MECHANISMS OF MHC CLASS-I - RESTRICTED ANTIGEN-PROCESSING

Classical class I molecules assemble in the endoplasmic reticulum (ER) with peptides mostly generated from cytosolic proteins by the proteas ome. The activity of the proteasome can be modulated by a variety of a ccessory protein complexes. A subset of the proteasome beta-subunits ( LMP2, LMP7, and MECL-1) and one of the accessory complexes, PA28, are upregulated by gamma-interferon and affect the generation of peptides to promote more efficient antigen recognition. The peptides are transl ocated into the ER by the transporter associated with antigen processi ng (TAP). A transient complex containing a class I heavy chain-beta(2) microglobulin (beta(2)m) dimer is assembled onto the TAP molecule by successive interactions with the ER chaperones calnexin and calreticul in and a specialized molecule, tapasin. Peptide binding releases the c lass I-beta(2)m dimer for transport to the cell surface, while lack of binding results in proteasome-mediated degradation. The products of c ertain nonclassical MHC-linked class I genes bind peptides in a simila r way. A homologous set of beta(2)m-associated membrane glycoproteins, the CD1 molecules, appears to bind lipid-based ligands within the end ocytic pathway.