Cyclophilin 40 (CyP40, rotamase D, PPID) was first described as a cyclosporine A (CsA) binding protein and part of the inactivated estrogen receptor complex. CyP 40 functions include contributing to protein folding, ligand binding, and nuclear localization of glucocorticoid, estrogen and progesterone receptors. One of the important roles of CyP 40 is to help with the assembling of heat shock protein Hsp90 in chaperone protein-folding machinery.
Many functions of Hsp90 are dependent on its association with co-chaperone proteins. Co-chaperones mediate various aspects of Hsp90 function, including the association of Hsp90 with client proteins and the regulation of Hsp90 ATPase activity. CyP 40 is a member of the immunophilin family of Hsp90 co-chaperones.This family is characterized by its association with Hsp90-steroid hormone receptor complexes containing client proteins such as the glucocorticoid, estrogen, progesterone, and androgen receptors. Immunophilin co-chaperones are important in cancer, as CyP40 has been shown to promote the proliferation of androgen-dependent and androgen-independent prostate cancer cell lines.
A recent study showed that CyP40 mRNA was over-expressed in breast cancer tissues as compared with normal breast control tissues, and CyP 40 mRNA was ubiquitously expressed in 10 breast cancer cell lines. CyP40 also plays an important role in chronic hepatitis virus (HCV) replication. Baker et al. demonstrated that CyP 40 is capable of disaggregating amyloid fibrils in vitro and data from this study implicate that CyP 40 is a potential therapeutic intervention for tauopathies and other amyloidogenic disorders.