Axons branch wildly without kinesin-5 (bottom).
The spindle brake is kinesin-5. Unlike most microtubule-based motors, the “cargo” of kinesin-5 is more microtubules. In dividing cells, this motor bundles oppositely oriented spindle microtubules and seems to help drive them apart. Recent work shows that kinesin-5 can also prevent them from sliding past each other too quickly, thus preventing premature pole separation.
Kinesin-5 also has a strong presence in developing neurons, which are done dividing and would thus seem not to need a spindle brake. Because its inhibition creates longer axons, Myers and Baas imagined that kinesin-5 normally transports short microtubule building blocks from the axon back to the cell body. In its absence, they figured, more blocks would be available for axon growth.
Instead, the authors found...
