Budding of retroviruses utilizing divergent L domains requires nucleocapsid

We recently reported that human immunodeficiency virus type 1 (HIV-1) carrying PTAP and LYPX(n)L L domains ceased budding when the nucleocapsid (NC) domain was mutated, suggesting a role for NC in HIV-1 release. Here we investigated whether NC involvement in virus release is a property specific to HIV-1 or a general requirement of retroviruses. Specifically, we examined a possible role for NC in the budding of retroviruses relying on divergent L domains and structurally homologous NC domains that harbor diverse protein sequences. We found that NC is critical for the release of viruses utilizing the PTAP motif whether it functions within its native Gag in simian immunodeficiency virus cpzGAB2 (SIVcpzGAB2) or SIVsmmE543 or when it is transplanted into the heterologous Gag protein of equine infectious anemia virus (EIAV). In both cases, virus release was severely diminished even though NC mutant Gag proteins retained the ability to assemble spherical particles. Moreover, budding-defective NC mutants, which displayed particles tethered to the plasma membrane, were triggered to release virus when access to the cell endocytic sorting complex required for transport pathway was restored (i.e., in trans expression of Nedd4.2s). We also examined the role of NC in the budding of EIAV, a retrovirus relying exclusively on the (L)YPX(n)L-type L domain. We found that EIAV late budding defects were rescued by overexpression of the isolated Alix Bro1 domain (Bro1). Bro1-mediated rescue of EIAV release required the wild-type NC. EIAV NC mutants lost interactions with Bro1 and failed to produce viruses despite retaining the ability to self-assemble. Together, our studies establish a role for NC in the budding of retroviruses harboring divergent L domains and evolutionarily diverse NC sequences, suggesting the utilization of a common conserved mechanism and/or cellular factor rather than a specific motif.     

Mutations in the PPPY motif of vesicular stomatitis virus matrix protein reduce virus budding by inhibiting a late step in virion release

The N terminus of the matrix (M) protein of vesicular stomatitis virus (VSV) and of other rhabdoviruses contains a highly conserved PPPY sequence (or PY motif) similar to the late (L) domains in the Gag proteins of some retroviruses. These L domains in retroviral Gag proteins are required for efficient release of virus particles. In this report, we show that mutations in the PPPY sequence of the VSV M protein reduce virus yield by blocking a late stage in virus budding. We also observed a delay in the ability of mutant viruses to cause inhibition of host gene expression compared to wild-type (WT) VSV. The effect of PY mutations on virus budding appears to be due to a block at a stage just prior to virion release, since electron microscopic examination of PPPA mutant-infected cells showed a large number of assembled virions at the plasma membrane trapped in the process of budding. Deletion of the glycoprotein (G) in addition to these mutations further reduced the virus yield to less than 1% of WT levels, and very few particles were assembled at the cell surface. This observation suggested that G protein aids in the initial stage of budding, presumably during the formation of the bud site. Overall, our results confirm that the PPPY sequence of the VSV M protein possesses L domain activity analogous to that of the retroviral Gag proteins.     

In vivo interference of Rous sarcoma virus budding by cis expression of a WW domain

For all enveloped viruses, the actual mechanism by which nascent virus particles separate or "pinch off" from the cell surface is largely unknown. In the case of retroviruses, the Gag protein drives the budding process, and the virus release step is directed by the late (L) assembly domain within Gag. A PPPPY motif within the L domain of Rous sarcoma virus (RSV) was previously characterized as being critical for the release of virions and shown to interact in vitro with the WW domain of Yes-associated protein (Yap). To determine whether WW domain-L domain interactions can occur in vivo, we attempted to interfere with the host cell machinery normally recruited to the site of budding by inserting this WW domain in different locations within Gag. At a C-terminal location, the WW(Yap) domain had no effect on budding, suggesting that the intervening I domains (which provide the major region of Gag-Gag interaction) prevent its access to the L domain. When positioned on the other side of the I domains closer to the L domain, the WW(Yap) domain resulted in a dramatic interference of particle release, and confocal microscopy revealed a block to budding on the plasma membrane. Budding was restored by attachment of the heterologous L domain of human immunodeficiency virus type 1 Gag, which does not bind WW(Yap). These findings suggest that cis expression of WW domains can interfere with RSV particle release in vivo via specific, high-affinity interactions at the site of assembly on the plasma membrane, thus preventing host factor accessibility to the L domain and subsequent virus-cell separation. In addition, they suggest that L domain-specific host factors function after Gag proteins begin to interact.     

