Binding of calmodulin changes the calcineurin regulatory region to a less dynamic conformation

Calcineurin (CN) is a Ca(2+)/calmodulin (CaM) activated serine/threonine phosphatase, and its regulatory region (CNRR) plays a critical role in the coupling of Ca(2+) signals to cellular responses. Ca(2+)/CaM binds to the CNRR, resulting in a conformational change that removes an autoinhibitory domain (CN467-486) from the active site of the phosphatase and activates the enzyme. However, almost the entire regulatory region (CN374-521) is not visible in the electron density maps of reported structures. In this study, we produced separate CN fragments corresponding to the CNRR (CNRR381-521, CN residues 381-521) and determined the binding affinity of CNRR381-521 for Ca(2+)/CaM using isothermal titration calorimetry (ITC). The structural change in CNRR381-521 induced by Ca(2+)/CaM binding was also investigated by Fourier transform infrared spectroscopy (FT-IR). The results indicate that Ca(2+)/CaM binding to CNRR381-521 is an exothermic reaction with a dissociation constant of 2.0×10(-6) M. Binding of calmodulin changes the calcineurin regulatory region to a less dynamic conformation.