FIGURE 5.

Actin-activated ATPase activity of S1 with increasing concentrations of activating NEM-S1 in the presence of Ca²⁺. The transition of the regulated acto-S1 complex to maximal ATPase activity with NEM-S1 binding was blunted with the mutants. (A) The rates of mutant 45 (solid bars) and WT (shaded bars) with increasing NEM-S1 concentration; (B) mutant 43/45 (solid bars) and WT (shaded bars); (C) mutant 43/45/144 (solid bars) and WT (shaded bars). All rates were corrected for the ATPase rate of S1 and NEM-S1. (D) The ratio between the mean rates of the various mutants to the wild-type with (mutant 45)/(WT), (mutant 43/45)/(WT), and (mutant 43/45/144)/(WT) at different NEM-S1 concentrations (0, 2, and 7 μM). Data for panels A, B, and C are presented as mean values ± SD with the number of trials in parentheses. Conditions are: 25°C, pH 7, 0.1 μM S1, 10 μM actin, 1 mM ATP, 10 mM MOPS, 3 mM MgCl₂, 33 mM NaCl, 1 mM dithiothreitol, 0.5 mM CaCl₂. Actin concentration was increased by an amount equal to the NEM-S1 added to maintain the same free actin concentration. Actin/tropomyosin/troponin was 7:1.5:1.5.