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The lentiviral accessory protein Vpx is critical for viral infection of myeloid cells and acts by hijacking CRL4(DCAF1) E3 ubiquitin ligase to induce the degradation of the host restriction factor SAMHD1. It has been observed that the sequences from HIV-2 and SIVsmm/SIVmac Vpx contain a poly-proline tail which is distinct from other SIV Vpx proteins. However, the role of this region in Vpx function is controversial. Herein, we found proteasome-dependent degradation of a Vpx mutant lacking the poly-proline tail in the nucleus in a CRL4(DCAF1) E3 ligase-independent fashion. Unlike wild-type Vpx, the poly-proline tail mutant Vpx is partly defective in enhancing viral infection in macrophages. Our findings suggest that during Vpx evolution, Vpx of the HIV-2/SIVsm/SIVmac lineage is targeted by a CRL4(DCAF1) E3 ligase-independent ubiquitination pathway, and have gained this interesting region, allowing them to maintain nuclear accumulation as part of their adaptation to host cell regulation.

Human APOBEC3G and other APOBEC3 cytidine deaminases inhibit a variety of retroviruses, including Vif-deficient HIV-1. These host proteins are packaged into viral particles and inhibit the replication of virus in new target cells. A3G and A3F are known to be efficiently packaged into HIV-1 virions by binding to 7SL RNA through the Gag NC domain; however, the packaging mechanisms of other APOBEC3 proteins are poorly defined. We have now demonstrated that APOBEC3C (A3C) can be efficiently packaged into HIV-1 virions that are deficient for viral genomic RNA. Inhibition of the encapsidation of 7SL RNA into HIV-1 virions blocked the packaging of A3G, but not A3C. While the NC domain is required for efficient packaging of A3G, deletion of this domain had little effect on A3C packaging into HIV-1 Gag particles. A3C interacted with HIV-1 Gag which was MA domain-dependent and RNA-dependent. Deletion of the MA domain of HIV-1 Gag inhibited A3C but not A3G packaging into HIV-1 Gag particles. Thus, A3G and A3C have evolved to use distinct mechanisms for targeting retroviruses.

APOBEC3G (A3G) and related cytidine deaminases are potent inhibitors of retroviruses. HIV-1 Vif hijacks the cellular Cul5-E3 ubiquitin ligase to degrade APOBEC3 proteins and render them ineffective against these viruses. Here, we report that HIV-1 Vif is a novel zinc-binding protein containing an H-x5-C-x17–18-C-x3–5-H motif that is distinct from other recognized classes of zinc fingers. Zinc-binding stabilized a conserved hydrophobic interface within the HCCH motif that is critical for Vif-Cul5 E3 assembly and Vif function. An N-terminal region in the first Cullin repeat of Cul5, which is dispensable for adaptor ElonginC binding, was required for interaction with Vif. This region is the most divergent sequence between Cul2 and Cul5, a factor that may contribute to the selection of Cul5 and not Cul2 by Vif. This is the first example of a zinc-binding substrate receptor responsible for the assembly of a Cullin-RING ligase, representing a new target for antiviral development.

Loss of T cell homeostasis usually precedes the onset of AIDS. We hypothesized that rapid progressors may be transmitted with HIV-1 that is particularly able to perturb T cell homeostasis. To this end, we have tested two transmitted, syncytium-inducing (SI) viral isolates from a rapid progressor in two thymus models. One of the isolates (R3A) exhibited markedly rapid kinetics of replication and thymocyte depletion. These phenotypes mapped to the envelope, as a recombinant NL4-3 virus encoding the R3A envelope had similar phenotypes, even in the absence of nef. Notably, the viruses with high pathogenic activity in the thymus (R3A and NL4-R3A) did not show enhanced replication or cytopathicity in PHA-stimulated PBMCs. Furthermore, NL4-R3A did not enhance replication of the coinfected NL4-3 virus in the thymus, suggesting an intrinsic advantage of the R3A envelope. The R3A envelope showed higher entry activity in infecting human T cells and in depleting CD4+ thymocytes when expressed in trans. These data suggest that SI viruses with unique envelope functions which can overcome barriers to transmission may hasten disease progression by perturbing T cell homeostasis.

