Enhanced Catalysis of Ribonuclease B Folding by the Interaction of Calnexin or Calreticulin with ERp57

The endoplasmic reticulum is the site of folding, disulfide bond formation, and N-glycosylation of secretory proteins. Correctly folded proteins are exported from the endoplasmic reticulum, whereas incorrectly folded proteins are retained by a quality control system. The type I membrane-protein calnexin and its soluble homologue calreticulin are constituents of this system that recognize monoglucosylated N-linked glycans that are present on unfolded glycoproteins. Although several components of the quality control apparatus are well characterized, it is not known whether and how they interact with enzymes that catalyze protein folding. The endoplasmic reticulum protein ERp57 is homologous to protein-disulfide isomerase and can be cross-linked to the same monoglucosylated glycoproteins that bind to calnexin and calreticulin. The present study demonstrates that the disulfide isomerase activity of ERp57 on the refolding of monoglucosylated ribonuclease B is much greater when this glycoprotein is associated with calnexin or calreticulin. This result is in contrast to protein-disulfide isomerase, whose activity on monoglucosylated ribonuclease B is decreased in the presence of these lectins. No direct binding of monoglucosylated ribonuclease B or monoglucosylated glycans to ERp57 could be detected, but we show that ERp57 interacts directly with calnexin.