1. A protein component different from actin or myosin A was extracted from the residue of rabbit skeletal muscle.
2. In the presence of the component, synthetic actomyosin became very sensitive to the calcium-related actions,
i.e., the syneresis-stimulating action of calucium and the relax-ing action of calcium-removing agents such as chelating agents or the physiological relax-ing factor.
3. When myosin B was treated with a low concentration of trypsin, it became in-sensitive to the relaxing action of calcium-removing agents like synthetic actomyosin. The addition of the component to the trypsin-treated myosin B recovered the sensitivity to the calcium-related actions. Essentially the same restits were obtained with myofibrils.
4. It is concluced that the nature of myosin B responsive to the calcium-related actions is due to the presence of the compo-nent in myosin B.
5. Various analyses conducted with a purified preparation of the component,
i.e., the amino acid composition, the viscosity-ionic strength relationship, the binding capacity to F-actin and the sensitivity to trypsin digestion, indicated that the component resembled tropo-myosin. However, tropomyosin prepared by the routine procedures did not show the functions exhibited by the newly discovered component.
We would like to express our sincere thanks to Dr. K. Maruyama for his invaluable advice and discussions. This work is supported in part by re-search grants of the U. S. Public Health Service, AM-04810, from National Institute of Arthritis and Meta-bolic Diseases, of Rockefeller Foundation, RF 60141, and of Pharmacological Research Foundation, Tokyo.
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