A proline-rich motif within the matrix protein of vesicular stomatitis virus and rabies virus interacts with WW domains of cellular proteins: implications for viral budding

The matrix (M) protein of rhabdoviruses has been shown to play a key role in virus assembly and budding; however, the precise mechanism by which M mediates these processes remains unclear. We have associated a highly conserved, proline-rich motif (PPxY or PY motif, where P denotes proline, Y represents tyrosine, and x denotes any amino acid) of rhabdoviral M proteins with a possible role in budding mediated by the M protein. Point mutations that disrupt the PY motif of the M protein of vesicular stomatitis virus (VSV) have no obvious effect on membrane localization of M but instead lead to a decrease in the amount of M protein released from cells in a functional budding assay. Interestingly, the PPxY sequence within rhabdoviral M proteins is identical to that of the ligand which interacts with WW domains of cellular proteins. Indeed, results from two in vitro binding assays demonstrate that amino acids 17 through 33 and 29 through 44, which contain the PY motifs of VSV and rabies virus M proteins, respectively, mediate interactions with WW domains of specific cellular proteins. Point mutations that disrupt the consensus PY motif of VSV or rabies virus M protein result in a significant decrease in their ability to interact with the WW domains. These properties of the PY motif of rhabdovirus M proteins are strikingly analogous to those of the late (L) budding domain identified in the gag-specific protein p2b of Rous sarcoma virus. Thus, it is possible that rhabdoviruses may usurp host proteins to facilitate the budding process and that late stages in the budding process of rhabdoviruses and retroviruses may have features in common.     

Late domain function identified in the vesicular stomatitis virus M protein by use of rhabdovirus-retrovirus chimeras

Little is known about the mechanisms used by enveloped viruses to separate themselves from the cell surface at the final step of budding. However, small sequences in the Gag proteins of several retroviruses (L domains) have been implicated in this process. A sequence has been identified in the M proteins of rhabdoviruses that closely resembles the PPPPY motif in the L domain of Rous sarcoma virus (RSV), an avian retrovirus. To evaluate whether the PPPY sequence in vesicular stomatitis virus (VSV) M protein has an activity analogous to that of the retroviral sequence, M-Gag chimeras were characterized. The N-terminal 74 amino acids of the VSV (Indiana) M protein, including the PPPY motif, was able to replace the L domain of RSV Gag and allow the assembly and release of virus-like particles. Alanine substitutions in the VSV PPPY motif severely compromised the budding activity of this hybrid protein but not that of another chimera which also contained the RSV PPPPY sequence. We conclude that this VSV sequence is functionally homologous to the RSV L domain in promoting virus particle release, making this the first example of such an activity in a virus other than a retrovirus. Both the RSV and VSV motifs have been shown to interact in vitro with certain cellular proteins that contain a WW interaction module, suggesting that the L domains are sites of interaction with unknown host machinery involved in virus release.     

