The HSV-1 mechanisms of cell-to-cell spread and fusion are critically dependent on host PTP1B.

All herpesviruses have mechanisms for passing through cell junctions, which exclude neutralizing antibodies and offer a clear path to neighboring, uninfected cells. In the case of herpes simplex virus type 1 (HSV-1), direct cell-to-cell transmission takes place between epithelial cells and sensory neurons, where latency is established. The spreading mechanism is poorly understood, but mutations in four different HSV-1 genes can dysregulate it, causing neighboring cells to fuse to produce syncytia.

Structural Model of the Tubular Assembly of the Rous Sarcoma Virus Capsid Protein.

The orthoretroviral capsid protein (CA) assembles into polymorphic capsids, whose architecture, assembly, and stability are still being investigated. The N-terminal and C-terminal domains of CA (NTD and CTD, respectively) engage in both homotypic and heterotypic interactions to create the capsid. Hexameric turrets formed by the NTD decorate the majority of the capsid surface.

Contributions of Charged Residues in Structurally Dynamic Capsid Surface Loops to Rous Sarcoma Virus Assembly.

Unlabelled: Extensive studies of orthoretroviral capsids have shown that many regions of the CA protein play unique roles at different points in the virus life cycle. The N-terminal domain (NTD) flexible-loop (FL) region is one such example: exposed on the outer capsid surface, it has been implicated in Gag-mediated particle assembly, capsid maturation, and early replication events. We have now defined the contributions of charged residues in the FL region of the Rous sarcoma virus (RSV) CA to particle assembly.

Atomic Modeling of an Immature Retroviral Lattice Using Molecular Dynamics and Mutagenesis.

Defining the molecular interaction between Gag proteins in an assembled hexagonal lattice of immature retrovirus particles is crucial for elucidating the mechanisms of virus assembly and maturation. Recent advances in cryo-electron microscopy have yielded subnanometer structural information on the morphology of immature Gag lattices, making computational modeling and simulations feasible for investigating the Gag-Gag interactions at the atomic level. We have examined the structure of Rous sarcoma virus (RSV) using all-atom molecular dynamics simulations and in vitro assembly, to create the first all-atom model of an immature retroviral lattice.

Potential role for CA-SP in nucleating retroviral capsid maturation.

Unlabelled: During virion maturation, the Rous sarcoma virus (RSV) capsid protein is cleaved from the Gag protein as the proteolytic intermediate CA-SP. Further trimming at two C-terminal sites removes the spacer peptide (SP), producing the mature capsid proteins CA and CA-S. Abundant genetic and structural evidence shows that the SP plays a critical role in stabilizing hexameric Gag interactions that form immature particles.

A two-pronged structural analysis of retroviral maturation indicates that core formation proceeds by a disassembly-reassembly pathway rather than a displacive transition.

Retrovirus maturation involves sequential cleavages of the Gag polyprotein, initially arrayed in a spherical shell, leading to formation of capsids with polyhedral or conical morphology. Evidence suggests that capsids assemble de novo inside maturing virions from dissociated capsid (CA) protein, but the possibility persists of a displacive pathway in which the CA shell remains assembled but is remodeled. Inhibition of the final cleavage between CA and spacer peptide SP1/SP blocks the production of mature capsids.

Lethal mutations in the major homology region and their suppressors act by modulating the dimerization of the rous sarcoma virus capsid protein C-terminal domain.

An infective retrovirus requires a mature capsid shell around the viral replication complex. This shell is formed by about 1500 capsid protein monomers, organized into hexamer and pentamer rings that are linked to each other by the dimerization of the C-terminal domain (CTD). The major homology region (MHR), the most highly conserved protein sequence across retroviral genomes, is part of the CTD.

Suppression of a morphogenic mutant in Rous sarcoma virus capsid protein by a second-site mutation: a cryoelectron tomography study.

Retrovirus assembly is driven by polymerization of the Gag polyprotein as nascent virions bud from host cells. Gag is then processed proteolytically, releasing the capsid protein (CA) to assemble de novo inside maturing virions. CA has N-terminal and C-terminal domains (NTDs and CTDs, respectively) whose folds are conserved, although their sequences are divergent except in the 20-residue major homology region (MHR) in the CTD.

Retroviral capsid assembly: a role for the CA dimer in initiation.

In maturing retroviral virions, CA protein assembles to form a capsid shell that is essential for infectivity. The structure of the two folded domains [N-terminal domain (NTD) and C-terminal domain (CTD)] of CA is highly conserved among various retroviruses, and the capsid assembly pathway, although poorly understood, is thought to be conserved as well. In vitro assembly reactions with purified CA proteins of the Rous sarcoma virus (RSV) were used to define factors that influence the kinetics of capsid assembly and provide insights into underlying mechanisms.

Visualization of a missing link in retrovirus capsid assembly.

