Amino acid composition of alpha1/alpha2 domains and cytoplasmic tail of MHC class I molecules determine their susceptibility to human cytomegalovirus US11-mediated down-regulation.

During co-evolution with its host, human cytomegalovirus has acquired multiple defense mechanisms to escape from immune recognition. In this study, we focused on US11, which binds to MHC class I heavy chains and mediates their dislocation to the cytosol and subsequent degradation by proteasomes. To examine which domains of class I heavy chains are involved in this process, we constructed chimeric HLA molecules of US11-sensitive and -insensitive class I molecules (HLA-A2 and HLA-G, respectively). Pulse-chase experiments were performed to evaluate protein stability and interactions between class I heavy chains and US11. Flow cytometry was employed to assess the effect of US11 on surface expression of the different chimeras. Our results indicate that the alpha1 and alpha2 domains of HLA molecules are important for the affinity of US11 association. However, the degradation efficiency seems to rely mostly on cytosolic tail residues. We found that the nonclassical HLA-G molecule is insensitive to US11-mediated degradation solely because it lacks essential tail residues. A deletion of the last two tail residues in full-length MHC class I molecules already caused a severe reduction in degradation efficiency. Altogether, our data provide new insights into the mechanism by which US11 down-regulates MHC class I molecules.