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is a nonclassical MHC Ib antigen, which has been implicated in both innate and adaptive immune responses, as well as embryonic development. Qa-2 has an unusual peptide binding specificity in that it requires two dominant C-terminal anchor residues and is capable of associating with a substantially more diverse array of peptide sequences than other nonclassical MHC.<h4>Results</h4>We have determined the crystal structure, to 2.3 A, of the Q9 gene of murine Qa-2 complexed with a self-peptide derived from the L19 ribosomal protein, which is abundant in the pool of peptides eluted from the Q9 groove. The 9 amino acid peptide is bound high in a shallow, hydrophobic binding groove of Q9, which is missing a C pocket. The peptide makes few specific contacts and exhibits extremely poor shape complementarity to the MHC groove, which facilitates the presentation of a degenerate array of sequences. The L19 peptide is in a centrally bulged conformation that is stabilized by intramolecular interactions from the invariant P7 histidine anchor residue and by a hydrophobic core of preferred secondary anchor residues that have minimal interaction with the MHC.<h4>Conclusions</h4>Unexpectedly, the preferred secondary peptide residues that exhibit tenuous contact with Q9 contribute significantly to the overall stability of the peptide-MHC complex. The structure of this complex implies a \"conformational\" selection by Q9 for peptide residues that optimally stabilize the large bulge rather than making intimate contact with the MHC pockets.","bibjson":{"author":[{"initials":"X","lastname":"He","name":"He X"},{"initials":"P","lastname":"Tabaczewski","name":"Tabaczewski P"},{"initials":"J","lastname":"Ho","name":"Ho J"},{"initials":"I","lastname":"Stroynowski","name":"Stroynowski I"},{"initials":"KC","lastname":"Garcia","name":"Garcia KC"}],"identifier":[{"id":"10.1016/s0969-2126(01)00689-x","type":"doi"},{"id":"11738047","type":"pubmed"}],"issue":["12"],"journal":{"iso_abbreviation":"Structure","name":""},"pages":["1213-24"],"title":"Promiscuous antigen presentation by the nonclassical MHC Ib Qa-2 is enabled by a shallow, hydrophobic groove and self-stabilized peptide conformation.","type":"article","url":"https://www.sciencedirect.com/science/article/abs/pii/S096921260100689X","volume":["9"],"year":[2001]},"in_pmc":"N","in_pmce":"N","open_access":"N"},"resolution":"2.30","same_as":{"pdbe":{"url":"https://www.ebi.ac.uk/pdbe/entry/pdb/1k8d"},"rcsb":{"url":"https://www.rcsb.org/structure/1k8d"}},"species":{"common_name":"Mouse","match_type":"histo:assign_species","scientific_name":"Mus musculus","slug":"mus_musculus"},"tcr":null,"title":"Qa-2 binding \"ILMEHIHKL\" at 2.30&#8491; resolution","unique_chain_count":3}}