Identification of domains in gag important for prototypic foamy virus egress

Sequence motifs (L domains) have been described in viral structural proteins. Mutations in these lead to a defect at a late stage in virus assembly and budding. For several viruses, recruitment of an endosomal sorting complexes required for transport 1 subunit (Tsg101), a component of the class E vacuolar protein sorting (EVPS) machinery, is a prerequisite for virion budding. To effect this, Tsg101 interacts with the PT/SAP L domain. We have identified candidate L-domain motifs, PSAP, PPPI, and YEIL, in the prototypic foamy virus (PFV) Gag protein, based on their homology to known viral L domains. Mutation of the PSAP and PPPI motifs individually reduced PFV egress, and their combined mutation had an additive effect. When PSAP was mutated, residual infectious PFV release was unaffected by dominant negative Vps4 (an ATPase involved in the final stages of budding), and sensitivity to dominant negative Tsg101 was dramatically reduced, suggesting that the PSAP motif functions as a conventional class E VPS-dependent L domain. Consistent with this notion, yeast two-hybrid analysis showed a PSAP motif-dependent interaction between PFV Gag and Tsg101. Surprisingly, PFV release which is dependent on the PPPI motif was Vps4-independent and was partially inhibited by dominant negative Tsg101, suggesting that PPPI functions by an unconventional mechanism to facilitate PFV egress. Mutation of the YEIL sequence completely abolished particle formation and also reduced the rate of Gag processing by the viral protease, suggesting that the integrity of YEIL is required at an assembly step prior to budding and YEIL is not acting as an L domain.     

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.     

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.     

Functions of early (AP-2) and late (AIP1/ALIX) endocytic proteins in equine infectious anemia virus budding

The proline-rich L domains of human immunodeficiency virus 1 (HIV-1) and other retroviruses interact with late endocytic proteins during virion assembly and budding. In contrast, the YPDL L domain of equine infectious anemia virus (EIAV) is apparently unique in its reported ability to interact both with the mu2 subunit of the AP-2 adaptor protein complex and with ALG-2-interacting protein 1 (AIP1/Alix) protein factors involved in early and late endosome formation, respectively. To define further the mechanisms by which EIAV adapts vesicle trafficking machinery to facilitate virion production, we have examined the specificity of EIAV p9 binding to endocytic factors and the effects on virion production of alterations in early and late endocytic protein expression. The results of these studies demonstrated that (i) an approximately 300-residue region of AIP1/Alix-(409-715) was sufficient for binding to the EIAV YPDL motif; (ii) overexpression of AIP1/Alix or AP-2 mu2 subunit specifically inhibited YPDL-mediated EIAV budding; (iii) virion budding from a replication-competent EIAV variant with its L domain replaced by the HIV PTAP sequence was inhibited by wild type or mutant mu2 to a level similar to that observed when a dominant-negative mutant of Tsg101 was expressed; and (iv) overexpression or siRNA silencing of AIP1/Alix and AP-2 revealed additive suppression of YPDL-mediated EIAV budding. Taken together, these results indicated that both early and late endocytic proteins facilitate EIAV production mediated by either YPDL or PTAP L domains, suggesting a comprehensive involvement of endocytic factors in retroviral assembly and budding that can be accessed by distinct L domain specificities.     

Roles of the three L‐domains in β‐retrovirus budding

Retroviral Gag protein plays a critical role during the late stage of virus budding and possesses a so‐called L‐domain containing PT/SAP, PPxY, YxxL or FPIV motifs that are critical for efficient budding. Mason–Pfizer monkey virus (M‐PMV) contains PSAP, PPPY, and YADL sequences in Gag. This study was performed to investigate the roles of these three L‐domain‐like sequences in virus replication in three different cell lines, 293T, COS‐7 and HeLa cells. It was found that the PPxY motif plays an essential role in progeny virus production as a major L‐domain in all three cell lines. The PSAP sequence was shown to function as an additional L‐domain in HeLa cells and to promote efficient release of M‐PMV; however, this sequence was dispensable for M‐PMV production in 293T and COS‐7 cells, suggesting that the role of the PSAP motif as an L‐domain in M‐PMV budding is cell type‐dependent. Viruses possessing multiple L‐domains appear to change the L‐domain usage to replicate in various cells. On the other hand, the YADL motif was required for M‐PMV production as a transport signal of Gag to the plasma membrane, but not as an L‐domain.     

