Actomyosin Contractile Ring Research Articles

Two S. pombe septation phases differ in ingression rate, septum structure, and response to F-actin loss.

In fission yeast, cytokinesis requires a contractile actomyosin ring (CR) coupled to membrane and septum ingression. Septation proceeds in two phases. In anaphase B, the septum ingresses slowly. During telophase, the ingression rate increases, and the CR becomes dispensable. Here, we explore the relationship between the CR and septation by analyzing septum ultrastructure, ingression, and septation proteins in cells lacking F-actin. We show that the two phases of septation correlate with septum maturation and the response of cells to F-actin removal. During the first phase, the septum is immature and, following F-actin removal, rapidly loses the Bgs1 glucan synthase from the membrane edge and fails to ingress. During the second phase, the rapidly ingressing mature septum can maintain a Bgs1 ring and septum ingression without F-actin, but ingression becomes Cdc42 and exocyst dependent. Our results provide new insights into fungal cytokinesis and reveal the dual function of CR as an essential landmark for the concentration of Bgs1 and a contractile structure that maintains septum shape and synthesis.

The phosphatase inhibitor Sds23 regulates cell division symmetry in fission yeast.

Animal and fungal cells divide through the assembly, anchoring, and constriction of a contractile actomyosin ring (CAR) during cytokinesis. The timing and position of the CAR must be tightly controlled to prevent defects in cell division, but many of the underlying signaling events remain unknown. The conserved heterotrimeric protein phosphatase PP2A controls the timing of events in mitosis, and upstream pathways including Greatwall–Ensa regulate PP2A activity. A role for PP2A in CAR regulation has been less clear, although loss of PP2A in yeast causes defects in cytokinesis. Here, we report that Sds23, an inhibitor of PP2A family protein phosphatases, promotes the symmetric division of fission yeast cells through spatial control of cytokinesis. We found that sds23∆ cells divide asymmetrically due to misplaced CAR assembly, followed by sliding of the CAR away from its assembly site. These mutant cells exhibit delayed recruitment of putative CAR anchoring proteins including the glucan synthase Bgs1. Our observations likely reflect a broader role for regulation of PP2A in cell polarity and cytokinesis because sds23∆ phenotypes were exacerbated when combined with mutations in the fission yeast Ensa homologue, Igo1. These results identify the PP2A regulatory network as a critical component in the signaling pathways coordinating cytokinesis.

Sliding filament and fixed filament mechanisms contribute to ring tension in the cytokinetic contractile ring.

A fundamental challenge in cell biology is to understand how cells generate actomyosin-based contractile force. Here we study the actomyosin contractile ring that divides cells during cytokinesis and generates tension by a mechanism that remains poorly understood. Long ago a muscle-like sliding filament mechanism was proposed, but evidence for sarcomeric organization in contractile rings is lacking. We develop a coarse-grained model of the fission yeast cytokinetic ring, incorporating the two myosin-II isoforms Myo2 and Myp2 and severely constrained by experimental data. The model predicts that ring tension is indeed generated by a sliding filament mechanism, but a spatially and temporally homogeneous version of that in muscle. In this mechanism all pairs of oppositely oriented actin filaments are rendered tense as they are pulled toward one another and slide through clusters of myosin-II. The mechanism relies on anchoring of actin filament barbed ends to the plasma membrane, which resists lateral motion and enables filaments to become tense when pulled by myosin-II. A second fixed filament component is independent of lateral anchoring, generated by chains of like-oriented actin filaments. Myo2 contributes to both components, while Myp2 contributes to the sliding filament component only. In the face of instabilities inherent to actomyosin contractility, organizational homeostasis is maintained by rapid turnover of Myo2 and Myp2, and by drag forces that resist lateral motion of actin, Myo2 and Myp2. Thus, sliding and fixed filament mechanisms contribute to tension in the disordered contractile ring without the need for the sarcomeric architecture of muscle.

Construction and application of actin fluorescent marker in Verticillium dahliae Kleb.

