Maruyama K
Biochim Biophys Acta. 1981 Jan 30;667(1):139-42. doi: 10.1016/0005-2795(81)90074-x.
At low ionic strength, removal of trace amounts of Ca2+ enhanced the polymerization of actin, and that of Mg2+ inhibited it. The actions of the divalent metal ions did not influence the elongation process of preformed nuclei. Thus it appears that the metal ions affect the nucleation step of actin polymerization.