CALNEXIN INFLUENCES FOLDING OF HUMAN CLASS-I HISTOCOMPATIBILITY PROTEINS BUT NOT THEIR ASSEMBLY WITH BETA(2)-MICROGLOBULIN

Class I major histocompatibility complex heavy chains bind to calnexin before associating with beta(2)-microglobulin (beta(2)m) and peptides . Calnexin has been shown to retain in the endoplasmic reticulum those class I heavy chains which have not assembled properly and, thus, to serve as a quality control mechanism. In addition, calnexin may direct the folding of class I subunits or their subsequent assembly. We aske d whether calnegin plays a role in the initial folding of HLA-B0702 h eavy chains by assessing disulfide bond formation in vivo. Our results show that class I heavy chains form intrachain disulfide bonds very s oon after translation, and that calnexin is bound to both reduced and oxidized forms during this process. When a cell-permeable reducing age nt, dithiothreitol, was added to cells, disulfide bond formation in ne wly synthesized heavy chains was substantially blocked, as was their a ssociation with calnexin. The reducing agent appeared to affect calnex in directly, since binding was similarly abolished to a subset of prot eins which do not contain internal disulfide bonds. Addition of the gl ucosidase inhibitor castanospermine to cells, shown previously to disr upt calnexin binding to ligands, slowed formation of disulfide bonds b ut did not decrease the amount of assembled heavy chain-beta(2)m compl exes that formed. Our data suggest that calnexin can promote disulfide bond formation in class I heavy chains but does not directly facilita te subsequent binding to beta(2)m.