Characterization of structural and functional domains of the anillin-related protein Mid1p that contribute to cytokinesis in fission yeast

Anillin-related Mid1 as an adaptive and multimodal contractile ring anchoring protein: A simulation study

Cytokinesis of animal and fungi cells depends crucially on the anillin scaffold proteins. Fission yeast anillin-related Mid1 anchors cytokinetic ring precursor nodes to the membrane. However, it is unclear if both of its Pleckstrin Homology (PH) and C2 C-terminal domains bind to the membrane as monomers or dimers, and if one domain plays a dominant role. We studied Mid1 membrane binding with all-atom molecular dynamics near a membrane with yeast-like lipid composition. In simulations with the full C terminal region started away from the membrane, Mid1 binds through the disordered L3 loop of C2 in a vertical orientation, with the PH away from the membrane. However, a configuration with both C2 and PH initially bound to the membrane remains associated with the membrane. Simulations of C2-PH dimers show extensive asymmetric membrane contacts. These multiple modes of binding may reflect Mid1's multiple interactions with membranes, node proteins, and ability to sustain mechanical forces.

The role of anillin/Mid1p during medial division and cytokinesis: from fission yeast to cancer cells

Cytokinesis is the final stage of cell division cycle when cellular constituents are separated to produce two daughter cells. This process is driven by the formation and constriction of a contractile ring. Progression of these events is controlled by mechanisms and proteins that are evolutionary conserved in eukaryotes from fungi to humans. Genetic and molecular studies in different model organisms identified essential cytokinesis genes, with several conserved proteins, including the anillin/Mid1p proteins, constituting the core cytokinetic machinery. The fission yeast Schizosaccharomyces pombe represents a well-established model organism to study eukaryotic cell cycle regulation. Cytokinesis in fission yeast and mammalian cells depends on the placement, assembly, maturation, and constriction of a medially located actin-myosin contractile ring (ACR). Here, we review aspects of the ACR assembly and cytokinesis process in fission yeast and consider the regulation of such events in mammalian cells. First, we briefly describe the role of anillin during mammalian ACR assembly and cytokinesis. Second, we describe different aspects of the anillin-like protein Mid1p regulation during the S. pombe cell cycle, including its structure, function, and phospho-regulation. Third, we briefly discuss Mid1pindependent ACR assembly in S. pombe. Fourth, we highlight emerging studies demonstrating the roles of anillin in human tumourigenesis introducing anillin as a potential drug target for cancer treatment. Collectively, we provide an overview of the current understanding of medial division and cytokinesis in S. pombe and suggest the implications of these observations in other eukaryotic organisms, including humans.

The number of cytokinesis nodes in mitotic fission yeast scales with cell size

Cytokinesis nodes are assemblies of stoichiometric ratios of proteins associated with the plasma membrane, which serve as precursors for the contractile ring during cytokinesis by fission yeast. The total number of nodes is uncertain, because of the limitations of the methods used previously. Here, we used the ~140 nm resolution of Airyscan super-resolution microscopy to measure the fluorescence intensity of small, single cytokinesis nodes marked with Blt1-mEGFP in live fission yeast cells early in mitosis. The ratio of the total Blt1-mEGFP fluorescence in the broad band of cytokinesis nodes to the average fluorescence of a single node gives about 190 single cytokinesis nodes in wild-type fission yeast cells early in mitosis. Most, but not all of these nodes condense into a contractile ring. The number of cytokinesis nodes scales with cell size in four strains tested, although large diameter rga4Δ mutant cells form somewhat fewer cytokinesis nodes than expected from the overall trend. The Pom1 kinase restricts cytokinesis nodes from the ends of cells, but the surface density of Pom1 on the plasma membrane around the equators of cells is similar with a wide range of node numbers, so Pom1 does not control cytokinesis node number. However, when the concentrations of either kinase Pom1 or kinase Cdr2 were varied with the nmt1 promoter, the numbers of cytokinesis nodes increased above a baseline of about ~190 with the total cellular concentration of either kinase.

