Structure of the meiotic spindle

The meiotic spindle is made up of shorter microtubules than previously believed, suggest results from Ge Yang, Gaudenz Danuser (Scripps Research Institute, La Jolla, CA), Ben Houghtaling, Tarun Kapoor (Rockefeller University, New York, NY), and colleagues. Current models of the spindle, as a bipolar array of overlapping filaments extending from opposite spindle poles, will require revision.

To get a closer look at the architecture of the meiotic spindle, Yang et al. incorporated labeled tubulin subunits into the spindle in a cell-free system. By refining their fluorescent speckle microscopy techniques, the authors were able for the first time to track individual tubulin subunits (seen as speckles) in a single tubulin polymer.

The authors identified pairs of speckles representing subunits on the same filament. Speckle separation supplied them with the minimum length of that filament. They then fitted a mathematical model to these observed lengths to predict overall filament lengths: most filaments were only ∼40% of the total spindle length. The short filaments were also scattered throughout the spindle. The researchers now propose that the spindle is a tiled array of overlapping short filaments.

The group next examined how spindle-associated proteins might control filament and spindle size. By inhibiting microtubule motor proteins, they found that dynein–dynactin limited individual fiber lengths and thus overall spindle length. Kinesin 5 activity limited the overlap between fibers by sliding them apart. “Our work suggests the spindle is a self-organizing system, whose stability and functional characteristics are built on these kind of local interactions,” says Kapoor.

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