FKBP

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.

FKBP52

FK506-binding protein 4 is a protein that in humans is encoded by the FKBP4 gene.

The protein encoded by this gene is a member of the immunophilin protein family, which play a role in immunoregulation and basic cellular processes involving protein folding and trafficking. This encoded protein is a cis-trans prolyl isomerase that binds to the immunosuppressants FK506 and rapamycin. It has high structural and functional similarity to FK506-binding protein 1A (FKBP1A), but unlike FKBP1A, this protein does not have immunosuppressant activity when complexed with FK506. It interacts with interferon regulatory factor-4 and plays an important role in immunoregulatory gene expression in B and T lymphocytes. This encoded protein is known to associate with phytanoyl-CoA alpha-hydroxylase. It can also associate with two heat shock proteins (hsp90 and hsp70) and thus may play a role in the intracellular trafficking of hetero-oligomeric forms of the steroid hormone receptors. This protein correlates strongly with adeno-associated virus type 2 vectors (AAV) resulting in a significant increase in AAV-mediated transgene expression in human cell lines. Thus this encoded protein is thought to have important implications for the optimal use of AAV vectors in human gene therapy. This gene has been found to have multiple polyadenylation sites.

FKBP6

FK506 binding protein 6, also known as FKBP6, is a human gene. The encoded protein shows structural homology to FKBP immunophilins, which bind to the immunosuppressants FK506 and rapamycin.

FKBP6 is essential for homologous chromosome pairing in meiosis during spermatogenesis. Targeted inactivation of FKBP6 in mice results in infertile males, but apparently normal females. Rats with spermatogenic failure at meiosis were found to have a deletion in the exon 8 portion of the FKBP6 gene. Mutations in this gene have been associated with male infertility in humans.

FKBP6 is deleted in Williams syndrome, however this hemizygous loss of FKBP6 is not associated with infertility.

FKBP6 contains 3 α-helices and 11 β-sheet strands, and as a FK506-family protein, has been shown to be a potent immunosuppressant which can assist in the prevention of allograft rejections.

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