Abstract

The bound Ca of actin readily exchanges with free Ca in G-actin but not in F-actin. The rates of exchange of the Ca and ATP bound to G-actin are similar. As the polymerization of G-actin proceeds, its Ca exchangeability decreases. Chelating agents and various other reagents which remove the bound ATP from G-actin concomitantly remove its bound Ca. Mercurials which inhibit the polymerization of G-actin also remove its bound Ca, whereas those —SH group reagents which do not inhibit polymerization do not affect the bound Ca. A relationship is shown between Ca content and characteristic properties of G-actin. The bound Ca, like the bound ADP, of F-actin withstands most of the treatments which remove the bound Ca from G-actin. It can be removed, however, after depolymerization of F-actin. Both G- and F-actins lose their bound Ca in the acid pH range; the removal of the bound Ca is accompanied by the removal of the bound ATP or the bound ADP; the characteristic properties of actin are simultaneously lost. The removal of the bound Ca from actin is irreversible. The bound Ca of actin is not exchangeable, displaceable, or removable during the actomyosin-catalyzed hydrolysis of ATP at low ionic strength.

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