Centralspindlin regulates ECT2 and RhoA accumulation at the equatorial cortex during cytokinesis

The RhoGAP ARHGAP19 controls cytokinesis and chromosome segregation in T lymphocytes

Small GTP-binding proteins of the Rho family orchestrate the cytoskeleton remodelling events required for cell division. Guanine nucleotide exchange factors (GEFs) and GTPase-activating proteins (GAPs) promote cycling of Rho GTPases between the active GTP-bound and the inactive GDP-bound conformations. We report that ARHGAP19, a previously uncharacterised protein, is predominantly expressed in hematopoietic cells and has an essential role in the division of T lymphocytes. Overexpression of ARHGAP19 in lymphocytes delays cell elongation and cytokinesis. Conversely, silencing of ARHGAP19 or expression of a GAP-deficient mutant induces precocious mitotic cell elongation and cleavage furrow ingression, as well as excessive blebbing. In relation to these phenotypes, we show that ARHGAP19 acts as a GAP for RhoA, and controls recruitment of citron and myosin II to the plasma membrane of mitotic lymphocytes as well as Rock2-mediated phosphorylation of vimentin, which is crucial to maintain the stiffness and shape of lymphocytes. In addition to its effects on cell shape, silencing of ARHGAP19 in lymphocytes also impairs chromosome segregation.

Single cell pattern formation and transient cytoskeletal arrays

A major goal of developmental biology is to explain the emergence of pattern in cell layers, tissues and organs. Developmental biologists now accept that reaction diffusion-based mechanisms are broadly employed in developing organisms to direct pattern formation. Here we briefly consider these mechanisms and then apply some of the concepts derived from them to several processes that occur in single cells: wound repair, yeast budding, and cytokinesis. Two conclusions emerge from this analysis: first, there is considerable overlap at the level of general mechanisms between developmental and single cell pattern formation; second, dynamic structures based on the actin cytoskeleton may be far more ordered than is generally recognized.

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.

Aurora B but not rho/MLCK signaling is required for localization of diphosphorylated myosin II regulatory light chain to the midzone in cytokinesis

Non-muscle myosin II is stimulated by monophosphorylation of its regulatory light chain (MRLC) at Ser19 (1P-MRLC). MRLC diphosphorylation at Thr18/Ser19 (2P-MRLC) further enhances the ATPase activity of myosin II. Phosphorylated MRLCs localize to the contractile ring and regulate cytokinesis as subunits of activated myosin II. Recently, we reported that 2P-MRLC, but not 1P-MRLC, localizes to the midzone independently of myosin II heavy chain during cytokinesis in cultured mammalian cells. However, the mechanism underlying the distinct localization of 1P- and 2P-MRLC during cytokinesis is unknown. Here, we showed that depletion of the Rho signaling proteins MKLP1, MgcRacGAP, or ECT2 inhibited the localization of 1P-MRLC to the contractile ring but not the localization of 2P-MRLC to the midzone. In contrast, depleting or inhibiting a midzone-localizing kinase, Aurora B, perturbed the localization of 2P-MRLC to the midzone but not the localization of 1P-MRLC to the contractile ring. We did not observe any change in the localization of phosphorylated MRLC in myosin light-chain kinase (MLCK)-inhibited cells. Furrow regression was observed in Aurora B- and 2P-MRLC-inhibited cells but not in 1P-MRLC-perturbed dividing cells. Furthermore, Aurora B bound to 2P-MRLC in vitro and in vivo. These results suggest that Aurora B, but not Rho/MLCK signaling, is essential for the localization of 2P-MRLC to the midzone in dividing HeLa cells.

Leukemia-associated RhoGEF (LARG) is a novel RhoGEF in cytokinesis and required for the proper completion of abscission

This study demonstrates a novel and unexpected role in cytokinesis for leukemia-associated RhoGEF (LARG). Depletion of LARG results in delayed abscission, and thus LARG is the first RhoGEF to be implicated in late cytokinesis.

Proper completion of mitosis requires the concerted effort of multiple RhoGEFs. Here we show that leukemia-associated RhoGEF (LARG), a RhoA-specific RGS-RhoGEF, is required for abscission, the final stage of cytokinesis, in which the intercellular membrane is cleaved between daughter cells. LARG colocalizes with α-tubulin at the spindle poles before localizing to the central spindle. During cytokinesis, LARG is condensed in the midbody, where it colocalizes with RhoA. HeLa cells depleted of LARG display apoptosis during cytokinesis with unresolved intercellular bridges, and rescue experiments show that expression of small interfering RNA–resistant LARG prevents this apoptosis. Moreover, live cell imaging of LARG-depleted cells reveals greatly delayed fission kinetics in abscission in which a population of cells with persistent bridges undergoes apoptosis; however, the delayed fission kinetics is rescued by Aurora-B inhibition. The formation of a Flemming body and thinning of microtubules in the intercellular bridge of cells depleted of LARG is consistent with a defect in late cytokinesis, just before the abscission event. In contrast to studies of other RhoGEFs, particularly Ect2 and GEF-H1, LARG depletion does not result in cytokinetic furrow regression nor does it affect internal mitotic timing. These results show that LARG is a novel and temporally distinct RhoGEF required for completion of abscission.

The Aurora-B-mediated phosphorylation of SHCBP1 regulates cytokinetic furrow ingression

Centralspindlin, which is composed of MgcRacGAP and MKLP1, is essential for central spindle formation and cytokinetic furrow ingression. MgcRacGAP utilizes its GAP domain to inactivate Rac1 and induce furrow ingression in mammalian cells. In this report, we present a novel regulatory mechanism for furrowing that is mediated by the phosphorylation of SHC SH2-domain binding protein 1 (SHCBP1), a binding partner of centralspindlin, by Aurora B (AurB). AurB phosphorylates Ser634 of SHCBP1 during mitosis. We generated a phosphorylation site mutant, S634A-SHCBP1, which was prematurely recruited to the central spindle during anaphase and inhibited furrowing. An in vitro GAP assay demonstrated that SHCBP1 can suppress the MgcRacGAP-mediated inactivation of Rac1. In addition, the inhibition of Rac1 activity rescued the furrowing defect induced by S634A-SHCBP1 expression. Thus, AurB phosphorylates SHCBP1 to prevent the premature localization of SHCBP1 to the central spindle and ensures that MgcRacGAP inactivates Rac1 to promote the ingression of the cytokinetic furrow.

