A MAP's stabilizing mechanism

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...