Centriolin anchoring of exocyst and SNARE complexes at the midbody is required for secretory-vesicle-mediated abscission

Role of spindle asymmetry in cellular dynamics

The mitotic spindle is mostly perceived as a symmetric structure. However, in many cell divisions, the two poles of the spindle organize asters with different dynamics, associate with different biomolecules or subcellular domains, and perform different functions. In this chapter, we describe some of the most prominent examples of spindle asymmetry. These are encountered during cell-cycle progression in budding and fission yeast and during asymmetric cell divisions of stem cells and embryos. We analyze the molecular mechanisms that lead to generation of spindle asymmetry and discuss the importance of spindle-pole differentiation for the correct outcome of cell division.

Polarity and differential inheritance--universal attributes of life?

When and why did cell polarization arise? Recent work in bacteria and yeast suggests that polarization may have evolved to restrict senescence to one daughter during division by enabling the differential segregation of damaged material. In more complex organisms, polarity functions have diversified to permit the differential inheritance of centrosomes, RNAs, proteins, and membranes, which is essential for the generation of diverse cell types from stem cells and for morphogenesis.

Midbody ring disposal by autophagy is a post-abscission event of cytokinesis

At the end of cytokinesis, the dividing cells are connected by an intercellular bridge, containing the midbody along with a single, densely ubiquitylated, circular structure called the midbody ring (MR). Recent studies revealed that the MR serves as a target site for membrane delivery and as a physical barrier between the prospective daughter cells. The MR materializes in telophase, localizes to the intercellular bridge during cytokinesis, and moves asymmetrically into one cell after abscission. Daughter cells rarely accumulate MRs of previous divisions, but how these large structures finally disappear remains unknown. Here, we show that MRs are discarded by autophagy, which involves their sequestration into autophagosomes and delivery to lysosomes for degradation. Notably, autophagy factors, such as the ubiquitin adaptor p62 (Refs 4, 5) and the ubiquitin-related protein Atg8 (ref. 6), associate with the MR during abscission, suggesting that autophagy is coupled to cytokinesis. Moreover, MRs accumulate in cells of patients with lysosomal storage disorders, indicating that defective MR disposal is characteristic of these diseases. Thus our findings suggest that autophagy has a broader role than previously assumed, and that cell renovation by clearing from superfluous large macromolecular assemblies, such as MRs, is an important autophagic function.

Coordination of cytokinesis and cell separation by endosomal targeting of a Cdc42-specific guanine nucleotide exchange factor in Ustilago maydis

The small GTPase Cdc42 is a key regulator of cell polarity and cytoskeletal organization in most eukaryotic cells. In Ustilago maydis, Cdc42 and the guanine nucleotide exchange factor (GEF) Don1 regulate cytokinesis and cell separation. Don1 belongs to the FGD1 family of Cdc42-specific GEFs that are characterized by a C-terminal lipid-binding FYVE domain. Although the FGD1/frabin family of Rho-GEFs is evolutionary conserved from fungi to mammals the role of the FYVE domain for its biological function is unknown. Here, we show that the FYVE domain is specific for phosphatidylinositol-3-phosphate (PtdIns(3)P) and targets Don1 to endosomal vesicles. During cytokinesis asymmetric accumulation of Don1-containing vesicles occurs at the site of septation. We could show that FYVE-dependent localization is critical for the function of Don1 at normal expression levels but can be compensated for by overexpression of Don1 lacking a functional FYVE domain. Our results demonstrate that endosomal compartmentalization of a Cdc42-specific exchange factor is involved in the coordination of cytokinesis and cell separation.

Sec6p anchors the assembled exocyst complex at sites of secretion

The exocyst is an essential protein complex required for targeting and fusion of secretory vesicles to sites of exocytosis at the plasma membrane. To study the function of the exocyst complex, we performed a structure-based mutational analysis of the Saccharomyces cerevisiae exocyst subunit Sec6p. Two "patches" of highly conserved residues are present on the surface of Sec6p; mutation of either patch does not compromise protein stability. Nevertheless, replacement of SEC6 with the patch mutants results in severe temperature-sensitive growth and secretion defects. At nonpermissive conditions, although trafficking of secretory vesicles to the plasma membrane is unimpaired, none of the exocyst subunits are polarized. This is consistent with data from other exocyst temperature-sensitive mutants, which disrupt the integrity of the complex. Surprisingly, however, these patch mutations result in mislocalized exocyst complexes that remain intact. Our results indicate that assembly and polarization of the exocyst are functionally separable events, and that Sec6p is required to anchor exocyst complexes at sites of secretion.

Structural basis for midbody targeting of spastin by the ESCRT-III protein CHMP1B

The ESCRT machinery, including ESCRT-III, localizes to the midbody and participates in the membrane abscission step of cytokinesis. The ESCRT-III protein CHMP1B is required for recruitment of the MIT domain-containing protein spastin, a microtubule severing enzyme, to the midbody. The 2.5 Å structure of the C-terminal tail of CHMP1B with the MIT domain of spastin reveals a specific, high-affinity complex involving a non-canonical binding site between the first and third helices of the MIT domain. The structural interface is twice as large as that of the MIT domain of VPS4-CHMP complex, consistent with the high affinity of the interaction. A series of unique hydrogen bonding interactions and close packing of small side-chains discriminate against the other ten human ESCRT-III subunits. Point mutants in the CHMP1B binding site of spastin block recruitment of spastin to the midbody.

Membrane traffic and polarization of lipid domains during cytokinesis

Growing evidence indicates that membrane traffic plays a crucial role during the late post-furrowing steps of cytokinesis in animal cells. Indeed, both endocytosis and exocytosis contribute to stabilizing the intercellular bridge that connects the daughter cells and to the final abscission in diverse organisms. The need for several intracellular transport routes probably reflects the complex events that occur during the late cytokinesis steps such as local remodelling of the plasma membrane composition, removal of components required for earlier steps of cytokinesis and membrane sealing that leads to daughter cell separation. In this mini-review, I will focus on recent evidence showing that endocytic pathways, such as the Rab35-regulated recycling pathway, contribute to the establishment of a PtdIns(4,5)P(2) lipid domain at the intercellular bridge which is involved in the localization of cytoskeletal elements essential for the late steps of cytokinesis. Possible cross-talk between Rab35 and other endocytic pathways involved in cytokinesis are also discussed.

