A high-resolution shape fitting and simulation demonstrated equatorial cell surface softening during cytokinesis and its promotive role in cytokinesis

Different models for animal cell cytokinesis posit that the stiffness of the equatorial cortex is either increased or decreased relative to the stiffness of the polar cortex. A recent work has suggested that the critical cytokinesis signaling complex centralspindlin may reduce the stiffness of the equatorial cortex by inactivating the small GTPase Rac. To determine if such a reduction occurs and if it depends on centralspindlin, we devised a method to estimate cortical bending stiffness with high spatio-temporal resolution from in vivo cell shapes. Using the early Caenorhabditis elegans embryo as a model, we show that the stiffness of the equatorial cell surface is reduced during cytokinesis, whereas the stiffness of the polar cell surface remains stiff. The equatorial reduction of stiffness was compromised in cells with a mutation in the gene encoding the ZEN-4/kinesin-6 subunit of centralspindlin. Theoretical modeling showed that the absence of the equatorial reduction of stiffness could explain the arrest of furrow ingression in the mutant. By contrast, the equatorial reduction of stiffness was sufficient to generate a cleavage furrow even without the constriction force of the contractile ring. In this regime, the contractile ring had a supportive contribution to furrow ingression. We conclude that stiffness is reduced around the equator in a centralspindlin-dependent manner. In addition, computational modeling suggests that proper regulation of stiffness could be sufficient for cleavage furrow ingression.     

KLIF-associated cytoskeletal proteins in Trypanosoma brucei regulate cytokinesis by promoting cleavage furrow positioning and ingression

Cytokinesis in the early divergent protozoan Trypanosoma brucei occurs from the anterior cell tip of the new-flagellum daughter toward the nascent posterior end of the old-flagellum daughter of a dividing biflagellated cell. The cleavage furrow ingresses unidirectionally along the preformed cell division fold and is regulated by an orphan kinesin named kinesin localized to the ingressing furrow (KLIF) that localizes to the leading edge of the ingressing furrow. Little is known about how furrow ingression is controlled by KLIF and whether KLIF interacts with and cooperates with other cytokinesis regulatory proteins to promote furrow ingression. Here, we investigated the roles of KLIF in cleavage furrow ingression and identified a cohort of KLIF-associated cytoskeletal proteins as essential cytokinesis regulators. By genetic complementation, we demonstrated the requirement of the kinesin motor activity, but not the putative tropomyosin domain, of KLIF in promoting furrow ingression. We further showed that depletion of KLIF impaired the resolution of the nascent posterior of the old-flagellar daughter cell, thereby stalking cleavage furrow ingression at late stages of cytokinesis. Through proximity biotinylation, we identified a subset of cytoskeleton-associated proteins (CAPs) as KLIF-proximal proteins, and functional characterization of these cytoskeletal proteins revealed the essential roles of CAP46 and CAP52 in positioning the cleavage furrow and the crucial roles of CAP42 and CAP50 in promoting cleavage furrow ingression. Together, these results identified multiple cytoskeletal proteins as cytokinesis regulators and uncovered their essential and distinct roles in cytokinesis.     

Interactions between myosin and actin crosslinkers control cytokinesis contractility dynamics and mechanics

INTRODUCTION: Contractile networks are fundamental to many cellular functions, particularly cytokinesis and cell motility. Contractile networks depend on myosin-II mechanochemistry to generate sliding force on the actin polymers. However, to be contractile, the networks must also be crosslinked by crosslinking proteins, and to change the shape of the cell, the network must be linked to the plasma membrane. Discerning how this integrated network operates is essential for understanding cytokinesis contractility and shape control. Here, we analyzed the cytoskeletal network that drives furrow ingression in Dictyostelium. RESULTS: We establish that the actin polymers are assembled into a meshwork and that myosin-II does not assemble into a discrete ring in the Dictyostelium cleavage furrow of adherent cells. We show that myosin-II generates regional mechanics by increasing cleavage furrow stiffness and slows furrow ingression during late cytokinesis as compared to myoII nulls. Actin crosslinkers dynacortin and fimbrin similarly slow furrow ingression and contribute to cell mechanics in a myosin-II-dependent manner. By using FRAP, we show that the actin crosslinkers have slower kinetics in the cleavage furrow cortex than in the pole, that their kinetics differ between wild-type and myoII null cells, and that the protein dynamics of each crosslinker correlate with its impact on cortical mechanics. CONCLUSIONS: These observations suggest that myosin-II along with actin crosslinkers establish local cortical tension and elasticity, allowing for contractility independent of a circumferential cytoskeletal array. Furthermore, myosin-II and actin crosslinkers may influence each other as they modulate the dynamics and mechanics of cell-shape change.     

