A Rho for the kinetochore

Chromosome alignment on the spindle (top) is lost if Cdc42 is inactivated (bottom). N ar um iy a/ M ac m ill an Kyoto, Japan), and colleagues reveal microtubules at the cell cortex, are also needed for microtubule–kinetochore attachments, thus uniting two previously distant fields of study. Rho GTPases are needed during cytokinesis, when Rho helps build a contractile actin ring. Another Rho family member, Cdc42, is now shown to be required for nuclear division. Without its activity, chromosomes failed to make bipolar attachments. Some chromosomes were attached to one pole or the other, suggesting that correction of monoorientation, rather than capture itself, is controlled by the GTPase. Rho GTPases control both cortical and kinetochore capture by activating a formin homologue, mDia. At the cortex in yeast, an mDia1 homologue polymerizes long actin tracks that EB1-bound microtubules use to find their way to the cortex. At the mammalian kinetochore, mDia3 was needed for Cdc42-regulated biorientation. Narumiya would be surprised if actin tracks also guide microtubules to kinetochores, but since he does not know yet what the kinetochore target of mDia3 is, “we do not exclude any possibility,” he says.