The chromosomal passenger complex (CPC): from easy rider to the godfather of mitosis

Successful cell division requires the precise and timely coordination of chromosomal, cytoskeletal and membrane trafficking events. These processes are regulated by the competing actions of protein kinases and phosphatases. Aurora B is one of the most intensively studied kinases. In conjunction with inner centromere protein (INCENP), borealin (also known as Dasra) and survivin it forms the chromosomal passenger complex (CPC). This complex targets to different locations at differing times during mitosis, where it regulates key mitotic events: correction of chromosome-microtubule attachment errors; activation of the spindle assembly checkpoint; and construction and regulation of the contractile apparatus that drives cytokinesis. Our growing understanding of the CPC has seen it develop from a mere passenger riding on the chromosomes to one of the main controllers of mitosis.

Centralspindlin: at the heart of cytokinesis

The final step in the cell cycle is the formation of two genetically identical daughter cells by cytokinesis. At the heart of cytokinesis in animal cells is the centralspindlin complex which is composed of two proteins, a kinesin-like protein, Mitotic kinesin-like protein 1, and a Rho GTPase activating protein (RhoGAP), CYK-4. Through its targeted localization to a narrow region of antiparallel microtubule overlap immediately following chromosome segregation, centralspindlin initiates central spindle assembly. Centralspindlin has several critical functions during cell division including positioning of the division plane, regulation of Rho family GTPases, as well as midbody assembly and abscission. In this review, we will examine the biochemistry of centralspindlin and its multiple functions during cell division. Remarkably, several of its critical functions are somewhat unexpected. Although endowed with motor domains, centralspindlin has an important role in generating stable, antiparallel microtubule bundles. Although it contains a Rho family GAP domain, it has a central role in the activation of RhoA during cytokinesis. Finally, centralspindlin functions as a motor protein complex, as a scaffold protein for key regulators of abscission and as a conventional RhoGAP. Because of these diverse functions, centralspindlin lies at the heart of the cytokinetic mechanism.

Comparing contractile apparatus-driven cytokinesis mechanisms across kingdoms

Cytokinesis is the final stage of the cell cycle during which a cell physically divides into two daughters through the assembly of new membranes (and cell wall in some cases) between the forming daughters. New membrane assembly can either proceed centripetally behind a contractile apparatus, as in the case of prokaryotes, archaea, fungi, and animals or expand centrifugally, as in the case of higher plants. In this article, we compare the mechanisms of cytokinesis in diverse organisms dividing through the use of a contractile apparatus. While an actomyosin ring participates in cytokinesis in almost all centripetally dividing eukaryotes, the majority of bacteria and archaea (except Crenarchaea) divide using a ring composed of the tubulin-related protein FtsZ. Curiously, despite molecular conservation of the division machinery components, division site placement and its cell cycle regulation occur by a variety of unrelated mechanisms even among organisms from the same kingdom. While molecular motors and cytoskeletal polymer dynamics contribute to force generation during eukaryotic cytokinesis, cytoskeletal polymer dynamics alone appears to be sufficient for force generation during prokaryotic cytokinesis. Intriguingly, there are life forms on this planet that appear to lack molecules currently known to participate in cytokinesis and how these cells perform cytokinesis remains a mystery waiting to be unravelled.

Rho GTPases in animal cell cytokinesis: an occupation by the one percent

Rho GTPases are molecular switches that elicit distinct effects on the actomyosin cytoskeleton to accurately promote cytokinesis. Although they represent less than 1% of the human genome, Rho GTPases exert disproportionate control over cell division. Crucial to this master regulatory role is their localized occupation of specific domains of the cell to ensure the assembly of a contractile ring at the proper time and place. RhoA occupies the division plane and is the central positive Rho family regulator of cytokinesis. Rac1 is a negative regulator of cytokinesis and is inactivated within the division plane while active Rac1 occupies the cell poles. Cdc42 regulation during cytokinesis is less studied, but thus far a clear role has only been shown during polar body emission. Here we review what is known about the function of Rho family GTPases during cell division, as well as their upstream regulators and known downstream cytokinetic effectors.

What Drosophila spermatocytes tell us about the mechanisms underlying cytokinesis

Cytokinesis separates the genomic material and organelles of a dividing cell equitably into two physically distinct daughter cells at the end of cell division. This highly choreographed process involves coordinated reorganization and regulated action of the actin and microtubule cytoskeletal systems, an assortment of motor proteins, and membrane trafficking components. Due to their large size, the ease with which exquisite cytological analysis may be performed on them, and the availability of numerous mutants and other genetic tools, Drosophila spermatocytes have provided an excellent system for exploring the mechanistic basis for the temporally programmed and precise spatially localized events of cytokinesis. Mutants defective in male meiotic cytokinesis can be easily identified in forward genetic screens by the production of multinucleate spermatids. In addition, the weak spindle assembly checkpoint in spermatocytes, which causes only a small delay of anaphase onset in the presence of unattached chromosomes, allows investigation of whether gene products required for spindle assembly and chromosome segregation are also involved in cytokinesis. Perhaps due to the large size of spermatocytes and the requirement for two rapid-fire rounds of division without intervening S or growth phases during meiosis, male meiotic mutants have also revealed much about molecular mechanisms underlying new membrane addition during cytokinesis. Finally, cell type-specific differences in the events that set up and complete cytokinesis are emerging from comparison of spermatocytes with cells undergoing mitosis either elsewhere in the organism or in tissue culture.

Centralspindlin and chromosomal passenger complex behavior during normal and Rappaport furrow specification in echinoderm embryos

The chromosomal passenger (CPC) and Centralspindlin complexes are essential for organizing the anaphase central spindle and providing cues that position the cytokinetic furrow between daughter nuclei. However, echinoderm zygotes are also capable of forming "Rappaport furrows" between asters positioned back-to-back without intervening chromosomes. To understand how these complexes contribute to normal and Rappaport furrow formation, we studied the localization patterns of Survivin and mitotic-kinesin-like-protein1 (MKLP1), members respectively of the CPC and the Centralspindlin complex, and the effect of CPC inhibition on cleavage in mono- and binucleate echinoderm zygotes. In zygotes, Survivin initially localized to metaphase chromosomes, upon anaphase onset relocalized to the central spindle and then, together with MKLP1 spread towards the equatorial cortex in an Aurora-dependent manner. Inhibition of Aurora kinase activity resulted in disruption of central spindle organization and furrow regression, although astral microtubule elongation and furrow initiation were normal. In binucleate cells containing two parallel spindles MKLP1 and Survivin localized to the plane of the former metaphase plate, but were not observed in the secondary cleavage plane formed between unrelated spindle poles, except when chromosomes were abnormally present there. However, the secondary furrow was sensitive to Aurora inhibition, indicating that Aurora kinase may still contribute to furrow ingression without chromosomes nearby. Our results provide insights that reconcile classic micromanipulation studies with current molecular understanding of furrow specification in animal cells.

