The highly conserved C-terminal dileucine motif in the cytosolic domain of the human immunodeficiency virus type 1 envelope glycoprotein is critical for its association with the AP-1 clathrin adaptor [correction of adapter]

Rab7A is required for efficient production of infectious HIV-1

Retroviruses take advantage of cellular trafficking machineries to assemble and release new infectious particles. Rab proteins regulate specific steps in intracellular membrane trafficking by recruiting tethering, docking and fusion factors, as well as the actin- and microtubule-based motor proteins that facilitate vesicle traffic. Using virological tests and RNA interference targeting Rab proteins, we demonstrate that the late endosome-associated Rab7A is required for HIV-1 propagation. Analysis of the late steps of the HIV infection cycle shows that Rab7A regulates Env processing, the incorporation of mature Env glycoproteins into viral particles and HIV-1 infectivity. We also show that siRNA-mediated Rab7A depletion induces a BST2/Tetherin phenotype on HIV-1 release. BST2/Tetherin is a restriction factor that impedes HIV-1 release by tethering mature virus particles to the plasma membrane. Our results suggest that Rab7A contributes to the mechanism by which Vpu counteracts the restriction factor BST2/Tetherin and rescues HIV-1 release. Altogether, our results highlight new roles for a major regulator of the late endocytic pathway, Rab7A, in the late stages of the HIV-1 replication cycle.

Human immunodeficiency virus (HIV) propagation requires the assistance of host cell factors at all stages of the infection cycle. HIV exploits components of the cellular membrane sorting machinery for its assembly, budding and release. Rab GTPases are key regulators of membrane-trafficking events, including exocytosis and endocytosis, in eukaryotic cells. Here we show that the late endosome associated Rab7A plays a major role in HIV-1 replication. We find that Rab7A regulates the production of infectious HIV-1 particles at two critical stages. First, Rab7A is required for efficient Env processing and, thus, for the incorporation of mature HIV-1 envelope glycoproteins into virions. Second, Rab7A contributes to the mechanism that counteracts the restriction imposed on HIV-1 release by the cellular restriction factor BST2/Tetherin that physically tethers viral particles to the plasma membrane of infected cells. Altogether these data highlight new roles for a major player of the late endocytic pathway, Rab7A, in the late stages of the HIV-1 replication cycle.

The regulated secretory pathway in CD4(+) T cells contributes to human immunodeficiency virus type-1 cell-to-cell spread at the virological synapse

Direct cell-cell spread of Human Immunodeficiency Virus type-1 (HIV-1) at the virological synapse (VS) is an efficient mode of dissemination between CD4(+) T cells but the mechanisms by which HIV-1 proteins are directed towards intercellular contacts is unclear. We have used confocal microscopy and electron tomography coupled with functional virology and cell biology of primary CD4(+) T cells from normal individuals and patients with Chediak-Higashi Syndrome and report that the HIV-1 VS displays a regulated secretion phenotype that shares features with polarized secretion at the T cell immunological synapse (IS). Cell-cell contact at the VS re-orientates the microtubule organizing center (MTOC) and organelles within the HIV-1-infected T cell towards the engaged target T cell, concomitant with polarization of viral proteins. Directed secretion of proteins at the T cell IS requires specialized organelles termed secretory lysosomes (SL) and we show that the HIV-1 envelope glycoprotein (Env) localizes with CTLA-4 and FasL in SL-related compartments and at the VS. Finally, CD4(+) T cells that are disabled for regulated secretion are less able to support productive cell-to-cell HIV-1 spread. We propose that HIV-1 hijacks the regulated secretory pathway of CD4(+) T cells to enhance its dissemination.

HIV-1 envelope glycoprotein biosynthesis, trafficking, and incorporation

The HIV-1 envelope (Env) glycoproteins play an essential role in the virus replication cycle by mediating the fusion between viral and cellular membranes during the entry process. The Env glycoproteins are synthesized as a polyprotein precursor (gp160) that is cleaved by cellular proteases to the mature surface glycoprotein gp120 and the transmembrane glycoprotein gp41. During virus assembly, the gp120/gp41 complex is incorporated as heterotrimeric spikes into the lipid bilayer of nascent virions. These gp120/gp41 complexes then initiate the infection process by binding receptor and coreceptor on the surface of target cells. Much is currently known about the HIV-1 Env glycoprotein trafficking pathway and the structure of gp120 and the extracellular domain of gp41. However, the mechanism by which the Env glycoprotein complex is incorporated into virus particles remains incompletely understood. Genetic data support a major role for the cytoplasmic tail of gp41 and the matrix domain of Gag in Env glycoprotein incorporation. Still to be defined are the identities of host cell factors that may promote Env incorporation and the role of specific membrane microdomains in this process. Here, we review our current understanding of HIV-1 Env glycoprotein trafficking and incorporation into virions.

Mutagenesis of tyrosine and di-leucine motifs in the HIV-1 envelope cytoplasmic domain results in a loss of Env-mediated fusion and infectivity

Background

The gp41 component of the Human Immunodeficiency Virus (HIV) envelope glycoprotein (Env) contains a long cytoplasmic domain (CD) with multiple highly conserved tyrosine (Y) and dileucine (LL) motifs. Studies suggest that the motifs distal to major endocytosis motif (Y₇₁₂HRL), located at residues 712-715 of Env, may contribute to Env functionality in the viral life cycle. In order to examine the biological contribution of these motifs in the biosynthesis, transport, and function of Env, we constructed two panels of mutants in which the conserved Y- and LL-motifs were sequentially substituted by alternative residues, either in the presence or absence of Y₇₁₂. Additional mutants targeting individual motifs were then constructed.

Results

All mutant Envs, when expressed in the absence of other viral proteins, maintained at least WT levels of Env surface staining by multiple antibodies. The Y₇₁₂ mutation (Y712C) contributed to at least a 4-fold increase in surface expression for all mutants containing this change. Sequential mutagenesis of the Y- and LL-motifs resulted in a generally progressive decrease in Env fusogenicity. However, additive mutation of dileucine and tyrosine motifs beyond the tyrosine at residue 768 resulted in the most dramatic effects on Env incorporation into virions, viral infectivity, and virus fusion with target cells.

Conclusions

From the studies reported here, we show that mutations of the Y- and LL-motifs, which effectively eliminate the amphipathic nature of the lytic peptide 2 (LLP2) domain or disrupt YW and LL motifs in a region spanning residues 795-803 (YWWNLLQYW), just C-terminal of LLP2, can dramatically interfere with biological functions of HIV-1 Env and abrogate virus replication. Because these mutant proteins are expressed at the cell surface, we conclude that tyrosine and di-leucine residues within the cytoplasmic domain of gp41 play critical roles in HIV-1 replication that are distinct from that of targeting the plasma membrane.

Mechanisms for Env glycoprotein acquisition by retroviruses

A mandatory step in the formation of an infectious retroviral particle is the acquisition of its envelope glycoprotein (Env). This step invariably occurs by Env positioning itself in the host membrane at the location of viral budding and being incorporated along with the host membrane into the viral particle. In some ways, this step of the viral life cycle would appear to be imprecise. There is no specific sequence in Env or in the retroviral structural protein, Gag, that is inherently required for the production of an infectious Env-containing particle. Additionally, Env-defective proviruses can efficiently produce infectious particles with any of a number of foreign retroviral Env glycoproteins or even glycoproteins from unrelated viral families, a process termed pseudotyping. However, mounting evidence suggests that Env incorporation is neither passive nor random. Rather, several redundant mechanisms appear to contribute to the carefully controlled process of Env acquisition, many of which are apparently used by a wide variety of enveloped viruses. This review presents and discusses the evidence for these different mechanisms contributing to incorporation.

T cell polarization at the virological synapse

Cell-to-cell spread of HIV-1 between CD4(+) T cells takes place at multimolecular structures called virological synapses. A defining feature of the virological synapse is polarization of viral assembly and budding at sites of T cell-T cell contact. Recent work is beginning to address how viral proteins are targeted to the virological synapse and the molecular mechanisms that regulate HIV-1 egress by cell-to-cell spread. This review discusses our current understanding of these processes and considers how T cell polarization during other forms of intercellular communication may provide insight into HIV-1 assembly and dissemination.

