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.