Calcineurin is a Ser/Thr, calcium and calmodulin-dependent protein phosphatase that plays an essential role in the T cell activation pathway. Calcineurin (also referred to as protein phosphatase 2B) is composed of two subunits; calcineurin A (CnA), a 60 kDa catalytic subunit which is highly homologous to protein phosphatases 1 and 2A; and a 19 kDa regulatory subunit, calcineurin B (CnB) which, like calmodulin, contains four EF-hand calcium binding sites. Calcineurin-mediated dephosphorylation of the nuclear factor of activated T-cells (NF-AT) is essential for NF-AT activation, nuclear translocation, and early gene expression in T-cells. The immunosuppressive drugs, cyclosporin A and FK506, when bound to the immunophilins, cyclophilin and FKBP respectively, inhibit dephosphorylation and activation of NF-AT by calcineurin. This inhibition is caused by the immunosuppressant/immunophilin complex simultaneously binding to both CnA and CnB which does not inhibit the phosphatase activity of CnA, but rather blocks access of the substrate to the catalytic domain of CnA.View more
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