HIV-1 assembly in macrophages

The molecular mechanisms involved in the assembly of newly synthesized Human Immunodeficiency Virus (HIV) particles are poorly understood. Most of the work on HIV-1 assembly has been performed in T cells in which viral particle budding and assembly take place at the plasma membrane. In contrast, few studies have been performed on macrophages, the other major target of HIV-1. Infected macrophages represent a viral reservoir and probably play a key role in HIV-1 physiopathology. Indeed macrophages retain infectious particles for long periods of time, keeping them protected from anti-viral immune response or drug treatments. Here, we present an overview of what is known about HIV-1 assembly in macrophages as compared to T lymphocytes or cell lines.

Early electron microscopy studies suggested that viral assembly takes place at the limiting membrane of an intracellular compartment in macrophages and not at the plasma membrane as in T cells. This was first considered as a late endosomal compartment in which viral budding seems to be similar to the process of vesicle release into multi-vesicular bodies. This view was notably supported by a large body of evidence involving the ESCRT (Endosomal Sorting Complex Required for Transport) machinery in HIV-1 budding, the observation of viral budding profiles in such compartments by immuno-electron microscopy, and the presence of late endosomal markers associated with macrophage-derived virions. However, this model needs to be revisited as recent data indicate that the viral compartment has a neutral pH and can be connected to the plasma membrane via very thin micro-channels. To date, the exact nature and biogenesis of the HIV assembly compartment in macrophages remains elusive. Many cellular proteins potentially involved in the late phases of HIV-1 cycle have been identified; and, recently, the list has grown rapidly with the publication of four independent genome-wide screens. However, their respective roles in infected cells and especially in macrophages remain to be characterized. In summary, the complete process of HIV-1 assembly is still poorly understood and will undoubtedly benefit from the ongoing explosion of new imaging techniques allowing better time-lapse and quantitative studies.

Investigation of low-dose ritonavir on human peripheral blood mononuclear cells using gene expression whole genome microarrays

Ritonavir is a protease inhibitor associated with metabolic abnormalities and cardiovascular disease. We have investigated the effects of low-dose ritonavir treatment on gene expression in peripheral blood mononuclear cells (PBMC) of 10 healthy donors. Results using whole genome Illumina microarrays show that ritonavir modulates a number of genes implicated in lipid metabolism, inflammation and atherosclerosis. These candidate genes are dual specificity phosphatase 1 DUSP1), Kelch domain containing 3 (KLHDC3), neutral cholesterol ester hydrolase 1 (NCEH1) and acyl-CoA synthetase short-chain family member 2 (ACSS2). Validation experiments using quantitative PCR showed that ritonavir (at 100 mg once daily and 100 mg twice daily significantly down-regulated these 4 selected candidate genes in 20 healthy individuals. Lower expression levels of these 4 candidate genes, known to play a critical role in inflammation, lipid metabolism and atherosclerosis, may explain ritonavir adverse effects in patients.

The cytoplasmic domain of human immunodeficiency virus type 1 transmembrane protein gp41 harbors lipid raft association determinants

The molecular basis for localization of the human immunodeficiency virus type 1 envelope glycoprotein (Env) in detergent-resistant membranes (DRMs), also called lipid rafts, still remains unclear. The C-terminal cytoplasmic tail of gp41 contains three membrane-interacting, amphipathic alpha-helical sequences, termed lentivirus lytic peptide 2 (LLP-2), LLP-3, and LLP-1, in that order. Here we identify determinants in the cytoplasmic tail which are crucial for Env's association with Triton X-100-resistant rafts. Truncations of LLP-1 greatly reduced Env localization in lipid rafts, and the property of Gag-independent gp41 localization in rafts was conserved among different strains. Analyses of mutants containing single deletions or substitutions in LLP-1 showed that the alpha-helical structure of the LLP-1 hydrophobic face has a more-critical role in Env-raft associations than that of the hydrophilic face. With the exception of a Pro substitution for Val-833, all Pro substitution and charge-inverting mutants showed wild-type virus-like one-cycle viral infectivity, replication kinetics, and Env incorporation into the virus. The intracellular localization and cell surface expression of mutants not localized in lipid rafts, such as the TM844, TM813, 829P, and 843P mutants, were apparently normal compared to those of wild-type Env. Cytoplasmic subdomain targeting analyses revealed that the sequence spanning LLP-3 and LLP-1 could target a cytoplasmic reporter protein to DRMs. Mutations of LLP-1 that affected Env association with lipid rafts also disrupted the DRM-targeting ability of the LLP-3/LLP-1 sequence. Our results clearly demonstrate that LLP motifs located in the C-terminal cytoplasmic tail of gp41 harbor Triton X-100-resistant raft association determinants.

Identification of the cellular prohibitin 1/prohibitin 2 heterodimer as an interaction partner of the C-terminal cytoplasmic domain of the HIV-1 glycoprotein

Our studies aim to elucidate the functions carried out by the very long, and in its length highly conserved, C-terminal cytoplasmic domain (Env-CT) of the HIV-1 glycoprotein. Mass spectrometric analysis of cellular proteins bound to a tagged version of the HIV Env-CT led to the identification of the prohibitin 1 and 2 proteins (Phb1 and Phb2). These ubiquitously expressed proteins, which exist as stable heterodimers, have been shown to have multiple functions within cells and to localize to multiple cellular and extracellular compartments. The specificity of binding of the Phb1/Phb2 complex to the Env-CT was confirmed in various manners, including coimmunoprecipitation with authentic provirally encoded, full-length Env. Strong binding was dependent on Env residues 790 to 800 and could be severely inhibited by the double mutation L799R/L800Q but not by mutation of these amino acids individually. Analysis of the respective mutant virions revealed that their different abilities to bind Phb1/Phb2 correlated with their replicative properties. Thus, mutated virions with single mutations [HIV-Env-(L799R) and HIV-Env-(L800Q)] replicated similarly to wild-type HIV, but HIV-Env-(L799R/L800Q) virions, which cannot bind Phb1/Phb2, exhibited a cell-dependent replicative phenotype similar to that of HIV-Env-Tr712, lacking the entire Env-CT domain. Thus, replicative spread was achieved, although somewhat delayed, in "permissive" MT-4 cells but failed to occur in "nonpermissive" H9 T cells. These results point to binding of the Phb1/Phb2 complex to the Env-CT as being of importance for replicative spread in nonpermissive cells, possibly by modulating critical Phb-dependent cellular process(es).

Matrix and envelope coevolution revealed in a patient monitored since primary infection with human immunodeficiency virus type 1

Lentiviruses, including human immunodeficiency virus type 1 (HIV-1), typically encode envelope glycoproteins (Env) with long cytoplasmic tails (CTs). The strong conservation of CT length in primary isolates of HIV-1 suggests that this factor plays a key role in viral replication and persistence in infected patients. However, we report here the emergence and dominance of a primary HIV-1 variant carrying a natural 20-amino-acid truncation of the CT in vivo. We demonstrated that this truncation was deleterious for viral replication in cell culture. We then identified a compensatory amino acid substitution in the matrix protein that reversed the negative effects of CT truncation. The loss or rescue of infectivity depended on the level of Env incorporation into virus particles. Interestingly, we found that a virus mutant with defective Env incorporation was able to spread by cell-to-cell transfer. The effects on viral infectivity of compensation between the CT and the matrix protein have been suggested by in vitro studies based on T-cell laboratory-adapted virus mutants, but we provide here the first demonstration of the natural occurrence of similar mechanisms in an infected patient. Our findings provide insight into the potential of HIV-1 to evolve in vivo and its ability to overcome major structural alterations.

