Background: HIV-1 replication in primary human T-cells and monocytes requires VIF. VIF is required in these cells to overcome the inhibitory activity of the cellular protein ABOBEC-3G/CEM15, a protein that belongs to a family of RNA editing enzymes. The mechanism by which APOBEC3G interferes with HIV-1 replication is not understood. To begin to get insight into its function, we identified and characterized the murine homologue of APOBEC3G and tested its ability to inhibit HIV-1 replication and for VIF to act on it.

Methods: cDNAs corresponding to murine homologues of human APOBEC3G were amplified by RT-PCR from murine lymphoid organs and rodent cell lines. The cDNAs were cloned into an expression vector and expressed with an epitope tag in transfected cells. The VIF co-factor activity of the protein was measured using a rapid VIF activity assay in which virion infectivity was monitored with a LTR-Luciferase reporter cell line. Real-time Q-PCR of newly infected cells was used to determine the step at which co-factor activity is manifested. Northern and real-time Q-PCR analysis were used to characterize the cell-type expression of the murine homologues.

Results: Murine APOBEC3G/CEM15 was a potent inhibitor of HIV-1 virion infectivity. Interestingly, it was more potent than the human protein. As little as 1 ng of the murine cofactor cDNA vector effectively blocked HIV-1 infectivity. Moreover, HIV-1 VIF was unable to reverse the inhibitory activity of the murine cofactor unless it was expressed at a very low level. Expression of the gene in the animal was restricted to lymphoid tissues including spleen, thymus, and lymph nodes, and in addition, heart and lung. Real-time PCR analysis of newly infected cells showed that the mouse co-factor acted to block the initiation of reverse transcription. APOBEC3G derived from Chinese hamster or rat did not have inhibitory activity on HIV-1 virion infectivity. Interestingly, murine APOBEC3G was able to partially block HIV-1 replication in target cells, a finding that is at odds with the activity of the human homologue.

Conclusions: Murine APOBEC3G is a potent inhibitor of HIV-1 replication and is largely independent of VIF. This protein may play a role in determining the species specificity of HIV-1 replication in mouse and other species. In addition, the findings separate 2 functions of APOBEC3G, one that inhibits virion infectivity and one that is targeted by VIF.