A host cytidine deaminase, APOBEC3G (A3G), inhibits replication of human 
immunodeficiency virus type 1 (HIV-1) by incorporating into virions in the 
absence of the virally encoded Vif protein (Deltavif virions), at least in part 
by causing G-to-A hypermutation. To gain insight into the antiretroviral 
function of A3G, we determined the quantities of A3G molecules that are 
incorporated in Deltavif virions. We combined three experimental 
approaches-reversed-phase high-pressure liquid chromatography (HPLC), 
scintillation proximity assay (SPA), and quantitative immunoblotting-to 
determine the molar ratio of A3G to HIV-1 capsid protein in Deltavif virions. 
Our studies revealed that the amount of the A3G incorporated into Deltavif 
virions was proportional to the level of its expression in the viral producing 
cells, and the ratio of the A3G to Gag in the Deltavif virions produced from 
activated human peripheral blood mononuclear cells (PBMC) was approximately 
1:439. Based on previous estimates of the stoichiometry of HIV-1 Gag in virions 
(1400-5000), we conclude that approximately 7 (+/-4) molecules of A3G are 
incorporated into Deltavif virions produced from human PBMCs. These results 
indicate that virion incorporation of only a few molecules of A3G is sufficient 
to inhibit HIV-1 replication.