The Late Domain of Human Immunodeficiency Virus Type 1 p6 Promotes Virus Release in a Cell Type-Dependent Manner

The p6 domain of human immunodeficiency virus type 1 (HIV-1) is located at the C terminus of the Gag precursor protein Pr55(Gag). Previous studies indicated that p6 plays a critical role in HIV-1 particle budding from virus-expressing HeLa cells. In this study, we performed a detailed mutational analysis of the N terminus of p6 to map the sequences required for efficient virus release. We observed that the highly conserved P-T/S-A-P motif located near the N terminus of p6 is remarkably sensitive to change; even conservative mutations in this sequence imposed profound virus release defects in HeLa cells. In contrast, single and double amino acid substitutions outside the P-T/S-A-P motif had no significant effect on particle release. The introduction of stop codons one or two residues beyond the P-T/S-A-P motif markedly impaired virion release, whereas truncation four residues beyond P-T/S-A-P had no effect on particle production in HeLa cells. By examining the effects of p6 mutation in biological and biochemical analyses and by electron microscopy, we defined the role of p6 in particle release and virus replication in a panel of T-cell and adherent cell lines and in primary lymphocytes and monocyte-derived macrophages. We demonstrated that the effects of p6 mutation on virus replication are markedly cell type dependent. Intriguingly, even in T-cell lines and primary lymphocytes in which p6 mutations block virus replication, these changes had little or no effect on particle release. However, p6-mutant particles produced in T-cell lines and primary lymphocytes exhibited a defect in virion-virion detachment, resulting in the production of tethered chains of virions. Virus release in monocyte-derived macrophages was markedly inhibited by p6 mutation. To examine further the cell type-specific virus release defect in HeLa versus T cells, transient heterokaryons were produced between HeLa cells and the Jurkat T-cell line. These heterokaryons display a T-cell-like phenotype with respect to the requirement for p6 in particle release. The results described here define the role of p6 in virus replication in a wide range of cell types and reveal a strong cell type-dependent requirement for p6 in virus particle budding.     

Annexin 2 Is Not Required for Human Immunodeficiency Virus Type 1 Particle Production but Plays a Cell Type-Dependent Role in Regulating Infectivity

