Modulator protein as a Ca2+-dependent activator of rabbit skeletal myosin light-chain kinase. Purification and characterization.

The Ca2+-dependent activator protein of myosin light-chain kinase, which was identified as the bovine brain modulator protein of cyclic nucleotide phosphodiesterase, was isolated from rabbit skeletal muscle. The purified protein binds about 2 Ca2+ per mol in a medium containing 5 mM MgCl2, 10 micron CaCl2, and 0.1 M KCl at pH 6.8. The Ca2+ binding caused a conformational change of the activator protein which was measured by difference ultraviolet absorption spectroscopy. In the same Ca2+ concentration range as that causing the conformational change, activation of myosin light-chain kinase was observed.