Temporal Control of Contractile Ring Assembly by Plo1 Regulation of Myosin II Recruitment by Mid1/Anillin

Abstract

Summary In eukaryotes, cytokinesis generally involves an acto-myosin ring, the contraction of which promotes daughter cells segregation. Assembly of the contractile ring is tightly controlled in space and time (see[1–4] for reviews). In the fission yeast, contractile ring components are first organized by the Anillin-like protein Mid1 [5, 6] and Cdr2 kinase [7–9] into medial cortical nodes. These nodes then coalesce laterally into a functional compact contractile ring [10–13]. Although Mid1 is present at the medial cortex throughout G2 phase [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 [15] has long been implicated in Mid1 regulation [16]: Plo1 activates Mid1 nuclear export at mitotic onset [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.

Mid1/anillin and the spatial regulation of cytokinesis in fission yeast

“…3). Plo1 phosphorylates Mid1 in the region of the first NES and mutations of these sites to alanine affect Mid1 export from the nucleus during mitosis [Almonacid et al, 2011]. This formally demonstrates that Plo1 controls Mid1 export from the nucleus.…”

“…3). The phosphorylation of Plo1 binding site also allows Plo1 localization to the contractile ring during mitosis [Almonacid et al, 2011].…”

“…3). The phosphorylation of Plo1 binding site also allows Plo1 localization to the contractile ring during mitosis [Almonacid et al, 2011].Contrary to other anillin-family members which exit the nucleus upon nuclear envelope breakdown, Mid1 contains two nuclear export sequences (NES) required for efficient translocation of the protein from the nucleus to the cortex upon entry into mitosis [Paoletti and Chang, 2000] (see Fig. 3).…”

“…In particular, Plo1 promotes Mid1 nuclear export at the beginning of mitosis ]. However, recent work in our laboratory has shown that Plo1 also acts as an activator of Mid1, triggering the recruitment of contractile ring components at mitosis onset [Almonacid et al, 2011].One protein directly recruited by Mid1 is the IQGAP protein Rng2, a multidomain protein essential for contractile ring assembly [Eng et al, 1998;Takaine et al, 2009]. The characterization of a novel mutant of Rng2 showed that this protein is indeed involved in division plane positioning [Padmanabhan et al, 2011].…”

“…In fact, Rng2 recruitment is a prerequisite for myosin II recruitment to medial cortical nodes [Laporte et al, 2010;Padmanabhan et al, 2011]. This event is triggered by the Plo1-dependent phosphorylation of Mid1 N-terminal fragment which can bind Rng2 directly in vitro [Almonacid et al, 2011]. Noteworthy, the phosphorylation by Plo1 of the same set of amino acids within Mid1 1-100 fragment thus controls at the same time Mid1 activation and Mid1 export from the nucleus, directly linking these two events.…”