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Retrovirus immature particle morphology consists of a membrane enclosed, pleomorphic, spherical and incomplete lattice of Gag hexamers. Previously, we demonstrated that human immunodeficiency virus type 2 (HIV-2) immature particles possess a distinct and extensive Gag lattice morphology. To better understand the nature of the continuously curved hexagonal Gag lattice, we have used single particle cryo-electron microscopy with a retrovirus to determine the HIV-2 Gag lattice structure for immature virions. The reconstruction map at 5.5 Å resolution revealed a stable, wineglass-shaped Gag hexamer structure with structural features consistent with other lentiviral immature Gag structures. Cryo-electron tomography provided evidence for nearly complete ordered Gag lattice structures in HIV-2 immature particles. We also solved a 1.98 Å resolution crystal structure of the carboxyl-terminal domain (CTD) of the HIV-2 capsid (CA) protein that identified a structured helix 12 supported via an interaction of helix 10 in the absence of the SP1 region of Gag. Residues at the helix 10-12 interface proved critical in maintaining HIV-2 particle release and infectivity. Taken together, our findings provide the first 3D organization of HIV-2 immature Gag lattice and important insights into both HIV Gag lattice stabilization and virus maturation.

In the original publication, the names of the second and third authors were incorrectly published.

The retroviral Gag protein is the main structural protein responsible for virus particle assembly and release. Like human immunodeficiency virus type 1 (HIV-1) Gag, human T-cell leukemia virus type 1 (HTLV-1) has a structurally conserved capsid (CA) domain, including a β-hairpin turn and a centralized coiled-coil-like structure of six α helices in the CA amino-terminal domain (NTD), as well as four α-helices in the CA carboxy-terminal domain (CTD). CA drives Gag oligomerization, which is critical for both immature Gag lattice formation and particle production. The HIV-1 CA CTD has previously been shown to be a primary determinant for CA-CA interactions, and while both the HTLV-1 CA NTD and CTD have been implicated in Gag-Gag interactions, our recent observations have implicated the HTLV-1 CA NTD as encoding key determinants that dictate particle morphology. Here, we have conducted alanine-scanning mutagenesis in the HTLV-1 CA NTD nucleotide-encoding sequences spanning the loop regions and amino acids at the beginning and ends of α-helices due to their structural dissimilarity from the HIV-1 CA NTD structure. We analyzed both Gag subcellular distribution and efficiency of particle production for these mutants. We discovered several important residues (i.e., M17, Q47/F48, and Y61). Modeling implicated that these residues reside at the dimer interface (i.e., M17 and Y61) or at the trimer interface (i.e., Q47/F48). Taken together, these observations highlight the critical role of the HTLV-1 CA NTD in Gag-Gag interactions and particle assembly, which is, to the best of our knowledge, in contrast to HIV-1 and other retroviruses.IMPORTANCE Retrovirus particle assembly and release from infected cells is driven by the Gag structural protein. Gag-Gag interactions, which form an oligomeric lattice structure at a particle budding site, are essential to the biogenesis of an infectious virus particle. The CA domain of Gag is generally thought to possess the key determinants for Gag-Gag interactions, and the present study has discovered several critical amino acid residues in the CA domain of HTLV-1 Gag, an important cancer-causing human retrovirus, which are distinct from that of HIV-1 as well as other retroviruses studied to date. Altogether, our results provide important new insights into a poorly understood aspect of HTLV-1 replication that significantly enhances our understanding of the molecular nature of Gag-Gag interaction determinants crucial for virus particle assembly.

The retrovirus capsid core is a metastable structure that disassembles during the early phase of viral infection after membrane fusion. The core is intact and permeable to essential nucleotides during reverse transcription, but it undergoes disassembly for nuclear entry and genome integration. Increasing or decreasing the stability of the capsid core has a substantial negative impact on virus infectivity, which makes the core an attractive anti-viral target. The retrovirus capsid core also encounters a variety of virus- and organism-specific host cellular factors that promote or restrict viral replication. This review describes the structural elements fundamental to the formation and stability of the capsid core. The physical and chemical properties of the capsid core that are critical to its functional role in reverse transcription and interaction with host cellular factors are highlighted to emphasize areas of current research.

The vault complex is the largest cellular ribonucleoprotein complex ever characterized and is present across diverse Eukarya. Despite significant information regarding the structure, composition and evolutionary conservation of the vault, little is know about the complex’s actual biological function. To determine if intracellular vaults are morphologically similar to previously studied purified and recombinant vaults, we have used electron cryo-tomography to characterize the vault complexes found in the thin edges of primary human cells growing in tissue culture. Our studies confirm that intracellular vaults are similar in overall size and shape to purified and recombinant vaults previously analyzed. Results from subtomogram averaging indicate that densities within the vault lumen are not ordered, but randomly distributed. We also observe that vaults located in the extreme periphery of the cytoplasm predominately associate with granule-like structures and actin. Our ultrastructure studies augment existing biochemical, structural and genetic information on the vault, and provide important intracellular context for the ongoing efforts to understand the biological function of the native cytoplasmic vault.