Endosome positioning during cytokinesis

In mammalian cells, completion of cytokinesis relies on targeted delivery of recycling membranes to the midbody. At this step of mitosis, recycling endosomes are organized as clusters located at the mitotic spindle poles as well as at both sides of the midbody. However, the mechanism that controls endosome positioning during cytokinesis is not known. Here, we discuss the possible mechanisms that drive the formation of endosomal clusters and the importance of this process for the targeted delivery of recycling membranes to the midbody.

Mechanisms regulating targeting of recycling endosomes to the cleavage furrow during cytokinesis

Recently, recycling endosomes have emerged as a key components required for the successful completion of cytokinesis. Furthermore, FIP3 (family of Rab11-interacting protein 3), a Rab11 GTPase-binding protein, has been implicated in targeting the recycling endosomes to the midbody of dividing cells. Previously, we have shown that FIP3/Rab11-containing endosomes associate with centrosomes until anaphase, at which time they translocate to the cleavage furrow. At telophase, FIP3/Rab11-containing endosomes move from the furrow into the midbody, and this step is required for abscission. While several other proteins were implicated in regulating FIP3 targeting to the cleavage furrow, the mechanisms regulating the dynamics of FIP3-containing endosomes during mitosis have not been defined. To identify the factors regulating FIP3 targeting to the furrow, we used a combination of siRNA (small interfering RNA) screens and proteomic analysis to identify Cyk-4/MgcRacGAP (GTPase-activating protein) and kinesin I as FIP3-binding proteins. Furthermore, kinesin I mediates the transport of FIP3-containing endosomes to the cleavage furrow. Once in the furrow, FIP3 binds to Cyk-4 as part of centralspindlin complex and accumulates at the midbody. Finally, we demonstrated that ECT2 regulates FIP3 association with the centralspindlin complex. Thus we propose that kinesin I, in concert with centralspindlin complex, plays a role in temporal and spatial regulation of endosome transport to the cleavage furrow during cytokinesis.

An overview of the fission yeast septation initiation network (SIN)

The fission yeast septation initiation network, or SIN, is a signal transduction network that is required for septum formation in Schizosaccharomyces pombe. Its activity is tightly regulated through the cell cycle, to ensure proper co-ordination of mitosis and cytokinesis. SIN signalling requires three protein kinases for its function and is mediated by a ras-superfamily GTPase. We discuss the elements of the SIN and how they are regulated.

Spastin couples microtubule severing to membrane traffic in completion of cytokinesis and secretion

Mutations in the gene encoding the microtubule (MT)-severing protein spastin are the most common cause of hereditary spastic paraplegia, a genetic condition in which axons of the corticospinal tracts degenerate. We show that not only does endogenous spastin colocalize with MTs, but that it is also located on the early secretory pathway, can be recruited to endosomes and is present in the cytokinetic midbody. Spastin has two main isoforms, a 68 kD full-length isoform and a 60 kD short form. These two isoforms preferentially localize to different membrane traffic pathways with 68 kD spastin being principally located at the early secretory pathway, where it regulates endoplasmic reticulum-to-Golgi traffic. Sixty kiloDalton spastin is the major form recruited to endosomes and is also present in the midbody, where its localization requires the endosomal sorting complex required for transport-III-interacting MIT domain. Loss of midbody MTs accompanies the abscission stage of cytokinesis. In cells lacking spastin, a MT disruption event that normally accompanies abscission does not occur and abscission fails. We suggest that this event represents spastin-mediated MT severing. Our results support a model in which membrane traffic and MT regulation are coupled through spastin. This model is relevant in the axon, where there also is co-ordinated MT regulation and membrane traffic.

The anaphase promoting complex targeting subunit Ama1 links meiotic exit to cytokinesis during sporulation in Saccharomyces cerevisiae

Ascospore formation in yeast is accomplished through a cell division in which daughter nuclei are engulfed by newly formed plasma membranes, termed prospore membranes. Closure of the prospore membrane must be coordinated with the end of meiosis II to ensure proper cell division. AMA1 encodes a meiosis-specific activator of the anaphase promoting complex (APC). The activity of APC(Ama1) is inhibited before meiosis II, but the substrates specifically targeted for degradation by Ama1 at the end of meiosis are unknown. We show here that ama1Delta mutants are defective in prospore membrane closure. Ssp1, a protein found at the leading edge of the prospore membrane, is stabilized in ama1Delta mutants. Inactivation of a conditional form of Ssp1 can partially rescue the sporulation defect of the ama1Delta mutant, indicating that an essential function of Ama1 is to lead to the removal of Ssp1. Depletion of Cdc15 causes a defect in meiotic exit. We find that prospore membrane closure is also defective in Cdc15 and that this defect can be overcome by expression of a form of Ama1 in which multiple consensus cyclin-dependent kinase phosphorylation sites have been mutated. These results demonstrate that APC(Ama1) functions to coordinate the exit from meiosis II with cytokinesis.

