The regulatory domains of CNA have different effects on the inhibition of CN activity by FK506 and CsA.

Calcineurin (CN) is the common receptor for two immunophilin-immunosuppressant complexes, Cyp-CsA and FKBP-FK506. Calcineurin is composed of a catalytic subunit (CNA) and a regulatory subunit (CNB). CNA contains the catalytic domain and three regulatory domains: a CNB-binding domain (BBH, 350-370), a calmodulin- binding domain (CBD, 389-413), and an autoinhibitory domain (AID, 457-482). To investigate the effects of these three regulatory domains on the inhibition of CN by the two drugs we constructed three C-terminal deletion mutants: CNAabc (1-456), CNAab (1-388) and CNAa (1-347). Inhibition of CNA and its derivatives by the two drugs was examined and compared with inhibition by peptides (AID [457-482] and LCBD [389-456], CBD and the extension of the AID were included). Our results show that the BBH is critical for inhibition of CN by Cyp-CsA and FKBP-FK506. The LCBD has no effect and the AID reduces the inhibition of CN by two complexes. In addition, LCBD and AID as autoinhibitors may inhibit enzyme activity via different sites.