMed]
119. Egan MA, Carruth LM, Rowell JF, Yu X, Siliciano RF. 1996. Human immunodeficiency virus type 1 envelope protein endocytosis mediated by a highly conserved intrinsic internalization signal in the cytoplasmic domain of gp41 is suppressed in the presence of the Pr55gag precursor protein. J. Virol. 70:6547–6556. [PMC free article] [PubMed]
120. Chertova E, Bess JWJ, Crise BJ, Sowder IR, Schaden TM, Hilburn JM, Hoxie JA, Benveniste RE, Lifson JD, Henderson LE, Arthur LO. 2002. Envelope glycoprotein incorporation, not shedding of surface envelope glycoprotein (gp120/SU), is the primary determinant of SU content of purified human immunodeficiency virus type 1 and simian immunodeficiency virus. J. Virol. 76:5315–5325. [PMC free article] [PubMed]
121. Dorfman T, Mammano F, Haseltine WA, Göttlinger HG. 1994. Role of the matrix protein in the virion association of the human immunodeficiency virus type 1 envelope glycoprotein. J. Virol. 68:1689–1696. [PMC free article] [PubMed]
122. González SA, Affranchino JL, Gelderblom HR, Burny A. 1993. Assembly of the matrix protein of simian immunodeficiency virus into virus-like particles. Virology 194:548–556. [PubMed]
123. Manrique JM, Celma CCP, Hunter E, Affranchino JL, González SA. 2003. Positive and negative modulation of virus infectivity and envelope glycoprotein incorporation into virions by amino acid substitutions at the N terminus of the simian immunodeficiency virus matrix protein. J. Virol. 77:10881–10888. [PMC free article] [PubMed]
124. Yu X, Yuan X, Matsuda Z, Lee TH, Essex M. 1992. The matrix protein of human immunodeficiency virus type 1 is required for incorporation of viral envelope protein into mature virions. J. Virol. 66:4966–4971. [PMC free article] [PubMed]
125. Göttlinger HG, Sodroski JG, Haseltine WA. 1989. Role of capsid precursor processing and myristoylation in morphogenesis and infectivity of human immunodeficiency virus type 1. Proc. Natl. Acad. Sci. U. S. A. 86:5781–5785. [PubMed]
126. Hill CP, Worthylake D, Bancroft DP, Christensen AM, Sundquist WI. 1996. Crystal structures of the trimeric human immunodeficiency virus type 1 matrix protein: implications for membrane association and assembly. Proc. Natl. Acad. Sci. U. S. A. 93:3099–3104. [PubMed]
127. Massiah MA, Starich MR, Paschall C, Summers MF, Christensen AM, Sundquist WI. 1994. Three-dimensional structure of the human immunodeficiency virus type 1 matrix protein. J. Mol. Biol. 244:198–223. [PubMed]
128. Veronese FD, Copeland TD, Oroszlan S, Gallo RC, Sarngadharan MG. 1988. Biochemical and immunological analysis of human immunodeficiency virus gag gene products p17 and p24. J. Virol. 62:795–801. [PMC free article] [PubMed]
129. Zhou W, Parent LJ, Wills JW, Resh MD. 1994. Identification of a membrane-binding domain within the amino-terminal region of human immunodeficiency virus type 1 Gag protein which interacts with acidic phospholipids. J. Virol. 68:2556–2569. [PMC free article] [PubMed]
130. Cannon PM, Matthews S, Clark N, Byles ED, Iourin O, Hockley DJ, Kingsman SM, Kingsman AJ. 1997. Structure-function studies of the human immunodeficiency virus type 1 matrix protein, p17. J. Virol. 71:3474–3483. [PMC free article] [PubMed]
