Sequential steps in human immunodeficiency virus particle maturation revealed by alterations of individual Gag polyprotein cleavage sites

Assembly and release of human immunodeficiency virus type 1 Gag proteins containing tandem repeats of the matrix protein coding sequences in the matrix domain

We have constructed human immunodeficiency virus (HIV) gag mutants by increasing the matrix protein (MA) sequences via tandemly repeated duplication of the central 107-MA codons. Instead of a total of 132 amino acid residues for the wild-type MA, the resultant mutants designated as MA2, MA3, and MA4 contained a total of 242, 352, and 462 codons in the MA domains, respectively. Analysis indicated that the addition of 110 or 220 amino acid residues to the MA did not significantly affect the assembly, release, and processing of particles; however, particle production was markedly reduced when another copy of 110 residues was added to the MA. Subcellular fractionation analysis suggested that the MA tandem repeat mutations enhanced the Gag membrane affinity, in a manner which correlated with the copy number of MA sequences. The effects of enhanced membrane affinity were substantially reduced when sequences downstream of the capsid (CA) domain were deleted. Sucrose density gradient fractionation analysis showed that particles produced by the large insertion mutants possessed wild-type (wt) HIV particle density. Truncation of sequences downstream of the nucleocapsid (NC) domains of the mutants did not influence the budding of particles. In contrast, particle budding was severely impaired when sequences downstream of the CA domain were truncated. Particle densities for the large Gag proteins, which were truncated at the C-terminus of CA, were about 1.12-1.14 g/ml lower than that for wt. Our results suggest that the HIV MA domain could adopt insertions of large protein sequences, and strongly support the proposal that the NC and p2 domains play a crucial role in the process of correct Gag protein packing.

Role of Vif in stability of the human immunodeficiency virus type 1 core

The Vif protein of human immunodeficiency virus type 1 (HIV-1) is important for virion infectivity. Previous studies have shown that vif-defective virions exhibit structural abnormalities in the virus core and are defective in the ability to complete proviral DNA synthesis in acutely infected cells. We developed novel assays to assess the relative stability of the core in HIV-1 virions. Using these assays, we examined the role of Vif in the stability of the HIV-1 core. The integrity of the core was examined following virion permeabilization or removal of the lipid envelope and treatment with various triggers, including S100 cytosol, deoxynucleoside triphosphates, detergents, NaCl, and buffers of different pH to mimic aspects of the uncoating and disassembly process which occurs after virus entry but preceding or during reverse transcription. vif mutant cores were more sensitive to disruption by all triggers tested than wild-type cores, as determined by endogenous reverse transcriptase (RT) assays, biochemical analyses, and electron microscopy. RT and the p7 nucleocapsid protein were released more readily from vif mutant virions than from wild-type virions, suggesting that the internal nucleocapsid is less stably packaged in the absence of Vif. Purified cores could be isolated from wild-type but not vif mutant virions by sedimentation through detergent-treated gradients. These results demonstrate that Vif increases the stability of virion cores. This may permit efficient viral DNA synthesis by preventing premature degradation or disassembly of viral nucleoprotein complexes during early events after virus entry.

In vitro processing of human immunodeficiency virus type 1 Gag virus-like particles

Human immunodeficiency virus (HIV) Gag proteins are assembled into virus particles and then cleaved by the virion-associated HIV protease. Concomitant with Gag processing, doughnut-like HIV particles (the immature form) are converted to particles containing condensed cores (the mature form). Here we describe the in vitro processing of immature HIV Gag virus-like particles (VLP) by exogenously added HIV protease. Following delipidization, sequential processing of immature VLP showed that the matrix (MA)/capsid (CA) junction was cleaved faster than the CA/nucleocapsid (NC) junction, an altered order of processing when compared with authentic processing. When the in vitro processed VLP were analyzed on density gradients, most of the MA, CA-p15 intermediate, and NC were detected as a highly multimeric form, equivalent to the unprocessed VLP. In contrast, CA was found as a monomer dissociated from the multimeric CA-p15 following cleavage of the CA/NC junction. Electron microscopy revealed that the in vitro processing was accompanied by conversion of the doughnut-like particles to particles containing condensed cores and spherical outer shells. The cores, however, lacked core shells, which are normally observed for authentic HIV, suggesting that the in vitro processing of immature VLP failed to produce core shells.