During assembly and budding of retroviruses, host cell proteins are incorporated into viral particles. Identification of virion-associated proteins may help pinpoint key cellular components required for virus production and function. The cellular protein annexin 2 (Anx2) is incorporated into HIV-1 particles, and knockdown of Anx2 has been reported to cause defects in Gag processing and infectivity of HIV-1 particles in macrophages. Here, we tested whether Anx2 was required for HIV-1 production in other cell types capable of producing HIV-1 virions. Endogenous Anx2 levels were knocked down by ∼98% using lentivirus encoding short hairpin RNAs (shRNAs) or small interfering RNAs (siRNAs) targeting Anx2. Under these conditions, there was no reduction in HIV-1 virus-like particle (VLP) production in either COS-1, 293T, or Jurkat T cells or primary human monocyte-derived macrophages (MDMs). Murine embryonic fibroblasts derived from Anx2^−/− mice produced the same levels of VLPs as matched cells from wild-type mice. The calcium-mediated spike in VLP production still occurred in Anx2-depleted COS-1 cells, and there was no apparent alteration in the intracellular Gag localization. Overexpression of Anx2 in trans had no effect on Gag processing or VLP production. Neither Anx2 depletion nor Anx2 overexpression altered the infectivity of HIV-1 particles produced by COS-1 or 293T cells. However, supernatants containing virus from Anx2 siRNA-treated primary human MDMs exhibited decreased infectivity. These data indicate that Anx2 is not required for HIV-1 assembly or Gag processing but rather plays a cell type-dependent role in regulating production of infectious HIV-1 by macrophages.The Gag polyprotein generates the key structural proteins for all retroviruses. Gag is necessary and sufficient for the formation of virus-like particles (VLPs), which are morphologically similar to immature virions. Following its synthesis in the cytoplasm, HIV-1 Gag is trafficked to sites of particle production on membranes. Viral particle production depends on Gag-membrane interactions mediated by the myristoylated MA domain of Gag (18, 22, 31) and Gag-Gag interactions mediated by the CA and NC domains. Budding and release of the new virion are mediated by the Gag p6 domain. For successful particle production to occur, HIV-1 Gag must also interact with numerous host cell proteins and protein complexes. Identification of these interactions provides a crucial window into determining Gag trafficking intermediates as well as clues to the mechanism of virion production.The host cell protein annexin 2 (Anx2) has recently attracted attention for its potential to regulate key processes in both cells and viruses (9, 14, 17, 24). Anx2 belongs to a family of conserved calcium-regulated proteins and interacts with actin, membranes, and negatively charged phospholipids. The major protein binding partner for Anx2 is p11, also known as S100A10. Two populations of Anx2 have been identified: a heterotetrameric complex with two molecules of Anx2 and two molecules of p11 (found predominantly at the plasma membrane) and a monomeric form found mainly in the cytoplasm. Anx2 performs multiple functions in the cell, including regulation of actin-based dynamics, fibrinolysis, calcium-mediated exocytosis, and transport of intermediates from early to late endosomes (10, 14-16) Anx2 also enhances binding and fusion of cytomegalovirus with phospholipid membranes (21). In addition, Anx2 can be detected within influenza virus particles (28), where it has been shown to aid in virus replication (9).Several lines of evidence suggest that Anx2 may play a role in HIV-1 biogenesis. Both Anx2 and its binding partner p11 are incorporated in HIV-1 particles produced by macrophages (2). Anx2 interacts with Gag in macrophages, and annexin 2 knockdown has been reported to cause defective Gag processing and reduced infectivity of the released particles (24). Blockade of Anx2 function, with either anti-Anx2 antibody or small interfering RNA (siRNA)-mediated knockdown, results in suppression of HIV-1 infection in macrophages (11). Anx2 also binds to Gag in 293T cells, and expression of Anx2 in trans in these cells has been reported to lead to increased Gag processing and HIV-1 production (7). Taken together, these findings suggest that Anx2 might play a universal role in Gag trafficking and particle production. To test this hypothesis, we exploited methods to efficiently knock down Anx2 expression and determined the effect of Anx2 knockdown in a variety of cell lines capable of producing HIV-1 virions. Here we show that, in the absence of Anx2 expression, HIV-1 Gag is expressed, trafficked, and capable of mediating viral particle formation in a manner similar to that of control cells expressing Anx2. However, a cell type-dependent effect of Anx2 depletion on HIV-1 infectivity was detected in primary human monocyte-derived macrophages (MDMs). These findings suggest that Anx2 might be a macrophage-specific host cell factor that regulates HIV-1 infectivity.     

Particle Size Determinants in the Human Immunodeficiency Virus Type 1 Gag Protein

The retroviral Gag protein plays the central role in the assembly process and can form membrane-enclosed, virus-like particles in the absence of any other viral products. These particles are similar to authentic virions in density and size. Three small domains of the human immunodeficiency virus type 1 (HIV-1) Gag protein have been previously identified as being important for budding. Regions that lie outside these domains can be deleted without any effect on particle release or density. However, the regions of Gag that control the size of HIV-1 particles are less well understood. In the case of Rous sarcoma virus (RSV), the size determinant maps to the CA (capsid) and adjacent spacer sequences within Gag, but systematic mapping of the HIV Gag protein has not been reported. To locate the size determinants of HIV-1, we analyzed a large collection of Gag mutants. To our surprise, all mutants with defects in the MA (matrix), CA, and the N-terminal part of NC (nucleocapsid) sequences produced dense particles of normal size, suggesting that oncoviruses (RSV) and lentiviruses (HIV-1) have different size-controlling elements. The most important region found to be critical for determining HIV-1 particle size is the p6 sequence. Particles lacking all or small parts of p6 were uniform in size distribution but very large as measured by rate zonal gradients. Further evidence for this novel function of p6 was obtained by placing this sequence at the C terminus of RSV CA mutants that produce heterogeneously sized particles. We found that the RSV-p6 chimeras produced normally sized particles. Thus, we present evidence that the entire p6 sequence plays a role in determining the size of a retroviral particle.     

