HIV-1 Gag release from yeast reveals ESCRT interaction with the Gag N-terminal protein region

The HIV-1 protein Gag assembles at the plasma membrane and drives virion budding, assisted by the cellular endosomal complex required for transport (ESCRT) proteins. Two ESCRT proteins, TSG101 and ALIX, bind to the Gag C-terminal p6 peptide. TSG101 binding is important for efficient HIV-1 release, but how ESCRTs contribute to the budding process and how their activity is coordinated with Gag assembly is poorly understood. Yeast, allowing genetic manipulation that is not easily available in human cells, has been used to characterize the cellular ESCRT function. Previous work reported Gag budding from yeast spheroplasts, but Gag release was ESCRT-independent. We developed a yeast model for ESCRT-dependent Gag release. We combined yeast genetics and Gag mutational analysis with Gag-ESCRT binding studies and the characterization of Gag-plasma membrane binding and Gag release. With our system, we identified a previously unknown interaction between ESCRT proteins and the Gag N-terminal protein region. Mutations in the Gag-plasma membrane–binding matrix domain that reduced Gag-ESCRT binding increased Gag-plasma membrane binding and Gag release. ESCRT knockout mutants showed that the release enhancement was an ESCRT-dependent effect. Similarly, matrix mutation enhanced Gag release from human HEK293 cells. Release enhancement partly depended on ALIX binding to p6, although binding site mutation did not impair WT Gag release. Accordingly, the relative affinity for matrix compared with p6 in GST-pulldown experiments was higher for ALIX than for TSG101. We suggest that a transient matrix-ESCRT interaction is replaced when Gag binds to the plasma membrane. This step may activate ESCRT proteins and thereby coordinate ESCRT function with virion assembly.     

Functional interchangeability of late domains, late domain cofactors and ubiquitin in viral budding

The membrane scission event that separates nascent enveloped virions from host cell membranes often requires the ESCRT pathway, which can be engaged through the action of peptide motifs, termed late (L-) domains, in viral proteins. Viral PTAP and YPDL-like L-domains bind directly to the ESCRT-I and ALIX components of the ESCRT pathway, while PPxY motifs bind Nedd4-like, HECT-domain containing, ubiquitin ligases (e.g. WWP1). It has been unclear precisely how ubiquitin ligase recruitment ultimately leads to particle release. Here, using a lysine-free viral Gag protein derived from the prototypic foamy virus (PFV), where attachment of ubiquitin to Gag can be controlled, we show that several different HECT domains can replace the WWP1 HECT domain in chimeric ubiquitin ligases and drive budding. Moreover, artificial recruitment of isolated HECT domains to Gag is sufficient to stimulate budding. Conversely, the HECT domain becomes dispensable if the other domains of WWP1 are directly fused to an ESCRT-1 protein. In each case where budding is driven by a HECT domain, its catalytic activity is essential, but Gag ubiquitination is dispensable, suggesting that ubiquitin ligation to trans-acting proteins drives budding. Paradoxically, however, we also demonstrate that direct fusion of a ubiquitin moiety to the C-terminus of PFV Gag can also promote budding, suggesting that ubiquitination of Gag can substitute for ubiquitination of trans-acting proteins. Depletion of Tsg101 and ALIX inhibits budding that is dependent on ubiquitin that is fused to Gag, or ligated to trans-acting proteins through the action of a PPxY motif. These studies underscore the flexibility in the ways that the ESCRT pathway can be engaged, and suggest a model in which the identity of the protein to which ubiquitin is attached is not critical for subsequent recruitment of ubiquitin-binding components of the ESCRT pathway and viral budding to proceed.     

Development of a rapid and convenient method for the quantification of HIV-1 budding

In cells, the expression of Gag protein, one of the major structural proteins of retroviruses, is sufficient for budding virus-like particles (VLPs) from the cell surface. We have previously reported that spheroplasts of Saccharomyces cerevisiae expressing HIV-1 Gag proteins from an episomal plasmid constitutively released a large amount of VLPs into culture media; however, commercially available ELISA kits which detect mature capsid of HIV-1 could not detect uncleaved 55-kDa Gag proteins released from budding yeast. We therefore developed a method to quantitate VLP levels released from budding yeast by using fusion protein from HIV-1 Gag and Firefly Luciferase. This system is useful for screening cellular factor(s) involved in retrovirus budding from S. cerevisiae.     

