Antiviral Function of APOBEC3G Can Be Dissociated from Cytidine Deaminase Activity

Abstract

The antiretroviral activity of the cellular enzyme APOBEC3G has been attributed to the excessive deamination of cytidine (C) to uridine (U) in minus strand reverse transcripts, a process resulting in guanosine (G) to adenosine (A) hypermutation of plus strand DNAs. The HIV-1 Vif protein counteracts APOBEC3G by inducing proteasomal degradation and exclusion from virions through recruitment of a cullin5 ECS E3 ubiquitin ligase complex. APOBEC3G belongs to the APOBEC protein family, members of which possess consensus (H/C)-(A/V)-E-(X)24-30-P-C-(X)2-C cytidine deaminase motifs. Earlier analyses of APOBEC-1 have defined specific residues that are important for zinc coordination, proton transfer, and, therefore, catalysis within this motif. Because APOBEC3G contains two such motifs, we used site-directed mutagenesis of conserved residues to assess each region's contribution to anti-HIV-1 activity. Surprisingly, whereas either the N- or C-terminal domain could confer antiviral function in tissue culture-based infectivity assays, only an intact C-terminal motif was essential for DNA mutator activity. These findings reveal the nonequivalency of APOBEC3G's N- and C-terminal domains and imply that APOBEC3G-mediated DNA editing may not always be necessary for antiviral activity. Accordingly, we propose that APOBEC3G can achieve an anti-HIV-1 effect through an undescribed mechanism that is distinct from cytidine deamination.

Enzymatically Active APOBEC3G Is Required for Efficient Inhibition of Human Immunodeficiency Virus Type 1

“…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).…”

“…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).…”

Restriction of Equine Infectious Anemia Virus by Equine APOBEC3 Cytidine Deaminases

“…Because of the difficulties in detecting eqA3-derived cytidine deamination in EIAV genomes, we studied the relevance of the zinc coordination domain for the antiviral activity. It has been reported that cytidine deamination is largely dispensable for the inhibition HIV-1 ⌬vif by human A3F and A3G (4,30,46,61). But controversially, several groups have reported a significant drop in inhibition observed when active-site mutants of human A3G were analyzed (8,9,22,23,56,75).…”

“…Incorporated A3G specifically deaminates cytosine residues to uracil in growing singlestranded DNA during reverse transcription, leading to HIV genome degradation or hypermutation (5,25,39,48,49,98). More recent studies indicate that deaminase-independent mechanisms might also be involved in antiviral activity of A3 (4,27,28,30,54,61). The amount of encapsidated A3G in wild-type (wt) HIV-1 virions is dramatically reduced by a Vifdependent degradation via the ubiquitination-proteasome pathway (50,79,94,95).…”

Interferon-inducible expression of APOBEC3 editing enzymes in human hepatocytes and inhibition of hepatitis B virus replication

“…[20][21][22][23] Recent evidence suggests that some of the HIV restriction exerted by A3G may be independent of its cytidine deaminase activity. [24][25][26] G-to-A hypermutated HBV genomes have been detected in the plasma of HBV-infected patients, suggesting that APOBEC3 editing enzymes have edited the minus strand cDNA during HBV replication. [27][28][29] In hepatoma cells, transfection of A3B, A3C, A3F, or A3G induced extensive hypermutations in a minor fraction (approximately 10 Ϫ3 ) of HBV genomes that was detected by a novel polymerase chain reaction technique (3D-PCR) designed to selectively amplify AT-hypermutated sequences.…”