Definition of the interacting interfaces of Apobec3G and HIV-1 Vif using MAPPIT mutagenesis analysis

The host restriction factor Apobec3G is a cytidine deaminase that incorporates into HIV-1 virions and interferes with viral replication. The HIV-1 accessory protein Vif subverts Apobec3G by targeting it for proteasomal degradation. We studied the Apobec3G homomerisation and the interaction of Apobec3G with Vif in detail.

We used the MAPPIT two-hybrid technique to analyse the Apobec3G-Apobec3G and the Apobec3G-Vif interactions in intact human cells. MAPPIT is based on the functional complementation of a cytokine receptor signalling pathway.

We propose a model in which Apobec3G N-terminal domains symmetrically interact via a head-to-head interface containing residues 122 RLYYFW 127. Mutations in the head-to-head interface abrogate the Apobec3G-Apobec3G interaction. All mutations that inhibit Apobec3G-Apobec3G binding also inhibit the Apobec3G-Vif interaction, indicating that the head-to head interface plays an important role in the interaction with Vif. Only the D128K, P129A and T32Q mutations specifically affect the Apobec3G-Vif association. In our model, D128, P129 and T32 cluster at the edge of the head-to-head interface, possibly forming a Vif binding site composed of two Apobec3G molecules.

We propose that Vif either binds directly at the Apobec3G head-to-head interface or associates with an RNA-stabilized Apobec3G oligomer.