Human immunodeficiency virus type 1 Gag engages the Bro1 domain of ALIX/AIP1 through the nucleocapsid. J Virol. 2008;82:1389-1398. [PMID: 18032513 DOI: 10.1128/jvi.01912-07]

Structure of the Bro1 domain protein BROX and functional analyses of the ALIX Bro1 domain in HIV-1 budding

Background

Bro1 domains are elongated, banana-shaped domains that were first identified in the yeast ESCRT pathway protein, Bro1p. Humans express three Bro1 domain-containing proteins: ALIX, BROX, and HD-PTP, which function in association with the ESCRT pathway to help mediate intraluminal vesicle formation at multivesicular bodies, the abscission stage of cytokinesis, and/or enveloped virus budding. Human Bro1 domains share the ability to bind the CHMP4 subset of ESCRT-III proteins, associate with the HIV-1 NC^Gag protein, and stimulate the budding of viral Gag proteins. The curved Bro1 domain structure has also been proposed to mediate membrane bending. To date, crystal structures have only been available for the related Bro1 domains from the Bro1p and ALIX proteins, and structures of additional family members should therefore aid in the identification of key structural and functional elements.

Methodology/Principal Findings

We report the crystal structure of the human BROX protein, which comprises a single Bro1 domain. The Bro1 domains from BROX, Bro1p and ALIX adopt similar overall structures and share two common exposed hydrophobic surfaces. Surface 1 is located on the concave face and forms the CHMP4 binding site, whereas Surface 2 is located at the narrow end of the domain. The structures differ in that only ALIX has an extended loop that projects away from the convex face to expose the hydrophobic Phe105 side chain at its tip. Functional studies demonstrated that mutations in Surface 1, Surface 2, or Phe105 all impair the ability of ALIX to stimulate HIV-1 budding.

Conclusions/Significance

Our studies reveal similarities in the overall folds and hydrophobic protein interaction sites of different Bro1 domains, and show that a unique extended loop contributes to the ability of ALIX to function in HIV-1 budding.

Role of HIV-1 RNA and protein determinants for the selective packaging of spliced and unspliced viral RNA and host U6 and 7SL RNA in virus particles

HIV-1 particles contain RNA species other than the unspliced viral RNA genome. For instance, viral spliced RNAs and host 7SL and U6 RNAs are natural components that are non-randomly incorporated. To understand the mechanism of packaging selectivity, we analyzed the content of a large panel of HIV-1 variants mutated either in the 5'UTR structures of the viral RNA or in the Gag-nucleocapsid protein (GagNC). In parallel, we determined whether the selection of host 7SL and U6 RNAs is dependent or not on viral RNA and/or GagNC. Our results reveal that the polyA hairpin in the 5'UTR is a major packaging determinant for both spliced and unspliced viral RNAs. In contrast, 5'UTR RNA structures have little influence on the U6 and 7SL RNAs, indicating that packaging of these host RNAs is independent of viral RNA packaging. Experiments with GagNC mutants indicated that the two zinc-fingers and N-terminal basic residues restrict the incorporation of the spliced RNAs, while favoring unspliced RNA packaging. GagNC through the zinc-finger motifs also restricts the packaging of 7SL and U6 RNAs. Thus, GagNC is a major contributor to the packaging selectivity. Altogether our results provide new molecular insight on how HIV selects distinct RNA species for incorporation into particles.

The Phe105 loop of Alix Bro1 domain plays a key role in HIV-1 release

Alix and cellular paralogs HD-PTP and Brox contain N-terminal Bro1 domains that bind ESCRT-III CHMP4. In contrast to HD-PTP and Brox, expression of the Bro1 domain of Alix alleviates HIV-1 release defects that result from interrupted access to ESCRT. In an attempt to elucidate this functional discrepancy, we solved the crystal structures of the Bro1 domains of HD-PTP and Brox. They revealed typical "boomerang" folds they share with the Bro1 Alix domain. However, they each contain unique structural features that may be relevant to their specific function(s). In particular, phenylalanine residue in position 105 (Phe105) of Alix belongs to a long loop that is unique to its Bro1 domain. Concurrently, mutation of Phe105 and surrounding residues at the tip of the loop compromise the function of Alix in HIV-1 budding without affecting its interactions with Gag or CHMP4. These studies identify a new functional determinant in the Bro1 domain of Alix.

The role of cellular factors in promoting HIV budding

Human immunodeficiency virus type 1 (HIV-1) becomes enveloped while budding through the plasma membrane, and the release of nascent virions requires a membrane fission event that separates the viral envelope from the cell surface. To facilitate this crucial step in its life cycle, HIV-1 exploits a complex cellular membrane remodeling and fission machinery known as the endosomal sorting complex required for transport (ESCRT) pathway. HIV-1 Gag directly interacts with early-acting components of this pathway, which ultimately triggers the assembly of the ESCRT-III membrane fission complex at viral budding sites. Surprisingly, HIV-1 requires only a subset of ESCRT-III components, indicating that the membrane fission reaction that occurs during HIV-1 budding differs in crucial aspects from topologically related cellular abscission events.

Host factors mediating HIV-1 replication

Human immunodeficiency virus type 1(HIV-1) infection is the leading cause of death worldwide in adults attributable to infectious diseases. Although the majority of infections are in sub-Saharan Africa and Southeast Asia, HIV-1 is also a major health concern in most countries throughout the globe. While current antiretroviral treatments are generally effective, particularly in combination therapy, limitations exist due to drug resistance occurring among the drug classes. Traditionally, HIV-1 drugs have targeted viral proteins, which are mutable targets. As cellular genes mutate relatively infrequently, host proteins may prove to be more durable targets than viral proteins. HIV-1 replication is dependent upon cellular proteins that perform essential roles during the viral life cycle. Maraviroc is the first FDA-approved antiretroviral drug to target a cellular factor, HIV-1 coreceptor CCR5, and serves to intercept viral-host protein-protein interactions mediating entry. Recent large-scale siRNA and shRNA screens have revealed over 1000 candidate host factors that potentially support HIV-1 replication, and have implicated new pathways in the viral life cycle. These host proteins and cellular pathways may represent important targets for future therapeutic discoveries. This review discusses critical cellular factors that facilitate the successive steps in HIV-1 replication.

Host factors involved in retroviral budding and release

The plasma membrane is the final barrier that enveloped viruses must cross during their egress from the infected cell. Here, we review recent insights into the cell biology of retroviral assembly and release; these insights have driven a new understanding of the host proteins, such as the ESCRT machinery, that are used by retroviruses to promote their final separation from the host cell. We also review antiviral host factors such as tetherin, which can directly inhibit the release of retroviral particles. These studies have illuminated the role of the lipid bilayer as the unexpected target for virus restriction by the innate immune response.

