Catching augmin in the act

Kamasaki et al. observe links between microtubules within the mitotic spindle that might allow these microtubules to grow in the right direction.

Most spindle microtubules extend from the centrosomes, and some sprout near the chromosomes. But researchers have discovered a third category of microtubules that grow within the spindle. The augmin complex and the γ-tubulin ring complex (γ-TuRC) team up to spur the formation of these microtubules, but the mechanism remains uncertain. In the cell’s crowded middle, microtubules within the spindle are difficult to distinguish by fluorescence microscopy.

Kamasaki et al. used electron tomography to visualize part of the spindle in human cells. The researchers then painstakingly identified and traced every microtubule. The team detected microtubules whose ends were distributed within the spindle, with their minus ends pointing toward the spindle pole. However, these microtubules were less than half as abundant if the cell lacked augmin.

Augmin could potentially recruit γ-TuRC to the wall of an existing “mother” microtubule, leading to the initiation of a new “daughter” microtubule within the spindle. In control cells, the tomographs revealed examples of connections between a mother microtubule and the end of a daughter microtubule, but these links were rare in cells missing augmin. Kamasaki et al. showed that the microtubule connections often contained a rod-shaped structure that could be the augmin complex, although the researchers were unable to confirm its identity. Mother and daughter microtubules tended to be almost parallel, potentially explaining why the fibers adopt the correct orientation within the spindle.