The actin-binding and bundling protein,EPLIN, is required for cytokinesis

Cytokinesis involves two phases: 1) membrane ingression followed by 2) membrane abscission. The ingression phase generates a cleavage furrow and this requires co-operative function of the actin-myosin II contractile ring and septin filaments. We demonstrate that the actin-binding protein, EPLIN, locates to the cleavage furrow during cytokinesis and this is possibly via association with the contractile ring components, myosin II, and the septin, Sept2. Depletion of EPLIN results in formation of multinucleated cells and this is associated with inefficient accumulation of active myosin II (MRLCS19) and Sept2 and their regulatory small GTPases, RhoA and Cdc42, respectively, to the cleavage furrow during the final stages of cytokinesis. We suggest that EPLIN may function during cytokinesis to maintain local accumulation of key cytokinesis proteins at the furrow.     

Furrow Constriction in Animal Cell Cytokinesis

Cytokinesis is the process of physical cleavage at the end of cell division; it proceeds by ingression of an acto-myosin furrow at the equator of the cell. Its failure leads to multinucleated cells and is a possible cause of tumorigenesis. Here, we calculate the full dynamics of furrow ingression and predict cytokinesis completion above a well-defined threshold of equatorial contractility. The cortical acto-myosin is identified as the main source of mechanical dissipation and active forces. Thereupon, we propose a viscous active nonlinear membrane theory of the cortex that explicitly includes actin turnover and where the active RhoA signal leads to an equatorial band of myosin overactivity. The resulting cortex deformation is calculated numerically, and reproduces well the features of cytokinesis such as cell shape and cortical flows toward the equator. Our theory gives a physical explanation of the independence of cytokinesis duration on cell size in embryos. It also predicts a critical role of turnover on the rate and success of furrow constriction. Scaling arguments allow for a simple interpretation of the numerical results and unveil the key mechanism that generates the threshold for cytokinesis completion: cytoplasmic incompressibility results in a competition between the furrow line tension and the cell poles’ surface tension.     

Furrow Constriction in Animal Cell Cytokinesis

Cytokinesis is the process of physical cleavage at the end of cell division; it proceeds by ingression of an acto-myosin furrow at the equator of the cell. Its failure leads to multinucleated cells and is a possible cause of tumorigenesis. Here, we calculate the full dynamics of furrow ingression and predict cytokinesis completion above a well-defined threshold of equatorial contractility. The cortical acto-myosin is identified as the main source of mechanical dissipation and active forces. Thereupon, we propose a viscous active nonlinear membrane theory of the cortex that explicitly includes actin turnover and where the active RhoA signal leads to an equatorial band of myosin overactivity. The resulting cortex deformation is calculated numerically, and reproduces well the features of cytokinesis such as cell shape and cortical flows toward the equator. Our theory gives a physical explanation of the independence of cytokinesis duration on cell size in embryos. It also predicts a critical role of turnover on the rate and success of furrow constriction. Scaling arguments allow for a simple interpretation of the numerical results and unveil the key mechanism that generates the threshold for cytokinesis completion: cytoplasmic incompressibility results in a competition between the furrow line tension and the cell poles’ surface tension.     

Endomitotic Megakaryocytes that Form a Bipolar Spindle Exhibit Cleavage Furrow Ingression Followed by Furrow Regression