ALIX/AIP1 is required for NP incorporation into Mopeia virus Z-induced virus-like particles

During virus particle assembly, the arenavirus nucleoprotein (NP) associates with the viral genome to form nucleocapsids, which ultimately become incorporated into new virions at the cell membrane. Virion release is facilitated by the viral matrix Z protein through its interaction with the cellular endosomal sorting complex required for transport (ESCRT) machinery. However, the mechanism of nucleocapsid incorporation into virions is not well understood. Here, we demonstrate that ALIX/AIP1, an ESCRT-associated host protein, is required for the incorporation of the NP of Mopeia virus, a close relative of Lassa virus, into Z-induced virus-like particles (VLPs). Furthermore, we show that the Bro1 domain of ALIX/AIP1 interacts with the NP and Z proteins simultaneously, facilitating their interaction, and we identify residues 342 to 399 of NP as being necessary for its interaction with ALIX/AIP1. Our observations suggest a potential role for ALIX/AIP1 in linking Mopeia virus NP to Z and the budding apparatus, thereby promoting NP incorporation into virions.

Basic residues in the nucleocapsid domain of Gag are critical for late events of HIV-1 budding

The p6 region of HIV-1 Gag contains two late (L) domains, PTAP and LYPXnL, that bind the cellular proteins Tsg101 and Alix, respectively. These interactions are thought to recruit members of the host fission machinery (ESCRT) to facilitate HIV-1 release. Here we report a new role for the p6-adjacent nucleocapsid (NC) domain in HIV-1 release. The mutation of basic residues in NC caused a pronounced decrease in virus release from 293T cells, although NC mutant Gag proteins retained the ability to interact with cellular membranes and RNAs. Remarkably, electron microscopy analyses of these mutants revealed arrested budding particles at the plasma membrane, analogous to those seen following the disruption of the PTAP motif. This result indicated that the basic residues in NC are important for virus budding. When analyzed in physiologically more relevant T-cell lines (Jurkat and CEM), NC mutant viruses remained tethered to the plasma membrane or to each other by a membranous stalk, suggesting membrane fission impairment. Remarkably, NC mutant release defects were alleviated by the coexpression of a Gag protein carrying a wild-type (WT) NC domain but devoid of all L domain motifs and by providing alternative access to the ESCRT pathway, through the in trans expression of the ubiquitin ligase Nedd4.2s. Since NC mutant Gag proteins retained the interaction with Tsg101, we concluded that NC mutant budding arrests might have resulted from the inability of Gag to recruit or utilize members of the host ESCRT machinery that act downstream of Tsg101. Together, these data support a model in which NC plays a critical role in HIV-1 budding.

The ESCRT machinery: a cellular apparatus for sorting and scission

The ESCRT (endosomal sorting complex required for transport) machinery is a group of multisubunit protein complexes conserved across phyla that are involved in a range of diverse cellular processes. ESCRT proteins regulate the biogenesis of MVBs (multivesicular bodies) and the sorting of ubiquitinated cargos on to ILVs (intraluminal vesicles) within these MVBs. These proteins are also recruited to sites of retroviral particle assembly, where they provide an activity that allows release of these retroviruses. More recently, these proteins have been shown to be recruited to the intracellular bridge linking daughter cells at the end of mitosis, where they act to ensure the separation of these cells through the process of cytokinesis. Although these cellular processes are diverse, they share a requirement for a topologically unique membrane-fission step for their completion. Current models suggest that the ESCRT machinery catalyses this membrane fission.

Function of a retrotransposon nucleocapsid protein

Long terminal repeat (LTR) retrotransposons are not only the ancient predecessors of retroviruses, but they constitute significant fractions of the genomes of many eukaryotic species. Studies of their structure and function are motivated by opportunities to gain insight into common functions of retroviruses and retrotransposons, diverse mechanisms of intracellular genomic mobility, and host factors that diminish or enhance retrotransposition. This review focuses on the nucleocapsid (NC) protein of a Saccharomyces cerevisiae LTR retrotransposon, the metavirus, Ty3. Retrovirus NC promotes genomic (g)RNA dimerization and packaging, tRNA primer annealing, reverse transcription strand transfers, and host protein interactions with gRNA. Studies of Ty3 NC have revealed key roles for Ty3 NC in formation of retroelement assembly sites (retrosomes), and in chaperoning primer tRNA to both dimerize and circularize Ty3 gRNA. We speculate that Ty3 NC, together with P-body and stress-granule proteins, plays a role in transitioning Ty3 RNA from translation template to gRNA, and that interactions between the acidic spacer domain of Ty3 Gag3 and the adjacent basic NC domain control condensation of the virus-like particle.

Features, processing states, and heterologous protein interactions in the modulation of the retroviral nucleocapsid protein function

Retroviral nucleocapsid (NC) is central to viral replication. Nucleic acid chaperoning is a key function for NC through the action of its conserved basic amino acids and zinc-finger structures. NC manipulates genomic RNA from its packaging in the producer cell to reverse transcription into the infected host cell. This chaperone function, in conjunction with NC's aggregating properties, is up-modulated by successive NC processing events, from the Gag precursor to the fully mature protein, resulting in the condensation of the nucleocapsid within the capsid shell. Reverse transcription also depends on NC processing, whereas this process provokes NC dissociation from double-stranded DNA, leading to a preintegration complex (PIC), competent for host chromosomal integration. In addition NC interacts with cellular proteins, some of which are involved in viral budding, and also with several viral proteins. All of these properties are reviewed here, focusing on HIV-1 as a paradigmatic reference and highlighting the plasticity of the nucleocapsid architecture.

Identification and structural characterization of the ALIX-binding late domains of simian immunodeficiency virus SIVmac239 and SIVagmTan-1

Retroviral Gag proteins contain short late-domain motifs that recruit cellular ESCRT pathway proteins to facilitate virus budding. ALIX-binding late domains often contain the core consensus sequence YPX(n)L (where X(n) can vary in sequence and length). However, some simian immunodeficiency virus (SIV) Gag proteins lack this consensus sequence, yet still bind ALIX. We mapped divergent, ALIX-binding late domains within the p6(Gag) proteins of SIV(mac239) ((40)SREKPYKEVTEDLLHLNSLF(59)) and SIV(agmTan-1) ((24)AAGAYDPARKLLEQYAKK(41)). Crystal structures revealed that anchoring tyrosines (in lightface) and nearby hydrophobic residues (underlined) contact the ALIX V domain, revealing how lentiviruses employ a diverse family of late-domain sequences to bind ALIX and promote virus budding.

