Class IMHC molecules: assembly and antigen presentation

Abstract

Summary: Several years ago, the only factor known to be necessary for the assembly and surface expression of class I MHC was pjm; even for β₂m, it was unclear at what point in class I maturation its role was played. Recent experiments that employed attachment of an endoplasmic reticulum (ER) retention signal to β₂m have shown that the point of time at which β₂m is required is while the class I heavy chain is in the ER. Later association between β₂m and class I is not vital in order for properly folded class I to be expressed at β₂m cell surface. After crystallization of the first class I MHC molecule, it was reahed that not only is antigen presented by class I, but that antigen is presented in the form of a peptide that stabilizes the class I structure and allows its transit to the cell surface. Class I allelic differences influence interactions with both peptide and β₂m, with likely consequences for the ability of the class I heavy chains to present antigen through alternative pathways. Furthermore, it is now also clear that formation of appropriate disulfide bonds in the class I heavy chain is needed before class I can bind peptide antigen securely, a process that may he assisted by an ER chaperone. Many different proteins that are resident in the ER, such as calnexin, transporter associated with antigen processing (TAP), caheticulin, and tapasin, have been found to be integral to class I assembly. TAP, tapasin, and calreticulin hind preferentially to the open form of class I, which can be distinguished with the use of a monoclonal antibody specific for this form. Calreticulin and calnexin contrast in their interactions with class I, despite other similarities between these two chaperones. Overall, class I MHC assembly is now understood to involve the interplay of multiple intra- and intermolecular events in a defined chronological order which ensure continual reporting of cellular contents to cytotoxic T lymphocytes.