a cytoskeletal protein implicated in the regulation of actomyosin interactions in smooth muscle and nonmuscle cells. May act as a bridge between myosin and actin filaments. Stimulates actin binding of tropomyosin which increases the stabilization of actin filament structure. In muscle tissues, inhibits the actomyosin ATPase by binding to F-actin. This inhibition is attenuated by calcium-calmodulin and is potentiated by tropomyosin. Interacts with actin, myosin, two molecules of tropomyosin and with calmodulin. Also plays an essential role during cellular mitosis and receptor capping. Five alternatively spliced isoforms have been described. Note: This description may include information from UniProtKB.