sTM-dependent modulation of probe binding to mIIaQQQ. Stopped-flow fluorescence traces of the binding of I-2581 to mIIaQQQ at final concentrations of 0.5 and 0.3 μm, respectively, are illustrated at final sTM concentrations of 0, 0.1, 0.2, and 0.5 μm (bottom to top). Inset, fractional amplitudes of the slow phase (○) and the fast phase (●) as a function of sTM.