title: Activation of smooth muscle myosin Mg2+-ATPase by native thin filaments and actin/tropomyosin.
abstract: Application of the myosin competition test (Lehman, W., and Szent-Györgyi, A. G. (1975) J. Gen. Physiol. 66, 1-30) to chicken gizzard actomyosin indicated that this smooth muscle contains a thin filament-linked regulatory mechanism. Chicken gizzard thin filaments, isolated as described previously (Marston, S. B., and Lehman, W. (1985) Biochem. J. 231, 517-522), consisted almost exclusively of actin, tropomyosin, caldesmon, and an unidentified 32-kilodalton polypeptide in molar ratios of 1:1/6:1/26:1/17, respectively. When reconstituted with phosphorylated gizzard myosin, these thin filaments conferred Ca2+ sensitivity (67.8 +/- 2.1%; n = 5) on the myosin Mg2+-ATPase. On the other hand, no Ca2+ sensitivity of the myosin Mg2+-ATPase was observed when purified gizzard actin or actin plus tropomyosin was reconstituted with phosphorylated gizzard myosin. Native thin filaments were rendered essentially free of caldesmon and the 32-kilodalton polypeptide by extraction with 25 mM MgCl2. When reconstituted with phosphorylated gizzard myosin, caldesmon-free thin filaments and native thin filaments exhibited approximately the same Ca2+ sensitivity (45.1 and 42.7%, respectively). The observed Ca2+ sensitivity appears, therefore, not to be due to caldesmon. Only trace amounts of two Ca2+-binding proteins could be detected in native thin filaments. These were identified as calmodulin (present at a molar ratio to actin of 1:733) and the 20-kilodalton light chain of myosin (present at a molar ratio to actin of 1:270). The Ca2+ sensitivity observed in an in vitro system reconstituted from gizzard thin filaments and either skeletal myosin or phosphorylated gizzard myosin is due, therefore, to calmodulin and/or an unidentified minor protein component of the thin filaments which may be an actin-binding protein involved in regulating actin filament structure in a Ca2+-dependent manner.
mesh_terms: D000199:Actins / Q000378:metabolism*; D000818:Animals; D017301:Ca(2+) Mg(2+)-ATPase / Q000378:metabolism*; D002118:Calcium / Q000378:metabolism; D002135:Calcium-Binding Proteins / Q000378:metabolism; D002148:Calmodulin-Binding Proteins / Q000378:metabolism; D002417:Cattle; D002645:Chickens; D004789:Enzyme Activation; D005895:Gizzard, Avian / Q000032:analysis; D008970:Molecular Weight; D009130:Muscle, Smooth / Q000201:enzymology*; D009218:Myosins / Q000378:metabolism*; D011817:Rabbits; D014335:Tropomyosin / Q000378:metabolism*
chemical_list: D000199:Actins; D002135:Calcium-Binding Proteins; D002148:Calmodulin-Binding Proteins; D014335:Tropomyosin; D017301:Ca(2+) Mg(2+)-ATPase; D009218:Myosins; D002118:Calcium