Localized mRNA is the norm

Structure of the meiotic spindle T he 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 fi laments 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 refi ning their fl uorescent speckle microscopy techniques, the authors were able for the fi rst time to track individual tubulin subunits (seen as speckles) in a single tubulin polymer. The authors identifi ed pairs of speckles representing subunits on the same fi lament. Speckle separation supplied them with the minimum length of that fi lament. They then fi tted a mathematical model to these observed lengths to predict overall fi lament lengths: most fi laments were only 40% of the total spindle length. The short fi laments were also scattered throughout the spindle. The researchers now propose that the spindle is a tiled array of overlapping short fi laments. The group next examined how spindle-associated proteins might control fi lament and spindle size. By inhibiting microtubule motor proteins, they found that dynein–dynactin limited individual fi ber lengths and thus overall spindle length. Kinesin 5 activity limited the overlap between fi bers 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.