Abstract
Human immunodeficiency virus type 1 (HIV-1) requires the cellular transcription factor core binding factor subunit β (CBFβ) to stabilize its viral infectivity factor (Vif) protein and neutralize the APOBEC3 restriction factors. CBFβ normally heterodimerizes with the RUNX family of transcription factors, enhancing their stability and DNA-binding affinity. To test the hypothesis that Vif may act as a RUNX mimic to bind CBFβ, we generated a series of CBFβ mutants at the RUNX/CBFβ interface and tested their ability to stabilize Vif and impact transcription at a RUNX-dependent promoter. While several CBFβ amino acid substitutions disrupted promoter activity, none of these impacted the ability of CBFβ to stabilize Vif or enhance degradation of APOBEC3G. A mutagenesis screen of CBFβ surface residues identified a single amino acid change, F68D, that disrupted Vif binding and its ability to degrade APOBEC3G. This mutant still bound RUNX and stimulated RUNX-dependent transcription. These separation-of-function mutants demonstrate that HIV-1 Vif and the RUNX transcription factors interact with cellular CBFβ on genetically distinct surfaces.