Detection of a Trimeric Human Immunodeficiency Virus Type 1 Gag Intermediate Is Dependent on Sequences in the Matrix Protein, p17

Previous studies have shown that single amino acid changes in the amino-terminal matrix (MA) domain, p17, of the human immunodeficiency virus type 1 Gag precursor Pr55, can abrogate virion particle assembly. In the three-dimensional structure of MA such mutations lie in a single helix spanning residues 54 to 68, suggesting a key role for this helix in the assembly process. The fundamental nature of this involvement, however, remains poorly understood. In the present study, the essential features of the MA helix required for virus assembly have been investigated through the analysis of a further 15 site-directed mutants. With previous mutants that failed to assemble, residues mapped as critical for assembly were all located on the hydrophobic face of the helix and had a key role in stabilizing the trimeric interface. This implies a role for the MA trimer in virus assembly. We support this interpretation by showing that purified MA is trimeric in solution and that mutations that prevent virus assembly also prevent trimerization. Trimerization in solution was also a property of a larger MA-capsid (CA) Gag molecule, while under the same conditions CA only was a monomer. These data suggest that Gag trimerization driven by the MA domain is an intermediate stage in normal virion assembly and that it relies, in turn, on an MA conformation dependent on the hydrophobic core of the molecule.     

The I Domain Is Required for Efficient Plasma Membrane Binding of Human Immunodeficiency Virus Type 1 Pr55Gag

The interaction of the human immunodeficiency virus type 1 (HIV-1) Pr55^Gag molecule with the plasma membrane of an infected cell is an essential step of the viral life cycle. Myristic acid and positively charged residues within the N-terminal portion of MA constitute the membrane-binding domain of Pr55^Gag. A separate assembly domain, termed the interaction (I) domain, is located nearer the C-terminal end of the molecule. The I domain is required for production of dense retroviral particles, but has not previously been described to influence the efficiency of membrane binding or the subcellular distribution of Gag. This study used a series of Gag-green fluorescent protein fusion constructs to define a region outside of MA which determines efficient plasma membrane interaction. This function was mapped to the nucleocapsid (NC) region of Gag. The minimal region in a series of C-terminally truncated Gag proteins conferring plasma membrane fluorescence was identified as the N-terminal 14 amino acids of NC. This same region was sufficient to create a density shift in released retrovirus-like particles from 1.13 to 1.17 g/ml. The functional assembly domain previously termed the I domain is thus required for the efficient plasma membrane binding of Gag, in addition to its role in determining the density of released particles. We propose a model in which the I domain facilitates the interaction of the N-terminal membrane-binding domain of Pr55^Gag with the plasma membrane.     

Binding of the Human Immunodeficiency Virus Type 1 Gag Protein to the Viral RNA Encapsidation Signal in the Yeast Three-Hybrid System

We have used the yeast three-hybrid system (D. J. SenGupta, B. Zhang, B. Kraemer, P. Pochart, S. Fields, and M. Wickens, Proc. Natl. Acad. Sci. USA 93:8496–8501, 1996) to study binding of the human immunodeficiency virus type 1 (HIV-1) Gag protein to the HIV-1 RNA encapsidation signal (HIVΨ). Interaction of these elements results in the activation of a reporter gene in the yeast Saccharomyces cerevisiae. Using this system, we have shown that the HIV-1 Gag protein binds specifically to a 139-nucleotide fragment of the HIVΨ signal containing four stem-loop structures. Mutations in either the Gag protein or the encapsidation signal that have been shown previously to impair this interaction reduced the activation of the reporter gene. Interestingly, the nucleocapsid portion of Gag retained the RNA binding activity but lost its specificity compared to the full-length Gag. These results demonstrate the utility of this system and suggest that a variety of genetic analyses could be performed to study Gag-encapsidation signal interactions.     

