Fig. 31.22 Crystal structure of the complex of an MHC-I molecule with the bound nonapeptide Leu-Leu-Phe-Gly-Tyr-Pro-Val-Tyr-Val (gray, yellow-gray surface) and the T cell receptor. On the left (a) the entire structure is shown. On the right (b) a section around the peptide binding site is displayed. The MHC molecule consists of a heavy chain with α1, α2, and α3 domains (purple) and the βm light chain (blue). The pleated sheet-like structure of the α1/α2 domain forms an elongated bowl open at the top. It is bounded by two long, parallel α-helices (yellow). It accommodates the antigenic peptide fragment in its bowl-shaped opening and presents its upper surface to the T cell receptor. This heterodimeric receptor, composed of an α chain (light blue) and a β chain (gray), also has a folded sheet-like structure. With its hypervariable loops CDR3α and CDR3β, it recognizes the amino acid tyrosine of the antigenic peptide, especially at position 5.