To ensure the accurate transmission of the genome to daughter cells during mitosis, cell division (cytokinesis) must only occur after chromosome separation is complete–but how is the timing of cytokinesis controlled? On page 601, Viesturs Simanis and colleagues explore the regulation of the GTPase Spg1p, a key effector of cytokinesis that localises to the spindle pole body in fission yeast. The authors investigate the steady-state levels of Cdc16p and Byr4p, two proteins that together inactivate Spg1p by promoting hydrolysis of its bound GTP. Using a series of mutant S. pombe strains, the authors show that the majority of Byr4p in the cell forms a complex with Spg1p. Any Byr4p that does not bind Spg1p is degraded by the proteasome in a Cdc16p-dependent manner. Consequently, the steady-state levels of Byr4p and Spg1p are equivalent; this might, the authors suggest, protect Spg1p from inappropriate activation during interphase. Moreover, the phosphorylation state of Byr4p changes during the cell cycle, suggesting an additional mechanism by which its activity is modulated. These results underscore the complexity of the regulatory mechanisms that control cytokinesis.