Protein Disulfide Isomerase/P4HB: ProductsP4HB (Prolyl 4-hydroxylase beta chain; also PDI) is a 60 kDa member of the protein disulfide isomerase family. As an intracellular homodimer, it forms a tetrameric complex with P4H alpha chains to form an active prolyl 4 hydroxylase. This catalyses the hydroxylation of proline in collagen. On the cell surface, it reduces disulfide bonds in HIV that allow the virus to fuse with CXCR4 and enter susceptible cells. Mature human P4HB is 491 amino acids (aa) in length. It contains two TRX domains (aa 25-134 and 368-475) plus an ER retention sequence (aa 505-508). There is one potential isoform that shows an 11 aa substitution for the first 162 amino acids. Over aa 18-505, human P4HB shares 94% aa identity with mouse P4HB.