Both the PPPY and PTAP motifs are involved in human T-cell leukemia virus type 1 particle release

In retroviruses, the late (L) domain has been defined as a conserved motif in the Gag polyprotein precursor that, when mutated, leads to the emergence of virus particles that fail to pinch off from the plasma membrane. These domains have been observed to contain the PPXY, PTAP, or YXXL motifs. The deltaretroviruses, which include bovine leukemia virus (BLV) and human T-cell leukemia virus type 1 (HTLV-1) and HTLV-2, have a conserved PPPY motif in the C-terminal region of the matrix (MA) domain of Gag, while HTLV-1 also encodes a PTAP motif in MA. In this study, we analyzed the roles of the PPPY and PTAP motifs in the C terminus of MA in HTLV-1 particle release. Mutation of either motif (i.e., PPPY changed to APPY or PTAP changed to PTRP) reduced budding efficiencies. Particle buds and electron-dense regions of plasma membrane were observed by electron microscopy. When the locations of PPPY and PTAP were switched, particle release was eliminated. Intriguingly, the replacement of the PTAP motif with either the PPPY or YPDL motifs did not influence the release of virus particles, but the replacement of the PPPY motif with either PTAP or YPDL eliminated particle production. This indicates that the role that PPPY plays in HTLV-1 budding cannot be replaced with either PTAP or YPDL. A similar observation was made with the BLV PPPY motif. Finally, HTLV-1 particle release was found to be sensitive to proteasome inhibitors, implicating a role for ubiquitin in HTLV-1 budding. In summary, our observations indicate that (i) the PPPY motif plays a crucial role in virus budding and (ii) the PTAP motif plays a more subtle role in HTLV-1 particle release. Each of these motifs may play an important role in virus release from specific cell types and therefore be important in efficient virus spread and transmission.     

Rhabdoviruses and the cellular ubiquitin-proteasome system: a budding interaction

The matrix (M) proteins of vesicular stomatitis virus (VSV) and rabies virus (RV) play a key role in both assembly and budding of progeny virions. A PPPY motif (PY motif or late-budding domain) is conserved in the M proteins of VSV and RV. These PY motifs are important for virus budding and for mediating interactions with specific cellular proteins containing WW domains. The PY motif and flanking sequences of the M protein of VSV were used as bait to screen a mouse embryo cDNA library for cellular interactors. The mouse Nedd4 protein, a membrane-localized ubiquitin ligase containing multiple WW domains, was identified from this screen. Ubiquitin ligase Rsp5, the yeast homolog of Nedd4, was able to interact both physically and functionally with full-length VSV M protein in a PY-dependent manner. Indeed, the VSV M protein was multiubiquitinated by Rsp5 in an in vitro ubiquitination assay. To demonstrate further that ubiquitin may be involved in the budding process of rhabdoviruses, proteasome inhibitors (e.g., MG132) were used to decrease the level of free ubiquitin in VSV- and RV-infected cells. Viral titers measured from MG132-treated cells were reproducibly 10- to 20-fold lower than those measured from untreated control cells, suggesting that free ubiquitin is important for efficient virus budding. Last, release of a VSV PY mutant was not inhibited in the presence of MG132, signifying that the functional L domain of VSV is required for the inhibitory effect exhibited by MG132. These data suggest that the cellular ubiquitin-proteasome machinery is involved in the budding process of VSV and RV.     

Recruitment of the adaptor protein 2 complex by the human immunodeficiency virus type 2 envelope protein is necessary for high levels of virus release

The envelope (Env) protein of human immunodeficiency virus type 2 (HIV-2) and the HIV-1 Vpu protein stimulate the release of retroviral particles from human cells that restrict virus production, an activity that we call the enhancement of virus release (EVR). We have previously shown that two separate domains in the HIV-2 envelope protein are required for this activity: a glycine-tyrosine-x-x-hydrophobic (GYxxtheta) motif in the cytoplasmic tail and an unmapped region in the ectodomain of the protein. We here report that the cellular partner of the GYxxtheta motif is the adaptor protein complex AP-2. The mutation of this motif or the depletion of AP-2 by RNA interference abrogated EVR activity and changed the cellular distribution of the Env from a predominantly punctate pattern to a more diffuse distribution. Since the L domain of equine infectious anemia virus (EIAV) contains a Yxxtheta motif that interacts with AP-2, we used both wild-type and L domain-defective particles of HIV-1 and EIAV to examine whether the HIV-2 Env EVR function was analogous to L domain activity. We observed that the production of all particles was stimulated by HIV-2 Env or Vpu, suggesting that the L domain and EVR activities play independent roles in the release of retroviruses. Interestingly, we found that the cytoplasmic tail of the murine leukemia virus (MLV) Env could functionally substitute for the HIV-2 Env tail, but it did so in a manner that did not require a Yxxtheta motif or AP-2. The cellular distribution of the chimeric HIV-2/MLV Env was significantly less punctate than the wild-type Env, although confocal analysis revealed an overlap in the steady-state locations of the two proteins. Taken together, these data suggest that the essential GYxxtheta motif in the HIV-2 Env tail recruits AP-2 in order to direct Env to a cellular pathway or location that is necessary for its ability to enhance virus release but that an alternate mechanism provided by the MLV Env tail can functionally substitute.     

