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Microtubules (red) are bound but not broken by spastin (green) when one hand of spastin is mutated.

It's easier to snap a twig with two hands than one, and the same goes for snapping microtubules. By binding to tubulin in two places, a microtubule-severing protein can exert the force necessary to pull the polymer apart, according to White et al. (page 995).

Microtubule-severing spastin is a member of the AAA ATPase family, which often breaks down multiprotein complexes. The team now shows that spastin, like other AAAs, forms ring-shaped hexamers. They find that spastin must use two different binding domains—one on the outside of the ring and one on the loop domains inside the ring—to lock onto tubulin and cause severing.

The spastin ring is too small to fit around the whole microtubule. Instead, the team shows, spastin binds to two spots on the microtubule, one of which they identify as the extreme COOH-terminal amino acids of the tubulin tail that sticks out from the surface of the microtubule polymer. The authors found that spastin only bound to the polymerized form of tubulin. They therefore suggest that spastin, by grabbing hold of two parts of tubulin, might pull the protein into a conformation that would release tubulin from the microtubule polymer as well as from spastin itself.

Some AAAs, such as those of the proteasome, unfold and thus destroy their targets. Other AAAs pull off individual components from multiprotein complexes, thus allowing the components to be reused. The team is currently trying to determine which of these mechanisms spastin uses.