Lysines close to the Rous sarcoma virus late domain critical for budding

Post-Translational Modifications of Retroviral HIV-1 Gag Precursors: An Overview of Their Biological Role

Protein post-translational modifications (PTMs) play key roles in eukaryotes since they finely regulate numerous mechanisms used to diversify the protein functions and to modulate their signaling networks. Besides, these chemical modifications also take part in the viral hijacking of the host, and also contribute to the cellular response to viral infections. All domains of the human immunodeficiency virus type 1 (HIV-1) Gag precursor of 55-kDa (Pr55^Gag), which is the central actor for viral RNA specific recruitment and genome packaging, are post-translationally modified. In this review, we summarize the current knowledge about HIV-1 Pr55^Gag PTMs such as myristoylation, phosphorylation, ubiquitination, sumoylation, methylation, and ISGylation in order to figure out how these modifications affect the precursor functions and viral replication. Indeed, in HIV-1, PTMs regulate the precursor trafficking between cell compartments and its anchoring at the plasma membrane, where viral assembly occurs. Interestingly, PTMs also allow Pr55^Gag to hijack the cell machinery to achieve viral budding as they drive recognition between viral proteins or cellular components such as the ESCRT machinery. Finally, we will describe and compare PTMs of several other retroviral Gag proteins to give a global overview of their role in the retroviral life cycle.

L-Lysine as the Molecule Influencing Selective Brain Activity in Pain-Induced Behavior of Rats

Lysine-rich proteins are some of the most important proteins of neurons and it has become necessary to investigate the possible role of L-lysine as a brain functioning regulator. The purpose of our study is to identify the characteristics and the mechanisms of L-lysine effects on the different types of pain-induced behavior in the stimulation of tail and foot-shock models in 210 adult male Wistar rats. L-lysine was administered in intraperitoneal or intracerebroventricular injections in doses of 0.15-50.0 µg/kg. When a tail is irritated, L-lysine was found to enhance pain sensitivity and affective defense after both intraperitoneal and intracerebroventricular administration. In the case of unavoidable painful irritation of a pair of rats with both types of L-lysine administration, there was no direct correlation of the severity of pain with defensive reactions and outbursts of aggression. This indicates a more complex integration of the activity of brain structures in this situation of animal interaction, which was confirmed by the results of the direct amino acid action on the periventricular brain structures. Our findings show that L-lysine influences the selective brain activity in dependence on the biological significance of pain-induced behavior.

Growing functions of the ESCRT machinery in cell biology and viral replication

The vast expansion in recent years of the cellular processes promoted by the endosomal sorting complex required for transport (ESCRT) machinery has reinforced its identity as a modular system that uses multiple adaptors to recruit the core membrane remodelling activity at different intracellular sites and facilitate membrane scission. Functional connections to processes such as the aurora B-dependent abscission checkpoint also highlight the importance of the spatiotemporal regulation of the ESCRT machinery. Here, we summarise the role of ESCRTs in viral budding, and what we have learned about the ESCRT pathway from studying this process. These advances are discussed in the context of areas of cell biology that have been transformed by research in the ESCRT field, including cytokinetic abscission, nuclear envelope resealing and plasma membrane repair.

Virus budding and the ESCRT pathway

Enveloped viruses escape infected cells by budding through limiting membranes. In the decade since the discovery that HIV recruits cellular ESCRT (endosomal sorting complexes required for transport) machinery to facilitate viral budding, this pathway has emerged as the major escape route for enveloped viruses. In cells, the ESCRT pathway catalyzes analogous membrane fission events required for the abscission stage of cytokinesis and for a series of "reverse topology" vesiculation events. Studies of enveloped virus budding are therefore providing insights into the complex cellular mechanisms of cell division and membrane protein trafficking (and vice versa). Here, we review how viruses mimic cellular recruiting signals to usurp the ESCRT pathway, discuss mechanistic models for ESCRT pathway functions, and highlight important research frontiers.

CRM1-dependent trafficking of retroviral Gag proteins revisited

We analyzed the nuclear trafficking ability of Gag proteins from six retroviral genera. Contrary to a previous report, human immunodeficiency virus type 1 (HIV-1) Gag showed no propensity to cycle through the nucleus. The only Gag protein that displayed CRM1-dependent nuclear cycling was that of Rous sarcoma virus (RSV). Surprisingly, this cycling could be eliminated without compromising infectivity by replacing the RSV Gag N-terminal matrix (MA) domain with HIV MA.

