J. Biochem, 1990, Vol. 107, No. 5 671-679
© 1990 Japanese Biochemical Society
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In Vitro Motility of Skeletal Muscle Myosin and Its Proteolytic Fragments1
Department of Molecular Biology, Faculty of Science, Nagoya University Chikusa-ku, Nagoya, Aichi 464
2To whom correspondence should be addressed.
We have compared actin-activated myosin ATPase activity, myosin binding to actin, and the velocity of myosin-induced actin sliding in order to understand the mechanism of myosin motility. In our in vitro assay, F-actin slides at a constant velocity, regardless of length. The F-actin could slide over myosin heads at KC1 concentrations below a critical value (60 mM with myosin and HMM, 100 mM with S-1), and the sliding velocities were quite similar below the critical KC1 concentration. However, at KC1 concentrations close to the critical value, the sliding F-actin is attached to only one or a few particular points on the surface, each of which perhaps consists of a single head of myosin. The KATPase values for actin-activated ATPase were 
300 µM for S-l and 200 µM with HMM below the critical KC1 concentration, and µ 5, 000 µM above the critical KC1 concentration. This increase in KATPase is due to a drastic reduction in the binding affinity of myosin heads to F-actin, as determined by a proteolytic digestion method and direct observation by fluorescence microscopy. We also show that the Vmax of actin-activated myosin ATPase activity decreases steadily with increasing KC1 concentration, even though the velocity of F-actin sliding remains unchanged. This result provides evidence that the ATPase activity is not necessarily linked to motility. We discuss possible models that do not require a tight coupling between myosin ATPase and motility.
1This study was supported by a grant (No. 6048509, to S.H.F.) from the Ministry of Education, Science and Culture of Japan.
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