The human APOBEC3 proteins are a family of DNA-editing enzymes that play an important role in the innate immune response against retroviruses and retrotransposons. APOBEC3G is a member of this family that inhibits HIV-1 replication in the absence of the viral infectivity factor Vif. Inhibition of HIV replication occurs by both deamination of viral single-stranded DNA and a deamination-independent mechanism. Efficient deamination requires rapid binding to and dissociation from ssDNA. However, a relatively slow dissociation rate is required for the proposed deaminase-independent roadblock mechanism in which APOBEC3G binds the viral template strand and blocks reverse transcriptase- catalysed DNA elongation. Here, we show that APOBEC3G initially binds ssDNA with rapid on-off rates and subsequently converts to a slowly dissociating mode. In contrast, an oligomerization-deficient APOBEC3G mutant did not exhibit a slow off rate. We propose that catalytically active monomers or dimers slowly oligomerize on the viral genome and inhibit reverse transcription.
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The authors thank D. Pollpeter, M.H. Malim and D. Rueda for valuable discussions, and M.F. Goodman for his generous gift of the F126A/W127A mutant clone. This work was supported in part by the National Institutes of Health (GM072462 to M.C.W. and GM065056 to K.M-F.) and the National Science Foundation (MCB-1243883 to M.C.W.), the Japan Society for the Promotion of Science (JSPS; KAKENHI_24590568 to Y.I.), and in part by funds from the NIH Intramural Research Program (NICHD; to J.G.L.). K.R.C. was supported by the NSF IGERT Program (DGE-0504331).