Fig. 2.
Effect of Pi on elongation of actin filaments. Conditions were as in Fig. 1. To maintain constant ionic strength, the concentration of sulfate was varied such that the total concentration of Pi and sulfate was 160 mM, except the samples with <20 mM Pi, which did not have sulfate. (A) Dependence of the rate of elongation of barbed ends on the concentration of Pi. Red circles, 1 μM Mg-ATP-actin monomers; blue circles, 1 μM Mg-ADP-actin monomers. The curves are drawn through the data points. (B) Four different concentrations of Mg-ADP-actin monomers. The curves are global fits to the three highest ADP-actin concentrations (see Discussion and SI Text). (C) Barbed end elongation rate as a function of the concentration of Mg-ADP-actin monomers over a range of Pi concentrations. Black line and circles, 0 mM; broken line and brown circles, 5 mM; red line and circles, 20 mM; other Pi concentrations are defined by symbols on the graph. Error bars are ±1 standard deviation of the mean rates. The lines intersect the x axis at the critical concentrations. (D) Dependence of the rate of pointed end elongation on the concentration of Pi. Red squares, 1 μM Mg-ATP-actin; blue squares, 1 μM Mg-ADP-actin. (E) Dependence of the rate of pointed end elongation by four concentrations of Mg-ADP-actin monomers on Pi concentration. The curves are global fits to the three highest ADP-actin concentrations (see Discussion). (F) As in C, for the pointed end.