The Tem1 small GTPase controls actomyosin and septin dynamics during cytokinesis

Cytokinesis in budding yeast involves an actomyosin-based ring which assemb les in a multistepped fashion during the cell cycle and constricts during c ytokinesis. ill this report, we have investigated the structural and regula tory events that occur at the onset of cytokinesis. The septins, which form an hour-glass like structure during early stages of the cell cycle, underg o dynamic rearrangements prior to cell division: the hourglass structure sp lits into two separate rings. The contractile ring, localized between the s eptin double rings, immediately undergoes contraction, Septin ring splittin g is independent of actomyosin ring contraction as it still occurs in mutan ts where contraction fails. We hypothesize that septin ring splitting map r emove a structural barrier for actomyosin ring to contract. Because the Tem 1 small GTPase (Tem1p) is required for the completion of mitosis, we invest igated its role in regulating septin and actomyosin ring dynamics in the ba ckground of the net1-1 mutation, which bypasses the anaphase cell cycle arr est in Tem1-deficient cells. We show that Tem1p plays a specific role in cy tokinesis in addition to its function in tell cycle progression. Tem1p is n ot required for the assembly of the actomyosin ring but controls actomyosin and septin dynamics during cytokinesis.