Regulated Degradation of the HIV-1 Vpu Protein through a βTrCP-Independent Pathway Limits the Release of Viral Particles

Viral protein U (Vpu) of HIV-1 has two known functions in replication of the virus: degradation of its cellular receptor CD4 and enhancement of viral particle release. Vpu binds CD4 and simultaneously recruits the βTrCP subunit of the SCF(βTrCP) ubiquitin ligase complex through its constitutively phosphorylated DS(52)GXXS(56) motif. In this process, Vpu was found to escape degradation, while inhibiting the degradation of βTrCP natural targets such as β-catenin and IκBα. We further addressed this Vpu inhibitory function with respect to the degradation of Emi1 and Cdc25A, two βTrCP substrates involved in cell-cycle progression. In the course of these experiments, we underscored the importance of a novel phosphorylation site in Vpu. We show that, especially in cells arrested in early mitosis, Vpu undergoes phosphorylation of the serine 61 residue, which lies adjacent to the βTrCP-binding motif. This phosphorylation event triggers Vpu degradation by a βTrCP-independent process. Mutation of Vpu S61 in the HIV-1 provirus extends the half-life of the protein and significantly increases the release of HIV-1 particles from HeLa cells. However, the S61 determinant of regulated Vpu turnover is highly conserved within HIV-1 isolates. Altogether, our results highlight a mechanism where differential phosphorylation of Vpu determines its fate as an adaptor or as a substrate of distinct ubiquitin ligases. Conservation of the Vpu degradation determinant, despite its negative effect on virion release, argues for a role in overall HIV-1 fitness.