Rescue of HIV-1 release by targeting widely divergent NEDD4-type ubiquitin ligases and isolated catalytic HECT domains to Gag

Retroviruses engage the ESCRT pathway through late assembly (L) domains in Gag to promote virus release. HIV-1 uses a PTAP motif as its primary L domain, which interacts with the ESCRT-I component Tsg101. In contrast, certain other retroviruses primarily use PPxY-type L domains, which constitute ligands for NEDD4-type ubiquitin ligases. Surprisingly, although HIV-1 Gag lacks PPxY motifs, the release of HIV-1 L domain mutants is potently enhanced by ectopic NEDD4-2s, a native isoform with a naturally truncated C2 domain that appears to account for the residual titer of L domain-defective HIV-1. The reason for the unique potency of the NEDD4-2s isoform has remained unclear. We now show that the naturally truncated C2 domain of NEDD4-2s functions as an autonomous Gag-targeting module that can be functionally replaced by the unrelated Gag-binding protein cyclophilin A (CypA). The residual C2 domain of NEDD4-2s was sufficient to transfer the ability to stimulate HIV-1 budding to other NEDD4 family members, including the yeast homologue Rsp5, and even to isolated catalytic HECT domains. The isolated catalytic domain of NEDD4-2s also efficiently promoted HIV-1 budding when targeted to Gag via CypA. We conclude that the regions typically required for substrate recognition by HECT ubiquitin ligases are all dispensable to stimulate HIV-1 release, implying that the relevant target for ubiquitination is Gag itself or can be recognized by divergent isolated HECT domains. However, the mere ability to ubiquitinate Gag was not sufficient to stimulate HIV-1 budding. Rather, our results indicate that the synthesis of K63-linked ubiquitin chains is critical for ubiquitin ligase-mediated virus release.

To promote its escape from cells, HIV-1 hijacks cellular budding machinery through so-called L domains in its structural Gag protein. However, HIV-1 lacks a type of L domain that recruits NEDD4 ubiquitin ligases, a family of cellular enzymes that attach one or more copies of a small protein called ubiquitin to other proteins. Surprisingly, one NEDD4 family member, which is known as NEDD4-2s and stands out because its membrane-binding domain is uniquely truncated, can nevertheless potently stimulate HIV-1 release. Our study reveals that NEDD4-2s can do this because its altered membrane-binding domain allows it to associate with HIV-1 Gag. Remarkably, when tagged with the altered membrane-binding domain of NEDD4-2s, even a distantly related yeast protein becomes capable of stimulating the release of HIV-1. We also show that only the portion of NEDD4-2s that acts as an enzyme is required when targeted to HIV-1 Gag in an alternative manner. Taken together, our findings indicate that it is not simply the ability to attach ubiquitin to Gag, but rather the ability to form a particular type of ubiquitin chain in the immediate vicinity of Gag, that is critical to stimulate virus release.

The ESCRT complexes

The ESCRT machinery consists of the peripheral membrane protein complexes ESCRT-0, -I, -II, -III, and Vps4-Vta1, and the ALIX homodimer. The ESCRT system is required for degradation of unneeded or dangerous plasma membrane proteins; biogenesis of the lysosome and the yeast vacuole; the budding of most membrane enveloped viruses; the membrane abscission step in cytokinesis; macroautophagy; and several other processes. From their initial discovery in 2001-2002, the literature on ESCRTs has grown exponentially. This review will describe the structure and function of the six complexes noted above and summarize current knowledge of their mechanistic roles in cellular pathways and in disease.

The ESCRT-associated protein Alix recruits the ubiquitin ligase Nedd4-1 to facilitate HIV-1 release through the LYPXnL L domain motif

The p6 region of HIV-1 Gag contains two late (L) domains, PTAP and LYPX(n)L, that bind Tsg101 and Alix, respectively. Interactions with these two cellular proteins recruit members of the host's fission machinery (ESCRT) to facilitate HIV-1 release. Other retroviruses gain access to the host ESCRT components by utilizing a PPXY-type L domain that interacts with cellular Nedd4-like ubiquitin ligases. Despite the absence of a PPXY motif in HIV-1 Gag, interaction with the ubiquitin ligase Nedd4-2 was recently shown to stimulate HIV-1 release. We show here that another Nedd4-like ubiquitin ligase, Nedd4-1, corrected release defects resulting from the disruption of PTAP (PTAP(-)), suggesting that HIV-1 Gag also recruits Nedd4-1 to facilitate virus release. Notably, Nedd4-1 remediation of HIV-1 PTAP(-) budding defects is independent of cellular Tsg101, implying that Nedd4-1's function in HIV-1 release does not involve ESCRT-I components and is therefore distinct from that of Nedd4-2. Consistent with this finding, deletion of the p6 region decreased Nedd4-1-Gag interaction, and disruption of the LYPX(n)L motif eliminated Nedd4-1-mediated restoration of HIV-1 PTAP(-). This result indicated that both Nedd4-1 interaction with Gag and function in virus release occur through the Alix-binding LYPX(n)L motif. Mutations of basic residues located in the NC domain of Gag that are critical for Alix's facilitation of HIV-1 release, also disrupted release mediated by Nedd4-1, further confirming a Nedd4-1-Alix functional interdependence. In fact we found that Nedd4-1 binds Alix in both immunoprecipitation and yeast-two-hybrid assays. In addition, Nedd4-1 requires its catalytic activity to promote virus release. Remarkably, RNAi knockdown of cellular Nedd4-1 eliminated Alix ubiquitination in the cell and impeded its ability to function in HIV-1 release. Together our data support a model in which Alix recruits Nedd4-1 to facilitate HIV-1 release mediated through the LYPX(n)L/Alix budding pathway via a mechanism that involves Alix ubiquitination.