Ubiquitin Is Covalently Attached to the p6Gag Proteins of Human Immunodeficiency Virus Type 1 and Simian Immunodeficiency Virus and to the p12Gag Protein of Moloney Murine Leukemia Virus

Host proteins are incorporated into retroviral virions during assembly and budding. We have examined three retroviruses, human immunodeficiency virus type 1 (HIV-1), simian immunodeficiency virus (SIV), and Moloney murine leukemia virus (Mo-MuLV), for the presence of ubiquitin inside each of these virions. After a protease treatment to remove exterior viral as well as contaminating cellular proteins, the proteins remaining inside the virion were analyzed. The results presented here show that all three virions incorporate ubiquitin molecules at approximately 10% of the level of Gag found in virions. In addition to free ubiquitin, covalent ubiquitin-Gag complexes were detected, isolated, and characterized from all three viruses. Our immunoblot and protein sequencing results on treated virions showed that approximately 2% of either HIV-1 or SIV p6^Gag was covalently attached to a single ubiquitin molecule inside the respective virions and that approximately 2 to 5% of the p12^Gag in Mo-MuLV virions was monoubiquitinated. These results show that ubiquitination of Gag is conserved among these retroviruses and occurs in the p6^Gag portion of the Gag polyprotein, a region that is likely to be involved in assembly and budding.     

Human Immunodeficiency Virus Type 1 Matrix Protein Interacts with Cellular Protein HO3

The matrix (MA) protein of human immunodeficiency virus type 1 (HIV-1) plays a critical role in virion morphogenesis and fulfills important functions during the early steps of infection. In an effort to identify cellular partners of MA, a Saccharomyces cerevisiae two-hybrid screen was utilized. A specific interaction between MA and HO3, a putative histidyl-tRNA synthetase, was demonstrated in this system. HO3-specific mRNA was detected in several tissues relevant for HIV infection, such as spleen, thymus, and peripheral blood lymphocytes, as well as in a number of T-lymphoid-cell lines. The binding of MA to HO3 was confirmed in transfected cells by coimmunoprecipitation. This interaction was abrogated by replacing two lysine residues at positions 26 and 27 of MA by threonine (MA_KK^27TT). HO3 localized both to the cytoplasm and to the nucleus of acutely transfected 293T cells. When overexpressed in HIV-1-producing cells, HO3 was incorporated into wild-type virions but not in ones containing the dilysine-mutated variant of MA. Correspondingly, overexpression of HO3 in virus producer cells enhanced the infectivity of wild-type but not MA_KK^27AA HIV-1 particles. The stimulating effect of HO3 was independent from the presence of Envelope, Vpr, or Vpu. Taken together, these results suggest that HO3, through its recognition of MA, plays a role in the life cycle of HIV-1.     

Analysis of the Assembly Function of the Human Immunodeficiency Virus Type 1 Gag Protein Nucleocapsid Domain

