ERp57 functions as a subunit of specific complexes formed with the ER lectins calreticulin and calnexin

ERp57 is a lumenal protein of the endoplasmic reticulum (ER) and a member o f the protein disulfide isomerase (PDI) family. In contrast to archetypal P DI, ERp57 interacts specifically with newly synthesized glycoproteins. In t his study we demonstrate that ERp57 forms discrete complexes with the ER le ctins, calnexin and calreticulin. Specific ERp57/calreticulin complexes exi st in canine pancreatic microsomes, as demonstrated by SDS-PAGE after cross linking, and by native electrophoresis in the absence of cross-linking. Af ter in vitro translation and import into microsomes, radiolabeled ERp57 can be cross-linked to endogenous calreticulin and calnexin while radiolabeled PDI cannot. Likewise, radiolabeled calreticulin is cross-linked to endogen ous ERp57 but not PDI. Similar results were obtained in Lec23 cells, which lack the glucosidase I necessary to produce glycoprotein substrates capable of binding to calnexin and calreticulin. This observation indicates that E Rp57 interacts with both of the ER lectins in the absence of their glycopro tein substrate. This result was confirmed by a specific interaction between in vitro synthesized calreticulin and ERp57 prepared in solution in the ab sence of other ER components. We conclude that ERp57 forms complexes with b oth calnexin and calreticulin and propose that it is these complexes that c an specifically modulate glycoprotein folding within the ER lumen.