Mitotic cells regulate the lengths of their astral microtubules to keep the spindle steady during cytokinesis, Rankin and Wordeman report.

Astral microtubules emanate from the spindle poles toward the cell cortex and help to position the cleavage furrow at the end of mitosis by confining actomyosin contraction to the cell equator: when microtubules are shortened with depolymerizing drugs, contraction occurs all around the cell cortex. Rankin and Wordeman took the opposite approach to studying astral microtubules—lengthening them by depleting the depolymerizing motor protein MCAK or treating cells with the stabilizing drug taxol.

Cytokinetic furrows formed correctly but mitotic spindles oscillated wildly between would-be daughter cells. Spindle rocking correlated with the formation of membrane blebs—protrusions at the cell poles where cortical actomyosin was disrupted. Mitotic spindles were sucked into the blebs before actin and myosin reassembled to squeeze the membrane back into shape, pushing the spindle into blebs on the opposing side. Inhibiting myosin activity stopped spindle rocking in MCAK-depleted cells.

Rankin and Wordeman suggest that membrane blebs form during cytokinesis because the polar cortical cytoskeleton is weakened as actin and myosin relocate to the growing cleavage furrow. Blebbing occurs in wild-type cells too, but is particularly hazardous in the absence of MCAK because the cells' extended astral microtubules widen the blebs to a size large enough to initiate spindle oscillations. Aster size must therefore be tightly regulated to position the cleavage furrow without exacerbating membrane blebs.

J. Cell Biol.