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). May be involved with other chaperones in the structural modification of the TG precursor in hormone biogenesis. Also acts a structural subunit of various enzymes such as prolyl 4-hydroxylase and microsomal triacylglycerol transfer protein MTTP. Homodimer. Monomers and homotetramers may also occur. Also constitutes the structural subunit of prolyl 4-hydroxylase and 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. Binds to CD4, and upon HIV-1 binding to the cell membrane, is part of a P4HB/PDI-CD4-CXCR4-gp120 complex. Belongs to the protein disulfide isomerase family. Note: This description may include information from UniProtKB.
SS: The number of records in which this modification site was determined using site-specific methods. SS methods include amino acid sequencing, site-directed mutagenesis, modification site-specific antibodies, specific MS strategies, etc.