HIV-1 Vif inhibits G to A hypermutations catalyzed by virus-encapsidated APOBEC3G to maintain HIV-1 infectivity.

Background: HIV-1 viral infectivity factor (Vif) is an essential accessory protein for HIV-1 replication. The predominant function of Vif is to counteract Apolipoprotein B mRNA-editing enzyme-catalytic polypeptide-like 3G (APOBEC3G, A3G), a potent host restriction factor that inhibits HIV-1 replication. Vif mediates the proteasomal degradation of A3G and inhibits A3G translation, thus diminishing the pool of A3G that is available to be packaged into budding virion. Although Vif is robust in degrading A3G, the protection provided against A3G is not absolute. Clinical and laboratory evidence have shown that A3G is not completely excluded from HIV-1 viral particles during HIV-1 replication. It remains unclear why the viral samples are still infectious when A3G has been packaged into the virions.

Results: In this study, we provide evidence that Vif continues to protect HIV-1 from the deleterious effects of A3G, even after packaging of A3G has occurred. When equal amounts of A3G were packaged into budding virions, the virus expressing functional Vif was more infectious and incurred fewer G to A hypermutations in the second round of infection compared to Vif-deficient virus. A Vif mutant with a defect in viral packaging showed a reduced ability to protect the HIV-1 genome from G to A hypermutations.

Conclusion: Our data suggest that even packaged A3G is still under the tyranny of Vif. Our work brings to light an additional caveat for any therapy that hopes to exploit the Vif-A3G axis. The ideal strategy would not only enhance A3G viral packaging, but also reduce HIV-1 Vif viral encapsidation.