The tightly bound divalent cation regulates actin polymerization

The polymerization characteristics of Ca ++-actin and Mg ++-actin were studied by measuring initial rates of polymerization upon addition of phalloidin-stabilized nuclei and neutral salt. Under conditions where the effects of divalent cation exchange were minimized, CaCl 2 and MgCl 2 were found to be equally effective in polymerizing actin. Mg ++-actin was found to nucleate and polymerize more readily than Ca ++-actin, having a forward rate constant about twice that of Ca ++-actin under a variety of polymerizing conditions. The critical concentration for Ca ++-actin is approximately 20 times that for Mg ++-actin under equivalent conditions. These data imply that the polymer of Mg ++-actin must be more stable than that of Ca ++-actin, having a depolymerization rate constant about 10 fold lower. Since Mg ++ is probably the tightly-bound cation in vivo , whereas Ca ++-actin has been more widely studied in vitro , it would appear that actin in its physiological state is probably more polymerizable and more stable in the polymer form than previously considered.