THE MOLECULAR CHAPERONE CALNEXIN FACILITATES FOLDING AND ASSEMBLY OF CLASS-I HISTOCOMPATIBILITY MOLECULES

Calnexin, a membrane protein of the endoplasmic reticulum, is generall y thought to function as a molecular chaperone, based on indirect or c orrelative evidence, To examine calnexin's functions more directly, we reconstituted the assembly of class I histocompatibility molecules in the absence or presence of calnexin in Drosophila melanogaster cells, Calnexin enhanced the assembly of class I heavy chains with beta(2)-m icroglobulin as much as 5-fold, The improved assembly appeared largely due to more efficient folding of heavy chains, as evidenced by increa sed reactivity with a conformation-sensitive monoclonal antibody and b y a reduction in the level of aggregates. Similar findings were obtain ed in mouse or human cells when the interaction of calnexin with class I heavy chains was prevented by treatment with the oligosaccharide pr ocessing inhibitor castanospermine. The ability of calnexin to facilit ate heavy chain folding and to prevent the formation of aggregates pro vides compelling evidence that calnexin functions as a bona fide molec ular chaperone.