Disulfide isomerase which catalyzes the formation, isomerization, and reduction or oxidation of disulfide bonds (By similarity). Associates with calcitriol, the active form of vitamin D3 which mediates the action of this vitamin on cells (By similarity). Association with calcitriol does not affect its enzymatic activity (By similarity). Catalyzes the rearrangement of -S-S- bonds in proteins. Seems to be inhibited by acidic phospholipids. 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 (By similarity). Interacts with MZB1 in a calcium-dependent manner (By similarity). Interacts with SERPINA2 and with the S and Z variants of SERPINA1. Interacts with ATP2A2 (By similarity). In caput epididymal spermatozoa, detected in the head, mid and principal pieces. In cauda epididymal spermatozoa detected only in the acrosome (at protein level). Belongs to the protein disulfide isomerase family. Was originally thought to be a phosphatidyl-inositol 4,5-bisphosphate phosphodiesterase type I (phospholipase C-alpha) then was thought (PubMed:1321829 and PubMed:1330685) to be a thiol protease.