Myosin light chain kinase from skeletal muscle regulates an ATP-dependent interaction between actin and myosin by binding to actin

Myosin light chain kinase (MLCK) has been purified from various muscles as an enzyme to phosphorylate myosin light chains. While the regulatory role o f smooth muscle MLCK is well understood, the role of skeletal muscle MLCK i n the regulation of contraction has not been fully characterized. Such char acterization of skeletal muscle MLCK is difficult because skeletal muscle m yosin interacts with actin whether or not the myosin is phosphorylated. Tak ing the hint from our recent finding that smooth muscle MLCK inhibits the a ctin-myosin interaction by binding to actin (Kohama et al., Biochem Biophys Res Commun 184: 1204-1211, 1992), we investigated the regulatory role of t he actin-binding activity of MLCK from chicken breast muscle in the actin-m yosin interaction. The amount of MLCK that bound to act in increased with i ncreases in the concentration of MLCK. However, MLCK hardly bound to myosin . The actin-binding activity of MLCK was affected when Ca2+ and calmodulin (Ca2+-CaM) were present. The effect of MLCK on the actin-myosin interaction was examined by an in vitro motility assay; the movement of actin-filament s on a myosin-coated glass surface was inhibited by increasing the concentr ation of MLCK. When CaM was present, the inhibition was overcome in a Ca2+- dependent manner at mu M levels. The inhibition of the movement by MLCK and the recovery from the inhibition by Ca2+-CaM were not altered whether we u se phosphorylated or unphosphorylated myosin for the assay, ruling out the involvement of the kinase activity of MLCK. (Mol Cell Biochem 190: 85-90, 1 999).