Protein disulfide isomerases, such as PDIA6, are endoplasmic reticulum resident proteins that catalyze formation, reduction, and isomerization of disulfide bonds in proteins and are thought to play a role in folding of disulfide-bonded proteins The deduced 440-amino acid protein has a calculated molecular mass of 48.1 kD. It has a putative N-terminal signal sequence, followed by 2 thioredoxin like domains and a C-terminal ER retention signal. Mutation analysis revealed that the first thioredoxin-like motif of P5 was more important than the second for isomerase activity, and that the first cysteine in each motif was necessary for isomerase activity. Thioredoxin motif mutants of P5 lacking isomerase activity retained chaperone activity with citrate synthase as substrate, indicating that, like PDI, the isomerase and chaperone activities of P5 are likely independent.