Soluble CD4 and CD4-mimetic compounds inhibit HIV-1 infection by induction of a short-lived activated state

Binding to the CD4 receptor induces conformational changes in the human immunodeficiency virus (HIV-1) gp120 exterior envelope glycoprotein. These changes allow gp120 to bind the coreceptor, either CCR5 or CXCR4, and prime the gp41 transmembrane envelope glycoprotein to mediate virus–cell membrane fusion and virus entry. Soluble forms of CD4 (sCD4) and small-molecule CD4 mimics (here exemplified by JRC-II-191) also induce these conformational changes in the HIV-1 envelope glycoproteins, but typically inhibit HIV-1 entry into CD4-expressing cells. To investigate the mechanism of inhibition, we monitored at high temporal resolution inhibitor-induced changes in the conformation and functional competence of the HIV-1 envelope glycoproteins that immediately follow engagement of the soluble CD4 mimics. Both sCD4 and JRC-II-191 efficiently activated the envelope glycoproteins to mediate infection of cells lacking CD4, in a manner dependent on coreceptor affinity and density. This activated state, however, was transient and was followed by spontaneous and apparently irreversible changes of conformation and by loss of functional competence. The longevity of the activated intermediate depended on temperature and the particular HIV-1 strain, but was indistinguishable for sCD4 and JRC-II-191; by contrast, the activated intermediate induced by cell-surface CD4 was relatively long-lived. The inactivating effects of these activation-based inhibitors predominantly affected cell-free virus, whereas virus that was prebound to the target cell surface was mainly activated, infecting the cells even at high concentrations of the CD4 analogue. These results demonstrate the ability of soluble CD4 mimics to inactivate HIV-1 by prematurely triggering active but transient intermediate states of the envelope glycoproteins. This novel strategy for inhibition may be generally applicable to high–potential-energy viral entry machines that are normally activated by receptor binding.

Human immunodeficiency virus type 1 (HIV-1) is the cause of the global AIDS epidemic. HIV-1 gains entry into its target cells by fusing with the cell membrane, a process that begins with the interaction of the viral envelope glycoproteins with cell-surface receptors. HIV-1 uses two receptors on the target cell: CD4 and CCR5/CXCR4. Binding of the virus to the primary receptor, CD4, primes the viral envelope glycoproteins to mediate the fusion of the viral membrane and the membrane of the target cell. Soluble forms of the CD4 receptor and small molecules that mimic the effects of CD4 can inhibit virus infection; however, how this inhibition occurs is still unknown. In this report, we show that soluble mimics of CD4 inhibit HIV-1 infection by prematurely triggering the viral envelope glycoproteins. The unstable activated state of the virus lasts only a few minutes, after which the virus loses the ability to infect cells. This novel strategy for inhibition may be generally applicable to other viruses besides HIV-1, some of which are also activated by binding to their receptors.

Human immunodeficiency virus type-1 gag and host vesicular trafficking pathways

The Gag protein of HIV-1 directs the particle assembly process. Gag recruits components of the cellular vesicular trafficking machinery in order to traverse the cytoplasm of the cell and reach the particle assembly site. The plasma membrane is the primary site of particle assembly in most cell types, while in macrophages an unusual intracellular membrane-bound compartment bearing markers of late endosomes and the plasma membrane is the predominant assembly site. Plasma membrane specificity of assembly may be directed by components of lipid rafts and the cytoplasmic leaflet component PI(4,5)P(2). Recent work has highlighted the role of adaptor protein complexes, protein sorting and recycling pathways, components of the multivesicular body, and cellular motor proteins in facilitating HIV assembly and budding. This review presents an overview of the relevant vesicular trafficking pathways and describes the individual components implicated in interactions with Gag.

Roles of the interactions between Env and Gag proteins in the HIV-1 replication cycle

The Env and Gag proteins of HIV-1 are the two major structural proteins of this retrovirus. The interactions between Env and Gag proteins and their regulation in HIV-1 are required for several steps of the replication cycle, involving not only virus assembly, specifically Env incorporation, but also entry steps after virus maturation. A large number of host factors and certain membrane microdomains appear to engage both in transport/trafficking of Env and/or Gag proteins, and in the interactions of these two proteins. The present review briefly summarizes our current knowledge regarding the roles of the interactions between Env and Gag proteins in the virus replication cycle.

Nef can enhance the infectivity of receptor-pseudotyped human immunodeficiency virus type 1 particles

Nef is an accessory protein of human immunodeficiency virus type 1 (HIV-1) that enhances the infectivity of progeny virions when expressed in virus-producing cells. The requirement for Nef for optimal infectivity is, at least in part, determined by the envelope (Env) glycoprotein, because it can be eliminated by pseudotyping HIV-1 particles with pH-dependent Env proteins. To investigate the role of Env in the function of Nef, we have examined the effect of Nef on the infectivity of Env-deficient HIV-1 particles pseudotyped with viral receptors for cells expressing cognate Env proteins. We found that Nef significantly enhances the infectivity of CD4-chemokine receptor pseudotypes for cells expressing HIV-1 Env. Nef also increased the infectivity of HIV-1 particles pseudotyped with Tva, the receptor for subgroup A Rous sarcoma virus (RSV-A), even though Nef had no effect if the pH-dependent Env protein of RSV-A was used for pseudotyping. However, Nef does not always enhance viral infectivity if the normal orientation of the Env-receptor interaction is reversed, because the entry of Env-deficient HIV-1 into cells expressing the vesicular stomatitis virus G protein was unaffected by Nef. Together, our results demonstrate that the presence of a viral Env protein during virus production is not required for the ability of Nef to increase viral infectivity. Furthermore, since the infectivity of Tva pseudotypes was blocked by inhibitors of endosomal acidification, we conclude that low-pH-dependent entry does not always bypass the requirement for Nef.

The membrane-proximal external region of the human immunodeficiency virus type 1 envelope: dominant site of antibody neutralization and target for vaccine design

Enormous efforts have been made to produce a protective vaccine against human immunodeficiency virus type 1; there has been little success. However, the identification of broadly neutralizing antibodies against epitopes on the highly conserved membrane-proximal external region (MPER) of the gp41 envelope protein has delineated this region as an attractive vaccine target. Furthermore, emerging structural information on the MPER has provided vaccine designers with new insights for building relevant immunogens. This review describes the current state of the field regarding (i) the structure and function of the gp41 MPER; (ii) the structure and binding mechanisms of the broadly neutralizing antibodies 2F5, 4E10, and Z13; and (iii) the development of an MPER-targeting vaccine. In addition, emerging approaches to vaccine design are presented.

Susceptibility to virus-cell fusion at the plasma membrane is reduced through expression of HIV gp41 cytoplasmic domains

The cytoplasmic tail of the HIV transmembrane protein plays an important role in viral infection. In this study we analyzed the role of retroviral cytoplasmic tails in modulating the cytoskeleton and interfering with virus-cell fusion. HeLaP4 cells expressing different HIV cytoplasmic tail constructs showed reduced acetylated tubulin levels whereas the cytoplasmic tail of MLV did not alter microtubule stability indicating a unique function for the lentiviral cytoplasmic tail. The effect on tubulin is mediated through the membrane proximal region of the HIV cytoplasmic tail and was independent of membrane localization. Site-directed mutagenesis identified three motifs in the HIV-2 cytoplasmic tail required to effect the reduction in acetylated tubulin. Both the YxxPhi domain and amino acids 21 to 45 of the HIV-2 cytoplasmic tail need to be present to change the level of acetylated tubulin in transfected cells. T-cells stably expressing one HIV-2 cytoplasmic tail derived construct showed also a reduction in acetylated tubulin thus confirming the importance of this effect not only for HeLaP4 and 293T cells. Challenge experiments using transiently transfected HeLaP4 cells and T cells stably expressing an HIV cytoplasmic tail construct revealed both reduced virus-cell fusion and replication of HIV-1(NL4.3) compared to control cells. In the virus-cell fusion assay only virions pseudotyped with either HIV or MLV envelopes showed reduced fusion efficiency, whereas VSV-G pseudotyped virions where not affected by the expression of HIV derived cytoplasmic tail constructs, indicating that fusion at the plasma but not endosomal membrane is affected. Overexpression of human histone-deacetylase 6 (HDAC6) and constitutively active RhoA resulted in a reduction of acetylated tubulin and reduced virus-cell fusion as significant as that observed following expression of HIV cytoplasmic tail constructs. Inhibition of HDAC6 showed a strong increase in acetylated tubulin and increase of virus-cell fusion confirming the correlation between post-translational modification of tubulin and virus-cell fusion. These results thus identify tubulin and its post-translational modification as a new cellular target for interference with HIV-cell fusion.

