APOBEC3G (APO3G), a cytidine deaminase with two zinc finger domains, inhibits human immunodeficiency virus type 1 replication in the absence of Vif. Here, we provide a comprehensive molecular analysis of the deaminase and nucleic acid binding activities of human APO3G using a pure system containing only one protein component, i.e., highly purified, catalytically active enzyme expressed in a baculovirus system. We demonstrate that APO3G deaminates cytosines in single-stranded DNA (ssDNA) only, whereas it binds efficiently to ssDNA and ssRNA, about half as well to a DNA/RNA hybrid, and poorly to double-stranded DNA and RNA. In addition, the base specificities for deamination and binding of ssDNA are not correlated. The minimum length required for detection of APO3G binding to an ssDNA oligonucleotide in an electrophoretic mobility shift assay is 16 nucleotides. Interestingly, if nucleocapsid protein and APO3G are present in the same reaction, we find that they do not interfere with each other's binding to RNA and a complex containing the RNA and both proteins is formed. Finally, we also identify the functional activities of each zinc finger domain. Thus, although both zinc finger domains have the ability to bind nucleic acids, the first zinc finger contributes more to binding and APO3G encapsidation into virions than finger two. In contrast, deamination is associated exclusively with the second zinc finger. Moreover, zinc finger two is more important than finger one for the antiviral effect, demonstrating a correlation between deaminase and antiviral activities.

APOBEC3G (APO3G) is a host cytidine deaminase that is incorporated into human immunodeficiency virus type 1 (HIV-1) particles. We report here that viral RNA promotes stable association of APO3G with HIV-1 nucleoprotein complexes (NPC). A target sequence located within the 5-untranslated region of the HIV-1 RNA was identified to be necessary and sufficient for efficient APO3G packaging. Fine mapping revealed a sequence normally involved in viral genomic RNA dimerization and Gag binding to be important for APO3G packaging and association with viral NPC. Our data suggest that packaging of APO3G into HIV-1 NPC is enhanced by viral RNA.Replication of human immunodeficiency virus type 1 (HIV-1) in primary cells is dependent on the expression of Vif protein, which counteracts the activity of the host cytidine deaminases APOBEC3G (APO3G) and APOBEC3F (4,25,29,32). In the absence of Vif, APO3G is incorporated into virus particles (11,16,19,20,26,27,30), resulting in hypermutation of the viral genome (19) or degradation of mutated cDNA (14, 18, 31) via a DNA repair mechanism (reviewed in references 3 and 12). Interestingly, human APO3G is not only packaged into human immunodeficiency viruses but also incorporated into simian immunodeficiency viruses and murine leukemia virus (9,18,19). Packaging of APO3G into such diverse viruses suggests that it either is a relatively nonspecific process or involves signals shared by these viruses. APO3G can bind RNA in vitro (10). Indeed, several reports have noted that the presence of viral RNA enhanced APO3G encapsidation (28); however, others (17, 23) suggested that viral RNA was not essential for APO3G packaging (2,5,8,17,23,28).To further study the role of viral RNA in the packaging of APO3G into HIV-1 virions, we first compared the packaging of APO3G into either the wild-type infectious NL4-3 virus or a helper virus (C-Help) whose RNA genome is not packaged due to a 33-base deletion in the putative RNA packaging signal (21). Virus stocks were prepared by transient cotransfection of HeLa cells with either the pNL4-3 plasmid (1) or the C-Help vector DNA and the APO3G-expressing plasmid pcDNA-APO3G (11). Viruses were collected 48 h after transfection and purified by two rounds of sucrose gradient centrifugation. Cell lysates and concentrated virus preparations were analyzed by immunoblotting (Fig. 1A). We found that packaging of APO3G into helper virus was reduced by Ͼ3.5-fold compared to packaging into NL4-3 virus (Fig. 1B). Thus, viral RNA contributes to the specific packaging of APO3G into HIV-1 virions, consistent with data reported by Svarovskaia et al. (28).If encapsidation of APO3G and viral RNA are linked, the APO3G packaging defect observed with the helper virus ( Fig. 1A and B) should be overcome by the coexpression of packaging-competent vector-derived RNA. To test this hypothesis, several packaging vectors were constructed based on the lentiviral packaging vector pHRЈCMVGFP (15). This vector contains both HIV-1 long terminal repeats (LTRs), the 5Ј-untranslated region, 35...