Structure and self-association of the Rous sarcoma virus capsid protein

BACKGROUND

The capsid protein (CA) of retroviruses, such as Rous sarcoma virus (RSV), consists of two independently folded domains. CA functions as part of a polyprotein during particle assembly and budding and, in addition, forms a shell encapsidating the genomic RNA in the mature, infectious virus.

RESULTS

The structures of the N- and C-terminal domains of RSV CA have been determined by X-ray crystallography and solution nuclear magnetic resonance (NMR) spectroscopy, respectively. The N-terminal domain comprises seven alpha helices and a short beta hairpin at the N terminus. The N-terminal domain associates through a small, tightly packed, twofold symmetric interface within the crystal, different from those previously described for other retroviral CAs. The C-terminal domain is a compact bundle of four alpha helices, although the last few residues are disordered. In dilute solution, RSV CA is predominantly monomeric. We show, however, using electron microscopy, that intact RSV CA can assemble in vitro to form both tubular structures constructed from toroidal oligomers and planar monolayers. Both modes of assembly occur under similar solution conditions, and both sheets and tubes exhibit long-range order.

CONCLUSIONS

The tertiary structure of CA is conserved across the major retroviral genera, yet sequence variations are sufficient to cause change in associative behavior. CA forms the exterior shell of the viral core in all mature retroviruses. However, the core morphology differs between viruses. Consistent with this observation, we find that the capsid proteins of RSV and human immunodeficiency virus type 1 exhibit different associative behavior in dilute solution and assemble in vitro into different structures.

Comparison of human immunodeficiency virus type 1 Pr55(Gag) and Pr160(Gag-pol) processing intermediates that accumulate in primary and transformed cells treated with peptidic and nonpeptidic protease inhibitors

Human immunodeficiency virus type 1 (HIV-1) produces two polyproteins, Pr55(Gag) and Pr160(Gag-Pol), that are cleaved into mature functional subunits by the virally encoded protease. Drugs that inhibit this protease are an important part of anti-HIV therapy. We studied the ordered accumulation of Gag and Gag-Pol processing intermediates by variably blocking the protease with HIV-1 protease inhibitors (PIs). Variable protease inhibition caused accumulation of a complex pattern of processing intermediates, which was the same after incubating HIV-1-infected cells with increasing concentrations of either one of the peptidomimetic inhibitors indinavir, saquinavir (SQV), ritonavir (RTV), nelfinavir, and SC-52151 or one of the nonpeptidomimetic inhibitors DMP450, DMP323, PNU-140135, and PNU-109112 for 3 days. The patterns of Gag and Gag-Pol processing intermediate accumulation were nearly identical when the following were compared: cell- versus virion-associated proteins, HIV-1-infected transformed cell lines versus primary human peripheral blood mononuclear cells (PBMCs) and HIV-1(MN) versus HIV-1(IIIB) virus strains. RTV was a more potent inhibitor of p24 production in PBMCs than SQV by approximately 7-fold, whereas SQV was a more potent inhibitor in transformed cells than RTV by approximately 30-fold. Although the antiretroviral potency of HIV-1 PIs may change as a function of cell type, the polyprotein intermediates that accumulate with increasing drug concentrations are the same. These results support sequential processing of Gag and Gag-Pol polyproteins by the HIV-1 protease and may have important implications for understanding common cross-resistance pathways.

