Background:  The human APOBEC3G protein is a potent restrictor of retroviral infection in the absence of the viral counter-defense protein Vif. APOBEC3G mediates restriction by deaminating retroviral minus strand cytosines to uracils, lesions that manifest as genomic strand G®A hypermutations and lead to viral inactivation. Although APOBEC3G restricts a broad number of retroviral substrates—including HIV, SIV, MLV, and EIAV—it is not clear whether this is its primary physiological function.

Methods:  We tested whether APOBEC3 proteins possess the ability to restrict mobility of the endogenous yeast retroelement Ty1.

Results:  We report that APOBEC3G can be functionally expressed in yeast, triggering a 25-fold increase in the frequency of mutation to canavanine-resistance. This effect was exacerbated in a uracil DNA glycosylase-deficient background, indicating that most of the lesions are repaired by the base excision pathway. All of these mutations mapped to the nuclear CAN1 gene and were exclusively C/G®T/A transition mutations within a 5’-CC consensus, identical to hotspots observed in retroviral cDNA. This mutator activity was not perturbed by HIV-1 Vif, suggesting a requirement for other factors in yeast. However, expression of APOBEC3G or its homolog APOBEC3F strongly inhibited the mobility of the endogenous yeast retrotransposon Ty1. Thus, in the absence of potential APOBEC3 regulatory mechanisms, which may exist in model human systems but not in heterologous systems like yeast, APOBEC3G expression can cause genomic alterations and can inhibit the mobility of endogenous retroelements.

Conclusions:  These data expand the range of APOBEC3 targets and indicate that this innate cellular defense may be part of a more general mobile nucleic acid restriction mechanism poised to withstand internal as well as external assaults.

Keywords: HIV-1; innate immunity; host factors