The central region of human immunodeficiency virus type 1 p6 protein (Gag residues S14-I31) is dispensable for the virus in vitro

How HIV-1 Gag Manipulates Its Host Cell Proteins: A Focus on Interactors of the Nucleocapsid Domain

The human immunodeficiency virus (HIV-1) polyprotein Gag (Group-specific antigen) plays a central role in controlling the late phase of the viral lifecycle. Considered to be only a scaffolding protein for a long time, the structural protein Gag plays determinate and specific roles in HIV-1 replication. Indeed, via its different domains, Gag orchestrates the specific encapsidation of the genomic RNA, drives the formation of the viral particle by its auto-assembly (multimerization), binds multiple viral proteins, and interacts with a large number of cellular proteins that are needed for its functions from its translation location to the plasma membrane, where newly formed virions are released. Here, we review the interactions between HIV-1 Gag and 66 cellular proteins. Notably, we describe the techniques used to evidence these interactions, the different domains of Gag involved, and the implications of these interactions in the HIV-1 replication cycle. In the final part, we focus on the interactions involving the highly conserved nucleocapsid (NC) domain of Gag and detail the functions of the NC interactants along the viral lifecycle.

C-Terminal HIV-1 Transframe p6* Tetrapeptide Blocks Enhanced Gag Cleavage Incurred by Leucine Zipper Replacement of a Deleted p6* Domain

HIV-1 protease (PR) functions as a homodimer mediating virus maturation following virus budding. Gag-Pol dimerization is believed to trigger embedded PR activation by promoting PR dimer formation. Early PR activation can lead to markedly reduced virus yields due to premature Gag cleavage. The p6* peptide, located between Gag and PR, is believed to ensure virus production by preventing early PR maturation. Studies aimed at finding supporting evidence for this proposal are limited due to a reading frame overlap between p6* and the p6gag budding domain. To determine if p6* affects virus production via the modulation of PR activation, we engineered multiple constructs derived from Dp6*PR (an assembly- and processing-competent construct with Pol fused at the inactivated PR C terminus). The data indicated that a p6* deletion adjacent to active PR significantly impaired virus processing. We also observed that the insertion of a leucine zipper (LZ) dimerization motif in the deleted region eliminated virus production in a PR activity-dependent manner, suggesting that the LZ insertion triggered premature PR activation by facilitating PR dimer formation. As few as four C-terminal p6* residues remaining at the p6*/PR junction were sufficient to restore virus yields, with a Gag processing profile similar to that of the wild type. Our study provides supporting evidence in a virus assembly context that the C-terminal p6* tetrapeptide plays a role in preventing premature PR maturation.IMPORTANCE Supporting evidence for the assumption that p6* retards PR maturation in the context of virus assembly is lacking. We found that replacing p6* with a leucine zipper peptide abolished virus assembly due to the significant enhancement of Gag cleavage. However, as few as four C-terminal p6* residues remaining in the deleted region were sufficient for significant PR release, as well as for counteracting leucine zipper-incurred premature Gag cleavage. Our data provide evidence that (i) p6* ensures virus assembly by preventing early PR activation and (ii) four C-terminal p6* residues are critical for modulating PR activation. Current PR inhibitor development efforts are aimed largely at mature PR, but there is a tendency for HIV-1 variants that are resistant to multiple protease inhibitors to emerge. Our data support the idea of modulating PR activation by targeting PR precursors as an alternative approach to controlling HIV-1/AIDS.

Understanding HIV-1 protease autoprocessing for novel therapeutic development

In the infected cell, HIV-1 protease (PR) is initially synthesized as part of the GagPol polyprotein. PR autoprocessing is a virus-specific process by which the PR domain embedded in the precursor catalyzes proteolytic reactions responsible for liberation of free mature PRs, which then recognize and cleave at least ten different peptide sequences in the Gag and GagPol polyproteins. Despite extensive structure and function studies of the mature PRs as well as the successful development of ten US FDA-approved catalytic-site inhibitors, the precursor autoprocessing mechanism remains an intriguing yet-to-be-solved puzzle. This article discusses current understanding of the autoprocessing mechanism, in an effort to prompt the development of novel anti-HIV drugs that selectively target precursor autoprocessing.