Proline residues in the HIV-1 NH2-terminal capsid domain: structure determinants for proper core assembly and subsequent steps of early replication

Recent analyses suggest that the p24 capsid (p24(CA)) domain of the HIV-1 group-specific antigen (Gag) may be divided into two structurally and functionally distinct moieties: (i) an amino-terminal portion, previously shown to bind the cellular chaperone cyclophilin A, and (ii) a carboxy-terminal domain, known to contribute to the interaction of the Gag and Gag-Pol precursors during the early assembly process. In order to gain deeper insight into the role of the amino-terminal domain of the p24(CA) protein during viral replication, eight highly conserved proline residues known to promote turns and to terminate alpha-helices within the p24 tertiary structure were replaced by a leucine residue (P-position-L). Following transfection of the proviral constructs in COS7 cells, the majority of the mutants resembled wild-type viruses with respect to the assembly and release of virions. However, although the released particles contained wild-type levels of genomic viral RNA, the mature products of the Gag and Gag-Pol polyproteins as well as the Env glycoproteins-all of them, except mutant P225L-were either noninfectious or severely affected in their replicative capacity. Entry assays monitoring the process of viral DNA synthesis led to the classification of selected provirus mutants into four different phenotypes: (i) mutant P225L was infectious and allowed complete reverse transcription including formation of 2-LTR circles; (ii) mutants P149L, P170L, and P217L failed to form 2-LTR circles; (iii) mutant P222L displayed a severe defect in binding and incorporating cyclophilin A into virions, was delayed with respect to DNA polymerization, and failed to form a 2-LTR replication intermediate; and (iv) mutant P133L was unable even to synthesize a first-strand cDNA product. All replication-defective mutants were characterized by severe alterations in the stability of virion cores, which were in two cases reflected by visible changes in the core morphology. These results suggest that proline residues in the NH(2)-terminal capsid domain represent critical structure determinants for proper formation of functional virion cores and subsequent stages of early replication.

Biochemical and structural analysis of isolated mature cores of human immunodeficiency virus type 1

Mature human immunodeficiency virus type 1 (HIV-1) particles contain a cone-shaped core structure consisting of the internal ribonucleoprotein complex encased in a proteinaceous shell derived from the viral capsid protein. Because of their very low stability after membrane removal, HIV-1 cores have not been purified in quantities sufficient for structural and biochemical analysis. Based on our in vitro assembly experiments, we have developed a novel method for isolation of intact mature HIV-1 cores. Concentrated virus suspensions were briefly treated with nonionic detergent and immediately centrifuged in a microcentrifuge for short periods of time. The resuspended pellet was subsequently analyzed by negative-stain and thin-section electron microscopy and by immunoelectron microscopy. Abundant cone-shaped cores as well as tubular and aberrant structures were observed. Stereo images showed that core structures preserved their three-dimensional architecture and exhibited a regular substructure. Detailed analysis of 155 cores revealed an average length of ca. 103 nm, an average diameter at the base of ca. 52 nm, and an average angle of 21.3 degrees. There was significant variability in all parameters, indicating that HIV cores are not homogeneous. Immunoblot analysis of core preparations allowed semiquantitative estimation of the relative amounts of viral and cellular proteins inside the HIV-1 core, yielding a model for the topology of various proteins inside the virion.

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.

