Vif Overcomes the Innate Antiviral Activity of APOBEC3G by Promoting Its Degradation in the Ubiquitin-Proteasome Pathway

Abstract

Viruses must overcome diverse intracellular defense mechanisms to establish infection. The Vif (virion infectivity factor) protein of human immunodeficiency virus 1 (HIV-1) acts by overcoming the antiviral activity of APOBEC3G (CEM15), a cytidine deaminase that induces G to A hypermutation in newly synthesized viral DNA. In the absence of Vif, APOBEC3G incorporation into virions renders HIV-1 non-infectious. We report here that Vif counteracts the antiviral activity of APOBEC3G by targeting it for destruction by the ubiquitin-proteasome pathway. Vif forms a complex with APOBEC3G and enhances APOBEC3G ubiquitination, resulting in reduced steady-state APOBEC3G levels and a decrease in protein half-life. Furthermore, Vif-dependent degradation of APOBEC3G is blocked by proteasome inhibitors or ubiquitin mutant K48R. A mutation of highly conserved cysteines or the deletion of a conserved SLQ(Y/F)LA motif in Vif results in mutants that fail to induce APOBEC3G degradation and produce non-infectious HIV-1; however, mutations of conserved phosphorylation sites in Vif that impair viral replication do not affect APOBEC3G degradation, suggesting that Vif is important for other functions in addition to inducing proteasomal degradation of APOBEC3G. Vif is monoubiquitinated in the absence of APOBEC3G but is polyubiquitinated and rapidly degraded when APOBEC3G is coexpressed, suggesting that coexpression accelerates the degradation of both proteins. These results suggest that Vif functions by targeting APOBEC3G for degradation via the ubiquitin-proteasome pathway and implicate the proteasome as a site of dynamic interplay between microbial and cellular defenses.

APOBEC proteins and intrinsic resistance to HIV-1 infection

“…Perhaps the simplest way to prevent A3G action would, therefore, be to block virion encapsidation-and this is what Vif does. Though there have been reports of Vif directly impeding packaging (Mariani et al 2003;Opi et al 2007), Vif's principal function is to induce the polyubiquitylation and subsequent proteasomal degradation of A3G, thereby depleting the pool of cytosolic A3G available for incorporation into assembling virus particles (Conticello et al 2003;Marin et al 2003;Sheehy et al 2003;Stopak et al 2003;Yu et al 2003;Mehle et al 2004b;Kobayashi et al 2005). Vif accomplishes this by simultaneously binding to both A3G and the cullin5-elongin B/C-Rbx ubiquitin ligase, thus serving as an adaptor to recruit the ligase complex to its substrate ( Yu et al 2003; figure 2).…”

Stimulation of Cell Surface CCR5 and CD40 Molecules by Their Ligands or by HSP70 Up-Regulates APOBEC3G Expression in CD4+ T Cells and Dendritic Cells

“…A3G was also demonstrated in testes, ovaries, and a number of tumor cell lines (14). There is limited information concerning the factors that control expression of A3G, but its level of mRNA does not change with HIV-1 infection (11,16). A3G inhibits both the R5 and X4 strains of HIV-1 (5).…”

Enzymatically Active APOBEC3G Is Required for Efficient Inhibition of Human Immunodeficiency Virus Type 1

“…It is now well established that HIV-1 Vif can counteract the human cytidine deaminase APOBEC3G (APO3G). The inhibition of APO3G's antiviral effects has been attributed to a reduction in cellular expression of APO3G protein, which is due to Vif-mediated degradation of APO3G by cytoplasmic proteasomes (6,20,25,27,34,37,43). On the other hand, we recently found that Vif could prevent encapsidation of a degradation-resistant APO3G variant, suggesting that Vif can inhibit the APO3G antiviral activity through multiple independent mechanisms (29).…”