Exosomes and HIV Gag bud from endosome-like domains of the T cell plasma membrane

Exosomes are secreted, single membrane organelles of approximately 100 nm diameter. Their biogenesis is typically thought to occur in a two-step process involving (1) outward vesicle budding at limiting membranes of endosomes (outward = away from the cytoplasm), which generates intralumenal vesicles, followed by (2) endosome-plasma membrane fusion, which releases these internal vesicles into the extracellular milieu as exosomes. In this study, we present evidence that certain cells, including Jurkat T cells, possess discrete domains of plasma membrane that are enriched for exosomal and endosomal proteins, retain the endosomal property of outward vesicle budding, and serve as sites of immediate exosome biogenesis. It has been hypothesized that retroviruses utilize the exosome biogenesis pathway for the formation of infectious particles. In support of this, we find that Jurkat T cells direct the key budding factor of HIV, HIV Gag, to these endosome-like domains of plasma membrane and secrete HIV Gag from the cell in exosomes.     

Identification of an inhibitory budding signal that blocks the release of HIV particles and exosome/microvesicle proteins

Animal cells bud exosomes and microvesicles (EMVs) from endosome and plasma membranes. The combination of higher-order oligomerization and plasma membrane binding is a positive budding signal that targets diverse proteins into EMVs and retrovirus particles. Here we describe an inhibitory budding signal (IBS) from the human immunodeficiency virus (HIV) Gag protein. This IBS was identified in the spacer peptide 2 (SP2) domain of Gag, is activated by C-terminal exposure of SP2, and mediates the severe budding defect of p6-deficient and PTAP-deficient strains of HIV. This IBS also impairs the budding of CD63 and several other viral and nonviral EMV proteins. The IBS does not prevent cargo delivery to the plasma membrane, a major site of EMV and virus budding. However, the IBS does inhibit an interaction between EMV cargo proteins and VPS4B, a component of the endosomal sorting complexes required for transport (ESCRT) machinery. Taken together, these results demonstrate that inhibitory signals can block protein and virus budding, raise the possibility that the ESCRT machinery plays a role in EMV biogenesis, and shed new light on the role of the p6 domain and PTAP motif in the biogenesis of HIV particles.     

Sequence of human immunodeficiency virus type 1 (HIV-1) Gag localization and oligomerization monitored with live confocal imaging of a replication-competent, fluorescently tagged HIV-1

The assembly of infectious human immunodeficiency virus (HIV) requires that Gag transport and oligomerization be coordinated with its association with other viral proteins, viral RNAs, and cellular membranes. We have developed a replication-competent HIV type 1 molecular clone that carries a Gag-internal or interdomain green fluorescent protein (iGFP) fusion to reveal a physiologically accurate temporal sequence of Gag localization and oligomerization during the formation of infectious HIV. This recombinant HIV is as infectious as native HIV in single-round infectivity assays, validating its use for trafficking studies. It replicates robustly in permissive MT4 cells and is infectious, yet it spreads poorly in other T-cell lines. Immunofluorescence of Gag-iGFP showed a pattern very similar to that of native Gag. However, the intense plasma membrane Gag-iGFP fluorescence contrasts markedly with its immunofluorescence at this site, indicating that many Gag epitopes can be masked by oligomerization. Consistent with this, fluorescence resonance energy transfer studies visualized intense Gag oligomerization at the plasma membrane and weaker oligomerization at cytoplasmic sites. Four-dimensional, time-lapse confocal imaging reveals a temporal progression of Gag distribution over hours in which Gag is initially diffusely localized within the cytoplasm. Plasma membrane signals then accumulate as Gag levels increase and vesicular association appears late, only after plasma membrane site signals have reached high intensity. Lastly, the cell rounds up and HIV protease activation induces diffuse fluorescence throughout the cell. These distinct phases reveal a natural progression of Gag trafficking during the viral gene expression program. HIV Gag-iGFP is a useful tool for dissecting mechanisms of viral assembly and transmission.     