Tsg101 is recruited by a late domain of the nucleocapsid protein to support budding of Marburg virus-like particles

The nucleoprotein NP of Marburg virus (MARV) is the major component of the viral nucleocapsid, which also consists of the viral proteins VP35, L, and VP30, as well as the viral genome. During virus assembly at the plasma membrane, the nucleocapsids are enwrapped by the major matrix protein VP40 and the viral envelope, which contains the transmembrane glycoprotein GP. Upon recombinant expression, VP40 alone is able to induce the formation and release of virus-like particles (VLPs) that closely resemble the filamentous morphology of MARV particles. Release of these VP40-induced VLPs is partially dependent on the cellular ESCRT machinery, which interacts with a late-domain motif in VP40. Coexpression with NP significantly enhances the budding of VP40-induced VLPs by an unknown mechanism. In the present study we analyzed the impact of late domains present in NP on the release of VLPs. We observed that the ESCRT I protein Tsg101 was recruited by NP into NP-induced inclusions in the perinuclear region. In the presence of VP40, NP was then recruited to VP40-positive membrane clusters and, in turn, recruited Tsg101 via a C-terminal PSAP late-domain motif in NP. This PSAP motif also mediated a dramatically enhanced incorporation of Tsg101 into VLPs, and its deletion significantly diminished the positive effect of NP on the release of VLPs. Taken together, these data indicate that NP enhances budding of VLPs by recruiting Tsg101 to the VP40-positive budding site through a PSAP late-domain motif.

Human immunodeficiency virus type 1 nucleocapsid p1 confers ESCRT pathway dependence

To facilitate the release of infectious progeny virions, human immunodeficiency virus type 1 (HIV-1) exploits the Endosomal Sorting Complex Required for Transport (ESCRT) pathway by engaging Tsg101 and ALIX through late assembly (L) domains in the C-terminal p6 domain of Gag. However, the L domains in p6 are known to be dispensable for efficient particle production by certain HIV-1 Gag constructs that have the nucleocapsid (NC) domain replaced by a foreign dimerization domain to substitute for the assembly function of NC. We now show that one such L domain-independent HIV-1 Gag construct (termed Z(WT)) that has NC-p1-p6 replaced by a leucine zipper domain is resistant to dominant-negative inhibitors of the ESCRT pathway that block HIV-1 particle production. However, Z(WT) became dependent on the presence of an L domain when NC-p1-p6 was restored to its C terminus. Furthermore, when the NC domain was replaced by a leucine zipper, the p1-p6 region, but not p6 alone, conferred sensitivity to inhibition of the ESCRT pathway. In an authentic HIV-1 Gag context, the effect of an inhibitor of the ESCRT pathway on particle production could be alleviated by deleting a portion of the NC domain together with p1. Together, these results indicate that the ESCRT pathway dependence of HIV-1 budding is determined, at least in part, by the NC-p1 region of Gag.

An LYPSL late domain in the gag protein contributes to the efficient release and replication of Rous sarcoma virus

The efficient release of newly assembled retrovirus particles from the plasma membrane requires the recruitment of a network of cellular proteins (ESCRT machinery) normally involved in the biogenesis of multivesicular bodies and in cytokinesis. Retroviruses and other enveloped viruses recruit the ESCRT machinery through three classes of short amino acid consensus sequences termed late domains: PT/SAP, PPXY, and LYPX(n)L. The major late domain of Rous sarcoma virus (RSV) has been mapped to a PPPY motif in Gag that binds members of the Nedd4 family of ubiquitin ligases. RSV Gag also contains a second putative late domain motif, LYPSL, positioned 5 amino acids downstream of PPPY. LYPX(n)L motifs have been shown to support budding in other retroviruses by binding the ESCRT adaptor protein Alix. To investigate a possible role of the LYPSL motif in RSV budding, we constructed PPPY and LYPSL mutants in the context of an infectious virus and then analyzed the budding rates, spreading profiles, and budding morphology. The data imply that the LYPSL motif acts as a secondary late domain and that its role in budding is amplified in the absence of a fully functional PPPY motif. The LYPXL motif proved to be a stronger late domain when an aspartic acid was substituted for the native serine, recapitulating the properties of the LYPDL late domain of equine infectious anemia virus. The overexpression of human Alix in the absence of a fully functional PPPY late domain partially rescued both the viral budding rate and viral replication, supporting a model in which the RSV LYPSL motif mediates budding through an interaction with the ESCRT adaptor protein Alix.

Coinfection of hepatic cell lines with human immunodeficiency virus and hepatitis B virus leads to an increase in intracellular hepatitis B surface antigen

Liver-related mortality is increased in the setting of HIV-hepatitis B virus (HBV) coinfection. However, interactions between HIV and HBV to explain this observation have not been described. We hypothesized that HIV infection of hepatocytes directly affects the life cycle of HBV. We infected human hepatic cell lines expressing HBV (Hep3B and AD38 cells) or not expressing HBV (Huh7, HepG2, and AD43 cells) with laboratory strains of HIV (NL4-3 and AD8), as well as a vesicular stomatitis virus (VSV)-pseudotyped HIV expressing enhanced green fluorescent protein (EGFP). Following HIV infection with NL4-3 or AD8 in hepatic cell lines, we observed a significant increase in HIV reverse transcriptase activity which was infectious. Despite no detection of surface CD4, CCR5, and CXCR4 by flow cytometry, AD8 infection of AD38 cells was inhibited by maraviroc and NL4-3 was inhibited by AMD3100, demonstrating that HIV enters AD38 hepatic cell lines via CCR5 or CXCR4. High-level infection of AD38 cells (50%) was achieved using VSV-pseudotyped HIV. Coinfection of the AD38 cell line with HIV did not alter the HBV DNA amount or species as determined by Southern blotting or nucleic acid signal amplification. However, coinfection with HIV was associated with a significant increase in intracellular HBsAg when measured by Western blotting, quantitative HBsAg, and fluorescence microscopy. We conclude that HIV infection of HBV-infected hepatic cell lines significantly increased intracellular HBsAg but not HBV DNA synthesis and that increased intrahepatic HBsAg secondary to direct infection by HIV may contribute to accelerated liver disease in HIV-HBV-coinfected individuals.

A role for the C terminus of Mopeia virus nucleoprotein in its incorporation into Z protein-induced virus-like particles

Arenaviruses are enveloped, negative-strand RNA viruses. For several arenaviruses, virus-like particle (VLP) formation requires the viral matrix Z protein. However, the mechanism by which viral ribonucleoprotein complexes are incorporated into virions is poorly understood. Here, we show that the expression of the Z protein and nucleoprotein (NP) of Mopeia virus, a close relative of the pathogenic Lassa virus, resulted in the highly selective incorporation of the NP protein into Z protein-induced VLPs. Moreover, the Z protein promoted the association of NP with cellular membranes, suggesting that the association of NP, Z, and the cellular membranes may facilitate the efficient incorporation of NP into VLPs. By employing a series of NP deletion constructs and testing their VLP incorporation, we further demonstrated an important role for the C-terminal half of NP in its incorporation into VLPs.

