P4HB is a multifunctional protein catalyzes the formation, breakage and rearrangement of disulfide bonds. At the cell surface, P4HB seems to act as a reductase that cleaves disulfide bonds of proteins attached to the cell. Inside the cell, it seems to form/rearrange disulfide bonds of nascent proteins. At high concentrations, P4HB functions as a chaperone that inhibits aggregation of misfolded proteins. At low concentrations, it facilitates aggregation (anti-chaperone activity) [taken from the Universal Protein Resource (UniProt) www.uniprot.org/uniprot/P07237].