The evolution, complex structures and function of septin proteins

The septin family is a conserved GTP-binding protein family and was originally discovered through genetic screening for budding yeast mutants. Septins are implicated in many cellular processes in fungi and metazoa. The function of septins usually depends on septin assembling into oligomeric complexes and highly ordered polymers. The expansion of the septin gene number in vertebrates increased the complex diversity of septins. In this review, we first discuss the evolution, structures and assembly of septin proteins in yeast and metazoa. Then, we review the function of septin proteins in cytokinesis, membrane remodeling and compartmentalization.

EFFECT OF LATRUNCULIN B AND BREFELDIN A ON CYTOKINESIS IN THE BROWN ALGA SCYTOSIPHON LOMENTARIA (SCYTOSIPHONALES, PHAEOPHYCEAE)(1)

In zygotes of the brown alga Scytosiphon lomentaria (Lyngb.) Link, cytokinesis proceeds by growth of membranous sacs, which are formed by fusion of Golgi vesicles and flat cisternae accumulated at the future cytokinetic plane. It has been reported that depolymerization of actin filaments by latrunculin B does not inhibit mitosis. However, this molecule prevents the formation of the actin plate, which appears at the region of intermingled microtubules from each centrosome just before and during cytokinesis. In this study, zygotes treated with latrunculin B were observed using EM. Remarkably, this reagent inhibited the formation of flat cisternae. Golgi vesicles gathered around the midzone between the two daughter nuclei and fused with the plasma membrane there. As a result, the plasma membrane invaginated, in a complicated manner, into the cytoplasm. However, these invaginations of the plasma membrane never produced a continuous partition membrane. The ultrastructure of zygotes treated with brefeldin A, which prevents Golgi-mediated secretion, was also examined. Flat cisternae appeared at the future cytokinetic plane, and a new cell partition membrane was formed. However, the partition membrane became thick, because it was filled with amorphous material rather than the normal rigid fibrous material. These results suggested that actin is involved in the formation of flat cisternae, where it is necessary for completion of the new cell partition membrane, and that Golgi vesicles may play an important role in the deposition of cell wall material.

TWO SNOW SPECIES OF THE QUADRIFLAGELLATE GREEN ALGA CHLAINOMONAS (CHLOROPHYTA, VOLVOCALES): ULTRASTRUCTURE AND PHYLOGENETIC POSITION WITHIN THE CHLOROMONAS CLADE(1)

The quadriflagellate snow alga Chlainomonas Christen, distributed in New Zealand and North America, has several unusual structural attributes. A process assumed to be cytokinesis involves extrusion of protoplasm from the parent through a narrow canal, C. kolii (J. T. Hardy et Curl) Hoham produces a net-like outer envelope rather than a cell wall, and the flagellar basal apparatus of Chlainomonas consists of two semi-independent pairs of basal bodies. Structural connections between basal body pairs appear minimal, but a connecting system different from that observed in other genera exists within each pair. Phylogenetic analysis using rbcL sequences places Chlainomonas in the Chloromonas clade, other known members of which are all biflagellate. Chlainomonas is split into two robust lineages, with New Zealand collections sharing an origin with northern North American collections. Although the quadriflagellate condition is regarded as ancestral in the Chlorophyceae, we speculate-based on ultrastructural and molecular data presented here-that Chlainomonas represents a derived form that has arisen from fusion of two ancestral biflagellate cells. Other explanations (for example, that Chlainomonas represents a diploid form of a biflagellate species) are remotely possible but are presently at odds with extensive observations of field material. Improvements in techniques for experimental manipulation of these sensitive cryophiles will be required to fully characterize their structure and progress our understanding of their biology.

Differential regulation of centrosome integrity by DNA damage response proteins

MDC1 and BRIT1 have been shown to function as key regulators in response to DNA damage. However, their roles in centrosomal regulation haven't been elucidated. In this study, we demonstrated the novel functions of these two molecules in regulating centrosome duplication and mitosis. We found that MDC1 and BRIT1 were integral components of the centrosome that colocalize with gamma-tubulin. Depletion of either protein led to centrosome amplification. However, the mechanisms that allow them to maintain centrosome integrity are different. MDC1-depleted cells exhibited centrosome overduplication, leading to multipolar mitosis, chromosome missegregation, and aneuploidy, whereas BRIT1 depletion led to misaligned spindles and/or lagging chromosomes with defective spindle checkpoint activation that resulted in defective cytokinesis and polyploidy. We further illustrated that both MDC1 and BRIT1 were negative regulators of Aurora A and Plk1, two centrosomal kinases involved in centrosome maturation and spindle assembly. Moreover, the levels of MDC1 and BRIT1 inversely correlated with centrosome amplification, defective mitosis and cancer metastasis in human breast cancer. Together, MDC1 and BRIT1 may function as tumor-suppressor genes, at least in part by orchestrating proper centrosome duplication and mitotic spindle assembly.

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.

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.

Characterization and evolution of the cell cycle-associated mob domain-containing proteins in eukaryotes

The MOB family includes a group of cell cycle-associated proteins highly conserved throughout eukaryotes, whose founding members are implicated in mitotic exit and co-ordination of cell cycle progression with cell polarity and morphogenesis. Here we report the characterization and evolution of the MOB domain-containing proteins as inferred from the 43 eukaryotic genomes so far sequenced. We show that genes for Mob-like proteins are present in at least 41 of these genomes, confirming the universal distribution of this protein family and suggesting its prominent biological function. The phylogenetic analysis reveals five distinct MOB domain classes, showing a progressive expansion of this family from unicellular to multicellular organisms, reaching the highest number in mammals. Plant Mob genes appear to have evolved from a single ancestor, most likely after the loss of one or more genes during the early stage of Viridiplantae evolutionary history. Three of the Mob classes are widespread among most of the analyzed organisms. The possible biological and molecular function of Mob proteins and their role in conserved signaling pathways related to cell proliferation, cell death and cell polarity are also presented and critically discussed.

Identification of Rab11 as a small GTPase binding protein for the Evi5 oncogene

The Evi5 oncogene has recently been shown to regulate the stability and accumulation of critical G(1) cell cycle factors including Emi1, an inhibitor of the anaphase-promoting complex/cyclosome, and cyclin A. Sequence analysis of the amino terminus of Evi5 reveals a Tre-2, Bub2, Cdc16 domain, which has been shown to be a binding partner and GTPase-activating protein domain for the Rab family of small Ras-like GTPases. Here we describe the identification of Evi5 as a candidate binding protein for Rab11, a GTPase that regulates intracellular transport and has specific roles in endosome recycling and cytokinesis. By yeast two-hybrid analysis, immunoprecipitation, and Biacore analysis, we demonstrate that Evi5 binds Rab11a and Rab11b in a GTP-dependent manner. However, Evi5 displays no activation of Rab11 GTPase activity in vitro. Evi5 colocalizes with Rab11 in vivo, and overexpression of Rab11 perturbs the localization of Evi5, redistributing it into Rab11-positive recycling endosomes. Interestingly, in vitro binding studies show that Rab11 effector proteins including FIP3 compete with Evi5 for binding to Rab11, suggesting a partitioning between Rab11-Evi5 and Rab11 effector complexes. Indeed, ablation of Evi5 by RNA interference causes a mislocalization of FIP3 at the abscission site during cytokinesis. These data demonstrate that Evi5 is a Rab11 binding protein and that Evi5 may cooperate with Rab11 to coordinate vesicular trafficking, cytokinesis, and cell cycle control independent of GTPase-activating protein function.