Previous studies have shown that in addition to its function in specific RNA encapsidation, the human immunodeficiency virus type 1 (HIV-1) nucleocapsid (NC) is required for efficient virus particle assembly. However, the mechanism by which NC facilitates the assembly process is not clearly established. Formally, NC could act by constraining the Pr55^gag polyprotein into an assembly-competent conformation or by masking residues which block the assembly process. Alternatively, the capacity of NC to bind RNA or make interprotein contacts might affect particle assembly. To examine its role in the assembly process, we replaced the NC domain in Pr55^gag with polypeptide domains of known function, and the chimeric proteins were analyzed for their abilities to direct the release of virus-like particles. Our results indicate that NC does not mask inhibitory domains and does not act passively, by simply providing a stable folded monomeric structure. However, replacement of NC by polypeptides which form interprotein contacts permitted efficient virus particle assembly and release, even when RNA was not detected in the particles. These results suggest that formation of interprotein contacts by NC is essential to the normal HIV-1 assembly process.Human immunodeficiency virus type 1 (HIV-1) encodes three major genes, gag, pol, and env, which are commonly found in all mammalian retroviruses. It also encodes accessory genes whose protein products are important for regulation of its life cycle (6, 30, 35). However, of all the genes encoded by HIV-1, only the protein product of the gag gene has been found to be necessary and sufficient for the assembly of virus-like particles (11, 13, 17, 22, 32, 33). The HIV-1 Gag protein initially is expressed as a 55-kDa polyprotein precursor (Pr55^gag), but during or shortly after particle release, Pr55^gag ordinarily is cleaved by the viral protease (PR). The products of the protease action are the four major viral proteins matrix (MA), capsid (CA), nucleocapsid (NC), and p6, and the two spacer polypeptides p2 and p1, which represent sequences between CA and NC and between NC and p6, respectively (15, 19, 23, 30).The HIV-1 nucleocapsid proteins have two Cys-X₂-Cys-X₄-His-X₄-Cys (Cys-His) motifs, reminiscent of the zinc finger motifs found in many DNA binding proteins, and NC has been shown to facilitate the specific encapsidation of HIV-1 genomic RNAs. In addition to its encapsidation function, NC influences virus particle assembly (7, 10, 17, 21, 40). In particular, Gag proteins lacking the NC domain fail to assemble virus particles efficiently. Nevertheless, some chimeric Gag proteins which carry foreign sequences in place of NC have been shown to assemble and release virus particles at wild-type (wt) levels (2, 37, 40). Thus, it appears that in some circumstances, the role that NC plays in virus particle assembly can be replaced. To date, it is not clear how NC affects particle assembly, although several possibilities might be envisioned. One possibility is that deletion of NC unmasks inhibitory sequences in p2 or the C terminus of CA. Alternatively, NC may simply provide a stable monomeric folded structure which locks CA or other Gag domains into an assembly-competent conformation. Another possibility is that NC facilitates assembly by forming essential protein-protein contacts between neighbor Pr^gag molecules, as suggested in cross-linking studies (21). Finally, the assembly role of NC may stem from its RNA binding capabilities, a hypothesis supported by studies of Campbell and Vogt (5), which have shown that RNA facilitates the in vitro assembly of retroviral Gag proteins into higher-order structures.To distinguish among possible mechanisms by which NC facilitates HIV-1 assembly, we replaced NC with polypeptides having known structural characteristics and examined particle assembly directed by these chimeric proteins. Using this approach, we have found that NC does not play a passive role in HIV-1 assembly as either a mask to assembly inhibitor domains or a nonspecific, stably folded structure. Rather, sequences known to form strong interprotein contacts were observed to enhance assembly, suggesting a similar role for the NC domain itself. With several assembly-competent chimeric proteins, we detected no particle-associated RNAs. These results suggest that while RNA may be essential to virus assembly in the context of the wt Pr55^gag protein, it is dispensable for formation of virus-like particles from chimeric proteins.     

Cleavage of Human Immunodeficiency Virus Type 1 Proteinase from the N-Terminally Adjacent p6* Protein Is Essential for Efficient Gag Polyprotein Processing and Viral Infectivity

Maturation of infectious human immunodeficiency virus (HIV) particles requires proteolytic cleavage of the structural polyproteins by the viral proteinase (PR), which is itself encoded as part of the Gag-Pol polyprotein. Expression of truncated PR-containing sequences in heterologous systems has mostly led to the autocatalytic release of an 11-kDa species of PR which is capable of processing all known cleavage sites on the viral precursor proteins. Relatively little is known about cleavages within the nascent virus particle, on the other hand, and controversial results concerning the active PR species inside the virion and the relative activities of extended PR species have been reported. Here, we report that HIV type 1 (HIV-1) particles of four different strains obtained from different cell lines contain an 11-kDa PR, with no extended PR proteins detectable. Furthermore, mutation of the N-terminal PR cleavage site leading to production of an N-terminally extended 17-kDa PR species caused a severe defect in Gag polyprotein processing and a complete loss of viral infectivity. We conclude that N-terminal release of PR from the HIV-1 polyprotein is essential for viral replication and suggest that extended versions of PR may have a transient function in the proteolytic cascade.     

The C-Terminal Half of the Human Immunodeficiency Virus Type 1 Gag Precursor Is Sufficient for Efficient Particle Assembly

Human immunodeficiency virus type 1 particle assembly is directed by the Gag polyprotein Pr55^gag, the precursor for the matrix (MA), capsid (CA), and nucleocapsid proteins of the mature virion. We now show that CA sequences N terminal to the major homology region (MHR), which form a distinct domain, are dispensable for particle formation. However, slightly larger deletions which extend into the MHR severely impair particle production. Remarkably, a deletion which removed essentially all MA and CA sequences between the N-terminal myristyl anchor and the MHR reduced the yield of extracellular particles only moderately. Particle formation even exceeded wild-type levels when additional MA sequences, either from the N or the C terminus of the domain, were retained. We conclude that no distinct region between the myristyl anchor and the MHR is required for efficient particle assembly or release.     