Identification of human VPS37C, a component of endosomal sorting complex required for transport-I important for viral budding

Endosomal sorting complex required for transport-I (ESCRT-I) is one of three defined protein complexes in the class E vacuolar protein sorting (VPS) pathway required for the sorting of ubiquitinated transmembrane proteins into internal vesicles of multivesicular bodies. In yeast, ESCRT-I is composed of three proteins, VSP23, VPS28, and VPS37, whereas in mammals only Tsg101(VPS23) and VPS28 were originally identified as ESCRT-I components. Using yeast two-hybrid screens, we identified one of a family of human proteins (VPS37C) as a Tsg101-binding protein. VPS37C can form a ternary complex with Tsg101 and VPS28 by binding to a domain situated toward the carboxyl terminus of Tsg101 and binds to another class E VPS factor, namely Hrs. In addition, VPS37C is recruited to aberrant endosomes induced by overexpression of Tsg101, Hrs, or dominant negative form of the class E VPS ATPase, VPS4. Enveloped viruses that encode PTAP motifs to facilitate budding exploit ESCRT-I as an interface with the class E VPS pathway, and accordingly, VPS37C is recruited to the plasma membrane along with Tsg101 by human immunodeficiency virus, type 1 (HIV-1) Gag. Moreover, direct fusion of VPS37C to HIV-1 Gag obviates the requirement for a PTAP motif to induce virion release. Depletion of VPS37C from cells does not inhibit murine leukemia virus budding, which is not mediated by ESCRT-I, however, if murine leukemia virus budding is engineered to be ESCRT-I-dependent, then it is inhibited by VPS37C depletion, and this inhibition is accentuated if VPS37B is simultaneously depleted. Thus, this study identifies VPS37C as a functional component of mammalian ESCRT-I.     

Biogenesis of the posterior pole is mediated by the exosome/microvesicle protein-sorting pathway

Biogenesis of the posterior pole is critical to directed cell migration and other polarity-dependent processes. We show here that proteins are targeted to the posterior pole on the basis of higher order oligomerization and plasma membrane binding, the same elements that target proteins to exosomes/microvesicles (EMVs), HIV, and other retrovirus particles. We also demonstrate that the polarization of the EMV protein-sorting pathway can occur in morphologically non-polarized cells, defines the site of uropod formation, is induced by increased expression of EMV cargo proteins, and is evolutionarily conserved between humans and the protozoan Dictyostelium discoideum. Based on these results, we propose a mechanism of posterior pole biogenesis in which elevated levels of EMV cargoes (i) polarize the EMV protein-sorting pathway, (ii) generate a nascent posterior pole, and (iii) prime cells for signal-induced biogenesis of a uropod. This model also offers a mechanistic explanation for the polarized budding of EMVs and retroviruses, including HIV.     

Pdr5-mediated multidrug resistance requires the CPY-vacuolar sorting protein Vps3: are xenobiotic compounds routed from the vacuole to plasma membrane transporters for efflux?

In Saccharomyces cerevisiae several members of the ATP-binding cassette transporter superfamily efflux a broad range of xenobiotic substrates from cells. The vacuole also plays a critical role in multidrug resistance. Mutations in genes such as VPS3 that are essential for vacuolar acidification and carboxypeptidase Y vacuolar protein-sorting are multidrug sensitive. A similar phenotype is also observed with deletions of VPS15, VPS34, and VPS38, which encode essential members of the carboxypeptidase Y vacuolar protein-sorting pathway. Prior to the work described herein, detoxification by transporters and the vacuole were presumed to function independently. We demonstrate that this is not the case. Significantly, Vps3 has an epistatic relationship with Pdr5, a major yeast multidrug transporter. Thus, a double pdr5, vps3 deletion mutant is no more multidrug sensitive than its isogenic single-mutant counterparts. Subcellular fractionation experiments and analysis of purified plasma membrane vesicles indicate, however, that a vps3 mutation does not affect the membrane-localization or ATPase activity of Pdr5 even though rhodamine 6G efflux is reduced significantly. This suggests that Vps3 and probably other members of the carboxypeptidase Y vacuolar protein-sorting pathway are required for relaying xenobiotic compounds to transporters in the membrane.     

