A kinetic comparison between Mg-actin and Ca-actin.

The kinetics of the elongation reaction in the polymerization of actin containing tightly-bound Mg2+ (Mg-actin) or tightly-bound Ca2+ (Ca-actin) have been studied. The reaction was monitored using the increase in fluorescence intensity of N-(1-pyrenyl)iodoacetamide-labelled monomeric actin as a measure of polymer formation. The actin nucleation reaction was circumvented by the addition of phalloidin-stabilized actin nuclei. Elongation rates were obtained at various actin concentrations and at various temperatures for polymerization induced by the presence of different monovalent and divalent salt concentrations. Values for the relative rate constant of forward polymerization (mk+) for Mg-actin were found to be larger than those for Ca-actin under similar conditions (m = number of polymer ends). The critical actin concentration (Cc) of Mg-actin is lower than the Cc for Ca-actin, as were estimates of the relative rate constant of depolymerization (mk-). The temperature dependence of Cc, mk+ and mk- for Mg-actin was different from that for Ca-actin, further suggesting a difference in monomeric properties due to the type of divalent cation tightly bound to actin. Estimates of the activation enthalpy change for the forward reaction in the G in equilibrium F transformation were similar for both types of actin, but the activation enthalpy change for the depolymerization of Mg-actin was significantly larger than that for Ca-actin.