Figure 4.

Binding of excess S1 to pyrene-labeled actin filaments containing tropomyosin and troponin in the absence of ATP at very low Ca²⁺. (A) Averages of ≥5 time courses of S1 binding to actin filaments containing troponin with wild-type troponin T, curve 1, or Δ14 troponin T, curve 2 are shown. Exponential fits are superimposed on the curves. Inset: expanded view of initial 0.2 s shown with the major rapid exponential phase extrapolated back to illustrate the lag. (B) Lag time preceding the exponential phase against troponin type is shown. (C) Apparent rate constants for the major rapid exponential phase of binding against troponin variant are shown. Error bars shown are the SDs. Conditions are as follows: at 25°C, 0.2 μM pyrene-actin (40% labeled), 0.086 μM tropomyosin, and 0.086 μM troponin were rapidly mixed with 2 μM myosin S1 in a buffer containing 152 mM KCl, 20 mM MOPS buffer (pH 7), 4 mM MgCl₂, 1 mM dithiothreitol, and 2 mM EGTA.