This multifunctional protein catalyzes the formation, breakage and rearrangement of disulfide bonds. At the cell surface, seems to act as a reductase that cleaves disulfide bonds of proteins attached to the cell. May therefore cause structural modifications of exofacial proteins. Inside the cell, seems to form/rearrange disulfide bonds of nascent proteins. At high concentrations, functions as a chaperone that inhibits aggregation of misfolded proteins. At low concentrations, facilitates aggregation (anti-chaperone activity). Also acts a structural subunit of various enzymes such as prolyl 4-hydroxylase (By similarity). Catalyzes the rearrangement of -S-S- bonds in proteins. Heterodimer; heterodimerizes with the protein microsomal triglyceride transfer MTTP. Homodimer. Monomers and homotetramers may also occur. Also constitutes the structural subunit of prolyl 4-hydroxylase. Stabilizes this enzyme and retains it in the ER without contributing to the catalytic activity. Binds UBQLN1 (By similarity). Belongs to the protein disulfide isomerase family.