Terminal cytokinesis events uncovered after an RNAi screen

KIF14 and citron kinase act together to promote efficient cytokinesis

Multiple mitotic kinesins and microtubule-associated proteins (MAPs) act in concert to direct cytokinesis (Glotzer, M. 2005. Science. 307:1735–1739). In anaphase cells, many of these proteins associate with an antiparallel array of microtubules termed the central spindle. The MAP and microtubule-bundling protein PRC1 (protein-regulating cytokinesis 1) is one of the key molecules required for the integrity of this structure (Jiang, W., G. Jimenez, N.J. Wells, T.J. Hope, G.M. Wahl, T. Hunter, and R. Fukunaga. 1998. Mol. Cell. 2:877–885; Mollinari, C., J.P. Kleman, W. Jiang, G. Schoehn, T. Hunter, and R.L. Margolis. 2002. J. Cell Biol. 157:1175–1186). In this study, we identify an interaction between endogenous PRC1 and the previously uncharacterized kinesin KIF14 as well as other mitotic kinesins (MKlp1/CHO1, MKlp2, and KIF4) with known functions in cytokinesis (Hill, E., M. Clarke, and F.A. Barr. 2000. EMBO J. 19:5711–5719; Matuliene, J., and R. Kuriyama. 2002. Mol. Biol. Cell. 13:1832–1845; Kurasawa, Y., W.C. Earnshaw, Y. Mochizuki, N. Dohmae, and K. Todokoro. 2004. EMBO J. 23:3237–3248). We find that KIF14 targets to the central spindle via its interaction with PRC1 and has an essential function in cytokinesis. In KIF14-depleted cells, citron kinase but not other components of the central spindle and cleavage furrow fail to localize. Furthermore, the localization of KIF14 and citron kinase to the central spindle and midbody is codependent, and they form a complex depending on the activation state of citron kinase. Contrary to a previous study (Di Cunto, F., S. Imarisio, E. Hirsch, V. Broccoli, A. Bulfone, A. Migheli, C. Atzori, E. Turco, R. Triolo, G.P. Dotto, et al. 2000. Neuron. 28:115–127), we find a general requirement for citron kinase in human cell division. Together, these findings identify a novel pathway required for efficient cytokinesis.

Developmental control of nuclear morphogenesis and anchoring by charleston, identified in a functional genomic screen of Drosophila cellularisation

Morphogenesis of epithelial tissues relies on the precise developmental control of cell polarity and architecture. In the early Drosophila embryo, the primary epithelium forms during cellularisation, following a tightly controlled genetic programme where specific sets of genes are upregulated. Some of them, for example, control membrane invagination between the nuclei anchored at the apical surface of the syncytium. We used microarrays to describe the global programme of gene expression underlying cellularisation and identified distinct classes of upregulated genes during this process. Fifty-seven genes were then tested functionally by RNAi. We found six genes affecting various aspects of cellular architecture: membrane growth, organelle transport or organisation and junction assembly. We focus here on charleston (char), a new regulator of nuclear morphogenesis and of apical nuclear anchoring. In char-depleted embryos, the nuclei fail to maintain their elongated shape and, instead, become rounded. In addition, together with a disruption of the centrosome-nuclear envelope interaction, the nuclei lose their regular apical anchoring. These nuclear defects perturb the regular columnar organisation of epithelial cells in the embryo. Although microtubules are required for both nuclear morphogenesis and anchoring, char does not control microtubule organisation and association to the nuclear envelope. We show that Char is lipid anchored at the nuclear envelope by a farnesylation group, and localises at the inner nuclear membrane together with Lamin. Our data suggest that Char forms a scaffold that regulates nuclear architecture to constrain nuclei in tight columnar epithelial cells. The upregulation of Char during cellularisation and gastrulation reveals the existence of an as yet unknown developmental control of nuclear morphology and anchoring in embryonic epithelia.

A role for the Rab6A' GTPase in the inactivation of the Mad2-spindle checkpoint

The two isoforms of the Rab6 GTPase, Rab6A and Rab6A', regulate a retrograde transport route connecting early endosomes and the endoplasmic reticulum via the Golgi complex in interphasic cells. Here we report that when Rab6A' function is altered cells are unable to progress normally through mitosis. Such cells are blocked in metaphase, despite displaying a normal Golgi fragmentation and with the Mad2-spindle checkpoint activated. Furthermore, the Rab6 effector p150(Glued), a subunit of the dynein/dynactin complex, remains associated with some kinetochores. A similar phenotype was observed when GAPCenA, a GTPase-activating protein of Rab6, was depleted from cells. Our results suggest that Rab6A' likely regulates the dynamics of the dynein/dynactin complex at the kinetochores and consequently the inactivation of the Mad2-spindle checkpoint. Rab6A', through its interaction with p150(Glued) and GAPCenA, may thus participate in a pathway involved in the metaphase/anaphase transition.

