Arenaviruses are enveloped viruses with a bi-segmented negative stranded RNA genome and a life-cycle restricted to the cell cytoplasm [3
]. Each genomic RNA segment, L (ca.
7.3 kb) and S (ca.
3.5 kb), uses an ambisense coding strategy to direct the synthesis of two polypeptides that are encoded in opposite orientations, separated by a non-coding intergenic region (IGR) with a predicted stable hairpin folding structure (). The S RNA encodes the viral glycoprotein precursor (GPC) and the nucleoprotein (NP). GPC is post-translationally cleaved by the cellular site 1 protease (S1P), also known as pro-protein convertase (PC) subtilisin-kexin-isozyme-1 (SKI-1), to yield the two mature virion glycoproteins GP1 and GP2. The L RNA encodes the viral RNA-dependent RNA polymerase (RdRp, or L polymerase), and the small (11 kDa) really interesting new gene (RING) finger protein Z.
Figure 2. Virion structure and genome organization of arenaviruses. Each genome segment S (ca. 3.5 kb) and L (ca. 7.5 kb) uses an ambisense coding strategy to direct the synthesis of two different viral polypeptides: S encodes GP and NP, and L encodes the L polymerase (more ...)
The NP and L mRNAs are transcribed into a genomic-complementary mRNA, whereas the GPC and Z coding regions are not translated directly from genomic RNA, but rather from genomic-sense mRNAs that are transcribed using the corresponding antigenome RNA species as templates, which also function as replicative intermediates. The IGR acts as a bona fide transcription termination signal for the virus polymerase [18
Trimers of GP1/GP2 associate via ionic interactions to form the spikes that decorate the virus surface (). GP1 is located at the top of the spike and mediates virus interaction with host cell surface receptors. The cellular receptor for LCMV and LASV is α-dystroglycan (α-DG) [20
]. Upon initial attachment to the target cell, LCMV virions are taken up in smooth-walled vesicles, which are not associated with clathrin [22
]. In contrast, the NW arenaviruses GTOV, JUNV, MACV and Sabia (SABV) can use human transferrin receptor 1 as a cellular receptor [23
] and clathrin-dependent endocytosis has been reported for JUNV [24
]. Fusion between the viral and cell membranes is triggered by the acidic environment of the late endosome, which is thought to trigger conformational changes in the arenavirus GP [25
], exposing a fusogenic peptide that mediates fusion of the virion and host cell membranes [28
As with many other enveloped negative stranded (NS) RNA viruses, formation and cell release (budding) of arenavirus infectious progeny requires that assembled viral ribonucleoproteins (RNPs) associate at the cell surface with membranes that are enriched in viral GPs. For many enveloped NS RNA viruses this association and subsequent viral budding is mediated by a matrix (M) protein that acts as a bridge between the RNP and GP. We [30
] and others [31
] have shown that Z is the main driving force for arenavirus budding and that this process is mediated by the Z proline-rich late domain motifs (PTAP and PPPY) known to control budding of several other viruses via interaction with specific host cell proteins [33
]. The view of Z as the arenavirus counterpart of the M protein found in many other NS RNA viruses is consistent with ultrastructural data on arenavirus virions determined by cryo-electron microscopy [34
], as well as genetic and biochemical evidence of a GP-Z association [35
], and the ability of Z to inhibit RNA synthesis by the arenavirus polymerase [36
Arenavirus Reverse Genetics
The inability to genetically manipulate the arenavirus genome has hampered studies aimed at understanding its molecular and cell biology of arenaviruses, as well as their pathogenesis. The development of reverse genetics systems for several arenaviruses including LCMV has facilitated the investigation of the cis
-acting sequences and trans
-acting factors that control arenavirus replication and gene expression, as well as assembly and budding [37
]. Moreover, the ability to rescue infectious arenaviruses including LCMV [41
], JUNV [43
] and Pichinde virus (PICV) [44
] entirely from cloned complementary DNAs (cDNAs) has provided investigators with new avenues to start examining the relationship between predetermined mutations within the virus genome and specific phenotypes, which in the case of LCMV can be assessed in the context of virus infection of its natural host the mouse. This development has provided investigators with novel and unique opportunities to investigate arenavirus-host interactions that influence the variable outcome of infection, which can range from virus control by host defenses to severe acute disease and long-term chronic infection associated with subclinical disease. Likewise, these advances in arenavirus genetics have opened up new approaches for the development of novel vaccine strategies and screening methods to identify and evaluate novel anti-arenaviral drugs targeting specific steps of the virus life-cycle.