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.     

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.     

Regulation of human T-cell leukemia virus type 1 (HTLV-1) budding by ubiquitin ligase Nedd4

The Gag protein of human T-cell leukemia virus type 1 (HTLV-1) contains the conserved sequences PPxY and PTAP, which are putative viral motifs required for budding (L-domain motifs). We show here that the PPxY motif, but not the PTAP motif, is essential for HTLV-1 virion budding from the plasma membrane. In addition, we show that overexpression of Nedd4 enhances HTLV-1 budding and that Nedd4 interacts with Gag via its WW domain. The HECT domain of Nedd4 is also required for budding. These results indicate that Nedd4 or a Nedd4-related ubiquitin ligase plays a critical role in HTLV-1 budding.     

Role of Nedd4 and ubiquitination of Rous sarcoma virus Gag in budding of virus-like particles from cells

Rous sarcoma virus (RSV) budding requires an interaction of the L domain within the p2b region of Gag with cellular Nedd4-family E3 ubiquitin protein ligases. Members of our laboratories previously demonstrated that overexpression of a fragment of the chicken Nedd4-like protein (LDI-1 WW) inhibits Gag release in a dominant-negative manner (A. Kikonyogo, F. Bouamr, M. L. Vana, Y. Xiang, A. Aiyar, C. Carter, and J. Leis, Proc. Natl. Acad. Sci. USA 98:11199-11204, 2001). We have now identified the complete 3' end of LDI-1 and determined that it has a C-terminal ubiquitin ligase HECT domain, similar to other Nedd4 family members. While overexpression of the full-length LDI-1 clone (LDI-1 FL) had little effect on Gag budding, an LDI-1 FL mutant with a substitution in the HECT domain catalytic site blocked Gag release, similar to LDI-1 WW. The coexpression of Gag and hemagglutinin-tagged ubiquitin (HA-Ub) resulted in the detection of mono- and polyubiquitinated forms of Gag in cells and mostly monoubiquitinated Gag in virus-like particles (VLPs). When the Nedd4-binding site (L domain) was deleted, ubiquitinated Gag was not detected. Interestingly, the release of Gag with ubiquitin covalently linked to the C terminus (Gag-Ub) was still blocked by LDI-1 WW. To understand the mechanism of this inhibition, we examined cells expressing Gag and LDI-1 WW by electron microscopy. In the presence of LDI-1 WW, VLPs were found in electron-dense inclusion bodies in the cytoplasm of transfected cells. In contrast, when cells that coexpressed Gag-Ub and LDI-1 WW were examined, inclusion bodies were detected but did not contain VLPs. These results indicate that the ubiquitination of Gag is dependent upon Nedd4 binding to the L domain and suggest that Nedd4 has additional functions during RSV release besides the ubiquitination of Gag.     

Multiple interactions between the ESCRT machinery and arrestin-related proteins: implications for PPXY-dependent budding