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

During determination of the cell division plane, an actomyosin contractile ring is induced at the equatorial cell cortex by signals from the mitotic apparatus and contracts to cause cleavage furrow progression. Although the small GTPase RhoA is known to regulate the progression, probably by controlling actin filament assembly and enhancing actomyosin interaction, any involvement of RhoA in division plane determination is unknown. In this study, using a trichloroacetic acid (TCA) fixation protocol we recently developed, we show that RhoA accumulates at the equatorial cortex before furrow initiation and continues to concentrate at the cleavage furrow during cytokinesis. We also demonstrate that both Rho activity and microtubule organization are required for RhoA localization and proper furrowing. Selective disruption of microtubule organization revealed that both astral and central spindle microtubules can recruit RhoA at the equatorial cortex. We find that centralspindlin and ECT2 are required for RhoA localization and furrowing. Centralspindlin is localized both to central spindle microtubules and at the tips of astral microtubules near the equatorial cortex and recruits ECT2. Positional information for division plane determination from microtubules is transmitted to the cell cortex to organize actin cytoskeleton through a mechanism involving these proteins.

Identification of Drosophila gene products required for phagocytosis of Candida albicans

Phagocytosis is a highly conserved aspect of innate immunity. We used Drosophila melanogaster S2 cells as a model system to study the phagocytosis of Candida albicans, the major fungal pathogen of humans, by screening an RNAi library representing 7,216 fly genes conserved among metazoans. After rescreening the initial genes identified and eliminating certain classes of housekeeping genes, we identified 184 genes required for efficient phagocytosis of C. albicans. Diverse biological processes are represented, with actin cytoskeleton regulation, vesicle transport, signaling, and transcriptional regulation being prominent. Secondary screens using Escherichia coli and latex beads revealed several genes specific for C. albicans phagocytosis. Characterization of one of those gene products, Macroglobulin complement related (Mcr), shows that it is secreted, that it binds specifically to the surface of C. albicans, and that it promotes its subsequent phagocytosis. Mcr is closely related to the four Drosophila thioester proteins (Teps), and we show that TepII is required for efficient phagocytosis of E. coli (but not C. albicans or Staphylococcus aureus) and that TepIII is required for the efficient phagocytosis of S. aureus (but not C. albicans or E. coli). Thus, this family of fly proteins distinguishes different pathogens for subsequent phagocytosis.

Mcr and the closely related Drosophila Tep proteins (proteins similar to mammalian secreted immune complement) bind to the surface of invading microbes and are required to promote the phagocytosis of specific pathogens.

Midbodies and phragmoplasts: analogous structures involved in cytokinesis

Cytokinesis is an event common to all organisms that involves the precise coordination of independent pathways involved in cell-cycle regulation and microtubule, membrane, actin and organelle dynamics. In animal cells, the spindle midzone/midbody with associated endo-membrane system are required for late cytokinesis events, including furrow ingression and scission. In plants, cytokinesis is mediated by the phragmoplast, an array of microtubules, actin filaments and associated molecules that act as a framework for the future cell wall. In this article (which is part of the Cytokinesis series), we discuss recent studies that highlight the increasing number of similarities in the components and function of the spindle midzone/midbody in animals and the phragmoplast in plants, suggesting that they might be analogous structures.

Cytokinesis: welcome to the Rho zone

Cytokinesis follows nuclear division and generates two distinct daughter cells, each replete with a full complement of the genome and cytoplasmic organelles. Members of the Rho family of GTPases are crucial regulators of this process in a wide variety of species. In many cell types, cytokinesis is mediated by a discretely localized contractile ring that is rich in actin and myosin. In this article (which is part of the Cytokinesis series), we review recent studies in animal cells that have shown that local assembly of the contractile ring is mediated by a discrete pool of GTP-bound, active RhoA. Advances in detecting the active pool of RhoA have allowed insights into the mechanisms and the molecules that promote the accumulation of active RhoA at the correct time and place in the cell.

