Ring around the tubule

The kinesin protein family is in the trucking business, hauling vesicles and other cellular cargo along the microtubules. The family's black sheep is kinesin-13, which disassembles the filaments while riding on them. Tan et al. report on page 25 that kinesin-13 is unusual in another way. The protein forms rings and spirals around microtubules. The structures might help it maintain its position as a tubule shrinks.

By shortening microtubules, kinesin-13 helps chromosomes to go their separate ways during mitosis. But how kinesin-13 performs the job is mysterious. The researchers spotted the rings and spirals when they combined microtubules with the motor domain of kinesin-13, the protein segment that latches onto the filament. Other types of kinesin did not coil up, the scientists showed. Molecular reconstructions suggest that each ring consists of several kinesin-13 molecules encircled by a strand...