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;Is a microtubule, actin filament, etc.) is discussed from a gen eral theoretical viewpoint open to experimental test. An elementary but fun damental "barometric" relation for the driving force is introduced that (i) applies to a range of kinetic and stochastic models of catalytic motor pro teins, (ii) is consistent with more elaborate expressions that entail furth er; explicit assumptions for the representation of externally applied loads and, (iii) sufficiently close to thermal equilibrium, satisfies an Einstei n;type relation in terms of the observable velocity and dispersion, or diff usion coefficient, of the (load-free) motor protein on its track. Even in t he simplest two-state kinetic models, the predicted velocity-vs.-load plots (that are observationally accessible) exhibit a variety of contrasting sha pes that can include nonmonotonic behavior. Previously suggested bounds on the driving force are shown to be inapplicable in general by considering di screte jump models which feature waiting-time distributions. Some compariso ns with experiment are sketched. (C) 1999 Elsevier Science B.V. All rights reserved.