title: The mechanism of Ca2+ regulation of vascular smooth muscle thin filaments by caldesmon and calmodulin.
abstract: The interactions of vascular smooth muscle caldesmon with actin, tropomyosin, and calmodulin were determined under conditions in which the four proteins can form reconstituted Ca2+-sensitive smooth muscle thin filaments. Caldesmon bound to actin in a complex fashion with high affinity sites (K = 10(7) M-1) saturating at a stoichiometry of 1 per 28 actins, and lower affinity sites at 1 per 7 actins. The affinity of binding was increased in the presence of tropomyosin, and this could be attributed to a direct interaction between caldesmon and tropomyosin which was demonstrated using caldesmon cross-linked to Sepharose. In the presence of tropomyosin, occupancy of the high affinity sites was associated with inhibition of actin-activated myosin MgATPase activity. Caldesmon was found to bind to calmodulin in the presence of Ca2+, with an affinity of 10(6) M-1. The binding of Ca2+ X calmodulin to caldesmon was associated with the neutralization of inhibition of actin-tropomyosin. Ca2+ X calmodulin binding reduced but did not abolish the binding of caldesmon to actin-tropomyosin. From this data we have proposed a model for smooth muscle thin filaments in which Ca2+ regulates activity by converting the inhibited actin-tropomyosin-caldesmon complex to the active complexes, actin-tropomyosin-caldesmon-calmodulin X Ca2+ and actin-tropomyosin.
mesh_terms: D008841:Actin Cytoskeleton / Q000378:metabolism*; D000199:Actins / Q000378:metabolism; D000818:Animals; D017301:Ca(2+) Mg(2+)-ATPase / Q000037:antagonists & inhibitors; D002118:Calcium / Q000378:metabolism / Q000494:pharmacology*; D002147:Calmodulin / Q000378:metabolism* / Q000494:pharmacology; D002148:Calmodulin-Binding Proteins / Q000378:metabolism* / Q000494:pharmacology; D003599:Cytoskeleton / Q000378:metabolism*; D004789:Enzyme Activation / Q000187:drug effects; D009131:Muscle, Smooth, Vascular / Q000378:metabolism*; D009218:Myosins / Q000378:metabolism; D012756:Sheep; D014335:Tropomyosin / Q000378:metabolism
chemical_list: D000199:Actins; D002147:Calmodulin; D002148:Calmodulin-Binding Proteins; D014335:Tropomyosin; D017301:Ca(2+) Mg(2+)-ATPase; D009218:Myosins; D002118:Calcium