Actin filaments play an important role in fungal life processes such as growth, development and cytokinesis. The expression vector pSULPH-Lifeact-mCherry of fluorescent mCherry-labeled actin was transferred into Verticillium dahliae Kleb. wild type V592 by the genetic transformation system mediated by Agrobacterium tumefaciens to obtain the stable fluorescent labeled actin strain V592/Lifeact-mCherry. Then we detected its biological phenotype and the dynamic changes of actin fluorescence during the process of spore germination, mycelial growth and development. There was no significant difference in the colony morphology, colonial growth rate, sporulation and germination rate between the fluorescent labeled actin strain and the wild type. The actin fluorescence signal was observed at the tip of the conidia and hyphae and the septum clearly. Actin participated in the formation of the contractile actomyosin ring (CAR) during cytokinesis by observing the dynamic behavior of the actin in the process of hyphal septum formation. The fluorescent labeled actin strain can be used to study the dynamics of actin in fungal development to provide theoretical and practical support for further study of the mechanism of actin in fungal development and pathogenesis.

A direct interaction between survivin and myosin II is required for cytokinesis.

An acto-myosin contractile ring, which forms after anaphase onset and is highly regulated in time and space, mediates cytokinesis, the final step of mitosis. The chromosomal passenger complex (CPC), composed of Aurora-B kinase, INCENP, borealin and survivin (also known as BIRC5), regulates various processes during mitosis, including cytokinesis. It is not understood, however, how CPC regulates cytokinesis. We show that survivin binds to non-muscle myosin II (NMII), regulating its filament assembly. Survivin and NMII interact mainly in telophase, and Cdk1 regulates their interaction in a mitotic-phase-specific manner, revealing the mechanism for the specific timing of survivin-NMII interaction during mitosis. The survivin-NMII interaction is indispensable for cytokinesis, and its disruption leads to multiple mitotic defects. We further show that only the survivin homodimer binds to NMII, attesting to the biological importance for survivin homodimerization. We suggest a novel function for survivin in regulating the spatio-temporal formation of the acto-NMII contractile ring during cytokinesis and we elucidate the role of Cdk1 in regulating this process.This article has an associated First Person interview with the first author of the paper.

First person – Aryeh Babkoff

ABSTRACT First Person is a series of interviews with the first authors of a selection of papers published in Journal of Cell Science, helping early-career researchers promote themselves alongside their papers. Aryeh Babkoff is first author on ‘A direct interaction between survivin and myosin II is required for cytokinesis’, published in JCS. Aryeh is a MD/PhD candidate in the lab of Prof. Shoshana Ravid at The Institute of Medical Research Israel-Canada, The Hebrew University-Hadassah Medical School, Jerusalem, Israel, investigating the regulation of the actomyosin contractile ring during mitosis.

Septin clearance from the division site triggers cytokinesis in budding yeast.

In many eukaryotic cells cytokinesis involves a contractile actomyosin ring (CAR) that drives cleavage furrow ingression. What triggers CAR constriction at a precise time of the cell cycle and how constriction is coupled to chromosome segregation are fundamental questions. In the budding yeast Saccharomyces cerevisiae, CAR assembly strictly requires a rigid septin collar that forms at the bud neck early during the cell cycle. At the time of cytokinesis, a sudden remodelling of the septin collar occurs, leading to its splitting into two separate rings that sandwich the CAR. We have shown that septin displacement during splitting is an essential prerequisite for CAR constriction [Tamborrini et al., Nat Commun. 9(1):4308]. Thus, cytokinesis in budding yeast is a two-step mechanism: during the first step, the septin collar organizes the assembly of the cytokinetic machinery at the right place while restraining CAR-driven membrane ingression; during the second step, a confined eviction of septins from the division site during septin ring splitting triggers CAR constriction. Our data further indicate that septin ring splitting is prompted by the Mitotic Exit Network (MEN), and in particular by its downstream phosphatase Cdc14, independently of its mitotic exit function. Surprisingly, MEN signalling at spindle pole bodies (SPBs) is critical for septin ring splitting and cytokinesis. Ubiquitination of the MEN anchor at SPBs by the Dma1/2 ubiquitin ligase attenuates MEN signalling and could have a decisive role in coupling cytokinesis to chromosome and organelle segregation. Altogether, our data emphasize the importance of septin ring splitting, which has been mysterious so far, and highlight a novel mechanism to prevent CAR constriction and cytokinesis in unpropitious conditions.

A unique kinesin-like protein, Klp8, is involved in mitosis and cell morphology through microtubule stabilization.

