Catalyzes the rearrangement of -S-S- bonds in proteins. Part of a large chaperone multiprotein complex comprising DNAJB11, HSP90B1, HSPA5, HYOU, PDIA2, PDIA4, PDIA6, PPIB, SDF2L1, UGGT1 and very small amounts of ERP29, but not, or at very low levels, CALR nor CANX. Component of a complex containing at least CRELD2, MANF, MATN3 and PDIA4 (By similarity). Identified by mass spectrometry in melanosome fractions from stage I to stage IV (PubMed:17081065). Belongs to the protein disulfide isomerase family. Was originally thought to be a deoxycytidine kinase.