Focal adhesion components are essential for mammalian cell cytokinesis

The final stages in mammalian cytokinesis are poorly understood. Previously, we reported that the ADP-ribosyltransferase activity of Pseudomonas aeruginosa type III secreted toxin ExoT inhibits late stages of cytokinesis. Given that Crk adaptor proteins are the major substrates of ExoT ADP-ribosyltransferase activity, we tested the involvement of Crk in cytokinesis. We report that the focal adhesion-associated proteins, Crk and paxillin are essential for completion of cytokinesis. When their function is absent, the cytoplasmic bridge fails to resolve and the daughter cells fuse to form a binucleated cell. During cytokinesis, Crk is required for syntaxin-2 recruitment to the midbody, while paxillin is required for both Crk and syntaxin-2 localization to this compartment. Our data demonstrate that the subcellular localization and the activity of RhoA and citron K, which are essential for early stages of cytokinesis, are not dependent on paxillin, Crk or syntaxin-2. These studies reveal a novel role for Crk and paxillin in cytokinesis and suggest that focal adhesion complex, as a unit, may partake in this fundamental cellular process.

Dishevelled links basal body docking and orientation in ciliated epithelial cells

Some epithelia contain cells with multiple motile cilia that beat in a concerted manner. New tools and experimental systems have facilitated molecular studies of cilium biogenesis and the coordinated planar polarization of cilia that leads to their concerted motility. A recent elegant study using embryonic frog epidermis demonstrates that Dishevelled, a key regulator of both the Wnt-beta-catenin and planar cell polarity pathways, controls both the docking and planar polarization of ciliary basal bodies.

Distinct roles of RalA and RalB in the progression of cytokinesis are supported by distinct RalGEFs

The Ras family G-proteins RalA and RalB make critical non-overlapping contributions to the generation of a tumorigenic regulatory network, supporting bypass of the normal restraints on both cell proliferation and survival. The Sec6/8 complex, or exocyst, has emerged as a principal direct effector complex for Ral GTPases. Here, we show that RalA and RalB support mitotic progression through mobilization of the exocyst for two spatially and kinetically distinct steps of cytokinesis. RalA is required to tether the exocyst to the cytokinetic furrow in early cytokinesis. RalB is then required for recruitment of the exocyst to the midbody of this bridge to drive abscission and completion of cytokinesis. The collaborative action of RalA and RalB is specified by discrete subcellular compartmentalization and unique pairs of RalGEF proteins that provide inputs from both Ras-family protein-dependent and protein-independent regulatory cues. This suggests that Ral GTPases integrate diverse upstream signals to choreograph multiple roles for the exocyst in mitotic progression.

Functional involvement of human discs large tumor suppressor in cytokinesis

Cytokinesis is the final step of cell division that completes the separation of two daughter cells. We found that the human discs large (hDlg) tumor suppressor homologue is functionally involved in cytokinesis. The guanylate kinase (GUK) domain of hDlg mediates the localization of hDlg to the midbody during cytokinesis, and over-expression of the GUK domain in U2OS and HeLa cells impaired cytokinesis. Mouse embryonic fibroblasts (MEFs) derived from dlg mutant mice contained an increased number of multinucleated cells and showed reduced proliferation in culture. A kinesin-like motor protein, GAKIN, which binds directly to the GUK domain of hDlg, exhibited a similar intracellular distribution pattern with hDlg throughout mitosis and localized to the midbody during cytokinesis. However, the targeting of hDlg and GAKIN to the midbody appeared to be independent of each other. The midbody localization of GAKIN required its functional kinesin-motor domain. Treatment of cells with the siRNA specific for hDlg and GAKIN caused formation of multinucleated cells and delayed cytokinesis. Together, these results suggest that hDlg and GAKIN play functional roles in the maintenance of midbody architecture during cytokinesis.

Dynamic regulation of ubiquitylation and deubiquitylation at the central spindle during cytokinesis

During cytokinesis, the central spindle, a bundle of interdigitated anti-parallel microtubules between separating chromosomes, recruits various cytokinetic regulator proteins to the cleavage region. Here, we show that the level of protein ubiquitylation is strikingly and transiently elevated in Aurora B kinase-positive double-band regions of the central spindle during cytokinesis. Two deubiquitylating enzymes UBPY and AMSH, which act on endosomes in interphase, were also recruited to the cleavage region. Whereas UBPY was detected only in the final stage of cytokinesis at the midbody, AMSH localized to a ring structure surrounding the mitotic kinesin MKLP1-positive region of the central spindle and midbody throughout cytokinesis. Depletion of cellular UBPY or AMSH led to defects in cytokinesis. VAMP8, a v-SNARE required for vesicle fusion in cytokinesis, localized to the central spindle region positive for ubiquitylated proteins, and underwent ubiquitylation and deubiquitylation by both UBPY and AMSH. Our results thus implicate the ubiquitylation/deubiquitylation of proteins including VAMP8 in cytokinesis.

Involvement of Exoc3l, a protein structurally related to the exocyst subunit Sec6, in insulin secretion

The exocyst is an octameric complex involved in docking or tethering of secretory vesicles to fusion sites of the plasma membrane. Sec6 is the core subunit of the exocyst complex. Here we identify an isoform of Sec6, deposited as Exocyst complex component 3-like (Exoc3l) in the database, by in silico screening using rat Sec6 as a probe. The amino acid sequence of Exoc3l has 31% identity and 53% similarity with that of Sec6. RT-PCR analysis reveals that Exoc3l is expressed in insulin-secreting MIN6 cells as well as in various tissues including pancreatic islets and brain. In co-immunoprecipitation experiments, Exoc3l was found to interact with Sec5, Sec8, and Sec10, all of which are binding partners of Sec6 in the exocyst complex. Furthermore, overexpression of a deletion mutant of Exoc3l in MIN6 cells suppressed glucose-stimulated secretion. These results suggest that Exoc3l is involved in regulated exocytosis of insulin granules.