131. Fäcke M, Janetzko A, Shoeman RL, Kräusslich HG. 1993. A large deletion in the matrix domain of the human immunodeficiency virus gag gene redirects virus particle assembly from the plasma membrane to the endoplasmic reticulum. J. Virol. 67:4972–4980. [PMC free article] [PubMed]
132. Gallina A, Mantoan G, Rindi G, Milanesi G. 1994. Influence of MA internal sequences, but not of the myristylated N-terminus sequence, on the budding site of HIV-1 Gag protein. Biochem. Biophys. Res. Commun. 204:1031–1038. [PubMed]
133. Ono A, Orenstein JM, Freed EO. 2000. Role of the Gag matrix domain in targeting human immunodeficiency virus type 1 assembly. J. Virol. 74:2855–2866. [PMC free article] [PubMed]
134. Alcamí J, Laín de Lera T, Folgueira L, Pedraza MA, Jacque JM, Bachelerie F, Noriega AR, Hay RT, Harrich D, Gaynor RB. 1995. Absolute dependence on kappa B responsive elements for initiation and Tat-mediated amplification of HIV transcription in blood CD4 T lymphocytes. EMBO J. 14:1552–1560. [PubMed]
135. Alexander L, Illyinskii PO, Lang SM, Means RE, Lifson J, Mansfield K, Desrosiers RC. 2003. Determinants of increased replicative capacity of serially passaged simian immunodeficiency virus with nef deleted in rhesus monkeys. J. Virol. 77:6823–6835. [PMC free article] [PubMed]
136. Chukkapalli V, Hogue IB, Boyko V, Hu Ono W-SA. 2008. Interaction between the human immunodeficiency virus type 1 Gag matrix domain and phosphatidylinositol-(4,5)-bisphosphate is essential for efficient Gag membrane binding. J. Virol. 82:2405–2417. [PMC free article] [PubMed]
137. Ghanam RH, Samal AB, Fernandez TF, Saad JS. 2012. Role of the HIV-1 matrix protein in Gag intracellular trafficking and targeting to the plasma membrane for virus assembly. Front. Microbiol. 3:55 doi:10.3389/fmicb.2012.00055. [PMC free article] [PubMed]
138. Ono A, Ablan SD, Lockett SJ, Nagashima K, Freed EO. 2004. Phosphatidylinositol (4,5) bisphosphate regulates HIV-1 Gag targeting to the plasma membrane. Proc. Natl. Acad. Sci. U. S. A. 101:14889–14894. [PubMed]
139. Saad JS, Miller J, Tai J, Kim A, Ghanam RH, Summers MF. 2006. Structural basis for targeting HIV-1 Gag proteins to the plasma membrane for virus assembly. Proc. Natl. Acad. Sci. U. S. A. 103:11364–11369. [PubMed]
140. Murakami T, Freed EO. 2000. Genetic evidence for an interaction between human immunodeficiency virus type 1 matrix and alpha-helix 2 of the gp41 cytoplasmic tail. J. Virol. 74:3548–3554. [PMC free article] [PubMed]
141. Freed EO, Martin MA. 1995. Virion incorporation of envelope glycoproteins with long but not short cytoplasmic tails is blocked by specific, single amino acid substitutions in the human immunodeficiency virus type 1 matrix. J. Virol. 69:1984–1989. [PMC free article] [PubMed]
142. González SA, Burny A, Affranchino JL. 1996. Identification of domains in the simian immunodeficiency virus matrix protein essential for assembly and envelope glycoprotein incorporation. J. Virol. 70:6384–6389. [PMC free article] [PubMed]
143. Alfadhli A, Barklis RL, Barklis E. 2009. HIV-1 matrix organizes as a hexamer of trimers on membranes containing phosphatidylinositol-(4,5)-bisphosphate. Virology 387:466–472. [PMC free article] [PubMed]
144. Bauby H, Lopez-Verges S, Hoeffel G, Delcroix-Genete D, Janvier K, Mammano F, Hosmalin A, Berlioz-Torrent C. 2010. TIP47 is required for the production of infectious HIV-1 particles from primary macrophages. Traffic 11:455–467. [PubMed]
