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1.  The 64-Kilodalton Capsid Protein Homolog of Beet Yellows Virus Is Required for Assembly of Virion Tails 
Journal of Virology  2003;77(4):2377-2384.
The filamentous virion of the closterovirus Beet yellows virus (BYV) consists of a long body formed by the major capsid protein (CP) and a short tail composed of the minor capsid protein (CPm) and the virus-encoded Hsp70 homolog. By using nano-liquid chromatography-tandem mass spectrometry and biochemical analyses, we show here that the BYV 64-kDa protein (p64) is the fourth integral component of BYV virions. The N-terminal domain of p64 is exposed at the virion surface and is accessible to antibodies and mild trypsin digestion. In contrast, the C-terminal domain is embedded in the virion and is inaccessible to antibodies or trypsin. The C-terminal domain of p64 is shown to be homologous to CP and CPm. Mutation of the signature motifs of capsid proteins of filamentous RNA viruses in p64 results in the formation of tailless virions, which are unable to move from cell to cell. These results reveal the dual function of p64 in tail assembly and BYV motility and support the concept of the virion tail as a specialized device for BYV cell-to-cell movement.
PMCID: PMC141117  PMID: 12551975
2.  Regulation of Closterovirus Gene Expression Examined by Insertion of a Self-Processing Reporter and by Northern Hybridization 
Journal of Virology  1999;73(10):7988-7993.
A reporter open reading frame (ORF) coding for a fusion of bacterial β-glucuronidase (GUS) with a proteinase domain (Pro) derived from tobacco etch potyvirus was utilized for tagging individual genes of beet yellows closterovirus (BYV). Insertion of this reporter ORF between the first and second codons of the BYV ORFs encoding the HSP70 homolog (HSP70h), a major capsid protein (CP), and a 20-kDa protein (p20) resulted in the expression of the processed GUS-Pro reporter from corresponding subgenomic RNAs. The high sensitivity of GUS assays permitted temporal analysis of reporter accumulation, revealing early expression from the HSP70h promoter, followed by the CP promoter and later the p20 promoter. The kinetics of transcription of the remaining BYV genes encoding a 64-kDa protein (p64), a minor capsid protein (CPm), and a 21-kDa protein (p21) were examined via Northern blot analysis. Taken together, the data indicated that the temporal regulation of BYV gene expression includes early (HSP70h, CPm, CP, and p21 promoters) and late (p64 and p20 promoters) phases. It was also demonstrated that the deletion of six viral genes that are nonessential for RNA amplification resulted in a dramatic increase in the level of transcription from one of the two remaining subgenomic promoters. Comparison with other positive-strand RNA viruses producing multiple subgenomic RNAs showed the uniqueness of the pattern of closterovirus transcriptional regulation.
PMCID: PMC112813  PMID: 10482546
3.  Interaction between Long-Distance Transport Factor and Hsp70-Related Movement Protein of Beet Yellows Virus 
Journal of Virology  2002;76(21):11003-11011.
Systemic spread of viruses in plants involves local movement from cell to cell and long-distance transport through the vascular system. The cell-to-cell movement of the Beet yellows virus (BYV) is mediated by a movement protein that is an Hsp70 homolog (Hsp70h). This protein is required for the assembly of movement-competent virions that incorporate Hsp70h. By using the yeast two-hybrid system, in vitro coimmunoprecipitation, and in planta coexpression approaches, we show here that the Hsp70h interacts with a 20-kDa BYV protein (p20). We further demonstrate that p20 is associated with the virions presumably via binding to Hsp70h. Genetic and immunochemical analyses indicate that p20 is dispensable for assembly and cell-to-cell movement of BYV but is required for the long-distance transport of virus through the phloem. These results reveal a novel activity for the Hsp70h that provides a molecular link between the local and systemic spread of a plant virus by docking a long-distance transport factor to virions.
PMCID: PMC136651  PMID: 12368343
4.  Crinivirus replication and host interactions 
Criniviruses comprise one of the genera within the family Closteroviridae. Members in this family are restricted to the phloem and rely on whitefly vectors of the genera Bemisia and/or Trialeurodes for plant-to-plant transmission. All criniviruses have bipartite, positive-sense single-stranded RNA genomes, although there is an unconfirmed report of one having a tripartite genome. Lettuce infectious yellows virus (LIYV) is the type species of the genus, the best studied so far of the criniviruses and the first for which a reverse genetics system was developed. LIYV RNA 1 encodes for proteins predicted to be involved in replication, and alone is competent for replication in protoplasts. Replication results in accumulation of cytoplasmic vesiculated membranous structures which are characteristic of most studied members of the Closteroviridae. These membranous structures, often referred to as Beet yellows virus (BYV)-type vesicles, are likely sites of RNA replication. LIYV RNA 2 is replicated in trans when co-infecting cells with RNA 1, but is temporally delayed relative to RNA 1. Efficient RNA 2 replication also is dependent on the RNA 1-encoded RNA-binding protein, P34. No LIYV RNA 2-encoded proteins have been shown to affect RNA replication, but at least four, CP (major coat protein), CPm (minor coat protein), Hsp70h, and P59 are virion structural components and CPm is a determinant of whitefly transmissibility. Roles of other LIYV RNA 2-encoded proteins are largely as yet unknown, but P26 is a non-virion protein that accumulates in cells as characteristic plasmalemma deposits which in plants are localized within phloem parenchyma and companion cells over plasmodesmata connections to sieve elements. The two remaining crinivirus-conserved RNA 2-encoded proteins are P5 and P9. P5 is 39 amino acid protein and is encoded at the 5′ end of RNA 2 as ORF 1 and is part of the hallmark closterovirus gene array. The orthologous gene in BYV has been shown to play a role in cell-to-cell movement and indicated to be localized to the endoplasmic reticulum as a Type III integral membrane protein. The other small protein, P9, is encoded by ORF 4 overlaps with ORF 3 that encodes the structural protein, P59. P9 seems to be unique to viruses in the genus Crinivirus, as no similar protein has been detected in viruses of the other two genera of the Closteroviridae.
PMCID: PMC3657685  PMID: 23730299
phloem-limited; plasmalemma deposit; whitefly vector; Crinivirus; quintuple gene block
5.  Class VIII Myosins Are Required for Plasmodesmatal Localization of a Closterovirus Hsp70 Homolog▿  
Journal of Virology  2008;82(6):2836-2843.
