BETA(2)-MICROGLOBULIN AND CALNEXIN CAN INDEPENDENTLY PROMOTE FOLDING AND DISULFIDE BOND FORMATION IN CLASS-I HISTOCOMPATIBILITY PROTEINS

Class I histocompatibility proteins fold and assemble with beta(2)-mic roglobulin (beta(2)m) into heterodimers before binding short peptides in the endoplasmic reticulum. Here, we show that class I proteins rapi dly form disulfide bonds, and that the process is highly reversible in Daudi cells lacking beta(2)m. Three distinct class I protein conforma tions are present in equal amounts in these cells, each associated wit h the molecular chaperone calnexin. When binding of calnexin is inhibi ted by the glucosidase inhibitor castanospermine, fully oxidized class I proteins are no longer detected, suggesting that calnexin is requir ed for completion of folding. However, in Daudi cells transfected to e xpress beta(2)m, castanospermine decreases only slightly the levels of fully oxidized class I proteins, indicating that folding is much less dependent on calnexin in the presence of beta(2)m. Furthermore, calre ticulin, a chaperone with functional similarities to calnexin, associa tes with class I molecules in beta(2)m-positive cells, but not in Daud i cells, consistent with completion of folding and disulfide bond form ation of class I heavy chains before binding to calreticulin occurs. T his study demonstrates that calnexin and beta(2)m can function indepen dently to promote folding of class I heavy chains prior to formation o f stable class I dimers. (C) 1997 Elsevier Science Ltd.