In Vitro Assembly Properties of Human Immunodeficiency Virus Type 1 Gag Protein Lacking the p6 Domain

Human immunodeficiency virus type 1 (HIV-1) normally assembles into particles of 100 to 120 nm in diameter by budding through the plasma membrane of the cell. The Gag polyprotein is the only viral protein that is required for the formation of these particles. We have used an in vitro assembly system to examine the assembly properties of purified, recombinant HIV-1 Gag protein and of Gag missing the C-terminal p6 domain (Gag Δp6). This system was used previously to show that the CA-NC fragment of HIV-1 Gag assembled into cylindrical particles. We now report that both HIV-1 Gag and Gag Δp6 assemble into small, 25- to 30-nm-diameter spherical particles in vitro. The multimerization of Gag Δp6 into units larger than dimers and the formation of spherical particles required nucleic acid. Removal of the nucleic acid with NaCl or nucleases resulted in the disruption of the multimerized complexes. We conclude from these results that (i) N-terminal extension of HIV-1 CA-NC to include the MA domain results in the formation of spherical, rather than cylindrical, particles; (ii) nucleic acid is required for the assembly and maintenance of HIV-1 Gag Δp6 virus-like particles in vitro and possibly in vivo; (iii) a wide variety of RNAs or even short DNA oligonucleotides will support assembly; (iv) protein-protein interactions within the particle must be relatively weak; and (v) recombinant HIV-1 Gag Δp6 and nucleic acid are not sufficient for the formation of normal-sized particles.     

Human Immunodeficiency Virus Type 1 Gag Polyprotein Multimerization Requires the Nucleocapsid Domain and RNA and Is Promoted by the Capsid-Dimer Interface and the Basic Region of Matrix Protein

The human immunodeficiency virus type 1 (HIV-1) Gag polyprotein directs the formation of virions from productively infected cells. Many gag mutations disrupt virion assembly, but little is known about the biochemical effects of many of these mutations. Protein-protein interactions among Gag monomers are believed to be necessary for virion assembly, and data suggest that RNA may modify protein-protein interactions or even serve as a bridge linking Gag polyprotein monomers. To evaluate the primary sequence requirements for HIV-1 Gag homomeric interactions, a panel of HIV-1 Gag deletion mutants was expressed in bacteria and evaluated for the ability to associate with full-length Gag in vitro. The nucleocapsid protein, the major RNA-binding domain of Gag, exhibited activity comparable to that of the complete polyprotein. In the absence of the nucleocapsid protein, relatively weak activity was observed that was dependent upon both the capsid-dimer interface and basic residues within the matrix domain. The relevance of the in vitro findings was confirmed with an assay in which nonmyristylated mutant Gags were assessed for the ability to be incorporated into virions produced by wild-type Gag expressed in trans. Evidence of the importance of RNA for Gag-Gag interaction was provided by the demonstration that RNase impairs the Gag-Gag interaction and that HIV-1 Gag interacts efficiently with Gags encoded by distantly related retroviruses and with structurally unrelated RNA-binding proteins. These results are consistent with models in which Gag multimerization involves indirect contacts via an RNA bridge as well as direct protein-protein interactions.     

Functional Analysis of the Core Human Immunodeficiency Virus Type 1 Packaging Signal in a Permissive Cell Line

Packaging of type C retrovirus genomic RNAs into budding virions requires a highly specific interaction between the viral Gag precursor and unique cis-acting packaging signals on the full-length RNA genome, allowing the selection of this RNA species from among a pool of spliced viral RNAs and similar cellular RNAs. This process is thought to involve RNA secondary and tertiary structural motifs since there is little conservation of the primary sequence of this region between retroviruses. To confirm RNA secondary structures, which we and others have predicted for this region, disruptive, compensatory, and deletion mutations were introduced into proviral constructs, which were then assayed in a permissive cell line. Disruption of either of two predicted stem-loops was found to greatly reduce RNA encapsidation and replication, whereas compensatory mutations restoring base pairing to these stem-loops had a wild-type phenotype. A GGNGR motif was identified in the loops of three hairpins in this region. Results were consistent with the hypothesis that the process of efficient RNA encapsidation is linked to dimerization. Replication and encapsidation were shown to occur at a reduced rate in the absence of the previously described kissing hairpin motif.     

