Calcium ion and troponin: Professor S. Ebashi's epoch-making achievement

The processes by which Professor Setsuro Ebashi accomplished his great work are described. Independently of Marsh, Ebashi discovered the relaxing factor in homogenized muscle and showed that it has a lipid-containing particulate fraction with ATPase activity, later identified as the sarcoplasmic reticulum. He then solved the mechanism of relaxation of the relaxing factor through the following findings. A minute amount of calcium ion (Ca2+) is necessary for the physiological contractile reaction. The relaxing factor strongly accumulates Ca2+ in the presence of ATP and sufficiently removes Ca2+ from the contractile system to bring about relaxation. Ebashi found that the contractile reaction of myosin and actin is regulated by Ca2+ only in the presence of a tropomyosin-like protein factor, which he later showed to be a complex of tropomyosin and a new protein, troponin. He proved that troponin is the Ca2+-receptive protein and proposed the correct scheme for the molecular mechanism of regulation of contraction and relaxation.