The Hsp70 homolog (Hsp70h) of Beet yellows virus (BYV) functions in virion assembly and cell-to-cell movement and is autonomously targeted to plasmodesmata in association with the actomyosin motility system (A. I. Prokhnevsky, V. V. Peremyslov, and V. V. Dolja, J. Virol. 79:14421-14428, 2005). Myosins are a diverse category of molecular motors that possess a motor domain and a tail domain involved in cargo binding. Plants have two classes of myosins, VIII and XI, whose specific functions are poorly understood. We used dominant negative inhibition to identify myosins required for Hsp70h localization to plasmodesmata. Six full-length myosin cDNAs from the BYV host plant Nicotiana benthamiana were sequenced and shown to encode apparent orthologs of the Arabidopsis thaliana myosins VIII-1, VIII-2, VIII-B, XI-2, XI-F, and XI-K. We found that the ectopic expression of the tail domains of each of the class VIII, but not the class XI, myosins inhibited the plasmodesmatal localization of Hsp70h. In contrast, the overexpression of the motor domains or the entire molecules of the class VIII myosins did not affect Hsp70h targeting. Further mapping revealed that the minimal cargo-binding part of the myosin VIII tails was both essential and sufficient for the inhibition of the proper Hsp70h localization. Interestingly, plasmodesmatal localization of the Tobacco mosaic virus movement protein and Arabidopsis protein RGP2 was not affected by myosin VIII tail overexpression. Collectively, our data implicate class VIII myosins in protein delivery to plasmodesmata and suggest that more than one mechanism of such delivery exist in plants.
PMCID: PMC2258991  PMID: 18199648
6.  ATPγS Disrupts Human Immunodeficiency Virus Type 1 Virion Core Integrity 
Journal of Virology  2005;79(9):5557-5567.
Heat shock protein 70 (Hsp70) is incorporated within the membrane of primate lentiviral virions. Here we demonstrate that Hsp70 is also incorporated into oncoretroviral virions and that it remains associated with membrane-stripped human immunodeficiency virus type 1 (HIV-1) virion cores. To determine if Hsp70 promotes virion infectivity, we attempted to generate Hsp70-deficient virions with gag deletion mutations, Hsp70 transdominant mutants, or RNA interference, but these efforts were confounded, largely because they disrupt virion assembly. Given that polypeptide substrates are bound and released by Hsp70 in an ATP-hydrolytic reaction cycle, we supposed that incubation of HIV-1 virions with ATP would perturb Hsp70 interaction with substrates in the virion and thereby decrease infectivity. Treatment with ATP or ADP had no observable effect, but ATPγS and GTPγS, nucleotide triphosphate analogues resistant to Hsp70 hydrolysis, dramatically reduced the infectivity of HIV-1 and murine leukemia virus virions. ATPγS-treated virions were competent for fusion with susceptible target cells, but viral cDNA synthesis was inhibited to an extent that correlated with the magnitude of decrease in infectivity. Intravirion reverse transcription by HIV-1, simian immunodeficiency virus, or murine leukemia virus was also inhibited by ATPγS. The effects of ATPγS on HIV-1 reverse transcription appeared to be indirect, resulting from disruption of virion core morphology that was evident by transmission electron microscopy. Consistent with effects on capsid conformation, ATPγS-treated viruslike particles failed to saturate host antiviral restriction activity. Our observations support a model in which the catalytic activity of virion-associated Hsp70 is required to maintain structural integrity of the virion core.
PMCID: PMC1082765  PMID: 15827170
7.  An Examination of the Electrostatic Interactions Between the N-Terminal Tail of the Brome Mosaic Virus Coat Protein and Encapsidated RNAs 
Journal of Molecular Biology  2012;419(5):284-300.
The coat protein of positive-stranded RNA viruses often contains a positively-charged tail that extends toward the center of the capsid and interacts with the viral genome. Electrostatic interaction between the tail and the RNA has been postulated as a major force in virus assembly and stabilization. The goal of this work is to examine the correlation between electrostatic interaction and the amount of RNA packaged in the tripartite Brome Mosaic Virus (BMV). Nanoindentation experiment using atomic force microscopy showed that the stiffness of BMV virions with different RNAs varied by a ten-fold higher range than would be predicted by electrostatics. BMV mutants with decreased positive charges encapsidated lower amounts of RNA while mutants with increased positive charges packaged additional RNAs up to ~900 nucleotides. However, the extra RNAs included truncated BMV RNAs, an additional copy of RNA4, potential cellular RNAs, or a combination of the three, indicating that change in the charge of the capsid could result in several different outcomes in RNA encapsidation. In addition, mutant with specific arginines changed to lysines in the capsid also exhibited defects in the specific encapsidation of one or more of the four BMV positive-strand RNAs. The experimental results indicate that electrostatics is a major component in RNA encapsidation, but was unable to account for all of the observed effects on RNA encapsidation. Thermodynamic modeling incorporating the electrostatics was able to predict the approximate length of the RNA to be encapsidated for the majority of mutant virions, but not for a mutant with extreme clustered positive charges. Cryo-electron microscopy of virions that encapsidated an additional copy of RNA4 revealed that, despite the increase in RNA encapsidated, the capsid structure was minimally changed. These results experimentally demonstrated the impact of electrostatics and additional restraints in the encapsidation of BMV RNAs, which could be applicable to other viruses.
PMCID: PMC3360812  PMID: 22472420
Brome Mosaic Virus; coat protein; capsid; RNA encapsidation; electrostatic interaction; thermodynamic modeling; virions; cryo-EM
8.  Recruitment of the Host Plant Heat Shock Protein 70 by Tomato Yellow Leaf Curl Virus Coat Protein Is Required for Virus Infection 
PLoS ONE  2013;8(7):e70280.
A functional capsid protein (CP) is essential for host plant infection and insect transmission of Tomato yellow leaf curl virus (TYLCV) and other monopartite begomoviruses. We have previously shown that TYLCV CP specifically interacts with the heat shock protein 70 (HSP70) of the virus insect vector, Bemisia tabaci. Here we demonstrate that during the development of tomato plant infection with TYLCV, a significant amount of HSP70 shifts from a soluble form into insoluble aggregates. CP and HSP70 co-localize in these aggregates, first in the cytoplasm, then in the nucleus of cells associated with the vascular system. CP-HSP70 interaction was demonstrated by co-immunopreciptation in cytoplasmic - but not in nuclear extracts from leaf and stem. Inhibition of HSP70 expression by quercetin caused a decrease in the amount of nuclear CP aggregates and a re-localization of a GFP-CP fusion protein from the nucleus to the cytoplasm. HSP70 inactivation resulted in a decrease of TYLCV DNA levels, demonstrating the role of HSP70 in TYLCV multiplication in planta. The current study reveals for the first time the involvement of plant HSP70 in TYLCV CP intracellular movement. As described earlier, nuclear aggregates contained TYLCV DNA-CP complexes and infectious virions. Showing that HSP70 localizes in these large nuclear aggregates infers that these structures operate as nuclear virus factories.