Late domains are short peptide sequences encoded by enveloped viruses to promote the final separation of the nascent virus from the infected cell. These amino acid motifs facilitate viral egress by interacting with components of the ESCRT (endosomal sorting complex required for transport) machinery, ultimately leading to membrane scission by recruiting ESCRT-III to the site of viral budding. PPXY late (L) domains present in viruses such as murine leukemia virus (MLV) or human T-cell leukemia virus type 1 (HTLV-1) access the ESCRT pathway via interaction with HECT ubiquitin ligases (WWP1, WWP2, and Itch). However, the mechanism of ESCRT-III recruitment in this context remains elusive. In this study, we tested the arrestin-related trafficking (ART) proteins, namely, ARRDC1 (arrestin domain-containing protein 1) to ARRDC4 and TXNIP (thioredoxin-interacting protein), for their ability to function as adaptors between HECT ubiquitin ligases and the core ESCRT machinery in PPXY-dependent budding. We present several lines of evidence in support of such a role: ARTs interact with HECT ubiquitin ligases, and they also exhibit multiple interactions with components of the ESCRT pathway, namely, ALIX and Tsg101, and perhaps with an as yet unidentified factor. Additionally, the ARTs can be recruited to the site of viral budding, and their overexpression results in a PPXY-specific inhibition of MLV budding. Lastly, we show that WWP1 changes the ubiquitination status of ARRDC1, suggesting that the ARTs may provide a platform for ubiquitination in PPXY-dependent budding. Taken together, our results support a model whereby ARTs are involved in PPXY-mediated budding by interacting with HECT ubiquitin ligases and providing several alternative routes for ESCRT-III recruitment.     

The yeast Npi1/Rsp5 ubiquitin ligase lacking its N-terminal C2 domain is competent for ubiquitination but not for subsequent endocytosis of the gap1 permease

The yeast ubiquitin ligase Npi1/Rsp5 and its mammalian homologue Nedd4 are involved in ubiquitination of various cell surface proteins, these being subsequently internalized by endocytosis and degraded in the vacuole/lysosome. Both enzymes consist of an N-terminal C2 domain, three to four successive WW(P) domains, and a C-terminal catalytic domain (HECT) containing a highly conserved cysteine residue involved in ubiquitin thioester formation. In this study, we show that the conserved cysteine of the HECT domain is required for yeast cell viability and for ubiquitination and subsequent endocytosis of the Gap1 permease. In contrast, the C2 domain of Npi1/Rsp5 is not essential to cell viability. Its deletion impairs internalization of Gap1, without detectably affecting ubiquitination of the permease. This suggests that Npi1/Rsp5 participates, via its C2 domain, in endocytosis of ubiquitinated permeases.     

A single ubiquitin is sufficient for cargo protein entry into MVBs in the absence of ESCRT ubiquitination

ESCRTs (endosomal sorting complexes required for transport) bind and sequester ubiquitinated membrane proteins and usher them into multivesicular bodies (MVBs). As Ubiquitin (Ub)-binding proteins, ESCRTs themselves become ubiquitinated. However, it is unclear whether this regulates a critical aspect of their function or is a nonspecific consequence of their association with the Ub system. We investigated whether ubiquitination of the ESCRTs was required for their ability to sort cargo into the MVB lumen. Although we found that Rsp5 was the main Ub ligase responsible for ubiquitination of ESCRT-0, elimination of Rsp5 or elimination of the ubiquitinatable lysines within ESCRT-0 did not affect MVB sorting. Moreover, by fusing the catalytic domain of deubiquitinating peptidases onto ESCRTs, we could block ESCRT ubiquitination and the sorting of proteins that undergo Rsp5-dependent ubiquitination. Yet, proteins fused to a single Ub moiety were efficiently delivered to the MVB lumen, which strongly indicates that a single Ub is sufficient in sorting MVBs in the absence of ESCRT ubiquitination.     

A dual role for K63-linked ubiquitin chains in multivesicular body biogenesis and cargo sorting

In yeast, the sorting of transmembrane proteins into the multivesicular body (MVB) internal vesicles requires their ubiquitylation by the ubiquitin ligase Rsp5. This allows their recognition by the ubiquitin-binding domains (UBDs) of several endosomal sorting complex required for transport (ESCRT) subunits. K63-linked ubiquitin (K63Ub) chains decorate several MVB cargoes, and accordingly we show that they localize prominently to the class E compartment, which accumulates ubiquitylated cargoes in cells lacking ESCRT components. Conversely, yeast cells unable to generate K63Ub chains displayed MVB sorting defects. These properties are conserved among eukaryotes, as the mammalian melanosomal MVB cargo MART-1 is modified by K63Ub chains and partly missorted when the genesis of these chains is inhibited. We show that all yeast UBD-containing ESCRT proteins undergo ubiquitylation and deubiquitylation, some being modified through the opposing activities of Rsp5 and the ubiquitin isopeptidase Ubp2, which are known to assemble and disassemble preferentially K63Ub chains, respectively. A failure to generate K63Ub chains in yeast leads to an MVB ultrastructure alteration. Our work thus unravels a double function of K63Ub chains in cargo sorting and MVB biogenesis.     