The nucleocapsid protein of HIV-1 as a promising therapeutic target for antiviral drugs

The nucleocapsid protein (NCp7) is a major HIV-1 structural protein that plays key roles in viral replication, mainly through its conserved zinc fingers that direct specific interactions with the viral nucleic acids. Owing to its high degree of conservation and critical functions, NCp7 represents a target of choice for drugs that can potentially complement HAART, thus possibly impairing the circulation of drug-resistant HIV-1 strains. Zinc ejectors showing potent antiretroviral activity were developed, but early generations suffered from limited selectively and significant toxicity. Compounds with improved selectivity have been developed and are being explored as topical microbicide candidates. Several classes of molecules inhibiting the interaction of NCp7 with the viral nucleic acids have also been developed. Although small molecules would be more suited for drug development, most molecules selected by screening showed limited antiretroviral activity. Peptides and RNA aptamers appear to be more promising, but the mechanism of their antiretroviral activity remains elusive. Substantial and more concerted efforts are needed to further develop anti-HIV drugs targeting NCp7 and bring them to the clinic.

Late domain-independent rescue of a release-deficient Moloney murine leukemia virus by the ubiquitin ligase itch

Moloney murine leukemia virus (MoMLV) Gag utilizes its late (L) domain motif PPPY to bind members of the Nedd4-like ubiquitin ligase family. These interactions recruit components of the cell's budding machinery that are critical for virus release. MoMLV Gag contains two additional L domains, PSAP and LYPAL, that are believed to drive residual MoMLV release via interactions with cellular proteins Tsg101 and Alix, respectively. We found that overexpression of Tsg101 or Alix failed to rescue the release of PPPY-deficient MoMLV via these other L domains. However, low-level expression of the ubiquitin ligase Itch potently rescued the release and infectivity of MoMLV lacking PPPY function. In contrast, other ubiquitin ligases such as WWP1, Nedd4.1, Nedd4.2, and Nedd4.2s did not rescue this release-deficient virus. Efficient rescue required the ubiquitin ligase activity of Itch and an intact C2 domain but not presence of the endophilin-binding site. Additionally, we found Itch to immunoprecipitate with MoMLV Gag lacking the PPPY motif and to be incorporated into rescued MoMLV particles. The PSAP and LYPAL motifs were dispensable for Itch-mediated virus rescue, and their absence did not affect the incorporation of Itch into the rescued particles. Itch-mediated rescue of release-defective MoMLV was sensitive to inhibition by dominant-negative versions of ESCRT-III components and the VPS4 AAA ATPase, indicating that Itch-mediated correction of MoMLV release defects requires the integrity of the host vacuolar sorting protein pathway. RNA interference knockdown of Itch suppressed the residual release of the MoMLV lacking the PPPY motif. Interestingly, Itch stimulation of the PPPY-deficient MoMLV release was accompanied by the enhancement of Gag ubiquitination and the appearance of new ubiquitinated Gag proteins in virions. Together, these results suggest that Itch can facilitate MoMLV release in an L domain-independent manner via a mechanism that requires the host budding machinery and involves Gag ubiquitination.

No strings attached: the ESCRT machinery in viral budding and cytokinesis

Since the initial discovery of the endosomal sorting complex required for transport (ESCRT) pathway, research in this field has exploded. ESCRT proteins are part of the endosomal trafficking system and play a crucial role in the biogenesis of multivesicular bodies by functioning in the formation of vesicles that bud away from the cytoplasm. Subsequently, a surprising role for ESCRT proteins was defined in the budding step of some enveloped retroviruses, including HIV-1. ESCRT proteins are also employed in this outward budding process, which results in the resolution of a membranous tether between the host cell and the budding virus particle. Remarkably, it has recently been described that ESCRT proteins also have a role in the topologically equivalent process of cell division. In the same way that viral particles recruit the ESCRT proteins to the site of viral budding, ESCRT proteins are also recruited to the midbody - the site of release of daughter cell from mother cell during cytokinesis. In this Commentary, we describe recent advances in the understanding of ESCRT proteins and how they act to mediate these diverse processes.

Functional role of Alix in HIV-1 replication

Retroviral Gag proteins encode small peptide motifs known as late domains that promote the release of virions from infected cells by interacting directly with host cell factors. Three types of retroviral late domains, with core sequences P(T/S)AP, YPX(n)L, and PPPY, have been identified. HIV-1 encodes a primary P(T/S)AP-type late domain and an apparently secondary late domain sequence of the YPX(n)L type. The P(T/S)AP and YPX(n)L motifs interact with the endosomal sorting factors Tsg101 and Alix, respectively. Although biochemical and structural studies support a direct binding between HIV-1 p6 and Alix, the physiological role of Alix in HIV-1 biology remains undefined. To elucidate the function of the p6-Alix interaction in HIV-1 replication, we introduced a series of mutations in the p6 Alix binding site and evaluated the effects on virus particle production and virus replication in a range of cell types, including physiologically relevant primary T cells and macrophages. We also examined the effects of the Alix binding site mutations on virion morphogenesis and single-cycle virus infectivity. We determined that the p6-Alix interaction plays an important role in HIV-1 replication and observed a particularly severe impact of Alix binding site mutations when they were combined with mutational inactivation of the Tsg101 binding site.

Quantitative fluorescence resonance energy transfer microscopy analysis of the human immunodeficiency virus type 1 Gag-Gag interaction: relative contributions of the CA and NC domains and membrane binding

The human immunodeficiency virus type 1 structural polyprotein Pr55(Gag) is necessary and sufficient for the assembly of virus-like particles on cellular membranes. Previous studies demonstrated the importance of the capsid C-terminal domain (CA-CTD), nucleocapsid (NC), and membrane association in Gag-Gag interactions, but the relationships between these factors remain unclear. In this study, we systematically altered the CA-CTD, NC, and the ability to bind membrane to determine the relative contributions of, and interplay between, these factors. To directly measure Gag-Gag interactions, we utilized chimeric Gag-fluorescent protein fusion constructs and a fluorescence resonance energy transfer (FRET) stoichiometry method. We found that the CA-CTD is essential for Gag-Gag interactions at the plasma membrane, as the disruption of the CA-CTD has severe impacts on FRET. Data from experiments in which wild-type (WT) and CA-CTD mutant Gag molecules are coexpressed support the idea that the CA-CTD dimerization interface consists of two reciprocal interactions. Mutations in NC have less-severe impacts on FRET between normally myristoylated Gag proteins than do CA-CTD mutations. Notably, when nonmyristoylated Gag interacts with WT Gag, NC is essential for FRET despite the presence of the CA-CTD. In contrast, constitutively enhanced membrane binding eliminates the need for NC to produce a WT level of FRET. These results from cell-based experiments suggest a model in which both membrane binding and NC-RNA interactions serve similar scaffolding functions so that one can functionally compensate for a defect in the other.

