Molecular motors and the forces they exert

The stochastic driving force that is exerted by a single molecular motor (e.g., a kinesin, or myosin protein molecule) moving on a periodic molecular track (such as a microtubule, actin filament, etc.) is discussed from a general theoretical viewpoint open to experimental test. An elementary but fundamental “barometric” relation for the driving force is introduced that (i) applies to a range of kinetic and stochastic models of catalytic motor proteins, (ii) is consistent with more elaborate expressions that entail further, explicit assumptions for the representation of externally applied loads and, (iii) sufficiently close to thermal equilibrium, satisfies an Einstein-type relation in terms of the observable velocity and dispersion, or diffusion coefficient, of the (load-free) motor protein on its track. Even in the simplest two-state kinetic models, the predicted velocity-vs.-load plots (that are observationally accessible) exhibit a variety of contrasting shapes that can include nonmonotonic behavior. Previously suggested bounds on the driving force are shown to be inapplicable in general by considering discrete jump models which feature waiting-time distributions. Some comparisons with experiment are sketched.