Abstract: Protein disulfide isomerase (Pdi1p) is a folding assistant of the endoplasmic reticulum (ER) that catalyzes disulfide formation and the isomerization of incorrect disulfides. Its disulfide forming activity is its essential function in S. cerevisiae. A truncation mutant (Pdi1a') that is competent in disulfide formation but deficient in catalyzing isomerization has only a small effect on growth even though the maturation of isomerase-requiring substrates (carboxypeptidase Y) is impaired (Xiao, R., Wilkinson, B., Solovyov, A., Winther, J. R., Holmgren, A., Lundstrom-Ljung, J., and Gilbert, H. F. (2004) J. Biol. Chem. 279, 49780-49786). We show here that there are multiple ways to compensate for defects in disulfide formation and isomerization in the ER. Genes of the unfolded protein response are induced and deletions of the non-essential IRE1 or HAC1 genes are synthetically lethal. Diploid synthetic lethality analysis by microarray (dSLAM) using PDIa' and a temperature sensitive mutant of PDIa' as query mutations reveals a group of 130 synthetically lethal genes. Only ten of these correspond to genes clearly associated with the unfolded protein response. More than half are involved in vesicle traffic, not only out of and into the ER, but anterograde and retrograde traffic from most cellular compartments. This suggests that defects in protein maturation in one intracellular compartment may be compensated for by adjusting vesicular traffic patterns throughout the cell.