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

The terminal step in cytokinesis, called abscission, requires resolution of the membrane connection between two prospective daughter cells. Our previous studies demonstrated that the coiled-coil protein centriolin localized to the midbody during cytokinesis and was required for abscission. Here we show that centriolin interacts with proteins of vesicle-targeting exocyst complexes and vesicle-fusion SNARE complexes. These complexes require centriolin for localization to a unique midbody-ring structure, and disruption of either complex inhibits abscission. Exocyst disruption induces accumulation of v-SNARE-containing vesicles at the midbody ring. In control cells, these v-SNARE vesicles colocalize with a GFP-tagged secreted polypeptide. The vesicles move to the midbody ring asymmetrically from one prospective daughter cell; the GFP signal is rapidly lost, suggesting membrane fusion; and subsequently the cell cleaves at the site of vesicle delivery/fusion. We propose that centriolin anchors protein complexes required for vesicle targeting and fusion and integrates membrane-vesicle fusion with abscission.

Proteomic analysis of isolated chlamydomonas centrioles reveals orthologs of ciliary-disease genes

BACKGROUND

The centriole is one of the most enigmatic organelles in the cell. Centrioles are cylindrical, microtubule-based barrels found in the core of the centrosome. Centrioles also act as basal bodies during interphase to nucleate the assembly of cilia and flagella. There are currently only a handful of known centriole proteins.

RESULTS

We used mass-spectrometry-based MudPIT (multidimensional protein identification technology) to identify the protein composition of basal bodies (centrioles) isolated from the green alga Chlamydomonas reinhardtii. This analysis detected the majority of known centriole proteins, including centrin, epsilon tubulin, and the cartwheel protein BLD10p. By combining proteomic data with information about gene expression and comparative genomics, we identified 45 cross-validated centriole candidate proteins in two classes. Members of the first class of proteins (BUG1-BUG27) are encoded by genes whose expression correlates with flagellar assembly and which therefore may play a role in ciliogenesis-related functions of basal bodies. Members of the second class (POC1-POC18) are implicated by comparative-genomics and -proteomics studies to be conserved components of the centriole. We confirmed centriolar localization for the human homologs of four candidate proteins. Three of the cross-validated centriole candidate proteins are encoded by orthologs of genes (OFD1, NPHP-4, and PACRG) implicated in mammalian ciliary function and disease, suggesting that oral-facial-digital syndrome and nephronophthisis may involve a dysfunction of centrioles and/or basal bodies.

CONCLUSIONS

By analyzing isolated Chlamydomonas basal bodies, we have been able to obtain the first reported proteomic analysis of the centriole.

Cleavage furrow formation and ingression during animal cytokinesis: a microtubule legacy

Cytokinesis ensures the proper partitioning of the nuclear and cytoplasmic contents into independent daughter cells at the end of cell division. Although the metazoan mitotic spindle has been implicated in the placement and advancement of the cleavage furrow, the molecules responsible for these processes have remained elusive. Recent studies have provided insights into the role of different microtubule structures and associated proteins in cleavage furrow positioning and ingression together with the signalling events that regulate the dynamics of the equatorial cell cortex during cytokinesis. We try to unify these findings into a general model of cytokinesis in which both astral and central spindle microtubules have the ability to induce furrowing. We further propose that the evolutionarily conserved centralspindlin complex serves as a master controller of cell cleavage in Drosophila by promoting both furrow formation and ingression. The same mechanism might be conserved in other organisms.

Septins: traffic control at the cytokinesis intersection

The physical division of one cell into two requires the highly orchestrated separation of genetic and cytoplasmic contents during M phase of the cell cycle. Mitosis, the physical segregation of the genetic material of a cell into two daughter cells, has traditionally received more attention than cytokinesis, the partitioning of the cytoplasmic contents, yet clearly the two processes must be intimately co-ordinated and tightly regulated. While plant cells divide by the formation of a membranous cell barrier called the phragmoplast, animal cell division is largely driven by contraction of an actomyosin ring. However, recent evidence has suggested that membranes derived from one or more intracellular compartments are also required to break the cytoplasmic bridge connecting two dividing cells during late telophase. In this review, we focus on studies of animal cell cytokinesis that support a requirement for specific endomembrane fusion during fission, define molecular components of the membrane fusion apparatus that may be involved and point to possible roles for an emerging family of cytoskeletal proteins, the septins, in this process.

