Human cytidine deaminase apolipoprotein B mRNA-editing catalytic polypeptide-like 3G (APOBEC3G [A3G]) and other APOBEC3 proteins (25) are related to a family of proteins that also includes apolipoprotein B-editing catalytic subunit 1 (APOBEC1), APOBEC2, and activation-induced cytidine deaminase (AID) (23,66). These proteins have cytidine deaminase activities that modify RNA or DNA. A3G was the first APOBEC3 protein to be identified as a potent inhibitor of HIV-1 in the absence of Vif (59). A major outcome of virion packaging of A3G is the induction of C-to-U mutations in the minus-strand viral DNA during reverse transcription (22,32,42,43,63,73,77). Virion-packaged A3G and A3F can also reduce the accumulation of viral DNA (3,21,27,40,45,57,71) and the formation of proviral DNA (40,45).

APOBEC3G (A3G) is a single-stranded DNA cytidine deaminase that targets retroviral minus-strand DNA and has potent antiviral activity against diverse retroviruses. However, the mechanisms of A3G antiviral functions are incompletely understood. Here we demonstrate that A3G, A3F, and, to a lesser extent, the noncatalytic A3GC291S block human immunodeficiency virus type 1 (HIV-1) replication by interfering with proviral DNA formation. In HIV-1 virions, A3G interacted with HIV-1 integrase and nucleocapsid, key viral factors for reverse transcription and integration. Unlike A3G, the weak antiviral A3C cytidine deaminase did not interact with either of these factors and did not affect viral reverse transcription or proviral DNA formation. Thus, multiple steps of the HIV-1 replication cycle, most noticeably the formation of proviral DNA, are inhibited by both cytidine deamination-dependent and -independent mechanisms.

The human cytidine deaminase APOBEC3G (A3G) and other APOBEC3 proteins exhibit differential inhibitory activities against diverse endogenous retroelements and retroviruses, including Vif-deficient human immunodeficiency virus type 1. The potential inhibitory activity of human APOBEC proteins against long interspersed element 1 (LINE-1) has not been fully evaluated. Here, we demonstrate inhibition of LINE-1 by multiple human APOBEC3 cytidine deaminases, including previously unreported activity for A3DE and A3G. More ancient members of APOBEC, cytidine deaminases AID and APOBEC2, had no detectable activity against LINE-1. A3A, which did not form high-molecular-mass (HMM) complexes and interacted poorly with P bodies, was the most potent inhibitor of LINE-1. A3A specifically recognizes LINE-1 RNA but not the other cellular RNAs tested. However, in the presence of LINE-1, A3A became associated with HMM complexes containing LINE-1 RNA. The ability of A3A to recognize LINE-1 RNA required its catalytic domain and was important for its LINE-1 suppression. Although the mechanism of LINE-1 restriction did not seem to involve DNA editing, A3A inhibited the accumulation of nascent LINE-1 DNA, suggesting interference with LINE-1 reverse transcription and/or integration or intracellular movement of LINE-1 ribonucleoprotein. Thus, association with P bodies or cellular HMM complexes could not predict the potency of APOBEC3 anti-LINE-1 activities. The catalytic domain of APOBEC3 proteins may be important for proper folding and target factors such as RNA or protein interaction in addition to cytidine deamination.A3G and other APOBEC3 proteins (22) belong to a family of proteins that also includes activation-induced cytidine deaminase (AID), apolipoprotein B-editing catalytic subunit 1 (APOBEC1), and APOBEC2 (1,16,19,35,39,45). These proteins have cytidine deaminase activities that can modify RNA or DNA. A3G was the first APOBEC3 protein to be identified as a potent inhibitor of human immunodeficiency virus type 1 (HIV-1) in the absence of Vif (41). Subsequently, several other human APOBEC3 proteins, including APOBE C3A (A3A), APOBEC3B (A3B), APOBEC3C (A3C), APOB EC3DE (A3DE), and APOBEC3F (A3F), were identified as broad antiviral factors against HIV-1, simian immunodeficiency virus, murine leukemia virus, and various endogenous retroelements (3, 4, 11-14, 24, 26, 37, 40, 48, 49, 53) as well as hepatitis B virus (44).Like AID, which edits single-stranded immunoglobulin gene DNA, APOBEC3 proteins prefer minus-strand retroviral DNAs as targets (1,10,16,19,29,35,39,45,51). In the absence of Vif-induced A3G degradation in virus-producing cells, virion-packaged A3G induces C-to-U mutations in minus-strand viral DNA during reverse transcription (18,25,30,31,43,50,52). Both cytidine deamination-dependent and -independent antiviral functions of APOBEC3 proteins have been reported (2,8,17,20,28,33,36,44).The potential inhibitory activity of certain human APOBEC3 proteins against LINE-1 has been reported. A3A and A3B are potent inhibitors...

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-x(5)-C-x(17-18)-C-x(3-5)-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.