Understanding mechanochemical coupling in kinesins using first-passage-time processes.

Kinesins are processive motor proteins that move along microtubules in a stepwise manner, and their motion is powered by the hydrolysis of ATP. Recent experiments have investigated the coupling between the individual steps of single kinesin molecules and ATP hydrolysis, taking explicitly into account forward steps, backward steps, and detachments. A theoretical study of mechanochemical coupling in kinesins, which extends the approach used successfully to describe the dynamics of motor proteins, is presented. The possibility of irreversible detachments of kinesins from the microtubules is explicitly taken into account. Using the method of first-passage times, experimental data on the mechanochemical coupling in kinesins are fully described using the simplest two-state model. It is shown that the dwell times for the kinesin to move one step forward or backward, or to dissociate irreversibly, are the same, although the probabilities of these events are different. It is concluded that the current theoretical view-that only the forward motion of the motor protein molecule is coupled to ATP hydrolysis--is consistent with all available experimental observations for kinesins.