The retrovirus capsid core is a metastable structure that disassembles during the early phase of viral infection after membrane fusion. The core is intact and permeable to essential nucleotides during reverse transcription, but it undergoes disassembly for nuclear entry and genome integration. Increasing or decreasing the stability of the capsid core has a substantial negative impact on virus infectivity, which makes the core an attractive anti-viral target. The retrovirus capsid core also encounters a variety of virus- and organism-specific host cellular factors that promote or restrict viral replication. This review describes the structural elements fundamental to the formation and stability of the capsid core. The physical and chemical properties of the capsid core that are critical to its functional role in reverse transcription and interaction with host cellular factors are highlighted to emphasize areas of current research.

In the original publication, the names of the second and third authors were incorrectly published.

Human T-cell leukemia virus type 1 (HTLV-1) is the etiological agent of adult T-cell leukemia (ATL) and HTLV-1-associated myelopathy/tropical spastic paraparesis (HAM/TSP). HTLV-1 cell-to-cell transmission is dependent on the release of infectious virus particles into the virological synapse. The HTLV-1 particle structure is still poorly understood, and previous studies analyzed viruses produced by transformed lymphocytic cell lines chronically infected with HTLV-1, particularly the MT-2 cell line, which harbors truncated proviruses and expresses aberrant forms of the Gag protein. In this study, we demonstrate that the chronically infected SP cell line harbors a relatively low number of proviruses, making it a more promising experimental system for the study of the HTLV-1 particle structure. We first identified the genomic sites of integration and characterized the genetic structure of the gag region in each provirus. We also determined that despite encoding a truncated Gag protein, only the full-length Gag protein was incorporated into virus particles. Cryo-transmission electron microscopy analyses of the purified virus particles revealed three classes of particles based upon capsid core morphology: complete cores, incomplete cores, and particles without distinct electron densities that would correlate with the capsid region of a core structure. Observed cores were generally polygonal, and virus particles were on average 115 nm in diameter. These data corroborate particle morphologies previously observed for MT-2 cells and provide evidence that the known poor infectivity of HTLV-1 particles may correlate with HTLV-1 particle populations containing few virus particles possessing a complete capsid core structure. IMPORTANCE Studies of retroviral particle core morphology have demonstrated a correlation between capsid core stability and the relative infectivity of the virus. In this study, we used cryo-transmission electron microscopy to demonstrate that HTLV-1 particles produced from a distinct chronically infected cell line are polymorphic in nature, with many particles lacking organized electron densities that would correlate with a complete core structure. These findings have important implications for infectious HTLV-1 spread, particularly in the context of cell-to-cell transmission, a critical step in HTLV-1 transmission and pathogenesis.

The capsid domain (CA) of the retroviral Gag protein is a primary determinant of Gag oligomerization, which is a critical step for immature Gag lattice formation and virus particle budding. Although the human immunodeficiency virus type 1 (HIV-1) CA carboxy-terminal domain (CTD) is essential for CA-CA interactions, the CA CTD has been suggested to be largely dispensable for human T-cell leukemia virus type 1 (HTLV-1) particle biogenesis. To more clearly define the roles of the HTLV-1 CA amino-terminal domain (NTD) and CA CTD in particle biogenesis, we generated and analyzed a panel of Gag proteins with chimeric HIV-1/HTLV-1 CA domains. Subcellular distribution and protein expression levels indicated that Gag proteins with a chimeric HIV-1 CA NTD/HTLV-1 CA CTD did not result in Gag oligomerization regardless of the parent Gag background. Furthermore, chimeric Gag proteins with the HTLV-1 CA NTD produced particles phenotypically similar to HTLV-1 immature particles, highlighting the importance of the HTLV-1 CA NTD in HTLV-1 immature particle morphology. Taken together, these observations support the conclusion that the HTLV-1 CA NTD can functionally replace the HIV-1 CA CTD, but the HIV-1 CA NTD cannot replace the HTLV-1 CA CTD, indicating that the HTLV-1 CA subdomains provide distinct contributions to Gag-Gag oligomerization, particle morphology, and biogenesis. Furthermore, we have shown for the first time that HIV-1 and HTLV-1 Gag domains outside the CA (e.g., matrix and nucleocapsid) impact Gag oligomerization as well as immature particle size and morphology. IMPORTANCE A key aspect in virus replication is virus particle assembly, which is a poorly understood process for most viruses. For retroviruses, the Gag structural protein is the primary driver of virus particle biogenesis, and the CA CTD is the primary determinant of Gag-Gag interactions for HIV-1. In this study, the HTLV-1 capsid amino-terminal domain was found to provide distinct contributions to Gag-Gag oligomerization, particle morphology, and biogenesis. This study provides information that will aid efforts for discovery of therapeutic targets for intervention.