Characterization of a new 5' splice site within the caprine arthritis encephalitis virus genome: evidence for a novel auxiliary protein

BACKGROUND

Lentiviral genomes encode multiple structural and regulatory proteins. Expression of the full complement of viral proteins is accomplished in part by alternative splicing of the genomic RNA. Caprine arthritis encephalitis virus (CAEV) and maedi-visna virus (MVV) are two highly related small-ruminant lentiviruses (SRLVs) that infect goats and sheep. Their genome seems to be less complex than those of primate lentiviruses since SRLVs encode only three auxiliary proteins, namely, Tat, Rev, and Vif, in addition to the products of gag, pol, and env genes common to all retroviruses. Here, we investigated the central part of the SRLV genome to identify new splice elements and their relevance in viral mRNA and protein expression.

RESULTS

We demonstrated the existence of a new 5' splice (SD) site located within the central part of CAEV genome, 17 nucleotides downstream from the SD site used for the rev mRNA synthesis, and perfectly conserved among SRLV strains. This new SD site was found to be functional in both transfected and infected cells, leading to the production of a transcript containing an open reading frame generated by the splice junction with the 3' splice site used for the rev mRNA synthesis. This open reading frame encodes two major protein isoforms of 18- and 17-kDa, named Rtm, in which the N-terminal domain shared by the Env precursor and Rev proteins is fused to the entire cytoplasmic tail of the transmembrane glycoprotein. Immunoprecipitations using monospecific antibodies provided evidence for the expression of the Rtm isoforms in infected cells. The Rtm protein interacts specifically with the cytoplasmic domain of the transmembrane glycoprotein in vitro, and its expression impairs the fusion activity of the Env protein.

CONCLUSION

The characterization of a novel CAEV protein, named Rtm, which is produced by an additional multiply-spliced mRNA, indicated that the splicing pattern of CAEV genome is more complex than previously reported, generating greater protein diversity. The high conservation of the SD site used for the rtm mRNA synthesis among CAEV and MVV strains strongly suggests that the Rtm protein plays a role in SRLV propagation in vivo, likely by competing with Env protein functions.

Immunogenicity and protective efficacy of Gag/Pol/Env vaccines derived from temporal isolates of SIVmne against cognate virus challenge

BACKGROUND

We used the SIVmne model to examine the relative immunogenicity and protective efficacy of vaccines derived from temporal isolates of lentivirus infection. SIVmne170 is a molecular clone isolated from a pig-tailed macaque 170 weeks after inoculation with SIVmneCL8.

METHODS

We immunized pig-tailed macaques with Gag/Pol/Env vaccines derived from CL8 or 170 and examined their protective efficacy against CL8, 170, or chimeras 8/170 and 170/8, containing the 5' or 3' half of the respective parental genomes.

RESULTS

As expected, CL8 vaccines protected animals against the CL8, but not the 170 virus. Surprisingly, 170 vaccines not only failed to protect against the 170 virus, but also the less pathogenic CL8. Chimeric virus challenges revealed that the envelope antigen of CL8 represents an important target for protective immunity.

CONCLUSIONS

These results underscore the potential importance of targeting transmitted viruses through judicious choice of immunogens from early isolates for vaccine development.

The dominant-negative action of a fusion protein containing the cytoplasmic domain of human immunodeficiency virus type 1 transmembrane protein gp41 in virus replication

We previously described a novel mode of downregulation of human immunodeficiency virus type 1 (HIV-1) Gag expression by a cytoplasmic domain fusion protein of the envelope (Env) transmembrane protein, beta-galactosidase (beta-gal)/706-856, which contains the cytoplasmic tail of gp41 fused at the C terminus of Escherichia coli beta-gal. In the present study, we showed that this mediator conferred a dose-dependent dominant interference with virus infectivity. In the context of an HIV-1 provirus, this inhibitor downregulated steady-state Env expression. Paradoxically, Env overexpression suppressed beta-gal/706-856-mediatd Gag downregulation. Sucrose gradient ultracentrifugation and confocal microscopy revealed that Gag, Env, and beta-gal/706-856 had stable interactions and formed aggregated complexes in perinuclear regions. Moreover, Env overexpression hindered colocalization of Gag with beta-gal/706-856 in the perinuclear region. Further cytoplasmic domain mapping analyses showed a correlation between the ability of cytoplasmic subdomains to downregulate Gag expression and the ability of these subdomains to stably interact with Gag. These studies show that redirection of Gag from its cytoplasmic synthesis site to a perinuclear compartment is a prerequisite for beta-gal/706-856-mediated Gag downregulation. The results also illustrate that the dynamic interplay among Gag, Env, and beta-gal/706-856 can modulate Gag and Env expression, thus controlling HIV-1 infection.

Maturation-dependent human immunodeficiency virus type 1 particle fusion requires a carboxyl-terminal region of the gp41 cytoplasmic tail

Lentiviruses, including human immunodeficiency virus type 1 (HIV-1), typically encode fusion glycoproteins with long cytoplasmic tails (CTs). We previously reported that immature HIV-1 particles are inhibited for fusion with target cells by a mechanism requiring the 152-amino-acid CT of gp41. The gp41 CT was also shown to mediate the detergent-resistant association of the HIV-1 envelope glycoprotein complex with immature HIV-1 particles, indicating that the gp41 CT forms a stable complex with Gag in immature virions. In the present study, we analyzed the effects of progressive truncations and point mutations in the gp41 CT on the fusion of mature and immature HIV-1 particles with target cells. We also determined the effects of these mutations on the detergent-resistant association of gp41 with immature HIV-1 particles. Removal of the C-terminal 28 amino acids relieved the dependence of HIV-1 fusion on maturation. However, a mutant Env protein lacking this region remained associated with immature HIV-1 particles treated with nonionic detergent. Further mutational analysis of the C-terminal region of gp41 revealed two specific sequences required for maturation-dependent HIV-1 fusion. Collectively, our results demonstrate that the extreme C terminus of gp41 plays a key role in coupling HIV-1 fusion competence to virion maturation. They further indicate that the stable association of gp41 with Gag in immature virions is not sufficient for inhibition of immature HIV-1 particle fusion.

Differential functional phenotypes of two primary HIV-1 strains resulting from homologous point mutations in the LLP domains of the envelope gp41 intracytoplasmic domain

We previously reported that selected mutations of highly conserved arginine residues within the LLP regions of HIV-1(ME46) gp41 had diverse effects on Env function. In the current study, we sought to test if the observed LLP mutant phenotypes would be similar in HIV-1(89.6). The results of the current studies revealed that the LLP-1 mutations conferred reduced Env incorporation, infectivity, and replication phenotypes in both viruses, while homologous LLP-2 mutations had differential phenotypical effects between the two strains. In particular, several of the 89.6 LLP-2 mutant viruses were replication defective in CEMX174 cells despite having increased levels of Env incorporation, and with both strains, there were differential effects on infectivity. This comparison of homologous point mutations in two different strains of HIV supports the role of LLPs as determinants of Env function, but reveals for the first time the influence of virus strain on LLP mutant phenotypes.