Slow immunological progression in HIV-1 CRF07_BC-infected injecting drug users

Human immunodeficiency virus type 1 (HIV-1) circulating recombinant form (CRF) 07_BC has caused serious HIV-1 epidemics among injecting drug users (IDUs) in East Asia. Little is known about the characteristics of the virus and its impact on disease progression among the infected individuals. In this study, we compared immunological progression between 423 IDUs infected with CRF07_BC and 194 men who have sex with men (MSM) with primary subtype B infection, and a representative full-length CRF07_BC molecular clone, pCRF07_BC, was constructed to characterize the virus. We found that IDUs infected with CRF07_BC had significantly slower immunological progression in the Cox proportional hazards model (hazard ratio: 0.30; 95% confidence interval: 0.13–0.69; P=0.004). The constructed recombinant CRF07_BC viruses had a reduced processing of the Gag/Gag-Pol polyproteins, a decreased incorporation of Vpr in the virus particle, tethering of virus particles on the plasma membrane and decreased virus growth kinetics. These phenotypes are related to the unique 7-amino acid deletion in the p6 of CRF07_BC, since complementation of the 7-amino acid in pCRF07_BC could improve the defective phenotypes. In summary, compared with MSM infected with HIV-1 subtype B, IDUs infected with CRF07_BC had slower immunological progression, which is likely correlated with interference of virus particle maturation by the 7-amino acid deletion in p6.

Transmission of new CRF07_BC strains with 7 amino acid deletion in Gag p6

A 7 amino acid deletion in Gag p6 (P6delta7) emerged in Chinese prevalent HIV-1 strain CRF07_BC from different epidemic regions. It is important to determine whether this mutation could be transmitted and spread. In this study, HIV-1 Gag sequences from 5 different epidemic regions in China were collected to trace the transmission linkage and to analyze genetic evolution of P6delta7 strains. The sequence analysis demonstrated that P6delta7 is a CRF07_BC specific deletion, different P6delta7 strains could be originated from different parental CRF07_BC recombinants in different epidemic regions, and the transmission of P6delta7 strain has occurred in IDU populations. This is for the first time to identify the transmission linkage for P6delta7 strains and serves as a wake-up call for further monitoring in the future; In addition, P6delta7 deletion may represent an evolutionary feature which might exert influence on the fitness of CRF07_BC strain.

Uncoupling human immunodeficiency virus type 1 Gag and Pol reading frames: role of the transframe protein p6* in viral replication

Apart from its regulatory role in protease (PR) activation, little is known about the function of the human immunodeficiency virus type 1 transframe protein p6* in the virus life cycle. p6* is located between the nucleocapsid and PR domains in the Gag-Pol polyprotein precursor and is cleaved by PR during viral maturation. We have recently reported that the central region of p6* can be extensively mutated without abolishing viral infectivity and replication in vitro. However, mutagenesis of the entire p6*-coding sequence in the proviral context is not feasible without affecting the superimposed frameshift signal or the overlapping p1-p6(gag) sequences. To overcome these limitations, we created a novel NL4-3-derived provirus by displacing the original frameshift signal to the 3' end of the gag gene, thereby uncoupling the p6* gene sequence from the p1-p6(gag) reading frame. The resulting virus (AL) proved to be replication competent in different cell cultures and thus represents an elegant tool for detailed analysis of p6* function. Hence, extensive deletions or substitutions were introduced into the p6* gene sequence of the AL provirus, and effects on particle release, protein processing, and viral infectivity were evaluated. Interestingly, neither the deletion of 63% of all p6* residues nor the partial substitution by a heterologous sequence affected virus growth and infectivity, suggesting that p6* is widely dispensable for viral in vitro replication. However, the insertion of a larger reporter sequence interfered with virus production and maturation, implying that the length or conformation of this spacer region might be critical for p6* function.

