The stoichiometry of caldesmon and actin in chicken gizzard thin filaments
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Regulation of smooth muscle contraction has been known to inovlve two distinct mechanisms. The role of myosin phosphorylation and dephosphorylation in the control of vertebrate smooth muscle contraction has been well documented. Recent evidence also suggests the existence of a thin filament-linked regulatory system in smooth muscle. Dual regulation of smooth muscle contraction may allow smooth muscle to vary tension output over a wide range of stretch and to maintain developed tension at low energy cost. Since the discovery of caldesmon in chicken gizzard smooth muscles, this protein was subsequently shown to be an actin and calmodulin binding protein. Since this protein was shown to be present in the thin filaments of smooth muscle in relatively large amounts, it has been proposed that caldesmon may be involved in thin filament linked regulation of smooth muscle contraction. While caldesmon has been shown to inhibit actin-activated myosin ATPase activity and to crosslink F-actin filaments in vitro, the precise function and action of caldesmon in vivo is uncertain. One approach to understand the mechanism of caldesmon mediated effects in smooth muscle is to construct a thin filament structural model. A model of thin filaments may provide insight on how contractile proteins interact during contraction and how thin filament associated proteins, possibily caldesmon may regulate this process. In this study, the stoichiometry of thin filament components of chicken gizzard smooth muscles is evaluated by quantitative gel densitometry. This showed an actin:tropomyosin:caldesmon ratio of 28:4:1. Together with results obtained from electron microscopic and biochemical studies, the stoichiometry obtained in this study will be used to formulate possible model of smooth muscle thin filaments.
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