PMCID: PMC3720902  PMID: 23894631
9.  Human immune response to Mycobacterium tuberculosis antigens. 
Infection and Immunity  1991;59(2):665-670.
Little is known about the immunodominant or protective antigens of Mycobacterium tuberculosis in humans. Cell-mediated immunity is necessary for protection, and healthy tuberculin-positive individuals are relatively resistant to exogenous reinfection. We compared the targets of the cell-mediated immune response in healthy tuberculin-positive individuals to those of tuberculosis patients and tuberculin-negative persons. By using T-cell Western blotting (immunoblotting) of nitrocellulose-bound M. tuberculosis culture filtrate, peaks of T-cell blastogenic activity were identified in the healthy tuberculin reactors at 30, 37, 44, 57, 64, 71 and 88 kDa. Three of these fractions (30, 64, and 71 kDa) coincided with previously characterized proteins: antigen 6/alpha antigen, HSP60, and HSP70, respectively. The blastogenic responses to purified M. tuberculosis antigen 6/alpha antigen and BCG HSP60 were assessed. When cultured with purified antigen 6/alpha antigen, lymphocytes of healthy tuberculin reactors demonstrated greater [3H]thymidine incorporation than either healthy tuberculin-negative controls or tuberculous patients (8,113 +/- 1,939 delta cpm versus 645 +/- 425 delta cpm and 1,019 +/- 710 delta cpm, respectively; P less than 0.01). Healthy reactors also responded to HSP60, although to a lesser degree than antigen 6/alpha antigen (4,276 +/- 1,095 delta cpm; P less than 0.05). Partially purified HSP70 bound to nitrocellulose paper elicited a significant lymphocyte blastogenic response in two of six of the tuberculous patients but in none of the eight healthy tuberculin reactors. Lymphocytes of none of five tuberculin-negative controls responded to recombinant antigens at 14 or 19 kDa or to HSP70. Antibody reactivity generally was inversely correlated with blastogenic response: tuberculous sera had high titer antibody to M. tuberculosis culture filtrate in a range from 35 to 180 kDa. This is the first systematic evaluation of the human response to a panel of native and recombinant antigens in healthy tuberculin reactors and tuberculous patients. Antigens which stimulated prominent lymphocyte blastogenic responses were identified in seven fractions on T-cell Western blot analysis. Two of these may represent previously characterized proteins; the others may contain immunodominant proteins that will require further characterization.
PMCID: PMC257808  PMID: 1898911
10.  Genes Required for Replication of the 15.5-Kilobase RNA Genome of a Plant Closterovirus 
Journal of Virology  1998;72(7):5870-5876.
A full-length cDNA clone of beet yellows closterovirus (BYV) was engineered and used to map functions involved in the replication of the viral RNA genome and subgenomic RNA formation. Among 10 open reading frames (ORFs) present in BYV, ORFs 1a and 1b suffice for RNA replication and transcription. The proteins encoded in these ORFs harbor putative methyltransferase, RNA helicase, and RNA polymerase domains common to Sindbis virus-like viruses and a large interdomain region that is unique to closteroviruses. The papain-like leader proteinase (L-Pro) encoded in the 5′-proximal region of ORF 1a was found to have a dual function in genome amplification. First, the autocatalytic cleavage between L-Pro and the remainder of the ORF 1a product was essential for replication of RNA. Second, an additional L-Pro function that was separable from proteolytic activity was required for efficient RNA accumulation. The deletion of a large, ∼5.6-kb, 3′-terminal region coding for a 6-kDa hydrophobic protein, an HSP70 homolog, a 64-kDa protein, minor and major capsid proteins, a 20-kDa protein, and a 21-kDa protein (p21) resulted in replication-competent RNA. However, examination of mutants with replacements of start codons in each of these seven 3′-terminal ORFs revealed that p21 functions as an enhancer of genome amplification. The intriguing analogies between the genome organization and replicational requirements of plant closteroviruses and animal coronavirus-like viruses are discussed.
PMCID: PMC110390  PMID: 9621048
11.  An interdomain sector mediating allostery in Hsp70 molecular chaperones 
The Hsp70 family of molecular chaperones provides a well defined and experimentally powerful model system for understanding allosteric coupling between different protein domains.New extensions to the statistical coupling analysis (SCA) method permit identification of a group of co-evolving amino-acid positions—a sector—in the Hsp70 that is associated with allosteric function.Literature-based and new experimental studies support the notion that the protein sector identified through SCA underlies the allosteric mechanism of Hsp70.This work extends the concept of protein sectors by showing that two non-homologous protein domains can share a single sector when the underlying biological function is defined by the coupled activity of the two domains.
Allostery is a biologically critical property by which distantly positioned functional surfaces on proteins functionally interact. This property remains difficult to elucidate at a mechanistic level (Smock and Gierasch, 2009) because long-range coupling within proteins arises from the cooperative action of groups of amino acids. As a case study, consider the Hsp70 molecular chaperones, a large and diverse family of two-domain allosteric proteins required for cellular viability in nearly every organism (Figure 1) (Mayer and Bukau, 2005). In the ADP-bound state, the two domains act independently, the C-terminal substrate-binding domain displays a stable configuration in which the so-called ‘lid' region is docked against the β-sandwich subdomain, and substrates bind with relatively high affinity (Figure 1A) (Moro et al, 2003; Swain et al, 2007; Bertelsen et al, 2009). Exchange of ADP for ATP in the N-terminal nucleotide-binding domain causes significant local and propagated conformational change, formation of an interface with the substrate-binding domain, opening of the lid subdomain, and a decrease in the binding affinity for substrates (Figure 1B) (Rist et al, 2006; Swain et al, 2007). Upon ATP hydrolysis by the nucleotide-binding domain, Hsp70 is returned to the ADP-bound configuration suitable for another round of substrate binding and release. This process of cyclical substrate binding and release underlies all biological functions of Hsp70 proteins.
What is the structural basis for the long-range functional coupling within Hsp70? When allostery is a conserved property of a protein family, one approach to this problem is to analyze the correlated evolution of amino acids in the family—the expected statistical signature of cooperative action of protein residues (Lockless and Ranganathan, 1999; Kass and Horovitz, 2002; Suel et al, 2003). Previous work using an implementation of this concept (the statistical coupling analysis or SCA) showed that proteins contain sparse networks of co-evolving amino acids termed ‘sectors' that link protein active sites with distinct functional surfaces through the protein core (Halabi et al, 2009). This architecture is consistent with known allosteric mechanisms in protein domains (Suel et al, 2003; Halabi et al, 2009).
