Myosin light chain kinase as a multifunctional regulatory protein of smooth muscle contraction

Myosin light chain kinase (MLCK) is a regulatory protein for smooth muscle contraction, which acts by phosphorylating 20-kDa myosin light chain (MLC20 ) to activate the myosin ATPase activity. Although this mode of action is w ell-established, there are numerous reports of smooth muscle contraction th at is not associated with MLC20 phosphorylation. The kinase activity for th e phosphorylation is localized at the central part of MLCK, which is also f urnished with actin-binding activity at its N terminal and myosin-binding a ctivity at its C terminal. This article overviews as to how such multifunct ional properties of MLCK modify the actin-myosin interaction and presents o ur observations that the phosphorylation is not obligatory in induction of smooth muscle contraction.