APOBEC3G: an intracellular centurion

The intrinsic antiretroviral factor APOBEC3G (A3G) is highly active against HIV-1 and other retroviruses. In different cell types, A3G is expressed in high-molecular-mass (HMM) RNA–protein complexes or low-molecular-mass (LMM) forms displaying different biological activities. In resting CD4 T cells, a LMM form of A3G potently restricts HIV-1 infection soon after virion entry. However, when T cells are activated, LMM A3G is recruited into HMM complexes that include Staufen-containing RNA granules. These complexes are probably nucleated by the induced expression of Alu/hY retroelement RNAs that accompany T-cell activation. HMM A3G sequesters these retroelement RNAs away from the nuclear long interspersed nuclear element-derived enzymes required for Alu/hY retrotransposition. Human immunodeficiency virus (HIV) exploits this ‘window of opportunity’ provided by the loss of LMM A3G in activated CD4 T cells to productively infect these cells. During HIV virion formation, newly synthesized LMM A3G is preferentially encapsidated but only under conditions where Vif is absent and thus not able to target A3G for proteasome-mediated degradation. Together, these findings highlight the discrete functions of the different forms of A3G. LMM A3G opposes the external threat posed by exogenous retroviruses, while HMM A3G complexes oppose the internal threat posed by the retrotransposition of select types of retroelements

APOBEC proteins and intrinsic resistance to HIV-1 infection

Members of the APOBEC family of cellular polynucleotide cytidine deaminases, most notably APOBEC3G and APOBEC3F, are potent inhibitors of HIV-1 infection. Wild type HIV-1 infections are largely spared from APOBEC3G/F function through the action of the essential viral protein, Vif. In the absence of Vif, APOBEC3G/F are encapsidated by budding virus particles leading to excessive cytidine (C) to uridine (U) editing of negative sense reverse transcripts in newly infected cells. This registers as guanosine (G) to adenosine (A) hypermutations in plus-stranded cDNA. In addition to this profoundly debilitating effect on genetic integrity, APOBEC3G/F also appear to inhibit viral DNA synthesis by impeding the translocation of reverse transcriptase along template RNA. Because the functions of Vif and APOBEC3G/F proteins oppose each other, it is likely that fluctuations in the Vif–APOBEC balance may influence the natural history of HIV-1 infection, as well as viral sequence diversification and evolution. Given Vif's critical role in suppressing APOBEC3G/F function, it can be argued that pharmacologic strategies aimed at restoring the activity of these intrinsic anti-viral factors in the context of infected cells in vivo have clear therapeutic merit, and therefore deserve aggressive pursuit