Elongated Stu2p (top), upon binding tubulin, forms a compact complex (bottom).

The Stu2p microtubule-associated protein (MAP) clamps around tubulin heterodimers, as revealed by electron micrograph images from Al-Bassam et al. (page 1009). Stu2p and other members of the XMAP215 family may thus capture and deliver tubulin to the growing ends of microtubules.

Although Stu2p stabilizes microtubules in vivo, it has a destabilizing effect in vitro. This as-yet unexplained difference has made it difficult to determine how the XMAP215 family works at a mechanistic level. The new studies suggest that their stabilizing ability stems from their interaction with free tubulin α/β heterodimers.

Tubulin heterodimers, the authors found, associate with dimers of Stu2p, as shown by affinity chromatography. This association is necessary for Stu2p's stabilizing ability, as a mutant that bound to filament ends but not to tubulin dimers caused microtubule shortening. Electron micrographs revealed that the long, open dimers of free Stu2p closed up and clamped around tubulin upon binding.

Earlier experiments suggested that Stu2p is transported along microtubules to the growing ends. If Stu2p brings along its captured tubulin, it would increase the local concentration of available new subunits. Microtubule stabilization by the XMAP215 family might therefore result from increased polymerization rates.

The authors are now trying to trim down the Stu2p–tubulin complex into a version that can be crystallized. They hope that higher resolution images will identify interactions that trigger the release of tubulin and free Stu2p for another round of capture. If the interaction is stronger in vitro than in vivo, Stu2p might sequester rather than deliver tubulin, thereby explaining its in vitro destabilizing effects.