Phosphorylation of NtMAP65-1 by a MAP kinase down-regulates its activity of microtubule bundling and stimulates progression of cytokinesis of tobacco cells

The tobacco mitogen-activated protein kinase (MAPK) cascade, which includes MAPK NRK1/NTF6, positively regulates expansion of the cytokinetic machinery known as the phragmoplast, which is followed by the synthesis of cell plates for completion of cell division. However, molecular events lying between the MAPK and phragmoplast expansion were not known. Here, we show that NRK1/NTF6 phosphorylates the threonine residue at position 579 in NtMAP65-1a, a microtubule-associated (MT-associated) protein. Levels of phosphorylated NtMAP65-1 increase during late M phase of the cell cycle, when NRK1/NTF6 is activated. Phosphorylated NtMAP65-1 is concentrated at the equator of phragmoplast, as is NRK1/NTF6. Overexpression of mutant forms of NtMAP65-1a that cannot be phosphorylated by NRK1 delays progression of the M phase and phragmoplast expansion, also rendering phragmoplast structures resistant to an MT-depolymerizing drug. Phosphorylation of NtMAP65-1 by NRK1/NTF6 down-regulates its MT-bundling activity in vitro. These results suggest that phosphorylation of NtMAP65-1 by NRK1/NTF6 also reduces its MT-bundling activity in vivo, which enhances destabilization and turnover of MTs at the phragmoplast equator, perhaps facilitating phragmoplast expansion.

Activation of the human FP prostanoid receptor disrupts mitosis progression and generates aneuploidy and polyploidy

Studies have shown prostaglandin F(2alpha) (PGF(2alpha)) to be an endogenous tumor promoter in mouse models of skin carcinogenesis; however, the mechanisms by which PGF(2alpha) affects cell cycle events remain unknown. Here we performed cell cycle analyses on HEK cells stably expressing the human FP receptor and found that treatment with PGF(2alpha) delays mitosis and is associated with an increased expression of cyclin B1 and Cdc2 kinase activity. In addition, multipolar spindles and misaligned chromosomes were observed in a significant proportion of cells treated with PGF(2alpha). Defective cytokinesis was also observed which resulted in gross aneuploidy and polyploidy. Expression of dominant negative Rho attenuated the cell cycle delay and prevented the generation of micronuclei following treatment with PGF(2alpha). This suggests that FP receptor activation of Rho signaling by PGF(2alpha) can interfere with nuclear division. Aneuploidy is associated with genomic instability and may underlie the tumor-promoting properties of PGF(2alpha).

Rab11-FIP3 and FIP4 interact with Arf6 and the exocyst to control membrane traffic in cytokinesis

The dual Rab11/Arf binding proteins, family of Rab11-interacting proteins FIP3 and FIP4 function in the delivery of recycling endosomes to the cleavage furrow and are, together with Rab11, essential for completion of abscission, the terminal step of cytokinesis. Here, we report that both FIP3 and FIP4 bind Arf6 in a nucleotide-dependent manner but exhibit differential affinities for Rab11 and Arf6. Both FIP3 and FIP4 can form ternary complexes with Rab11 and Arf6. Arf6 is localised to the furrow and midbody and we show that Arf6-GTP functions to localise FIP3 and FIP4 to midbodies during cytokinesis. Exo70p, a component of the Exocyst complex, also localises to the furrow of dividing cells and interacts with Arf6. We show that depletion of Exo70p leads to cytokinesis failure and an impairment of FIP3 and Rab11 localisation to the furrow and midbody. Moreover, Exo70p co-immunoprecipitates FIP3 and FIP4. Hence, we propose that FIP3 and FIP4 serve to couple Rab11-positive vesicle traffic from recycling endosomes to the cleavage furrow/midbody where they are tethered prior to fusion events via interactions with Arf6 and the Exocyst.

A cytokinetic function of Drosophila ORC6 protein resides in a domain distinct from its replication activity

Coordination between separate pathways may be facilitated by the requirements for common protein factors, a finding congruent with the link between proteins regulating DNA replication with other important cellular processes. We report that the smallest of Drosophila origin recognition complex subunits, Orc6, was found in embryos and cell culture localized to the cell membrane and cleavage furrow during cell division as well as in the nucleus. A two-hybrid screen revealed that Orc6 interacts with the Drosophila peanut (pnut), a member of the septin family of proteins important for cell division. This interaction, mediated by a distinct C-terminal domain of Orc6, was substantiated in Drosophila cells by coimmunoprecipitation from extracts and cytological methods. Silencing of Orc6 expression with double-stranded RNA resulted in a formation of multinucleated cells and also reduced DNA replication. Deletion of the C-terminal Orc6-peanut interaction domain and subsequent overexpression of the Orc6 mutant protein resulted in the formation of multinucleated cells that had replicated DNA. This mutant protein does not localize to the membrane or cleavage furrows. These results suggest that Orc6 has evolved a domain critical mainly for cytokinesis.

Asymmetric localization of flotillins/reggies in preassembled platforms confers inherent polarity to hematopoietic cells

Hematopoietic cells have long been defined as round, nonpolar cells that show uniform distribution of cell surface-associated molecules. However, recent analyses of the immunological synapse and the importance of lipid microdomains in signaling have shed new light on the aspect of lymphocyte polarization during the activation processes, but none of the molecules implicated so far in either the activation process or the microdomain residency are known to have a preferential localization in nonactivated cells. Chemical crosslinking and fluorescence resonance energy transfer methods have allowed the visualization of certain glycosylphosphatidylinositol-anchored proteins in lipid rafts but so far no microdomain resident protein has been shown to exist as visible stable platforms in the membrane. We report here that two lipid microdomain resident proteins, flotillins/reggies, form preassembled platforms in hematopoietic cells. These platforms recruit signaling molecules upon activation through lipid rafts. The preassembled platforms significantly differ from the canonical cholesterol-dependent "lipid rafts," as they are resistant to cholesterol-disrupting agents. Most evidence for the functional relevance of microdomains in living cells remains indirect. Using laser scanning confocal microscopy, we show that these proteins exist as stable, microscopically patent domains localizing asymmetrically to one pole of the cell. We present evidence that the asymmetric concentration of these microdomain resident proteins is built up during cytokinesis.