Changes in Ect2 localization couple actomyosin-dependent cell shape changes to mitotic progression

As they enter mitosis, animal cells undergo profound actin-dependent changes in shape to become round. Here we identify the Cdk1 substrate, Ect2, as a central regulator of mitotic rounding, thus uncovering a link between the cell-cycle machinery that drives mitotic entry and its accompanying actin remodeling. Ect2 is a RhoGEF that plays a well-established role in formation of the actomyosin contractile ring at mitotic exit, through the local activation of RhoA. We find that Ect2 first becomes active in prophase, when it is exported from the nucleus into the cytoplasm, activating RhoA to induce the formation of a mechanically stiff and rounded metaphase cortex. Then, at anaphase, binding to RacGAP1 at the spindle midzone repositions Ect2 to induce local actomyosin ring formation. Ect2 localization therefore defines the stage-specific changes in actin cortex organization critical for accurate cell division.

Proper positioning of the cleavage furrow requires α-actinin to regulate the specification of different populations of microtubules

Proper positioning of the cleavage furrow is essential for successful cell division. The mitotic spindle, which consists of dynamic astral microtubules and stable equatorial microtubules is responsible for this process. However, little is known about how microtubules are regulated in a time- and region-dependent manner. Here, we show that α-actinin-regulated cortical actin filament integrity is crucial to specify different populations of microtubules during cell division in mammalian cells. Depletion of α-actinin caused aberrant recruitment of centralspindlin, but not aurora B or PRC1, to the tips of astral microtubules, leading to a stable association of astral microtubules with the cortex and induction of ectopic furrowing. Depletion of α-actinin also caused impaired assembly of midzone microtubules, leading to a failure of relocation of aurora B to midzone. Our findings unveil an unexpected yet crucial role for an actin crosslinking protein in the regulation of the localization of the microtubule-associated cytokinetic regulator.

Cytokinesis in animal cells

Cytokinesis, the final step in cell division, partitions the contents of a single cell into two. In animal cells, cytokinesis occurs through cortical remodeling orchestrated by the anaphase spindle. Cytokinesis relies on a tight interplay between signaling and cellular mechanics and has attracted the attention of both biologists and physicists for more than a century. In this review, we provide an overview of four topics in animal cell cytokinesis: (a) signaling between the anaphase spindle and cortex, (b) the mechanics of cortical remodeling, (c) abscission, and (d) regulation of cytokinesis by the cell cycle machinery. We report on recent progress in these areas and highlight some of the outstanding questions that these findings bring into focus.

Molecular control of animal cell cytokinesis

Cytokinesis is the process by which mitotic cells physically split in two following chromosome segregation. Dividing animal cells first ingress a cytokinetic furrow and then separate the plasma membrane by abscission. The general cytological events and several conserved molecular factors involved in cytokinesis have been known for many years. However, recent progress in microscopy, chemical genetics, biochemical reconstitution and biophysical methodology has tremendously increased our understanding of the underlying molecular mechanisms. We discuss how recent insights have led to refined models of the distinct steps of animal cell cytokinesis, including anaphase spindle reorganization, division plane specification, actomyosin ring assembly and contraction, and abscission. We highlight how molecular signalling pathways coordinate the individual events to ensure faithful partitioning of the genome to emerging daughter cells.

Role of RhoA-specific guanine exchange factors in regulation of endomitosis in megakaryocytes

Polyploidization can precede the development of aneuploidy in cancer. Polyploidization in megakaryocytes (Mks), in contrast, is a highly controlled developmental process critical for efficient platelet production via unknown mechanisms. Using primary cells, we demonstrate that the guanine exchange factors GEF-H1 and ECT2, which are often overexpressed in cancer and are essential for RhoA activation during cytokinesis, must be downregulated for Mk polyploidization. The first (2N-4N) endomitotic cycle requires GEF-H1 downregulation, whereas subsequent cycles (>4N) require ECT2 downregulation. Exogenous expression of both GEF-H1 and ECT2 prevents endomitosis, resulting in proliferation of 2N Mks. Furthermore, we have shown that the mechanism by which polyploidization is prevented in Mks lacking Mkl1, which is mutated in megakaryocytic leukemia, is via elevated GEF-H1 expression; shRNA-mediated GEF-H1 knockdown alone rescues this ploidy defect. These mechanistic insights enhance our understanding of normal versus malignant megakaryocytopoiesis, as well as aberrant mitosis in aneuploid cancers.

Polo-like kinase 1, on the rise from cell cycle regulation to prostate cancer development

Polo-like kinase 1 (Plk1), a well-characterized member of serine/threonine kinases Plk family, has been shown to play pivotal roles in mitosis and cytokinesis in eukaryotic cells. Recent studies suggest that Plk1 not only controls the process of mitosis and cytokinesis, but also, going beyond those previously described functions, plays critical roles in DNA replication and Pten null prostate cancer initiation. In this review, we briefly summarize the functions of Plk1 in mitosis and cytokinesis, and then mainly focus on newly discovered functions of Plk1 in DNA replication and in Pten-null prostate cancer initiation. Furthermore, we briefly introduce the architectures of human and mouse prostate glands and the possible roles of Plk1 in human prostate cancer development. And finally, the newly chemotherapeutic development of small-molecule Plk1 inhibitors to target Plk1 in cancer treatment and their translational studies are also briefly reviewed.

Phosphorylation of myosin II regulatory light chain controls its accumulation, not that of actin, at the contractile ring in HeLa cells

During cytokinesis in eukaryotic cells, an actomyosin-based contractile ring (CR) is assembled along the equator of the cell. Myosin II ATPase activity is stimulated by the phosphorylation of the myosin II regulatory light chain (MRLC) in vitro, and phosphorylated MRLC localizes at the CR in various types of cells. Previous studies have determined that phosphorylated MRLC plays an important role in CR furrowing. However, the role of phosphorylated MRLC in CR assembly remains unknown. Here, we have used confocal microscopy to observe dividing HeLa cells expressing fluorescent protein-tagged MRLC mutants and actin during CR assembly near the cortex. Di-phosphomimic MRLC accumulated at the cell equator earlier than non-phosphorylatable MRLC and actin. Interestingly, perturbation of myosin II activity by non-phosphorylatable MRLC expression or treatment with blebbistatin, a myosin II inhibitor, did not alter the time of actin accumulation at the cell equator. Furthermore, inhibition of actin polymerization by treatment with latrunculin A had no effect on MRLC accumulation at the cell equator. Taken together, these data suggest that phosphorylated MRLC temporally controls its own accumulation, but not that of actin, in cultured mammalian cells.

