P4HB, the beta subunit of prolyl 4-hydroxylase, a highly abundant member of the PDI family. P4HB is a multifunctional enzyme, involved in hydroxylation of prolyl residues in preprocollagen, as well as continual rearrangement of disulfide bonds. Other known functions include its ability to act as a chaperone that inhibits aggregation of misfolded proteins in a concentration-dependent manner, its ability to bind thyroid hormone, its role in both the influx and efflux of S-nitrosothiol-bound nitric oxide, and its function as a subunit of the microsomal triglyceride transfer protein complex.