Structural definition of the H-2K^d peptide-binding motif

Classic major histocompatibility complex (MHC) proteins associate with antigen- and self-derived peptides in an allele-specific manner. Herein we present the crystal structure of the MHC class I protein H-2K^d (K^d) expressed by BALB/c mice in complex with an antigenic peptide derived from influenza A/PR/8/34 nucleoprotein (Flu, residues 147-155, TYQRTRALV). Analysis of our structure in conjunction with the sequences of naturally processed epitopes provides a comprehensive understanding of the dominant K^d peptide-binding motif. We find that Flu residues Tyr ^P2, Thr^P5, and Val^P9 are sequestered into the B, C, and F pockets of the K^d groove, respectively. The shape and chemistry of the polymorphic B pocket make it an optimal binding site for the side chain of Tyr^P2 as the dominant anchoring residue of nonameric peptides. The non-polar F pocket limits the amino acid repertoire at P9 to hydrophobic residues such as Ile, Leu, or Val, whereas the C pocket restricts the size of the P5-anchoring side chain. We also show that Flu is accommodated in the complex through an unfavorable kink in the otherwise extended peptide backbone due to the presence of a prominent ridge in the K^d groove. Surprisingly, this backbone conformation is strikingly similar to D ^b-presented peptides despite the fact that these proteins employ distinct motif-anchoring strategies. The results presented in this study provide a solid foundation for the understanding of K^d-restricted antigen presentation and recognition events.