Both daughter cells traffic and exocytose membrane at the cleavage furrow during mammalian cytokinesis

Membrane trafficking during cytokinesis is not well understood. We used advanced live cell imaging techniques to track exocytosis of single vesicles to determine whether constitutively exocytosed membrane is focally delivered to the cleavage furrow. Ultrasensitive three-dimensional confocal time-lapse imaging of the temperature-sensitive membrane cargo protein vesicular stomatitis virus protein–yellow fluorescent protein revealed that vesicles from both daughter cells traffic out of the Golgi and into the furrow, following curvilinear paths. Immunolocalization and photobleaching experiments indicate that individual vesicles accumulate at the midbody and generate a reserve vesicle pool that is distinct from endosomal and lysosomal compartments. Total internal reflection fluorescence microscopy imaging provided direct evidence that Golgi-derived vesicles from both daughter cells not only traffic to the furrow region but dock and fuse there, supporting a symmetrically polarized exocytic delivery model. In contrast, quantitative analysis of midbody abscission showed inheritance of the midbody remnant by one daughter cell, indicating that cytokinesis is composed of both symmetrical and asymmetrical stages.

The meiosis-specific Sid2p-related protein Slk1p regulates forespore membrane assembly in fission yeast

Cytokinesis in all organisms involves the creation of membranous barriers that demarcate individual daughter cells. In fission yeast, a signaling module termed the septation initiation network (SIN) plays an essential role in the assembly of new membranes and cell wall during cytokinesis. In this study, we have characterized Slk1p, a protein-kinase related to the SIN component Sid2p. Slk1p is expressed specifically during meiosis and localizes to the spindle pole bodies (SPBs) during meiosis I and II in a SIN-dependent manner. Slk1p also localizes to the forespore membrane during sporulation. Cells lacking Slk1p display defects associated with sporulation, leading frequently to the formation of asci with smaller and/or fewer spores. The ability of slk1 Delta cells to sporulate, albeit inefficiently, is fully abolished upon compromise of function of Sid2p, suggesting that Slk1p and Sid2p play overlapping roles in sporulation. Interestingly, increased expression of the syntaxin Psy1p rescues the sporulation defect of sid2-250 slk1 Delta. Thus, it is likely that Slk1p and Sid2p play a role in forespore membrane assembly by facilitating recruitment of components of the secretory apparatus, such as Psy1p, to allow membrane expansion. These studies thereby provide a novel link between the SIN and vesicle trafficking during cytokinesis.

Endopolyploidy in irradiated p53-deficient tumour cell lines: persistence of cell division activity in giant cells expressing Aurora-B kinase

Recent findings including computerised live imaging suggest that polyploidy cells transiently emerging after severe genotoxic stress (and named 'endopolyploid cells') may have a role in tumour regrowth after anti-cancer treatment. Until now, mostly the factors enabling metaphase were studied in them. Here we investigate the mitotic activities and the role of Aurora-B, in view of potential depolyploidisation of these cells, because Aurora-B kinase is responsible for coordination and completion of mitosis. We observed that endopolyploid giant cells are formed via different means in irradiated p53 tumours, by: (1) division/fusion of daughter cells creating early multi-nucleated cells; (2) asynchronous division/fusion of sub-nuclei of these multi-nucleated cells; (3) a series of polyploidising mitoses reverting replicative interphase from aborted metaphase and forming giant cells with a single nucleus; (4) micronucleation of arrested metaphases enclosing genome fragments; or (5) incomplete division in the multi-polar mitoses forming late multi-nucleated giant cells. We also observed that these activities can release para-diploid cells, although infrequently. While apoptosis typically occurs after a substantial delay in these cells, we also found that approximately 2% of the endopolyploid cells evade apoptosis and senescence arrest and continue some form of mitotic activity. We describe here that catalytically active Aurora-B kinase is expressed in the nuclei of many endopolyploid cells in interphase, as well as being present at the centromeres, mitotic spindle and cleavage furrow during their attempted mitotes. The totally micronucleated giant cells (containing sub-genomic fragments in multiple micronuclei) represented only the minor fraction which failed to undergo mitosis, and Aurora-B was absent from it. These observations suggest that most endopolyploid tumour cells are not reproductively inert and that Aurora-B may contribute to the establishment of resistant tumours post-irradiation.

Sequential Cyk-4 binding to ECT2 and FIP3 regulates cleavage furrow ingression and abscission during cytokinesis

Cytokinesis is a highly regulated and dynamic event that involves the reorganization of the cytoskeleton and membrane compartments. Recently, FIP3 has been implicated in targeting of recycling endosomes to the mid-body of dividing cells and is found required for abscission. Here, we demonstrate that the centralspindlin component Cyk-4 is a FIP3-binding protein. Furthermore, we show that FIP3 binds to Cyk-4 at late telophase and that centralspindlin may be required for FIP3 recruitment to the mid-body. We have mapped the FIP3-binding region on Cyk-4 and show that it overlaps with the ECT2-binding domain. Finally, we demonstrate that FIP3 and ECT2 form mutually exclusive complexes with Cyk-4 and that dissociation of ECT2 from the mid-body at late telophase may be required for the recruitment of FIP3 and recycling endosomes to the cleavage furrow. Thus, we propose that centralspindlin complex not only regulates acto-myosin ring contraction but also endocytic vesicle transport to the cleavage furrow and it does so through sequential interactions with ECT2 and FIP3.

Endocytic traffic in animal cell cytokinesis

Cytokinesis is the final step of mitosis whereby two daughter cells physically separate. It is initiated by the assembly of an actomyosin contractile ring at the mitotic cell equator, which constricts the cytoplasm between the two reforming nuclei resulting in the formation of a narrow intercellular bridge filled with central spindle microtubule bundles. Cytokinesis terminates with the cleavage of the intercellular bridge in a poorly understood process called abscission. Recent work has highlighted the importance of membrane trafficking events occurring from membrane compartments flanking the bridge to the central midbody region. In particular, polarized delivery of endocytic recycling membranes is essential for completion of animal cell cytokinesis. Why endocytic traffic occurs within the intercellular bridge remains largely mysterious and its significance for cytokinesis will be discussed.

