Structural basis for activation of calcineurin by calmodulin

Abstract

The highly conserved phosphatase calcineurin plays vital roles in numerous processes including T-cell activation, development and function of the central nervous system, and cardiac growth. It is activated by the calcium sensor calmodulin. Calmodulin binds to a regulatory domain within calcineurin, causing a conformational change that displaces an autoinhibitory domain from the active site, resulting in activation of the phosphatase. This is the same general mechanism by which calmodulin activates calmodulin-dependent protein kinases. Previously published data has hinted that the regulatory domain of calcineurin is intrinsically disordered. In this work we demonstrate that the regulatory domain is unstructured and that it folds upon binding calmodulin, ousting the autoinhibitory domain from the catalytic site. The regulatory domain is 95 residues long, with the autoinhibitory domain attached to its C-terminal end and the 24 residue calmodulin binding region towards the N-terminal end. This is unlike the calmodulin-dependent protein kinases which have calmodulin binding sites and autoinhibitory domains immediately adjacent in sequence. Our data demonstrate that not only does the calmodulin binding region fold, but that an ~25-30 residue region between it and the autoinhibitory domain also folds, resulting in over half of the regulatory domain adopting α-helical structure. This appears to be the first observation of calmodulin inducing folding of this scale outside of its binding site on a target protein.