Megakaryocyte (MK) differentiation is marked by the development of progressive polyploidy, due to repeated incomplete cell cycles in which mitosis is aborted during anaphase, a process termed endomitosis. We have postulated that anaphase in endomitotic MKs diverges from diploid mitosis at a point distal to the assembly of the midzone, possibly involving impaired cleavage furrow progression. To define the extent of furrow initiation and ingression in endomitosis, we performed time-lapse imaging of MKs expressing yellow fluorescent protein (YFP)-tubulin and monitored shape change as they progressed through anaphase. We found that in early endomitotic cells that have a bipolar spindle, cleavage furrows form that can undergo significant ingression, but furrows regress to produce polyploid cells. Compared to cells that divide, cells that exhibit furrow regression have a slower rate of furrow ingression and do not furrow as deeply. More highly polyploid MKs undergoing additional endomitotic cycles also show measurable furrowing that is followed by regression, but the magnitude of the shape change is less than seen in the early MKs. This suggests that in the earliest endomitotic cycles when there is formation of a bipolar spindle, the failure of cytokinesis occurs late, following assembly and initial constriction of the actin/myosin ring, whereas in endomitotic MKs that are already polyploid there is secondary inhibition of furrow progression. This behavior of furrow ingression followed by regression may explain why midbody remnants are occasionally observed in polyploid MKs. This finding has important implications for the potential mechanisms for cytokinesis failure in endomitosis.     

Polo-like kinase 1 triggers the initiation of cytokinesis in human cells by promoting recruitment of the RhoGEF Ect2 to the central spindle

Cytokinesis of animal cells requires ingression of the actomyosin-based contractile ring between segregated sister genomes. Localization of the RhoGEF Ect2 to the central spindle at anaphase promotes local activation of the RhoA GTPase, which induces assembly and ingression of the contractile ring. Here we have used BI 2536, an inhibitor of the mitotic kinase Plk1, to analyze the functions of this enzyme during late mitosis in human cells. We show that Plk1 acts after Cdk1 inactivation and independently from Aurora B to promote RhoA accumulation at the equator, contractile ring formation, and cleavage furrow ingression. Inhibition of Plk1 abolishes the interaction of Ect2 with its activator and midzone anchor, HsCyk-4, thereby preventing localization of Ect2 to the central spindle. We propose that late mitotic Plk1 activity promotes recruitment of Ect2 to the central spindle, triggering the initiation of cytokinesis and contributing to cleavage plane specification in human cells.     

Targeting Aurora B to the Equatorial Cortex by MKlp2 Is Required for Cytokinesis

Although Aurora B is important in cleavage furrow ingression and completion during cytokinesis, the mechanism by which kinase activity is targeted to the cleavage furrow and the molecule(s) responsible for this process have remained elusive. Here, we demonstrate that an essential mitotic kinesin MKlp2 requires myosin-II for its localization to the equatorial cortex, and this event is required to recruit Aurora B to the equatorial cortex in mammalian cells. This recruitment event is also required to promote the highly focused accumulation of active RhoA at the equatorial cortex and stable ingression of the cleavage furrow in bipolar cytokinesis. Specifically, in drug-induced monopolar cytokinesis, targeting Aurora B to the cell cortex by MKlp2 is essential for cell polarization and furrow formation. Once the furrow has formed, MKlp2 further recruits Aurora B to the growing furrow. This process together with continuous Aurora B kinase activity at the growing furrow is essential for stable furrow propagation and completion. In contrast, a MKlp2 mutant defective in binding myosin-II does not recruit Aurora B to the cell cortex and does not promote furrow formation during monopolar cytokinesis. This mutant is also defective in maintaining the ingressing furrow during bipolar cytokinesis. Together, these findings reveal that targeting Aurora B to the cell cortex (or the equatorial cortex) by MKlp2 is essential for the maintenance of the ingressing furrow for successful cytokinesis.     

On the mechanism of cleavage furrow ingression in Dictyostelium

The ability of Dictyostelium cells to divide without myosin II in a cell cycle-coupled manner has opened two questions about the mechanism of cleavage furrow ingression. First, are there other possible functions for myosin II in this process except for generating contraction of the furrow by a sliding filament mechanism? Second, what could be an alternative mechanical basis for the furrowing? Using aberrant changes of the cell shape and anomalous localization of the actin-binding protein cortexillin I during asymmetric cytokinesis in myosin II-deficient cells as clues, it is proposed that myosin II filaments act as a mechanical lens in cytokinesis. The mechanical lens serves to focus the forces that induce the furrowing to the center of the midzone, a cortical region where cortexillins are enriched in dividing cells. Additionally, continual disassembly of a filamentous actin meshwork at the midzone is a prerequisite for normal ingression of the cleavage furrow and a successful cytokinesis. If this process is interrupted, as it occurs in cells that lack cortexillins, an overassembly of filamentous actin at the midzone obstructs the normal cleavage. Disassembly of the crosslinked actin network can generate entropic contractile forces in the cortex, and may be considered as an alternative mechanism for driving ingression of the cleavage furrow. Instead of invoking different types of cytokinesis that operate under attached and unattached conditions in Dictyostelium, it is anticipated that these cells use a universal multifaceted mechanism to divide, which is only moderately sensitive to elimination of its constituent mechanical processes.     

