MG2+ INHIBITS FORMATION OF 4CA2+-CALMODULIN-ENZYME COMPLEX AT LOWER CA2+ CONCENTRATION - H-1 AND CD-113 NMR-STUDIES

Our previous H-1 NMR studies indicated that when mastoparan (MP) is added to Ca2+-half-saturated calmodulin (2Ca2+-CaM) in the absence of Mg2+, ions, Ca2+ ions transfer from the C-terminal-half domain of CaM not interacting with MP to the N-terminal-half domain of CaM interacting with MP at lower MP concentrations (Ohki, S., Yazawa, M., Yagi, K., and Hikichi, K. (1991b) J. Biochem. (Tokyo) 110, 737-742). As a consequence, the active form of 4Ca2+-CaM.MP complex is formed. In the present study, we studied the effect of Mg2+ ions on Ca2+ transfer. In the presence of Mg2+ ions, such Ca2+ transfer does not occur. The effects of Mg2+ ions are also studied by observing Cd-113 NMR in the presence of M13, the 26-residue peptide of the CaM-binding region of myosin light chain kinase. The Cd-113 NMR results show that Mg2+ ions prevent to form the active complex. Mg2+ plays an important role as an inactivating factor to CaM.