However, the principle of co-evolution of protein residues need not be limited to the study of individual protein domains. Indeed, conserved allosteric coupling between two (or more) non-homologous domains implies the existence of shared sectors that span functional sites on different domains. Here, we test this concept by extending the SCA method to consider the allosteric mechanism acting between the two domains of the Hsp70 proteins. Hsp70-like proteins include not only the allosteric Hsp70s, but also the Hsp110s—homologs that contain both domains and are regarded as structural models for Hsp70s, but that do not exhibit allosteric coupling. In this study, we take advantage of the functional divergence between the Hsp70s and Hsp110s to reveal patterns of co-evolution between amino acids that are specifically associated with the allosteric mechanism.
To identify the allosteric sector in Hsp70, we used SCA to compute a weighted correlation matrix, C̃, that describes the co-evolution of every pair of amino-acids positions in a sequence alignment of 926 members of the Hsp70/110 family. We then applied a mathematical method known as singular value decomposition to simultaneously evaluate the pattern of divergence between sequences and the pattern of co-evolution between amino-acid positions. The basic idea is that if the pattern of sequence divergence is able to classify members of a protein family into distinct functional subgroups, then we can rigorously identify the group of co-evolving residues that correspond to the underlying mechanism. Figure 2A shows the principal axis of sequence variation in the Hsp70/110 family, showing a clear separation of the allosteric (Hsp70) and non-allosteric (Hsp110) members of this family. The corresponding axis of co-evolution between amino-acid positions reveals a subset of Hsp70/110 positions (∼20%, 115 residues out of 605 total) that underlie the divergence of Hsp70 and Hsp110 proteins (Figure 2B). These positions derive roughly equally from the nucleotide-binding domain (in blue, 56 positions) and the substrate-binding domain (in green, 59 positions) and are more conserved within the Hsp70 sub-family. These results define a protein sector that is predicted to underlie the allosteric mechanism of Hsp70.
What is the structural arrangement of the putative allosteric sector within the Hsp70 protein? Consistent with a function in allosteric coupling, the 115 sector residues form a physically contiguous network of atoms, linking the ATP-binding site on the nucleotide-binding domain to the substrate recognition site on the substrate-binding domain through the interdomain interface (Figure 2C). The physical connectivity is remarkable given that only ∼20% of overall Hsp70 residues is involved (Figure 2B). Thus, functionally coupled but non-homologous protein domains can share a single sector of co-evolving residues that connects their respective functional sites.
We compared the Hsp70 sector mapping with the large body of biochemical studies that have been carried out in this family. We find strong experimental support for the involvement of sector positions in the Hsp70 allosteric mechanism in several regions: (1) within the ATP-binding site, (2) at the interface linking the two domains, and (3) within the β-sandwich core of the substrate-binding domain. The sector analysis also makes predictions about the involvement of some previously untested residues; we show that mutations at two such sites in fact reduce the allosteric coupling within Hsp70 in vitro and fail to complement a DnaK knockout strain of E. coli in a stress-response assay. Taken together, we conclude that sector positions are associated with the allosteric mechanism of Hsp70.
This work also adds a new finding with regard to the concept of protein sectors. Previous work showed that multiple quasi-independent sectors, each of which contributes a different aspect of function, are possible within a single protein domain (Halabi et al, 2009). This work shows that a single sector can also span two different protein domains when biological function (here, nucleotide-dependent substrate binding) arises from their coupled action. This result emphasizes the point that sectors are units of functional selection and are not obviously related to traditional hierarchies of structural organization in proteins. An interesting possibility is that evolution of allostery between proteins might evolve through the joining of protein sectors, a conjecture that can be tested in future work.
Allosteric coupling between protein domains is fundamental to many cellular processes. For example, Hsp70 molecular chaperones use ATP binding by their actin-like N-terminal ATPase domain to control substrate interactions in their C-terminal substrate-binding domain, a reaction that is critical for protein folding in cells. Here, we generalize the statistical coupling analysis to simultaneously evaluate co-evolution between protein residues and functional divergence between sequences in protein sub-families. Applying this method in the Hsp70/110 protein family, we identify a sparse but structurally contiguous group of co-evolving residues called a ‘sector', which is an attribute of the allosteric Hsp70 sub-family that links the functional sites of the two domains across a specific interdomain interface. Mutagenesis of Escherichia coli DnaK supports the conclusion that this interdomain sector underlies the allosteric coupling in this protein family. The identification of the Hsp70 sector provides a basis for further experiments to understand the mechanism of allostery and introduces the idea that cooperativity between interacting proteins or protein domains can be mediated by shared sectors.
PMCID: PMC2964120  PMID: 20865007
allostery; chaperone; co-evolution; SCA; sector
12.  The M-domain Controls the Hsp104 Protein-remodeling Activity in an Hsp70/Hsp40-dependent Manner 
Journal of molecular biology  2010;402(1):30-37.
Yeast Hsp104 is a ring-forming, ATP-dependent protein disaggregase that, together with the cognate Hsp70 chaperone system, has the remarkable ability to rescue stress damaged proteins from a previously aggregated state. Both up-stream and down-stream functions for the Hsp70 system have been reported, but it remains unclear how Hsp70/Hsp40 is coupled to the Hsp104 protein remodeling activity.
Hsp104 is a multi-domain protein that possesses an N-terminal domain, an M-domain, and two tandem AAA+ domains. The M-domain forms an 85-Å long coiled-coil and is a hallmark of the Hsp104 chaperone family. While the 3D structure of Hsp104 has been determined, the function of the M-domain is unclear. Here, we demonstrate that the M-domain is essential for protein disaggregation but dispensable for the Hsp104 ATPase and substrate translocating activities. Remarkably, replacing the Hsp104 M-domain against that of bacterial ClpB, and vice versa, switches the species-specificity so that our chimeras now cooperate with the non-cognate Hsp70/DnaK chaperone system. Our results demonstrate that the M-domain controls the Hsp104 protein remodeling activities in an Hsp70/Hsp40-dependent manner, which is required to unleash the Hsp104 protein disaggregating activity.
PMCID: PMC2938849  PMID: 20654624
Molecular chaperone; AAA+ machine; protein disaggregase; Hsp104, ClpB
13.  Hsp90/Cdc37 Chaperone/co-chaperone complex, a novel junction anticancer target elucidated by the mode of action of herbal drug Withaferin A 
BMC Bioinformatics  2011;12(Suppl 1):S30.