Role of ubiquitin in parainfluenza virus 5 particle formation

Ubiquitin is important for the budding of many retroviruses and other enveloped viruses, but the precise role of ubiquitin in virus budding remains unclear. Here, we characterized the ubiquitination of the matrix (M) protein of a paramyxovirus, parainfluenza virus 5 (PIV5). The PIV5 M protein (but not the PIV5 nucleocapsid protein) was found to be targeted for monoubiquitination in transfected mammalian cells. Major sites of ubiquitin attachment identified by mass spectrometry analysis were lysine residues at amino acid positions 79/80, 130, and 247. The cumulative mutation of lysine residues 79, 80, and 130 to arginines led to an altered pattern of M protein ubiquitination and impaired viruslike particle (VLP) production. However, the cumulative mutation of lysine residues 79, 80, 130, and 247 to arginines restored M protein ubiquitination and VLP production, suggesting that ubiquitin is attached to alternative sites on the M protein when the primary ones have been removed. Additional lysine residues were targeted for mutagenesis based on the UbiPred algorithm. An M protein with seven lysine residues changed to arginines exhibited altered ubiquitination and poor VLP production. A recombinant virus encoding an M protein with seven lysines mutated was generated, and this virus exhibited a 6-fold-reduced maximum titer, with the defect being attributed mainly to the budding of noninfectious particles. The recombinant virus was assembly deficient, as judged by the redistribution of viral M and hemagglutinin-neuraminidase proteins in infected cells. Similar assembly defects were observed for the wild-type (wt) virus after treatment with a proteasome inhibitor. Collectively, these findings suggest that the monoubiquitination of the PIV5 M protein is important for proper virus assembly and for the budding of infectious particles.

Viral takeover of the host ubiquitin system

Like the other more well-characterized post-translational modifications (phosphorylation, methylation, acetylation, acylation, etc.), the attachment of the 76 amino acid ubiquitin (Ub) protein to substrates has been shown to govern countless cellular processes. As obligate intracellular parasites, viruses have evolved the capability to commandeer many host processes in order to maximize their own survival, whether it be to increase viral production or to ensure the long-term survival of latently infected host cells. The first evidence that viruses could usurp the Ub system came from the DNA tumor viruses and Adenoviruses, each of which use Ub to dysregulate the host cell cycle (Scheffner et al., 1990; Querido et al., 2001). Today, the list of viruses that utilize Ub includes members from almost every viral class, encompassing both RNA and DNA viruses. Among these, there are examples of Ub usage at every stage of the viral life cycle, involving both ubiquitination and de-ubiquitination. In addition to viruses that merely modify the host Ub system, many of the large DNA viruses encode their own Ub modifying machinery. In this review, we highlight the latest discoveries regarding the myriad ways that viruses utilize Ub to their advantage.

How ubiquitin functions with ESCRTs

The endosomal-sorting complex required for transport (ESCRT) apparatus has multiple ubiquitin (Ub)-binding domains and participates in a wide variety of cellular processes. Many of these ESCRT-dependent processes are keenly regulated by Ub, which serves as a lysosomal-sorting signal for membrane proteins targeted into multivesicular bodies (MVBs) and which may serve as a mediator of viral budding from the cell surface. Hints that both ESCRTs and Ub work together in the processes such as cytokinesis, transcription and autophagy are beginning to emerge. Here, we explore the relationship between ESCRTs and Ub in MVB sorting and viral budding.

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.

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.

The release of an enveloped virus particle from an infected cell requires the separation of the viral and cell membranes. Many enveloped viruses accomplish this by parasitizing a set of cellular proteins, termed the ESCRT pathway, that normally separates cellular membranes from each other. In some cases, viral structural proteins encode peptides motifs that bind directly to, and thereby recruit, the ESCRT machinery. Alternatively, viruses can recruit enzymes, termed ubiquitin ligases, that bind to other proteins, and catalyze the addition of ubiquitin to them. It has, heretofore, been somewhat unclear precisely how the recruitment of ubiquitin ligases leads to the engagement of the ESCRT machinery. We show that the simple recruitment of a fragment of a ubiquitin ligase that is responsible for the addition of ubiquitin to other proteins is sufficient to drive virus particle release, even when it is not possible to attach ubiquitin to viral proteins. Paradoxically, we also found that simple attachment of ubiquitin to the same viral protein can also drive particle release. These results show that there is flexibility in the ways in which the ESCRT machinery can be recruited and how ubiquitin can be co-opted to enable this.