Nef is an accessory protein of human immunodeficiency viruses that promotes viral replication and progression to AIDS through interference with various host trafficking and signaling pathways. A key function of Nef is the down-regulation of the coreceptor CD4 from the surface of the host cells. Nef-induced CD4 down-regulation involves at least two independent steps as follows: acceleration of CD4 endocytosis by a clathrin/AP-2-dependent pathway and targeting of internalized CD4 to multivesicular bodies (MVBs) for eventual degradation in lysosomes. In a previous work, we found that CD4 targeting to the MVB pathway was independent of CD4 ubiquitination. Here, we report that this targeting depends on a direct interaction of Nef with Alix/AIP1, a protein associated with the endosomal sorting complexes required for transport (ESCRT) machinery that assists with cargo recruitment and intraluminal vesicle formation in MVBs. We show that Nef interacts with both the Bro1 and V domains of Alix. Depletion of Alix or overexpression of the Alix V domain impairs lysosomal degradation of CD4 induced by Nef. In contrast, the V domain overexpression does not prevent cell surface removal of CD4 by Nef or protein targeting to the canonical ubiquitination-dependent MVB pathway. We also show that the Nef-Alix interaction occurs in late endosomes that are enriched in internalized CD4. Together, our results indicate that Alix functions as an adaptor for the ESCRT-dependent, ubiquitin-independent targeting of CD4 to the MVB pathway induced by Nef.

Nef is an important player for viral infectivity and AIDS progression, but the mechanisms involved are not completely understood. It was previously demonstrated that Nef interacts with GagPol through p6*-Protease region. Because p6* and Protease are involved in processing, we explored the effect of Nef on viral Protease activity and virion assembly. Using in vitro assays, we observed that Nef is highly capable of inhibiting Protease activity. The IC50 for nef-deficient viruses in drug susceptibility assays were 1.7- to 3.5-fold higher than the wild-type counterpart varying with the type of the Protease inhibitor used. Indicating that, in the absence of Nef, Protease is less sensitive to Protease inhibitors. We compared the protein content between wild-type and nef-deficient mature viral particles by gradient sedimentation and observed up to 2.7-fold reduction in the Integrase levels in nef-deficient mature particles. This difference in levels of Integrase correlated with the difference in infectivity levels of wild type and nef-deficient viral progeny. In addition, an overall decrease in the production of mature particles was detected in nef-deficient viruses. Collectively, our data support the hypothesis that the decreased infectivity typical of nef-deficient viruses is due to an abnormal function of the viral Protease, which is in turn associated with less mature particles being produced and the loss of Integrase content in these particles, and these results may characterize Nef as a regulator of viral Protease activity.

Human and Simian Immunodeficiency Viruses (HIV and SIV) are the etiological agents of the Acquired Immunodeficiency Syndrome (AIDS) in humans and the Simian AIDS (SAIDS) in macaques, respectively. HIVs and SIVs are members of the Retroviridae family, Lentivirus genera, and are considered complex retroviruses since its genome organization predicts the presence of at least 6 open reading frames (ORFs) in addition to the main Gag, Pol and Env ORFs present in the genomes of all retroviruses. These additional ORFs code for both regulatory (Tat and Rev) and accessory (Nef, Vif, Vpr, Vpu and Vpx) viral proteins and are all organized from the 5’ half of the genome in a way that overlap both with each other and with the Pol and Env ORFs and the non-coding 3’ Long Terminal Repeat (LTR) region (Figure 1). To ensure its expression and to achieve an optimal production of the viral progeny, complex mechanisms have evolved in these viruses that tightly control the expression of these ORFs during the viral replication cycle. The existence of such a number of viral proteins in addition to the viral structural (Gag and Env) and enzymatic (Pol) proteins allows the virus to explore new mechanisms to control the different steps of the replication cycle and to avoid the host cell defense. In this chapter we shall review the different steps of the HIV and SIV replication cycle with emphasis in the role taken by the viral accessory protein Nef, in both subverting the host cell machinery and influencing the function and activation of viral structural and enzymatic proteins in order to optimize viral progeny production as well as in evading the host cell defenses. Lentiviral accessory proteins Vif, Vpr, Vpu and Nef were classically regarded as nonessential for virus production and/or infectivity since laboratory adapted HIV strains lacking the expression of these proteins could still replicate to several levels (Adachi et al., 1991). Since then, several studies demonstrated the crucial importance of these proteins to the efficient replication, infectivity and spread of both HIV and SIV (Kirchhoff, 2010). Vif (Aguiar and Peterlin, 2008) and Vpu (Adachi et al., 1991) have now been acknowledged as crucial viral factors that counteract the host cell innate defense. Vif interacts and prompts the degradation of a family of cytidine deaminases DNA/RNA editing enzymes, known as Apoliprotein B mRNA-editing Enzymes (APOBECs), that otherwise would inhibit HIV and SIV replication by causing hypermutation of nascent