HSPs (Heat shock proteins) are highly conserved ubiquitous proteins among species which are involved in maintaining appropriate folding and conformation of other proteins and are thus referred to as molecular chaperones. Hsp90 (Heat-shock protein 90 kDa) is one of a group of molecular chaperones responsible for managing protein folding and quality control in cell environment. However it is also involved in the maturation and stabilization of a wide range of oncogenic client proteins which are crucial for oncogenesis and malignant progression. Hsp90 requires a series of co-chaperones to assemble into a super-chaperone complex for its function. These co-chaperones bind and leave the complex at various stages to regulate the chaperoning process. Arresting the chaperone cycle at these stages by targeting different co-chaperone/Hsp90 interactions seems to be quite a viable alternative and is likely to achieve similar consequences as that of Hsp90 direct inhibition with added favors of high specificity and reduced side effect profile. The study conducted here is an attempt to explore the potential of Withania somnifera’s major constituent WA (Withaferin A) in attenuating the Hsp90/Cdc37 chaperone/co-chaperone interactions for enhanced tumor arresting activity and to elucidate the underlying mode of action using computational approaches.
Formation of active Hsp90/Cdc37 complex is one of the essential steps for facilitation of chaperone client interaction, non-assembly of which can lead to prevention of the chaperone-client association resulting in apoptosis of tumor cells. From our flexible docking analysis of WA into active Hsp90/Cdc37 complex in which key interfacing residues of the complex were kept flexible, disruption of the active association complex can be discerned. While docking of WA into segregated Hsp90 leaves the interface residues untouched. Thus the molecular docking analysis of WA into Hsp90 and active Hsp90/Cdc37 complex conducted in this study provides significant evidence in support of the proposed mechanism of chaperone assembly suppression by inhibition or disruption of active Hsp90/Cdc37 complex formation being accounted by non-assembly of the catalytically active Hsp90/Cdc37 complex. Results from the molecular dynamics simulations in water show that the trajectories of the protein complexed with ligand WA are stable over a considerably long time period of 4 ns, with the energies of the complex being lowered in comparison to the un-docked association complex, suggesting the thermodynamic stability of WA complexed Hsp90/Cdc37.
The molecular chaperone Hsp90 has been a promising target for cancer therapy. Cancer is a disease marked by genetic instability. Thus specific inhibition of individual proteins or signalling pathways holds a great potential for subversion of this genetic plasticity of cancers. This study is a step forward in this direction. Our computational analysis provided a rationalization to the ability of naturally occurring WA to alter the chaperone signalling pathway. The large value of binding energy involved in binding of WA to the active Hsp90/Cdc37 complex consolidates the thermodynamic stability of the binding. Our docking results obtained substantiate the hypothesis that WA has the potential to inhibit the association of chaperone (Hsp90) to its co-chaperone (Cdc37) by disrupting the stability of attachment of Hsp90 to Cdc37. Conclusively our results strongly suggest that withaferin A is a potent anticancer agent as ascertained by its potent Hsp90-client modulating capability.
PMCID: PMC3044286  PMID: 21342561
14.  Actin Cytoskeleton Is Involved in Targeting of a Viral Hsp70 Homolog to the Cell Periphery 
Journal of Virology  2005;79(22):14421-14428.
The cell-to-cell movement of plant viruses involves translocation of virus particles or nucleoproteins to and through the plasmodesmata (PDs). As we have shown previously, the movement of the Beet yellows virus requires the concerted action of five viral proteins including a homolog of cellular ∼70-kDa heat shock proteins (Hsp70h). Hsp70h is an integral component of the virus particles and is also found in PDs of the infected cells. Here we investigate subcellular distribution of Hsp70h using transient expression of Hsp70h fused to three spectrally distinct fluorescent proteins. We found that fluorophore-tagged Hsp70h forms motile granules that are associated with actin microfilaments, but not with microtubules. In addition, immobile granules were observed at the cell periphery. A pairwise appearance of these granules at the opposite sides of cell walls and their colocalization with the movement protein of Tobacco mosaic virus indicated an association of Hsp70h with PDs. Treatment with various cytoskeleton-specific drugs revealed that the intact actomyosin motility system is required for trafficking of Hsp70h in cytosol and its targeting to PDs. In contrast, none of the drugs interfered with the PD localization of Tobacco mosaic virus movement protein. Collectively, these findings suggest that Hsp70h is translocated and anchored to PDs in association with the actin cytoskeleton.
PMCID: PMC1280222  PMID: 16254376
15.  Insight into DNA and Protein Transport in Double-stranded DNA Viruses: The Structure of Bacteriophage N4 
Journal of molecular biology  2008;378(3):726-736.
Bacteriophage N4 encapsidates a 3,500 amino acid-long DNA-dependent RNA polymerase (vRNAP), which is injected into the host along with the N4 genome upon infection. The three-dimensional structures of wild-type and mutant N4 viruses lacking gp17, gp50, or gp65 were determined by cryo-electron microscopy. The virion has an icosahedral capsid with T = 9 quasi-symmetry that encapsidates well-organized dsDNA and vRNAP. The tail, attached at a unique pentameric vertex of the head, consists of a neck, twelve appendages, and six ribbons that constitute a non-contractile sheath around a central tail tube. Comparison of wild-type and mutant virus structures in conjunction with bioinformatics established the identity and virion locations of the major capsid protein (gp56), a decorating protein (gp17), the vRNAP (gp50), the tail sheath (gp65), the appendages (gp66), and the portal protein (gp59). The N4 virion organization provides insights into its assembly, and suggests a mechanism for genome and vRNAP transport strategies utilized by this unique system.
PMCID: PMC2396777  PMID: 18374942
Bacteriophage N4; cryo-electron microscopy; DNA and protein transport; T = 9 quasi-symmetry; virion-encapsidated DNA-dependent RNA polymerase
16.  The Smallest Capsid Protein Mediates Binding of the Essential Tegument Protein pp150 to Stabilize DNA-Containing Capsids in Human Cytomegalovirus 
PLoS Pathogens  2013;9(8):e1003525.