APOBEC3G (APO3G) is a cellular cytidine deaminase with potent antiviral activity. Initial studies of the function of APO3G demonstrated extensive mutation of the viral genome, suggesting a model in which APO3G's antiviral activity is due to hypermutation of the viral genome. Recent studies, however, found that deaminase-defective APO3G mutants transiently expressed in virus-producing cells exhibited significant antiviral activity, suggesting that the antiviral activity of APO3G could be dissociated from its deaminase activity. To directly compare the antiviral activities of wild-type (wt) and deaminase-defective APO3G, we used two approaches: (i) we titrated wt and deaminase-defective APO3G in transient-transfection studies to achieve similar levels of virus-associated APO3G and (ii) we constructed stable cell lines and selected clones expressing comparable amounts of wt and deaminase-defective APO3G. Viruses produced under these conditions were tested for viral infectivity. The results from the two approaches were consistent and suggested that the antiviral activity of deaminase-defective APO3G was significantly lower than that of wt APO3G. We conclude that efficient inhibition of vif-defective human immunodeficiency virus type 1 requires catalytically active APO3G.The human immunodeficiency virus type 1 (HIV-1) accessory protein Vif plays an important role in regulating virus infectivity (8,38). 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). In the absence of Vif, APO3G is efficiently packaged into HIV virions and inhibits virus replication. A number of studies reported that the presence of APO3G in the virus can result in hypermutation of the viral minus-strand cDNA during reverse transcription (11,18,23,24,42,45), inhibition of reverse transcription (9), tRNA annealing or tRNA processing (10, 26), DNA strand transfer (19, 26), or integration (22, 26).Some of these effects do not require catalytically active APO3G (19,22), and several reports suggested that deaminase-defective APO3G and APO3F have antiviral activity when transiently coexpressed with HIV-1 in 293T cells (3,12,28,35). Our own data concerning the antiviral properties of the deaminase-defective APO3G C288S/C291A mutant supported these conclusions (30). However, in our previous study we found that comparable inhibition of viral infectivity required higher levels of deaminase-defective APO3G protein than that of wild type (wt) (30). The purpose of the current study was to characterize in more detail the antiviral properties of deaminase-defective APO3G. We p...

APOBEC3G (APO3G) is a cytidine deaminase that restricts replication of vif-defective human immunodeficiency virus type 1 (HIV-1). Like other members of the cellular deaminase family, APO3G has the propensity to form homo-multimers. In the current study, we investigated the functional determinants for multimerization of human APO3G and studied the role of APO3G multimerization for catalytic activity, virus encapsidation, and antiviral activity. We found that human APO3G is capable of forming multimeric complexes in transfected HeLa cells. Interestingly, multimerization of APO3G was exquisitely sensitive to RNase treatment, suggesting that interaction of APO3G subunits is facilitated or stabilized by an RNA bridge. Mutation of a conserved cysteine residue (C97) that is part of an N-terminal zinc-finger motif in APO3G abolished multimerization of APO3G; however, the C97 mutation inhibited neither in vitro deaminase activity nor antiviral function of APO3G. These results suggest that monomeric APO3G is both catalytically active and has antiviral activity. Interference studies employing either catalytically inactive or packaging-incompetent APO3G variants suggest that wild-type APO3G is packaged into HIV-1 particles in monomeric form. These results provide novel insights into the catalytic function and antiviral property of APO3G and demonstrate an important role for C97 in the RNA-dependent multimerization of this protein.APOBEC3G (APO3G) belongs to a family of cytidine deaminases that in humans includes APOBEC1, APOBEC2, eight APOBEC3 variants designated APOBEC3A through -3H, and APOBEC4, as well as activation-induced deaminase (7,12,30,40). The activity of APO3G interferes with the replication of a broad range of retroviruses and can also block hepatitis B virus (39), whose replication cycle involves the reverse transcription of a pregenomic RNA intermediate (33). Interestingly, human immunodeficiency virus type 1 (HIV-1) Vif can counteract the antiviral activity of APO3G by inhibiting its incorporation into virions (13,24,34,36). This inhibition is at least in part due to a reduction in cellular APO3G expression, which has been attributed primarily to Vif-mediated degradation of APO3G by cytoplasmic proteasomes (6,19,24,25,34,36,44).It is now well established that packaging of APO3G into virus particles can result in hypermutation of the viral minus-strand cDNA during reverse transcription (10,17,22,23,43,45). Interestingly, human APO3G is not only packaged into human immunodeficiency viruses but is also incorporated into simian immunodeficiency viruses and murine leukemia virus (10,22,23). The efficient packaging of APO3G into such diverse viruses could suggest that the mechanism of APO3G packaging is either relatively nonspecific or requires signals shared by these viruses. The observation that deletions in various regions of APO3G can prevent packaging into virus particles (4, 18) argues in favor of a specific packaging mechanism. APO3G binds to RNA in vitro, a property that it shares with other members of the...