A widely conserved bacterial cell division protein that promotes assembly of the tubulin-like protein FtsZ

Cell division in bacteria is mediated by the tubulin-like protein FtsZ, which assembles into a structure known as the Z ring at the future site of cytokinesis. We report the discovery of a Z-ring-associated protein in Bacillus subtilis called ZapA. ZapA was found to colocalize with the Z ring in vivo and was capable of binding to FtsZ and stimulating the formation of higher-order assemblies of the cytokinetic protein in vitro. The absence of ZapA alone did not impair cell viability, but the absence of ZapA in combination with the absence of a second, dispensable division protein EzrA caused a severe block in cytokinesis. The absence of ZapA also caused lethality in cells producing lower than normal levels of FtsZ or lacking the division-site-selection protein DivIVA. Conversely, overproduction of ZapA reversed the toxicity of excess levels of the division inhibitor MinD. In toto, the evidence indicates that ZapA is part of the cytokinetic machinery of the cell and acts by promoting Z-ring formation. Finally, ZapA is widely conserved among bacteria with apparent orthologs in many species, including Escherichia coli, in which the orthologous protein exhibited a strikingly similar pattern of subcellular localization to that of ZapA. Members of the ZapA family of proteins are likely to be a common feature of the cytokinetic machinery in bacteria.

Microtubule capture by the cleavage apparatus is required for proper spindle positioning in yeast

Cell division is the result of two major cytoskeletal events: partition of the chromatids by the mitotic spindle and cleavage of the cell by the cytokinetic apparatus. Spatial coordination of these events ensures that each daughter cell inherits a nucleus. Here we show that, in budding yeast, capture and shrinkage of astral microtubules at the bud neck is required to position the spindle relative to the cleavage apparatus. Capture required the septins and the microtubule-associated protein Kar9. Like Kar9-defective cells, cells lacking the septin ring failed to position their spindle correctly and showed an increased frequency of nuclear missegregation. Microtubule attachment at the bud neck was followed by shrinkage and a pulling action on the spindle. Enhancement of microtubule shrinkage at the bud neck required the Par-1-related, septin-dependent kinases (SDK) Hsl1 and Gin4. Neither the formin Bnr1 nor the actomyosin contractile ring was required for either microtubule capture or microtubule shrinkage. Together, our results indicate that septins and septin-dependent kinases may coordinate microtubule and actin functions in cell division.

Novel complex integrating mitochondria and the microtubular cytoskeleton with chromosome remodeling and tumor suppressor RASSF1 deduced by in silico homology analysis, interaction cloning in yeast, and colocalization in cultured cells

Availability of the complete sequence of the human genome and sequence homology analysis has accelerated new protein discovery and clues to protein function. Protein-protein interaction cloning suggests multisubunit complexes and pathways. Here, we combine these molecular approaches with cultured cell colocalization analysis to suggest a novel complex and a pathway that integrate the mitochondrial location and the microtubular cytoskeleton with chromosome remodeling, apoptosis, and tumor suppression based on a novel leucine-rich pentatricopeptide repeat-motif-containing protein (LRPPRC) that copurified with the fibroblast growth factor receptor complex. One round of interaction cloning and sequence homology analysis defined a primary LRPPRC complex with novel subunits cat eye syndrome chromosome region candidate 2 (CECR2), ubiquitously expressed transcript (UXT), and chromosome 19 open reading frames 5 (C19ORF5) but still of unknown function. Immuno, deoxyribonucleic acid (DNA), and green fluorescent protein (GFP) tag colocalization analyses revealed that LRPPRC appears in both cytosol and nuclei of cultured cells, colocalizes with mitochondria and beta-tubulin rather than with alpha-actin in the cytosol of interphase cells, and exhibits phase-dependent organization around separating chromosomes in mitotic cells. GFP-tagged CECR2B was strictly nuclear and colocalized with condensed DNA in apoptotic cells. GFP-tagged UXT and GFP-tagged C19ORF5 appeared in both cytosol and nuclei and colocalized with LRPPRC and beta-tubulin. Cells exhibiting nuclear C19ORF5 were apoptotic. Screening for interactive substrates with the primary LRPPRC substrates in the human liver complementary DNA library revealed that CECR2B interacted with chromatin-associated TFIID-associated protein TAFII30 and ribonucleic acid splicing factor SRP40, UXT bridged to CBP/p300-binding factor CITED2 and kinetochore-associated factor BUB3, and C19ORF5 complexed with mitochondria-associated NADH dehydrogenase I and cytochrome c oxidase I. C19ORF5 also interacted with RASSF1, providing a bridge to apoptosis and tumor suppression.

Identification of a molecular target of psychosine and its role in globoid cell formation

Globoid cell leukodystrophy (GLD) is characterized histopathologically by apoptosis of oligodendrocytes, progressive demyelination, and the existence of large, multinuclear (globoid) cells derived from perivascular microglia. The glycosphingolipid, psychosine (d-galactosyl-beta-1,1' sphingosine), accumulates to micromolar levels in GLD patients who lack the degradative enzyme galactosyl ceramidase. Here we document that an orphan G protein-coupled receptor, T cell death-associated gene 8, is a specific psychosine receptor. Treatment of cultured cells expressing this receptor with psychosine or structurally related glycosphingolipids results in the formation of globoid, multinuclear cells. Our discovery of a molecular target for psychosine suggests a mechanism for the globoid cell histology characteristic of GLD, provides a tool with which to explore the disjunction of mitosis and cytokinesis in cell cultures, and provides a platform for developing a medicinal chemistry for psychosine.

Centrosomes enhance the fidelity of cytokinesis in vertebrates and are required for cell cycle progression

When centrosomes are destroyed during prophase by laser microsurgery, vertebrate somatic cells form bipolar acentrosomal mitotic spindles (Khodjakov, A., R.W. Cole, B.R. Oakley, and C.L. Rieder. 2000. Curr. Biol. 10:59-67), but the fate of these cells is unknown. Here, we show that, although these cells lack the radial arrays of astral microtubules normally associated with each spindle pole, they undergo a normal anaphase and usually produce two acentrosomal daughter cells. Relative to controls, however, these cells exhibit a significantly higher (30-50%) failure rate in cytokinesis. This failure correlates with the inability of the spindle to properly reposition itself as the cell changes shape. Also, we destroyed just one centrosome during metaphase and followed the fate of the resultant acentrosomal and centrosomal daughter cells. Within 72 h, 100% of the centrosome-containing cells had either entered DNA synthesis or divided. By contrast, during this period, none of the acentrosomal cells had entered S phase. These data reveal that the primary role of the centrosome in somatic cells is not to form the spindle but instead to ensure cytokinesis and subsequent cell cycle progression.

