Role of caldesmon in the Ca2+ regulation of smooth muscle thin filaments

Smooth muscle thin filaments are made up of actin, tropomyosin, caldesmon, and a Ca2+-binding protein and their interaction with myosin is Ca2+-regulated. We suggested that Ca2+ regulation by caldesmon and Ca2+-calmodulin is achieved by controlling the state of thin filament through a cooperative-allosteric mechanism homologous to troponin-tropomyosin in striated muscles. In the present work, we have tested this hypothesis. We monitored directly the thin filament transition between the ON and OFF state using the excimer fluorescence of pyrene iodoacetamide (PIA)-labeled smooth muscle alpha alpha-tropomyosin homodimers. In steady state fluorescence measurements, myosin subfragment 1 (S1) cooperatively switches the thin filaments to the ON state, and this is exhibited as an increase in the excimer fluorescence. In contrast, caldesmon decreases the excimer fluorescence, indicating a switch of the thin filament to the OFF state. Addition of Ca2+-calmodulin increases the excimer fluorescence, indicating a switch of the thin filament to the ON state. The excimer fluorescence was also used to monitor the kinetics of the ON-OFF transition in a stopped-flow apparatus. When ATP induces S1 dissociation from actin-PIA-tropomyosin, the transition to the OFF state is delayed until all S1 molecules are dissociated actin. In contrast, caldesmon switches the thin filament to the OFF state in a cooperative way, and no lag is displayed in the time course of the caldesmon-induced fluorescence decrease. We have also studied caldesmon and Ca2+-calmodulin-caldesmon binding to actin-tropomyosin in the ON and OFF states. The results are used to discuss both caldesmon inhibition and Ca2+-calmodulin- caldesmon activation of actin-tropomyosin.