ISOLATION AND CHARACTERIZATION OF A YEAST GENE, MPD1, THE OVEREXPRESSION OF WHICH SUPPRESSES INVIABILITY CAUSED BY PROTEIN DISULFIDE-ISOMERASE DEPLETION

MPD1, a yeast gene the overexpression of which suppresses the inviabil ity caused by the loss of protein disulfide isomerase (PDI) was isolat ed and characterized. The MPD1 gene product retained a single disulfid e isomerase active site sequence (APWCGHCK), an N-terminal putative si gnal sequence, and a C-terminal endoplasmic reticulum (ER) retention s ignal, and was a novel member of the PDI family, The gene product, ide ntified in yeast extract, contained core size carbohydrates. MPD1 was not essential for growth, but overexpression of the gene suppressed th e maturation defect of carboxypeptidase Y caused by PDI1 deletion, ind icative of the related function to PDI in the yeast ER.