βTrCP is Required for HIV-1 Vpu Modulation of CD4, GaLV Env, and BST-2/Tetherin

The Human immunodeficiency virus-1 (HIV-1) accessory protein Vpu modulates numerous proteins, including the host proteins CD4 and BST-2/tetherin. Vpu interacts with the Skp, Cullin, F-Box (SCF) ubiquitin ligase through interactions with the F-Box protein βTrCP (1 and/or 2). This interaction is dependent on phosphorylation of S(52,56) in Vpu. Mutation of S(52,56), or inhibition of the SCF, abolishes most Vpu activity against CD4 and partly reduces activity against BST-2/tetherin. Recently, Vpu has also been reported to interact with the clathrin adapter proteins AP-1 and AP-2, and these interactions were also found to be required for BST-2/tetherin antagonism in an S(52,56) -dependent manner. In assays where HIV-1 is pseudotyped with gibbon ape leukemia virus (GaLV Env), Vpu has also been found to prevent GaLV Env from being incorporated into viral particles, but the mechanism for this antagonism is not fully understood. To clarify the role of the βTrCPs in Vpu function we used CRISPR/Cas9 to generate a clonal cell line lacking both βTrCP-1 and -2. Vpu activity against CD4 and GaLV Env was abolished in this cell line, and activity against BST-2/tetherin reduced significantly. Mutation of the S(52,56) residues no longer affected Vpu activity against BST-2/tetherin in this cell line. These data suggest that the primary role of the S(52,56) residues in antagonism of CD4, GaLV Env, and BST-2/tetherin is to recruit the SCF/βTrCP ubiquitin ligase.