An acidic cluster of the cytoplasmic tail of the RD114 virus glycoprotein controls assembly of retroviral envelopes

Retroviral core proteins, Gag and envelope (Env) glycoproteins are expressed from distinct cellular areas and therefore need to encounter to assemble infectious particles. The intrinsic cell localisation properties of either viral component or their capacity to mutually interact determines the assembly of infectious particles. Here, we address how Env determinants and cellular sorting proteins allow the Env derived from gamma retroviruses, murine leukemia virus (MLV) and RD114, to travel to or from late endosomes (LE), which may represent the Env assembly site of retroviruses in some cells. The individual expression of MLV Env resulted in its accumulation in LE in contrast to RD114 Env that required the presence of gamma retroviral Gag proteins. To discriminate between intrinsic intracellular Env localisation and gamma retroviral Gag/Env interactions in influencing Env viral incorporation, we studied Env assembly on heterologous lentiviral particles on which they are passively recruited. We found that an acidic cluster present at the C-terminus of the RD114 Env cytoplasmic tail determines its sub-cellular localisation and retrograde transport. Mutation of this motif induced late endosomal concentration of the RD114 Env, correlating with increased viral incorporation and infectivity. Reciprocally, the reinforcement of a poorly functional acidic motif in the MLV Env resulted in a marked decrease of its late endosomal localisation, leading to weakly infectious lentiviral particles with low Env densities. Finally, through upregulation versus downregulation of its cellular expression, we show that phosphofurin acidic-cluster-sorting protein 1 (PACS-1) controls the function of the RD114 Env acidic cluster, assigning to this cellular effector a crucial role in modulation of Env assembly of some retroviruses.

The clathrin adaptor complex AP-1 binds HIV-1 and MLV Gag and facilitates their budding

Retroviral assembly is driven by Gag, and nascent viral particles escape cells by recruiting the machinery that forms intralumenal vesicles of multivesicular bodies. In this study, we show that the clathrin adaptor complex AP-1 is involved in retroviral release. The absence of AP-1mu obtained by genetic knock-out or by RNA interference reduces budding of murine leukemia virus (MLV) and HIV-1, leading to a delay of viral propagation in cell culture. In contrast, overexpression of AP-1mu enhances release of HIV-1 Gag. We show that the AP-1 complex facilitates retroviral budding through a direct interaction between the matrix and AP-1mu. Less MLV Gag is found associated with late endosomes in cells lacking AP-1, and our results suggest that AP-1 and AP-3 could function on the same pathway that leads to Gag release. In addition, we find that AP-1 interacts with Tsg101 and Nedd4.1, two cellular proteins known to be involved in HIV-1 and MLV budding. We propose that AP-1 promotes Gag release by transporting it to intracellular sites of active budding, and/or by facilitating its interactions with other cellular partners.

Human immunodeficiency virus type 1 assembly, budding, and cell-cell spread in T cells take place in tetraspanin-enriched plasma membrane domains

Human immunodeficiency virus type-1 (HIV-1) egress from infected CD4+ T cells is thought to be via assembly and budding at the plasma membrane and may involve components of the T-cell secretory apparatus, including tetraspanins. However, many studies on HIV-1 assembly have examined the trafficking of viral proteins in isolation, and most have used immortalized epithelial, fibroblastic, or hematopoietic cell lines that may not necessarily reflect natural infection of susceptible T cells. Here we have used immunofluorescence and cryoimmunoelectron microscopy (CEM) to examine protein transport during HIV-1 assembly in productively infected Jurkat CD4+ T cells and primary CD4+ T cells. The HIV-1 envelope glycoprotein (Env) and the core protein (Gag) colocalize strongly with CD63 and CD81 and less strongly with CD9, whereas no colocalization was seen between Env or Gag and the late endosome/lysosomal marker Lamp2. CEM revealed incorporation of CD63 and CD81 but not Lamp2 into virions budding at the plasma membrane, and this was supported by immunoprecipitation studies, confirming that HIV-1 egress in T cells is trafficked via tetraspanin-enriched membrane domains (TEMs) that are distinct from lysosomal compartments. CD63, CD81, and, to a lesser extent, CD9 were recruited to the virological synapse (VS), and antibodies against these tetraspanins reduced VS formation. We propose that HIV-1 promotes virus assembly and cell-cell transfer in T cells by targeting plasma membrane TEMs.

Requirement for an intact T-cell actin and tubulin cytoskeleton for efficient assembly and spread of human immunodeficiency virus type 1

Human immunodeficiency virus type 1 (HIV-1) infection of CD4(+) T cells leads to the production of new virions that assemble at the plasma membrane. Gag and Env accumulate in the context of lipid rafts at the inner and outer leaflets of the plasma membrane, respectively, forming polarized domains from which HIV-1 buds. HIV-1 budding can result in either release of cell-free virions or direct cell-cell spread via a virological synapse (VS). The recruitment of Gag and Env to these plasma membrane caps in T cells is poorly understood but may require elements of the T-cell secretory apparatus coordinated by the cytoskeleton. Using fixed-cell immunofluorescence labeling and confocal microscopy, we observed a high percentage of HIV-1-infected T cells with polarized Env and Gag in capped, lipid raft-like assembly domains. Treatment of infected T cells with inhibitors of actin or tubulin remodeling disrupted Gag and Env compartmentalization within the polarized raft-like domains. Depolymerization of the actin cytoskeleton reduced Gag release and viral infectivity, and actin and tubulin inhibitors reduced Env incorporation into virions. Live- and fixed-cell confocal imaging and assay of de novo DNA synthesis by real-time PCR allowed quantification of HIV-1 cell-cell transfer. Inhibition of actin and tubulin remodeling in infected cells interfered with cell-cell spread across a VS and reduced new viral DNA synthesis. Based on these data, we propose that HIV-1 requires both actin and tubulin components of the T-cell cytoskeleton to direct its assembly and budding and to elaborate a functional VS.

Kinetic studies of HIV-1 and HIV-2 envelope glycoprotein-mediated fusion

Background

HIV envelope glycoprotein (Env)-mediated fusion is driven by the concerted coalescence of the HIV gp41 N-helical and C-helical regions, which results in the formation of 6 helix bundles. Kinetics of HIV Env-mediated fusion is an important determinant of sensitivity to entry inhibitors and antibodies. However, the parameters that govern the HIV Env fusion cascade have yet to be fully elucidated. We address this issue by comparing the kinetics HIV-1_IIIB Env with those mediated by HIV-2 from two strains with different affinities for CD4 and CXCR4.

Results

HIV-1 and HIV-2 Env-mediated cell fusion occurred with half times of about 60 and 30 min, respectively. Binding experiments of soluble HIV gp120 proteins to CD4 and co-receptor did not correlate with the differences in kinetics of fusion mediated by the three different HIV Envs. However, escape from inhibition by reagents that block gp120-CD4 binding, CD4-induced CXCR4 binding and 6-helix bundle formation, respectively, indicated large difference between HIV-1 and HIV-2 envelope glycoproteins in their CD4-induced rates of engagement with CXCR4.

Conclusion

The HIV-2 Env proteins studied here exhibited a significantly reduced window of time between the engagement of gp120 with CD4 and exposure of the CXCR4 binding site on gp120 as compared with HIV-1_IIIB Env. The efficiency with which HIV-2 Env undergoes this CD4-induced conformational change is the major cause of the relatively rapid rate of HIV-2 Env mediated-fusion.

A conserved dileucine motif mediates clathrin and AP-2-dependent endocytosis of the HIV-1 envelope protein

During the assembly of enveloped viruses viral and cellular components essential for infectious particles must colocalize at specific membrane locations. For the human and simian immunodeficiency viruses (HIV and SIV), sorting of the viral envelope proteins (Env) to assembly sites is directed by trafficking signals located in the cytoplasmic domain of the transmembrane protein gp41 (TM). A membrane proximal conserved GYxxØ motif mediates endocytosis through interaction with the clathrin adaptor AP-2. However, experiments with SIV(mac239) Env indicate the presence of additional signals. Here we show that a conserved C-terminal dileucine in HIV(HxB2) also mediates endocytosis. Biochemical and morphological assays demonstrate that the C-terminal dileucine motif mediates internalization as efficiently as the GYxxØ motif and that both must be removed to prevent Env internalization. RNAi experiments show that depletion of the clathrin adaptor AP-2 leads to increased plasma membrane expression of HIV Env and that this adaptor is required for efficient internalization mediated by both signals. The redundancy of conserved endocytosis signals and the role of the SIV(mac239) Env GYxxØ motif in SIV pathogenesis, suggest that these motifs have functions in addition to endocytosis, possibly related to Env delivery to the site of viral assembly and/or incorporation into budding virions.