Kinesins are microtubule (MT)-based motors involved in various cellular functions including intracellular transport of vesicles and organelles, and dynamics of chromosomes during cell division. The fission yeast Schizosaccharomyces pombe expresses nine kinesin-like proteins (klps). Klp8 is one of them and has not been characterized yet though it has been reported to localize at the division site. Here, we studied function and localization of Klp8 in S. pombe cells. The gene klp8+ was not essential for both viability and cytoskeletal organization. Klp8-YFP was concentrated as medial cortical dots during interphase, and organized into a ring at the division site during mitosis. The Klp8 ring seemed to be localized in the space between the actomyosin contractile ring and the plasma membrane. The Klp8 ring shrank as cytokinesis proceeded. In klp8-deleted (Δ) cells, the speed of spindle elongation during anaphase B was slowed down. Overproduction of Klp8 caused bent or elongated cells, in which MTs were abnormally elongated and less dynamic than those in normal cells. Deletion of klp8+ gene suppressed the delay in mitotic entry in blt1Δ cells. These results suggest that Klp8 is involved in mitosis and cell morphology through MT stabilization.

Hof1 of Aspergillus nidulans is Essential for Cytokinesis and Septum Formation

Cytokinesis is an essential component of cellular development in filamentous fungi. The process parallels that of cell division in animal cells in requiring assembly and constriction of a contractile actomyosin ring (CAR) in the cell periphery. However, the process differs from that in animals by involving the simultaneous construction of a chitin‐rich cross wall termed a septum, which separates semi‐independent compartments of the fungal hypha. Through randomized mutations, we have generated a temperature‐sensitive mutant defective in septum formation, designated strain RCH59. Mendelian crossing confirmed that the temperature sensitive phenotype results from a single locus mutation. Next Generation Whole Genome Sequencing identified a mutation that is located in gene AN4566 encoding the homolog of Saccharomyces cerevisiae's Hof1. The mutation in Aspergillus nidulans’ version of Hof1 (AnHof1) is at base 2436 out of 5536 and is a transition from cytosine to thymine. The mutation is predicted to remove an Arginine and introduce a premature stop codon after residue 738. The missing 342 C‐terminal residues includes the entire SH3 domain of AnHof1. We cloned the wildtype version of AnHof1 into plasmid pRG3 and complemented the phenotype in RCH59, confirming that the mutation occurs in AnHof1. We also deleted AnHof1 to demonstrate that a AnHof1 minus strain has an aseptate phenotype. In the yeasts (S. cerevisiae and S. pombe), Hof1 forms a ring structure that co‐localizes with the CAR and is involved with mediating the cytoskeletal rearrangements necessary for cytokinesis. We GFP tagged AnHof1 to demonstrate that the protein product localizes to sites of septum formation and co‐localizes with actin during at least part of CAR assembly and constriction. In further work, we have generated double‐mutant strains in which a GFP‐tagged septation‐related protein is expressed in the RCH59 strain to demonstrate a network of genetics interactions between AnHof1 and other proteins involved in septation.Support or Funding InformationThis research was supported by NSF grant RUI‐0742907 to TWH and LJH and NSF grant RUI‐1615192 to LJH and TWH.This abstract is from the Experimental Biology 2019 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.

Analyzing Protein Kinase C Interactions With Rho‐type GTPases in the Filamentous Fungus Aspergillus nidulans

Cell division in filamentous fungi involves coalescence and subsequent constriction of a contractile actomyosin ring (CAR), as in other fungi and metazoans. In fungi, constriction of the CAR is followed by deposition of cell wall material resulting in a septum. In the model organism, Aspergillus nidulans, many proteins are involved in septation including protein kinase C (PkcA), the formin SepA, and the GTPase Rho4. The current study focuses on the relationships among PkcA, SepA, and Rho‐type GTPases during septation. PkcA contains two putative N‐terminal Rho binding domains designated HR1A and HR1B. The formin SepA belongs to the Diaphanous‐related family of formins that contain an N‐terminal Rho binding domain that upon binding of the appropriate Rho GTPase results in formin activation. Reports from another group suggest that in A. nidulans Rho4 is likely the GTPase required for SepA activation. In previous work we showed that PkcA both physically and functionally interacts with SepA via one of its three formin homology domains (FH2), and in the sepA1 temperature sensitive mutant (FH2 domain‐L1369S) which is aseptate at elevated temperature, PkcA fails to localize to septation sites at elevated temperature. Here we report via yeast two‐hybrid assay that PkcA interacts with SepA via one of its putative Rho binding domains (HR1A), and that expressing extra copies of pkcA in a rho4‐null mutant increased colony diameter and improved sporulation. PkcA localization to septation sites is reduced in a rhoA mutant while septation is unaffected. The spatiotemporal relationship among PkcA, SepA, Rho4, and RhoA will be further investigated via yeast two‐hybrid, bimolecular complementation assays, and localization of fluorescence labeled proteins in mutant strains.Support or Funding InformationSupport for this work was provided by the National Science Foundation Grant 1615192 (Loretta Jackson‐Hayes, PI).This abstract is from the Experimental Biology 2019 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.