Grp/DChk1 is required for G2-M checkpoint activation in Drosophila S2 cells, whereas Dmnk/DChk2 is dispensable

Cell-cycle checkpoints are signal-transduction pathways required to maintain genomic stability in dividing cells. Previously, it was reported that two kinases essential for checkpoint signalling, Chk1 and Chk2 are structurally conserved. In contrast to yeast, Xenopus and mammals, the Chk1- and Chk2-dependent pathways in Drosophila are not understood in detail. Here, we report the function of these checkpoint kinases, referred to as Grp/DChk1 and Dmnk/DChk2 in Drosophila Schneider's cells, and identify an upstream regulator as well as downstream targets of Grp/DChk1. First, we demonstrate that S2 cells are a suitable model for G(2)/M checkpoint studies. S2 cells display Grp/DChk1-dependent and Dmnk/DChk2-independent cell-cycle-checkpoint activation in response to hydroxyurea and ionizing radiation. S2 cells depleted for Grp/DChk1 using RNA interference enter mitosis in the presence of impaired DNA integrity, resulting in prolonged mitosis and mitotic catastrophe. Grp/DChk1 is phosphorylated in a Mei-41/DATR-dependent manner in response to hydroxyurea and ionizing radiation, indicating that Mei-41/ATR is an upstream component in the Grp/DChk1 DNA replication and DNA-damage-response pathways. The level of Cdc25(Stg) and phosphorylation status of Cdc2 are modulated in a Grp/DChk1-dependent manner in response to hydroxyurea and irradiation, indicating that these cell-cycle regulators are downstream targets of the Grp/DChk1-dependent DNA replication and DNA-damage responses. By contrast, depletion of Dmnk/DChk2 by RNA interference had little effect on checkpoint responses to hydroxyurea and irradiation. We conclude that Grp/DChk1, and not Dmnk/DChk2, is the main effector kinase involved in G(2)/M checkpoint control in Drosophila cells.

Distinct pathways control recruitment and maintenance of myosin II at the cleavage furrow during cytokinesis

The correct localization of myosin II to the equatorial cortex is crucial for proper cell division. Here, we examine a collection of genes that cause defects in cytokinesis and reveal with live cell imaging two distinct phases of myosin II localization. Three genes in the rho1 signaling pathway, pebble (a Rho guanidine nucleotide exchange factor), rho1, and rho kinase, are required for the initial recruitment of myosin II to the equatorial cortex. This initial localization mechanism does not require F-actin or the two components of the centralspindlin complex, the mitotic kinesin pavarotti/MKLP1 and racGAP50c/CYK-4. However, F-actin, the centralspindlin complex, formin (diaphanous), and profilin (chickadee) are required to stably maintain myosin II at the furrow. In the absence of these latter genes, myosin II delocalizes from the equatorial cortex and undergoes highly dynamic appearances and disappearances around the entire cell cortex, sometimes associated with abnormal contractions or blebbing. Our findings support a model in which a rho kinase-dependent event, possibly myosin II regulatory light chain phosphorylation, is required for the initial recruitment to the furrow, whereas the assembly of parallel, unbranched actin filaments, generated by formin-mediated actin nucleation, is required for maintaining myosin II exclusively at the equatorial cortex.

Cytokinesis depends on the motor domains of myosin-II in fission yeast but not in budding yeast

Budding yeast possesses one myosin-II, Myo1p, whereas fission yeast has two, Myo2p and Myp2p, all of which contribute to cytokinesis. We find that chimeras consisting of Myo2p or Myp2p motor domains fused to the tail of Myo1p are fully functional in supporting budding yeast cytokinesis. Remarkably, the tail alone of budding yeast Myo1p localizes to the contractile ring, supporting both its constriction and cytokinesis. In contrast, fission yeast Myo2p and Myp2p require both the catalytic head domain as well as tail domains for function, with the tails providing distinct functions (Bezanilla and Pollard, 2000). Myo1p is the first example of a myosin whose cellular function does not require a catalytic motor domain revealing a novel mechanism of action for budding yeast myosin-II independent of actin binding and ATPase activity.

