Effects of Various Concentrations of MgATP on the Superprecipitation and ATPase Activity of Scallop Striated Muscle Myosin B1

1. The superprecipitation and ATPase activity of scallop striated muscle myosin B showed essentially the same biphasic pattern as that of vertebrate skeletal muscle myosin B in its responses to changes in the MgATP concentration. Ca1+ had no effect at MgATP concen-trations less than 10~6 M. 2. The superprecipitation of gizzard myosin B showed a weak biphasic pattern, but its ATPase activity showed no sign of a biphasic pattern up to 3 x 10" * M MgATP, in accord with the result ofBremel(1974). 3. Thus scallop striated muscle is classified into the same group as vertebrate skeletal muscle as far as its response to changes in MgATP concentration is concerned. Gizzard is distinct from the other two kinds of muscle. The contraction of muscle is regulated by very small amounts of Cal+ (1-3). For this regulation, two different kinds of mechanism have been demon-strated. One is the troponin-tropomyosin system, first found in vertebrate skeletal muscle (4-6), and now shown to be widely distributed in the animal kingdom, not only in deuterostomia but also in protostomia (7, 8). The other is the so-called 'myosin-linked ' system, first found in scallop striated muscle (7) and later in different kinds of protostomia (8). Molluscan muscle was first thought to depend entirely on the myosin-linke