Biochemistry. Processive motor movement.

Permeating throughout a eukaryotic cell is a lattice of filamentous tracks called microtubules, upon which molecular motors travel, moving cargo about. In this transport system, the molecular motor kinesin-1 carries relatively large loads (molecular complexes, membranous vesicles, and organelles), its motion powered by the energy liberated from hydrolyzing adenosine triphosphate (ATP) (1). In fact, kinesin-1 can move processively for long distances (over a micrometer) along a microtubule without falling off. This feat requires coordinating the two motor domains (“heads”) of kinesin so that one head is always attached to the microtubule while the other is detached and moving further along the track, in the direction of movement. The proposed molecular mechanism underlying this processive motion has been highly debated. One controversial point has been how an operation called “gating” proceeds, that is, how the ATPase cycle of a kinesin head is stalled until a specific binding or conformational change needed for motility occurs. On page 120 of this issue, Alonso et al. (2) show that a microtubule track is not necessary for such gating to occur. This throws a surprising wrench into existing models of kinesin movement and suggests that the track is not required to produce the nonequivalence of the heads that is needed for processive movement.