Drosophila aurora B kinase is required for histone H3 phosphorylation and condensin recruitment during chromosome condensation and to organize the central spindle during cytokinesis

Aurora/Ipl1-related kinases are a conserved family of enzymes that have multiple functions during mitotic progression. Although it has been possible to use conventional genetic analysis to dissect the function of aurora, the founding family member in Drosophila (Glover, D.M., M.H. Leibowitz, D.A. McLean, and H. Parry. 1995. Cell. 81:95-105), the lack of mutations in a second aurora-like kinase gene, aurora B, precluded this approach. We now show that depleting Aurora B kinase using double-stranded RNA interference in cultured Drosophila cells results in polyploidy. aurora B encodes a passenger protein that associates first with condensing chromatin, concentrates at centromeres, and then relocates onto the central spindle at anaphase. Cells depleted of the Aurora B kinase show only partial chromosome condensation at mitosis. This is associated with a reduction in levels of the serine 10 phosphorylated form of histone H3 and a failure to recruit the Barren condensin protein onto chromosomes. These defects are associated with abnormal segregation resulting from lagging chromatids and extensive chromatin bridging at anaphase, similar to the phenotype of barren mutants (Bhat, M.A., A.V. Philp, D.M. Glover, and H.J. Bellen. 1996. Cell. 87:1103-1114.). The majority of treated cells also fail to undertake cytokinesis and show a reduced density of microtubules in the central region of the spindle. This is accompanied by a failure to correctly localize the Pavarotti kinesin-like protein, essential for this process. We discuss these conserved functions of Aurora B kinase in chromosome transmission and cytokinesis.

The cytokinesis gene KEULE encodes a Sec1 protein that binds the syntaxin KNOLLE

KEULE is required for cytokinesis in Arabidopsis thaliana. We have positionally cloned the KEULE gene and shown that it encodes a Sec1 protein. KEULE is expressed throughout the plant, yet appears enriched in dividing tissues. Cytokinesis-defective mutant sectors were observed in all somatic tissues upon transformation of wild-type plants with a KEULE-green fluorescent protein gene fusion, suggesting that KEULE is required not only during embryogenesis, but at all stages of the plant's life cycle. KEULE is characteristic of a Sec1 protein in that it appears to exist in two forms: soluble or peripherally associated with membranes. More importantly, KEULE binds the cytokinesis-specific syntaxin KNOLLE. Sec1 proteins are key regulators of vesicle trafficking, capable of integrating a large number of intra- and/or intercellular signals. As a cytokinesis-related Sec1 protein, KEULE appears to represent a novel link between cell cycle progression and the membrane fusion apparatus.

A novel function of Saccharomyces cerevisiae CDC5 in cytokinesis

Coordination of mitotic exit with timely initiation of cytokinesis is critical to ensure completion of mitotic events before cell division. The Saccharomyces cerevisiae polo kinase Cdc5 functions in a pathway leading to the degradation of mitotic cyclin Clb2, thereby permitting mitotic exit. Here we provide evidence that Cdc5 also plays a role in regulating cytokinesis and that an intact polo-box, a conserved motif in the noncatalytic COOH-terminal domain of Cdc5, is required for this event. Depletion of Cdc5 function leads to an arrest in cytokinesis. Overexpression of the COOH-terminal domain of Cdc5 (cdc5DeltaN), but not the corresponding polo-box mutant, resulted in connected cells. These cells shared cytoplasms with incomplete septa, and possessed aberrant septin ring structures. Provision of additional copies of endogenous CDC5 remedied this phenotype, suggesting a dominant-negative inhibition of cytokinesis. The polo-box-dependent interactions between Cdc5 and septins (Cdc11 and Cdc12) and genetic interactions between the dominant-negative cdc5DeltaN and Cyk2/Hof1 or Myo1 suggest that direct interactions between cdc5DeltaN and septins resulted in inhibition of Cyk2/Hof1- and Myo1-mediated cytokinetic pathways. Thus, we propose that Cdc5 may coordinate mitotic exit with cytokinesis by participating in both anaphase promoting complex activation and a polo-box-dependent cytokinetic pathway.

The NPK1 mitogen-activated protein kinase kinase kinase is a regulator of cell-plate formation in plant cytokinesis

Mitogen-activated protein kinase (MAPK) cascades play important roles not only in the transduction of extracellular signals but in the progression of the cell cycle. However, evidence for their role in cytokinesis is limited. Here, we show that a tobacco MAPK kinase kinase (MAPKKK), designated NPK1, is required for cytokinesis. The activity of NPK1 increases in the late M phase of the tobacco cell cycle. During expansion of a new cross-wall (cell plate) toward the cell cortex, NPK1 is consistently localized to the equatorial zone of the phragmoplast, the cytokinetic apparatus where the cell plate is formed. Expression of a kinase-negative mutant of NPK1 results in the generation of multinucleate cells with incomplete cell plates. Phragmoplasts can be formed, but its expansion toward the cell cortex is also blocked. Thus, our results indicate that the NPK1 MAPKKK is essential for the formation of the cell plate, especially for its lateral growth.

Tangled1: a microtubule binding protein required for the spatial control of cytokinesis in maize

Spatial control of cytokinesis in plant cells depends on guidance of the cytokinetic apparatus, the phragmoplast, to a cortical "division site" established before mitosis. Previously, we showed that the Tangled1 (Tan1) gene of maize is required for this process during maize leaf development (Cleary, A.L., and L.G. Smith. 1998. Plant Cell. 10:1875-1888.). Here, we show that the Tan1 gene is expressed in dividing cells and encodes a highly basic protein that can directly bind to microtubules (MTs). Moreover, proteins recognized by anti-TAN1 antibodies are preferentially associated with the MT-containing cytoskeletal structures that are misoriented in dividing cells of tan1 mutants. These results suggest that TAN1 protein participates in the orientation of cytoskeletal structures in dividing cells through an association with MTs.

Human survivin is a kinetochore-associated passenger protein

Survivin, a dimeric baculovirus inhibitor of apoptosis repeat (BIR) motif protein that is principally expressed in G2 and mitosis, has been associated with protection against apoptosis of cells that exit mitosis aberrantly. Mammalian survivin has been reported to associate with centrosomes and with the mitotic spindle. We have expressed a human hemagglutinin-tagged survivin plasmid to determine its localization, and find instead that it clearly acts as a passenger protein. In HeLa cells, survivin first associates with the kinetochores, and then translocates to the spindle midzone during anaphase and, finally, to the midbody during cell cleavage. Its localization is similar to that of TD-60, a known passenger protein. Both a point mutation in the baculovirus IAP repeat motif (C84A) and a COOH-terminal deletion mutant (Delta106) of survivin fail to localize to either kinetochores or midbodies, but neither interferes with cell cleavage. The interphase localization of survivin is cell cycle regulated since in permanently transfected NIH3T3 cells it is excluded from the nuclei until G2, where it localizes with centromeres. Survivin remains associated with mitotic kinetochores when microtubule assembly is disrupted and its localization is thus independent of microtubules. We conclude that human survivin is positioned to have an important function in the mechanism of cell cleavage.