A Highlights from MBoC Selection: Calcium spikes accompany cleavage furrow ingression and cell separation during fission yeast cytokinesis

The role of calcium signaling in cytokinesis has long remained ambiguous. Past studies of embryonic cell division discovered that calcium concentration increases transiently at the division plane just before cleavage furrow ingression, suggesting that these calcium transients could trigger contractile ring constriction. However, such calcium transients have only been found in animal embryos and their function remains controversial. We explored cytokinetic calcium transients in the fission yeast Schizosaccharomyces pombe by adopting GCaMP, a genetically encoded calcium indicator, to determine the intracellular calcium level of this model organism. We validated GCaMP as a highly sensitive calcium reporter in fission yeast, allowing us to capture calcium transients triggered by osmotic shocks. We identified a correlation between the intracellular calcium level and cell division, consistent with the existence of calcium transients during cytokinesis. Using time-lapse microscopy and quantitative image analysis, we discovered calcium spikes both at the start of cleavage furrow ingression and the end of cell separation. Inhibition of these calcium spikes slowed the furrow ingression and led to frequent lysis of daughter cells. We conclude that like the larger animal embryos, fission yeast triggers calcium transients that may play an important role in cytokinesis (197).     

Localization and activation of the ARF6 GTPase during cleavage furrow ingression and cytokinesis

The ARF6 GTPase mediates cell shape changes in interphase cells through its effects on membrane cycling and actin remodeling. In this study, we focus our attention on the dynamics of cell division and present evidence supporting a novel role for ARF6 during cleavage furrow ingression and cytokinesis. We demonstrate that endogenous ARF6 redistributes during mitosis and concentrates near the cleavage furrow during telophase. Constitutively activated ARF6 localizes to the plasma membrane at the site of cleavage furrow ingression and midbody formation, and dominant negative ARF6 remains cytoplasmic. By using a novel pull-down assay for ARF6-GTP, we find an abrupt, but transient, increase in ARF6-GTP levels as cells progress through cytokinesis. Whereas high levels of expression of a GTPase-defective ARF6 mutant induce aberrant phenotypes in cells at cytokinesis, cells expressing low levels of ARF6 mutants do not display a significant mitotic delay or cytokinesis defect, presumably due to compensatory or redundant mechanisms that allow cytokinesis to proceed when the ARF6 GTPase cycle is disrupted. Finally, actin accumulation and phospholipid metabolism at the cleavage furrow are unchanged in cells expressing ARF6 mutants, suggesting that ARF6 may be involved in membrane remodeling during cytokinesis via effector pathways that are distinct from those operative in interphase cells.     

Anillin is a scaffold protein that links RhoA, actin, and myosin during cytokinesis

Cell division after mitosis is mediated by ingression of an actomyosin-based contractile ring. The active, GTP-bound form of the small GTPase RhoA is a key regulator of contractile-ring formation. RhoA concentrates at the equatorial cell cortex at the site of the nascent cleavage furrow. During cytokinesis, RhoA is activated by its RhoGEF, ECT2. Once activated, RhoA promotes nucleation, elongation, and sliding of actin filaments through the coordinated activation of both formin proteins and myosin II motors (reviewed in [1, 2]). Anillin is a 124 kDa protein that is highly concentrated in the cleavage furrow in numerous animal cells in a pattern that resembles that of RhoA [3-7]. Although anillin contains conserved N-terminal actin and myosin binding domains and a PH domain at the C terminus, its mechanism of action during cytokinesis remains unclear. Here, we show that human anillin contains a conserved C-terminal domain that is essential for its function and localization. This domain shares homology with the RhoA binding protein Rhotekin and directly interacts with RhoA. Further, anillin is required to maintain active myosin in the equatorial plane during cytokinesis, suggesting it functions as a scaffold protein to link RhoA with the ring components actin and myosin. Although furrows can form and initiate ingression in the absence of anillin, furrows cannot form in anillin-depleted cells in which the central spindle is also disrupted, revealing that anillin can also act at an early stage of cytokinesis.     