Targeting of the RhoGEF Ect2 to the equatorial membrane controls cleavage furrow formation during cytokinesis

In animal cells, formation of the cytokinetic furrow requires activation of the GTPase RhoA by the guanine nucleotide exchange factor Ect2. How Ect2, which is associated with the spindle midzone, controls RhoA activity at the equatorial cortex during anaphase is not understood. Here, we show that Ect2 concentrates at the equatorial membrane during cytokinesis in live cells. Ect2 membrane association requires a pleckstrin homology domain and a polybasic cluster that bind to phosphoinositide lipids. Both guanine nucleotide exchange function and membrane targeting of Ect2 are essential for RhoA activation and cleavage furrow formation in human cells. Membrane localization of Ect2 is spatially confined to the equator by centralspindlin, Ect2's spindle midzone anchor complex, and is temporally coordinated with chromosome segregation through the activation state of CDK1. We propose that targeting of Ect2 to the equatorial membrane represents a key step in the delivery of the cytokinetic signal to the cortex.

PRC1 controls spindle polarization and recruitment of cytokinetic factors during monopolar cytokinesis

PRC1 and KIF4A are believed to play a critical role in organizing antiparallel microtubules of the central spindle. Separable and non­redundant roles for these proteins were uncovered using cells with compromised spindle bipolarity, in which cytokinesis can be induced by bypassing the spindle assembly checkpoint.

The central spindle is a postanaphase array of microtubules that plays an essential role in organizing the signaling machinery for cytokinesis. The model by which the central spindle organizes the cytokinetic apparatus is premised on an antiparallel arrangement of microtubules, yet cells lacking spindle bipolarity are capable of generating a distal domain of ectopic furrowing when forced into mitotic exit. Because protein regulator of cytokinesis (PRC1) and kinesin family member 4A (KIF4A) are believed to play a principal role in organizing the antiparallel midzone array, we sought to clarify their roles in monopolar cytokinesis. Although both factors localized to the distal ends of microtubules during monopolar cytokinesis, depletion of PRC1 and KIF4A displayed different phenotypes. Cells depleted of PRC1 failed to form a polarized microtubule array or ectopic furrows following mitotic exit, and recruitment of Aurora B kinase, male germ cell Rac GTPase-activating protein, and RhoA to the cortex was impaired. In contrast, KIF4A depletion impaired neither polarization nor ectopic furrowing, but it did result in elongated spindles with a diffuse distribution of cytokinetic factors. Thus, even in the absence of spindle bipolarity, PRC1 appears to be essential for polarizing parallel microtubules and concentrating the factors responsible for contractile ring assembly, whereas KIF4A is required for limiting the length of anaphase microtubules.

Roles of putative Rho-GEF Gef2 in division-site positioning and contractile-ring function in fission yeast cytokinesis

How Rho-GEFs and Rho GTPases regulate division-site selection during cytokinesis in fission yeast is unknown. The Rho-GEF Gef2 interacts with the anillin Mid1 to regulate contractile-ring positioning and assembly in coordination with the polo kinase Plo1. In addition, Gef2 is involved in contractile-ring stability and disassembly.

Cytokinesis is crucial for integrating genome inheritance and cell functions. In multicellular organisms, Rho-guanine nucleotide exchange factors (GEFs) and Rho GTPases are key regulators of division-plane specification and contractile-ring formation during cytokinesis, but how they regulate early steps of cytokinesis in fission yeast remains largely unknown. Here we show that putative Rho-GEF Gef2 and Polo kinase Plo1 coordinate to control the medial cortical localization and function of anillin-related protein Mid1. The division-site positioning defects of gef2∆ plo1-ts18 double mutant can be partially rescued by increasing Mid1 levels. We find that Gef2 physically interacts with the Mid1 N-terminus and modulates Mid1 cortical binding. Gef2 localization to cortical nodes and the contractile ring depends on its last 145 residues, and the DBL-homology domain is important for its function in cytokinesis. Our data suggest the interaction between Rho-GEFs and anillins is an important step in the signaling pathways during cytokinesis. In addition, Gef2 also regulates contractile-ring function late in cytokinesis and may negatively regulate the septation initiation network. Collectively, we propose that Gef2 facilitates and stabilizes Mid1 binding to the medial cortex, where the localized Mid1 specifies the division site and induces contractile-ring assembly.

Diphosphorylated but not monophosphorylated myosin II regulatory light chain localizes to the midzone without its heavy chain during cytokinesis

Myosin II is activated by the monophosphorylation of its regulatory light chain (MRLC) at Ser19 (1P-MRLC). Its ATPase activity is further enhanced by MRLC diphosphorylation at Thr18/Ser19 (2P-MRLC). As these phosphorylated MRLCs are colocalized with their heavy chains at the contractile ring in dividing cells, we believe that the phosphorylated MRLC acts as a subunit of the activated myosin II during cytokinesis. However, the distinct role(s) of 1P- and 2P-MRLC during cytokinesis has not been elucidated. In this study, a monoclonal antibody (4F12) specific for 2P-MRLC was raised and used to examine the roles of 2P-MRLC in cultured mammalian cells. Our confocal microscopic observations using 4F12 revealed that 2P-MRLC localized to the contractile ring, and, unexpectedly, to the midzone also. Interestingly, 2P-MRLC did not colocalize with 1P-MRLC, myosin II heavy chain, and F-actin at the midzone. These results suggest that 2P-MRLC has a role different from that of 1P-MRLC at the midzone, and is not a subunit of myosin II.

Chlamydia trachomatis inclusions induce asymmetric cleavage furrow formation and ingression failure in host cells

Chlamydia trachomatis infection has been suggested to induce host genome duplication and is linked to increased risks of cervical cancer. We describe here the mechanism by which Chlamydia causes a cleavage furrow defect that consistently results in the formation of multinucleated host cells, a phenomenon linked to tumorigenesis. Host signaling proteins essential for cleavage furrow initiation, ingression, and stabilization are displaced from one of the prospective furrowing cortices after Chlamydia infection. This protein displacement leads to the formation of a unique asymmetrical, unilateral cleavage furrow in infected human cells. The asymmetrical distribution of signaling proteins is caused by the physical presence of the Chlamydia inclusion at the cell equator. By using ingested latex beads, we demonstrate that the presence of a large vacuole at the cell equator is sufficient to cause furrow ingression failure and can lead to multinucleation. Interestingly, internalized latex beads of similar size do not localize to the cell equator as efficiently as Chlamydia inclusions; moreover, inhibition of bacterial protein synthesis with antibiotic reduces the frequency at which Chlamydia localizes to the cell equator. Together, these results suggest that Chlamydia effectors are involved in strategic positioning of the inclusion during cell division.

