Figure 2.

Tripartate complex formation between Nef or Vpu, cellular co-factors, and target proteins. Left: Nef binds differently to the AP-complexes depending on its target: when Nef downregulates CD4 it binds AP-2 via its ExxxLL motif and the α/σ2 subunits of AP-2; when Nef downregulates MHC-I it binds via unclear sequences to the µ subunit of AP-1, while directing a key tyrosine residue of MHC-I to the canonical binding site for Yxxφ motifs on the µ subunit. Right: Vpu binds an E3 ubiquitin ligase complex, which results in ubiquitination and downregulation of CD4 and BST-2. “X” refers to unidentified factors that might bind the cytoplasmic domain of Vpu or BST-2; in the case of BST-2 such factors could recognize ubiquitin.