Late assembly domain function can exhibit context dependence and involves ubiquitin residues implicated in endocytosis

Retroviral Gag polyproteins contain regions that promote the separation of virus particles from the plasma membrane and from each other. These Gag regions are often referred to as late assembly (L) domains. The L domain of human immunodeficiency virus type 1 (HIV-1) is in the C-terminal p6(gag) domain and harbors an essential P(T/S)APP motif, whereas the L domains of oncoretroviruses are in the N-terminal half of the Gag precursor and have a PPXY core motif. We recently observed that L domains induce the ubiquitination of a minimal HIV-1 Gag construct and that point mutations which abolish L domain activity prevent Gag ubiquitination. In that study, a peptide from the Ebola virus L domain with overlapping P(T/S)APP and PPXY motifs showed exceptional activity in promoting Gag ubiquitination and the release of virus-like particles. We now show that a substitution which disrupts the PPXY motif but leaves the P(T/S)APP motif intact abolishes L domain activity in the minimal Gag context, but not in the context of a near full-length HIV-1 Gag precursor. Our results reveal that the P(T/S)APP motif does not function autonomously and indicate that the HIV-1 nucleocapsid-p1 region, which is proximal to p6(gag), can cooperate with the conserved L domain core motif. We have also examined the effects of ubiquitin mutants on virus-like particle production, and the results indicate that residues required for the endocytosis function of ubiquitin are also involved in virus budding.     

Heterologous late-domain sequences have various abilities to promote budding of human immunodeficiency virus type 1

Retroviral late (L) domains present within Gag act in conjunction with cellular proteins to efficiently release virions from the surface of the cell. Three different critical core sequences have been identified as required elements for L-domain function: PPPY, PTAP (also PSAP), and YPDL, with different retroviruses utilizing one or two of these core sequences. The human immunodeficiency virus type 1 (HIV-1) L domain is centered around a PTAP sequence in the p6 region of Gag. To assess the ability of heterologous L-domain sequences to be functionally interchanged for those in full-length HIV-1, we produced a series of constructs that replaced PTAP-containing p6(Gag) sequences with those of PPPY- or YPDL-based L domains. While previous studies had found that L domains are interchangeable in other retroviruses, most of the sequences introduced into p6(Gag) failed to substitute for PTAP-mediated L-domain function. One exception was the 11-amino-acid p2b sequence of Rous sarcoma virus (RSV) Gag, which could fully restore HIV-1 budding, while a PPPPY sequence exchange alone did not. This suggests that the RSV L domain consists of more than simply its core L-domain sequence. The HIV-p2b chimera was as infectious as the wild type, produced normal virions, and was sensitive to proteasome inhibitors. These results show that L-domain sequences are not necessarily interchangeable. Thus, HIV-1 Gag might have a more stringent requirement for L-domain function than the other retroviruses previously studied.     