Conserved motifs within Ebola and Marburg virus VP40 proteins are important for stability, localization, and subsequent budding of virus-like particles

The filovirus VP40 protein is capable of budding from mammalian cells in the form of virus-like particles (VLPs) that are morphologically indistinguishable from infectious virions. Ebola virus VP40 (eVP40) contains well-characterized overlapping L domains, which play a key role in mediating efficient virus egress. L domains represent only one component required for efficient budding and, therefore, there is a need to identify and characterize additional domains important for VP40 function. We demonstrate here that the (96)LPLGVA(101) sequence of eVP40 and the corresponding (84)LPLGIM(89) sequence of Marburg virus VP40 (mVP40) are critical for efficient release of VP40 VLPs. Indeed, deletion of these motifs essentially abolished the ability of eVP40 and mVP40 to bud as VLPs. To address the mechanism by which the (96)LPLGVA(101) motif of eVP40 contributes to egress, a series of point mutations were introduced into this motif. These mutants were then compared to the eVP40 wild type in a VLP budding assay to assess budding competency. Confocal microscopy and gel filtration analyses were performed to assess their pattern of intracellular localization and ability to oligomerize, respectively. Our results show that mutations disrupting the (96)LPLGVA(101) motif resulted in both altered patterns of intracellular localization and self-assembly compared to wild-type controls. Interestingly, coexpression of either Ebola virus GP-WT or mVP40-WT with eVP40-DeltaLPLGVA failed to rescue the budding defective eVP40-DeltaLPLGVA mutant into VLPs; however, coexpression of eVP40-WT with mVP40-DeltaLPLGIM successfully rescued budding of mVP40-DeltaLPLGIM into VLPs at mVP40-WT levels. In sum, our findings implicate the LPLGVA and LPLGIM motifs of eVP40 and mVP40, respectively, as being important for VP40 structure/stability and budding.

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.

Hepatitis B virus replication and release are independent of core lysine ubiquitination

Ubiquitin conjugation to lysine residues regulates a variety of protein functions, including endosomal trafficking and degradation. While ubiquitin plays an important role in the release of many viruses, the requirement for direct ubiquitin conjugation to viral structural proteins is less well understood. Some viral structural proteins require ubiquitin ligase activity, but not ubiquitin conjugation, for efficient release. Recent evidence has shown that, like other viruses, hepatitis B virus (HBV) requires a ubiquitin ligase for release from the infected cell. The HBV core protein contains two lysine residues (K7 and K96), and K96 has been suggested to function as a potential ubiquitin acceptor site based on the fact that previous studies have shown that mutation of this amino acid to alanine blocks HBV release. We therefore reexamined the potential connection between core lysine ubiquitination and HBV replication, protein trafficking, and virion release. In contrast to alanine substitution, we found that mutation of K96 to arginine, which compared to alanine is more conserved but also cannot mediate ubiquitin conjugation, does not affect either virus replication or virion release. We also found that the core lysine mutants display wild-type sensitivity to the antiviral activity of interferon, which demonstrates that ubiquitination of core lysines does not mediate the interferon-induced disruption of HBV capsids. However, mutation of K96 to arginine alters the nuclear-cytoplasmic distribution of core, leading to an accumulation in the nucleolus. In summary, these studies demonstrate that although ubiquitin may regulate the HBV replication cycle, these mechanisms function independently of direct lysine ubiquitination of core protein.

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.

The Mechanism of Budding of Retroviruses From Cell Membranes

Retroviruses have evolved a mechanism for the release of particles from the cell membrane that appropriates cellular protein complexes, referred to as ESCRT-I, -II, -III, normally involved in the biogenesis of multivesicular bodies. Three different classes of late assembly (L) domains encoded in Gag, with core sequences of PPXY, PTAP, and YPXL, recruit different components of the ESCRT machinery to form a budding complex for virus release. Here, we highlight recent progress in identifying the role of different ESCRT complexes in facilitating budding, ubiquitination, and membrane targeting of avian sarcoma and leukosis virus (ASLV) and human immunodeficiency virus, type 1 (HIV-1). These findings show that retroviruses adopt parallel budding pathways by recruiting different host factors from common cellular machinery for particle release.