Lava lamp, a novel peripheral golgi protein, is required for Drosophila melanogaster cellularization

Drosophila cellularization and animal cell cytokinesis rely on the coordinated functions of the microfilament and microtubule cytoskeletal systems. To identify new proteins involved in cellularization and cytokinesis, we have conducted a biochemical screen for microfilament/microtubule-associated proteins (MMAPs). 17 MMAPs were identified; seven have been previously implicated in cellularization and/or cytokinesis, including KLP3A, Anillin, Septins, and Dynamin. We now show that a novel MMAP, Lava Lamp (Lva), is also required for cellularization. Lva is a coiled-coil protein and, unlike other proteins previously implicated in cellularization or cytokinesis, it is Golgi associated. Our functional analysis shows that cellularization is dramatically inhibited upon injecting anti-Lva antibodies (IgG and Fab) into embryos. In addition, we show that brefeldin A, a potent inhibitor of membrane trafficking, also inhibits cellularization. Biochemical analysis demonstrates that Lva physically interacts with the MMAPs Spectrin and CLIP190. We suggest that Lva and Spectrin may form a Golgi-based scaffold that mediates the interaction of Golgi bodies with microtubules and facilitates Golgi-derived membrane secretion required for the formation of furrows during cellularization. Our results are consistent with the idea that animal cell cytokinesis depends on both actomyosin-based contraction and Golgi-derived membrane secretion.

Localization and activity of myosin light chain kinase isoforms during the cell cycle

Phosphorylation on Ser 19 of the myosin II regulatory light chain by myosin light chain kinase (MLCK) regulates actomyosin contractility in smooth muscle and vertebrate nonmuscle cells. The smooth/nonmuscle MLCK gene locus produces two kinases, a high molecular weight isoform (long MLCK) and a low molecular weight isoform (short MLCK), that are differentially expressed in smooth and nonmuscle tissues. To study the relative localization of the MLCK isoforms in cultured nonmuscle cells and to determine the spatial and temporal dynamics of MLCK localization during mitosis, we constructed green fluorescent protein fusions of the long and short MLCKs. In interphase cells, localization of the long MLCK to stress fibers is mediated by five DXRXXL motifs, which span the junction of the NH(2)-terminal extension and the short MLCK. In contrast, localization of the long MLCK to the cleavage furrow in dividing cells requires the five DXRXXL motifs as well as additional amino acid sequences present in the NH(2)-terminal extension. Thus, it appears that nonmuscle cells utilize different mechanisms for targeting the long MLCK to actomyosin structures during interphase and mitosis. Further studies have shown that the long MLCK has twofold lower kinase activity in early mitosis than in interphase or in the early stages of postmitotic spreading. These findings suggest a model in which MLCK and the myosin II phosphatase (Totsukawa, G., Y. Yamakita, S. Yamashiro, H. Hosoya, D.J. Hartshorne, and F. Matsumura. 1999. J. Cell Biol. 144:735-744) act cooperatively to regulate the level of Ser 19-phosphorylated myosin II during mitosis and initiate cytokinesis through the activation of myosin II motor activity.

Polarized insertion of new membrane from a cytoplasmic reservoir during cleavage of the Drosophila embryo

Cellularization of the Drosophila embryo is a specialized form of cytokinesis that results in the formation of a polarized epithelium. The mechanisms of membrane growth during cytokinesis are largely unknown. It is also unclear whether membrane growth and polarization represent distinct processes that occur simultaneously or whether growth of the membrane is involved in the emergence of polarity. Using a combination of surface labeling and particles tracking techniques, we monitored the dynamics of marked membrane regions during cellularization. We find that the major source of membrane is intracellular, rather than in the form of a plasma membrane reservoir. Depolymerization of microtubules inhibits the export of a newly synthesized transmembrane protein from the Golgi apparatus to the plasma membrane and simultaneously blocks membrane growth. Membrane insertion occurs in a defined sequence at specific sites, first apical, then apical-lateral. Diffusion of the membrane appears insufficient to compete with the massive local insertion of new membrane. We thus identify a tightly regulated scheme of polarized membrane insertion during cellularization. We propose that such a mechanism could participate in the progressive emergence of apical-basal polarity.

A C. elegans patched gene, ptc-1, functions in germ-line cytokinesis

Patched (Ptc), initially identified in Drosophila, defines a class of multipass membrane proteins that control cell fate and cell proliferation. Biochemical studies in vertebrates indicate that the membrane proteins Ptc and Smoothened (Smo) form a receptor complex that binds Hedgehog (Hh) morphogens. Smo transduces the Hh signal to downstream effectors. The Caenorhabditis elegans genome encodes two Ptc homologs and one related pseudogene but does not encode obvious Hh or Smo homologs. We have analyzed ptc-1 by RNAi and mutational deletion and find that it is an essential gene, although the absence of ptc-1 has no detectable effect on body patterning or proliferation. Therefore, the C. elegans ptc-1 gene is functional despite the lack of Hh and Smo homologs. We find that the activity and expression of ptc-1 is essentially confined to the germ line and its progenitors. ptc-1 null mutants are sterile with multinucleate germ cells arising from a probable cytokinesis defect. We have also identified a surprisingly large family of PTC-related proteins containing sterol-sensing domains, including homologs of Drosophila dispatched, in C. elegans and other phyla. These results suggest that the PTC superfamily has multiple functions in animal development.

Microtubules remodel actomyosin networks in Xenopus egg extracts via two mechanisms of F-actin transport

Interactions between microtubules and filamentous actin (F-actin) are crucial for many cellular processes, including cell locomotion and cytokinesis, but are poorly understood. To define the basic principles governing microtubule/F-actin interactions, we used dual-wavelength digital fluorescence and fluorescent speckle microscopy to analyze microtubules and F-actin labeled with spectrally distinct fluorophores in interphase Xenopus egg extracts. In the absence of microtubules, networks of F-actin bundles zippered together or exhibited serpentine gliding along the coverslip. When microtubules were nucleated from Xenopus sperm centrosomes, they were released and translocated away from the aster center. In the presence of microtubules, F-actin exhibited two distinct, microtubule-dependent motilities: rapid ( approximately 250-300 nm/s) jerking and slow ( approximately 50 nm/s), straight gliding. Microtubules remodeled the F-actin network, as F-actin jerking caused centrifugal clearing of F-actin from around aster centers. F-actin jerking occurred when F-actin bound to motile microtubules powered by cytoplasmic dynein. F-actin straight gliding occurred when F-actin bundles translocated along the microtubule lattice. These interactions required Xenopus cytosolic factors. Localization of myosin-II to F-actin suggested it may power F-actin zippering, while localization of myosin-V on microtubules suggested it could mediate interactions between microtubules and F-actin. We examine current models for cytokinesis and cell motility in light of these findings.

Inhibition of cytokinesis by a lipid metabolite, psychosine

Although a number of cellular components of cytokinesis have been identified, little is known about the detailed mechanisms underlying this process. Here, we report that the lipid metabolite psychosine (galactosylsphingosine), derived from galactosylceramide, induced formation of multinuclear cells from a variety of nonadherent and adherent cells due to inhibition of cytokinesis. When psychosine was added to the human myelomonocyte cell line U937, which was the most sensitive among the cell lines tested, cleavage furrow formed either incompletely or almost completely. However, abnormal contractile movement was detected in which the cellular contents of one of the hemispheres of the contracting cell were transferred into its counterpart. Finally, the cleavage furrow disappeared and cytokinesis was reversed. Psychosine treatment also induced giant clots of actin filaments in the cells that probably consisted of small vacuoles with filamentous structures, suggesting that psychosine affected actin reorganization. These observations could account for the formation of multinuclear globoid cells in the brains of patients with globoid cell leukodystrophy, a neurological disorder characterized by the accumulation of psychosine due to galactosylceramidase deficiency.