Association of Nef with the human immunodeficiency virus type 1 core

Highly conserved among primate lentiviruses, the human immunodeficiency virus type 1 (HIV-1) Nef protein enhances viral infectivity by an unknown mechanism. Nef-defective virions are blocked at a stage of the HIV-1 life cycle between entry and reverse transcription, possibly virus uncoating. Nef is present in purified HIV-1 particles; however, it has not been determined whether Nef is specifically recruited into HIV-1 particles or whether virion-associated Nef plays a functional role in HIV-1 replication. To address the specificity and potential functionality of virion-associated Nef, we determined the subviral localization of Nef. HIV-1 cores were isolated by detergent treatment of concentrated virions followed by equilibrium density gradient sedimentation. Relative to HIV-1 virions, HIV-1 cores contained equivalent amounts of reverse transcriptase and integrase, decreased amounts of the viral matrix protein, and trace quantities of the viral transmembrane glycoprotein gp41. Examination of the particles by electron microscopy revealed cone-shaped structures characteristic of lentiviral cores. Similar quantities of proteolytically processed Nef protein were detected in gradient fractions of HIV-1 cores and intact virions. In addition, detergent-resistant subviral complexes isolated from immature HIV-1 particles contained similar quantities of Nef as untreated virions. These results demonstrate that Nef stably associates with the HIV-1 core and suggest that virion-associated Nef plays a functional role in accelerating HIV-1 replication.

Effect of substrate residues on the P2' preference of retroviral proteinases

The substrate sequence requirements for preference toward P2' Glu residue by human immunodeficiency virus type 1 (HIV-1) proteinase were studied in both the matrix protein/ capsid protein (MA/CA) and CA/p2 cleavage site sequence contexts. These sequences represent typical type 1 (-aromatic*Pro-) and type 2 (-hydrophobic* hydrophobic-) cleavage site sequences, respectively. While in the type 1 sequence context, the preference for P2' Glu over Ile or Gln was found to be strongly dependent on the ionic strength and the residues being outside the P2-P2' region of the substrate, it remained preferable in the type 2 substrates when typical type 1 substrate sequence residues were substituted into the outside regions. The pH profile of the specificity constants suggested a lower pH optimum for substrates having P2' Glu in contrast to those having uncharged residues, in both sequence contexts. The very low frequency of P2' Glu in naturally occurring retroviral cleavage sites of various retroviruses including equine infectious anemia virus (EIAV) and murine leukemia virus (MuLV) suggests that such a residue may not have a general regulatory role in the retroviral life cycle. In fact, unlike HIV-1 and HIV-2, EIAV and MuLV proteinases do not favor P2' Glu in either the MA/CA or CA/p2 sequence contexts.

Backbone dynamics of the N-terminal domain of the HIV-1 capsid protein and comparison with the G94D mutant conferring cyclosporin resistance/dependence

Nuclear magnetic resonance (NMR) (15)N relaxation methods have been used to characterize the backbone dynamics of the N-terminal core domain of the HIV-1 capsid protein (CA(151)). The domain, which has an unusually flat, triangular shape, tumbles in solution at 28 degrees C with an effective rotational correlation time of 11.5 ns. Relaxation data for backbone amides in the domain's seven alpha-helices are indicative of fully anisotropic rotational diffusion. The principal axes of the rotational diffusion tensor calculated from the NMR data are aligned to within 12-23 degrees of the principal axes of the inertial tensor, with the axis of fastest rotational diffusion coincident with that of minimal inertia, and vice versa. Large variations in the (15)N-(1)H nuclear Overhauser effects for individual amino acids correlate with the degree of convergence in the previously calculated NMR structure. In particular, the partially disordered residues Val86-Arg97 that contain the human cyclophilin A (CypA) packaging signal have (15)N heteronuclear NOEs and transversal relaxation rates consistent with a high degree of dynamic conformational averaging. The N-terminal domain of a CA mutant (G94D) that confers both resistance to and dependence on cyclosporin A analogues was also analyzed. Our results indicate that this mutation does not influence the conformation or dynamics of CA(151), and therefore probably affects the function of the protein by modifying essential intermolecular CA-CA interactions.