Divergent Bro1 domains share the capacity to bind human immunodeficiency virus type 1 nucleocapsid and to enhance virus-like particle production

To promote the release of infectious virions, human immunodeficiency virus type 1 (HIV-1) exploits the endosomal sorting complex required for transport (ESCRT) pathway by engaging Tsg101 and ALIX through late assembly (L) domains in p6 Gag. An LYPx(n)L motif in p6 serves as docking site for the central V domain of ALIX and is required for its ability to stimulate HIV-1 budding. Additionally, the nucleocapsid (NC) domain of Gag binds to the N-terminal Bro1 domain of ALIX, which connects ALIX to the membrane-deforming ESCRT-III complex via its CHMP4 subunits. Since the isolated Bro1 domain of ALIX is sufficient to markedly stimulate virus-like particle (VLP) production in a minimal Gag rescue assay, we examined whether the Bro1 domains of other human proteins possess a similar activity. We now show that the Bro1 domain-only protein Brox and the isolated Bro1 domains of HD-PTP and rhophilin all bind to HIV-1 NC. Furthermore, all shared the capacity to stimulate VLP production by a minimal HIV-1 Gag molecule, and Brox in particular was as potent as the Bro1 domain of ALIX in this assay. Unexpectedly, Brox retained significant activity even if its CHMP4 binding site was disrupted. Thus, the ability to assist in VLP production may be an intrinsic property of the boomerang-shaped Bro1 domain.

Exploring the functional interaction between POSH and ALIX and the relevance to HIV-1 release

BACKGROUND

The ALG2-interacting protein X (ALIX)/AIP1 is an adaptor protein with multiple functions in intracellular protein trafficking that plays a central role in the biogenesis of enveloped viruses. The ubiquitin E3-ligase POSH (plenty of SH3) augments HIV-1 egress by facilitating the transport of Gag to the cell membrane. Recently, it was reported, that POSH interacts with ALIX and thereby enhances ALIX mediated phenotypes in Drosophila.

RESULTS

In this study we identified ALIX as a POSH ubiquitination substrate in human cells: POSH induces the ubiquitination of ALIX that is modified on several lysine residues in vivo and in vitro. This ubiquitination does not destabilize ALIX, suggesting a regulatory function. As it is well established that ALIX rescues virus release of L-domain mutant HIV-1, HIV-1DeltaPTAP, we demonstrated that wild type POSH, but not an ubiquitination inactive RING finger mutant (POSHV14A), substantially enhances ALIX-mediated release of infectious virions derived from HIV-1DeltaPTAP L-domain mutant (YPXnL-dependent HIV-1). In further agreement with the idea of a cooperative function of POSH and ALIX, mutating the YPXnL-ALIX binding site in Gag completely abrogated augmentation of virus release by overexpression of POSH. However, the effect of the POSH-mediated ubiquitination appears to be auxiliary, but not necessary, as silencing of POSH by RNAi does not disturb ALIX-augmentation of virus release.

CONCLUSION

Thus, the cumulative results identified ALIX as an ubiquitination substrate of POSH and indicate that POSH and ALIX cooperate to facilitate efficient virus release. However, while ALIX is obligatory for the release of YPXnL-dependent HIV-1, POSH, albeit rate-limiting, may be functionally interchangeable.

The nucleocapsid region of HIV-1 Gag cooperates with the PTAP and LYPXnL late domains to recruit the cellular machinery necessary for viral budding

HIV-1 release is mediated through two motifs in the p6 region of Gag, PTAP and LYPX(n)L, which recruit cellular proteins Tsg101 and Alix, respectively. The Nucleocapsid region of Gag (NC), which binds the Bro1 domain of Alix, also plays an important role in HIV-1 release, but the underlying mechanism remains unclear. Here we show that the first 202 residues of the Bro1 domain (Bro(i)) are sufficient to bind Gag. Bro(i) interferes with HIV-1 release in an NC-dependent manner and arrests viral budding at the plasma membrane. Similar interrupted budding structures are seen following over-expression of a fragment containing Bro1 with the adjacent V domain (Bro1-V). Although only Bro1-V contains binding determinants for CHMP4, both Bro(i) and Bro1-V inhibited release via both the PTAP/Tsg101 and the LYPX(n)L/Alix pathways, suggesting that they interfere with a key step in HIV-1 release. Remarkably, we found that over-expression of Bro1 rescued the release of HIV-1 lacking both L domains. This rescue required the N-terminal region of the NC domain in Gag and the CHMP4 binding site in Bro1. Interestingly, release defects due to mutations in NC that prevented Bro1 mediated rescue of virus egress were rescued by providing a link to the ESCRT machinery via Nedd4.2s over-expression. Our data support a model in which NC cooperates with PTAP in the recruitment of cellular proteins necessary for its L domain activity and binds the Bro1-CHMP4 complex required for LYPX(n)L-mediated budding.

The ESCRT pathway and HIV-1 budding

HIV-1 Gag engages components of the ESCRT (endosomal sorting complex required for transport) pathway via so-called L (late-assembly) domains to promote virus budding. Specifically, the PTAP (Pro-Thr-Ala-Pro)-type primary L domain of HIV-1 recruits ESCRT-I by binding to Tsg101 (tumour susceptibility gene 101), and an auxiliary LYPX(n)L (Leu-Tyr-Pro-Xaa(n)-Leu)-type L domain recruits the ESCRT-III-binding partner Alix [ALG-2 (apoptosis-linked gene 2)-interacting protein X]. The structurally related CHMPs (charged multivesicular body proteins), which form ESCRT-III, are kept in an inactive state through intramolecular interactions, and become potent inhibitors of HIV-1 budding upon removal of an autoinhibitory region. In the absence of the primary L domain, HIV-1 budding is strongly impaired, but can be efficiently rescued through the overexpression of Alix. This effect of Alix depends on its ability to interact with CHMP4, suggesting that it is the recruitment of CHMPs that ultimately drives virus release. Surprisingly, HIV-1 budding defects can also be efficiently corrected by overexpressing Nedd (neural-precursor-cell-expressed developmentally down-regulated) 4-2s, a member of a family of ubiquitin ligases previously implicated in the function of PPXY (Pro-Pro-Xaa-Tyr)-type L domains, which are absent from HIV-1. At least under certain circumstances, Nedd4-2s stimulates the activity of PTAP-type L domains, raising the possibility that the ubiquitin ligase regulates the activity of ESCRT-I.

