During cytokinesis, an actin-based cleavage furrow separates the dividing cells. The furrow must be accurately positioned relative to the microtubule spindle but how this is achieved is unclear. On , Robert Saint and colleagues report that in Drosophila cells, a Rho-family GTPase-activating protein (RhoGAP) called Tumbleweed (Tum/RacGAP50C) acts as a furrow-positioning signal by linking anaphase microtubules to the assembly of the cytokinetic furrow at the cortex. The authors show that, in Drosophila embryos, cells that lack Tum do not form furrows and fail to localize key cytokinetic components correctly, including the Rho guanine-nucleotide-exchange factor (RhoGEF) Pebble (Pbl). Pbl is thought to initiate changes in Rho activity at the cell equator that culminate in furrow formation. Disruption of the Pbl-interacting domain of Tum also prevents furrow formation but not localization of Tum to the microtubules near where the furrow should form. The authors conclude that Tum is required early in cytokinesis in Drosophila and that the interaction between Tum and Pbl forms an essential link between the mitotic spindle and formation of the cleavage furrow.
