Human APOBEC3G can restrict retroviral infection in avian cells and acts independently of both UNG and SMUG1.

APOBEC3 proteins are mammal-specific cytidine deaminases that can restrict retroviral infection. The exact mechanism of the restriction remains unresolved, but one model envisions that uracilated retroviral cDNA, generated by cytidine deamination, is the target of cellular glycosylases. While restriction is unaffected by UNG deficiency, it has been suggested that the SMUG1 glycosylase might provide a backup. We found that retroviral restriction can be achieved by introducing human APOBEC3G into chicken cells (consistent with the components necessary for APOBEC3-mediated restriction predating mammalian evolution) and used this assay to show that APOBEC3G-mediated restriction can occur in cells deficient in both UNG and SMUG1.