The ect2 rho Guanine nucleotide exchange factor is essential for early mouse development and normal cell cytokinesis and migration

Ect2 is a member of the human Dbl family of guanine nucleotide exchange factors (RhoGEFs) that serve as activators of Rho family small GTPases. Although Ect2 is one of at least 25 RhoGEFs that can activate the RhoA small GTPase, cell culture studies using established cell lines determined that Ect2 is essential for mammalian cell cytokinesis and proliferation. To address the function of Ect2 in normal mammalian development, we performed gene targeting to generate Ect2 knockout mice. The heterozygous Ect2(+/-) mice showed normal development and life span, indicating that Ect2 haplodeficiency was not deleterious for development or growth. In contrast, Ect2(-/-) embryos were not found at birth or postimplantation stages. Ect2(-/-) blastocysts were recovered at embryonic day 3.5 but did not give rise to viable outgrowths in culture, indicating that Ect2 is required for peri-implantation development. To further assess the importance of Ect2 in normal cell physiology, we isolated primary fibroblasts from Ect2(fl/fl) embryos (MEFs) and ablated Ect2 using adenoviral delivery of Cre recombinase. We observed a significant increase in multinucleated cells and accumulation of cells in G2/M phase, consistent with a role for Ect2 in cytokinesis. Ect2 deficiency also caused enlargement of the cytoplasm and impaired cell migration. Finally, although Ect2-dependent activation of RhoA has been implicated in cytokinesis, Ect2 can also activate Rac1 and Cdc42 to cause growth transformation. Surprisingly, ectopic expression of constitutively activated RhoA, Rac1, or Cdc42, known substrates of Ect2, failed to phenocopy Ect2 and did not rescue the defect in cytokinesis caused by loss of Ect2. In summary, our results establish the unique role of Ect2 in development and normal cell proliferation.

Role of Survivin in cytokinesis revealed by a separation-of-function allele

A mutation in Drosophila Survivin that enables metaphase functions but impairs cytokinesis demonstrates a role for Survivin and CPC in Rho activation and contractile ring assembly and highlights striking differences in regulation of cytokinesis in different cell systems.

The chromosomal passenger complex (CPC), containing Aurora B kinase, Inner Centromere Protein, Survivin, and Borealin, regulates chromosome condensation and interaction between kinetochores and microtubules at metaphase, then relocalizes to midzone microtubules at anaphase and regulates central spindle organization and cytokinesis. However, the precise role(s) played by the CPC in anaphase have been obscured by its prior functions in metaphase. Here we identify a missense allele of Drosophila Survivin that allows CPC localization and function during metaphase but not cytokinesis. Analysis of mutant cells showed that Survivin is essential to target the CPC and the mitotic kinesin-like protein 1 orthologue Pavarotti (Pav) to the central spindle and equatorial cell cortex during anaphase in both larval neuroblasts and spermatocytes. Survivin also enabled localization of Polo kinase and Rho at the equatorial cortex in spermatocytes, critical for contractile ring assembly. In neuroblasts, in contrast, Survivin function was not required for localization of Rho, Polo, or Myosin II to a broad equatorial cortical band but was required for Myosin II to transition to a compact, fully constricted ring. Analysis of this “separation-of-function” allele demonstrates the direct role of Survivin and the CPC in cytokinesis and highlights striking differences in regulation of cytokinesis in different cell systems.

Proper regulation of Cdc42 activity is required for tight actin concentration at the equator during cytokinesis in adherent mammalian cells

Cytokinesis in mammalian cells requires actin assembly at the equatorial region. Although functions of RhoA in this process have been well established, additional mechanisms are likely involved. We have examined if Cdc42 is involved in actin assembly during cytokinesis. Depletion of Cdc42 had no apparent effects on the duration of cytokinesis, while overexpression of constitutively active Cdc42 (CACdc42) caused cytokinesis failure in normal rat kidney epithelial cells. Cells depleted of Cdc42 displayed abnormal cell morphology and caused a failure of tight accumulation of actin and RhoA at the equator. In contrast, in cells overexpressing CACdc42, actin formed abnormal bundles and RhoA was largely eliminated from the equator. Our results suggest that accurate regulation of Cdc42 activity is crucial for proper equatorial actin assembly and RhoA localization during cytokinesis. Notably, our observations also suggest that tight actin concentration is not essential for cytokinesis in adherent mammalian cells.

The chromosomal passenger complex and centralspindlin independently contribute to contractile ring assembly

In contrast to their sequential roles in midzone assembly, the CPC and centralspindlin act through independent mechanisms to regulate contractile ring assembly.

The chromosomal passenger complex (CPC) and centralspindlin are conserved cytokinesis regulators that localize to the spindle midzone, which forms between the separating chromosomes. Previous work placed the CPC and centralspindlin in a linear pathway that governs midzone formation. Using Caenorhabditis elegans embryos, we test whether there is a similar linear relationship between centralspindlin and the CPC in contractile ring constriction during cytokinesis. We show that simultaneous inhibition of the CPC kinase Aurora B^AIR-2 and the centralspindlin component MKLP1^ZEN-4 causes an additive constriction defect. Consistent with distinct roles for the proteins, inhibition of filamentous septin guanosine triphosphatases alleviates constriction defects in Aurora B^AIR-2–inhibited embryos, whereas inhibition of Rac does so in MKLP1^ZEN-4-inhibited embryos. Centralspindlin and the CPC are not required to enrich ring proteins at the cell equator but instead regulate formation of a compact mature ring. Therefore, in contrast to the linear midzone assembly pathway, centralspindlin and the CPC make independent contributions to control transformation of the sheet-like equatorial band into a ribbon-like contractile ring at the furrow tip.