Developmental regulation of central spindle assembly and cytokinesis during vertebrate embryogenesis

Mitosis and cytokinesis not only ensure the proper segregation of genetic information but also contribute importantly to morphogenesis in embryos. Cytokinesis is controlled by the central spindle, a microtubule-based structure containing numerous microtubule motors and microtubule-binding proteins, including PRC1. We show here that central spindle assembly and function differ dramatically between two related populations of epithelial cells in developing vertebrate embryos examined in vivo. Compared to epidermal cells, early neural epithelial cells undergo exaggerated anaphase chromosome separation, rapid furrowing, and a marked reduction of microtubule density in the spindle midzone. Cytokinesis in normal early neural epithelial cells thus resembles that in cultured vertebrate cells experimentally depleted of PRC1. We find that PRC1 mRNA and protein expression is surprisingly dynamic in early vertebrate embryos and that neural-plate cells contain less PRC1 than do epidermal cells. Expression of excess PRC1 ameliorates both the exaggerated anaphase and reduced midzone microtubule density observed in early neural epithelial cells. These PRC1-mediated modifications to the cytokinetic mechanism may be related to the specialization of the midbody in neural cells. These data suggest that PRC1 is a dose-dependent regulator of the central spindle in vertebrate embryos and demonstrate unexpected plasticity to fundamental mechanisms of cell division.

Breaking up is hard to do - membrane traffic in cytokinesis

Throughout normal development, and in aberrant conditions such as cancer, cells divide by a process called cytokinesis. Most textbooks suggest that animal cells execute cytokinesis using an actomyosin-containing contractile ring, whereas plant cells generate a new cell wall by the assembly of a novel membrane compartment using vesicle-trafficking machinery in an apparently distinct manner. Recent studies have shown that cytokinesis in animal and plant cells may not be as distinct as these models imply - both have an absolute requirement for vesicle traffic. Moreover, some of the key molecular components of cytokinesis have been identified, many of which are proteins that function to control membrane traffic. Here, we review recent advances in this area.

Tektin 2 is required for central spindle microtubule organization and the completion of cytokinesis

During anaphase, the nonkinetochore microtubules in the spindle midzone become compacted into the central spindle, a structure which is required to both initiate and complete cytokinesis. We show that Tektin 2 (Tek2) associates with the spindle poles throughout mitosis, organizes the spindle midzone microtubules during anaphase, and assembles into the midbody matrix surrounding the compacted midzone microtubules during cytokinesis. Tek2 small interfering RNA (siRNA) disrupts central spindle organization and proper localization of MKLP1, PRC1, and Aurora B to the midzone and prevents the formation of a midbody matrix. Video microscopy revealed that loss of Tek2 results in binucleate cell formation by aberrant fusion of daughter cells after cytokinesis. Although a myosin II inhibitor, blebbistatin, prevents actin-myosin contractility, the microtubules of the central spindle are compacted. Strikingly, Tek2 siRNA abolishes this actin-myosin-independent midzone microtubule compaction. Thus, Tek2-dependent organization of the central spindle during anaphase is essential for proper midbody formation and the segregation of daughter cells after cytokinesis.

A dual role for IQGAP1 in regulating exocytosis

Polarized secretion is a tightly regulated event generated by conserved, asymmetrically localized multiprotein complexes, and the mechanism(s) underlying its temporal and spatial regulation are only beginning to emerge. Although yeast Iqg1p has been identified as a positional marker linking polarity and exocytosis cues, studies on its mammalian counterpart, IQGAP1, have focused on its role in organizing cytoskeletal architecture, for which the underlying mechanism is unclear. Here, we report that IQGAP1 associates and co-localizes with the exocyst-septin complex, and influences the localization of the exocyst and the organization of septin. We further show that activation of CDC42 GTPase abolishes this association and inhibits secretion in pancreatic beta-cells. Whereas the N-terminus of IQGAP1 binds the exocyst-septin complex, enhances secretion and abrogates the inhibition caused by CDC42 or the depletion of IQGAP1, the C-terminus, which binds CDC42, inhibits secretion. Pulse-chase experiments indicate that IQGAP1 influences protein-synthesis rates, thus regulating exocytosis. We propose and discuss a model in which IQGAP1 serves as a conformational switch to regulate exocytosis.

Calcium signalling during the cleavage period of zebrafish development

Imaging studies, using both luminescent and fluorescent Ca(2+)-sensitive reporters, have revealed that during the first few meroblastic cleavages of the large embryos of teleosts, localized elevations of intracellular Ca(2+) accompany positioning, propagation, deepening and apposition of the cleavage furrows. Here, we will review the Ca(2+) transients reported during the cleavage period in these embryos, with reference mainly to that of the zebrafish (Danio rerio). We will also present the latest findings that support the proposal that Ca(2+) transients are an essential feature of embryonic cytokinesis. In addition, the potential upstream triggers and downstream targets of the different cytokinetic Ca(2+) transients will be discussed. Finally, we will present a hypothetical model that summarizes what has been suggested to be the various roles of Ca(2+) signalling during cytokinesis in teleost embryos.

Slk1 is a meiosis-specific Sid2-related kinase that coordinates meiotic nuclear division with growth of the forespore membrane

Septation and spore formation in fission yeast are compartmentalization processes that occur during the mitotic and meiotic cycles, and that are regulated by the septation initiation network (SIN). In mitosis, activation of Sid2 protein kinase transduces the signal from the spindle pole body (SPB) to the middle of the cell in order to promote the constriction of the actomyosin ring. Concomitant with ring contraction, membrane vesicles are added at the cleavage site to enable the necessary expansion of the cell membrane. In meiosis, the forespore membrane is synthesized from the outer layers of the SPB by vesicle fusion. This membrane grows and eventually engulfs each of the four haploid nuclei. The molecular mechanism that connects the SIN pathway with synthesis of the forespore membrane is poorly understood. Here, we describe a meiosis-specific Sid2-like kinase (Slk1), which is important for the coordination of the growth of the forespore membrane with the meiotic nuclear divisions. Slk1 and Sid2 are required for forespore membrane biosynthesis and seem to be the final output of the SIN pathway in meiosis.