Down-regulation of survivin by antisense oligonucleotides increases apoptosis, inhibits cytokinesis and anchorage-independent growth

Survivin, a member of the inhibitor of apoptosis protein (IAP) family, is detected in most common human cancers but not in adjacent normal cells. Previous studies suggest that survivin associates with the mitotic spindle and directly inhibits caspase activity. To further investigate the function of survivin, we used a survivin antisense (AS) oligonucleotide to downregulate survivin expression in normal and cancer cells. We found that inhibition of survivin expression increased apoptosis and polyploidy while decreasing colony formation in soft agar. Immunohistochemistry showed that cells without survivin can initiate the cleavage furrow and contractile ring, but cannot complete cytokinesis, thus resulting in multinucleated cells. These findings indicate that survivin plays important roles in a late stage of cytokinesis, as well as in apoptosis.

LIN-5 is a novel component of the spindle apparatus required for chromosome segregation and cleavage plane specification in Caenorhabditis elegans

Successful divisions of eukaryotic cells require accurate and coordinated cycles of DNA replication, spindle formation, chromosome segregation, and cytoplasmic cleavage. The Caenorhabditis elegans gene lin-5 is essential for multiple aspects of cell division. Cells in lin-5 null mutants enter mitosis at the normal time and form bipolar spindles, but fail chromosome alignment at the metaphase plate, sister chromatid separation, and cytokinesis. Despite these defects, cells exit from mitosis without delay and progress through subsequent rounds of DNA replication, centrosome duplication, and abortive mitoses. In addition, early embryos that lack lin-5 function show defects in spindle positioning and cleavage plane specification. The lin-5 gene encodes a novel protein with a central coiled-coil domain. This protein localizes to the spindle apparatus in a cell cycle- and microtubule-dependent manner. The LIN-5 protein is located at the centrosomes throughout mitosis, at the kinetochore microtubules in metaphase cells, and at the spindle during meiosis. Our results show that LIN-5 is a novel component of the spindle apparatus required for chromosome and spindle movements, cytoplasmic cleavage, and correct alternation of the S and M phases of the cell cycle.

The polo-box-dependent induction of ectopic septal structures by a mammalian polo kinase, plk, in Saccharomyces cerevisiae

Members of the polo subfamily of protein kinases play pivotal roles in cell-cycle control and proliferation. In addition to a high degree of sequence similarity in the kinase domain, polo kinases contain a strikingly conserved motif termed "polo-box" in the noncatalytic C-terminal domain. We have previously shown that the mammalian polo-like kinase Plk is a functional homolog of Saccharomyces cerevisiae Cdc5. Here, we show that, in a polo-box- and kinase activity-dependent manner, ectopic expression of Plk in budding yeast can induce a class of cells with abnormally elongated buds. In addition to localization at spindle poles and cytokinetic neck filaments, Plk induces and localizes to ectopic septin ring structures within the elongated buds. In contrast, mutations in the polo-box abolish both localization to, and induction of, septal structures. Consistent with the polo-box-dependent subcellular localization, the C-terminal domain of Plk, but not its polo-box mutant, is sufficient for subcellular localization. Our data suggest that Plk may contribute a signal to initiate or promote cytokinetic event(s) and that an intact polo-box is required for regulation of these cellular processes.

Phosphorylation of myosin-binding subunit (MBS) of myosin phosphatase by Rho-kinase in vivo

Rho-associated kinase (Rho-kinase), which is activated by the small GTPase Rho, phosphorylates myosin-binding subunit (MBS) of myosin phosphatase and thereby inactivates the phosphatase activity in vitro. Rho-kinase is thought to regulate the phosphorylation state of the substrates including myosin light chain (MLC), ERM (ezrin/radixin/moesin) family proteins and adducin by their direct phosphorylation and by the inactivation of myosin phosphatase. Here we identified the sites of phosphorylation of MBS by Rho-kinase as Thr-697, Ser-854 and several residues, and prepared antibody that specifically recognized MBS phosphorylated at Ser-854. We found by use of this antibody that the stimulation of MDCK epithelial cells with tetradecanoylphorbol-13-acetate (TPA) or hepatocyte growth factor (HGF) induced the phosphorylation of MBS at Ser-854 under the conditions in which membrane ruffling and cell migration were induced. Pretreatment of the cells with Botulinum C3 ADP-ribosyltransferase (C3), which is thought to interfere with Rho functions, or Rho-kinase inhibitors inhibited the TPA- or HGF-induced MBS phosphorylation. The TPA stimulation enhanced the immunoreactivity of phosphorylated MBS in the cytoplasm and membrane ruffling area of MDCK cells. In migrating MDCK cells, phosphorylated MBS as well as phosphorylated MLC at Ser-19 were localized in the leading edge and posterior region. Phosphorylated MBS was localized on actin stress fibers in REF52 fibroblasts. The microinjection of C3 or dominant negative Rho-kinase disrupted stress fibers and weakened the accumulation of phosphorylated MBS in REF52 cells. During cytokinesis, phosphorylated MBS, MLC and ERM family proteins accumulated at the cleavage furrow, and the phosphorylation level of MBS at Ser-854 was increased. Taken together, these results indicate that MBS is phosphorylated by Rho-kinase downstream of Rho in vivo, and suggest that myosin phosphatase and Rho-kinase spatiotemporally regulate the phosphorylation state of Rho-kinase substrates including MLC and ERM family proteins in vivo in a cooperative manner.

Parameters that specify the timing of cytokinesis

One model for the timing of cytokinesis is based on findings that p34(cdc2) can phosphorylate myosin regulatory light chain (LC20) on inhibitory sites (serines 1 and 2) in vitro (Satterwhite, L.L., M.H. Lohka, K.L. Wilson, T.Y. Scherson, L.J. Cisek, J.L. Corden, and T.D. Pollard. 1992. J. Cell Biol. 118:595-605), and this inhibition is proposed to delay cytokinesis until p34(cdc2) activity falls at anaphase. We have characterized previously several kinase activities associated with the isolated cortical cytoskeleton of dividing sea urchin embryos (Walker, G.R., C.B. Shuster, and D.R. Burgess. 1997. J. Cell Sci. 110:1373-1386). Among these kinases and substrates is p34(cdc2) and LC20. In comparison with whole cell activity, cortical H1 kinase activity is delayed, with maximum levels in cortices prepared from late anaphase/telophase embryos. To determine whether cortical-associated p34(cdc2) influences cortical myosin II activity during cytokinesis, we labeled eggs in vivo with [(32)P]orthophosphate, prepared cortices, and mapped LC20 phosphorylation through the first cell division. We found no evidence of serine 1,2 phosphorylation at any time during mitosis on LC20 from cortically associated myosin. Instead, we observed a sharp rise in serine 19 phosphorylation during anaphase and telophase, consistent with an activating phosphorylation by myosin light chain kinase. However, serine 1,2 phosphorylation was detected on light chains from detergent-soluble myosin II. Furthermore, cells arrested in mitosis by microinjection of nondegradable cyclin B could be induced to form cleavage furrows if the spindle poles were physically placed in close proximity to the cortex. These results suggest that factors independent of myosin II inactivation, such as the delivery of the cleavage stimulus to the cortex, determine the timing of cytokinesis.