Avian sarcoma virus and human immunodeficiency virus, type 1 use different subsets of ESCRT proteins to facilitate the budding process

Members of the Nedd4 family of E3 ubiquitin ligases bind the L domain in avian sarcoma virus (ASV) Gag and facilitate viral particle release. Translational fusion of ASV Gag with an L domain deletion (Deltap2b) to proteins that comprise ESCRT-I, -II, and -III (the endocytic sorting complexes required for transport) rescued both Gag ubiquitination and particle release from cells. The ESCRT-I factors Vps37C or Tsg101 were more effective in rescue of Gag/Deltap2b budding than the ESCRT-II factor Eap20 or the ESCRT-III component CHMP6. Thus ESCRT components can substitute for Nedd4 family members in ASV Gag release. Unlike wild type, ASV Gag/Deltap2b -ESCRT chimeras failed to co-immunoprecipitate with co-expressed hemagglutinin-tagged Nedd4, indicating that Nedd4 was not stably associated with these Gag fusions. Release of the Gag-ESCRT-I or -II fusions was inhibited by a dominant negative mutant of Vps4 ATPase similar to wild type ASV Gag. In contrast to ASV Gag, HIV-1 Gag containing an L domain inactivating mutation (P7L) was efficiently rescued by fusion to a component of ESCRT-III (Chmp6) but not ESCRT-II (Eap20). Depletion of the endogenous pool of Eap20 (ESCRT-II) had little effect on HIV-1 Gag release but blocked ASV Gag release. In contrast, depletion of the endogenous pool of Vps37C (ESCRT-I) had little effect on ASV but blocked HIV-1 Gag release. Furthermore, an N-terminal fragment of Chmp6 inhibited both HIV-1 and ASV Gag release in a dominant negative manner. Taken together, these results indicate that ASV and HIV-1 Gag utilize different combinations of ESCRT proteins to facilitate the budding process, although they share some common elements.

Functional replacement of a retroviral late domain by ubiquitin fusion

Retroviral Gag polyprotein precursors are both necessary and sufficient for the assembly and release of virus-like particles (VLPs) from infected cells. It is well established that small Gag-encoded motifs, known as late domains, promote particle release by interacting with components of the cellular endosomal sorting and ubiquitination machinery. The Gag proteins of a number of different retroviruses are ubiquitinated; however, the role of Gag ubiquitination in particle egress remains undefined. In this study, we investigated this question by using a panel of equine infectious anemia virus (EIAV) Gag derivatives bearing the wild-type EIAV late domain, heterologous retroviral late domains or no late domain. Ubiquitin was fused in cis to the C-termini of these Gag polyproteins, and the effects on VLP budding were measured. Remarkably, fusion of ubiquitin to EIAV Gag lacking a late domain (EIAV/DeltaYPDL-Ub) largely rescued VLP release. We also determined the effects of ubiquitin fusion on the sensitivity of particle release to budding inhibitors and to depletion of key endosomal sorting factors. Ubiquitin fusion rendered EIAV/DeltaYPDL-Ub sensitive to depletion of cellular endosomal sorting factors Tsg101 and Alix and to overexpression of dominant-negative fragments of Tsg101 and Alix. These findings demonstrate that ubiquitin can functionally compensate for the absence of a retroviral late domain and provide insights into the host-cell machinery engaged by ubiquitin during particle egress.

Cooperative role of the MHR and the CA dimerization helix in the maturation of the functional retrovirus capsid

The second helix in the C-terminal domain of retroviral capsid (CA) protein functions as the site of dimerization between subunits in capsid assembly and is believed to participate in a unique interface between Gag molecules in immature particles. This study reports isolation of two substitutions in the dimerization helix of Rous sarcoma virus CA protein that have the ability to suppress lethal defects in core maturation imposed by alterations to the major homology region (MHR) motif just upstream. Together with two previously published suppressors, these define an extended region of the dimerization helix that is unlikely to contribute directly to CA-CA contacts but whose assembly-competence may be strongly affected by conformation. The broad-spectrum suppression and temperature-sensitivity exhibited by some mutants argues that they act through modulation of protein conformation. These findings provide important biological evidence in support of a significant conformational change involving the dimerization helix and the MHR during maturation.

