Figure 3.

Mg²⁺-ATPase activities of wild-type (○, ●) and W441F (▵, ▴) HMMs as a function of KCl concentration. Assay conditions were 0.24 mg/ml HMM, 2 mM MgCl₂, 20 mM Tris⋅HCl (pH 7.5), 0.5 mM DTT, and 0.5 mM ATP with 4 μg/ml myosin light chain kinase, 1 μg/ml calmodulin, and 0.05 mM CaCl₂, or with 0.8 mM EGTA. Before the assay, HMM was phosphorylated by incubating for 15 min at 25°C in a low-salt medium (40 mM KCl) with myosin light chain kinase, calmodulin, and CaCl₂ (○, ▵). Each assay was started by adjusting the KCl concentration to an appropriate value. For unphosphorylated HMM, EGTA was added to the medium instead of myosin light chain kinase/calmodulin/CaCl₂ (●, ▴).