Molecular organization of cytokinesis node predicts the constriction rate of the contractile ring

The molecular organization of cytokinesis proteins governs contractile ring function. We used single molecule localization microscopy in live cells to elucidate the molecular organization of cytokinesis proteins and relate it to the constriction rate of the contractile ring. Wild-type fission yeast cells assemble contractile rings by the coalescence of cortical proteins complexes called nodes whereas cells without Anillin/Mid1p (Δmid1) lack visible nodes yet assemble contractile rings competent for constriction from the looping of strands. We leveraged the Δmid1 contractile ring assembly mechanism to determine how two distinct molecular organizations, nodes versus strands, can yield functional contractile rings. Contrary to previous interpretations, nodes assemble in Δmid1 cells. Our results suggest that Myo2p heads condense upon interaction with actin filaments and an excess number of Myo2p heads bound to actin filaments hinders constriction thus reducing the constriction rate. Our work establishes a predictive correlation between the molecular organization of nodes and the behavior of the contractile ring.

Anillin/Mid1p interacts with the ESCRT-associated protein Vps4p and mitotic kinases to regulate cytokinesis in fission yeast

Cytokinesis is the final stage of the cell cycle which separates cellular constituents to produce two daughter cells. Using the fission yeast Schizosaccharomyces pombe we have investigated the role of various classes of proteins involved in this process. Central to these is anillin/Mid1p which forms a ring-like structure at the cell equator that predicts the site of cell separation through septation in fission yeast. Here we demonstrate a direct physical interaction between Mid1p and the endosomal sorting complex required for transport (ESCRT)-associated protein Vps4p, a genetic interaction of the mid1 and vps4 genes essential for cell viability, and a requirement of Vps4p for the correct cellular localization of Mid1p. Furthermore, we show that Mid1p is phosphorylated by aurora kinase, a genetic interaction of the mid1 and the aurora kinase ark1 genes is essential for cell viability, and that Ark1p is also required for the correct cellular localization of Mid1p. We mapped the sites of phosphorylation of Mid1p by human aurora A and the polo kinase Plk1 and assessed their importance in fission yeast by mutational analysis. Such analysis revealed serine residues S332, S523 and S531 to be required for Mid1p function and its interaction with Vps4p, Ark1p and Plo1p. Combined these data suggest a physical interaction between Mid1p and Vps4p important for cytokinesis, and identify phosphorylation of Mid1p by aurora and polo kinases as being significant for this process.

Fission yeast Pak1 phosphorylates anillin-like Mid1 for spatial control of cytokinesis

Protein kinases direct polarized growth by regulating the cytoskeleton in time and space and could play similar roles in cell division. We found that the Cdc42-activated polarity kinase Pak1 colocalizes with the assembling contractile actomyosin ring (CAR) and remains at the division site during septation. Mutations in pak1 led to defects in CAR assembly and genetic interactions with cytokinesis mutants. Through a phosphoproteomic screen, we identified novel Pak1 substrates that function in polarized growth and cytokinesis. For cytokinesis, we found that Pak1 regulates the localization of its substrates Mid1 and Cdc15 to the CAR. Mechanistically, Pak1 phosphorylates the Mid1 N-terminus to promote its association with cortical nodes that act as CAR precursors. Defects in Pak1-Mid1 signaling lead to misplaced and defective division planes, but these phenotypes can be rescued by synthetic tethering of Mid1 to cortical nodes. Our work defines a new signaling mechanism driven by a cell polarity kinase that promotes CAR assembly in the correct time and place.

Cell Motility and Cytokinesis: From Mysteries to Molecular Mechanisms in Five Decades

This is the story of someone who has been fortunate to work in a field of research where essentially nothing was known at the outset but that blossomed with the discovery of profound insights about two basic biological processes: cell motility and cytokinesis. The field started with no molecules, just a few people, and primitive methods. Over time, technological advances in biophysics, biochemistry, and microscopy allowed the combined efforts of scientists in hundreds of laboratories to explain mysterious processes with molecular mechanisms that can be embodied in mathematical equations and simulated by computers. The success of this field is a tribute to the power of the reductionist strategy for understanding biology.