145. Robinson MS. 2004. Adaptable adaptors for coated vesicles. Trends Cell Biol. 14:167–174. [PubMed]
146. Lodge R, Lalonde Lemay J-PG, Cohen EA. 1997. The membrane-proximal intracytoplasmic tyrosine residue of HIV-1 envelope glycoprotein is critical for basolateral targeting of viral budding in MDCK cells. EMBO J. 16:695–705. [PubMed]
147. Deschambeault J, Lalonde JP, Cervantes-Acosta G, Lodge R, Cohen EA, Lemay G. 1999. Polarized human immunodeficiency virus budding in lymphocytes involves a tyrosine-based signal and favors cell-to-cell viral transmission. J. Virol. 73:5010–5017. [PMC free article] [PubMed]
148. Chen BK. 2012. T cell virological synapses and HIV-1 pathogenesis. Immunol. Res. 54:133–139. [PubMed]
149. Díaz E, Pfeffer SR. 1998. TIP47: a cargo selection device for mannose 6-phosphate receptor trafficking. Cell 93:433–443. [PubMed]
150. Bickel PE, Tansey JT, Welte MA. 2009. PAT proteins, an ancient family of lipid droplet proteins that regulate cellular lipid stores. Biochim. Biophys. Acta 1791:419–440. [PMC free article] [PubMed]
151. Carroll KS, Hanna J, Simon I, Krise J, Barbero P, Pfeffer SR. 2001. Role of Rab9 GTPase in facilitating receptor recruitment by TIP47. Science 292:1373–1376. [PubMed]
152. Murray JL, Mavrakis M, McDonald NJ, Yilla M, Sheng J, Bellini WJ, Zhao L, Le Doux JM, Shaw MW, Luo CC, Lippincott-Schwartz J, Sanchez A, Rubin DH, Hodge TW. 2005. Rab9 GTPase is required for replication of human immunodeficiency virus type 1, filoviruses, and measles virus. J. Virol. 79:11742–11751. [PMC free article] [PubMed]
153. Sivars U, Aivazian D, Pfeffer SR. 2003. Yip3 catalyses the dissociation of endosomal Rab-GDI complexes. Nature 425:856–859. [PubMed]
154. Blancou P, Evans DT, Desrosiers RC. 2005. PRA1 co-localizes with envelope but does not influence primate lentivirus production, infectivity or envelope incorporation. J. Gen. Virol. 86:1785–1790. [PubMed]
155. Bhattacharya J, Peters PJ, Clapham PR. 2004. Human immunodeficiency virus type 1 envelope glycoproteins that lack cytoplasmic domain cysteines: impact on association with membrane lipid rafts and incorporation onto budding virus particles. J. Virol. 78:5500–5506. [PMC free article] [PubMed]
156. Rousso I, Mixon MB, Chen BK, Kim PS. 2000. Palmitoylation of the HIV-1 envelope glycoprotein is critical for viral infectivity. Proc. Natl. Acad. Sci. U. S. A. 97:13523–13525. [PubMed]
157. Chan W-E, Lin H-H, Chen SS-L. 2005. Wild-type-like viral replication potential of human immunodeficiency virus type 1 envelope mutants lacking palmitoylation signals. J. Virol. 79:8374–8387. [PMC free article] [PubMed]
158. Yang P, Ai L-S, Huang S-C, Li H-F, Chan W-E, Chang C-W, Ko C-Y, Chen SS-L. 2010. The cytoplasmic domain of human immunodeficiency virus type 1 transmembrane protein gp41 harbors lipid raft association determinants. J. Virol. 84:59–75. [PMC free article] [PubMed]
159. Oeckinghaus A, Ghosh S. 2009. The NF-κB family of transcription factors and its regulation. Cold Spring Harb. Perspect. Biol. 1:a000034 doi:10.1101/cshperspect.a000034. [PMC free article] [PubMed]
160. Montano MA, Novitsky VA, Blackard JT, Cho NL, Katzenstein DA, Essex M. 1997. Divergent transcriptional regulation among expanding human immunodeficiency virus type 1 subtypes. J. Virol. 71:8657–8665. [PMC free article] [PubMed]
