A new stress protein: Synthesis of Schizosaccharomyces pombe UDP-Glc:glycoprotein glucosyltransferase mRNA is induced by stress conditions but the enzyme is not essential for cell viability

We have identified and begun the characterization of the gene encoding UDP-Glc:glycoprotein glucosyltransferase in Schizosaccharomyces pombe. This gene, here designated gpt1, codes for a polypeptide having a signal peptide of 18 amino acids followed by 1429 amino acids with no transmembrane domain, as expected for a soluble protein of the endoplasmic reticulum (ER). The C-terminal tetrapeptide PDEL most probably corresponds to a novel ER retention signal in this fission yeast. Synthesis of the corresponding mRNA was induced 2- to 9-fold by conditions known to affect glycoprotein folding in the ER (e.g. heat shock, culture in the presence of a Ca2+ ionophore, 2-mercaptoethanol or inhibitors of protein N-glycosylation such as tunicamycin or 2-deoxyglucose). This is the first evidence obtained in vivo that supports the proposed involvement of the enzyme in the quality control of glycoprotein folding in the ER. Thus far, the said involvement was inferred solely from the ability of the enzyme to glucosylate misfolded but not native glycoproteins in cell-free assays. The gpt1 gene was disrupted and gpt1(-) cells were found to be viable. Moreover, no significant differences in the growth rate patterns at 18, 28 or 39 degrees C or in cell morphology between gpt1(+) and gpt1(-) cells were observed, although they differed slightly in size.