ACTIN-ACTIVATED MG-ATPASE ACTIVITY OF DICTYOSTELIUM MYOSIN-II - EFFECTS OF FILAMENT FORMATION AND HEAVY-CHAIN PHOSPHORYLATION

The actin-activated Mg-ATPase activities of unphosphorylated and heavy chain phosphorylated Dictyostelium myosin II and of a Dictyostelium myosin II heavy meromyosin (HMM) fragment were examined at different Mg2+ and KCl concentrations. The Mg-ATPase activity of HMM displayed a maximum rate, V(max), of about 4.0/s and a K(app) (actin concentration required to achieve 1/2 V(max)) that increased from 8 to 300-mu-M as the KCl concentration increased from 0 to 120 mM. When assayed with greater than 5 mM Mg2+ and 0 mM KCl the unphosphorylated Dictyostelium myosin II yielded a K(app) of 0.25-mu-M and a V(max) of 2.8/s. At lower Mg2+ concentrations or with 50 mM KCl the data were not fit well by a single hyperbolic curve and K(app) increased to 25-100-mu-M. The increase in K(app) did not correlate with the loss of sedimentable filaments. At KCl concentrations above 100 mM V(max) increased to greater than 4/s. Heavy chain phosphorylated myosin (3.5 mol of phosphate/mol myosin) displayed a V(max) of about 5/s and a K(app) of 50-mu-M under all conditions tested. Thus, heavy chain phosphorylation inhibited the actin-activated Mg-ATPase activity of Dictyostelium myosin II in 5-10-mu-M Mg2+ and low ionic strength through an increase in K(app).