Dissection of the peptide binding grooves of seven subtypes of human histocompatibility leukocyte antigen (HLA)-B27 into the six specificity pockets defined by the 2.6-A structure of HLA-A*0201 revealed just one pocket, the B ("45") pocket, that is conserved among all the HLA-B27 subtypes. Functional studies of mutant HLA-B*2705 molecules with point substitutions in residues of the B pocket show that this structure, and the glutamine residue at position 45 in particular, plays a critical role in cell surface expression, peptide binding, and in the presentation of both exogenous and endogenous peptides by HLA-B*2705. We predict that the B pocket of HLA-B*2705 interacts with an amino acid side chain that anchors peptides in the binding groove, and that this peptide motif is present in most endogenously processed peptides that bind to all seven subtypes of HLA-B27.

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