Regions of Human Immunodeficiency Virus Type 1 nef Required for Function In Vivo

In vivo studies in monkeys and humans have indicated that immunodeficiency viruses with Nef deleted are nonpathogenic in immunocompetent hosts, and this has motivated a search for live attenuated vaccine candidates. However, the mechanisms of action of Nef remain elusive. To define the regions of human immunodeficiency virus type 1 (HIV-1) Nef which mediate in vivo pathogenicity, a series of mutated isogenic viruses were inoculated into human thymic implants in SCID-hu mice. Mutation of several regions, including the myristoylation site at the second glycine and a region encompassing amino acids 41 through 49 of Nef, profoundly affected pathogenicity. Surprisingly, mutations of prolines in either of the two distant PXXP SH3 binding domains did not affect pathogenicity, indicating that these regions are not required for Nef activity in developing T-lineage cells. These data suggest that some functions of Nef described in vitro may not be relevant for in vivo pathogenicity.     

Virion Incorporation of Human Immunodeficiency Virus Type 1 Nef Is Mediated by a Bipartite Membrane-Targeting Signal: Analysis of Its Role in Enhancement of Viral Infectivity

The nef gene of primate immunodeficiency viruses is essential for high-titer virus replication and AIDS pathogenesis in vivo. In tissue culture, Nef is not required for human immunodeficiency virus (HIV) infection but enhances viral infectivity. We and others have shown that Nef is incorporated into HIV-1 particles and cleaved by the viral proteinase. To determine the signal for Nef incorporation and to analyze whether virion-associated Nef is responsible for enhancement of infectivity, we generated a panel of nef mutants and analyzed them for virion incorporation of Nef and for their relative infectivities. We report that N-terminal truncations of Nef abolished its incorporation into HIV particles. Incorporation was reconstituted by targeting the respective proteins to the plasma membrane by using a heterologous signal. Mutational analysis revealed that both myristoylation and an N-terminal cluster of basic amino acids were required for virion incorporation and for plasma membrane targeting of Nef. Grafting the N-terminal anchor domain of Nef onto the green fluorescent protein led to membrane targeting and virion incorporation of the resulting fusion protein. These results indicate that Nef incorporation into HIV-1 particles is mediated by plasma membrane targeting via an N-terminal bipartite signal which is reminiscent of a Src homology region 4. Virion incorporation of Nef correlated with enhanced infectivity of the respective viruses in a single-round replication assay. However, the phenotypes of HIV mutants with reduced Nef incorporation only partly correlated with their ability to replicate in primary lymphocytes, indicating that additional or different mechanisms may be involved in this system.     

Functional Interaction of Human Immunodeficiency Virus Type 1 Vpu and Gag with a Novel Member of the Tetratricopeptide Repeat Protein Family

Viral protein U (Vpu) is a protein encoded by human immunodeficiency virus type 1 (HIV-1) that promotes the degradation of the virus receptor, CD4, and enhances the release of virus particles from cells. We isolated a cDNA that encodes a novel cellular protein that interacts with Vpu in vitro, in vivo, and in yeast cells. This Vpu-binding protein (UBP) has a molecular mass of 41 kDa and is expressed ubiquitously in human tissues at the RNA level. UBP is a novel member of the tetratricopeptide repeat (TPR) protein family containing four copies of the 34-amino-acid TPR motif. Other proteins that contain TPR motifs include members of the immunophilin superfamily, organelle-targeting proteins, and a protein phosphatase. UBP also interacts directly with HIV-1 Gag protein, the principal structural component of the viral capsid. However, when Vpu and Gag are coexpressed, stable interaction between UBP and Gag is diminished. Furthermore, overexpression of UBP in virus-producing cells resulted in a significant reduction in HIV-1 virion release. Taken together, these data indicate that UBP plays a role in Vpu-mediated enhancement of particle release.