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.

Ubiquitin-dependent virus particle budding without viral protein ubiquitination

An essential step in the release of an extracellular enveloped virus particle is a budding event that ultimately separates virion and host cell membranes. For many enveloped viruses, membrane fission requires the recruitment of the class E vacuolar protein sorting (VPS) machinery by short, virally encoded peptide sequences termed "late-budding" or "L" domains. Some L-domain peptide sequences (e.g., PSAP) bind directly to components of class E VPS machinery, whereas others (e.g., PPxY) access it indirectly by recruiting ubiquitin ligases. Additionally, ubiquitin itself is known to be generally important for the fission of virion from cellular membranes, and because ubiquitination of cellular transmembrane proteins can signal the recruitment of class E machinery, a popular model is that deposition of ubiquitin on viral structural proteins mediates class E machinery recruitment. To test this model, we took advantage of a retroviral Gag protein from the prototypic foamy virus (PFV) that is almost devoid of ubiquitin acceptors, and we engineered it to generate extracellular virus-like particles in the complete absence of other viral proteins. Notably, we found that particle budding, induced by a class E VPS machinery-binding L domain (PSAP), proceeded efficiently in the absence of ubiquitin acceptors in PFV Gag. Moreover, when particle release was engineered to be dependent on a viral PPXY motif, the requirement for a catalytically active ubiquitin ligase was maintained, irrespective of the presence or absence of ubiquitin acceptor sites in PFV Gag. Thus, in this model system, ubiquitin conjugation to transacting factors, not viral proteins, appears critical for ubiquitin-dependent enveloped viral particle release.

The role of ubiquitin in retroviral egress

HIV and many other enveloped viruses encode a late budding domain (L-domain) that recruits the cellular machinery that mediates the separation of the nascent virion from the infected cell. The ubiquitin-proteasome system has been implicated in the L-domain activity, but the exact role of ubiquitin transfer and ubiquitin-binding proteins in the last step of viral replication remains elusive. It is now widely accepted that the class E vacuolar protein sorting pathway mediates both viral budding and vesicle budding into the multivesicular bodies and, remarkably, both budding events share the same topology and similar requirements for ubiquitin. In this review, the role of ubiquitin in viral budding is discussed in the light of recent advances in the understanding of the cellular mechanisms that assist the last step of HIV-1 release.

The role of WWP1-Gag interaction and Gag ubiquitination in assembly and release of human T-cell leukemia virus type 1

The PPPY motif in the matrix (MA) domain of human T-cell leukemia virus type 1 (HTLV-1) Gag associates with WWP1, a member of the HECT domain containing family of E3 ubiquitin ligases. Mutation of the PPPY motif arrests particle assembly at an early stage and abolishes ubiquitination of MA. Similar effects are seen when Gag is expressed in the presence of a truncated form of WWP1 that lacks the catalytically active HECT domain (C2WW). To understand the role of ubiquitination in budding, we mutated the four lysines in MA to arginines and identified lysine 74 as the unique site of ubiquitination. Virus-like particles produced by the K74R mutant did not contain ubiquitinated MA and showed a fourfold reduction in the release of infectious particles. Furthermore, the K74R mutation rendered assembly hypersensitive to C2WW inhibition; K74R Gag budding was inhibited at significantly lower levels of expression of C2WW compared with wild-type Gag. This finding indicates that the interaction between Gag and WWP1 is required for functions other than Gag ubiquitination. Additionally, we show that the PPPY(-) mutant Gag exerts a strong dominant-negative effect on the budding of wild-type Gag, further supporting the importance of recruitment of WWP1 to achieve particle assembly.

Host factors exploited by retroviruses

Retroviruses make a long and complex journey from outside the cell to the nucleus in the early stages of infection, and then an equally long journey back out again in the late stages of infection. Ongoing efforts are identifying an enormous array of cellular proteins that are used by the viruses in the course of their travels. These host factors are potential new targets for therapeutic intervention.