HECT ubiquitin ligases link viral and cellular PPXY motifs to the vacuolar protein-sorting pathway

Many enveloped viruses exploit the class E vacuolar protein-sorting (VPS) pathway to bud from cells, and use peptide motifs to recruit specific class E VPS factors. Homologous to E6AP COOH terminus (HECT) ubiquitin ligases have been implicated as cofactors for PPXY motif-dependent budding, but precisely which members of this family are responsible, and how they access the VPS pathway is unclear. Here, we show that PPXY-dependent viral budding is unusually sensitive to inhibitory fragments derived from specific HECT ubiquitin ligases, namely WWP1 and WWP2. We also show that WWP1, WWP2, or Itch ubiquitin ligase recruitment promotes PPXY-dependent virion release, and that this function requires that the HECT ubiquitin ligase domain be catalytically active. Finally, we show that several mammalian HECT ubiquitin ligases, including WWP1, WWP2, and Itch are recruited to class E compartments induced by dominant negative forms of the class E VPS ATPase, VPS4. These data indicate that specific HECT ubiquitin ligases can link PPXY motifs to the VPS pathway to induce viral budding.     

Sprouty 2 binds ESCRT-II factor Eap20 and facilitates HIV-1 gag release

The four ESCRT (endocytic sorting complexes required for transport) complexes (ESCRT-0, -I, -II, and -III) normally operate sequentially in the trafficking of cellular cargo. HIV-1 Gag trafficking and release as virus-like particles (VLPs) require the participation of ESCRTs; however, its use of ESCRTs is selective and nonsequential. Specifically, Gag trafficking to release sites on the plasma membrane does not require ESCRT-0 or -II. It is known that a bypass of ESCRT-0 is achieved by the direct linkage of the ESCRT-I component, Tsg101, to the primary L domain motif (PTAP) in Gag and that bypass of ESCRT-II is achieved by the linkage of Gag to ESCRT-III through the adaptor protein Alix. However, the mechanism by which Gag suppresses the interaction of bound ESCRT-I with ESCRT-II is unknown. Here we show (i) that VLP release requires the steady-state level of Sprouty 2 (Spry2) in COS-1 cells, (ii) that Spry2 binds the ESCRT-II component Eap20, (iii) that binding Eap20 permits Spry2 to disrupt ESCRT-I interaction with ESCRT-II, and (iv) that coexpression of Gag with a Spry2 fragment that binds Eap20 increases VLP release. Spry2 also facilitated release of P7L-Gag (i.e., release in the absence of Tsg101 binding). In this case, rescue required the secondary L domain (YPX(n)L) in HIV-1 Gag that binds Alix and the region in Spry2 that binds Eap20. The results identify Spry2 as a novel cellular factor that facilitates release driven by the primary and secondary HIV-1 Gag L domains.     

The functionally exchangeable L domains in RSV and HIV-1 Gag direct particle release through pathways linked by Tsg101