Characterization of anillin mutants reveals essential roles in septin localization and plasma membrane integrity

Anillin is a conserved component of the contractile ring that is essential for cytokinesis, and physically interacts with three conserved cleavage furrow proteins, F-actin, myosin II and septins in biochemical assays. We demonstrate that the Drosophila scraps gene, identified as a gene involved in cellularization, encodes Anillin. We characterize defects in cellularization, pole cell formation and cytokinesis in a series of maternal effect and zygotic anillin alleles. Mutations that result in amino acid changes in the C-terminal PH domain of Anillin cause defects in septin recruitment to the furrow canal and contractile ring. These mutations also strongly perturb cellularization, altering the timing and rate of furrow ingression. They cause dramatic vesiculation of new plasma membranes, and destabilize the stalk of cytoplasm that normally connects gastrulating cells to the yolk mass. A mutation closer to the N terminus blocks separation of pole cells with less effect on cellularization, highlighting mechanistic differences between contractile processes. Cumulatively, our data point to an important role for Anillin in scaffolding cleavage furrow components, directly stabilizing intracellular bridges, and indirectly stabilizing newly deposited plasma membrane during cellularization.

Distinct roles for two C. elegans anillins in the gonad and early embryo

Anillins are conserved proteins that are important for stabilizing and remodeling the actin cytoskeleton. Anillins have been implicated in cytokinesis in several systems and in cellularization of the syncytial Drosophila embryo. Here, we examine the functions of three C. elegans proteins with homology to anillin (ANI-1, ANI-2 and ANI-3). We show that ANI-1 and ANI-2 contribute to embryonic viability by performing distinct functions in the early embryo and gonad, respectively. By contrast, ANI-3 appears to be dispensable for embryonic development. ANI-1 is essential for cortical ruffling and pseudocleavage, contractile events that occur in embryos prior to mitosis. ANI-1 is also required for the highly asymmetric cytokinetic events that extrude the two polar bodies during oocyte meiosis, but is dispensable for cytokinesis following mitotic chromosome segregation. During both meiosis and mitosis, ANI-1 targets the septins, but not myosin II, to the contractile ring and does not require either for its own targeting. In contrast to ANI-1, ANI-2 functions during oogenesis to maintain the structure of the rachis, the central core of cytoplasm that connects the developing oocytes in the syncytial gonad. In ANI-2-depleted worms, oocytes disconnect prematurely from the defective rachis, generating embryos of varying sizes. Our results highlight specialization of divergent anillin family proteins in the C. elegans life cycle and reveal conserved roles for this protein family in organizing syncytial structures and cortical contractility.

E-RNAi: a web application to design optimized RNAi constructs

RNA interference (RNAi) has become a powerful genetic approach to systematically dissect gene function on a genome-wide scale. Owing to the penetrance and efficiency of RNAi in invertebrates, model organisms such as Drosophila melanogaster and Caenorhabditis elegans have contributed significantly to the identification of novel components of diverse biological pathways, ranging from early development to fat storage and aging. For the correct assessment of phenotypes, a key issue remains the stringent quality control of long double-stranded RNAs (dsRNA) to calculate potential off-target effects that may obscure the phenotypic data. We here describe a web-based tool to evaluate and design optimized dsRNA constructs. Moreover, the application also gives access to published predesigned dsRNAs. The E-RNAi web application is available at .

Split decisions: coordinating cytokinesis in yeast

Cytokinesis in eukaryotes involves the regulated assembly and contraction of a ring comprising filamentous (F)-actin and myosin II. Assembly of the contractile ring occurs through the accumulation of cortical cues at the specified division plane, followed by recruitment of F-actin, myosin II and accessory proteins involved in generating the mature ring. Ring contraction is temporally regulated to occur only after chromosome segregation and, in yeast, it is controlled by a conserved signaling cascade that becomes active only after Cdk1-Cyclin-B inactivation. In this article (which is part of the Cytokinesis series), we discuss recent studies that have begun to clarify both the spatial and the temporal order of ring assembly and that have illuminated the signals that trigger ring contraction in yeast. These studies add to the growing knowledge of the processes that control eukaryotic cell division.

Anillin is a substrate of anaphase-promoting complex/cyclosome (APC/C) that controls spatial contractility of myosin during late cytokinesis

Anillin, an actin-binding protein localized at the cleavage furrow, is required for cytokinesis. Through an in vitro expression screen, we identified anillin as a substrate of the anaphase-promoting complex/cyclosome (APC/C), a ubiquitin ligase that controls mitotic progression. We found that the levels of anillin fluctuate in the cell cycle, peaking in mitosis and dropping drastically during mitotic exit. Ubiquitination of anillin required a destruction-box and was mediated by Cdh1, an activator of APC/C. Overexpression of Cdh1 reduced the levels of anillin, whereas inactivation of APC/C(Cdh1) increased the half-life of anillin. Functionally, anillin was required for the completion of cytokinesis. In anillin knockdown cells, the cleavage furrow ingressed but failed to complete the ingression. At late cytokinesis, the cytosol and DNA in knockdown cells underwent rapid myosin-based oscillatory movement across the furrow. During this movement, RhoA and active myosin were absent from the cleavage furrow, and myosin was redistributed to cortical patches, which powers the random oscillatory movement. We concluded that anillin functions to maintain the localization of active myosin, thereby ensuring the spatial control of concerted contraction during cytokinesis.

