Crystal structures of human calcineurin and the human FKBP12–FK506–calcineurin complex

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Abstract
CALCINEURIN (CaN) is a calcium- and ca 1modulin-dependent protein serine/threonine phosphatase which is critical for several important cellular processes, including T-cell activation1. CaN is the target of the immunosuppressive drugs cyclosporin A and FK506, which inhibit CaN after forming complexes with cyto-plasmic binding proteins (cyclophilin and FKBP12, respectively)2. We report here the crystal structures of full-length human CaN at 2.1 Å resolution and of the complex of human CaN with FKBP12-FK506 at 3.5 Å resolution. In the native CaN structure, an auto-inhibitory element binds at the Zn/Fe-containing active site. The metal-site geometry and active-site water structure suggest a catalytic mechanism involving nucleophilic attack on the substrate phosphate by a metal-activated water molecule. In the FKBP12–FK506–CaN complex, the auto-inhibitory element is displaced from the active site. The site of binding of FKBP12–FK506 appears to be shared by other non-competitive inhibitors of calcineurin, including a natural anchoring protein.