Human cytomegalovirus (HCMV) is a ubiquitous herpesvirus that causes birth defects in newborns and life-threatening complications in immunocompromised individuals. Among all human herpesviruses, HCMV contains a much larger dsDNA genome within a similarly-sized capsid compared to the others, and it was proposed to require pp150, a tegument protein only found in cytomegaloviruses, to stabilize its genome-containing capsid. However, little is known about how pp150 interacts with the underlying capsid. Moreover, the smallest capsid protein (SCP), while dispensable in herpes simplex virus type 1, was shown to play essential, yet undefined, role in HCMV infection. Here, by cryo electron microscopy (cryoEM), we determine three-dimensional structures of HCMV capsid (no pp150) and virion (with pp150) at sub-nanometer resolution. Comparison of these two structures reveals that each pp150 tegument density is composed of two helix bundles connected by a long central helix. Correlation between the resolved helices and sequence-based secondary structure prediction maps the tegument density to the N-terminal half of pp150. The structures also show that SCP mediates interactions between the capsid and pp150 at the upper helix bundle of pp150. Consistent with this structural observation, ribozyme inhibition of SCP expression in HCMV-infected cells impairs the formation of DNA-containing viral particles and reduces viral yield by 10,000 fold. By cryoEM reconstruction of the resulting “SCP-deficient” viral particles, we further demonstrate that SCP is required for pp150 functionally binding to the capsid. Together, our structural and biochemical results point to a mechanism whereby SCP recruits pp150 to stabilize genome-containing capsid for the production of infectious HCMV virion.
Author Summary
Human cytomegalovirus (HCMV) causes birth defects in newborns and life-threatening complications in immunocompromised individuals, such as AIDS patients and organ transplant recipients. The smallest capsid protein (SCP) – only 8 kDa molecular mass as compared to the 155 kDa major capsid protein – has been demonstrated to be essential for HCMV growth, but is dispensable in herpes simplex virus type 1. These seemingly contradictory observations have been a paradox. Here, we solve this paradox by high resolution cryo electron microscopy (cryoEM), in conjunction with functional studies using ribozyme inhibition. Our structural comparisons of HCMV virion and capsid reveal molecular interactions at the secondary structure level and suggest that SCP might contribute to capsid binding of pp150, an essential, cytomegalovirus-specific tegument protein. SCP-deficient particles generated by ribozyme inhibition of SCP-expression in HCMV-infected cells show no pp150 tegument density, demonstrating that SCP is required for the functional binding of pp150 to the capsid. Our results suggest that SCP recruits pp150 to stabilize the HCMV nucleocapsid to enable encapsidation of the genome, which is more densely packaged in HCMV than in other herpesviruses. Overall, this study not only resolves the above paradox, but also illustrates the passive acquisition of a new, essential function by SCP in the production of infectious HCMV virions.
PMCID: PMC3744435  PMID: 23966856
17.  Insights into Regulation and Function of the Major Stress-Induced hsp70 Molecular Chaperone In Vivo: Analysis of Mice with Targeted Gene Disruption of the hsp70.1 or hsp70.3 Gene 
Molecular and Cellular Biology  2001;21(24):8575-8591.
The murine hsp70 gene family includes the evolutionarily conserved hsp70.1 and hsp70.3 genes, which are the major proteins induced by heat and other stress stimuli. hsp70.1 and hsp70.3 encode identical proteins which protect cells and facilitate their recovery from stress-induced damage. While the hsp70 gene family has been widely studied and the roles of the proteins it encodes as molecular chaperones in a range of human pathologies are appreciated, little is known about the developmental regulation of hsp70.1 and hsp70.3 expression and the in vivo biological function of their products. To directly study the physiological role of these proteins in vivo, we have generated mice deficient in heat shock protein 70 (hsp70) by replacing the hsp70.1 or hsp70.3 gene with an in-frame β-galactosidase sequence. We report here that the expression of hsp70.1 and hsp70.3 is developmentally regulated at the transcriptional level, and an overlapping expression pattern for both genes is observed during embryo development and in the tissues of adult mice. hsp70.1−/− or hsp70.3−/− mice are viable and fertile, with no obvious morphological abnormalities. In late embryonic stage and adult mice, both genes are expressed constitutively in tissues exposed directly to the environment (the epidermis and cornea) and in certain internal organs (the epithelium of the tongue, esophagus, and forestomach, and the kidney, bladder, and hippocampus). Exposure of mice to thermal stress results in the rapid induction and expression of hsp70, especially in organs not constitutively expressing hsp70 (the liver, pancreas, heart, lung, adrenal cortex, and intestine). Despite functional compensation in the single-gene-deficient mice by the intact homologous gene (i.e., hsp70.3 in hsp70.1−/− mice and vice versa), a marked reduction in hsp70 protein expression was observed in tissues under both normal and heat stress conditions. At the cellular level, inactivation of hsp70.1 or hsp70.3 resulted in deficient maintenance of acquired thermotolerance and increased sensitivity to heat stress-induced apoptosis. The additive or synergistic effects exhibited by coexpression of both hsp70 genes, and the evolutionary significance of the presence of both hsp70 genes, is hence underlined.
PMCID: PMC100019  PMID: 11713291
18.  Association of heat shock protein 70 with enterovirus capsid precursor P1 in infected human cells. 
Journal of Virology  1992;66(3):1520-1527.
Members of the human heat shock (HSP) family of related proteins are involved in the intracellular folding, transport, and assembly of proteins and protein complexes. We have observed that human heat shock protein 70 (HSP70) is associated with the capsid precursor P1 of poliovirus and coxsackievirus B1 in infected HeLa cells. Antiserum generated against HSP70 coimmunoprecipitated the poliovirus protein P1, an intermediate in capsid assembly. Similarly, alpha-virion serum coimmunoprecipitated HSP70 from virus-infected cell extracts, but not from mock-infected cell extracts. The HSP70-P1 complex was stable in high-salt medium but was sensitive to incubation with 2 mM ATP, which is a characteristic of other known functional complexes between HSP70 and cellular proteins. The P1 in the complex was predominantly newly synthesized, and the half-life of complexed P1 was nearly twice as long as that of total P1. The HSP70-P1 complex was found to sediment at 3S to 6S, suggesting that it may be part of, or a precursor to, the "5S promoter particles" thought to be an assembly intermediate of picornaviruses. The finding that HSP70 was associated with the capsid precursors of at least two enteroviruses may suggest a functional role of these complexes in the viral life cycles.
PMCID: PMC240877  PMID: 1310763
19.  Evidence for a Stable Interaction of gp41 with Pr55Gag in Immature Human Immunodeficiency Virus Type 1 Particles 
Journal of Virology  2000;74(20):9381-9387.