Mutations within four distinct gag proteins are required to restore replication of human immunodeficiency virus type 1 after deletion mutagenesis within the dimerization initiation site

Human immunodeficiency virus type 1 (HIV-1) genomic RNA segments at nucleotide (nt) positions +240 to +274 are thought to form a stem-loop secondary structure, termed SL1, that serves as a dimerization initiation site for viral genomic RNA. We have generated two distinct deletion mutations within this region, termed BH10-LD3 and BH10-LD4, involving nt positions +238 to +253 and +261 to +274, respectively, and have shown that each of these resulted in significant diminutions in levels of viral infectiousness. However, long-term culture of each of these viruses in MT-2 cells resulted in a restoration of infectiousness, due to a series of compensatory point mutations within four distinct proteins that are normally cleaved from the Gag precursor. In the case of BH10-LD3, these four mutations were MA1, CA1, MP2, and MNC, and they involved changes of amino acid Val-35 to Ile within the matrix protein (MA), Ile-91 to Thr within the capsid (CA), Thr-12 to Ile within p2, and Thr-24 to Ile within the nucleocapsid (NC). The order in which these mutations were acquired by the mutated BH10-LD3 was MNC > CA1 > MP2 > MA1. The results of site-directed mutagenesis studies confirmed that each of these four substitutions contributed to the increased viability of the mutated BH10-LD3 viruses and that the MNC substitution, which was acquired first, played the most important role in this regard. Three point mutations, MP2, MNC, and MA2, were also shown to be sequentially acquired by viruses that had emerged in culture from the BH10-LD4 deletion. The first two of these were identical to those described above, while the last involved a change of Val-35 to Leu. All three of these substitutions were necessary to restore the infectiousness of mutated BH10-LD4 viruses to wild-type levels, although the MP2 mutation alone, but neither of the other two substitutions, was able to confer some viability on BH10-LD4 viruses. Studies of viral RNA packaging showed that the BH10-LD4 deletion only marginally impaired encapsidation while the BH10-LD3 deletion caused a severe deficit in this regard.

Deletion mutagenesis within the dimerization initiation site of human immunodeficiency virus type 1 results in delayed processing of the p2 peptide from precursor proteins

Previous work has shown that deletions of genomic segments at nucleotide (nt) positions +238 to +253, i.e., construct BH10-LD3, or nt positions +261 to +274, i.e., construct BH10-LD4, within the human immunodeficiency virus type 1 (HIV-1) dimerization initiation site (DIS) destroyed DIS secondary structure and dramatically reduced viral replication capacity. Surprisingly, two point mutations located within the viral peptide 2 (p2) and nucleocapsid (NC) protein termed MP2 and MNC, respectively, were able to compensate for this defect. Since the MP2 mutation involves an amino acid substitution near the cleavage site between p2 and NC, we investigated the effects of the above-mentioned deletions on the processing of Gag proteins. Immunoprecipitation assays performed with monoclonal antibodies against viral capsid (CA) (p24) protein showed that p2 was cleaved from CA with less efficiency in viruses that contained the LD3 and LD4 deletions than in wild-type viruses. The presence of the two compensatory mutations, MP2 and MNC, increased the efficiency of the cleavage of p2 from CA, but neither mutation alone had this effect or was sufficient to compensate for the observed impairment in infectiousness. A virus that contained both of the above-mentioned deletions within the DIS was also impaired in regard to processing and infectiousness, and it could likewise be compensated by the MP2 and MNC point mutations. These results suggest that the DIS region of HIV-1 RNA plays an important role in the processing of Gag proteins.

Cyclophilin A incorporation is not required for human immunodeficiency virus type 1 particle maturation and does not destabilize the mature capsid

The cellular protein cyclophilin A (CypA) is packaged into human immunodeficiency virus type 1 (HIV-1) virions through a specific interaction with the capsid (CA) domain of the Gag polyprotein. CypA is important for infectivity, but its role in viral replication is currently unknown. Previous reports suggested that CypA promotes uncoating or enhances maturation. We analyzed the morphology and capsid stability of HIV-1 variants defective in CypA binding and of virus grown in the presence of cyclosporin. Both cyclosporin treatment and alteration of Gly89 or Pro90 in the CypA-binding site of CA caused a 5- to 20-fold decrease in CypA incorporation. Virus produced from cyclosporin-treated cells and variants G89V and G89A were 10- to 100-fold less infectious but exhibited normal virion morphologies with regular cone-shaped capsids. Irregular capsid morphologies and lower infectivities were observed for some other variants in the CypA-binding region. Decreased CypA incorporation did not reduce the kinetics of intracellular polyprotein processing or of virus release. No increase in immature particles was observed. These results suggest that CypA does not promote virion maturation. Furthermore, detergent stripping of virus particles with various CypA contents revealed no difference in capsid stability. Based on these results and those reported in the accompanying paper, it appears likely that CypA also is not an uncoating factor. Alternative models for CypA function are discussed.

