Abstract
The dwell times between two successive steps of the two-headed molecular motor myosin V are governed by non-exponential distributions. These distributions have been determined experimentally for various control parameters such as nucleotide concentrations and external load force. First, we use a simplified network representation to determine the dwell time distributions of myosin V, with the associated dynamics described by a Markov process on networks with absorbing boundaries. Our approach provides a direct relation between the motor’s chemical kinetics and its stepping properties. In the absence of an external load, the theoretical distributions quantitatively agree with experimental findings for various nucleotide concentrations. Second, using a more complex branched network, which includes ADP release from the leading head, we are able to elucidate the motor’s gating effect. This effect is caused by an asymmetry in the chemical properties of the leading and the trailing head of the motor molecule. In the case of an external load acting on the motor, the corresponding dwell time distributions reveal details about the motor’s backsteps.
Highlights
The molecular motor myosin V is a dimeric protein with two identical motor domains or ‘heads’, each of which has a nucleotide binding pocket for the hydrolysis of ATP
We have focused on a network description of myosin V that consists of only four chemomechanical states, and calculated the dwell time distributions for this molecular motor
Our approach provides a direct relation between nucleotide binding and release rates, that are accessible via chemokinetic experiments, and the dwell times distributions as observed in single-molecule measurements of myosin V
Summary
The molecular motor myosin V is a dimeric protein with two identical motor domains or ‘heads’, each of which has a nucleotide binding pocket for the hydrolysis of ATP. Since the latter head stays at a fixed filament position, the motor’s center-of-mass is displaced by 36 nm during such a step This directed motion requires the coordination of the ATP hydrolysis by the two motor heads. The different ADP release rates for the trailing and leading head, that we will describe as gating, are essential for the coordination of the two heads. It has not been possible, so far, to directly measure these two rates for doubleheaded myosin V. Note that the latter term is used with a slightly different meaning by different authors
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