Rab-GTPases: dual roles in vesicular transport and viral replication

Productive viral infection is dependent upon post-entry migration of viruses/viral components to sites within a host cell that complement viral deficiencies. Delivery of virions or component proteins to appropriate sites within an infected cell is critical for completing successive stages in viral replication, including release into the cytoplasm, uncoating, genome replication, viral gene expression, assembly and budding. Vesicular transport is essential for steady-state cellular trafficking of membrane-associated proteins. Rab GTPases and their associated effectors are key regulators of vesicular transport pathways. In recent years, Rab proteins have been implicated in the endocytic or exocytic sorting of component viral proteins or intact viruses, most of which are known to be membrane-encapsulated and enveloped. This review will discuss the current understanding of how Rab GTPases and their effectors may regulate individual vesicular transport steps, and detail emerging discoveries examining how specific Rabs and effectors support viral replication.

Impact of natural polymorphism within the gp41 cytoplasmic tail of human immunodeficiency virus type 1 on the intracellular distribution of envelope glycoproteins and viral assembly

The motifs involved in the various functions of the human immunodeficiency virus type 1 (HIV-1) gp41 cytoplasmic tail (CT), particularly those related to the intracellular trafficking and assembly of envelope glycoproteins (Env) onto core particles, have generally been assessed with a restricted panel of T-cell laboratory-adapted virus strains. Here, we investigated gp41 CT sequences derived from individuals infected with HIV-1 viruses of various subtypes. We identified four patients harboring HIV variants with a natural polymorphism in the membrane-proximal tyrosine-based signal Y(712)SPL or the Y(802)W(803) diaromatic motif, which are two major determinants of Env intracellular trafficking. Confocal microscopy showed that the intracellular distribution of Env with a mutation in the tyrosine or diaromatic motif differed from that of Env with no mutation in these motifs. Surprisingly, the gp41 CTs of the primary viruses also had differential effects on the intracellular distribution of Env, independently of mutations in the tyrosine or diaromatic motifs, suggesting the involvement of additional determinants. Furthermore, analyses of virus replication kinetics indicated that the effects of mutations in the tyrosine or diaromatic motifs on viral replication depended on the gp41 CT context. These effects were at least partly due to differences in the efficiency of Env incorporation into virions. Thus, polymorphisms in primary HIV-1 gp41 CTs at the quasispecies or subtype level can influence the intracellular distribution of Env, its incorporation into virions, and viral replication capacity.

Luman, a new partner of HIV-1 TMgp41, interferes with Tat-mediated transcription of the HIV-1 LTR

In our search for new partners of the HIV-1 envelope glycoprotein (Env), we found that the cytoplasmic domain of the TMgp41 (TMgp41 CD) subunit of HIV-1 Env interacted with Luman, a transcription factor of the CREB/ATF family. Luman is anchored in the endoplasmic reticulum membrane and subjected to activation by regulated intramembrane proteolysis (RIP). The RIP process permits the release of the activated amino-terminal fragment of Luman into the cytoplasm, and its import into the nucleus. Here, we demonstrate that interaction between the TMgp41 CD and Luman requires a region encompassing the b-Zip and TM domains of Luman and decreases the stability of this factor. Moreover, we found that overexpression of a constitutively active form of Luman in cells transfected with HXB2R HIV-1 provirus decreased the intracellular expression of Gag and Env and led to a decrease in virion release. This negative effect of activated Luman on HIV-1 production was correlated to the inhibition of Tat transactivation of the HIV-1 LTR, which might be related to an interaction of activated Luman with Tat. Altogether, these results show that Luman acts as a partner of two major HIV-1 proteins: the TMgp41 Env subunit and Tat. The interaction between the TMgp41 subunit of Env and Luman affects the stability of the full-length Luman protein, the precursor of the activated, nuclear form of Luman, which acts negatively on Tat-mediated HIV-1 transactivation.

The conserved dileucine- and tyrosine-based motifs in MLV and MPMV envelope glycoproteins are both important to regulate a common Env intracellular trafficking

BACKGROUND

Retrovirus particles emerge from the assembly of two structural protein components, Gag that is translated as a soluble protein in the cytoplasm of the host cells, and Env, a type I transmembrane protein. Because both components are translated in different intracellular compartments, elucidating the mechanisms of retrovirus assembly thus requires the study of their intracellular trafficking.

RESULTS

We used a CD25 (Tac) chimera-based approach to study the trafficking of Moloney murine leukemia virus and Mason-Pfizer monkey virus Env proteins. We found that the cytoplasmic tails (CTs) of both Env conserved two major signals that control a complex intracellular trafficking. A dileucine-based motif controls the sorting of the chimeras from the trans-Golgi network (TGN) toward endosomal compartments. Env proteins then follow a retrograde transport to the TGN due to the action of a tyrosine-based motif. Mutation of either motif induces the mis-localization of the chimeric proteins and both motifs are found to mediate interactions of the viral CTs with clathrin adaptors.

CONCLUSION

This data reveals the unexpected complexity of the intracellular trafficking of retrovirus Env proteins that cycle between the TGN and endosomes. Given that Gag proteins hijack endosomal host proteins, our work suggests that the endosomal pathway may be used by retroviruses to ensure proper encountering of viral structural Gag and Env proteins in cells, an essential step of virus assembly.

The effect of the membrane-proximal tyrosine-based sorting signal of HIV-1 gp41 on viral infectivity depends on sequences within gp120

The cytoplasmic domain of the HIV-1 Env glycoprotein (gp41) contains sequences that affect the trafficking of Env within the host cell. We previously showed that the membrane-proximal tyrosine-based adaptor protein (AP)-binding signal of gp41 (Y712XXL) is required for optimal viral infectivity and entry into target cells. Because these effects were not attributable to an effect on the incorporation of Env into virions, we hypothesized that they involved targeting of viral assembly to specific endosomal membranes that conferred greater fusogenicity. To further elaborate this hypothesis, we mutated the C-terminal leucine-based AP-binding signal of gp41 (LL855/856). In contrast to Env Y712, the leucine signal was dispensable for viral infectivity in both single cycle assays and during spreading infections within cultures of peripheral blood mononuclear cells (PBMCs). To test the hypothesis that these AP-binding motifs target Env to endosomes during viral morphogenesis, we compared the subcellular localization of wild-type Env to mutants of the Y712 and LL855/856 signals. The results failed to support the hypothesis that these signals target viral assembly to specific endosomal membranes. Strikingly, in the context of a C2-V3 region that confers macrophage-tropism, mutation of Y712 no longer markedly affected viral infectivity in either single cycle assays or during spreading infection within PBMCs, and it did not impair viral entry. These data indicate that the importance of the tyrosine-based sorting signal in gp41 for optimal viral infectivity depends on sequences in gp120. This observation is consistent with the hypothesis that the Y712 residue is part of the ectodomain of gp41 in virion-associated Env. We speculate that as part of the ectodomain, Y712 could affect specifically the conformation of the more positively charged CXCR4-tropic V3 loop in a manner that augments viral fusogenicity and infectivity.

Characterization of HIV-1 envelope gp41 genetic diversity and functional domains following perinatal transmission

Background

HIV-1 envelope gp41 is a transmembrane protein that promotes fusion of the virus with the plasma membrane of the host cells required for virus entry. In addition, gp41 is an important target for the immune response and development of antiviral and vaccine strategies, especially when targeting the highly variable envelope gp120 has not met with resounding success. Mutations in gp41 may affect HIV-1 entry, replication, pathogenesis, and transmission. We, therefore, characterized the molecular properties of gp41, including genetic diversity, functional motifs, and evolutionary dynamics from five mother-infant pairs following perinatal transmission.

Results

The gp41 open reading frame (ORF) was maintained with a frequency of 84.17% in five mother-infant pairs' sequences following perinatal transmission. There was a low degree of viral heterogeneity and estimates of genetic diversity in gp41 sequences. Both mother and infant gp41 sequences were under positive selection pressure, as determined by ratios of non-synonymous to synonymous substitutions. Phylogenetic analysis of 157 mother-infant gp41 sequences revealed distinct clusters for each mother-infant pair, suggesting that the epidemiologically linked mother-infant pairs were evolutionarily closer to each other as compared with epidemiologically unlinked sequences. The functional domains of gp41, including fusion peptide, heptad repeats, glycosylation sites and lentiviral lytic peptides were mostly conserved in gp41 sequences analyzed in this study. The CTL recognition epitopes and motifs recognized by fusion inhibitors were also conserved in the five mother-infant pairs.