Assembly of infectious human immunodeficiency virus type 1 (HIV-1) virions requires incorporation of the viral envelope glycoproteins gp41 and gp120. Several lines of evidence have suggested that the cytoplasmic tail of the transmembrane glycoprotein, gp41, associates with Pr55Gag in infected cells to facilitate the incorporation of HIV-1 envelope proteins into budding virions. However, direct evidence for an interaction between gp41 and Pr55Gag in HIV-1 particles has not been reported. To determine whether gp41 is associated with Pr55Gag in HIV-1 particles, viral cores were isolated from immature HIV-1 virions by sedimentation through detergent. The cores contained a major fraction of the gp41 that was present on untreated virions. Association of gp41 with cores required the presence of the gp41 cytoplasmic tail. In HIV-1 particles containing a functional protease, a mutation that prevents cleavage of Pr55Gag at the matrix-capsid junction was sufficient for the detergent-resistant association of gp41 with the isolated cores. In addition to gp41, a major fraction of virion-associated gp120 was also detected on immature HIV-1 cores. Isolation of cores under conditions known to disrupt lipid rafts resulted in the removal of a raft-associated protein incorporated into virions but not the HIV-1 envelope proteins. These results provide biochemical evidence for a stable interaction between Pr55Gag and the cytoplasmic tail of gp41 in immature HIV-1 particles. Moreover, findings in this study suggest that the interaction of Pr55Gag with gp41 may regulate the function of the envelope proteins during HIV-1 maturation.
PMCID: PMC112366  PMID: 11000206
20.  Assembly of bacteriophage 80α capsids in a Staphylococcus aureus expression system 
Virology  2012;434(2):242-250.
80α is a temperate, double-stranded DNA bacteriophage of Staphylococcus aureus that can act as a “helper” for the mobilization of S. aureus pathogenicity islands (SaPIs), including SaPI1. When SaPI1 is mobilized by 80α, the SaPI genomes are packaged into capsids that are composed of phage proteins, but that are smaller than those normally formed by the phage. This size determination is dependent on SaPI1 proteins CpmA and CpmB. Here, we show that co-expression of the 80α capsid and scaffolding proteins in S. aureus, but not in E. coli, leads to the formation of procapsid-related structures, suggesting that a host co-factor is required for assembly. The capsid and scaffolding proteins also undergo normal N-terminal processing upon expression in S. aureus, implicating a host protease. We also find that SaPI1 proteins CpmA and CpmB promote the formation of small capsids upon co-expression with 80α capsid and scaffolding proteins in S. aureus.
PMCID: PMC3518739  PMID: 22980502
molecular piracy; virus assembly; size determination; procapsid; electron microscopy; scaffolding protein; ribosomal protein L27; co-expression; Staphylococcus aureus pathogenicity island 1
21.  A Mutation in the Lettuce Infectious Yellows Virus Minor Coat Protein Disrupts Whitefly Transmission but Not In Planta Systemic Movement▿  
Journal of Virology  2010;84(23):12165-12173.
The Lettuce infectious yellows virus (LIYV) RNA 2 mutant p1-5b was previously isolated from Bemisia tabaci-transmitted virus maintained in Chenopodium murale plants. p1-5b RNA 2 contains a single-nucleotide deletion in the minor coat protein (CPm) open reading frame (ORF) that is predicted to result in a frameshift and premature termination of the protein. Using the recently developed agroinoculation system for LIYV, we tested RNA 2 containing the p1-5b CPm mutant genotype (agro-pR6-5b) in Nicotiana benthamiana plants. We showed that plant infection triggered by agro-pR6-5b spread systemically and resulted in the formation of virions similar to those produced in p1-5b-inoculated protoplasts. However, virions derived from these mutant CPm genotypes were not transmitted by whiteflies, even though virion concentrations were above the typical transmission thresholds. In contrast, and as demonstrated for the first time, an engineered restoration mutant (agro-pR6-5bM1) was capable of both systemic movement in plants and whitefly transmission. These results provide strong molecular evidence that the full-length LIYV-encoded CPm is dispensable for systemic plant movement but is required for whitefly transmission.
PMCID: PMC2976407  PMID: 20861267
22.  The Major Structural Protein of African Swine Fever Virus, p73, Is Packaged into Large Structures, Indicative of Viral Capsid or Matrix Precursors, on the Endoplasmic Reticulum 
Journal of Virology  1998;72(6):5215-5223.
African swine fever virus (ASFV) is a large enveloped DNA virus that shares the striking icosahedral symmetry of iridoviruses. To understand the mechanism of assembly of ASFV, we have been studying the biosynthesis and subcellular distribution of p73, the major structural protein of ASFV. Sucrose density sedimentation of lysates prepared from infected cells showed that newly synthesized p73 was incorporated into a complex with a size of 150 to 250 kDa. p73 synthesized by in vitro translation migrated at 70 kDa, suggesting that cellular and/or viral proteins are required for the formation of the 150- to 250-kDa complex. During a 2-h chase, approximately 50% of the newly synthesized pool of p73 bound to the endoplasmic reticulum (ER). During this period, the membrane-bound pool of p73, but not the cytosolic pool, formed large complexes of approximately 50,000 kDa. The complexes were formed via assembly intermediates, and the entire membrane-associated pool of p73 was incorporated into the 50,000-kDa complex within 2 h. The 50,000-kDa complexes containing p73 were also detected in virions secreted from cells. Immunoprecipitation of sucrose gradients with sera taken from hyperimmune pigs suggested that p73 was the major component of the 50,000-kDa complex. It is possible, therefore, that the complex contains between 600 and 700 copies of p73. The kinetics of complex formation and envelopment of p73 were similar, and complex formation and envelopment were both reversibly inhibited by cycloheximide, suggesting a functional link between complex assembly and ASFV envelopment. A protease protection assay detected 50,000-kDa complexes on the inside and outside of the membranes forming the viral envelope. The identification of a complex containing p73 beneath the envelope of ASFV suggests that p73 may be a component of the inner core shell or matrix of ASFV. The outer pool may represent p73 within the outer capsid layer of the virus. In summary, the data suggest that the assembly of the inner core matrix and outer capsid of ASFV takes place on the ER membrane during envelopment and that these structures are not preassembled in the cytosol.
PMCID: PMC110101  PMID: 9573294
23.  A Critical Phenylalanine Residue in the Respiratory Syncytial Virus Fusion Protein Cytoplasmic Tail Mediates Assembly of Internal Viral Proteins into Viral Filaments and Particles 
mBio  2012;3(1):e00270-11.
Respiratory syncytial virus (RSV) is a single-stranded RNA virus in the Paramyxoviridae family that assembles into filamentous structures at the apical surface of polarized epithelial cells. These filaments contain viral genomic RNA and structural proteins, including the fusion (F) protein, matrix (M) protein, nucleoprotein (N), and phosphoprotein (P), while excluding F-actin. It is known that the F protein cytoplasmic tail (FCT) is necessary for filament formation, but the mechanism by which the FCT mediates assembly into filaments is not clear. We hypothesized that the FCT is necessary for interactions with other viral proteins in order to form filaments. In order to test this idea, we expressed the F protein with cytoplasmic tail (CT) truncations or specific point mutations and determined the abilities of these variant F proteins to form filaments independent of viral infection when coexpressed with M, N, and P. Deletion of the terminal three FCT residues (amino acids Phe-Ser-Asn) or mutation of the Phe residue resulted in a loss of filament formation but did not affect F-protein expression or trafficking to the cell surface. Filament formation could be restored by addition of residues Phe-Ser-Asn to an FCT deletion mutant and was unaffected by mutations to Ser or Asn residues. Second, deletion of residues Phe-Ser-Asn or mutation of the Phe residue resulted in a loss of M, N, and P incorporation into virus-like particles. These data suggest that a C-terminal Phe residue in the FCT mediates assembly through incorporation of internal virion proteins into virus filaments at the cell surface.