NDR Kinase Sid2 Drives Anillin-like Mid1 from the Membrane to Promote Cytokinesis and Medial Division Site Placement

In animals and fungi, cytokinesis is facilitated by the constriction of an actomyosin contractile ring (CR) [1]. In Schizosaccharomyces pombe, the CR forms mid-cell during mitosis from clusters of proteins at the medial cell cortex called nodes [2]. The anillin-like protein Mid1 localizes to nodes and is required for CR assembly at mid-cell [3]. When CR constriction begins, Mid1 leaves the division site. How Mid1 disassociates and whether this step is important for cytokinetic progression has been unknown. The septation initiation network (SIN), analogous to the Hippo pathway of multicellular organisms, is a signaling cascade that triggers node dispersal, CR assembly and constriction, and septum formation [4, 5]. We report that the terminal SIN kinase, Sid2 [6], phosphorylates Mid1 to drive its removal from the cortex at CR constriction onset. A Mid1 mutant that cannot be phosphorylated by Sid2 remains cortical during cytokinesis, over-accumulates in interphase nodes following cell division in a manner dependent on the SAD kinase Cdr2, advances the G2/M transition, precociously recruits other CR components to nodes, pulls Cdr2 aberrantly into the CR, and reduces rates of CR maturation and constriction. When combined with cdr2 mutants that affect node assembly or disassembly, gross defects in division site positioning result. Our findings identify Mid1 as a key Sid2 substrate for SIN-mediated remodeling of the division site for efficient cytokinesis and provide evidence that nodes serve to integrate signals coordinating cell cycle progression and cytokinesis.

ATP produced by anaerobic glycolysis is essential for enucleation of human erythroblasts

More than 2million human erythroblasts extrude their nuclei every second in bone marrow under hypoxic conditions (<7% O2). Enucleation requires specific signal transduction pathways and the local assembly of contractile actomyosin rings. However, the energy source driving these events has not yet been identified. We examined whether different O2 environments (hypoxic [5% O2] and normoxic [21% O2] conditions) affected human CD34+ cell erythroblast differentiation. We also investigated the regulatory mechanisms underlying energy production in erythroblasts during terminal differentiation under 5% or 21% O2 conditions. The results obtained revealed that the enucleation ratio and intracellular levels of adenosine triphosphate (ATP), lactate dehydrogenase (LDH) M3H, and hypoxia-inducible factor 1α in erythroblasts during terminal differentiation were higher under the 5% O2 condition than under the 21% O2 condition. We also found that the enzymatic inhibition of glyceraldehyde 3-phosphate dehydrogenase and LDH, key enzymes in anaerobic glycolysis, blocked the proliferation of colony-forming units-erythroid and enucleation of erythroblasts, and also reduced ATP levels in erythroblasts under both hypoxic and normoxic conditions. Under both conditions, phosphorylation of the Ser232, Ser293, and Ser300 residues in pyruvate dehydrogenase (inactive state of the enzyme) in erythroblasts was involved in regulating the pathway governing energy metabolism during erythroid terminal differentiation. This reaction may be mediated by pyruvate dehydrogenase kinase (PDK) 4, the major PDK isozyme expressed in erythroblasts undergoing enucleation. Collectively, these results suggest that ATP produced by anaerobic glycolysis is the main source of energy for human erythroblast enucleation in the hypoxic bone marrow environment.

IPIP27 Coordinates PtdIns(4,5)P2 Homeostasis for Successful Cytokinesis

SummaryDuring cytokinesis, an actomyosin contractile ring drives the separation of the two daughter cells. A key molecule in this process is the inositol lipid PtdIns(4,5)P2, which recruits numerous factors to the equatorial region for contractile ring assembly. Despite the importance of PtdIns(4,5)P2 in cytokinesis, the regulation of this lipid in cell division remains poorly understood. Here, we identify a role for IPIP27 in mediating cellular PtdIns(4,5)P2 homeostasis. IPIP27 scaffolds the inositol phosphatase oculocerebrorenal syndrome of Lowe (OCRL) by coupling it to endocytic BAR domain proteins. Loss of IPIP27 causes accumulation of PtdIns(4,5)P2 on aberrant endomembrane vacuoles, mislocalization of the cytokinetic machinery, and extensive cortical membrane blebbing. This phenotype is observed in Drosophila and human cells and can result in cytokinesis failure. We have therefore identified IPIP27 as a key modulator of cellular PtdIns(4,5)P2 homeostasis required for normal cytokinesis. The results indicate that scaffolding of inositol phosphatase activity is critical for maintaining PtdIns(4,5)P2 homeostasis and highlight a critical role for this process in cell division.

