Temporal control of contractile ring assembly by Plo1 regulation of myosin II recruitment by Mid1/anillin

In eukaryotes, cytokinesis generally involves an actomyosin ring, the contraction of which promotes daughter cell segregation. Assembly of the contractile ring is tightly controlled in space and time [1-4]. In the fission yeast, contractile ring components are first organized by the anillin-like protein Mid1 into medial cortical nodes [5-9]. These nodes then coalesce laterally into a functional contractile ring [10-13]. Although Mid1 is present at the medial cortex throughout G2 [14], recruitment of contractile ring components to nodes starts only at mitotic onset [12], indicating that this event is cell-cycle regulated. Polo kinases are key temporal coordinators of mitosis and cytokinesis [1], and the Polo-like kinase Plo1 is known to activate Mid1 nuclear export at mitotic onset [15, 16], coupling division plane specification to nuclear position [7]. Here we provide evidence that Plo1 also triggers the recruitment of contractile ring components into medial cortical nodes. Plo1 binds at least two independent sites on Mid1, including a consensus site phosphorylated by Cdc2. Plo1 phosphorylates several residues within the first 100 amino acids of Mid1, which directly interact with the IQGAP Rng2 [17], and influences the timing of myosin II recruitment. Plo1 thereby facilitates contractile ring assembly at mitotic onset.