ER60/ERp57 forms disulfide-bonded intermediates with MHC class I heavy chain

We demonstrated previously that ER60/ERp57 is recruited into early folding intermediates in the MHC class I assembly pathway (1). ER60 binding preceded the binding of beta-2-microglobulin and was dependent on the presence of properly trimmed oligosaccharide moieties on the class I heavy chain (HC). Hence, this interaction could not be temporally differentiated from that of calnexin. Here, we show that calnexin is required for the recruitment of ER60 into early folding complexes with the HC and that in the absence of calnexin, ER60 is replaced by another member of the thiol-reductase family, ERp72. We demonstrate that once recruited, ER60 associates covalently with HC via a disulfide bond. The ER60/HC complexes were also found in late assembly complexes, noncovalently bound to the transporter associated with antigen processing and accounted for approximately one-third of the ER60 associated with the transporter associated with antigen processing. These results demonstrate that ER60 assists in the creation of conserved disulfide bonds within the class I HC through the formation of a mixed disulfide intermediate.