Held back by a finger

The familiar arm-over-arm kinesin swing was recently challenged by proposals that KIF1A acts as a processive, but monomeric, kinesin motor. The proposal involved a rotation of the motor that, along with a microtubule-binding patch, would bias further movement in one direction.

But now Al-Bassam et al. add to the growing evidence that the KIF1A orthologue in worms, Unc104, may function as a dimer, like conventional kinesin (page 743). The monomer, they propose, is instead a regulated form whose full activity is only restored when motors are crowded onto cargo vesicles.

The unusual prospect of a monomeric motor, and expression problems that had led to the use of a KIF1A/conventional kinesin hybrid in the earlier work, led the authors to study Unc104. Images obtained by cryo-EM revealed a motor domain with a protruding...