HIV-1 Vpu interacts with CD4 in the endoplasmic reticulum and triggers CD4 degradation, presumably by proteasomes. Human beta TrCP identified by interaction with Vpu connects CD4 to this proteolytic machinery, and CD4-Vpu-beta TrCP ternary complexes have been detected by coimmunoprecipitation. beta TrCP binding to Vpu and its recruitment to membranes require two phosphoserine residues in Vpu essential for CD4 degradation. In beta TrCP, WD repeats at the C terminus mediate binding to Vpu, and an F box near the N terminus is involved in interaction with Skp1p, a targeting factor for ubiquitin-mediated proteolysis. An F-box deletion mutant of beta TrCP had a dominant-negative effect on Vpu-mediated CD4 degradation. These data suggest that beta TrCP and Skp1p represent components of a novel ER-associated protein degradation pathway that mediates CD4 proteolysis.
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