Identification of amino acids within the second alpha helical domain of the human immunodeficiency virus type 1 Vpu that are critical for preventing CD4 cell surface expression

Human immunodeficiency virus type 1 (HIV-1) encodes for a Vpu protein, which interacts with CD4 resulting in its degradation. In this study, we examined the role of the 10 amino acids within the predicted second α-helical domain of the subtype B Vpu cytoplasmic tail in CD4 down-modulation using a VpuEGFP reporter system. Our findings indicate that the invariant leucine at position 63 and, to a lesser extent, the valine at position 68 were required for CD4 down-modulation. Mutation of analogous L63 in Vpu proteins subtypes A2, B(YU-2), C, D, and H also abolished CD4 down-modulation from the cell surface. Co-immunoprecipitation analysis revealed that L63A and V68A mutants were capable of binding CD4 and still retained the ability to interact with h-β-TrCP1. Taken together, these results indicate that amino acid substitutions in the second α-helical domain that retain the predicted structure and binding to h-β-TrCP1 can influence Vpu-mediated CD4 degradation.