RhoA GTPase is dispensable for actomyosin regulation but is essential for mitosis in primary mouse embryonic fibroblasts

RhoA, the founding member of mammalian Rho GTPase family, is thought to be essential for actomyosin regulation. To date, the physiologic function of RhoA in mammalian cell regulation has yet to be determined genetically. Here we have created RhoA conditional knock-out mice. Mouse embryonic fibroblasts deleted of RhoA showed no significant change in actin stress fiber or focal adhesion complex formation in response to serum or LPA, nor any detectable change in Rho-kinase signaling activity. Concomitant knock-out or knockdown of RhoB and RhoC in the RhoA(-/-) cells resulted in a loss of actin stress fiber and focal adhesion similar to that of C3 toxin treatment. Proliferation of RhoA(-/-) cells was impaired due to a complete cell cycle block during mitosis, an effect that is associated with defective cytokinesis and chromosome segregation and can be readily rescued by exogenous expression of RhoA. Furthermore, RhoA deletion did not affect the transcriptional activity of Stat3, NFκB, or serum response factor, nor the expression of the cell division kinase inhibitor p21(Cip)1 or p27(Kip1). These genetic results demonstrate that in primary mouse embryonic fibroblasts, RhoA is uniquely required for cell mitosis but is redundant with related RhoB and RhoC GTPases in actomyosin regulation.

Mechanisms of contractile-ring assembly in fission yeast and beyond

Most eukaryotes including fungi, amoebas, and animal cells assemble an actin/myosin-based contractile ring during cytokinesis. The majority of proteins implied in ring formation, maturation, and constriction are evolutionarily conserved, suggesting that common mechanisms exist among these divergent eukaryotes. Here, we review the recent advances in positioning and assembly of the actomyosin ring in the fission yeast Schizosaccharomyces pombe, the budding yeast Saccharomyces cerevisiae, and animal cells. In particular, major findings have been made recently in understanding ring formation in genetically tractable S. pombe, revealing a dynamic and robust search, capture, pull, and release mechanism.

Role of a novel coiled-coil domain-containing protein CCDC69 in regulating central spindle assembly

The formation of the central spindle (or the spindle midzone) is essential for cytokinesis in animal cells. In this study, we report that coiled-coil domain-containing protein 69 (CCDC69) is implicated in controlling the assembly of central spindles and the recruitment of midzone components. Exogenous expression of CCDC69 in HeLa cells interfered with microtubule polymerization and disrupted the formation of bipolar mitotic spindles. Endogenous CCDC69 proteins were localized to the central spindle during anaphase. RNA interference (RNAi)-mediated knockdown of CCDC69 led to the formation of aberrant central spindles and disrupted the localization of midzone components such as aurora B kinase, protein regulator of cytokinesis 1 (PRC1), MgcRacGAP/HsCYK-4, and polo-like kinase 1 (Plk1) at the central spindle. Aurora B kinase was found to bind to CCDC69 and this binding depended on the coiled-coil domains at the C-terminus of CCDC69. Further, disruption of aurora B function in HeLa cells by treatment with a small chemical inhibitor led to the mislocalization of CCDC69 at the central spindle. Our results indicate that CCDC69 acts as a scaffold to regulate the recruitment of midzone components and the assembly of central spindles.

Cellular symmetry breaking during Caenorhabditis elegans development

The nematode worm Caenorhabditis elegans has produced a wellspring of insights into mechanisms that govern cellular symmetry breaking during animal development. Here we focus on two highly conserved systems that underlie many of the key symmetry-breaking events that occur during embryonic and larval development in the worm. One involves the interplay between Par proteins, Rho GTPases, and the actomyosin cytoskeleton and mediates asymmetric cell divisions that establish the germline. The other uses elements of the Wnt signaling pathway and a highly reiterative mechanism that distinguishes anterior from posterior daughter cell fates. Much of what we know about these systems comes from intensive study of a few key events-Par/Rho/actomyosin-mediated polarization of the zygote in response to a sperm-derived cue and the Wnt-mediated induction of endoderm at the four-cell stage. However, a growing body of work is revealing how C. elegans exploits elements/variants of these systems to accomplish a diversity of symmetry-breaking tasks throughout embryonic and larval development.

The transforming parasite Theileria co-opts host cell mitotic and central spindles to persist in continuously dividing cells

The protozoan parasite Theileria inhabits the host cell cytoplasm and possesses the unique capacity to transform the cells it infects, inducing continuous proliferation and protection against apoptosis. The transforming schizont is a multinucleated syncytium that resides free in the host cell cytoplasm and is strictly intracellular. To maintain transformation, it is crucial that this syncytium is divided over the two daughter cells at each host cell cytokinesis. This process was dissected using different cell cycle synchronization methods in combination with the targeted application of specific inhibitors. We found that Theileria schizonts associate with newly formed host cell microtubules that emanate from the spindle poles, positioning the parasite at the equatorial region of the mitotic cell where host cell chromosomes assemble during metaphase. During anaphase, the schizont interacts closely with host cell central spindle. As part of this process, the schizont recruits a host cell mitotic kinase, Polo-like kinase 1, and we established that parasite association with host cell central spindles requires Polo-like kinase 1 catalytic activity. Blocking the interaction between the schizont and astral as well as central spindle microtubules prevented parasite segregation between the daughter cells during cytokinesis. Our findings provide a striking example of how an intracellular eukaryotic pathogen that evolved ways to induce the uncontrolled proliferation of the cells it infects usurps the host cell mitotic machinery, including Polo-like kinase 1, one of the pivotal mitotic kinases, to ensure its own persistence and survival.

Still entangled: assembly of the central spindle by multiple microtubule modulators

The central spindle is a microtubule-based structure that assembles during anaphase of mitosis in animal cells and is essential for multiple steps of cytokinesis. Central spindle assembly occurs by the cooperative action of multiple microtubule motors and modulators. Here, we review the mechanism by which the central spindle is formed, the role of several key proteins in this process and how central spindle assembly is temporally and spatially coordinated with mitosis.

Cytokinetic abscission in animal cells

Cytokinesis leads to the separation of dividing cells, which in animal cells involves the contraction of an actin-myosin ring and subsequent fission during abscission. Abscission requires a series of dynamic events, including midbody-targeted vesicle secretion, specialization of plasma membrane domains, disassembly of midbody-associated microtubule bundles and plasma membrane fission. A large number of molecular factors required for abscission have been identified through localization, loss-of-function and proteomics studies, but their coordinate function in abscission is still poorly understood. Here, we review the structural elements and molecular factors known to contribute to abscission, and discuss their potential role in the context of proposed models for the abscission mechanism.

Rho and anillin-dependent control of mDia2 localization and function in cytokinesis

Diaphanous-related formin, mDia, is an actin nucleation/polymerization factor functioning downstream of the small GTPase Rho. We found that, in addition to the Rho GTPase-mediated activation, the interaction between mDia2 and anillin is required for the localization and function of mDia2 in cytokinesis.