Dynamics of membrane clathrin-coated structures during cytokinesis

Remodeling of cell membranes takes place during motile processes such as cell migration and cell division. Defects of proteins involved in membrane dynamics, including clathrin and dynamin, disrupt cytokinesis. To understand the function of clathrin-containing structures (CCS) in cytokinesis, we have expressed a green fluorescent protein (GFP) fusion protein of clathrin light chain a (GFP-clathrin) in NRK epithelial cells and recorded images of dividing cells near the ventral surface with a spinning disk confocal microscope. Punctate GFP-CCS underwent dynamic appearance and disappearance throughout the ventral surface. Following anaphase onset, GFP-CCS between separated chromosomes migrated toward the equator and subsequently disappeared in the equatorial region. Movements outside separating chromosomes were mostly random, similar to what was observed in interphase cells. Directional movements toward the furrow were dependent on both actin filaments and microtubules, while the appearance/disappearance of CCS was dependent on actin filaments but not on microtubules. These results suggest that CCS are involved in remodeling the plasma membrane along the equator during cytokinesis. Clathrin-containing structures may also play a role in transporting signaling or structural components into the cleavage furrow.     

Functional analysis of a human homologue of the Drosophila actin binding protein anillin suggests a role in cytokinesis

We have characterized a human homologue of anillin, a Drosophila actin binding protein. Like Drosophila anillin, the human protein localizes to the nucleus during interphase, the cortex following nuclear envelope breakdown, and the cleavage furrow during cytokinesis. Anillin also localizes to ectopic cleavage furrows generated between two spindles in fused PtK(1) cells. Microinjection of antianillin antibodies slows cleavage, leading to furrow regression and the generation of multinucleate cells. GFP fusions that contain the COOH-terminal 197 amino acids of anillin, which includes a pleckstrin homology (PH) domain, form ectopic cortical foci during interphase. The septin Hcdc10 localizes to these ectopic foci, whereas myosin II and actin do not, suggesting that anillin interacts with the septins at the cortex. Robust cleavage furrow localization requires both this COOH-terminal domain and additional NH(2)-terminal sequences corresponding to an actin binding domain defined by in vitro cosedimentation assays. Endogenous anillin and Hcdc10 colocalize to punctate foci associated with actin cables throughout mitosis and the accumulation of both proteins at the cell equator requires filamentous actin. These results indicate that anillin is a conserved cleavage furrow component important for cytokinesis. Interactions with at least two other furrow proteins, actin and the septins, likely contribute to anillin function.     

A Novel Guanine Nucleotide Exchange Factor,MYOGEF, is Required for Cytokinesis

The cleavage furrow is created by an actomyosin contractile ring that isregulated by small GTPase proteins such as Rac1 and RhoA. Guanine nucleotideexchange factors (GEFs) are positive regulators of the small GTPase proteins andhave been implicated as important factors in regulating cytokinesis. However, it isstill unclear how GEFs regulate the contractile ring during cytokinesis inmammalian cells. Here we report that a novel GEF, which is termed MyoGEF(myosin-interacting GEF), interacts with nonmuscle myosin II and exhibits activitytoward RhoA. MyoGEF and nonmuscle myosin II colocalize to the cleavage furrowin early anaphase cells. Disruption of MyoGEF expression in U2OS cells by RNAinterference (RNAi) results in the formation of multinucleated cells. These resultssuggest that MyoGEF, RhoA, and nonmuscle myosin II act as a functional unit atthe cleavage furrow to advance furrow ingression during cytokinesis.     

A role for sphingomyelin-rich lipid domains in the accumulation of phosphatidylinositol-4,5-bisphosphate to the cleavage furrow during cytokinesis

Cytokinesis is a crucial step in the creation of two daughter cells by the formation and ingression of the cleavage furrow. Here, we show that sphingomyelin (SM), one of the major sphingolipids in mammalian cells, is required for the localization of phosphatidylinositol-4,5-bisphosphate (PIP(2)) to the cleavage furrow during cytokinesis. Real-time observation with a labeled SM-specific protein, lysenin, revealed that SM is concentrated in the outer leaflet of the furrow at the time of cytokinesis. Superresolution fluorescence microscopy analysis indicates a transbilayer colocalization between the SM-rich domains in the outer leaflet and PIP(2)-rich domains in the inner leaflet of the plasma membrane. The depletion of SM disperses PIP(2) and inhibits the recruitment of the small GTPase RhoA to the cleavage furrow, leading to abnormal cytokinesis. These results suggest that the formation of SM-rich domains is required for the accumulation of PIP(2) to the cleavage furrow, which is a prerequisite for the proper translocation of RhoA and the progression of cytokinesis.     