Downregulation of protein 4.1R, a mature centriole protein, disrupts centrosomes, alters cell cycle progression, and perturbs mitotic spindles and anaphase

Centrosomes nucleate and organize interphase microtubules and are instrumental in mitotic bipolar spindle assembly, ensuring orderly cell cycle progression with accurate chromosome segregation. We report that the multifunctional structural protein 4.1R localizes at centrosomes to distal/subdistal regions of mature centrioles in a cell cycle-dependent pattern. Significantly, 4.1R-specific depletion mediated by RNA interference perturbs subdistal appendage proteins ninein and outer dense fiber 2/cenexin at mature centrosomes and concomitantly reduces interphase microtubule anchoring and organization. 4.1R depletion causes G(1) accumulation in p53-proficient cells, similar to depletion of many other proteins that compromise centrosome integrity. In p53-deficient cells, 4.1R depletion delays S phase, but aberrant ninein distribution is not dependent on the S-phase delay. In 4.1R-depleted mitotic cells, efficient centrosome separation is reduced, resulting in monopolar spindle formation. Multipolar spindles and bipolar spindles with misaligned chromatin are also induced by 4.1R depletion. Notably, all types of defective spindles have mislocalized NuMA (nuclear mitotic apparatus protein), a 4.1R binding partner essential for spindle pole focusing. These disruptions contribute to lagging chromosomes and aberrant microtubule bridges during anaphase/telophase. Our data provide functional evidence that 4.1R makes crucial contributions to the structural integrity of centrosomes and mitotic spindles which normally enable mitosis and anaphase to proceed with the coordinated precision required to avoid pathological events.

A dominant-negative ESCRT-III protein perturbs cytokinesis and trafficking to lysosomes

In eukaryotic cells, the completion of cytokinesis is dependent on membrane trafficking events to deliver membrane to the site of abscission. Golgi and recycling endosomal-derived proteins are required for the terminal stages of cytokinesis. Recently, protein subunits of the ESCRT (endosomal sorting complexes required for transport) that are normally involved in late endosome to lysosome trafficking have also been implicated in abscission. Here, we report that a subunit, CHMP3 (charged multivesicular body protein-3), of ESCRT-III localizes at the midbody. Deletion of the C-terminal autoinhibitory domain of CHMP3 inhibits cytokinesis. At the midbody, CHMP3 does not co-localize with Rab11, suggesting that it is not present on recycling endosomes. These results combined provide compelling evidence that proteins involved in late endosomal function are necessary for the end stages of cytokinesis.

Final stages of cytokinesis and midbody ring formation are controlled by BRUCE

Cytokinesis involves the formation of a cleavage furrow, followed by abscission, the cutting of the midbody channel, the final bridge between dividing cells. Recently, the midbody ring became known as central for abscission, but its regulation remains enigmatic. Here, we identify BRUCE, a 528 kDa multifunctional protein, which processes ubiquitin-conjugating activity, as a major regulator of abscission. During cytokinesis, BRUCE moves from the vesicular system to the midbody ring and serves as a platform for the membrane delivery machinery and mitotic regulators. Depletion of BRUCE in cell cultures causes defective abscission and cytokinesis-associated apoptosis, accompanied by a block of vesicular targeting and defective formation of the midbody and the midbody ring. Notably, ubiquitin relocalizes from midbody microtubules to the midbody ring during cytokinesis, and depletion of BRUCE disrupts this process. We propose that BRUCE coordinates multiple steps required for abscission and that ubiquitylation may be a crucial trigger.

Stem cells and niches: mechanisms that promote stem cell maintenance throughout life

Niches are local tissue microenvironments that maintain and regulate stem cells. Long-predicted from mammalian studies, these structures have recently been characterized within several invertebrate tissues using methods that reliably identify individual stem cells and their functional requirements. Although similar single-cell resolution has usually not been achieved in mammalian tissues, principles likely to govern the behavior of niches in diverse organisms are emerging. Considerable progress has been made in elucidating how the microenvironment promotes stem cell maintenance. Mechanisms of stem cell maintenance are key to the regulation of homeostasis and likely contribute to aging and tumorigenesis when altered during adulthood.

Spermatocyte cytokinesis requires rapid membrane addition mediated by ARF6 on central spindle recycling endosomes

The dramatic cell shape changes during cytokinesis require the interplay between microtubules and the actomyosin contractile ring, and addition of membrane to the plasma membrane. Numerous membrane-trafficking components localize to the central spindle during cytokinesis, but it is still unclear how this machinery is targeted there and how membrane trafficking is coordinated with cleavage furrow ingression. Here we use an arf6 null mutant to show that the endosomal GTPase ARF6 is required for cytokinesis in Drosophila spermatocytes. ARF6 is enriched on recycling endosomes at the central spindle, but it is required neither for central spindle nor actomyosin contractile ring assembly, nor for targeting of recycling endosomes to the central spindle. However, in arf6 mutants the cleavage furrow regresses because of a failure in rapid membrane addition to the plasma membrane. We propose that ARF6 promotes rapid recycling of endosomal membrane stores during cytokinesis, which is critical for rapid cleavage furrow ingression.

The role of FIP3-dependent endosome transport during cytokinesis

Mitosis is a complex cellular process that is completed by the final abscission step called cytokinesis. The many roles of cytoskeletal proteins in animal cell division have been studied extensively, and are essential for proper daughter cell segregation. Lately, the need for the membrane delivery to the cleavage furrow of dividing cells has been implicated as a necessary and important step in cytokinesis.1-4 Newly published work from several group demonstrate that endosomal membranes are required for cytokinesis and also provide insight into the targeting of these membranes to the cleavage furrow.5-8 Rab11 GTPase and its effector protein FIP3 were shown to play key roles in endosome targeting to the cleavage furrow in a spatially and temporally regulated manner.7-9 Thus, Rab11/FIP3 protein complex have emerged as a regulator of endocytic traffic, essential for the abscission step in cytokinesis.

