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 and following phosphorylation by FAM20C, functions as a chaperone that inhibits aggregation of misfolded proteins. At low concentrations, facilitates aggregation (anti-chaperone activity). May be involved with other chaperones in the structural modification of the TG precursor in hormone biogenesis. Also acts as a structural subunit of various enzymes such as prolyl 4-hydroxylase and microsomal triacylglycerol transfer protein MTTP. Receptor for LGALS9; the interaction retains P4HB at the cell surface of Th2 T helper cells, increasing disulfide reductase activity at the plasma membrane, altering the plasma membrane redox state and enhancing cell migration. 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. Interacts with P4HA2, forming a heterotetramer consisting of 2 alpha subunits (P4HA2) and 2 beta (P4HB), where P4HB plays the role of a structural subunit; this tetramer catalyzes the formation of 4-hydroxyproline in collagen (By similarity). Also constitutes the structural subunit of the microsomal triacylglycerol transfer protein MTTP in mammalian cells. Stabilizes both enzymes and retain them in the ER without contributing to the catalytic activity. Binds UBQLN1. Interacts with ERO1B. Interacts with ILDR2 (By similarity). Interacts with ERN1/IRE1A (via N-terminus); the interaction is enhanced by phosphorylation of P4HB by FAM20C in response to endoplasmic reticulum stress and results in attenuation of ERN1 activity (By similarity). Highly abundant. In some cell types, seems to be also secreted or associated with the plasma membrane, where it undergoes constant shedding and replacement from intracellular sources. Localizes near CD4-enriched regions on lymphoid cell surfaces. Colocalizes with MTTP in the endoplasmic reticulum. Phosphorylation of Ser-359 by FAM20C is induced by endoplasmic reticulum stress and results in a functional switch from oxidoreductase to molecular chaperone. It also promotes interaction with ERN1. Belongs to the protein disulfide isomerase family. Was originally (PubMed:8251535 and PubMed:3178809) thought to be identical to thyroxine deiodinase but this was later shown to be incorrect.