Myosin-Induced Detachment Causes Differences Between Ensemble and Single Molecule Myosin Kinetics

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Single molecule measurements of mechanochemistry have greatly increased our understanding of muscle contraction. However, since trillions of myosin molecules work together in muscle, extrapolation to in vivo function requires additional understanding of how motors behave in an ensemble. Early findings suggested that myosin behaves similarly at both the single molecule and ensemble levels; but more recent experiments suggest otherwise. Using a combination of simulation and theory, we show that the force-dependence of ADP release causes myosin-myosin interactions that make ensemble myosin behavior differ fundamentally from single myosin behavior.We use solution data to estimate the parameters of a simple 4-state kinetic model for actomyosin interaction. For smooth muscle myosin, we add the measured force-dependence of ADP release to this model. Simulations of the model successfully predict the results of four experiments: 1. single molecule measurements of step size and strong binding liftetime; 2. In vitro motility measurements of actin speed as a function of [ATP]; 3. In vitro motility measurements of actin speed at low myosin density as a function of actin filament length; and 4. laser trap measurements of velocity as a function of force for small myosin ensembles. For skeletal muscle myosin, we use a subset of these data to estimate the force-dependence of ADP release and successfully predict the remaining data. The model is therefore consistent with both single molecule and ensemble data.In the model, myosin binding to actin accelerates the detachment of previously bound myosin. This myosin-induced detachment causes strong binding lifetime to depend on the number of myosin molecules interacting with actin. Counter-intuitively, this result implies that even when actin speed is “detachment limited” (meaning speed equals myosin's step size times the ADP release rate), increasing the attachment rate can increase speed.