PPIase that catalyzes the cis-trans isomerization of proline imidic peptide bonds in oligopeptides and may therefore assist protein folding. Proposed to act as a co-chaperone in HSP90 complexes such as in unligated steroid receptors heterocomplexes. Different co-chaperones seem to compete for association with HSP90 thus establishing distinct HSP90-co-chaperone-receptor complexes with the potential to exert tissue-specific receptor activity control. May have a preference for estrogen receptor complexes and is not found in glucocorticoid receptor complexes. May be involved in cytoplasmic dynein-dependent movement of the receptor from the cytoplasm to the nucleus. May regulate MYB by inhibiting its DNA-binding activity. Involved in regulation of AHR signaling by promoting the formation of the AHR:ARNT dimer; the function is independent of HSP90 but requires the chaperone activity region. Involved in regulation of UV radiation-induced apoptosis. [protein]-peptidylproline (omega=180) = [protein]-peptidylproline (omega=0) Less sensitive to inhibition by cyclosporin A than is CYP-18. Identified in ESR1 or NR3C1/GCR steroid receptor-chaperone complexes. Found in HSP90 chaperone complexes with kinase clients LCK or EIF2AK1. Two monomers associate with one HSP90 homodimer. Interacts with HSP90AA1. Interacts with HSP90AB1; PPID and FKBP4 compete for binding to HSP90AB1 and the interaction is mutually exclusive with the PPID:HSPA8 interaction. Interacts with HSPA8; PPID and STIP1 but not FKBP4 compete for binding to HSPA8 and the interaction is mutually exclusive with the PPID:HSP90AB1 interaction. Interacts with S100A1 and S100A2; the interactions dissociate the PPID:HSP90AA1 interaction. Interacts with S100A6. Interacts with MYB, ILF2, XRCC6, RACK1 and RPS3. Interacts with cytoplasmic dynein 1 intermediate chain (DYNC1I1 or DYNC1I2). Belongs to the cyclophilin-type PPIase family. PPIase D subfamily. This protein should not be confused with mitochondrial peptidyl-prolyl cis-trans isomerase F (PPIF) which is often referred to as cyclophilin D or CypD.