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.     

Mutations in capsid major homology region affect assembly and membrane affinity of HIV-1 Gag

We introduced mutations into the HIV-1 major homology region (MHR; capsids 153-172) and adjacent C-terminal region to analyze their effects on virus-like particle (VLP) assembly, membrane affinity, and the multimerization of the Gag structural protein. Results indicate that alanine substitutions at K158, F168 or E175 significantly diminished VLP production. All assembly-defective Gag mutants had markedly reduced membrane-binding capacities, but results from a velocity sedimentation analysis suggest that most of the membrane-bound Gag proteins were present, primarily in a higher-order multimerized form. The membrane-binding capacity of the K158A, F168A, and E175A Gag proteins increased sharply upon removal of the MA globular domain. While demonstrating improved multimerization capability, the two MA-deleted versions of F168A and E175A did not show marked improvement in VLP production, presumably due to a defect in association with the raft-like membrane domain. However, K158A bound to detergent-resistant raft-like membrane; this was accompanied by noticeably improved VLP production following MA removal. Our results suggest that the HIV-1 MHR and adjacent downstream region facilitate multimerization and tight Gag packing. Enhanced Gag multimerization may help expose the membrane-binding domain and thus improve Gag membrane binding, thereby promoting Gag multimerization into higher-order assembly products.     

Live-cell visualization of dynamics of HIV budding site interactions with an ESCRT component

HIV (human immunodeficiency virus) diverts the cellular ESCRT (endosomal sorting complex required for transport) machinery to promote virion release from infected cells. The ESCRT consists of four heteromeric complexes (ESCRT-0 to ESCRT-III), which mediate different membrane abscission processes, most importantly formation of intralumenal vesicles at multivesicular bodies. The ATPase VPS4 (vacuolar protein sorting 4) acts at a late stage of ESCRT function, providing energy for ESCRT dissociation. Recruitment of ESCRT by late-domain motifs in the viral Gag polyprotein and a role of ESCRT in HIV release are firmly established, but the order of events, their kinetics and the mechanism of action of individual ESCRT components in HIV budding are unclear at present. Using live-cell imaging, we show late-domain-dependent recruitment of VPS4A to nascent HIV particles at the host cell plasma membrane. Recruitment of VPS4A was transient, resulting in a single or a few bursts of at least two to five VPS4 dodecamers assembling at HIV budding sites. Bursts lasted for ～35 s and appeared with variable delay before particle release. These results indicate that VPS4A has a direct role in membrane scission leading to HIV-1 release.     

The protein network of HIV budding

HIV release requires TSG101, a cellular factor that sorts proteins into vesicles that bud into multivesicular bodies (MVB). To test whether other proteins involved in MVB biogenesis (the class E proteins) also participate in HIV release, we identified 22 candidate human class E proteins. These proteins were connected into a coherent network by 43 different protein-protein interactions, with AIP1 playing a key role in linking complexes that act early (TSG101/ESCRT-I) and late (CHMP4/ESCRT-III) in the pathway. AIP1 also binds the HIV-1 p6(Gag) and EIAV p9(Gag) proteins, indicating that it can function directly in virus budding. Human class E proteins were found in HIV-1 particles, and dominant-negative mutants of late-acting human class E proteins arrested HIV-1 budding through plasmal and endosomal membranes. These studies define a protein network required for human MVB biogenesis and indicate that the entire network participates in the release of HIV and probably many other viruses.     