Conclusion

The maintenance of an intact envelope gp41 ORF with conserved functional domains and a low degree of genetic variability as well as positive selection pressure for adaptive evolution following perinatal transmission is consistent with an indispensable role of envelope gp41 in HIV-1 replication and pathogenesis.

Interactions between virus proteins and host cell membranes during the viral life cycle

The structure and function of cells are critically dependent on membranes, which not only separate the interior of the cell from its environment but also define the internal compartments. It is therefore not surprising that the major steps of the life cycle of viruses of animals and plants also depend on cellular membranes. Indeed, interactions of viral proteins with host cell membranes are important for viruses to enter into host cells, replicate their genome, and produce progeny particles. To replicate its genome, a virus first needs to cross the plasma membrane. Some viruses can also modify intracellular membranes of host cells to create a compartment in which genome replication will take place. Finally, some viruses acquire an envelope, which is derived either from the plasma membrane or an internal membrane of the host cell. This paper reviews recent findings on the interactions of viral proteins with host cell membranes during the viral life cycle.

Downregulation of human immunodeficiency virus type 1 Gag expression by a gp41 cytoplasmic domain fusion protein

The cytoplasmic domain of human immunodeficiency virus type 1 (HIV-1) envelope (Env) transmembrane protein gp41 interacts with the viral matrix MA protein, which facilitates incorporation of the trimeric Env complex into the virus. It is thus feasible to design an anti-HIV strategy targeting this interaction. We herein describe that Gag expression can be downregulated by a cytoplasmic domain fusion protein of the Env transmembrane protein, beta-galactosidase (beta-gal)/706-856, which contains the cytoplasmic tail of gp41 fused at the C terminus of Escherichia coli beta-gal. This mediator depleted intracellular Gag molecules in a dose-dependent manner. Sucrose gradient ultracentrifugation and confocal microscopy revealed that Gag and beta-gal/706-856 had stable interactions and formed aggregated complexes in perinuclear, intracellular sites. Pulse-chase and cycloheximide chase analyses demonstrated that this mediator enhanced unmyristylated Gag degradation. The results demonstrate a novel mode of HIV-1 Gag downregulation by directing Gag to an intracellular site via the interaction of Gag with a gp41 cytoplasmic domain fusion protein.

HIV interaction with endosomes in macrophages and dendritic cells

The human immunodeficiency virus type 1 (HIV-1) is an enveloped retrovirus that undergoes assembly at specific sites in infected cells. In macrophages, at least, this assembly occurs primarily on a subset of endocytic organelles that contain some of the markers found in late endosomes or multivesicular bodies (MVBs), in particular CD63. The budding of virus particles into endosomes has many features in common with the formation of exosomes and some limited biochemical comparison of HIV-1 particles produced from macrophages with exosomes suggests that the two have similar cellular origins. Here we show that macrophages infected with HIV contain large intracellular pools of infectious virus that can be released by homogenisation of intact cells. Immunoprecipitation experiments indicate this virus has a similar complement of cellular membrane proteins to viruses that can be recovered from the extracellular medium, further suggesting that viruses released from macrophages initially bud into endosomal organelles and are then released by fusion of these organelles with the plasma membrane.

Intracellular Versus Cell Surface Assembly of Retroviral Pseudotypes Is Determined by the Cellular Localization of the Viral Glycoprotein, Its Capacity to Interact with Gag, and the Expression of the Nef Protein

Retroviral Gag and Env glycoproteins (GPs) are expressed from distinct cellular areas and need to encounter to interact and assemble infectious particles. Retroviral particles may also incorporate GPs derived from other enveloped viruses via active or passive mechanisms, a process known as "pseudotyping." To further understand the mechanisms of pseudotyping, we have investigated the capacity of murine leukemia virus (MLV) or lentivirus core particles to recruit GPs derived from different virus families: the G protein of vesicular stomatitis virus (VSV-G), the hemagglutinin from an influenza virus, the E1E2 glycoproteins of hepatitis C virus (HCV-E1E2), and the retroviral Env glycoproteins of MLV and RD114 cat endogenous virus. The parameters that influenced the incorporation of viral GPs onto retroviral core particles were (i) the intrinsic cell localization properties of both viral GP and retroviral core proteins, (ii) the ability of the viral GP to interact with the retroviral core, and (iii) the expression of the lentiviral Nef protein. Whereas the hemagglutinin and VSV-G glycoproteins were recruited by MLV and lentivirus core proteins at the cell surface, the HCV and MLV GPs were most likely recruited in late endosomes. In addition, whereas these glycoproteins could be passively incorporated on either retrovirus type, the MLV GP was also actively recruited by MLV core proteins, which, through interactions with the cytoplasmic tail of the latter GP, induced its localization to late endosomal vesicles. Finally, the expression of Nef proteins specifically enhanced the incorporation of the retroviral GPs by increasing their localization in late endosomes.

Trafficking of viral membrane proteins

Many viruses express membrane proteins. For enveloped viruses in particular, membrane proteins are frequently structural components of the virus that mediate the essential tasks of receptor recognition and membrane fusion. The functional activities of these proteins require that they are sorted correctly in infected cells. These sorting events often depend on the ability of the virus to mimic cellular protein trafficking signals and to interact with the cellular trafficking machinery. Importantly, loss or modification of these signals can influence virus infectivity and pathogenesis.

Regulation of human immunodeficiency virus type 1 envelope glycoprotein fusion by a membrane-interactive domain in the gp41 cytoplasmic tail

Truncation of the human immunodeficiency virus (HIV) or simian immunodeficiency virus (SIV) gp41 cytoplasmic tail (CT) can modulate the fusogenicity of the envelope glycoprotein (Env) on infected cells and virions. However, the CT domains involved and the underlying mechanism responsible for this "inside-out" regulation of Env function are unknown. HIV and SIV CTs are remarkably long and contain amphipathic alpha-helical domains (LLP1, LLP2, and LLP3) that likely interact with cellular membranes. Using a cell-cell fusion assay and a panel of HIV Envs with stop codons at various positions in the CT, we show that truncations of gp41 proximal to the most N-terminal alpha helix, LLP2, increase fusion efficiency and expose CD4-induced epitopes in the Env ectodomain. These effects were not seen with a truncation distal to this domain and before LLP1. Using a dye transfer assay to quantitate fusion kinetics, we found that these truncations produced a two- to fourfold increase in the rate of fusion. These results were observed for X4-, R5-, and dual-tropic Envs on CXCR4- and CCR5-expressing target cells and could not be explained by differences in Env surface expression. These findings suggest that distal to the membrane-spanning domain, an interaction of the gp41 LLP2 domain with the cell membrane restricts Env fusogenicity during Env processing. As with murine leukemia viruses, where cleavage of a membrane-interactive R peptide at the C terminus is required for Env to become fusogenic, this restriction of Env function may serve to protect virus-producing cells from the membrane-disruptive effects of the Env ectodomain.