Regulation of myosin II during cytokinesis in higher eukaryotes

Cellular myosin II is the principal motor responsible for cytokinesis. In higher eukaryotes, phosphorylation of the regulatory light chain (MLC) of myosin II is a primary means of activating myosin II and is known to be crucial for the execution of cell division. Because signals transmitted by the mitotic spindle coordinate key spatial and temporal aspects of cytokinesis, such signals should ultimately function to activate myosin II. Thus, it follows that identification of regulatory factors involved in MLC phosphorylation should elucidate the nature of spindle-derived regulatory signals and lead to a model for how they control cytokinesis. However, the identity of these upstream molecules remains elusive. This review (which is part of the Cytokinesis series) summarizes current views of the regulatory pathway controlling MLC phosphorylation and features four candidate molecules that are likely immediate upstream myosin regulators. I discuss proposed functions for MLCK, ROCK, citron kinase and myosin phosphatase during cytokinesis and consider the possibility of a link between these molecules and the signals transmitted by the mitotic spindle.

G alpha12 interaction with alphaSNAP induces VE-cadherin localization at endothelial junctions and regulates barrier function

The involvement of heterotrimeric G proteins in the regulation of adherens junction function is unclear. We identified alphaSNAP as an interactive partner of G alpha12 using yeast two-hybrid screening. Glutathione S-transferase pull-down assays showed the selective interaction of alphaSNAP with G alpha12 in COS-7 as well as in human umbilical vein endothelial cells. Using domain swapping experiments, we demonstrated that the N-terminal region of G alpha12 (1-37 amino acids) was necessary and sufficient for its interaction with alphaSNAP. G alpha13 with its N-terminal extension replaced by that of G alpha12 acquired the ability to bind to alphaSNAP, whereas G alpha12 with its N terminus replaced by that of G alpha13 lost this ability. Using four point mutants of alphaSNAP, which alter its ability to bind to the SNARE complex, we determined that the convex rather than the concave surface of alphaSNAP was involved in its interaction with G alpha12. Co-transfection of human umbilical vein endothelial cells with G alpha12 and alphaSNAP stabilized VE-cadherin at the plasma membrane, whereas down-regulation of alphaSNAP with siRNA resulted in the loss of VE-cadherin from the cell surface and, when used in conjunction with G alpha12 overexpression, decreased endothelial barrier function. Our results demonstrate a direct link between the alpha subunit of G12 and alphaSNAP, an essential component of the membrane fusion machinery, and implicate a role for this interaction in regulating the membrane localization of VE-cadherin and endothelial barrier function.

Cell cycle goes global

The cell division cycle is one of the most intensively studied biological processes, yet, in spite of great effort, many questions remain as to how the cell cycle is controlled by cyclin-dependent kinases and other critical regulators. Recent functional genomic and proteomic approaches have yielded new insights into almost all aspects of cell cycle control, including transcriptional circuits, DNA replication, sister chromatid separation and regulation by environmental signals. Perhaps most notably, systematic analysis has begin to reveal meta-level connections between previously distinct sub-processes. As the interconnections between these huge datasets are beyond intuition, mathematical representation and automated analysis of functional genomic data is an urgent mandate.

The molecular requirements for cytokinesis

After anaphase onset, animal cells build an actomyosin contractile ring that constricts the plasma membrane to generate two daughter cells connected by a cytoplasmic bridge. The bridge is ultimately severed to complete cytokinesis. Myriad techniques have been used to identify proteins that participate in cytokinesis in vertebrates, insects, and nematodes. A conserved core of about 20 proteins are individually involved with cytokinesis in most animal cells. These components are found in the contractile ring, on the central spindle, within the RhoA pathway, and on vesicles that expand the membrane and sever the bridge. Cytokinesis involves additional proteins, but they, or their requirement in cytokinesis, are not conserved among animal cells.