Cell-type-dependent targeting of human immunodeficiency virus type 1 assembly to the plasma membrane and the multivesicular body

The human immunodeficiency virus type 1 (HIV-1) assembly-and-release pathway begins with the targeting of the Gag precursor to the site of virus assembly. The molecular mechanism by which Gag is targeted to the appropriate subcellular location remains poorly understood. Based on the analysis of mutant Gag proteins, we and others have previously demonstrated that a highly basic patch in the matrix (MA) domain of Gag is a major determinant of Gag transport to the plasma membrane. In this study, we determined that in HeLa and T cells, the MA mutant Gag proteins that are defective in plasma membrane targeting form virus particles in a CD63-positive compartment, defined as the late endosome or multivesicular body (MVB). Interestingly, we find that in primary human macrophages, both wild-type (WT) and MA mutant Gag proteins are targeted specifically to the MVB. Despite the fact that particle assembly in macrophages occurs at an intracellular site rather than at the plasma membrane, we observe that WT Gag expressed in this cell type is released as extracellular virions with high efficiency. These results demonstrate that Gag targeting to and assembly in the MVB are physiologically important steps in HIV-1 virus particle production in macrophages and that particle release in this cell type may follow an exosomal pathway. To determine whether Gag targeting to the MVB is the result of an interaction between the late domain in p6(Gag) and the MVB sorting machinery (e.g., TSG101), we examined the targeting and assembly of Gag mutants lacking p6. Significantly, the MVB localization of Gag was still observed in the absence of p6, suggesting that an interaction between Gag and TSG101 is not required for Gag targeting to the MVB. These data are consistent with a model for Gag targeting that postulates two different cellular binding partners for Gag, one on the plasma membrane and the other in the MVB.     

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.     

Tsg101: HIV-1's ticket to ride

Recent studies implicate the vacuolar protein-sorting pathway in the transport of the retroviral structural precursor (Gag) protein to its budding site on the plasma membrane of infected cells. This exploitation of the cellular endocytic trafficking machinery to release viral particles could lead to the identification of virus-specific modulators and provide opportunities to design new targeted anti-viral agents.     

Roles of matrix, p2, and N-terminal myristoylation in human immunodeficiency virus type 1 Gag assembly

Human immunodeficiency virus type 1 Gag protein is cotranslationally myristoylated at the N terminus and targeted to the plasma membrane, where virus particle assembly occurs. Particle assembly requires the ordered multimerization of Gag proteins, yet there is little direct evidence of intermediates of the reaction or of the domains that lead to each stage of the oligomerization process. In this study, following the expression in insect cells of C-terminally truncated Gag proteins and their purification, both the multimeric nature of each Gag protein and the ability to form Gag virus-like particles (VLP) were analyzed. Our results show that (i) the matrix (MA) domain forms a trimer and contributes to a similar level of oligomerization of the assembly-competent Gag; (ii) the p2 domain, located at the capsid/nucleocapsid junction, is essential for a higher order of multimerization (>1,000 kDa); (iii) the latter multimerization is accompanied by a change in Gag assembly morphology from tubes to spheres and results in VLP production; and (iv) N-terminal myristoylation is not required for either of the multimerization stages but plays a key role in conversion of these multimers to Gag VLP. We suggest that the Gag trimer and the > 1,000-kDa multimer are intermediates in the assembly reaction and form before Gag targeting to the plasma membrane. Our data identify a minimum of three stages for VLP development and suggest that each stage involves a separate domain, MA, p2, or N-terminal myristoylation, each of which contributes to HIV particle assembly.     