The functionally exchangeable L domains of HIV-1 and Rous sarcoma virus (RSV) Gag bind Tsg101 and Nedd4, respectively. Tsg101 and Nedd4 function in endocytic trafficking, and studies show that expression of Tsg101 or Nedd4 fragments interfere with release of HIV-1 or RSV Gag, respectively, as virus-like particles (VLPs). To determine whether functional exchangeability reflects use of the same trafficking pathway, we tested the effect on RSV Gag release of co-expression with mutated forms of Vps4, Nedd4 and Tsg101. A dominant-negative mutant of Vps4A, an AAA ATPase required for utilization of endosomal sorting proteins that was shown previously to interfere with HIV-1 budding, also inhibited RSV Gag release, indicating that RSV uses the endocytic trafficking machinery, as does HIV. Nedd4 and Tsg101 interacted in the presence or absence of Gag and, through its binding of Nedd4, RSV Gag interacted with Tsg101. Deletion of the N-terminal region of Tsg101 or the HECT domain of Nedd4 did not prevent interaction; however, three-dimensional spatial imaging suggested that the interaction of RSV Gag with full-length Tsg101 and N-terminally truncated Tsg101 was not the same. Co-expression of RSV Gag with the Tsg101 C-terminal fragment interfered with VLP release minimally; however, a significant fraction of the released VLPs was tethered to each other. The results suggest that, while Tsg101 is not required for RSV VLP release, alterations in the protein interfere with VLP budding/fission events. We conclude that RSV and HIV-1 Gag direct particle release through independent ESCRT-mediated pathways that are linked through Tsg101-Nedd4 interaction.     

Influenza virus hemagglutinin and neuraminidase, but not the matrix protein, are required for assembly and budding of plasmid-derived virus-like particles

For influenza virus, we developed an efficient, noncytotoxic, plasmid-based virus-like particle (VLP) system to reflect authentic virus particles. This system was characterized biochemically by analysis of VLP protein composition, morphologically by electron microscopy, and functionally with a VLP infectivity assay. The VLP system was used to address the identity of the minimal set of viral proteins required for budding. Combinations of viral proteins were expressed in cells, and the polypeptide composition of the particles released into the culture media was analyzed. Contrary to previous findings in which matrix (M1) protein was considered to be the driving force of budding because M1 was found to be released copiously into the culture medium when M1 was expressed by using the vaccinia virus T7 RNA polymerase-driven overexpression system, in our noncytotoxic VLP system M1 was not released efficiently into the culture medium. Additionally, hemagglutinin (HA), when treated with exogenous neuraminidase (NA) or coexpressed with viral NA, could be released from cells independently of M1. Incorporation of M1 into VLPs required HA expression, although when M1 was omitted from VLPs, particles with morphologies similar to those of wild-type VLPs or viruses were observed. Furthermore, when HA and NA cytoplasmic tail mutants were included in the VLPs, M1 failed to be efficiently incorporated into VLPs, consistent with a model in which the glycoproteins control virus budding by sorting to lipid raft microdomains and recruiting the internal viral core components. VLP formation also occurred independently of the function of Vps4 in the multivesicular body pathway, as dominant-negative Vps4 proteins failed to inhibit influenza VLP budding.     

The HECT E3 ubiquitin ligase Rsp5 is important for ubiquitin homeostasis in yeast

The HECT E3 ubiquitin ligase Rsp5, a yeast member of the Nedd4 family, has been implicated in many different aspects of cell physiology. Here, we present evidence that Rsp5 function is important for ubiquitin homeostasis. Several observations suggest that ubiquitin is limiting in the rsp5-1 mutant. Reduced synthesis of ubiquitin appears to contribute to ubiquitin depletion. A transient inhibition of general protein synthesis is observed in a wildtype strain upon heat-shock. While the wildtype cells quickly recover from this transient arrest, the rsp5-1 cells remain arrested. This suggests that Rsp5 is important for recovery from heat-induced protein synthesis arrest. Our results suggest that rsp5 phenotypes should be interpreted with caution, since some of the phenotypes could be simply the result of ubiquitin limitation.     

Characterization of multiple multivesicular body sorting determinants within Sna3: a role for the ubiquitin ligase Rsp5