Respiratory syncytial virus (RSV) is a leading cause of bronchiolitis and pneumonia in infants and the elderly worldwide. There is no licensed RSV vaccine and only limited therapeutics for use in infected patients. Many aspects of the RSV life cycle have been studied, but the mechanisms that drive RSV assembly at the cell surface are not well understood. This study provides evidence that a specific residue in the RSV fusion protein cytoplasmic tail coordinates assembly into viral filaments by mediating the incorporation of internal virion proteins. Understanding the mechanisms that drive RSV assembly could lead to targeted development of novel antiviral drugs. Moreover, since RSV exits infected cells in an ESCRT (endosomal sorting complexes required for transport)-independent manner, these studies may contribute new knowledge about a general strategy by which ESCRT-independent viruses mediate outward bud formation using viral protein-mediated mechanisms during assembly and budding.
PMCID: PMC3280462  PMID: 22318318
24.  Small Heat Shock Proteins Potentiate Amyloid Dissolution by Protein Disaggregases from Yeast and Humans 
PLoS Biology  2012;10(6):e1001346.
The authors define how small heat-shock proteins synergize to regulate the assembly and disassembly of a beneficial prion, and then they exploit this knowledge to identify the human amyloid depolymerase.
How small heat shock proteins (sHsps) might empower proteostasis networks to control beneficial prions or disassemble pathological amyloid is unknown. Here, we establish that yeast sHsps, Hsp26 and Hsp42, inhibit prionogenesis by the [PSI+] prion protein, Sup35, via distinct and synergistic mechanisms. Hsp42 prevents conformational rearrangements within molten oligomers that enable de novo prionogenesis and collaborates with Hsp70 to attenuate self-templating. By contrast, Hsp26 inhibits self-templating upon binding assembled prions. sHsp binding destabilizes Sup35 prions and promotes their disaggregation by Hsp104, Hsp70, and Hsp40. In yeast, Hsp26 or Hsp42 overexpression prevents [PSI+] induction, cures [PSI+], and potentiates [PSI+]-curing by Hsp104 overexpression. In vitro, sHsps enhance Hsp104-catalyzed disaggregation of pathological amyloid forms of α-synuclein and polyglutamine. Unexpectedly, in the absence of Hsp104, sHsps promote an unprecedented, gradual depolymerization of Sup35 prions by Hsp110, Hsp70, and Hsp40. This unanticipated amyloid-depolymerase activity is conserved from yeast to humans, which lack Hsp104 orthologues. A human sHsp, HspB5, stimulates depolymerization of α-synuclein amyloid by human Hsp110, Hsp70, and Hsp40. Thus, we elucidate a heretofore-unrecognized human amyloid-depolymerase system that could have applications in various neurodegenerative disorders.
Author Summary
Amyloid fibers are protein aggregates that are associated with numerous neurodegenerative diseases, including Parkinson's disease, for which there are no effective treatments. They can also play beneficial roles; in yeast, for example, they are associated with increased survival and the evolution of new traits. Amyloid fibers are also central to many revolutionary concepts and important questions in biology and nanotechnology, including long-term memory formation and versatile self-organizing nanostructures. Thus, there is an urgent need to understand how we can promote beneficial amyloid assembly, or reverse pathogenic assembly, at will. In this study, we define the mechanisms by which small heat-shock proteins synergize to regulate the assembly and disassembly of a beneficial yeast prion. We then exploit this knowledge to discover an amyloid depolymerase machinery that is conserved from yeast to humans. Remarkably, the human small heat shock protein, HspB5, stimulates Hsp110, Hsp70, and Hsp40 chaperones to gradually depolymerize amyloid fibers formed by α-synuclein (which are implicated in Parkinson's disease) from their ends on a biologically relevant timescale. This newly identified and highly conserved amyloid-depolymerase system could have important therapeutic applications for various neurodegenerative disorders.
PMCID: PMC3378601  PMID: 22723742
25.  Rab11-FIP1C and Rab14 Direct Plasma Membrane Sorting and Particle Incorporation of the HIV-1 Envelope Glycoprotein Complex 
PLoS Pathogens  2013;9(4):e1003278.
The incorporation of the envelope glycoprotein complex (Env) onto the developing particle is a crucial step in the HIV-1 lifecycle. The long cytoplasmic tail (CT) of Env is required for the incorporation of Env onto HIV particles in T cells and macrophages. Here we identify the Rab11a-FIP1C/RCP protein as an essential cofactor for HIV-1 Env incorporation onto particles in relevant human cells. Depletion of FIP1C reduced Env incorporation in a cytoplasmic tail-dependent manner, and was rescued by replenishment of FIP1C. FIP1C was redistributed out of the endosomal recycling complex to the plasma membrane by wild type Env protein but not by CT-truncated Env. Rab14 was required for HIV-1 Env incorporation, and FIP1C mutants incapable of binding Rab14 failed to rescue Env incorporation. Expression of FIP1C and Rab14 led to an enhancement of Env incorporation, indicating that these trafficking factors are normally limiting for CT-dependent Env incorporation onto particles. These findings support a model for HIV-1 Env incorporation in which specific targeting to the particle assembly microdomain on the plasma membrane is mediated by FIP1C and Rab14.
Author Summary
Enveloped viruses must develop strategies to ensure that a sufficient quantity of their receptor-binding envelope proteins are incorporated onto the surface of viruses as they form. The HIV envelope glycoprotein is specifically incorporated onto assembling virions in relevant cells such as T lymphocytes in a manner that requires its long cytoplasmic tail. The mechanism underlying this specific incorporation has remained unknown. Here, we identify a cellular trafficking pathway that is required for the incorporation of HIV envelope onto virions. A combination of the adaptor protein Rab11-FIP1C and Rab14 directs the envelope protein onto assembling virions, and loss of either of these host factors results in the production of virus particles lacking envelope. We also found that FIP1C was required for replication in T cell lines. This study identifies a trafficking complex required for HIV envelope incorporation and for the formation of infectious HIV particles.
PMCID: PMC3616983  PMID: 23592992

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