Adenosine Triphosphatase Activity and “Thick Filament” Formation of Chicken Gizzard Myosin in Low Salt Media1

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Abstract
The ATPase activity of chicken gizzard myosin was studied by varying the KCl concentration in the reaction medium. The following was thus found: (a) A sharp depression of the activity occurred when the KCl concentration was reduced to less than 0.3 m, showing the minimum activity around 0.15 m KCl. (b) The activity depression was removed by addition of urea or by papain-digestion, but not by addition of p-chloromercuribenzoate. (c) In the KCl concentration where the activity depression occurred, the ATPase reaction proceeded in two distinct phases; the activity was relatively high in the early phase of the reaction and declined into the later phase where the steady state reaction took place, (d) In the KCl concentrations higher than that particular concentration or in the presence of urea, the ATPase reaction proceeded in one phase, (e) The temperature dependence of the ATPase activity in the earlyphase was of an ordinary magnitude being approximately equal to that of the ATPase activityin 0.6 m KCl. In contrast, the temperature dependence of the activity in the later phase was unusually small. Gizzard myosin in various concentrations of KCl was also examined by measuring the turbidity and the light-scattering intensity, and by observation under an electron microscope. The following was thus found: (a) In the KCl concentration where the activity depression occurred, there was a stagnation in the turbidity decrease as the KCl concentration was gradually increased and also the formation of “thick filaments,” each of which was approximately 0.6–0.9 (μm in length and 20–30 nm in diameter with no central “bare zone.” (b) Addition of ATP induced dissociation of the thick filaments, and the dissociation occurred during the early phase of the ATPase reaction, (c) Moreover, the temperature dependence of the ATP induced dissociation rate was approximately equal to that of the ATPase activity in the earlyphase. On the basis of the findings mentioned above, it is concluded that the activity depression results from the ATP-induced dissociation of myosin filaments. Moreover, since high concentrations of KCl or urea also caused dissociation of myosin filaments and yet did not produce the activity depression, it was strongly suggested that gizzard myosin in the ATP-dissociated form must be different from that in the urea- or KCl-dissociated form, probably in the physical state of some myosin aggregates which were not detectable by the physical methods we used. As a side-observation, gizzard myosin filaments formed in the presence of ADP were found to be unusually long (longer than 2 μm), and they looked very similar to the particular filaments of skeletal myosin that were reported, by Moos, to be formed in the absence of the C protein.