A subset of proteins that transit the endosomal system are directed into the intralumenal vesicles of multivesicular bodies (MVBs). MVB formation is critical for a variety of cellular functions including receptor down-regulation, viral budding, antigen presentation, and the generation of lysosome-related organelles. Entry of transmembrane proteins into the intralumenal vesicles of a MVB is a highly regulated process that is positively modulated by covalent modification of cargoes with ubiquitin. To identify additional MVB sorting signals, we examined the previously described ubiquitination-independent MVB cargo Sna3. Although Sna3 ubiquitination is not essential, Sna3 MVB sorting is positively modulated by its ubiquitination. Examination of MVB sorting determinants within a form of Sna3 lacking all lysine residues identified two critical regions: an amino-terminal tyrosine-containing region and a carboxyl-terminal PPAY motif. This PPAY motif interacts with the WW domains of the ubiquitin ligase Rsp5, and mutations in either the WW or, surprisingly, the HECT domains of Rsp5 negatively impacted MVB targeting of lysine-minus Sna3. These data indicate that Rsp5 function is required for MVB targeting of Sna3 in a capacity beyond cargo ubiquitination. These results uncover a series of determinants impacting Sna3 MVB sorting, including unexpected roles for Rsp5.     

The ubiquitin ligase Rsp5p is required for modification and sorting of membrane proteins into multivesicular bodies

Precursor forms of vacuolar proteins with transmembrane domains, such as the carboxypeptidase S Cps1p and the polyphosphatase Phm5p, are selectively sorted in endosomal compartments to vesicles that invaginate, budding into the lumen of the late endosomes, resulting in the formation of multivesicular bodies (MVBs). These proteins are then delivered to the vacuolar lumen following fusion of the MVBs with the vacuole. The sorting of Cps1p and Phm5p to these structures is mediated by ubiquitylation, and in doa4 mutant cells, which have reduced level of free ubiquitin, these proteins are missorted to the vacuolar membrane. A RING-finger ubiquitin ligase Tul1p has been shown to participate in the ubiquitylation of Cps1p and Phm5p. We show here that the HECT-ubiquitin ligase Rsp5p is also required for the ubiquitylation of these proteins, and therefore for their sorting to MVBs. Rsp5p is an essential ubiquitin ligase containing an N-terminal C2 domain followed by three WW domains, and a C-terminal catalytic HECT domain. In cells with low levels of Rsp5p (npi1 mutant cells), vacuolar hydrolases do not reach the vacuolar lumen and are instead missorted to the vacuolar membrane. The C2 domain and both the second and third WW domains of Rsp5p are important determinants for sorting to MVBs. Ubiquitylation of Cps1p was strongly reduced in the npi1 mutant strain and ubiquitylation was completely abolished in the npi1 tul1Delta double mutant. These data demonstrate that Rsp5p plays a novel and key role in intracellular trafficking, and extend the currently very short list of substrates ubiquitylated in vivo by several different ubiquitin ligases acting cooperatively.     

Defect of human immunodeficiency virus type 2 Gag assembly in Saccharomyces cerevisiae

We have previously shown that the expression of human immunodeficiency virus type 1 (HIV-1) Gag protein in Saccharomyces cerevisiae spheroplasts produces Gag virus-like particles (VLPs) at the plasma membrane, indicating that yeast has all the host factors necessary for HIV-1 Gag assembly. Here we expand the study by using diverse primate lentiviral Gags and show that yeast does not support the production of HIV-2 or simian immunodeficiency virus SIVmac Gag VLPs but allows the production of SIVagm and SIVmnd Gag VLPs. Particle budding was observed at the surfaces of cells expressing SIVagm and SIVmnd Gags, but cells expressing HIV-2 and SIVmac Gags showed only membrane-ruffling structures, although they were accompanied with electron-dense submembrane layers, suggesting arrest at an early stage of particle budding. Comparison of HIV-1 and HIV-2 Gag expression revealed broadly equivalent levels of intracellular Gag expression and Gag N-terminal myristoylation in yeast. Both Gags showed the same membrane-binding ability and were incorporated into lipid raft fractions at a physiological concentration of salt. HIV-2 Gag, however, failed to form a high-order multimer and easily dissociated from the membrane, phenomena which were not observed in higher eukaryotic cells. A series of chimeric Gags between HIV-1 and HIV-2 and Gag mutants with amino acid substitutions revealed that a defined region in helix 2 of HIV-2 MA (located on the membrane-binding surface of MA) affects higher-order Gag assembly and particle production in yeast. Together, these data suggest that yeast may lack a host factor(s) for HIV-2 and SIVmac Gag assembly.