Abstract
It was recently established that physiological contraction is induced by Ca ion. However, the basic reaction of muscle contraction, viz., the interaction of pure actin and myosin, in the presence of ATP, is quite insensitive to Ca ion unlike a clude system such as myosin B. Prof. Ebashi inquired into this problem and discovered aprotein component which sensitized actomyosin to Ca. The component was similar to tropomyosin in its physicochemical nature but ordinary tropomyosin did not possess the sensitizing action. The component, therefore, was named “native tropomyosin”. Later, “native tropomyosin” was found to consist of tropomyosin and a new globular protein, troponin. Ca sensitivity of actin-myosin-interaction requires the presence of both troponin and tropomyosin.
Troponin binds Ca with a binding constant which is just reasonable to explain the effect of Ca on the contractile system. Further studies of relative sensitivity to Sr and Ca of heart and skeletal muscle and relative affinity of heart and skeletal muscle troponin to Sr and Ca led to a final conclusion that troponin is the Ca-receptive protein in the contractile system. Some difference in the physicochemical nature of troponin was found between in the presence and the absence of Ca ion, suggesting that some conformational change of troponin molecule was induced by Ca. Troponin was also shown to be distributed along the entire thin filament with the periodicity of about 400Å.
A possible mechanism as to how Ca and troponin could regulate the interaction of actin, myosin and ATP was discussed.
