| Humans express seven APOBEC3 (A3) proteins, a family of cytidine deaminases that convert deoxycytidine to deoxyuridine in single-stranded DNA. At least in part due to this enzymatic activity, APOBEC3 proteins inhibit the replication of multiple viruses and endogenous retro-elements. However, studies of APOBEC3G (A3G) and related cytidine deaminases suggest that these proteins also have the potential to edit the host genomic DNA. These studies raise the question of how A3 proteins target parasitic DNA while sparing host DNA and suggest that A3 deaminase activity may be regulated to achieve this specificity. Such regulatory mechanisms may not be fully recapitulated when A3 proteins are over-expressed in transformed cells. Thus, it is important to analyze deaminase activity of A3 proteins in their native context at physiological expression levels. Since primary human leukocytes express A3 proteins, I developed an assay to study A3 activity in extracts from these cells. I showed that the enzymatic activity of A3G in primary T cells is significantly lower than activity of A3G expressed by transfection in 293T cells, despite similar protein levels. Furthermore, active A3G from 293T cells was inhibited by T cell extracts, suggesting the presence of an inhibitory factor in T cells that regulates A3G deaminase activity post-translationally. Thus, although A3G is constitutively expressed in primary leukocytes, its activity is negatively regulated. In contrast, A3A is not typically expressed in human leukocytes, but several A3A-related proteins are strongly induced in monocytes by interferon alpha, resulting in a >10-fold increase in deaminase activity. Two of these proteins are generated via use of alternative translational start sites and differ significantly in their enzymatic activity, raising the possibility that they may perform different cellular functions. These functions likely include defense against multiple viral pathogens. Because many viruses induce interferon, A3A expression could represent a significant barrier to viral replication. Further studies are needed to identify viruses that can be inhibited by A3A or alternatively, viral mechanisms for evading this host defense. However, in addition to studies of the viral consequences of A3A expression, studies are also needed to identify effects these proteins may have on the host genome. |
