Dynamical role of "protein friction" in the sliding movement of protein motors in vitro.
When protein motors interact with a sliding cytoplasmic-filament through a weak-binding interaction (thus, without ATP splitting), this interaction cycle results in friction opposing the sliding movement. The friction is owing to the flexible nature of the heads of these motors as globular proteins. Under a certain condition, the friction becomes proportional to the sliding velocity. This viscous-like friction by protein motor is called protein friction. Since the protein friction is more than 10 times larger than the hydrodynamic viscous drag, we propose that the sliding velocity in the in vitro motility system is limited when the active sliding force generated by protein motors is balanced by the protein friction. The model of the protein friction hypothesis is consistent with many experimental data of the in vitro motility systems such as those of mixture experiments with different myosins and the ATP-concentration dependence of the sliding velocity. By relating the coefficient of the protein friction to the diffusion coefficient, we show that the model is consistent with the data on the one-dimensional Brownian movement of a microtubule on a dynein-coated glass surface in the presence of vanadate and ATP. The model also shows that the Brownian movement is driven directly by the thermally-generated structural fluctuations of the dynein heads rather than the atomic collision of solvent molecules. Thus, the model implies that the thermal structural fluctuations of the protein motor heads underlie the ATP-induced sliding movement by protein motors and hence protein motors are a Brownian actuator.