New kinesin (red) prefers to bind on the plus-end side of earlier kinesins (yellow).

Train travel might be more efficient if the tracks helped the train along. On page 691, Muto et al. show that, unlike trains, kinesin motors do get a boost from their tracks—in this case, microtubules.Muto and colleagues show that the binding of the plus end–directed motor kinesin to microtubules is cooperative. Like proverbial sheep, once one bead (or motor) bound to a filament, others followed suit in the same neighborhood. Cooperativity extended for several microns in either direction from the initial binding site, although newcomers were more than twice as likely to bind on the plus-end side of the earlier motors.

The findings suggest that microtubules are altered by kinesin in such a way that they attract more motors. ATP hydrolysis by kinesin was required for cooperativity, but how the energy is used is not known. The group is currently investigating how kinesin binding affects the movement of positive ions that surround the negatively charged microtubules. These changes may in turn alter kinesin motility.

The preference of kinesin for already-bound microtubules in vivo may establish a set of microtubules that is dedicated to vesicle trafficking. This idea fits well with previous findings that axonal microtubules carry either many vesicles or very few.