Modulator Protein as a Ca2+-Dependent Activator of Rabbit Skeletal Myosin Light-Chain Kinase

Abstract

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 μM CaCl₂, and 0.1 M KC1 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.