The ESCRT machinery: new functions in viral and cellular biology

The ESCRT (endosomal sorting complex required for transport) machinery consists of a number of cytosolic proteins that make up three functional subcomplexes: ESCRT-I, ESCRT-II and ESCRT-III. These proteins function in multivesicular body formation and cell division and are co-opted by enveloped retroviruses to facilitate viral egress. Analysis of these functions may help illuminate conserved mechanisms of ESCRT function.

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.

Cell biology of HIV-1 infection of macrophages

HIV infection of macrophages is a critically important component of viral pathogenesis and progression to AIDS. Although the virus follows the same life cycle in macrophages and T lymphocytes, several aspects of the virus-host relationship are unique to macrophage infection. Examples of these are the long-term persistence of productive infection, sustained by the absence of cell death, and the ability of progeny virus to bud into and accumulate in endocytic compartments designated multivesicular bodies (MVBs). Recently, the hypothesis that viral exploitation of the macrophage endocytic machinery is responsible for perpetuating the chronic state of infection unique to this cell type has been challenged in several independent studies employing a variety of experimental strategies. This review examines the evidence supporting and refuting the canonical hypothesis and highlights recently identified cellular factors that may contribute to the unique aspects of the HIV-macrophage interaction.

The HIV-1 p6/EIAV p9 docking site in Alix is autoinhibited as revealed by a conformation-sensitive anti-Alix monoclonal antibody

Alix [ALG-2 (apoptosis-linked gene 2)-interacting protein X], a component of the endosomal sorting machinery, contains a three-dimensional docking site for HIV-1 p6(Gag) or EIAV (equine infectious anaemia virus) p9(Gag), and binding of the viral protein to this docking site allows the virus to hijack the host endosomal sorting machinery for budding from the plasma membrane. In the present study, we identified a monoclonal antibody that specifically recognizes the docking site for p6(Gag)/p9(Gag) and we used this antibody to probe the accessibility of the docking site in Alix. Our results show that the docking site is not available in cytosolic or recombinant Alix under native conditions and becomes available upon addition of the detergent Nonidet P40 or SDS. In HEK (human embryonic kidney)-293 cell lysates, an active p6(Gag)/p9(Gag) docking site is specifically available in Alix from the membrane fraction. The findings of the present study demonstrate that formation or exposure of the p6(Gag)/p9(Gag) docking site in Alix is a regulated event and that Alix association with the membrane may play a positive role in this process.

Drug-associated changes in amino acid residues in Gag p2, p7(NC), and p6(Gag)/p6(Pol) in human immunodeficiency virus type 1 (HIV-1) display a dominant effect on replicative fitness and drug response

Regions of HIV-1 gag between p2 and p6(Gag)/p6(Pol), in addition to protease (PR), develop genetic diversity in HIV-1 infected individuals who fail to suppress virus replication by combination protease inhibitor (PI) therapy. To elucidate functional consequences for viral replication and PI susceptibility by changes in Gag that evolve in vivo during PI therapy, a panel of recombinant viruses was constructed. Residues in Gag p2/p7(NC) cleavage site and p7(NC), combined with residues in the flap of PR, defined novel fitness determinants that restored replicative capacity to the posttherapy virus. Multiple determinants in Gag have a dominant effect on PR phenotype and increase susceptibility to inhibitors of drug-resistant or drug-sensitive PR genes. Gag determinants of drug sensitivity and replication alter the fitness landscape of the virus, and viral replicative capacity can be independent of drug sensitivity. The functional linkage between Gag and PR provides targets for novel therapeutics to inhibit drug-resistant viruses.

ALIX-CHMP4 interactions in the human ESCRT pathway

The ESCRT pathway facilitates membrane fission events during enveloped virus budding, multivesicular body formation, and cytokinesis. To promote HIV budding and cytokinesis, the ALIX protein must bind and recruit CHMP4 subunits of the ESCRT-III complex, which in turn participate in essential membrane remodeling functions. Here, we report that the Bro1 domain of ALIX binds specifically to C-terminal residues of the human CHMP4 proteins (CHMP4A-C). Crystal structures of the complexes reveal that the CHMP4 C-terminal peptides form amphipathic helices that bind across the conserved concave surface of ALIX(Bro1). ALIX-dependent HIV-1 budding is blocked by mutations in exposed ALIX(Bro1) residues that help contribute to the binding sites for three essential hydrophobic residues that are displayed on one side of the CHMP4 recognition helix (M/L/IxxLxxW). The homologous CHMP1-3 classes of ESCRT-III proteins also have C-terminal amphipathic helices, but, in those cases, the three hydrophobic residues are arrayed with L/I/MxxxLxxL spacing. Thus, the distinct patterns of hydrophobic residues provide a "code" that allows the different ESCRT-III subunits to bind different ESCRT pathway partners, with CHMP1-3 proteins binding MIT domain-containing proteins, such as VPS4 and Vta1/LIP5, and CHMP4 proteins binding Bro1 domain-containing proteins, such as ALIX.

The host protein Staufen1 interacts with the Pr55Gag zinc fingers and regulates HIV-1 assembly via its N-terminus

Background

The formation of new infectious human immunodeficiency type 1 virus (HIV-1) mainly relies on the homo-multimerization of the viral structural polyprotein Pr55^Gag and on the recruitment of host factors. We have previously shown that the double-stranded RNA-binding protein Staufen 1 (Stau1), likely through an interaction between its third double-stranded RNA-binding domain (dsRBD3) and the nucleocapsid (NC) domain of Pr55^Gag, participates in HIV-1 assembly by influencing Pr55^Gag multimerization.

Results

We now report the fine mapping of Stau1/Pr55^Gag association using co-immunoprecipitation and live cell bioluminescence resonance energy transfer (BRET) assays. On the one hand, our results show that the Stau1-Pr55^Gag interaction requires the integrity of at least one of the two zinc fingers in the NC domain of Pr55^Gag but not that of the NC N-terminal basic region. Disruption of both zinc fingers dramatically impeded Pr55^Gag multimerization and virus particle release. In parallel, we tested several Stau1 deletion mutants for their capacity to influence Pr55^Gag multimerization using the Pr55^Gag/Pr55^Gag BRET assay in live cells. Our results revealed that a molecular determinant of 12 amino acids at the N-terminal end of Stau1 is necessary to increase Pr55^Gag multimerization and particle release. However, this region is not required for Stau1 interaction with the viral polyprotein Pr55^Gag.

