Disulfide isomerase which catalyzes the formation, isomerization, and reduction or oxidation of disulfide bonds (PubMed:7487104, PubMed:27897272). Associates with calcitriol, the active form of vitamin D3 which mediates the action of this vitamin on cells (PubMed:27897272). Association with calcitriol does not affect its enzymatic activity (PubMed:27897272). Catalyzes the rearrangement of -S-S- bonds in proteins. Subunit of the TAP complex, also known as the peptide loading complex (PLC). Can form disulfide-linked heterodimers with TAPBP. Interacts with ERP27 and CANX. Interacts with SERPINA2 and with the S and Z variants of SERPINA1. Interacts with ATP2A2. Identified by mass spectrometry in melanosome fractions from stage I to stage IV (PubMed:12643545). Detected in the flagellum and head region of spermatozoa (at protein level) (PubMed:20400973). Expressed in liver, stomach and colon (at protein level). Expressed in gastric parietal cells and chief cells (at protein level) (PubMed:24188822). Belongs to the protein disulfide isomerase family. Was originally thought to be a phosphatidylinositol 4,5-bisphosphate phosphodiesterase type I (phospholipase C-alpha).