Secretory mechanisms in cell-mediated cytotoxicity

Cytotoxic T lymphocytes (CTLs) play a critical role in the immune system; they are able to recognize and destroy virally infected and tumorigenic cells. Specific recognition of MHC class I-peptide complexes by the T cell receptor (TcR) results in precise delivery of lytic granules to the target cell, sparing neighboring cells and the CTL itself. Over the past 10 years various studies have eludicated the mechanisms that lead to the rapid polarization of the secretory apparatus in CTLs. These studies highlight similarities and differences between polarity and secretory mechanisms seen in other cell types and developmental systems. This review focuses on recent advances in our understanding of the molecular basis of polarized secretion from CTLs and the novel mechanism used by these cells to deliver their lethal hit.

Cytokinesis: placing and making the final cut

Cytokinesis is the process by which cells physically separate after the duplication and spatial segregation of the genetic material. A number of general principles apply to this process. First the microtubule cytoskeleton plays an important role in the choice and positioning of the division site. Once the site is chosen, the local assembly of the actomyosin contractile ring remodels the plasma membrane. Finally, membrane trafficking to and membrane fusion at the division site cause the physical separation of the daughter cells, a process termed abscission. Here we will discuss recent advances in our understanding of the mechanisms of cytokinesis in animals, yeast, and plants.

Conversion of plasma membrane topology during epithelial tube connection requires Arf-like 3 small GTPase in Drosophila

The development of tubular organs often involves the hollowing of cells into a torus (doughnut shape), as observed in blood vessel formation in vertebrates and tracheal development in insects. During the fusion of Drosophila tracheal branches, fusion cells located at the tip of migrating branches contact each other and form intracellular luminal cavities on opposite sides of the cells that open to connect the tubule lumens. This process involves the intracellular fusion of plasma membranes associated with microtubule tracks. Here, we studied the function of an evolutionarily conserved small GTPase, Arf-like 3, in branch fusion. Arf-like 3 is N-terminally acetylated, and associates with both intracellular vesicles and microtubules. In Arf-like 3 mutants, the cell adhesion of fusion cells, specification of apical membrane domains, and secretion of luminal extracellular matrix proceeded normally, but the luminal cavities did not open due to the failure of intracellular fusion of the plasma membranes. We present evidence that the Arf-like 3 mutation impairs the localized assembly of the exocyst complex, suggesting that the targeting of exocytosis machinery to specific apical domains is the key step in converting the plasma membrane topology in fusion cells.

Cooperative assembly of CYK-4/MgcRacGAP and ZEN-4/MKLP1 to form the centralspindlin complex

Cytokinesis in metazoan cells requires a set of antiparallel microtubules that become bundled upon anaphase onset to form a structure known as the central spindle. Bundling of these microtubules requires a protein complex, centralspindlin, that consists of the CYK-4/MgcRacGAP Rho-family GTPase-activating protein and the ZEN-4/MKLP1 kinesin-6 motor protein. Centralspindlin, but not its individual subunits, is sufficient to bundle microtubules in vitro. Here, we present a biochemical and genetic dissection of centralspindlin. We show that each of the two subunits of centralspindlin dimerize via a parallel coiled coil. The two homodimers assemble into a high-affinity heterotetrameric complex by virtue of two low-affinity interactions. Conditional mutations in the regions that mediate complex assembly can be readily suppressed by numerous second site mutations in the interacting regions. This unexpected plasticity explains the lack of primary sequence conservation of the regions critical for this essential protein-protein interaction.

Rab11 is required for membrane trafficking and actomyosin ring constriction in meiotic cytokinesis of Drosophila males

Rab11 is a small GTPase that regulates several aspects of vesicular trafficking. Here, we show that Rab11 accumulates at the cleavage furrow of Drosophila spermatocytes and that it is essential for cytokinesis. Mutant spermatocytes form regular actomyosin rings, but these rings fail to constrict to completion, leading to cytokinesis failures. rab11 spermatocytes also exhibit an abnormal accumulation of Golgi-derived vesicles at the telophase equator, suggesting a defect in membrane-vesicle fusion. These cytokinesis phenotypes are identical to those elicited by mutations in giotto (gio) and four wheel drive (fwd) that encode a phosphatidylinositol transfer protein and a phosphatidylinositol 4-kinase, respectively. Double mutant analysis and immunostaining for Gio and Rab11 indicated that gio, fwd, and rab11 function in the same cytokinetic pathway, with Gio and Fwd acting upstream of Rab11. We propose that Gio and Fwd mediate Rab11 recruitment at the cleavage furrow and that Rab11 facilitates targeted membrane delivery to the advancing furrow.

Human ESCRT and ALIX proteins interact with proteins of the midbody and function in cytokinesis

TSG101 and ALIX both function in HIV budding and in vesicle formation at the multivesicular body (MVB), where they interact with other Endosomal Sorting Complex Required for Transport (ESCRT) pathway factors required for release of viruses and vesicles. Proteomic analyses revealed that ALIX and TSG101/ESCRT-I also bind a series of proteins involved in cytokinesis, including CEP55, CD2AP, ROCK1, and IQGAP1. ALIX and TSG101 concentrate at centrosomes and are then recruited to the midbodies of dividing cells through direct interactions between the central CEP55 'hinge' region and GPP-based motifs within TSG101 and ALIX. ESCRT-III and VPS4 proteins are also recruited, indicating that much of the ESCRT pathway localizes to the midbody. Depletion of ALIX and TSG101/ESCRT-I inhibits the abscission step of HeLa cell cytokinesis, as does VPS4 overexpression, confirming a requirement for these proteins in cell division. Furthermore, ALIX point mutants that block CEP55 and CHMP4/ESCRT-III binding also inhibit abscission, indicating that both interactions are essential. These experiments suggest that the ESCRT pathway may be recruited to facilitate analogous membrane fission events during HIV budding, MVB vesicle formation, and the abscission stage of cytokinesis.