Fascetto interacting protein ensures proper cytokinesis and ploidy.

Cell division is critical for development, organ growth, and tissue repair. The later stages of cell division include the formation of the microtubule (MT)-rich central spindle in anaphase, which is required to properly define the cell equator, guide the assembly of the acto-myosin contractile ring and ultimately ensure complete separation and isolation of the two daughter cells via abscission. Much is known about the molecular machinery that forms the central spindle, including proteins needed to generate the antiparallel overlapping interzonal MTs. One critical protein that has garnered great attention is the protein regulator of cytokinesis 1, or Fascetto (Feo) in Drosophila, which forms a homodimer to cross-link interzonal MTs, ensuring proper central spindle formation and cytokinesis. Here, we report on a new direct protein interactor and regulator of Feo we named Feo interacting protein (FIP). Loss of FIP results in a reduction in Feo localization, rapid disassembly of interzonal MTs, and several defects related to cytokinesis failure, including polyploidization of neural stem cells. Simultaneous reduction in Feo and FIP results in very large, tumorlike DNA-filled masses in the brain that contain hundreds of centrosomes. In aggregate, our data show that FIP acts directly on Feo to ensure fully accurate cell division.

Increasing ergosterol levels delays formin-dependent assembly of F-actin cables and disrupts division plane positioning in fission yeast.

In most eukaryotes, cytokinesis is mediated by the constriction of a contractile acto-myosin ring (CR), which promotes the ingression of the cleavage furrow. Many components of the CR interact with plasma membrane lipids suggesting that lipids may regulate CR assembly and function. Although there is clear evidence that phosphoinositides play an important role in cytokinesis, much less is known about the role of sterols in this process. Here, we studied how sterols influence division plane positioning and CR assembly in fission yeast. We show that increasing ergosterol levels in the plasma membrane blocks the assembly of F-actin cables from cytokinetic precursor nodes, preventing their compaction into a ring. Abnormal F-actin cables form after a delay, leading to randomly placed septa. Since the formin Cdc12 was detected on cytokinetic precursors and the phenotype can be partially rescued by inhibiting the Arp2/3 complex, which competes with formins for F-actin nucleation, we propose that ergosterol may inhibit formin dependent assembly of F-actin cables from cytokinetic precursors.

Crosslinking activity of non-muscle myosin II is not sufficient for embryonic cytokinesis in C. elegans.

ABSTRACTCytokinesis in animal cells requires the assembly and constriction of a contractile actomyosin ring. Non-muscle myosin II is essential for cytokinesis, but the role of its motor activity remains unclear. Here, we examine cytokinesis in C. elegans embryos expressing non-muscle myosin motor mutants generated by genome editing. Two non-muscle motor-dead myosins capable of binding F-actin do not support cytokinesis in the one-cell embryo, and two partially motor-impaired myosins delay cytokinesis and render rings more sensitive to reduced myosin levels. Further analysis of myosin mutants suggests that it is myosin motor activity, and not the ability of myosin to crosslink F-actin, that drives the alignment and compaction of F-actin bundles during contractile ring assembly, and that myosin motor activity sets the pace of contractile ring constriction. We conclude that myosin motor activity is required at all stages of cytokinesis. Finally, characterization of the corresponding motor mutations in C. elegans major muscle myosin shows that motor activity is required for muscle contraction but is dispensable for F-actin organization in adult muscles.This article has an associated ‘The people behind the papers’ interview.

Heparan sulfate proteoglycan (HSPG) can take part in cell division: inside and outside.

