Myosin-Linked Regulation of Molluscan Muscle
In all muscles, contraction is triggered by an increase in intracellular free calcium. The precise nature of the response to elevated sarcoplasmic Ca2+ concentration, however, depends on the kind of regulatory proteins that sense and respond to intracellular concentrations of Ca2+. These proteins are diverse. In vertebrate striated muscles, as we have seen in Chap.4, the calcium sensor is troponin-C, located in the thin actin filaments. In such a thin filament, or actin-linked regulatory system, contractile activity is turned off by the tropomyosin-troponin system when it does not have calcium bound to it. In molluscan muscle, on the other hand, the calcium switch is incorporated into the myosin molecule. Regulation is thus myosin-linked. In contrast to vertebrate muscle, in molluscan muscle the same protein molecule serves both a contractile and regulatory function. In the relaxed state, the actin-activated myosin ATPase is repressed by an inhibitor, the so-called regulatory light chain of molluscan myosin (Sect.6.1). A different regulatory mechanism is found in the slow adductor smooth muscle of bivalves showing the “catch phenomenon” (Sect.6.2). Because of the ease with which the regulatory light chain can be reversibly removed from myosin, the striated adductor of the scallop (Pecten sp.) proved to be unusually suitable for studies of the calcium-regulated mechanisms (Chantler and Szent-Györgyi 1980; Szent-Györgyi and Chantler 1986).
KeywordsLight Chain Myosin Head Thick Filament Regulatory Light Chain Myosin Molecule
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