161. Nabel G, Baltimore D. 1987. An inducible transcription factor activates expression of human immunodeficiency virus in T cells. Nature 326:711–713. [PubMed]
162. Rodenburg CM, Li Y, Trask SA, Chen Y, Decker J, Robertson DL, Kalish ML, Shaw GM, Allen S, Hahn BH, Gao F., UNAIDS and NIAID Networks for HIV Isolation and Characterization 2001. Near full-length clones and reference sequences for subtype C isolates of HIV type 1 from three different continents. AIDS Res. Hum. Retroviruses 17:161–168. [PubMed]
163. van Opijnen T, Jeeninga RE, Boerlijst MC, Pollakis GP, Zetterberg V, Salminen M, Berkhout B. 2004. Human immunodeficiency virus type 1 subtypes have a distinct long terminal repeat that determines the replication rate in a host-cell-specific manner. J. Virol. 78:3675–3683. [PMC free article] [PubMed]
164. Regier DA, Desrosiers RC. 1990. The complete nucleotide sequence of a pathogenic molecular clone of simian immunodeficiency virus. AIDS Res. Hum. Retroviruses 6:1221–1231. [PubMed]
165. Bellas RE, Hopkins N, Li Y. 1993. The NF-κB binding site is necessary for efficient replication of simian immunodeficiency virus of macaques in primary macrophages but not in T cells in vitro. J. Virol. 67:2908–2913. [PMC free article] [PubMed]
166. Ilyinskii PO, Desrosiers RC. 1996. Efficient transcription and replication of simian immunodeficiency virus in the absence of NF-κB and Sp1 binding elements. J. Virol. 70:3118–3126. [PMC free article] [PubMed]
167. Leonard J, Parrott C, Buckler-White AJ, Turner W, Ross EK, Martin MA, Rabson AB. 1989. The NF-κB binding sites in the human immunodeficiency virus type 1 long terminal repeat are not required for virus infectivity. J. Virol. 63:4919–4924. [PMC free article] [PubMed]
168. Ross EK, Buckler-White AJ, Rabson AB, Englund G, Martin MA. 1991. Contribution of NF-κB and Sp1 binding motifs to the replicative capacity of human immunodeficiency virus type 1: distinct patterns of viral growth are determined by T-cell types. J. Virol. 65:4350–4358. [PMC free article] [PubMed]
169. Osborn L, Kunkel S, Nabel GJ. 1989. Tumor necrosis factor α and interleukin 1 stimulate the human immunodeficiency virus enhancer by activation of the nuclear factor κB. Proc. Natl. Acad. Sci. U. S. A. 86:2336–2340. [PubMed]
170. Perkins ND, Edwards NL, Duckett CS, Agranoff AB, Schmid RM, Nabel GJ. 1993. A cooperative interaction between NF-kappa B and Sp1 is required for HIV-1 enhancer activation. EMBO J. 12:3551–3558. [PubMed]
171. Folks TM, Clouse KA, Justement J, Rabson A, Duh E, Kehrl JH, Fauci AS. 1989. Tumor necrosis factor alpha induces expression of human immunodeficiency virus in a chronically infected T-cell clone. Proc. Natl. Acad. Sci. U. S. A. 86:2365–2368. [PubMed]
172. Griffin GE, Leung K, Folks TM, Kunkel S, Nabel GJ. 1989. Activation of HIV gene expression during monocyte differentiation by induction of NF-κB. Nature 339:70–73. [PubMed]
173. Adhikari A, Xu M, Chen ZJ. 2007. Ubiquitin-mediated activation of TAK1 and IKK. Oncogene 26:3214–3226. [PubMed]
174. Wertz IE, Dixit VM. 2010. Signaling to NF-κB: regulation by ubiquitination. Cold Spring Harb. Perspect. Biol. 2:a0033500 doi:10.1101/cshperspect.a003350. [PMC free article] [PubMed]