Opposing effects of human immunodeficiency virus type 1 matrix mutations support a myristyl switch model of gag membrane targeting

Targeting of the human immunodeficiency virus type 1 (HIV-1) Gag precursor Pr55(gag) to the plasma membrane, the site of virus assembly, is primarily mediated by the N-terminal matrix (MA) domain. N-myristylation of MA is essential for the stable association of Pr55(gag) with membranes and for virus assembly. We now show that single amino acid substitutions near the N terminus of MA can dramatically impair assembly without compromising myristylation. Subcellular fractionation demonstrated that Gag membrane binding was compromised to a similar extent as in the absence of the myristyl acceptor site, indicating that the myristyl group was not available for membrane insertion. Remarkably, the effects of the N-terminal modifications could be completely suppressed by second-site mutations in the globular core of MA. The compensatory mutations enhanced Gag membrane binding and increased viral particle yields above wild-type levels, consistent with an increase in the exposure of the myristyl group. Our results support a model in which the compact globular core of MA sequesters the myristyl group to prevent aberrant binding to intracellular membranes, while the N terminus is critical to allow the controlled exposure of the myristyl group for insertion into the plasma membrane.

Head-to-tail dimers and interdomain flexibility revealed by the crystal structure of HIV-1 capsid protein (p24) complexed with a monoclonal antibody Fab

The crystal structure of an intact molecule of HIV-1 capsid protein (p24) in complex with a monoclonal antibody fragment recognizing an epitope on the C-terminal domain has been determined at 3 A resolution. The helical N- and C-terminal domains of p24 are linked by an extended peptide forming a flexibly linked dumb-bell-shaped molecule 75 A in overall length. The p24 construct used is a variant with an N-terminal extension that mimics to some extent the Gag context of p24. We observed a novel head-to-tail dimer of p24 molecules which occurs through the formation of a substantial intermolecular interface between the N- and C-terminal domains. Comparison with previously observed p24 dimers shows that the same residues and secondary structural elements can partake in different interfaces revealing a remarkable stickiness and plasticity of the p24 molecule, properties which, combined with the inter-domain flexibility, are presumably important in the assembly and maturation of viral particles. Previous mutagenesis studies designed to test specific N-N and C-C homodimer interfaces do not discriminate fully against the possibility of the observed N-C interface.

Model for lentivirus capsid core assembly based on crystal dimers of EIAV p26

Two crystal forms of recombinant p26 capsid protein (CA) from the equine infectious anemia virus (EIAV) have in common an antiparallel four-helix bundle dimer interface between N-terminal domains (NTDs). The dimer interface provides a lenient scaffold to accommodate the wide sequence variation in these helices within lentivirus CA. Pairs of dimers weakly associate to form exact or approximate D2 symmetry tetramers. In one of the two crystal forms, the tetramers are linked via dimerization of C-terminal domains (CTDs). We propose that the observed NTD and CTD homodimer interactions are involved in the assembly of the lentivirus capsid. The NTD homodimer shape readily suggests a model for the mature capsid core, based on hexagonal packing with dimensions and surface topology resembling described EIAV capsid cores. Combining available data for human immunodeficiency virus and EIAV CA, we also propose an assembly pathway for maturation of the lentivirus capsid core following proteolytic cleavage of the gag polyprotein precursor.