Genetic Studies of the beta-hairpin loop of Rous sarcoma virus capsid protein

The first few residues of the Rous sarcoma virus (RSV) CA protein comprise a structurally dynamic region that forms part of a Gag-Gag interface in immature virus particles. Dissociation of this interaction during maturation allows refolding and formation of a beta-hairpin structure important for assembly of CA monomers into the mature capsid shell. A consensus binding site for the cellular Ubc9 protein was previously identified within this region, suggesting that binding of Ubc9 and subsequent small ubiquitin-like modifier protein 1 (SUMO-1) modification of CA may play a role either in regulating the assembly activity of CA in immature particles or mature cores or in controlling postentry function(s) during the establishment of infection. In the present study, mutations designed to eliminate the consensus binding site were used to dissect the potentially overlapping functions of these residues. The resulting replication defects could not be traced to a failure to form particles of normal composition but, rather, to a deficit in genome replication. Genetic suppressors of two detrimental beta-hairpin mutations improved infectivity without restoring the consensus site or creating a novel one elsewhere. Optimal restoration of infectivity to a Lys-to-Arg mutant required a combination of secondary changes, one on the surface of each domain of CA. Rather than arguing for a critical role of Ubc9 and SUMO in RSV replication, these findings provide strong support for a structural role of the N-terminal residues and a particularly striking example of long-range interactions between regions of CA in achieving a functional core competent for genome replication.

Cumulative mutations of ubiquitin acceptor sites in human immunodeficiency virus type 1 gag cause a late budding defect

The p6 domain of human immunodeficiency virus type 1 (HIV-1) Gag has long been known to be monoubiquitinated. We have previously shown that the MA, CA, and NC domains are also monoubiquitinated at low levels (E. Gottwein and H. G. Krausslich, J. Virol. 79:9134-9144, 2005). While several lines of evidence support a role for ubiquitin in virus release, the relevance of Gag ubiquitination is unclear. To directly address the function of Gag ubiquitination, we constructed Gag variants in which lysine residues in the NC, SP2, and p6 domains were mutated to arginine either in individual domains or in combination. Using these mutants, we showed that in addition to MA, CA, NC, and p6, SP2 is also mono- or di-ubiquitinated at levels comparable to those of the other domains. Replacement of all lysine residues in only one of the domains had minor effects on virus release, while cumulative mutations in NC and SP2 or in NC and p6 resulted in an accumulation of late budding structures, as observed by electron microscopy analysis. Strikingly, replacement of all lysine residues downstream of CA led to a significant reduction in virus release kinetics and a fivefold accumulation of late viral budding structures compared to wild-type levels. These results indicate that ubiquitination of lysine residues in Gag in the vicinity of the viral late domain is important for HIV-1 budding, while no specific lysine residue may be needed and individual domains can functionally substitute. This is consistent with Gag ubiquitination being functionally involved in a transient protein interaction network at the virus budding site.

YRKL sequence of influenza virus M1 functions as the L domain motif and interacts with VPS28 and Cdc42

Earlier studies have shown that the C-terminal half of helix 6 (H6) of the influenza A virus matrix protein (M1) containing the YRKL sequence is involved in virus budding (E. K.-W. Hui, S. Barman, T. Y. Yang, and D. P. Nayak, J. Virol. 77:7078-7092, 2003). In this report, we show that the YRKL sequence is the L domain motif of influenza virus. Like other L domains, YRKL can be inserted at different locations on the mutant M1 protein and can restore virus budding in a position-independent manner. Although YRKL is a part of the nuclear localization signal (NLS), the function of YRKL was independent of the NLS activity and the NLS function of M1 was not required for influenza virus replication. Some mutations in YRKL and the adjacent region caused a reduction in the virus titer by blocking virus release, and some affected virus morphology, producing elongated particles. Coimmunoprecipitation and Western blotting analyses showed that VPS28, a component of the ESCRT-I complex, and Cdc42, a member of the Rho family GTP-binding proteins, interacted with the M1 protein via the YRKL motif. In addition, depletion of VPS28 and Cdc42 by small interfering RNA resulted in reduction of influenza virus production. Moreover, overexpression of dominant-negative Cdc42 inhibited influenza virus replication, whereas a constitutively active Cdc42 mutant enhanced virus production in infected cells. These results indicated that VPS28, a component of ESCRT-I, and Cdc42, a small G protein, are associated with the M1 protein and involved in the influenza virus life cycle.

