Progress in crystallization of major histocompatibility complex class I in vertebrates

The major histocompatibility complex (MHC) of proteins that exists in all vertebrates is encoded by a cluster of genes associated with the immune response and related functions. MHC is divided into MHC I, II, and III; MHC I is involved in antigenic presentation, binding T cell receptors, and leading ultimately to specific cellular immune responses. The complicated functions of MHC I are determined by the nature of the complex. The crystal structure of MHC I has been solved for many animals, revealing the relationship between spatial structure and function. MHC I consists of an α heavy chain and a β2m light chain, both ligated non-covalently to a complex when a peptide is bound to the antigenic-binding groove. The α heavy chain is divided into an extracellular domain, a transmembrane domain, and an intracellular domain. The extracellular domain consists of sub-regions α1, α2, and α3. The α1 and α2 together form the antigenic-binding groove and bind antigenic peptides with 8–10 amino acid residues. MHC I can form a stable spatial structure; however, it should be noted that there are differences in the structure of MHC I among animal species, including anchored amino acids in binding peptides, binding sites, molecular distance, crystallization conditions, etc. Here, progress in determination of the crystal structure of human, mouse, chicken, non-human primate, and swine MHC I is described in detail.