175. Fisher AG, Ratner L, Mitsuya H, Marselle LM, Harper ME, Broder S, Gallo RC, Wong-Staal F. 1986. Infectious mutants of HTLV-III with changes in the 3′ region and markedly reduced cytopathic effects. Science 233:655–659. [PubMed]
176. Lee SJ, Hu W, Fisher AG, Looney DJ, Kao VF, Mitsuya H, Ratner L, Wong-Staal F. 1989. Role of the carboxy-terminal portion of the HIV-1 transmembrane protein in viral transmission and cytopathogenicity. AIDS Res. Hum. Retroviruses 5:441–449. [PubMed]
177. Miller MA, Cloyd MW, Liebmann J, Rinaldo CRJ, Islam KR, Wang SZ, Mietzner TA, Montelaro RC. 1993. Alterations in cell membrane permeability by the lentivirus lytic peptide (LLP-1) of HIV-1 transmembrane protein. Virology 196:89–100. [PubMed]
178. Miller MA, Garry RF, Jaynes JM, Montelaro RC. 1991. A structural correlation between lentivirus transmembrane proteins and natural cytolytic peptides. AIDS Res. Hum. Retroviruses 7:511–519. [PubMed]
179. Costin JM. 2007. Cytopathic mechanisms of HIV-1. Virol. J. 4:100 doi:10.1186/1743-422X-4-100. [PMC free article] [PubMed]
180. Costin JM, Rausch JM, Garry RF, Wimley WC. 2007. Viroporin potential of the lentivirus lytic peptide (LLP) domains of the HIV-1 gp41 protein. Virol. J. 4:123 doi:10.1186/1743-422X-4-123. [PMC free article] [PubMed]
181. Krebs J. 1998. The role of calcium in apoptosis. Biometals 11:375–382. [PubMed]
182. Cheung RK, Grinstein S, Gelfand EW. 1983. Permissive role of calcium in the inhibition of T cell mitogenesis by calmodulin antagonists. J. Immunol. 131:2291–2295. [PubMed]
183. Rao A, Luo C, Hogan PG. 1997. Transcription factors of the NFAT family: regulation and function. Annu. Rev. Immunol. 15:707–747. [PubMed]
184. Micoli KJ, Mamaeva O, Piller SC, Barker JL, Pan G, Hunter E, McDonald JM. 2006. Point mutations in the C-terminus of HIV-1 gp160 reduce apoptosis and calmodulin binding without affecting viral replication. Virology 344:468–479. [PMC free article] [PubMed]
185. Pan Z, Radding W, Zhou T, Hunter E, Mountz J, McDonald JM. 1996. Role of calmodulin in HIV-potentiated Fas-mediated apoptosis. Am. J. Pathol. 149:903–910. [PubMed]
186. Sasaki M, Uchiyama J, Ishikawa H, Matsushita S, Kimura G, Nomoto K, Koga Y. 1996. Induction of apoptosis by calmodulin-dependent intracellular Ca2+ elevation in CD4+ cells expressing gp160 of HIV. Virology 224:18–24. [PubMed]
187. McDougal JS, Mawle A, Cort SP, Nicholson JK, Cross GD, Scheppler-Campbell JA, Hicks D, Sligh J. 1985. Cellular tropism of the human retrovirus HTLV-III/LAV. I. Role of T cell activation and expression of the T4 antigen. J. Immunol. 135:3151–3162. [PubMed]
188. Stevenson M, Stanwick TL, Dempsey MP, Lamonica CA. 1990. HIV-1 replication is controlled at the level of T cell activation and proviral integration. EMBO J. 9:1551–1560. [PubMed]
189. Zack JA, Arrigo SJ, Weitsman SR, Go AS, Haislip A, Chen IS. 1990. HIV-1 entry into quiescent primary lymphocytes: molecular analysis reveals a labile, latent viral structure. Cell 61:213–222. [PubMed]
190. Zagury D, Bernard J, Leonard R, Cheynier R, Feldman M, Sarin PS, Gallo RC. 1986. Long-term cultures of HTLV-III-infected T cells: a model of cytopathology of T-cell depletion in AIDS. Science 231:850–853. [PubMed]
