Structural studies of class I major histocompatibility complex proteins: insights into antigen presentation.

The three-dimensional structures of three human and two murine class I molecules, in complex with single peptides and with mixtures of endogenous peptides, have now been determined to high resolution. These structures have afforded important insights into the way in which antigenic peptides are bound by an MHC class I molecule, and how a given MHC molecule can bind a large number and variety of peptides, for presentation to a T cell receptor. Peptides are bound in a cleft located in the alpha 1/alpha 2 domain of a class I molecule. They are tethered by an array of hydrogen bonding interactions many of which are conserved among the different structures. Binding is also accomplished through van der Waals interactions between two or three peptide residues and complementary pockets in the cleft. The location and the characteristics of its pockets are unique to a given MHC class I molecule, and so determine the identity of the anchor residues of the set of peptides that bind. The antigenic epitope recognized by the TCR consists of residues on the MHC as well as the side chains of those peptide residues that point out from the cleft. The various strategies used to expand the repertoire of peptides bound and presented are discussed.