Human APOBEC3B (A3B) has been described as a potent inhibitor of retroviral 
infection and retrotransposition. However, we found that the predominantly 
nuclear A3B only weakly restricted infection by HIV-1, HIV-1Δvif, and human 
T-cell leukemia virus type 1 (HTLV-1), while significantly inhibiting LINE-1 
retrotransposition. The chimeric construct A3G/B, in which the first 60 amino 
acids of A3B were replaced with those of A3G, restricted HIV-1, HIV-1Δvif, and 
HTLV-1 infection, as well as LINE-1 retrotransposition. In contrast to the 
exclusively cytoplasmic A3G, which is inactive against LINE-1 
retrotransposition, the A3G/B protein, while localized mainly to the cytoplasm, 
was also present in the nucleus. Further mutational analysis revealed that 
residues 18, 19, 22, and 24 in A3B were the major determinants for nuclear 
versus cytoplasmic localization and antiretroviral activity. HIV-1Δvif packages 
A3G, A3B, and A3G/B into particles with close-to-equal efficiencies. Mutation 
E68Q or E255Q in the active centers of A3G/B resulted in loss of the inhibitory 
activity against HIV-1Δvif, while not affecting activity against LINE-1 
retrotransposition. The low inhibition of HIV-1Δvif by A3B correlated with a low 
rate of G-to-A hypermutation. In contrast, viruses that had been exposed to 
A3G/B showed a high number of G-to-A transitions. The mutation pattern was 
similar to that previously reported for A3B, with a preference for the GA 
context. In summary, these observations suggest that changing 4 residues in the 
amino terminus of A3B not only retargets the protein from the nucleus to the 
cytoplasm but also enhances its ability to restrict HIV while retaining 
inhibition of retrotransposition.