Virion envelope content, infectivity, and neutralization sensitivity of simian immunodeficiency virus

A truncating E767stop mutation was introduced into the envelope glycoprotein of simian immunodeficiency virus (SIV) strain SIV239-M5 (moderately sensitive to antibody-mediated neutralization and lacking five sites for N-linked carbohydrate attachment) and strain SIV316 (very sensitive to neutralization, with eight amino acid changes from the neutralization-resistant parental molecular clone, SIV239). The truncating mutation increased Env content in virions, increased infectivity, and decreased sensitivity to antibody-mediated neutralization in both strains. However, the magnitude of the effect on infectivity and neutralization sensitivity differed considerably between the two strains. In the context of strain SIV239-M5, truncation increased Env content in virions approximately 10-fold and infectivity in a reporter cell assay 24-fold. The truncated SIV239-M5 was only slightly more resistant to neutralization by polyclonal monkey sera and by monoclonal antibodies than SIV239-M5 with a full-length envelope glycoprotein. In the context of strain SIV316, truncation increased infectivity a dramatic 480-fold, while envelope content in virions was increased only about 14-fold. This dramatic increase in infectivity cannot be simply explained by the increase in envelope content and is likely due to an increase in inherent infectivity, i.e., infectivity per spike, that results from truncation. The truncated SIV316 was extremely resistant to antibody-mediated neutralization. In fact, it was not neutralized by any of the antibodies tested. When increasing amounts of SIV316 envelope glycoprotein (full length) were provided in trans to SIV316, infectivity was increased and sensitivity to neutralization was decreased, but to nowhere near the degree that was obtained when truncated SIV316 envelope glycoprotein was used. Truncated forms of SIV239 and SIV239-M5 required higher levels of soluble CD4 for inhibition of infection than their nontruncated forms; truncated SIV316 did not. Our results suggest that envelope content in SIV virions, infectivity, and resistance to antibody-mediated neutralization can be increased not only by truncation of the cytoplasmic domain but also by provision of excess envelope in trans. The striking increase in infectivity that results from truncation in the context of SIV316 appears to be due principally to an increase in inherent infectivity per spike.

Rab9 GTPase is required for replication of human immunodeficiency virus type 1, filoviruses, and measles virus

Rab proteins and their effectors facilitate vesicular transport by tethering donor vesicles to their respective target membranes. By using gene trap insertional mutagenesis, we identified Rab9, which mediates late-endosome-to-trans-Golgi-network trafficking, among several candidate host genes whose disruption allowed the survival of Marburg virus-infected cells, suggesting that Rab9 is utilized in Marburg replication. Although Rab9 has not been implicated in human immunodeficiency virus (HIV) replication, previous reports suggested that the late endosome is an initiation site for HIV assembly and that TIP47-dependent trafficking out of the late endosome to the trans-Golgi network facilitates the sorting of HIV Env into virions budding at the plasma membrane. We examined the role of Rab9 in the life cycles of HIV and several unrelated viruses, using small interfering RNA (siRNA) to silence Rab9 expression before viral infection. Silencing Rab9 expression dramatically inhibited HIV replication, as did silencing the host genes encoding TIP47, p40, and PIKfyve, which also facilitate late-endosome-to-trans-Golgi vesicular transport. In addition, silencing studies revealed that HIV replication was dependent on the expression of Rab11A, which mediates trans-Golgi-to-plasma-membrane transport, and that increased HIV Gag was sequestered in a CD63+ endocytic compartment in a cell line stably expressing Rab9 siRNA. Replication of the enveloped Ebola, Marburg, and measles viruses was inhibited with Rab9 siRNA, although the non-enveloped reovirus was insensitive to Rab9 silencing. These results suggest that Rab9 is an important cellular target for inhibiting diverse viruses and help to define a late-endosome-to-plasma-membrane vesicular transport pathway important in viral assembly.

The C-terminal tail of the gp41 transmembrane envelope glycoprotein of HIV-1 clades A, B, C, and D may exist in two conformations: an analysis of sequence, structure, and function

In addition to the major ectodomain, the gp41 transmembrane glycoprotein of HIV-1 is now known to have a minor ectodomain that is part of the long C-terminal tail. Both ectodomains are highly antigenic, carry neutralizing and non-neutralizing epitopes, and are involved in virus-mediated fusion activity. However, data have so far been biologically based, and derived solely from T cell line-adapted (TCLA), B clade viruses. Here we have carried out sequence and theoretically based structural analyses of 357 gp41 C-terminal sequences of mainly primary isolates of HIV-1 clades A, B, C, and D. Data show that all these viruses have the potential to form a tail loop structure (the minor ectodomain) supported by three, β-sheet, membrane-spanning domains (MSDs). This means that the first (N-terminal) tyrosine-based sorting signal of the gp41 tail is situated outside the cell membrane and is non-functional, and that gp41 that reaches the cell surface may be recycled back into the cytoplasm through the activity of the second tyrosine-sorting signal. However, we suggest that only a minority of cell-associated gp41 molecules – those destined for incorporation into virions – has 3 MSDs and the minor ectodomain. Most intracellular gp41 has the conventional single MSD, no minor ectodomain, a functional first tyrosine-based sorting signal, and in line with current thinking is degraded intracellularly. The gp41 structural diversity suggested here can be viewed as an evolutionary strategy to minimize HIV-1 envelope glycoprotein expression on the cell surface, and hence possible cytotoxicity and immune attack on the infected cell.

Interaction of HIV-1 Gag with the clathrin-associated adaptor AP-2

The envelope glycoprotein (Env) of HIV-1 interacts with the clathrin-associated adaptor complex AP-2 during the late phase of the viral replication cycle. Upon its synthesis, Env, therefore, is retrieved from the cellular surface unless internalization is inhibited by viral Gag. Here we demonstrate that not only Env, but also HIV-1 Gag, specifically binds to AP-2. Gag-AP-2 association was found to depend on tyrosine residue 132 and valine residue 135 at the matrix-capsid junction in the Gag polyprotein. Results of a morphological analysis of viral egress from cells expressing dominant-negative AP-2 suggest an involvement of AP-2 in confining HIV-1 exit to distinct microdomains. Further, particle release from AP-2-mutant cells was enhanced compared to release from wild-type cells but the infectivity of virus released from these cells was moderately reduced. Together these data attribute a role to the AP-2 complex in the regulation of HIV-1 assembly/release.

Evaluating the immunogenicity of a disulfide-stabilized, cleaved, trimeric form of the envelope glycoprotein complex of human immunodeficiency virus type 1

The human immunodeficiency virus type 1 (HIV-1) envelope glycoprotein (Env) complex comprises three gp120 exterior glycoproteins each noncovalently linked to a gp41 transmembrane glycoprotein. Monomeric gp120 proteins can elicit antibodies capable of neutralizing atypically sensitive test viruses in vitro, but these antibodies are ineffective against representative primary isolates and the gp120 vaccines failed to provide protection against HIV-1 transmission in vivo. Alternative approaches to raising neutralizing antibodies are therefore being pursued. Here we report on the antibody responses generated in rabbits against a soluble, cleaved, trimeric form of HIV-1(JR-FL) Env. In this construct, the gp120 and gp41 moieties are covalently linked by an intermolecular disulfide bond (SOS gp140), and an I559P substitution has been added to stabilize gp41-gp41 interactions (SOSIP gp140). We investigated the value of DNA priming and compared the use of membrane-bound and soluble priming antigens and of repeat boosting with soluble and particulate protein antigen. Compared to monomeric gp120, SOSIP gp140 trimers elicited approximately threefold lower titers of anti-gp120 antibodies. Priming with DNA encoding a membrane-bound form of the SOS gp140 protein, followed by several immunizations with soluble SOSIP gp140 trimers, resulted in antibodies capable of neutralizing sensitive strains at high titers. A subset of these sera also neutralized, at lower titers, HIV-1(JR-FL) and some other primary isolates in pseudovirus and/or whole-virus assays. Neutralization of these viruses was immunoglobulin mediated and was predominantly caused by antibodies to gp120 epitopes, but not the V3 region.

PRA1 co-localizes with envelope but does not influence primate lentivirus production, infectivity or envelope incorporation

The results of yeast and mammalian two-hybrid assays previously indicated complex formation between prenylated Rab acceptor 1 (PRA1) and the cytoplasmic domain of gp41 (gp41CD) for both the human and simian immunodeficiency viruses [Evans, D. T., Tilman, K. C. & Desrosiers, R. C. (2002). J Virol 76, 327-337]. The assembly and release of infectious virus particles was studied under conditions of PRA1 overexpression in a transient transfection assay or suppression by RNA interference. Although a clear pattern of co-localization of PRA1 and gp41 was observed, no changes in virion release, infectivity or envelope content were observed as a result of either PRA1 suppression or overexpression. These data show that PRA1 co-localizes with gp41 inside cells and they are consistent with a direct or indirect interaction between these proteins. However, variation in the levels of PRA1 expression did not influence virion production, infectivity or envelope incorporation under the conditions of these assays.