191. Gilden J, Krummel MF. 2010. Control of cortical rigidity by the cytoskeleton: emerging roles for septins. Cytoskeleton (Hoboken) 67:477–486. [PMC free article] [PubMed]
192. Stolp BB, Fackler OTO. 2011. How HIV takes advantage of the cytoskeleton in entry and replication. Viruses 3:293–311. [PMC free article] [PubMed]
193. Taylor MP, Koyuncu OO, Enquist LW. 2011. Subversion of the actin cytoskeleton during viral infection. Nat. Rev. Microbiol. 9:427–439. [PMC free article] [PubMed]
194. Karlsson R, Pedersen ED, Wang Z, Brakebusch C. 2009. Rho GTPase function in tumorigenesis. Biochim. Biophys. Acta 1796:91–98. [PubMed]
195. Montaner S, Perona R, Saniger L, Lacal JC. 1999. Activation of serum response factor by RhoA is mediated by the nuclear factor-κB and C/EBP transcription factors. J. Biol. Chem. 274:8506–8515. [PubMed]
196. Wang L, Zhang H, Solski PA, Hart MJ, Der CJ, Su L. 2000. Modulation of HIV-1 replication by a novel RhoA effector activity. J. Immunol. 164:5369–5374. [PubMed]
197. Anastasiadis PZ. 2007. p120-ctn: a nexus for contextual signaling via Rho GTPases. Biochim. Biophys. Acta 1773:34–46. [PubMed]
198. Shapiro L, Weis WI. 2009. Structure and biochemistry of cadherins and catenins. Cold Spring Harb. Perspect. Biol. 1:a003053 doi:10.1101/cshperspect.a003053. [PMC free article] [PubMed]
199. Maiden SL, Hardin J. 2011. The secret life of α-catenin: moonlighting in morphogenesis. J. Cell Biol. 195:543–552. [PMC free article] [PubMed]
200. Asada R, Kanemoto S, Kondo S, Saito A, Imaizumi K. 2011. The signalling from endoplasmic reticulum-resident bZIP transcription factors involved in diverse cellular physiology. J. Biochem. 149:507–518. [PubMed]
201. Mishra S, Ande SR, Nyomba BLG. 2010. The role of prohibitin in cell signaling. FEBS J. 277:3937–3946. [PubMed]
202. Osman C, Merkwirth C, Langer T. 2009. Prohibitins and the functional compartmentalization of mitochondrial membranes. J. Cell Sci. 122:3823–3830. [PubMed]
203. Serra-Moreno R, Jia B, Breed M, Alvarez X, Evans DT. 2011. Compensatory changes in the cytoplasmic tail of gp41 confer resistance to tetherin/BST-2 in a pathogenic Nef-deleted SIV. Cell Host Microbe 9:46–57. [PMC free article] [PubMed]
204. Evans DT, Serra-Moreno R, Singh RK, Guatelli JC. 2010. BST-2/tetherin: a new component of the innate immune response to enveloped viruses. Trends Microbiol. 18:388–396. [PMC free article] [PubMed]
205. Neil SJD, Zang T, Bieniasz PD. 2008. Tetherin inhibits retrovirus release and is antagonized by HIV-1 Vpu. Nature 451:425–430. [PubMed]
206. Van Damme N, Goff D, Katsura C, Jorgenson RL, Mitchell R, Johnson MC, Stephens EB, Guatelli J. 2008. The interferon-induced protein BST-2 restricts HIV-1 release and is downregulated from the cell surface by the viral Vpu protein. Cell Host Microbe 3:245–252. [PMC free article] [PubMed]
207. Kestler HW, Ringler DJ, Mori K, Panicali DL, Sehgal PK, Daniel MD, Desrosiers RC. 1991. Importance of the nef gene for maintenance of high virus loads and for development of AIDS. Cell 65:651–662. [PubMed]
208. Baba TW, Liska V, Khimani AH, Ray NB, Dailey PJ, Penninck D, Bronson R, Greene MF, McClure HM, Martin LN, Ruprecht RM. 1999. Live attenuated, multiply deleted simian immunodeficiency virus causes AIDS in infant and adult macaques. Nat. Med. 5:194–203. [PubMed]

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