The Mason-Pfizer monkey virus PPPY and PSAP motifs both contribute to virus release

Late (L) domains are required for the efficient release of several groups of enveloped viruses. Three amino acid motifs have been shown to provide L-domain function, namely, PPXY, PT/SAP, or YPDL. The retrovirus Mason-Pfizer monkey virus (MPMV) carries closely spaced PPPY and PSAP motifs. Mutation of the PPPY motif results in a complete loss of virus release. Here, we show that the PSAP motif acts as an additional L domain and promotes the efficient release of MPMV but requires an intact PPPY motif to perform its function. Examination of HeLaP4 cells expressing PSAP mutant virus by electron microscopy revealed mostly late budding structures and chains of viruses accumulating at the cell surface with little free virus. In the case of the PPPY mutant virus, budding appeared to be mostly arrested at an earlier stage before induction of membrane curvature. The cellular protein TSG101, which interacts with the human immunodeficiency virus type 1 (HIV-1) PTAP L domain, was packaged into MPMV in a PSAP-dependent manner. Since TSG101 is crucial for HIV-1 release, this result suggests that the Gag-TSG101 interaction is responsible for the virus release function of the MPMV PSAP motif. Nedd4, which has been shown to interact with viral PPPY motifs, was also detected in MPMV particles, albeit at much lower levels. Consistent with a role of VPS4A in the budding of both PPPY and PTAP motif-containing viruses, the overexpression of ATPase-defective GFP-VPS4A fusion proteins blocked both wild-type and PSAP mutant virus release.     

Role of ESCRT-I in retroviral budding

Retroviral late-budding (L) domains are required for the efficient release of nascent virions. The three known types of L domain, designated according to essential tetrapeptide motifs (PTAP, PPXY, or YPDL), each bind distinct cellular cofactors. We and others have demonstrated that recruitment of an ESCRT-I subunit, Tsg101, a component of the class E vacuolar protein sorting (VPS) machinery, is required for the budding of viruses, such as human immunodeficiency virus type 1 (HIV-1) and Ebola virus, that encode a PTAP-type L domain, but subsequent events remain undefined. In this study, we demonstrate that VPS28, a second component of ESCRT-I, binds to a sequence close to the Tsg101 C terminus and is therefore recruited to the plasma membrane by HIV-1 Gag. In addition, we show that Tsg101 exhibits a multimerization activity. Using a complementation assay in which Tsg101 is artificially recruited to sites of HIV-1 assembly, we demonstrate that the integrity of the VPS28 binding site within Tsg101 is required for particle budding. In addition, mutation of a putative leucine zipper or residues important for Tsg101 multimerization also impairs the ability of Tsg101 to support HIV-1 budding. A minimal multimerizing Tsg101 domain is a dominant negative inhibitor of PTAP-mediated HIV-1 budding but does not inhibit YPDL-type or PPXY-type L-domain function. Nevertheless, YDPL-type L-domain activity is inhibited by expression of a catalytically inactive mutant of the class E VPS ATPase VPS4. These results indicate that all three classes of retroviral L domains require a functioning class E VPS pathway in order to effect budding. However, the PTAP-type L domain appears to be unique in its requirement for an intact, or nearly intact, ESCRT-I complex.     

Late domain-dependent inhibition of equine infectious anemia virus budding

The Gag proteins of a number of different retroviruses contain late or L domains that promote the release of virions from the plasma membrane. Three types of L domains have been identified to date: Pro-Thr-Ala-Pro (PTAP), Pro-Pro-X-Tyr, and Tyr-Pro-Asp-Leu. It has previously been demonstrated that overexpression of the N-terminal, E2-like domain of the endosomal sorting factor TSG101 (TSG-5') inhibits human immunodeficiency virus type 1 (HIV-1) release but does not affect the release of the PPPY-containing retrovirus murine leukemia virus (MLV), whereas overexpression of the C-terminal portion of TSG101 (TSG-3') potently disrupts both HIV-1 and MLV budding. In addition, it has been reported that, while the release of a number of retroviruses is disrupted by proteasome inhibitors, equine infectious anemia virus (EIAV) budding is not affected by these agents. In this study, we tested the ability of TSG-5', TSG-3', and full-length TSG101 (TSG-F) overexpression, a dominant negative form of the AAA ATPase Vps4, and proteasome inhibitors to disrupt the budding of EIAV particles bearing each of the three types of L domain. The results indicate that (i) inhibition by TSG-5' correlates with dependence on PTAP; (ii) the release of wild-type EIAV (EIAV/WT) is insensitive to TSG-3', whereas this C-terminal TSG101 fragment potently impairs the budding of EIAV when it is rendered PTAP or PPPY dependent; (iii) budding of all EIAV clones is blocked by dominant negative Vps4; and (iv) EIAV/WT release is not impaired by proteasome inhibitors, while EIAV/PTAP and EIAV/PPPY release is strongly disrupted by these compounds. These findings highlight intriguing similarities and differences in host factor utilization by retroviral L domains and suggest that the insensitivity of EIAV to proteasome inhibitors is conferred by the L domain itself and not by determinants in Gag outside the L domain.     

