TETRAMERIC CELL-SURFACE MHC CLASS-I MOLECULES

PURIFIED major histocompatibility complex (MHC) class I molecules have been studied at high resolution by X-ray crystallography 1; the structure is a complex of a single heavy chain, a beta-2-microglobulin light chain and a tightly bound peptide moiety. We show here that complete MHC class I molecules are post-translationally assembled into tetramers (made up of four heavy chains and four beta-2-microglobulin units) and that this tetrameric species is expressed on the cell surface. The multivalent tetrameric structure of class I molecules can be reconciled with models of T-cell activation that invoke antigen-receptor crosslinking, as opposed to models that depend on an allosteric change.