Calnexin and BiP interact with acid phosphatase independently of glucose trimming and reglucosylation in Schizosaccharomyces pombe

The association of newly synthesized glycoproteins with the ER molecular chaperones calnexin and immunoglobulin binding protein (BiP) has been well documented in a variety of higher eukaryotes. Here we report that Cnx1p, the calnexin homologue in Schizosaccharomyces pombe, associates with newly synthesized molecules of the secreted glycoprotein acid phosphatase. Unlike ligand binding to mammalian calnexin, glucose trimming and reglucosylation of acid phosphatase by UDP-Glc:glycoprotein glucosyltransferase were shown to be dispensable for its binding to Cnx1p. Thus, despite the essentiality of Cnx1p for S. pombe viability, the glucose trimming and reglucosylation cycle does not appear to be required for protein folding in the fission yeast. The association of core-glycosylated acid phosphatase with Cnx1p after exposure of cells to heat shock or to DTT was shown to be reversible. However, Cnx1p stably associated with unglycosylated acid phosphatase after treatment with the core-glycosylation inhibitor tunicamycin. BiP was found to coprecipitate with Cnx1p, under normal and stress conditions, and following inhibition of protein synthesis by cycloheximide. We postulate that Cnx1p and BiP are part of a complex that is involved in the folding of both core-glycosylated trimmed ligands and unglycosylated proteins.