A chemically reversible Brownian motor: Application to kinesin and ncd

Kinesin and nonclaret disjunctional protein (ncd) are two microtubule-based molecular motors that use energy from ATP hydrolysis to drive motion in op posite directions. They are structurally very similar and bind with similar orientations on microtubule. What is the origin of the different direction ality? Is it some subtle feature of the structure of the motor domains, not apparent in x-ray diffraction studies, or possibly some difference near th e neck regions far from the microtubule binding site? Perhaps because the m otors function as dimers, the explanation involves differences in the stren gth of the interaction between the two motor monomers themselves. Here we p resent another possibility, based on a Brownian ratchet, in which the direc tion of motion of the motor is controlled by the chemical mechanism of ATP hydrolysis and is an inherent property of a single head. In contrast to con ventional power stroke models, dissociation of the individual heads is not obligatory in the chemomechanical cycle, and the steps during which motion and force generation occurs are best described as one-dimensional thermally activated transitions that take place while both heads are attached to the microtubule. We show that our model is consistent with experiments on kine sin in which the velocity is measured as a function of external force and w ith the observed stiochiometry of one ATP/8-nm step at low load. Further, t he model provides a way of understanding recent experiments on the ATP depe ndence of the variance (randomness) of the distance moved in a given time.