Diaphanous-related formin, mDia, is an actin nucleation/polymerization factor functioning downstream of the small GTPase Rho. Although Rho is critically involved in cytokinesis, it remains elusive how Rho effectors and other regulators of cytoskeletons work together to accomplish this process. Here we focused on mDia2, an mDia isoform involved in cytokinesis of NIH 3T3 cells, and analyzed mechanisms of its localization in cytokinesis. We found that targeting of mDia2 to the cleavage furrow requires not only its binding to RhoA but also its diaphanous-inhibitory domain (DID). We then performed pulldown assays using a fragment containing the latter domain as a bait and identified anillin as a novel mDia2 interaction partner. The anillin-binding is competitive with the diaphanous autoregulatory domain (DAD) of mDia2 in its autoinhibitory interaction. A series of RNA interference and functional rescue experiments has revealed that, in addition to the Rho GTPase-mediated activation, the interaction between mDia2 and anillin is required for the localization and function of mDia2 in cytokinesis.

Polo kinase interacts with RacGAP50C and is required to localize the cytokinesis initiation complex

The assembly and constriction of an actomyosin contractile ring in cytokinesis is dependent on the activation of Rho at the equatorial cortex by a complex, here termed the cytokinesis initiation complex, between a microtubule-associated kinesin-like protein (KLP), a member of the RacGAP family, and the RhoGEF Pebble. Recently, the activity of the mammalian Polo kinase ortholog Plk1 has been implicated in the formation of this complex. We show here that Polo kinase interacts directly with the cytokinesis initiation complex by binding RacGAP50C. We find that a new domain of Polo kinase, termed the intermediate domain, interacts directly with RacGAP50C and that Polo kinase is essential for localization of the KLP-RacGAP centralspindlin complex to the cell equator and spindle midzone. In the absence of Polo kinase, RacGAP50C and Pav-KLP fail to localize normally, instead decorating microtubules along their length. Our results indicate that Polo kinase directly binds the conserved cytokinesis initiation complex and is required to trigger centralspindlin localization as a first step in cytokinesis.

Signalling through the RhoGEF Pebble in Drosophila

Small GTPase pathways of the Ras superfamily are implicated in a wide range of signalling processes in animal cells. Small GTPases control pathways by acting as molecular switches. They are converted from an inactive GDP-bound form to an active GTP-bound form by GTP exchange factors (GEFs). The spatial and temporal regulation of GEFs is a major component of the regulation of small GTPases. Here we review the role of the Drosophila RhoGEF, Pebble (the Drosophila ortholog of mammalian ECT2). We discuss its roles in cytokinesis and cell migration, highlighting the diversity with which Rho family signalling pathways operate in biological systems.

Mitotic down-regulation of p190RhoGAP is required for the successful completion of cytokinesis

p190RhoGAP-A (p190) is a GTPase-activating protein known to regulate actin cytoskeleton dynamics by decreasing RhoGTP levels through activation of Rho intrinsic GTPase activity. We have previously shown that p190 protein levels are cell cycle-regulated, decreasing in mitosis, and that this decrease is mediated by the ubiquitin-proteasome pathway. In addition, overexpression of p190 results in decreased RhoGTP levels at the cleavage furrow during cytokinesis, p190 and the RhoGEF Ect2 play opposing roles in cytokinesis, and sustained levels of p190 in mitosis are associated with cytokinesis failure, all findings that suggest but do not directly demonstrate that completion of cytokinesis is dependent on reduced levels of p190. Here we report, using an RNAi reconstitution approach with a degradation-resistant mutant, that decreased p190 levels are required for successful cytokinesis. We also show that the multinucleation phenotype is dependent on p190 RhoGAP activity, determine that the N-terminal GBDS1 region is necessary and sufficient for p190 mitotic ubiquitination and degradation, and identify four N-terminal residues as necessary for the degradation of p190 in mitosis. Our data indicate that in addition to activation of RhoGEF(s), reduction of RhoGAP (p190) is a critical mechanism by which increased RhoGTP levels are achieved in late mitosis, thereby ensuring proper cell division.

Polo-like kinase guides cytokinesis in Trypanosoma brucei through an indirect means

Polo-like kinase in Trypanosoma brucei (TbPLK) is confined to the flagellum attachment zone (FAZ) and regulates only cytokinetic initiation. However, it apparently diffuses into the cytoplasm before the trans-localization of chromosomal passenger complex (CPC) from the midzone of central spindle to FAZ, which is known to be required for initiating cytokinesis. Synchronized T. brucei procyclic cells treated with a TbPLK inhibitor, GW843682X (GW), in late S phase were found to go through a full cell cycle at a normal pace before being arrested at cytokinetic initiation in the second cycle. However, synchronized cells treated with GW in G(1) phase were arrested at cytokinetic initiation within the first cell cycle, suggesting that inhibition of TbPLK at its emergence blocks cytokinesis within the same cell cycle. To rule out potential off-target effects from GW, TbPLK RNA interference (RNAi) was induced to deplete TbPLK, and the progression of synchronized cells from late S phase was also found to be arrested at cytokinetic initiation within the first cell cycle. Apparently, TbPLK has accomplished its role in guiding cytokinesis before the late S phase, presumably by phosphorylating a certain substrate(s) during S phase, which may play a critical role in initiating the subsequent cytokinesis.

Actin cytoskeleton dynamics and the cell division cycle

The network of actin filaments is one of the crucial cytoskeletal structures contributing to the morphological framework of a cell and which participates in the dynamic regulation of cellular functions. In adherent cell types, cells adhere to the substratum during interphase and spread to assume their characteristic shape supported by the actin cytoskeleton. This actin cytoskeleton is reorganized during mitosis to form rounded cells with increased cortical rigidity. The actin cytoskeleton is re-established after mitosis, allowing cells to regain their extended shape and attachment to the substratum. The modulation of such drastic changes in cell shape in coordination with cell cycle progression suggests a tight regulatory interaction between cytoskeleton signalling, cell-cell/cell-matrix adhesions and mitotic events. Here, we review the contribution of the actin cytoskeleton to cell cycle progression with an emphasis on the effectors responsible for the regulation of the actin cytoskeleton and integration of their activities with the cell cycle machinery.