Conclusion

These data highlight that Stau1 is a modular protein and that Stau1 influences Pr55^Gag multimerization via 1) an interaction between its dsRBD3 and Pr55^Gag zinc fingers and 2) a regulatory domain within the N-terminus that could recruit host machineries that are critical for the completion of new HIV-1 capsids.

Refined study of the interaction between HIV-1 p6 late domain and ALIX

The interaction between the HIV-1 p6 late budding domain and ALIX, a class E vacuolar protein sorting factor, was explored by using the yeast two-hybrid approach. We refined the ALIX binding site of p6 as being the leucine triplet repeat sequence (Lxx)4 (LYPLTSLRSLFG). Intriguingly, the deletion of the C-terminal proline-rich region of ALIX prevented detectable binding to p6. In contrast, a four-amino acid deletion in the central hinge region of p6 increased its association with ALIX as shown by its ability to bind to ALIX lacking the proline rich domain. Finally, by using a random screening approach, the minimal ALIX391-510 fragment was found to specifically interact with this p6 deletion mutant. A parallel analysis of ALIX binding to the late domain p9 from EIAV revealed that p6 and p9, which exhibit distinct ALIX binding motives, likely bind differently to ALIX. Altogether, our data support a model where the C-terminal proline-rich domain of ALIX allows the access of its binding site to p6 by alleviating a conformational constraint resulting from the presence of the central p6 hinge.

Insulin-like growth factor II mRNA binding protein 1 associates with Gag protein of human immunodeficiency virus type 1, and its overexpression affects virus assembly

The assembly of human immunodeficiency virus type 1 (HIV-1) particles is driven by viral Gag protein. This function of Gag not only benefits from its self-multimerization property but also depends on its interaction with a number of cellular factors such as TSG101 and ALIX/AIP1 that promote virus budding and release from cell surfaces. However, interaction with Gag also allows some cellular factors such as APOBEC3G and Trim5alpha to access viral replication machinery and block viral replication. In this study, we report a new HIV-1 Gag-binding factor named insulin-like growth factor II mRNA binding protein 1 (IMP1). Gag-IMP1 interaction requires the second zinc finger of the nucleocapsid (NC) domain of Gag and the KH3 and KH4 domains of IMP1. A fourfold reduction of HIV-1 infectivity was seen with overexpression of the wild-type IMP1 and its mutant that is able to interact with Gag but not with overexpression of IMP1 mutants exhibiting Gag-binding deficiency. The decreased viral infectivity was further shown as a result of diminished viral RNA packaging, abrogated Gag processing on the cellular membranes, and impeded maturation of virus particles. Together, these results demonstrate that IMP1 interacts with HIV-1 Gag protein and is able to block the formation of infectious HIV-1 particles.

NEDD4L overexpression rescues the release and infectivity of human immunodeficiency virus type 1 constructs lacking PTAP and YPXL late domains

The cellular ESCRT pathway functions in membrane remodeling events that accompany endosomal protein sorting, cytokinesis, and enveloped RNA virus budding. In the last case, short sequence motifs (termed late domains) within human immunodeficiency virus type 1 (HIV-1) p6(Gag) bind and recruit two ESCRT pathway proteins, TSG101 and ALIX, to facilitate virus budding. We now report that overexpression of the HECT ubiquitin E3 ligase, NEDD4L/NEDD4-2, stimulated the release of HIV-1 constructs that lacked TSG101- and ALIX-binding late domains, increasing infectious titers >20-fold. Furthermore, depletion of endogenous NEDD4L inhibited the release of these crippled viruses and led to cytokinesis defects. Stimulation of virus budding was dependent upon the ubiquitin ligase activity of NEDD4L and required only the minimal HIV-1 Gag assembly regions, demonstrating that Gag has ubiquitin-dependent, cis-acting late domain activities located outside of the p6 region. NEDD4L stimulation also required TSG101 and resulted in ubiquitylation of several ESCRT-I subunits, including TSG101. Finally, we found that TSG101/ESCRT-I was required for efficient release of Mason-Pfizer monkey virus, which buds primarily by using a PPXY late domain to recruit NEDD4-like proteins. These observations suggest that NEDD4L and possibly other NEDD4-like proteins can ubiquitylate and activate ESCRT-I to function in virus budding.

Efficient and specific rescue of human immunodeficiency virus type 1 budding defects by a Nedd4-like ubiquitin ligase

To exit infected cells, human immunodeficiency virus type 1 (HIV-1) exploits the vacuolar protein-sorting pathway by engaging Tsg101 and ALIX through PTAP and LYPx(n)L late assembly (L) domains. In contrast, less-complex retroviruses often use PPxY L domains to recruit Nedd4 family ubiquitin ligases. Although HIV-1 Gag lacks PPxY motifs, we now show that the budding of various HIV-1 L-domain mutants is dramatically enhanced by ectopic Nedd4-2s, a native isoform with a truncated C2 domain. The effect of Nedd4-2s on HIV-1 budding required a catalytically active HECT domain and was specific, since other Nedd4 family proteins showed little activity and an unrelated retrovirus was not rescued. The residual C2 domain of Nedd4-2s was critical for the enhancement of HIV-1 budding and for the association of Nedd4-2s with Gag, as reflected by its incorporation into virus-like particles. Interestingly, the incorporation of Nedd4-2s also depended on its active site, indicating that the ability to form a thioester with ubiquitin was required. These data suggest a novel mechanism by which HIV-1 Gag can connect to cellular budding machinery.

Nucleocapsid mutations turn HIV-1 into a DNA-containing virus

Retroviruses replicate by converting their positive sense genomic RNA into double-stranded DNA that is subsequently integrated into the host genome. This conversion is catalyzed by reverse transcriptase (RT) early after virus entry into the target cell and is chaperoned by the nucleocapsid protein (NC). In HIV-1, NC is composed of small basic domains flanking two highly conserved CCHC zinc fingers that specifically interact with the genomic RNA and RT. Through specific interactions with the genomic RNA and RT, and possibly with cellular factors, the NC zinc fingers were found to play critical roles in HIV-1 assembly and budding, and later in proviral DNA synthesis and integration. Therefore, intact NC zinc fingers are needed throughout the virus replication cycle. Here, we report for the first time that deleting either one or the two NC zinc fingers leads to an unexpected premature viral DNA synthesis in virus producer cells and the production of noninfectious particles with a high level of viral DNA. In addition to providing the first example of reverse transcription during the late steps of HIV-1 replication, these findings emphasize the fact that the NC zinc fingers are a major target for new drugs against HIV-1.