Analysis of human immunodeficiency virus type 1 Gag ubiquitination

Ubiquitin is important for the release of human immunodeficiency virus type 1 (HIV-1) and several other retroviruses, but the functional significance of Gag ubiquitination is unknown. To address this problem, we decided to analyze Gag ubiquitination in detail. A low percentage of the HIV-1 p6 protein has previously been shown to be ubiquitinated, and published mutagenesis data suggested that Gag ubiquitination is largely lost upon mutation of the two lysine residues in p6. In this study, we show that Gag proteins lacking the p6 domain or the two lysine residues within p6 are ubiquitinated at levels comparable to those of the wild-type Gag protein. We detected monoubiquitinated forms of the matrix (MA), capsid (CA), and nucleocapsid (NC) proteins in mature virus preparations. Protease digestion of Gag polyproteins extracted from immature virions indicated that ubiquitinated MA, CA, and possibly NC are as abundant as ubiquitinated p6. The HIV-1 late-domain motifs PTAP and LRSLF were not required for Gag ubiquitination, and mutation of the PTAP motif even resulted in an increase in the amount of Gag-Ub conjugates detected. Finally, at steady state, ubiquitinated Gag proteins were not enriched in either membrane-associated or virus-derived Gag fractions. In summary, these results indicate that HIV-1 Gag can be monoubiquitinated in all domains and that ubiquitination of lysine residues outside p6 may thus contribute to viral release and/or infectivity.

The C-terminal half of TSG101 blocks Rous sarcoma virus budding and sequesters Gag into unique nonendosomal structures

Retroviral late domains (L domains) are short amino acid sequences in the Gag protein that facilitate the process of budding. L domains act by recruiting the ESCRT complexes, which normally function in the formation of multivesicular bodies. The PTAP late domain of human immunodeficiency virus (HIV) is believed to specifically recruit this machinery by binding the ESCRT protein TSG101. It was recently demonstrated that expression of a C-terminal fragment of TSG101 (TSG-3') blocked the budding of both PTAP-dependent and PPPY-dependent retroviruses. We show here that TSG-3' expression leads to the formation of large spherical entities that we call TICS (TSG-3'-induced cellular structures) in the cytoplasm. Rous sarcoma virus (RSV) and murine leukemia virus (MLV) Gag proteins are selectively recruited to these structures, but HIV type 1 Gag is completely excluded. Experiments with various HIV and RSV vector constructs as well as HIV and RSV chimeras suggest that recruitment to the TICS is late domain independent and does not involve recognition of any single amino acid sequence. TICS appear to have no limiting membrane and do not colocalize with markers for any membranous cellular compartment. Wild-type TSG101 is also recruited to TICS, but most other ESCRT proteins are excluded. These structures are similar in nature to aggresomes, colocalize with the aggresome marker GFP-250, and are highly enriched in ubiquitin but in other ways do not fully meet the description of aggresomes. We conclude that the block to retroviral budding by TSG-3' may be the result of its sequestration of Gag, depletion of free TSG101, or depletion of free ubiquitin.

Evidence for a new viral late-domain core sequence, FPIV, necessary for budding of a paramyxovirus

Enveloped virus budding has been linked to both the ubiquitin-proteasome pathway and the vacuolar protein-sorting pathway of cells. We show here for the paramyxovirus SV5 that proteasome inhibitors and expression of dominant-negative VPS4(E228Q) ATPase blocks budding. The SV5 matrix (M) protein lacks previously defined late domains (e.g., P[T/S]AP, PPxY, YPDL) that recruit cellular factors. We identified a new motif for budding (core sequence FPIV) that can compensate functionally for lack of a PTAP late domain in budding human immunodeficiency virus type 1 virus-like particles (VLPs). Mutagenesis experiments suggest the more general sequence O-P-x-V. The proline residue was found to be critically important for function of this sequence, as substitution of this proline in the SV5 M protein resulted in poor budding of SV5 VLPs and failure of recombinant SV5 virus to replicate normally. Adaptation of mutant virus occurred rapidly, resulting in new proline residues elsewhere in the M protein. We hypothesize that these proline residues act to partially restore virus budding by generation of new motifs that act as suboptimal late domains.