The small GTP-binding protein rho regulates cortical activities in cultured cells during division

We have investigated the role of the small GTP-binding protein Rho in cytokinesis by microinjecting an inhibitor, C3 ribosyltransferase, into cultured cells. Microinjection of C3 into prometaphase or metaphase normal rat kidney epithelial cells induced immediate and global cortical movement of actin toward the metaphase plate, without an apparent effect on the mitotic spindle. During anaphase, concentrated cortical actin filaments migrated with separating chromosomes, leaving no apparent concentration of actin filaments along the equator. Myosin II in injected epithelial cells showed a diffuse distribution throughout cell division. All treated, well-adherent cells underwent cleavage-like activities and most of them divided successfully. However, cytokinesis became abnormal, generating irregular ingressions and ectopic cleavage sites even when mitosis was blocked with nocodazole. The effects of C3 appeared to be dependent on cell adhesion; less adherent 3T3 fibroblasts exhibited irregular cortical ingression only when cells started to increase attachment during respreading, but managed to complete cytokinesis. Poorly adherent HeLa cells showed neither ectopic cleavage nor completion of cytokinesis. Our results indicate that Rho does not simply activate actin-myosin II interactions during cytokinesis, but regulates the spatial pattern of cortical activities and completion of cytokinesis possibly through modulating the mechanical strength of the cortex.

Dual function of Cyk2, a cdc15/PSTPIP family protein, in regulating actomyosin ring dynamics and septin distribution

We previously showed that the budding yeast Saccharomyces cerevisiae assembles an actomyosin-based ring that undergoes a contraction-like size change during cytokinesis. To learn more about the biochemical composition and activity of this ring, we have characterized the in vivo distribution and function of Cyk2p, a budding yeast protein that exhibits significant sequence similarity to the cdc15/PSTPIP family of cleavage furrow proteins. Video microscopy of cells expressing green fluorescent protein (GFP)-tagged Cyk2p revealed that Cyk2p forms a double ring that coincides with the septins through most of the cell cycle. During cytokinesis, however, the Cyk2 double ring merges with the actomyosin ring and exhibits a contraction-like size change that is dependent on Myo1p. The septin double ring, in contrast, does not undergo the contraction-like size change but the separation between the two rings increases during cytokinesis. These observations suggest that the septin-containing ring is dynamically distinct from the actomyosin ring and that Cyk2p transits between the two types of structures. Gene disruption of CYK2 does not affect the assembly of the actomyosin ring but results in rapid disassembly of the ring during the contraction phase, leading to incomplete cytokinesis, suggesting that Cyk2p has an important function in modulating the stability of the actomyosin ring during contraction. Overexpression of Cyk2p also blocks cytokinesis, most likely due to a loss of the septins from the bud neck, indicating that Cyk2p may also play a role in regulating the localization of the septins.

Role of polo kinase and Mid1p in determining the site of cell division in fission yeast

The fission yeast Schizosaccharomyces pombe divides symmetrically using a medial F-actin- based contractile ring to produce equal-sized daughter cells. Mutants defective in two previously described genes, mid1 and pom1, frequently divide asymmetrically. Here we present the identification of three new temperature-sensitive mutants defective in localization of the division plane. All three mutants have mutations in the polo kinase gene, plo1, and show defects very similar to those of mid1 mutants in both the placement and organization of the medial ring. In both cases, ring formation is frequently initiated near the cell poles, indicating that Mid1p and Plo1p function in recruiting medial ring components to the cell center. It has been reported previously that during mitosis Mid1p becomes hyperphosphorylated and relocates from the nucleus to a medial ring. Here we show that Mid1p first forms a diffuse cortical band during spindle formation and then coalesces into a ring before anaphase. Plo1p is required for Mid1p to exit the nucleus and form a ring, and Pom1p is required for proper placement of the Mid1p ring. Upon overexpression of Plo1p, Mid1p exits the nucleus prematurely and displays a reduced mobility on gels similar to that of the hyperphosphorylated form observed previously in mitotic cells. Genetic and two-hybrid analyses suggest that Plo1p and Mid1p act in a common pathway distinct from that involving Pom1p. Plo1p localizes to the spindle pole bodies and spindles of mitotic cells and also to the medial ring at the time of its formation. Taken together, the data indicate that Plo1p plays a role in the positioning of division sites by regulating Mid1p. Given its previously known functions in mitosis and the timing of cytokinesis, Plo1p is thus implicated as a key molecule in the spatial and temporal coordination of cytokinesis with mitosis.

AIR-2: An Aurora/Ipl1-related protein kinase associated with chromosomes and midbody microtubules is required for polar body extrusion and cytokinesis in Caenorhabditis elegans embryos

An emerging family of kinases related to the Drosophila Aurora and budding yeast Ipl1 proteins has been implicated in chromosome segregation and mitotic spindle formation in a number of organisms. Unlike other Aurora/Ipl1-related kinases, the Caenorhabditis elegans orthologue, AIR-2, is associated with meiotic and mitotic chromosomes. AIR-2 is initially localized to the chromosomes of the most mature prophase I-arrested oocyte residing next to the spermatheca. This localization is dependent on the presence of sperm in the spermatheca. After fertilization, AIR-2 remains associated with chromosomes during each meiotic division. However, during both meiotic anaphases, AIR-2 is present between the separating chromosomes. AIR-2 also remains associated with both extruded polar bodies. In the embryo, AIR-2 is found on metaphase chromosomes, moves to midbody microtubules at anaphase, and then persists at the cytokinesis remnant. Disruption of AIR-2 expression by RNA- mediated interference produces entire broods of one-cell embryos that have executed multiple cell cycles in the complete absence of cytokinesis. The embryos accumulate large amounts of DNA and microtubule asters. Polar bodies are not extruded, but remain in the embryo where they continue to replicate. The cytokinesis defect appears to be late in the cell cycle because transient cleavage furrows initiate at the proper location, but regress before the division is complete. Additionally, staining with a marker of midbody microtubules revealed that at least some of the components of the midbody are not well localized in the absence of AIR-2 activity. Our results suggest that during each meiotic and mitotic division, AIR-2 may coordinate the congression of metaphase chromosomes with the subsequent events of polar body extrusion and cytokinesis.