Structural biology of HIV

The human immunodeficiency virus (HIV) genome encodes a total of three structural proteins, two envelope proteins, three enzymes, and six accessory proteins. Studies over the past ten years have provided high-resolution three-dimensional structural information for all of the viral enzymes, structural proteins and envelope proteins, as well as for three of the accessory proteins. In some cases it has been possible to solve the structures of the intact, native proteins, but in most cases structural data were obtained for isolated protein domains, peptidic fragments, or mutants. Peptide complexes with two regulatory RNA fragments and a protein complex with an RNA recognition/encapsidation element have also been structurally characterized. This article summarizes the high-resolution structural information that is currently available for HIV proteins and reviews current structure-function and structure-biological relationships.

HIV-1 gag proteins: diverse functions in the virus life cycle

The Gag proteins of HIV-1, like those of other retroviruses, are necessary and sufficient for the assembly of virus-like particles. The roles played by HIV-1 Gag proteins during the life cycle are numerous and complex, involving not only assembly but also virion maturation after particle release and early postentry steps in virus replication. As the individual Gag domains carry out their diverse functions, they must engage in interactions with themselves, other Gag proteins, other viral proteins, lipid, nucleic acid (DNA and RNA), and host cell proteins. This review briefly summarizes our current understanding of how HIV-1 Gag proteins function in the virus life cycle.

Fusion between uninfected cells in retrovirus-induced fusion-from-within

We previously examined Moloney murine leukemia virus-induced fusion-from-within (FFWI) and fusion-from-without (FFWO) of SC-1 mouse cells. FFWI and FFWO can be distinguished by their stimulation by ionophores and polycations, respectively. FFWI is caused by infected cells. Normally, fusion between an infected cell and uninfected cells (heterofusions) is described, but we have surprisingly found that the infected cells also caused homofusion between uninfected cells in their vicinity (named neighbor homofusions). It was shown that neighbor homofusions were not induced by free virus particles (by FFWO). Transfectants expressing envelope proteins only induced heterofusions, indicating that virus production is necessary for the formation of neighbor homofusions. Both plasma membrane fragments and easily removable material from the surface of infected cells were able to induce fusion with the same stimulation pattern as FFWI and neighbor homofusion. These materials, especially the latter, have properties in common with virions, and it is discussed whether immature virions are involved in the formation of the neighbor homofusions.

N-Terminal extension of human immunodeficiency virus capsid protein converts the in vitro assembly phenotype from tubular to spherical particles

Expression of retroviral Gag polyproteins is sufficient for morphogenesis of virus-like particles with a spherical immature protein shell. Proteolytic cleavage of Gag into the matrix (MA), capsid (CA), nucleocapsid (NC), and p6 domains (in the case of human immunodeficiency virus [HIV]) leads to condensation to the mature cone-shaped core. We have analyzed the formation of spherical or cylindrical particles on in vitro assembly of purified HIV proteins or inside Escherichia coli cells. CA protein alone yielded cylindrical particles, while all N-terminal extensions of CA abolished cylinder formation. Spherical particles with heterogeneous diameters or amorphous protein aggregates were observed instead. Extending CA by 5 amino acids was sufficient to convert the assembly phenotype to spherical particles. Sequences C-terminal of CA were not required for sphere formation. Proteolytic cleavage of N-terminally extended CA proteins prior to in vitro assembly led to the formation of cylindrical particles, while proteolysis of in vitro assembly products caused disruption of spheres but not formation of cylinders. In vitro assembly of CA and extended CA proteins in the presence of cyclophilin A (CypA) at a CA-to-CypA molar ratio of 10:1 yielded significantly longer cylinders and heterogeneous spheres, while higher concentrations of CypA completely disrupted particle formation. We conclude that the spherical shape of immature HIV particles is determined by the presence of an N-terminal extension on the CA domain and that core condensation during virion maturation requires the liberation of the N terminus of CA.