Tropomyosin: Regulator of Actin Filaments
Cellular movement and function have long been known to depend on the actin cytoskeleton and its regulation. The actin cytoskeleton is the ultimate target of numerous cellular signaling pathways. The first signaling system understood in any detail was that of vertebrate skeletal muscle. Setsuro Ebashi, celebrated by this volume, was a pioneer through his role in showing that the calcium ion is the physiological regulator of muscle contraction followed by his landmark discovery and naming of troponin as the calcium ion receptor that regulates contraction through its interaction with tropomyosin and actin. Early work in the field is summarized in his remarkable 1968 review with M. Endo (Ebashi and Endo, 1968). There they put forth the evidence for a pathway by which activation of the muscle by an action potential would ultimately result in a contractile response consequent to the binding of calcium ion released from the sarcoplasmic reticulum to troponin bound to tropomyosin on the actin filament. The concept of a signaling cascade is now central to any thinking about signaling pathways as we attempt to understand such mechanisms at the molecular level. Whereas troponin is found only in striated muscles, tropomyosin is expressed in virtually all eucaryotic cells and is recognized to be a universal actin filament regulator, versatile in its function despite its deceptively simple coiled coil structure. In this chapter we give an overview of tropomyosin’s multiple regulatory roles and insights into aspects of the structural basis for its functions, focusing on vertebrate forms. As such, this is a personal view rather than a comprehensive review that can be found elsewhere (Perry, 2001; Gunning et al., 2005).
KeywordsActin Filament Myosin Head Coiled Coil Actin Binding Nonmuscle Cell
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