The genetic interaction of kar2 and wbp1 mutations. Distinct functions of binding protein BiP and N-linked glycosylation in the processing pathway of secreted proteins in Saccharomyces cerevisiae.

The endoplasmic binding protein BiP and N-linked glycosylation are proposed to be essential components in the processing pathway of secreted protein. In Saccharomyces cerevisiae, BiP is encoded by the KAR2 gene; WBP1 encodes an essential component of the N-oligosaccharyltransferase complex. wbp1 mutations result in reduced oligosaccharyltransferase activity and a temperature-sensitive phenotype. We show that a combination of kar2 and wbp1 mutations results in a synthetic phenotype with a strongly reduced growth rate at the permissive temperature. To investigate the role of N-linked glycosylation in BiP function, the processing of non-glycosylated carboxypeptidase was followed in different kar2 strains at the permissive temperature. In all kar2 strains, the processing of non-glycosylated carboxypeptidase Y was drastically reduced. A specific BiP/non-glycosylated carboxypeptidase Y complex was detected in kar2-159 and kar2-203 cells whereas the kar2-1 mutation did not result in such a complex. Our data show that BiP and N-linked glycosylation are directly involved in the processing of secreted proteins. The results support the hypothesis that BiP stabilizes the folding-competent and assembly-competent state of a polypeptide, whereas N-linked oligosaccharides are structural components required in the folding process after the polypeptide is released from BiP.