Wikipedia
FKBP, or FK506 binding protein, is a family of proteins that have prolyl isomerase activity and are related to the cyclophilins in function, though not in amino acid sequence. FKBPs have been identified in many eukaryotes from yeast to humans and function as protein folding chaperones for proteins containing proline residues. Along with cyclophilin, FKBPs belong to the immunophilin family.
FKBP12 is notable in humans for binding the immunosuppressant molecule tacrolimus (originally designated FK506), which is used in treating patients after organ transplant and patients suffering from autoimmune disorders. Tacrolimus has been found to reduce episodes of organ rejection over a related treatment, the drug ciclosporin, which binds cyclophilin. Both the FKBP-tacrolimus complex and the ciclosporin-cyclophilin complex inhibit a phosphatase called calcineurin, thus blocking signal transduction in the T- lymphocyte transduction pathway. This therapeutic role is not related to prolyl isomerase activity.