Caldesmon: A common actin‐linked regulatory protein in the smooth muscle and nonmuscle contractile system

Caldesmon was originally purified from gizzard smooth muscle as a major calmo-dulin-binding protein which also interacts with actin filaments. It has an alternative binding ability to either calmodulin or actin filaments depending upon the concentration of Ca2+(“flip-flop binding”). Two forms of caldesmon (Mr's in the range of 120–150 kDa and 70–80 kDa) have been demonstrated in a wide variety of smooth muscles and nonmuscle cells. Immunohistochemical studies suggest that caldesmon is colocalized with actin filaments in vivo. Considering its abundance, the Ca2+ -dependent flip-flop binding ability to either calmodulin or actin filaments, and its intracellular localization, caldesmon is expected to be involved in contractile events. Recent results from our laboratory have led to the conclusion that caldesmon regulates the smooth muscle and nonmuscle actin-myosin interaction and the smooth muscle actin-high Mr actin-binding protein (ABP or filamin) interactin in a flip-flop manner. It might function in cell motility by regulating the contractile system.