Synergism between the Calmodulin-binding and Autoinhibitory Domains on Calcineurin Is Essential for the Induction of Their Phosphatase Activity

Elevation of the intracellular calcium concentration is necessary for cell growth and the activation of several lymphokine genes. The immunosuppressive drugs cyclosporin A and FK506 profoundly inhibit the calcium-dependent signaling pathway in T lymphocytes by interfering with the activity of the calcium/calmodulin (CaM)-dependent serine/threonine phosphatase, calcineurin (CN). Little is known, however, about how activation of CN enzyme activity or interaction with its substrate, nuclear factor of activated T cell (NF-AT), is regulated. We show here that the binding of CaM to CN may affect the conformation of CN at both the CaM-binding and the autoinhibitory (AI) domains and that this is critical for activation of CN to dephosphorylate NF-AT. Dissociation of the AI domain from the enzyme active site on CN leads to expose a binding site for NF-AT at the N terminus of CN and allows the CN binding to NF-AT. Since the cytoplasmic form of NF-AT can interact with CN mutants lacking enzyme activity, the interaction of the two molecules is independent of CN enzyme activity and occurs prior to the dephosphorylation of NF-AT. Dephosphorylation converts NF-AT from the cytoplasmic to the nuclear form by exposing the DNA-binding domain on NF-AT, and the nuclear form of NF-AT brings CN together into the nuclei. We therefore propose that the activation of CN by the CaM binding independently regulates the interaction with NF-AT and the dephosphorylation of NF-AT.