Regulation of calcineurin by metal ions. Mechanism of activation by Ni²⁺ and an enhanced response to Ca²⁺/calmodulin

Calcineurin possesses phosphatase activity towards both protein (Stewart, A.A., Ingebritsen, T.S., Manalan, A., Klee, C.B., and Cohen, P. (1982) FEBS Lett. 137, 80-84)) and nonprotein substrates (Pallen, C.J., and Wang, J.H. (1983) J. Biol. Chem. 258, 8550-8553). These phosphatase activities are divalent cation-dependent and stimulated by calmodulin. We have utilized the nonprotein chromogenic substrate p-nitrophenyl phosphate to investigate the effects of several divalent metal ions on calcineurin activity and have found that Ni²⁺ is the best activator of calcineurin both in the presence and absence of calmodulin. A slightly less potent activator is Mn²⁺. Although the mechanisms and extents of activations stimulated by these two metal ions are different, we present evidence to suggest a competition for binding to the enzyme. Pretreatment of calcineurin with either of these two metal ions enhances the activation of calcineurin by Ca²⁺/calmodulin and may be a physiological mechanism by which calcineurin activity is regulated by Ca²⁺.