Mid1p/anillin and the septation initiation network orchestrate contractile ring assembly for cytokinesis

Abstract

In both animal cells and fungi, cytokinesis proceeds via a contractile actomyosin ring (CAR). Many CAR components and regulators are evolutionarily conserved. In Schizosaccharomyces pombe, the spatial cue for cytokinesis is provided by Mid1p/Anillin, whereas temporal coordination is ensured by the septation initiation network (SIN). However, neither Mid1p nor the SIN is considered to be essential for CAR assembly per se. Here, using 4D imaging, we reveal an unanticipated, novel role for the SIN in CAR assembly. We demonstrate that CAR assembly involves three, genetically separable steps: establishment of a cortical network of CAR proteins, its lateral condensation, and finally, the formation of a homogeneous CAR. We show that SIN mutants fail to form a homogeneous CAR; we identify hypophosphorylation and recruitment of the conserved PCH-family protein Cdc15p to the CAR as critical steps requiring SIN function. Furthermore, we show that in the absence of Mid1p, CAR assembly proceeds via an actomyosin filament, rather than a cortical network of CAR proteins. This mode of assembly is totally dependent on SIN signaling, thereby demonstrating a direct role for the SIN in CAR formation. Taken together, these data establish that Mid1p and the SIN are the key regulators that orchestrate CAR assembly.[Keywords: S. pombe; cytokinesis; septation initiation network; Mid1p/anillin; contractile actomyosin ring] Supplemental material is available at http://www.genesdev.org.

Role of the protein kinase Kin1 and nuclear centering in actomyosin ring formation in fission yeast

“…13 This broad band exactly reflect the early mitotic Mid1 cortical band. 8,13 The Rlc1-GFP signal then coalesces as a packed ring by lateral condensation, 9 followed by a constant diameter period and ring constriction.…”

“…At mitotic commitment, nuclear Mid1 completely exits the nucleus and associates with the cell cortex prior to chromosome segregation. [2][3][4][5][6][7][8] Recent studies 9,10 have suggested a model with two parallel non overlapping mechanisms for CAR formation. 11 First, Mid1 establishes a broad band of dots that dictates the site of 'cortical nodes' formation, which are protein complexes that include Rlc1-containing myosin II hexamers.…”

“…A homogeneous ring structure is achieved by the incorporation of the FER/CIP4 homology (FCH) domain protein Cdc15. 9 The fission yeast cell is rod-shaped and interphase cell growth, including plasma membrane extension and cell wall remodeling, is restricted to the opposite ends. Early in G 2 , growth is activated only in the mother-inherited 'old' cell end.…”

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

“…In this case, the contractile ring is formed from actomyosin filaments assembled randomly on the cortex, which eventually converge to assemble a contractile ring [Huang et al, 2008]. Contractile ring assembly is then strictly dependent on the activity of the SIN, [septation initiation network; Hachet and Simanis, 2008]. This signalling cascade, essential for cytokinesis, is activated by Plo1 kinase and controlled from the spindle pole body by the small GTPase Spg1 [Ohkura et al, 1995; Schmidt et al, 1997;Sohrmann et al, 1998; see Goyal et al, 2011 for a review].…”

“…This signalling cascade, essential for cytokinesis, is activated by Plo1 kinase and controlled from the spindle pole body by the small GTPase Spg1 [Ohkura et al, 1995; Schmidt et al, 1997;Sohrmann et al, 1998; see Goyal et al, 2011 for a review]. The SIN, whose activation can bypass the requirement for Plo1 for contractile ring assembly, induces the dephosphorylation of the F-BAR protein Cdc15 [Hachet and Simanis, 2008]. Cdc15 dephosphorylation has on the other hand been shown to promote the transition from a closed to an open conformation of the molecule, allowing its oligomerization and promoting its scaffolding activity.…”