Influence of extended mutations of the HIV-1 transframe protein p6 on Nef-dependent viral replication and infectivity in vitro

The HIV-1 transframe protein p6 known to modulate HIV-1 protease activation has been suggested to interact with the viral pathogenicity factor Nef. However, a potential interaction site in p6 has not been mapped so far. To evaluate effects of p6 modification on viral replication in light of Nef function, clustered substitutions were introduced into the central p6 region of the infectious provirus NL4-3 and virus growth and composition of the various mutants was analyzed in different cell cultures in the presence or absence of Nef. Whereas clustered p6 substitutions did neither affect particle incorporation of Nef, nor precursor maturation or viral infectivity, a simultaneous substitution of 40 of the total 56 p6 residues significantly diminished viral infectivity and replication in a Nef-independent manner. Furthermore, this extended modification was not capable of rescuing the negative effects of a transdominant Nef mutant on particle production suggesting that the proposed target for Nef interaction in Gag-Pol is located outside the modified p6 region. In sum these data strongly argue against a functional connection of the central p6 region and Nef during viral life cycle.

Analysis of HIV-1 CRF07_BC gag p6 sequences indicating novel deletions in the central region of p6

We amplified gag sequences from 66 individuals infected with HIV-1 CRF07_BC during 2003-2005 in the Xinjiang region of China. A novel deletion of 7aa (including a KELY motif) in the central region of the CRF07_BC gag p6 domain was detected, which has not been reported in other HIV-1 subtypes. Further deletions of up to 13aa (including KQE and KELY motifs) was also found in this domain, representing the biggest natural deletion up to now. Moreover, the CD4+ count and viral load level indicated that 1-13aa deletions in CRF07_BC gag p6 do not have a significant effect on viral replication and fitness.

Effects of human immunodeficiency virus type 1 transframe protein p6* mutations on viral protease-mediated Gag processing

The proteolytic processing of human immunodeficiency virus (HIV) particles mediated by the viral pol-encoded protease (PR) is essential for viral infectivity. The pol coding sequence partially overlaps with the gag coding sequence and is translated as a Gag-Pol polyprotein precursor. Within Gag-Pol, the C-terminal p6(gag) domain is replaced by a transframe peptide referred to as p6*, which separates the Gag nucleocapsid domain from PR. Several previous in vitro studies have ascribed a PR-suppression regulatory function to p6*. Here, it was demonstrated that an HIV-1 Gag-Pol lacking p6* is efficiently incorporated into virions when coexpressed with HIV-1 Gag precursor. However, the released virions are not processed appropriately and show a greatly reduced viral infectivity. This suggests that the p6* is indispensable during the process of PR-mediated virus particle maturation.

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

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

Contribution of the Gag-Pol transframe domain p6* and its coding sequence to morphogenesis and replication of human immunodeficiency virus type 1

The human immunodeficiency virus type-1 (HIV-1) transframe domain p6* is located between the nucleocapsid protein (NC) and the protease (PR) within the Gag-Pol precursor. This flexible, 68-amino-acid HIV-1 p6* domain has been suggested to negatively interfere with HIV PR activity in vitro proposing a contribution of either the C-terminal p6* tetrapeptide, internal cryptic PR cleavage sites, or a zymogen-related mechanism to a regulated PR activation. To assess these hypotheses in the viral context, a series of recombinant HX10-based provirus constructs has been established with clustered amino acid substitutions throughout the entire p6* coding sequence. Comparative analysis of the mutant proviral clones in different cell culture systems revealed that mutations within the well-conserved amino-terminal p6* region modified the Gag/Gag-Pol ratio and thus resulted in the release of viruses with impaired infectivity. Clustered amino acid substitutions destroying (i) the predicted cryptic PR cleavage sites or (ii) homologies to the pepsinogen propeptide did not influence viral replication in cell culture, whereas substitutions of the carboxyl-terminal p6* residues 62 to 68 altering proper release of the mature PR from the Gag-Pol precursor drastically reduced viral infectivity. Thus, the critical contribution of p6* and overlapping cis-acting sequence elements to timely regulated virus maturation and infectivity is closely linked to precise ribosomal frameshifting and proper N-terminal release of the viral PR from the Gag-Pol precursor, clearly disproving the hypothesis that sequence motifs in the central part of p6* modulate PR activation and viral infectivity.