The Functionally Important N-Terminal Half of Fission Yeast Mid1p Anillin Is Intrinsically Disordered and Undergoes Phase Separation

Division of fungal and animal cells depends on scaffold proteins called anillins. Cytokinesis by the fission yeast Schizosaccharomyces pombe is compromised by the loss of anillin Mid1p (Mid1, UniProtKB P78953 ), because cytokinesis organizing centers, called nodes, are misplaced and fail to acquire myosin-II, so they assemble slowly into abnormal contractile rings. The C-terminal half of Mid1p consists of lipid binding C2 and PH domains, but the N-terminal half (Mid1p-N452) performs most of the functions of the full-length protein. Little is known about the structure of the N-terminal half of Mid1p, so we investigated its physical properties using structure prediction tools, spectroscopic techniques, and hydrodynamic measurements. The data indicate that Mid1p-N452 is intrinsically disordered but moderately compact. Recombinant Mid1p-N452 purified from insect cells was phosphorylated, which weakens its tendency to aggregate. Purified Mid1p-N452 demixes into liquid droplets at concentrations far below its concentration in nodes. These physical properties are appropriate for scaffolding other proteins in nodes.

Identification and Characterization of the Anillin Gene in Silkworm

Anillin is an actin binding protein and plays crucial roles during mitotic cell cycle progression in metazoan. However, the sequence and functions of the Anillin gene have not been yet characterized in the silkworm, Bombyx mori. In this study, we cloned the full-length cDNA sequence of the silkworm Anillin (BmAnillin) gene. The deduced amino acid sequence for BmAnillin protein comprises an Anillin homology region (AHR) covering an Anillin homology domain and a pleckstrin homology domain. Phylogenetic analysis and multiple alignments of the Anillin genes from silkworm and other organisms indicated evolutionary conservation in the AHR containing conserved phosphorylation sites. Reverse transcription-PCR experiments confirmed that the BmAnillin gene was highly expressed during larval development of gonads in which cells undergo mitotic cycles and exhibited an unexpected high expression in silk gland with endocycle during larval molting. RNA interference-mediated knockdown of the BmAnillin gene in silkworm BmN4-SID1 cells derived from ovary disrupted chromosome separation and resulted in a loss of the F-actin filament at cleavage furrow during anaphase, suggesting that the BmAnillin gene is essential for cytokinesis in silkworm.

Molecular organization of cytokinesis nodes and contractile rings by super-resolution fluorescence microscopy of live fission yeast

Cytokinesis in animals, fungi, and amoebas depends on the constriction of a contractile ring built from a common set of conserved proteins. Many fundamental questions remain about how these proteins organize to generate the necessary tension for cytokinesis. Using quantitative high-speed fluorescence photoactivation localization microscopy (FPALM), we probed this question in live fission yeast cells at unprecedented resolution. We show that nodes, protein assembly precursors to the contractile ring, are discrete structural units with stoichiometric ratios and distinct distributions of constituent proteins. Anillin Mid1p, Fes/CIP4 homology-Bin/amphiphysin/Rvs (F-BAR) Cdc15p, IQ motif containing GTPase-activating protein (IQGAP) Rng2p, and formin Cdc12p form the base of the node that anchors the ends of myosin II tails to the plasma membrane, with myosin II heads extending into the cytoplasm. This general node organization persists in the contractile ring where nodes move bidirectionally during constriction. We observed the dynamics of the actin network during cytokinesis, starting with the extension of short actin strands from nodes, which sometimes connected neighboring nodes. Later in cytokinesis, a broad network of thick bundles coalesced into a tight ring around the equator of the cell. The actin ring was ∼125 nm wide and ∼125 nm thick. These observations establish the organization of the proteins in the functional units of a cytokinetic contractile ring.

The F-actin bundler α-actinin Ain1 is tailored for ring assembly and constriction during cytokinesis in fission yeast

The actomyosin contractile ring is a network of cross-linked actin filaments that facilitates cytokinesis in dividing cells. Contractile ring formation has been well characterized in Schizosaccharomyces pombe, in which the cross-linking protein α-actinin SpAin1 bundles the actin filament network. However, the specific biochemical properties of SpAin1 and whether they are tailored for cytokinesis are not known. Therefore we purified SpAin1 and quantified its ability to dynamically bind and bundle actin filaments in vitro using a combination of bulk sedimentation assays and direct visualization by two-color total internal reflection fluorescence microscopy. We found that, while SpAin1 bundles actin filaments of mixed polarity like other α-actinins, SpAin1 has lower bundling activity and is more dynamic than human α-actinin HsACTN4. To determine whether dynamic bundling is important for cytokinesis in fission yeast, we created the less dynamic bundling mutant SpAin1(R216E). We found that dynamic bundling is critical for cytokinesis, as cells expressing SpAin1(R216E) display disorganized ring material and delays in both ring formation and constriction. Furthermore, computer simulations of initial actin filament elongation and alignment revealed that an intermediate level of cross-linking best facilitates filament alignment. Together our results demonstrate that dynamic bundling by SpAin1 is important for proper contractile ring formation and constriction.