Prior to the cytokinesis, the cell-matrix interactions should be disrupted, and the mitotic cells round up. Prerequisite of mitosis, the centrosomes duplicate, spindle fibers are generated and move away from each other to opposite sides of the cells marking the cell poles. Later, an invagination in the plasma membrane is formed a few minutes after anaphase. This furrow ingression is driven by a contractile actomyosin ring, whose assembly is regulated by RhoA GTPase. At the completion of cytokinesis, the two daughter cells are still connected by a thin intercellular bridge, which is subjected to abscission, as the terminal step of cytokinesis. Here, it is overviewed, how syndecan-4, a transmembrane, heparan sulfate proteoglycan, can contribute to these processes in a phosphorylation-dependent manner.

The ARP2/3 complex prevents excessive formin activity during cytokinesis.

Cytokinesis completes cell division by constriction of an actomyosin contractile ring that separates the two daughter cells. Here we use the early Caenorhabditis elegans embryo to explore how the actin filament network in the ring and the surrounding cortex is regulated by the single cytokinesis formin CYK-1 and the ARP2/3 complex, which nucleate nonbranched and branched filaments, respectively. We show that CYK-1 and the ARP2/3 complex are the predominant F-actin nucleators responsible for generating distinct cortical F-actin architectures and that depletion of either nucleator affects the kinetics of cytokinesis. CYK-1 is critical for normal F-actin levels in the contractile ring, and acute inhibition of CYK-1 after furrow ingression slows ring constriction rate, suggesting that CYK-1 activity is required throughout ring constriction. Surprisingly, although the ARP2/3 complex does not localize in the contractile ring, depletion of the ARP2 subunit or treatment with ARP2/3 complex inhibitor delays contractile ring formation and constriction. We present evidence that the delays are due to an excess in formin-nucleated cortical F-actin, suggesting that the ARP2/3 complex negatively regulates CYK-1 activity. We conclude that the kinetics of cytokinesis are modulated by interplay between the two major actin filament nucleators.

Aspergillus nidulans protein kinase C forms a complex with the formin SepA that is involved in apical growth and septation

The Aspergillus nidulans orthologue of Protein kinase C (PkcA) and the A. nidulans formin SepA participate in polarized growth. PkcA localizes to growing hyphal apices and septation sites, and amino acid sequences within PkcA that are required for PkcA to localize to these sites of cell wall synthesis have been identified. SepA is associated with the contractile actomyosin ring (CAR), and it localizes at hyphal tips in association with the Spitzenkörper (SPK) and as an apical dome. A mutation in the sepA gene (sepA1) renders A. nidulans aseptate at elevated temperature. Progress towards understanding the spatiotemporal relationship between PkcA and SepA during polarized growth is presented here. Fluorescent chimeras of PkcA and SepA strongly overlapped in some hyphal tips in a dome pattern, while other tips displayed SepA SPK and PkcA dome localization within the same tip. At septation sites PkcA and SepA consistently colocalized through late stages of CAR constriction. Bimolecular fluorescence complementation experimental results provide evidence that SepA and PkcA are both present in complexes at both hyphal tip domes and at cortical rings. A Gal4-based yeast two-hybrid analysis confirmed the physical interaction between SepA and PkcA, and indicted that the FH2 domain of SepA is involved in its physical interaction with PkcA. A functional interaction between PkcA and SepA was shown through complementation of the pkcA calC2 mutant’s hypersensitivity to cell wall perturbing agents by overexpressed sepA and by the ability of the sepA1 mutation to block PkcA’s ability to form cortical rings. Taken together these results suggest that a PkcA/SepA complex is involved in polarized growth. Through experiments using the actin disrupter latrunculin B, evidence is presented suggesting that actin plays a role in the PkcA/SepA complex.

Recruitment of the mitotic exit network to yeast centrosomes couples septin displacement to actomyosin constriction

In many eukaryotic organisms cytokinesis is driven by a contractile actomyosin ring (CAR) that guides membrane invagination. What triggers CAR constriction at a precise time of the cell cycle is a fundamental question. In budding yeast CAR is assembled via a septin scaffold at the division site. A Hippo-like kinase cascade, the Mitotic Exit Network (MEN), promotes mitotic exit and cytokinesis, but whether and how these two processes are independently controlled by MEN is poorly understood. Here we show that a critical function of MEN is to promote displacement of the septin ring from the division site, which in turn is essential for CAR constriction. This is independent of MEN control over mitotic exit and involves recruitment of MEN components to the spindle pole body (SPB). Ubiquitination of the SPB scaffold Nud1 inhibits MEN signaling at the end of mitosis and prevents septin ring splitting, thus silencing the cytokinetic machinery.