Activation of actomyosin ATPase and tension development in smooth muscle is believed to be myosin-mediated via Ca2+ -calmodulin dependent phosphorylation of myosin light chain. However, existence of a thin filament based regulation, which complements the regulation via phosphorylation, is supported by biochemical studies. Tropomyosin, a protein located on the thin filament, potentiates the actin-activated ATP hydrolysis by myosin. Recent biochemical studies showed that the actin binding protein, caldesmon, inhibits the tropomyosin-enhanced actomyosin ATPase. In the presence of Ca2+, calmodulin binds to caldesmon and the caldesmon induced inhibition of the tropomyosin-enhanced actomyosin ATPase is reversed. Two of the major questions that pertain to the understanding of the roles of interactions of thin filament- associated proteins on the actomyosin ATPase and contraction in smooth muscle are: (a) How does caldesmon inhibit n potentiation of actin-activated ATP hydrolysis? and (b) How does Ca2+- calmodulin release the caldesmon induced inhibition? These two broad basic questions will be answered by experiments to obtain specific information on the interactions of thin-filament associated proteins on the actin and their effects on the binding of myosin heads to actin and ATP hydrolysis. Thin filament- associated proteins from medial smooth muscle of swine pulmonary arteries will be isolated. Actin filament will be reconstituted with other thin-filament proteins, tropomyosin, caldesmon and calmodulin, to test their roles in the regulation of actin- activated ATP hydrolysis by purified phosphorylated myosin, heavy meromyosin or subfragment-1. Data from these experiments will provide us with the basic information for understanding the protein-protein interactions on the thin filament and the actin- based regulation of actomyosin ATPase and the associated development and maintenance of force and relaxation in smooth muscle.

National Institute of Health (NIH)
National Heart, Lung, and Blood Institute (NHLBI)
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Physiology Study Section (PHY)
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University of Pennsylvania
Schools of Veterinary Medicine
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Horiuchi, K Y; Chacko, S (1991) The mechanism for the inhibition of actin-activated ATPase of smooth muscle heavy meromyosin by calponin. Biochem Biophys Res Commun 176:1487-93
Horiuchi, K Y; Samuel, M; Chacko, S (1991) Mechanism for the inhibition of acto-heavy meromyosin ATPase by the actin/calmodulin binding domain of caldesmon. Biochemistry 30:712-7
Chacko, S; Eisenberg, E (1990) Cooperativity of actin-activated ATPase of gizzard heavy meromyosin in the presence of gizzard tropomyosin. J Biol Chem 265:2105-10
Horiuchi, K Y; Chacko, S (1989) Caldesmon inhibits the cooperative turning-on of the smooth muscle heavy meromyosin by tropomyosin-actin. Biochemistry 28:9111-6
Katayama, E; Horiuchi, K Y; Chacko, S (1989) Characteristics of the myosin and tropomyosin binding regions of the smooth muscle caldesmon. Biochem Biophys Res Commun 160:1316-22
Horiuchi, K Y; Chacko, S (1988) Interaction between caldesmon and tropomyosin in the presence and absence of smooth muscle actin. Biochemistry 27:8388-93
Fillers, W S; Chacko, S (1987) Modulation of monomer-polymer equilibrium of phosphorylated smooth muscle myosin: effects on actin activation. Biochemistry 26:5896-903
Chacko, S; Miyata, H; Horiuchi, K Y (1987) Modulation of actomyosin ATPase by thin filament-associated proteins. Prog Clin Biol Res 245:143-58
Miyata, H; Chacko, S (1986) Role of tropomyosin in smooth muscle contraction: effect of tropomyosin binding to actin on actin activation of myosin ATPase. Biochemistry 25:2725-9
Horiuchi, K Y; Miyata, H; Chacko, S (1986) Modulation of smooth muscle actomyosin ATPase by thin filament associated proteins. Biochem Biophys Res Commun 136:962-8

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