Molecular control of fission yeast cytokinesis

Cytokinesis gives rise to two independent daughter cells at the end of the cell division cycle. The fission yeast Schizosaccharomyces pombe has emerged as one of the most powerful systems to understand how cytokinesis is controlled molecularly. Like in most eukaryotes, fission yeast cytokinesis depends on an acto-myosin based contractile ring that assembles at the division site under the control of spatial cues that integrate information on cell geometry and the position of the mitotic apparatus. Cytokinetic events are also tightly coordinated with nuclear division by the cell cycle machinery. These spatial and temporal regulations ensure an equal cleavage of the cytoplasm and an accurate segregation of the genetic material in daughter cells. Although this model system has specificities, the basic mechanisms of contractile ring assembly and function deciphered in fission yeast are highly valuable to understand how cytokinesis is controlled in other organisms that rely on a contractile ring for cell division.

Comparative biology of cell division in the fission yeast clade

Cytokinesis must be regulated in time and space in order to preserve genome integrity during cell proliferation and to allow daughter cells to adopt distinct fates and geometries during differentiation. The fission yeast Schizosaccharomyces pombe has been a popular model organism for understanding spatiotemporal regulation of cytokinesis in a symmetrically dividing cell. Recent work on another member of the same genus, Schisozaccharomyces japonicus, suggests that S. pombe may have evolved an unusual division site placement mechanism based on a recently duplicated anillin paralog. Here we discuss an extraordinary evolutionary plasticity of cytokinesis within the fission yeast clade and argue that the comparative cell biology approach may provide functional insights beyond those afforded by scrutinizing individual model species.

Molecular control of the Wee1 regulatory pathway by the SAD kinase Cdr2

Cell growth and division are tightly coordinated to maintain cell size constant during successive cell cycles. In Schizosaccharomyces pombe, the SAD kinase Cdr2 regulates the cell size at division and the positioning of the division plane. Cdr2 forms nodes on the medial cortex containing factors that constitute an inhibitory pathway for Wee1. This pathway is regulated by polar gradients of the DYRK kinase Pom1, and involves a direct inhibitor of Wee1, the SAD kinase Cdr1. Cdr2 also interacts with the anillin Mid1, which defines the division plane, and with additional components of the medial cortical nodes, including Blt1, which participate in the mitotic-promoting and cytokinetic functions of nodes. Here, we show that the interaction of Cdr2 with Wee1 and Mid1 requires the UBA domain of Cdr2, which is necessary for its kinase activity. In contrast, Cdr1 associates with the C-terminus of Cdr2, which is composed of basic and KA-1 lipid-binding domains. Mid1 also interacts with the C-terminus of Cdr2 and might bridge the N- and C-terminal domains, whereas Blt1 associates with the central spacer region. We propose that the association of Cdr2 effectors with different domains might constrain Cdr1 and Wee1 spatially to promote Wee1 inhibition upon Cdr2 kinase activation.

Mechanistic insights into the anchorage of the contractile ring by anillin and Mid1

Anillins and Mid1 are scaffold proteins that play key roles in anchorage of the contractile ring at the cell equator during cytokinesis in animals and fungi, respectively. Here, we report crystal structures and functional analysis of human anillin and S. pombe Mid1. The combined data show anillin contains a cryptic C2 domain and a Rho-binding domain. Together with the tethering PH domain, three membrane-associating elements synergistically bind to RhoA and phospholipids to anchor anillin at the cleavage furrow. Surprisingly, Mid1 also binds to the membrane through a cryptic C2 domain. Dimerization of Mid1 leads to high affinity and preference for PI(4,5)P2, which stably anchors Mid1 at the division plane, bypassing the requirement for Rho GTPase. These findings uncover the unexpected general machinery and the divergent regulatory logics for the anchorage of the contractile ring through the anillin/Mid1 family proteins from yeast to humans.

