Figure 4.

Time courses of pyrene tropomyosin fluorescence changes after rapid dissociation of S1 from S1-actin-pyrene-tropomyosin-troponin with ATP. (A) Absence of Ca²⁺. Pyrene tropomyosin transitions at 30°C, k_app = 70/s (a), 25°C, k_app= 28/s (b), 20°C, k_app = 21/s (c), and 1°C (d). (B) Saturating Ca²⁺ at 1°C, k_app = 268/s. The solid lines through the curves are simulations of Eq. 2. Conditions after mixing in the stopped flow: 4 μM F-actin, 3.6 μM S1, 0.86 μM pyrene-labeled tropomyosin, and 0.86 μM troponin in a buffer containing 20 mM MOPS, 142 mM KCl, 2 mM ATP, 2 mM EGTA, 6 mM MgCl₂, and 1 mM dithiothreitol. Other conditions are the same as in Fig. 1.