 In the ER, PPIB interacts with proteins such as P3H1, CRTAP, BiP, GRP94, PDI, and calreticulin to form foldase and chaperone complexes and facilitate protein folding, especially for type I collagen.This protein is the major PPIase for type I collagen, since the collagen contains an abundance of prolines that require cis-trans isomerization for proper folding.Thus, PPIB is essential for collagen biosynthesis and post-translational modification and affects fibril assembly, matrix cross-linking, and bone mineralization.