Retroviruses have differing requirements for proteasome function in the budding process

Proteasome inhibitors reduce the budding of human immunodeficiency virus types 1 (HIV-1) and 2, simian immunodeficiency virus, and Rous sarcoma virus. To investigate this effect further, we examined the budding of other retroviruses from proteasome inhibitor-treated cells. The viruses tested differed in their Gag organization, late (L) domain usage, or assembly site from those previously examined. We found that proteasome inhibition decreased the budding of murine leukemia virus (plasma membrane assembly, PPPY L domain) and Mason-Pfizer monkey virus (cytoplasmic assembly, PPPY L domain), similar to the reduction observed for HIV-1. Thus, proteasome inhibitors can affect the budding of a virus that assembles within the cytoplasm. However, the budding of mouse mammary tumor virus (MMTV; cytoplasmic assembly, unknown L domain) was unaffected by proteasome inhibitors, similar to the proteasome-independent budding previously observed for equine infectious anemia virus (plasma membrane assembly, YPDL L domain). Examination of MMTV particles detected Gag-ubiquitin conjugates, demonstrating that an interaction with the ubiquitination system occurs during assembly, as previously found for other retroviruses. For all of the cell lines tested, the inhibitor treatment effectively inactivated proteasomes, as measured by the accumulation of polyubiquitinated proteins. The ubiquitination system was also inhibited, as evidenced by the loss of monoubiquitinated histones from treated cells. These results and those from other viruses show that proteasome inhibitors reduce the budding of viruses that utilize either a PPPY- or PTAP-based L domain and that this effect does not depend on the assembly site or the presence of monoubiquitinated Gag in the virion.     

Defects in human immunodeficiency virus budding and endosomal sorting induced by TSG101 overexpression

Retrovirus budding is greatly stimulated by the presence of Gag sequences known as late or L domains. The L domain of human immunodeficiency virus type 1 (HIV-1) maps to a highly conserved Pro-Thr-Ala-Pro (PTAP) sequence in the p6 domain of Gag. We and others recently observed that the p6 PTAP motif interacts with the cellular endosomal sorting protein TSG101. Consistent with a role for TSG101 in virus release, we demonstrated that overexpressing the N-terminal, Gag-binding domain of TSG101 (TSG-5') suppresses HIV-1 budding by blocking L domain function. To elucidate the role of TSG101 in HIV-1 budding, we evaluated the significance of the binding between Gag and TSG-5' on the inhibition of HIV-1 release. We observed that a mutation in TSG-5' that disrupts the Gag/TSG101 interaction suppresses the ability of TSG-5' to inhibit HIV-1 release. We also determined the effect of overexpressing a panel of truncated TSG101 derivatives and full-length TSG101 (TSG-F) on virus budding. Overexpressing TSG-F inhibits HIV-1 budding; however, the effect of TSG-F on virus release does not require Gag binding. Furthermore, overexpression of the C-terminal portion of TSG101 (TSG-3') potently inhibits budding of not only HIV-1 but also murine leukemia virus. Confocal microscopy data indicate that TSG-F and TSG-3' overexpression induces an aberrant endosome phenotype; this defect is dependent upon the C-terminal, Vps-28-binding domain of TSG101. We propose that TSG-5' suppresses HIV-1 release by binding PTAP and blocking HIV-1 L domain function, whereas overexpressing TSG-F or TSG-3' globally inhibits virus release by disrupting the cellular endosomal sorting machinery. These results highlight the importance of TSG101 and the endosomal sorting pathway in virus budding and suggest that inhibitors can be developed that, like TSG-5', target HIV-1 without disrupting endosomal sorting.