Targeting APOBEC3A to the viral nucleoprotein complex confers antiviral activity

Background: APOBEC3 ( A3) proteins constitute a family of cytidine deaminases that provide intracellular resistance to retrovirus replication and to transposition of endogenous retroelements. A3A has significant homology to the C- terminus of A3G but has only a single cytidine deaminase active site ( CDA), unlike A3G, which has a second N- terminal CDA previously found to be important for Vif sensitivity and virus encapsidation. A3A is packaged into HIV-1 virions but, unlike A3G, does not have antiviral properties. Here, we investigated the reason for the lack of A3A antiviral activity. Results: Sequence alignment of A3G and A3A revealed significant homology of A3A to the C-terminal region of A3G. However, while A3G co-purified with detergent- resistant viral nucleoprotein complexes (NPC), virus- associated A3A was highly detergent-sensitive leading us to speculate that the ability to assemble into NPC may be a property conveyed by the A3G N-terminus. To test this model, we constructed an A3G-3A chimeric protein, in which the N-terminal half of A3G was fused to A3A. Interestingly, the A3G-3A chimera was packaged into HIV-1 particles and, unlike A3A, associated with the viral NPC. Furthermore, the A3G-3A chimera displayed strong antiviral activity against HIV-1 and was sensitive to inhibition by HIV-1 Vif. Conclusion: Our results suggest that the A3G N-terminal domain carries determinants important for targeting the protein to viral NPCs. Transfer of this domain to A3A results in A3A targeting to viral NPCs and confers antiviral activity.