The Rho-GEF Gef3 interacts with the septin complex and activates the GTPase Rho4 during fission yeast cytokinesis

Rho GTPases, activated by Rho guanine nucleotide exchange factors (GEFs), are conserved molecular switches for signal transductions that regulate diverse cellular processes, including cell polarization and cytokinesis. The fission yeast Schizosaccharomyces pombe has six Rho GTPases (Cdc42 and Rho1-Rho5) and seven Rho GEFs (Scd1, Rgf1-Rgf3, and Gef1-Gef3). The GEFs for Rho2-Rho5 have not been unequivocally assigned. In particular, Gef3, the smallest Rho GEF, was barely studied. Here we show that Gef3 colocalizes with septins at the cell equator. Gef3 physically interacts with septins and anillin Mid2 and depends on them to localize. Gef3 coprecipitates with GDP-bound Rho4 in vitro and accelerates nucleotide exchange of Rho4, suggesting that Gef3 is a GEF for Rho4. Consistently, Gef3 and Rho4 are in the same genetic pathways to regulate septum formation and/or cell separation. In gef3∆ cells, the localizations of two potential Rho4 effectors--glucanases Eng1 and Agn1--are abnormal, and active Rho4 level is reduced, indicating that Gef3 is involved in Rho4 activation in vivo. Moreover, overexpression of active Rho4 or Eng1 rescues the septation defects of mutants containing gef3∆. Together our data support that Gef3 interacts with the septin complex and activates Rho4 GTPase as a Rho GEF for septation in fission yeast.

Fission yeast profilin is tailored to facilitate actin assembly by the cytokinesis formin Cdc12

The evolutionarily conserved small actin-monomer binding protein profilin is believed to be a housekeeping factor that maintains a general pool of unassembled actin. However, despite similar primary sequences, structural folds, and affinities for G-actin and poly-L-proline, budding yeast profilin ScPFY fails to complement fission yeast profilin SpPRF temperature-sensitive mutant cdc3-124 cells. To identify profilin's essential properties, we built a combinatorial library of ScPFY variants containing either WT or SpPRF residues at multiple positions and carried out a genetic selection to isolate variants that support life in fission yeast. We subsequently engineered ScPFY(9-Mut), a variant containing nine substitutions in the actin-binding region, which complements cdc3-124 cells. ScPFY(9-Mut), but not WT ScPFY, suppresses severe cytokinesis defects in cdc3-124 cells. Furthermore, the major activity rescued by ScPFY(9-Mut) is the ability to enhance cytokinesis formin Cdc12-mediated actin assembly in vitro, which allows cells to assemble functional contractile rings. Therefore an essential role of profilin is to specifically facilitate formin-mediated actin assembly for cytokinesis in fission yeast.

Pom1 regulates the assembly of Cdr2-Mid1 cortical nodes for robust spatial control of cytokinesis

Pom1 regulation of Cdr2 membrane association and interaction with Mid1 prevents Cdr2 assembly into stable nodes in the cell tip region, which ensures proper positioning of cytokinetic ring precursors and accurate division plane positioning in fission yeast.

Proper division plane positioning is essential to achieve faithful DNA segregation and to control daughter cell size, positioning, or fate within tissues. In Schizosaccharomyces pombe, division plane positioning is controlled positively by export of the division plane positioning factor Mid1/anillin from the nucleus and negatively by the Pom1/DYRK (dual-specificity tyrosine-regulated kinase) gradients emanating from cell tips. Pom1 restricts to the cell middle cortical cytokinetic ring precursor nodes organized by the SAD-like kinase Cdr2 and Mid1/anillin through an unknown mechanism. In this study, we show that Pom1 modulates Cdr2 association with membranes by phosphorylation of a basic region cooperating with the lipid-binding KA-1 domain. Pom1 also inhibits Cdr2 interaction with Mid1, reducing its clustering ability, possibly by down-regulation of Cdr2 kinase activity. We propose that the dual regulation exerted by Pom1 on Cdr2 prevents Cdr2 assembly into stable nodes in the cell tip region where Pom1 concentration is high, which ensures proper positioning of cytokinetic ring precursors at the cell geometrical center and robust and accurate division plane positioning.

Cytokinetic nodes in fission yeast arise from two distinct types of nodes that merge during interphase

Two distinct classes of cortical nodes form separately during interphase in fission yeast cells and then merge at the cell equator by a diffuse-and-capture mechanism to prepare nodes to form the contractile ring for cytokinesis.