Cytokinesis failure in clathrin-minus cells is caused by cleavage furrow instability

The role of membrane traffic during cell division has only recently begun to be investigated. A growing number of trafficking proteins seem to be involved in the successful completion of cytokinesis. Clathrin was the first trafficking protein to be shown to be essential for cytokinesis in Dictyostelium. Here we investigate the nature of the cytokinesis defect of Dictyostelium clathrin null cells. We found that adherent clathrin null cells do form cleavage furrows but cannot maintain a consistent rate of furrow ingression. Clathrin null cells are completely defective in cytokinesis when placed in suspension. In these conditions, the cells develop an abnormal division morphology that consists of two lateral "furrows" on either side of a bulging equatorial region. Cells expressing GFP-myosin II were examined at various stages of cytokinesis. Clathrin null cells show multiple defects in myosin organization and localization that parallel the striking failure in furrow morphology. We postulate that this morphology is the result of contraction at the rear of the presumptive daughter cells in concert with incomplete furrow ingression.     

Inhibition of cyclin-dependent kinase 1 induces cytokinesis without chromosome segregation in an ECT2 and MgcRacGAP-dependent manner

Cleavage furrow formation marks the onset of cell division during early anaphase. The small GTPase RhoA and its regulators ECT2 and MgcRacGAP have been implicated in furrow ingression in mammalian cells, but the signaling upstream of these molecules remains unclear. We now show that the inhibition of cyclin-dependent kinase (Cdk)1 is sufficient to initiate cytokinesis. When mitotically synchronized cells were treated with the Cdk-specific inhibitor BMI-1026, the initiation of cytokinesis was induced precociously before chromosomal separation. Cytokinesis was also induced by the Cdk1-specific inhibitor purvalanol A but not by Cdk2/Cdk5- or Cdk4-specific inhibitors. Consistent with initiation of precocious cytokinesis by Cdk1 inhibition, introduction of anti-Cdk1 monoclonal antibody resulted in cells with aberrant nuclei. Depolymerization of mitotic spindles by nocodazole inhibited BMI-1026-induced precocious cytokinesis. However, in the presence of a low concentration of nocodazole, BMI-1026 induced excessive membrane blebbing, which appeared to be caused by formation of ectopic cleavage furrows. Depletion of ECT2 or MgcRacGAP by RNA interference abolished both of the phenotypes (precocious furrowing after nocodazole release and excessive blebbing in the presence of nocodazole). RNA interference of RhoA or expression of dominant-negative RhoA efficiently reduced both phenotypes. RhoA was localized at the cleavage furrow or at the necks of blebs. We propose that Cdk1 inactivation is sufficient to activate a signaling pathway leading to cytokinesis, which emanates from mitotic spindles and is regulated by ECT2, MgcRacGAP, and RhoA. Chemical induction of cytokinesis will be a valuable tool to study the initiation mechanism of cytokinesis.     

Augmin shapes the anaphase spindle for efficient cytokinetic furrow ingression and abscission

During anaphase, distinct populations of microtubules (MTs) form by either centrosome-dependent or augmin-dependent nucleation. It remains largely unknown whether these different MT populations contribute distinct functions to cytokinesis. Here we show that augmin-dependent MTs are required for the progression of both furrow ingression and abscission. Augmin depletion reduced the accumulation of anillin, a contractile ring regulator at the cell equator, yet centrosomal MTs were sufficient to mediate RhoA activation at the furrow. This defect in contractile ring organization, combined with incomplete spindle pole separation during anaphase, led to impaired furrow ingression. During the late stages of cytokinesis, astral MTs formed bundles in the intercellular bridge, but these failed to assemble a focused midbody structure and did not establish tight linkage to the plasma membrane, resulting in furrow regression. Thus augmin-dependent acentrosomal MTs and centrosomal MTs contribute to nonredundant targeting mechanisms of different cytokinesis factors, which are required for the formation of a functional contractile ring and midbody.