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Changes in crossbridge attachment in a myosin-regulated muscle


THE accompanying paper by Simmons and Szent-Györgyi1 presents a direct confirmation that regulation of activity in scallop muscles is mediated by the thick filaments: when one particular polypeptide (a regulatory light chain) is removed from the myosin molecules, tension development is no longer inhibited by the absence of calcium ions. Yet we still have no picture of the structural events associated with myosin-linked regulation, comparable with (for example) the ‘tropomyosin-shift’ model for the interactions of calcium, troponin and tropomyosin in actin-linked regulation (see, for example, Wakabayashi et al.2). We report here that removal of the regulatory light chain can lead to a structural change observed by X-ray diffraction. Our observations on the scallop may have a more general significance since myosin-linked regulation is found, in addition to actin-linked regulation, in many types of muscle3.

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