Equine infectious anemia virus utilizes host vesicular protein sorting machinery during particle release

A final step in retrovirus assembly, particle release from the cell, is modulated by a small motif in the Gag protein known as a late domain. Recently, human immunodeficiency virus type 1 (HIV-1) and Moloney murine leukemia virus (M-MuLV) were shown to require components of the cellular vacuolar protein sorting (VPS) machinery for efficient viral release. HIV-1 interacts with the VPS pathway via an association of HIV-1 Gag with TSG101, a component of the cellular complexes involved in VPS. Equine infectious anemia virus (EIAV) is unique among enveloped viruses studied to date because it utilizes a novel motif, YPDL in Gag, as a late domain. Our analysis of EIAV assembly demonstrates that EIAV Gag release is blocked by inhibition of the VPS pathway. However, in contrast to HIV-1, EIAV Gag release is insensitive to TSG101 depletion and EIAV particles do not contain significant levels of TSG101. Finally, we demonstrate that fusing EIAV Gag directly with another cellular component of the VPS machinery, VPS28, can restore efficient release of an EIAV Gag late-domain mutant. These results provide evidence that retroviruses can interact with the cellular VPS machinery in several different ways to accomplish particle release.     

An HRD/DER-independent ER quality control mechanism involves Rsp5p-dependent ubiquitination and ER-Golgi transport

There is increasing evidence that ubiquitination of receptors provides an important endosomal sorting signal. Here we report that mammalian class E vacuolar protein-sorting (vps) proteins recognize ubiquitin. Both tumor susceptibility gene 101 (TSG101)/human VPS (hVPS)28 and hepatocyte growth factor receptor substrate (Hrs) cytosolic complexes bind ubiquitin-agarose. TSG101 and hVPS28 are localized to endosomes that contain internalized EGF receptor and label strongly for ubiquitinated proteins. Microinjection of anti-hVPS28 specifically retards EGF degradation and leads to endosomal accumulation of ubiquitin-protein conjugates. Likewise, depletion of TSG101 impairs EGF trafficking and causes dramatic relocalization of ubiquitin to endocytic compartments. Similar defects are found in cells overexpressing Hrs, further emphasizing the links between class E protein function, receptor trafficking, and endosomal ubiquitination.     

Human immunodeficiency virus type 1 Gag contains a dileucine-like motif that regulates association with multivesicular bodies

Multivesicular bodies (MVBs) are cholesterol-enriched organelles formed by the endocytic pathway. The topology of vesicle formation in MVBs is identical to that of retroviral budding from the plasma membrane, and budding of human immunodeficiency virus type 1 (HIV-1) into MVBs in macrophages has recently been visualized. The Gag proteins from HIV-1, as well as many other retroviruses, contain short motifs that mediate interactions with MVBs and other endocytic components, suggesting that Gag proteins directly interface with the endocytic pathway. Here, we show that HIV-1 Gag contains an internalization signal that promotes endocytosis of a chimeric transmembrane fusion protein. Mutation of this motif within Gag strongly inhibits virus-like particle production. Moreover, wild-type Gag, but not the internalization-defective mutation, can be induced to accumulate within CD63-positive MVBs by treatment of cells with U18666A, a drug that redistributes cholesterol from the plasma membrane to MVBs. We propose that HIV-1 Gag contains a signal that promotes interaction with the cellular endocytic machinery and that the site of particle production is regulated by the subcellular distribution of cholesterol.     

Interrelationship between Cytoplasmic Retroviral Gag Concentration and Gag–Membrane Association

The early events in the retrovirus assembly pathway, particularly the timing and nature of Gag translocation from the site of protein translation to the inner leaflet of the plasma membrane, are poorly understood. We have investigated the interrelationship between cytoplasmic Gag concentration and plasma membrane association using complementary live-cell biophysical fluorescence techniques in real time with both human T-cell leukemia virus type 1 (HTLV-1) and human immunodeficiency virus type 1 (HIV-1) Gag proteins. In particular, dual-color, z-scan fluorescence fluctuation spectroscopy in conjunction with total internal reflection fluorescence and conventional, epi-illumination imaging were utilized. Our results demonstrate that HTLV-1 Gag is capable of membrane targeting and particle assembly at low (i.e., nanomolar) cytoplasmic concentrations and that there is a critical threshold concentration (approaching micromolar) prior to the observation of HIV-1 Gag associated with the plasma membrane. These observations imply fundamental differences between HIV-1 and HTLV-1 Gag trafficking and membrane association.