For a retrovirus such as HIV to be infectious, a properly formed capsid is needed; however, unusually among viruses, retrovirus capsids are highly variable in structure. According to the fullerene conjecture, they are composed of hexamers and pentamers of capsid protein (CA), with the shape of a capsid varying according to how the twelve pentamers are distributed and its size depending on the number of hexamers. Hexamers have been studied in planar and tubular arrays, but the predicted pentamers have not been observed.

Irregular and Semi-Regular Polyhedral Models for Rous Sarcoma Virus Cores.

Whereas many viruses have capsids of uniquely defined sizes that observe icosahedral symmetry, retrovirus capsids are highly polymorphic. Nevertheless, they may also be described as polyhedral foldings of a fullerene lattice on which the capsid protein (CA) is arrayed. Lacking the high order of symmetry that facilitates the reconstruction of icosahedral capsids from cryo-electron micrographs, the three-dimensional structures of individual retrovirus capsids may be determined by cryo-electron tomography, albeit at lower resolution.

Cooperative role of the MHR and the CA dimerization helix in the maturation of the functional retrovirus capsid.

The second helix in the C-terminal domain of retroviral capsid (CA) protein functions as the site of dimerization between subunits in capsid assembly and is believed to participate in a unique interface between Gag molecules in immature particles. This study reports isolation of two substitutions in the dimerization helix of Rous sarcoma virus CA protein that have the ability to suppress lethal defects in core maturation imposed by alterations to the major homology region (MHR) motif just upstream. Together with two previously published suppressors, these define an extended region of the dimerization helix that is unlikely to contribute directly to CA-CA contacts but whose assembly-competence may be strongly affected by conformation.

Critical role of conserved hydrophobic residues within the major homology region in mature retroviral capsid assembly.

During retroviral maturation, the CA protein undergoes dramatic structural changes and establishes unique intermolecular interfaces in the mature capsid shell that are different from those that existed in the immature precursor. The most conserved region of CA, the major homology region (MHR), has been implicated in both immature and mature assembly, although the precise contribution of the MHR residues to each event has been largely undefined. To test the roles of specific MHR residues in mature capsid assembly, an in vitro system was developed that allowed for the first-time formation of Rous sarcoma virus CA into structures resembling authentic capsids.

RSV capsid polymorphism correlates with polymerization efficiency and envelope glycoprotein content: implications that nucleation controls morphogenesis.

We used cryo-electron tomography to visualize Rous sarcoma virus, the prototypic alpharetrovirus. Its polyprotein Gag assembles into spherical procapsids, concomitant with budding. In maturation, Gag is dissected into its matrix, capsid protein (CA), and nucleocapsid moieties.

Overlapping roles of the Rous sarcoma virus Gag p10 domain in nuclear export and virion core morphology.

Nucleocytoplasmic shuttling of the Rous sarcoma virus (RSV) Gag polyprotein is an integral step in virus particle assembly. A nuclear export signal (NES) was previously identified within the p10 domain of RSV Gag. Gag mutants containing deletions of the p10 NES or mutations of critical hydrophobic residues at positions 219, 222, 225, or 229 become trapped within the nucleus and exhibit defects in the efficiency of virus particle release.

Genetic Studies of the beta-hairpin loop of Rous sarcoma virus capsid protein.

The first few residues of the Rous sarcoma virus (RSV) CA protein comprise a structurally dynamic region that forms part of a Gag-Gag interface in immature virus particles. Dissociation of this interaction during maturation allows refolding and formation of a beta-hairpin structure important for assembly of CA monomers into the mature capsid shell. A consensus binding site for the cellular Ubc9 protein was previously identified within this region, suggesting that binding of Ubc9 and subsequent small ubiquitin-like modifier protein 1 (SUMO-1) modification of CA may play a role either in regulating the assembly activity of CA in immature particles or mature cores or in controlling postentry function(s) during the establishment of infection.

Lysines close to the Rous sarcoma virus late domain critical for budding.

The release of retroviruses from the plasma membrane requires host factors that are believed to be recruited to the site of budding by the late (L) domain of the virus-encoded Gag protein. The L domain of Rous sarcoma virus (RSV) has been shown to interact with a ubiquitin (Ub) ligase, and budding of this virus is dependent on Ub. RSV is similar to other retroviruses in that it contains approximately 100 molecules of Ub, but it is unique in that none of these molecules has been found to be conjugated to Gag.

The small RING finger protein Z drives arenavirus budding: implications for antiviral strategies.

By using a reverse genetics system that is based on the prototypic arenavirus lymphocytic choriomeningitis virus (LCMV), we have identified the arenavirus small RING finger Z protein as the main driving force of virus budding. Both LCMV and Lassa fever virus (LFV) Z proteins exhibited self-budding activity, and both substituted efficiently for the late domain that is present in the Gag protein of Rous sarcoma virus. LCMV and LFV Z proteins contain proline-rich motifs that are characteristic of late domains.