The role of the nucleocapsid protein of HIV-1 Gag in virus assembly was investigated using Gag truncation mutants, a nucleocapsid deletion mutant, and point mutations in the nucleocapsid region of Gag, in transfected COS cells, and in stable T-cell lines. Consistent with previous investigations, a truncation containing only the matrix and capsid regions of Gag was unable to assemble efficiently into particles; also, the pelletable material released was lighter than the density of wild-type HIV-1. A deletion mutant lacking p7 nucleocapsid but containing the C-terminal p6 protein was also inefficient in particle release and released lighter particles, while a truncation containing only the first zinc finger of p7 could assemble more efficiently into virions. These results clearly show that p7 is indispensable for virus assembly and release. Some point mutations in the N-terminal basic domain and in the basic linker region between the two zinc fingers, which had been previously shown to have reduced RNA binding in vitro [Schmalzbauer, E., Strack, B., Dannull, J., Guehmann, S., and Moelling, K. (1996). J. Virol. 70: 771–777], were shown to reduce virus assembly dramatically when expressed in full-length viral clones. A fusion protein consisting of matrix and capsid fused to a heterologous viral protein known to have nonspecific RNA binding activity [Ribas, J. C., Fujimura, T., and Wickner, R. B. (1994) J. Biol. Chem. 269: 28420–28428] released pelletable material slightly more efficiently than matrix and capsid alone, and these particles had density higher than matrix and capsid alone. These results demonstrate the essential role of HIV-1 nucleocapsid in the virus assembly process and show that the positively charged N terminus of p7 is critical for this role.

For type-C and lentiviruses, including human immunodeficiency virus type 1 (HIV-1), the pathway of virus assembly remains poorly defined, and the assembly and budding of capsids are believed to occur simultaneously at the plasma membrane of the infected cell. We have now identified two putative HIV-1 assembly intermediate complexes in infected CD4+T cells. The first of these intermediates, a detergent-resistant complex (DRC), was identified as a large oligomer that had a density of 1.10–1.13 g/ml and was primarily composed of Pr55Gagand Pr160Gag-Polprecursors. The other putative intermediate was a detergent-sensitive complex (DSC) with a density of 1.15–1.17 g/ml, which apparently represented the products of extensive proteolytic processing of both the Pr55Gagand Pr160Gag-Polprecursors. Both complexes could be distinguished from released mature virions as well as immature viral particles. Surprisingly, the formation of DRC was not dependent upon the myristylation at the N-terminus of the Gag proteins, a signal required for plasma membrane targeting and virus production. However, the myristic acid modification was essential for the formation of DSC. These data suggest that interactions between individual Gag molecules and between Gag and Gag-Pol precursors may occur before their targeting to the plasma membrane during HIV-1 assembly. However, formation of the late virus assembly complex and productive processing of Pr55Gagand Pr160Gag-Polprecursors apparently do not occur until these precursors are targeted to the plasma membrane.

The viral infectivity factor gene ( vif ) of HIV-1 increases the infectivity of viral particles by inactivation of cellular anti-viral factors, and supports productive viral replication in primary human CD4 T cells and in certain non-permissive T cell lines. Here, we demonstrate that Vif also contributes to the arrest of HIV-1 infected cells in the G 2 phase of the cell cycle. Viruses deleted in Vif or Vpr induce less cell cycle arrest than wild-type virus, while cells infected with HIV-1 deleted in both Vif and Vpr have a cell cycle profile equivalent to that of uninfected cells. Furthermore, expression of Vif alone induces accumulation of cells in the G 2 phase of the cell cycle. These data demonstrate a novel role for Vif in cell cycle regulation and suggest that Vif and Vpr independently drive G 2 arrest in HIV-1 infected cells. Our results may have implications for the actions and interactions of key HIV-1 accessory proteins in AIDS pathogenesis.