Epithelial cell transforming protein 2 (ECT2) depletion blocks polar body extrusion and generates mouse oocytes containing two metaphase II spindles

Completion of the first meiosis in oocytes is achieved by the extrusion of the first polar body (PBI), a particular example of cell division. In mitosis, the small GTPase RhoA, which is activated by epithelial cell transforming protein 2 (ECT2), orchestrates contractile ring constriction, thus enabling cytokinesis. However, the involvement of this pathway in mammalian oocytes has not been established. To characterize the role of ECT2 in PBI emission in mouse oocytes, the small interfering RNA approach was employed. We found that ECT2 depletion significantly reduces PBI emission, induces first metaphase arrest, and generates oocytes containing two properly formed spindles of the second metaphase. Moreover, we describe, for the first time, that before PBI emission, RhoA forms a ring that is preceded by a dome-like accumulation at the oocyte cortex, next to the spindle. This unique mode of RhoA translocation failed to occur in the absence of ECT2. We further found that the Rho-dependent kinase, a main RhoA effector, is essential for PBI emission. In addition, we demonstrate herein that ECT2 is subjected to phosphorylation/dephosphorylation throughout meiosis in oocytes and further reveal that PBI emission is temporally associated with ECT2 dephosphorylation. Our data provide the first demonstration that an active cyclin-dependent kinase 1, the catalytic subunit of the maturation-promoting factor, phosphorylates ECT2 during the first meiotic metaphase and that cyclin-dependent kinase 1 inactivation at anaphase allows ECT2 dephosphorylation. In conclusion, our study demonstrates the indispensable role of the maturation-promoting factor/ECT2/RhoA pathway in PBI extrusion in mouse oocytes.

Understanding cytokinesis failure

Cytokinesis is the final step in cell division. The process begins during chromosome segregation, when the ingressing cleavage furrow begins to partition the cytoplasm between the nascent daughter cells. The process is not completed until much later, however, when the final cytoplasmic bridge connecting the two daughter cells is severed. Cytokinesis is a highly ordered process, requiring an intricate interplay between cytoskeletal, chromosomal and cell cycle regulatory pathways. A surprisingly broad range of additional cellular processes are also important for cytokinesis, including protein and membrane trafficking, lipid metabolism, protein synthesis and signaling pathways. As a highly regulated, complex process, it is not surprising that cytokinesis can sometimes fail. Cytokinesis failure leads to both centrosome amplification and production of tetraploid cells, which may set the stage for the development of tumor cells. However, tetraploid cells are abundant components of some normal tissues including liver and heart, indicating that cytokinesis is physiologically regulated. In this chapter, we summarize our current understanding of the mechanisms of cytokinesis, emphasizing steps in the pathway that may be regulated or prone to failure. Our discussion emphasizes findings in vertebrate cells although we have attempted to highlight important contributions from other model systems.

Action at a distance during cytokinesis

Animal cells decide where to build the cytokinetic apparatus by sensing the position of the mitotic spindle. Reflecting a long-standing presumption that a furrow-inducing stimulus travels from spindle to cortex via microtubules, debate continues about which microtubules, and in what geometry, are essential for accurate cytokinesis. We used live imaging in urchin and frog embryos to evaluate the relationship between microtubule organization and cytokinetic furrow position. In normal cells, the cytokinetic apparatus forms in a region of lower cortical microtubule density. Remarkably, cells depleted of astral microtubules conduct accurate, complete cytokinesis. Conversely, in anucleate cells, asters alone can support furrow induction without a spindle, but only when sufficiently separated. Ablation of a single centrosome displaces furrows away from the remaining centrosome; ablation of both centrosomes causes broad, inefficient furrowing. We conclude that the asters confer accuracy and precision to a primary furrow-inducing signal that can reach the cell surface from the spindle without transport on microtubules.

Clustering of centralspindlin is essential for its accumulation to the central spindle and the midbody

Cytokinesis in animal cells requires the central spindle and midbody, which contain prominent microtubule bundles. Centralspindlin, a heterotetrameric complex consisting of kinesin-6 and RhoGAP (Rho-family GTPase-activating protein) subunits, is essential for the formation of these structures. Centralspindlin becomes precisely localized to the central spindle, where it promotes the equatorial recruitment of important cytokinetic regulators. These include ECT2, the activator of the small GTPase RhoA, which controls cleavage furrow formation and ingression. Centralspindlin's own RhoGAP domain also contributes to furrow ingression. Finally, centralspindlin facilitates recruitment of the chromosome passenger complex and factors that control abscission. Despite the importance of localized accumulation of centralspindlin, the mechanism by which this motor protein complex suddenly concentrates to the center of interpolar microtubule bundles during anaphase is unclear. Here, we show that centralspindlin travels along central spindle microtubules as higher-order clusters. Clustering of centralspindlin is critical for microtubule bundling and motility along microtubules in vitro and for midbody formation in vivo. These data support a positive feedback loop of centralspindlin clustering and microtubule organization that may underlie its distinctive localization during cytokinesis.

Single molecule imaging reveals differences in microtubule track selection between Kinesin motors

Molecular motors differentially recognize and move cargo along discrete microtubule subpopulations in cells, resulting in preferential transport and targeting of subcellular cargoes.

Cells generate diverse microtubule populations by polymerization of a common α/β-tubulin building block. How microtubule associated proteins translate microtubule heterogeneity into specific cellular functions is not clear. We evaluated the ability of kinesin motors involved in vesicle transport to read microtubule heterogeneity by using single molecule imaging in live cells. We show that individual Kinesin-1 motors move preferentially on a subset of microtubules in COS cells, identified as the stable microtubules marked by post-translational modifications. In contrast, individual Kinesin-2 (KIF17) and Kinesin-3 (KIF1A) motors do not select subsets of microtubules. Surprisingly, KIF17 and KIF1A motors that overtake the plus ends of growing microtubules do not fall off but rather track with the growing tip. Selection of microtubule tracks restricts Kinesin-1 transport of VSVG vesicles to stable microtubules in COS cells whereas KIF17 transport of Kv1.5 vesicles is not restricted to specific microtubules in HL-1 myocytes. These results indicate that kinesin families can be distinguished by their ability to recognize microtubule heterogeneity. Furthermore, this property enables kinesin motors to segregate membrane trafficking events between stable and dynamic microtubule populations.

Eukaryotic cells assemble a variety of cytoskeletal structures from a set of highly conserved building blocks. For example, all microtubules are generated by the polymerization of a common α/β-tubulin subunit, yet cells can contain diverse, discrete populations of microtubule structures such as axonemes, spindles, and radial arrays. This diversity must be read and translated by cellular components in order to carry out population-specific functions. We use single-molecule imaging to study how molecular motors navigate the heterogeneous microtubule populations present in interphase cells. We show that different kinesin motors select different subpopulations of microtubules for transport. This selectivity, based solely on the motor-microtubule interface, may enable kinesin motors to segregate transport events to distinct microtubule populations and thus to target cargoes to specific subcellular destinations.