Activation of RalA Is Required for Insulin-Stimulated Glut4 Trafficking to the Plasma Membrane via the Exocyst and the Motor Protein Myo1c

Insulin stimulates glucose transport in muscle and adipose tissue by producing translocation of the glucose transporter Glut4. The exocyst, an evolutionarily conserved vesicle tethering complex, is crucial for targeting Glut4 to the plasma membrane. Here we report that insulin regulates this process via the G protein RalA, which is present in Glut4 vesicles and interacts with the exocyst in adipocytes. Insulin stimulates the activity of RalA in a PI 3-kinase-dependent manner. Disruption of RalA function by dominant-negative mutants or siRNA-mediated knockdown attenuates insulin-stimulated glucose transport. RalA also interacts with Myo1c, a molecular motor implicated in Glut4 trafficking. This interaction is modulated by Calmodulin, which functions as the light chain for Myo1c during insulin-stimulated glucose uptake. Thus, RalA serves two functions in insulin action: as a cargo receptor for the Myo1c motor, and as a signal for the unification of the exocyst to target Glut4 vesicles to the plasma membrane.

A novel Crumbs3 isoform regulates cell division and ciliogenesis via importin beta interactions

The Crumbs family of apical transmembrane proteins regulates apicobasal polarity via protein interactions with a conserved C-terminal sequence, ERLI. However, one of the mammalian Crumbs proteins, Crumbs3 (CRB3) has an alternate splice form with a novel C-terminal sequence ending in CLPI (CRB3-CLPI). We report that CRB3-CLPI localizes to the cilia membrane and a membrane compartment at the mitotic spindle poles. Knockdown of CRB3-CLPI leads to both a loss of cilia and a multinuclear phenotype associated with centrosomal and spindle abnormalities. Using protein purification, we find that CRB3-CLPI interacts with importin beta-1 in a Ran-regulated fashion. Importin beta-1 colocalizes with CRB3-CLPI during mitosis, and a dominant-negative form of importin beta-1 closely phenocopies CRB3-CLPI knockdown. Knockdown of importin beta-1 blocks targeting of CRB3-CLPI to the spindle poles. Our data suggest an expanded role for Crumbs proteins in polarized membrane targeting and cell division via unique interactions with importin proteins.

Molecular characterization of Rab11-FIP3 binding to ARF GTPases

Rab11-FIP3 is a Rab11-binding protein that has been implicated in regulating cytokinesis in mammalian cells. FIP3 functions by simultaneously interacting with Rab11 as well as Arf GTPases. However, unlike the interaction between Rab11 and FIP3, the structural basis of FIP3 binding to Arf GTPases has not yet been determined. The specificity of interaction between FIP3 and Arf GTPases remains controversial. While it was reported that FIP3 preferentially binds to Arf6 some data suggest that FIP3 can also interact with Arf5 and even possibly Arf4. The Arf-interaction motif on FIP3 also remains to be determined. Finally, the importance of Arf binding to FIP3 in regulating cell division and other cellular functions remains unclear. Here we used a combination of various biochemical techniques to measure the affinity of FIP3 binding to various Arfs and to demonstrate that FIP3 predominantly interacts with Arf6 in vitro and in vivo. In addition, we identified the motifs mediating Arf6 and FIP3 interaction and demonstrated that FIP3 binds to the Arf6 C-terminus rather than switch motifs. Finally we show that FIP3 and Arf6 binding is required for the targeting of Arf6 to the cleavage furrow during cytokinesis. Thus, we propose that FIP3 is a scaffolding protein that, in addition to regulating endosome targeting to the cleavage furrow, also is required for Arf6 recruitment to the midbody during late telophase.

Active ADP-ribosylation factor-1 (ARF1) is required for mitotic Golgi fragmentation

In mammalian cells the Golgi apparatus undergoes an extensive disassembly process at the onset of mitosis that is believed to facilitate equal partitioning of this organelle into the two daughter cells. However, the underlying mechanisms for this fragmentation process are so far unclear. Here we have investigated the role of the ADP-ribosylation factor-1 (ARF1) in this process to determine whether Golgi fragmentation in mitosis is mediated by vesicle budding. ARF1 is a small GTPase that is required for COPI vesicle formation from the Golgi membranes. Treatment of Golgi membranes with mitotic cytosol or with purified coatomer together with wild type ARF1 or its constitutive active form, but not the inactive mutant, converted the Golgi membranes into COPI vesicles. ARF1-depleted mitotic cytosol failed to fragment Golgi membranes. ARF1 is associated with Golgi vesicles generated in vitro and with vesicles in mitotic cells. In addition, microinjection of constitutive active ARF1 did not affect mitotic Golgi fragmentation or cell progression through mitosis. Our results show that ARF1 is active during mitosis and that this activity is required for mitotic Golgi fragmentation.

Parallels between cytokinesis and retroviral budding: a role for the ESCRT machinery

During cytokinesis, as dividing animal cells pull apart into two daughter cells, the final stage, termed abscission, requires breakage of the midbody, a thin membranous stalk connecting the daughter cells. This membrane fission event topologically resembles the budding of viruses, such as HIV-1, from infected cells. We found that two proteins involved in HIV-1 budding-tumor susceptibility gene 101 (Tsg101), a subunit of the endosomal sorting complex required for transport I (ESCRT-I), and Alix, an ESCRT-associated protein-were recruited to the midbody during cytokinesis by interaction with centrosome protein 55 (Cep55), a centrosome and midbody protein essential for abscission. Tsg101, Alix, and possibly other components of ESCRT-I were required for the completion of cytokinesis. Thus, HIV-1 budding and cytokinesis use a similar subset of cellular components to carry out topologically similar membrane fission events.