ABSTRACT
Calcium ions (Ca2+) has been recognized as an important component in muscle contraction and cell motility because it regulates a distinct class of protein kinases, myosin light chain kinases. Striated muscles, including cardiac and skeletal muscle tissues, have served as useful models for studying the mechanism by which Ca2+ regulates contractile activity via actin-myosin interactions. Myosin and actin also play important roles in nonmuscle contractile functions. Troponin is not present in smooth muscle, and the regulation of actin-myosin interactions by Ca2+ is more complex. Different biochemical mechanisms have been proposed and may be differentiated into two general classes involving thick and thin filament regulatory processes, respectively. The general properties of myosin light chain kinase activation and myosin light chain phosphorylation are similar in skeletal and smooth cells. It is clear that myosin phosphorylation does not play a dominant or obligatory role in contraction of this muscle as it does in smooth muscle contraction.
