Representatives of the apolipoprotein B mRNA editing enzyme catalytic polypeptide 3 (APOBEC3) family provide innate resistance to exogeneous and endogenous retroviruses (see Cullen 2006 for a recent review). Humans and other primates encode a cluster of seven different cytidine deaminases with APOBEC3G, APOBEC3F and APOBEC3B having some anti HIV-1 activity. Our understanding is most complete for APOBEC3G which has been described first and the reactions described herein will focus on this representative enzyme.

APOBEC3G is a cytoplasmic protein which strongly restricts replication of Vif deficient HIV-1 (Sheehy 2002). It is expressed in cell populations that are susceptible to HIV infection (e.g., T-lymphocytes and macrophages). In the producer cell, APOBEC3G is incorporated into budding HIV-1 particles through an interaction with HIV-1 gag nucleocapsid (NC) protein in a RNA-dependent fashion.

Within the newly infected cell (= target cell), virus-associated APOBEC3G regulates the infectivity of HIV-1 by deaminating cytidine to uracil in the minus-strand viral DNA intermediate during reverse transcription. Deamination results in the induction of G-to-A hypermutations in the plus-strand viral DNA which subsequently can either be integrated as a non-functional provirus or degraded before integration.