We investigated the assembly of cortical nodes that generate the cytokinetic contractile ring in fission yeast. Observations of cells expressing fluorescent fusion proteins revealed two types of interphase nodes. Type 1 nodes containing kinase Cdr1p, kinase Cdr2p, and anillin Mid1p form in the cortex around the nucleus early in G2. Type 2 nodes with protein Blt1p, guanosine triphosphate exchange factor Gef2p, and kinesin Klp8p emerge from contractile ring remnants. Quantitative measurements and computer simulations showed that these two types of nodes come together by a diffuse-and-capture mechanism: type 2 nodes diffuse to the equator and are captured by stationary type 1 nodes. During mitosis, cytokinetic nodes with Mid1p and all of the type 2 node markers incorporate into the contractile ring, whereas type 1 nodes with Cdr1p and Cdr2p follow the separating nuclei before dispersing into the cytoplasm, dependent on septation initiation network signaling. The two types of interphase nodes follow parallel branches of the pathway to prepare nodes for cytokinesis.

Cooperation between Rho-GEF Gef2 and its binding partner Nod1 in the regulation of fission yeast cytokinesis

Previous results showed that putative Rho-GEF Gef2 regulates division-site positioning during early cytokinesis in fission yeast. Here Nod1 is identified as a binding partner of Gef2. The two proteins form a complex to regulate division-site positioning and contractile-ring maintenance. In addition, Gef2 binds to GTPases Rho1, Rho4, and Rho5 in vitro.

Cytokinesis is the last step of the cell-division cycle, which requires precise spatial and temporal regulation to ensure genetic stability. Rho guanine nucleotide exchange factors (Rho GEFs) and Rho GTPases are among the key regulators of cytokinesis. We previously found that putative Rho-GEF Gef2 coordinates with Polo kinase Plo1 to control the medial cortical localization of anillin-like protein Mid1 in fission yeast. Here we show that an adaptor protein, Nod1, colocalizes with Gef2 in the contractile ring and its precursor cortical nodes. Like gef2∆, nod1∆ has strong genetic interactions with various cytokinesis mutants involved in division-site positioning, suggesting a role of Nod1 in early cytokinesis. We find that Nod1 and Gef2 interact through the C-termini, which is important for their localization. The contractile-ring localization of Nod1 and Gef2 also depends on the interaction between Nod1 and the F-BAR protein Cdc15, where the Nod1/Gef2 complex plays a role in contractile-ring maintenance and affects the septation initiation network. Moreover, Gef2 binds to purified GTPases Rho1, Rho4, and Rho5 in vitro. Taken together, our data indicate that Nod1 and Gef2 function cooperatively in a protein complex to regulate fission yeast cytokinesis.

Anillin-related protein Mid1p coordinates the assembly of the cytokinetic contractile ring in fission yeast

Anillin-like protein Mid1p coordinates contractile ring assembly in fission yeast by restricting precursors in nodes around the equator. Without Mid1p, contractile ring assembly is slow and unreliable because ring precursors are separated in nodes (Blt1p, Cdc12p) or strands (myosin-II, Rng2p, Cdc15p, actin filaments) scattered widely in the cortex.

In fission yeast cells cortical nodes containing the protein Blt1p and several kinases appear early in G2, mature into cytokinetic nodes by adding anillin Mid1p, myosin-II, formin Cdc12p, and other proteins, and condense into a contractile ring by movements that depend on actin and myosin-II. Previous studies concluded that cells without Mid1p lack cytokinetic nodes and assemble rings unreliably from myosin-II strands but left open questions. Why do strands form outside the equatorial region? Why is ring assembly unreliable without Mid1p? We found in Δmid1 cells that Cdc12p accumulates in cytokinetic nodes scattered in the cortex and produces actin filaments that associate with myosin-II, Rng2p, and Cdc15p to form strands located between the nodes. Strands incorporate nodes, and in ∼67% of cells, strands slowly close into rings that constrict without the normal ∼25-min maturation period. Ring assembly is unreliable and slow without Mid1p because the scattered Cdc12p nodes generate strands spread widely beyond the equator, and growing strands depend on random encounters to merge with other strands into a ring. We conclude that orderly assembly of the contractile ring in wild-type cells depends on Mid1p to recruit myosin-II, Rng2p, and Cdc15p to nodes and to place cytokinetic nodes around the cell equator.