Protein disulfide isomerase in spore germination and cell division.

Protein disulfide isomerase (PDI) is a protein of the endoplasmic reticulum (ER) that is essential for the unscrambling of nonnative disulfide bonds. Here, we have determined the importance of PDI to both spore germination and vegetative cell division. To vary the concentration of PDI in the ER, we used plasmids that direct the expression of rat PDI fused at its N terminus to either the alpha-factor pre-pro segment or the alpha-factor pre sequence, and fused at its C terminus to either the mammalian (KDEL) or the yeast (HDEL) ER retention signal. Classical yeast genetic (tetrad) analyses, and plasmid loss and plasmid shuffling experiments were used to evaluate the ability of these constructs to complement haploid Saccharomyces cerevisiae cells in which the endogenous PDI1 gene had been deleted. We find that basal levels of PDI in the ER are sufficient for vegetative growth. In contrast, high levels of PDI in the ER are required for efficient spore germination. Thus, catalysis of the unscrambling of nonnative disulfide bonds in cellular proteins is more important during spore germination than during vegetative cell division.