Spindles in flux

The formation and maintenance of a bipolar spindle in dividing eukaryotic cells involves multiple proteins that affect microtubule dynamics. The steady-state length of the metaphase spindle depends on the `microtubule flux' - a phenomenon in which tubulin subunits are simultaneously added at the plus ends and removed at the minus ends of microtubules. Joseph Laycock, Matthew Savoian and David Glover have investigated which proteins regulate this flux in Drosophila. On p. 2354, they report that Orbit, a highly conserved protein that stabilizes microtubules, interacts antagonistically with the kinesin-like microtubule depolymerase Klp10A to regulate spindle length, bipolarity and function. They find that knocking down Orbit alone by RNAi causes spindles to collapse and results in formation of monopolar spindles, mitotic arrest and apoptosis. Co-depletion of Orbit and Klp10A (but not other Drosophila microtubule depolymerases), however, rescues spindle bipolarity and the cells divide normally after extended periods in mitosis. Because these two proteins normally act antagonistically to drive microtubule flux, the authors conclude that microtubule flux in flies must therefore be dispensable for bipolar spindle formation and function.