Immunophilins are a family of soluble cytosolic receptors capable of binding to one of two major immunosuppressant agents: cyclosporin A (CsA) or FK506. Proteins that bind FK506 are termed FK506 Binding Proteins (FKBPs) and those that bind cyclosporin A are called cyclophilins (CyP).
Both CyP:CsA and FKBP:FK506 complexes have been shown to inhibit calcineurin, a calcium and calmodulin dependent protein phosphatase which has been implicated as an important signaling enzyme in T-cell activation, providing a possible mechanism of immunosuppression by CsA and FK506. Immunophilins function as peptidyl prolyl cis-trans-isomerases (PPIase) whose activity is inhibited by their respective immunosuppressant compounds. As PPIase's, immunophilins accelerate folding of some proteins both in vivo and in vitro by catalyzing slow steps in the initial folding and rearrangement of proline containing proteins.
CyP-40, a 40 kDa protein, shares significant homology with smaller CyPA (CyP-18) and FKBP59. CyP-40 exhibits the characteristic CsA binding and isomerase activity of CyP-18, though these activities appear to be less with CyP-40 than with Cyp-18. Like FKBP59, CyP-40 has been found in progesterone receptor complexes. CyP-40 is expressed at similar levels in many tissues.
Witting SR et al. Helper-dependent adenovirus-mediated short hairpin RNA expression in the liver activates the interferon response. J Biol Chem283:2120-8 (2008).
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