The force exerted by a molecular motor

The stochastic driving force exerted by a single molecular motor (e.g., a k inesin, or myosin) moving on a periodic molecular track (microtubule, actin filament, etc.) is discussed from a general viewpoint open to experimental test. An elementary "barometric" relation for the driving force is introdu ced that (i) applies to a range of kinetic and stochastic models, (ii) is c onsistent with more elaborate expressions entailing explicit representation s of externally applied loads, and (iii) sufficiently close to thermal equi librium, satisfies an Einstein-type relation in terms of the velocity and d iffusion coefficient of the (load-free) motor. Even in the simplest two-sta te models, the velocity-vs.-load plots exhibit a variety of contrasting sha pes (including nonmonotonic behavior). Previously suggested bounds on the d riving force are shown to be inapplicable in general by analyzing discrete jump models with waiting time distributions.