This forms the peptide-binding site . One Ig-like domain structure without transmembrane portion. It is held to the cell surface by noncovalent interaction with the 3 domain. NOT all peptides. The peptide-binding site is closed at both ends.
Short peptides are relatively extended, while long peptides bulge up in the middle.
Each MHC molecule binds to only certain shaped peptides, based on the shape of its peptide-binding site, which is determined by its aa sequence. Molecules and Disease Some MHC types are associated with susceptibility to certain diseases, others in disease resistance.