Roles of Rho-associated kinase in cytokinesis; mutations in Rho-associated kinase phosphorylation sites impair cytokinetic segregation of glial filaments

Rho-associated kinase (Rho-kinase), which is activated by the small GTPase Rho, regulates formation of stress fibers and focal adhesions, myosin fiber organization, and neurite retraction through the phosphorylation of cytoskeletal proteins, including myosin light chain, the ERM family proteins (ezrin, radixin, and moesin) and adducin. Rho-kinase was found to phosphorylate a type III intermediate filament (IF) protein, glial fibrillary acidic protein (GFAP), exclusively at the cleavage furrow during cytokinesis. In the present study, we examined the roles of Rho-kinase in cytokinesis, in particular organization of glial filaments during cytokinesis. Expression of the dominant-negative form of Rho-kinase inhibited the cytokinesis of Xenopus embryo and mammalian cells, the result being production of multinuclei. We then constructed a series of mutant GFAPs, where Rho-kinase phosphorylation sites were variously mutated, and expressed them in type III IF-negative cells. The mutations induced impaired segregation of glial filament (GFAP filament) into postmitotic daughter cells. As a result, an unusually long bridge-like cytoplasmic structure formed between the unseparated daughter cells. Alteration of other sites, including the cdc2 kinase phosphorylation site, led to no remarkable defect in glial filament separation. These results suggest that Rho-kinase is essential not only for actomyosin regulation but also for segregation of glial filaments into daughter cells which in turn ensures correct cytokinetic processes.

Drosophila polo kinase is required for cytokinesis

A number of lines of evidence point to a predominance of cytokinesis defects in spermatogenesis in hypomorphic alleles of the Drosophila polo gene. In the pre-meiotic mitoses, cytokinesis defects result in cysts of primary spermatocytes with reduced numbers of cells that can contain multiple centrosomes. These are connected by a correspondingly reduced number of ring canals, structures formed by the stabilization of the cleavage furrow. The earliest defects during the meiotic divisions are a failure to form the correct mid-zone and mid-body structures at telophase. This is accompanied by a failure to correctly localize the Pavarotti kinesin- like protein that functions in cytokinesis, and of the septin Peanut and of actin to be incorporated into a contractile ring. In spite of these defects, cyclin B is degraded and the cells exit M phase. The resulting spermatids are frequently binuclear or tetranuclear, in which case they develop either two or four axonemes, respectively. A significant proportion of spermatids in which cytokinesis has failed may also show the segregation defects previously ascribed to polo1 mutants. We discuss these findings in respect to conserved functions for the Polo-like kinases in regulating progression through M phase, including the earliest events of cytokinesis.

Role of the yeast Gin4p protein kinase in septin assembly and the relationship between septin assembly and septin function

To identify septin-interacting proteins in Saccharomyces cerevisiae, we screened for mutations that are synthetically lethal with a cdc12 septin mutation. One of the genes identified was GIN4, which encodes a protein kinase related to Hsl1p/Nik1p and Ycl024Wp in S. cerevisiae and to Nim1p/Cdr1p and Cdr2p in Schizosaccharomyces pombe. The Gin4p kinase domain displayed a two-hybrid interaction with the COOH-terminal portion of the Cdc3p septin, and Gin4p colocalized with the septins at the mother-bud neck. This localization depended on the septins and on the COOH-terminal (nonkinase) region of Gin4p, and overproduction of this COOH-terminal region led to a loss of septin organization and associated morphogenetic defects. We detected no effect of deleting YCL024W, either alone or in combination with deletion of GIN4. Deletion of GIN4 was not lethal but led to a striking reorganization of the septins accompanied by morphogenetic abnormalities and a defect in cell separation; however, remarkably, cytokinesis appeared to occur efficiently. Two other proteins that localize to the neck in a septin-dependent manner showed similar reorganizations and also appeared to remain largely functional. The septin organization observed in gin4Delta vegetative cells resembles that seen normally in cells responding to mating pheromone, and no Gin4p was detected in association with the septins in such cells. The organization of the septins observed in gin4Delta cells and in cells responding to pheromone appears to support some aspects of the model for septin organization suggested previously by Field et al. (Field, C.M., O. Al-Awar, J. Rosenblatt, M.L. Wong, B. Alberts, and T.J. Mitchison. 1996. J. Cell Biol. 133:605-616).

Polymerization of purified yeast septins: evidence that organized filament arrays may not be required for septin function

The septins are a family of proteins required for cytokinesis in a number of eukaryotic cell types. In budding yeast, these proteins are thought to be the structural components of a filament system present at the mother-bud neck, called the neck filaments. In this study, we report the isolation of a protein complex containing the yeast septins Cdc3p, Cdc10p, Cdc11p, and Cdc12p that is capable of forming long filaments in vitro. To investigate the relationship between these filaments and the neck filaments, we purified septin complexes from cells deleted for CDC10 or CDC11. These complexes were not capable of the polymerization exhibited by wild-type preparations, and analysis of the neck region by electron microscopy revealed that the cdc10Delta and cdc11Delta cells did not contain detectable neck filaments. These results strengthen the hypothesis that the septins are the major structural components of the neck filaments. Surprisingly, we found that septin dependent processes like cytokinesis and the localization of Bud4p to the neck still occurred in cdc10Delta cells. This suggests that the septins may be able to function in the absence of normal polymerization and the formation of a higher order filament structure.

Polar localization of the MinD protein of Bacillus subtilis and its role in selection of the mid-cell division site

Cell division in rod-shaped bacteria is initiated by formation of a ring of the tubulin-like protein FtsZ at mid-cell. Division site selection is controlled by a conserved division inhibitor MinCD, which prevents aberrant division at the cell poles. The Bacillus subtilis DivIVA protein controls the topological specificity of MinCD action. Here we show that DivIVA is targeted to division sites late in their assembly, after some MinCD-sensitive step requiring FtsZ and other division proteins has been passed. DivIVA then recruits MinD to the division sites preventing another division from taking place near the newly formed cell poles. Sequestration of MinD to the poles also releases the next mid-cell sites for division. Remarkably, this mechanism of DivIVA action is completely different from that of the equivalent protein MinE of Escherichia coli, even though both systems operate via the same division inhibitor MinCD.

imp2, a new component of the actin ring in the fission yeast Schizosaccharomyces pombe

Cytokinesis is the part of the cell cycle in which the cell is cleaved to form two daughter cells. The unicellular yeast, Schizosaccharomyces pombe is an excellent model organism in which to study cell division, since it shows the general features of eukaryotic cell division and is amenable to genetic analysis. In this manuscript we describe the isolation and characterization of a new protein, imp2, which is required for normal septation in fission yeast. imp2, which colocalizes with the medial ring during septation, is structurally similar to a group of proteins including the S. pombe cdc15 and the mouse PSTPIP that are localized to, and thought to be involved in actin ring organization. Cells in which the imp2 gene is deleted or overexpressed have septation and cell separation defects. An analysis of the actin cytoskeleton shows the lack of a medial ring in septating cells that overexpress imp2, and the appearance of abnormal medial ring structures in septated cells that lack imp2. These observations suggest that imp2 destabilizes the medial ring during septation. imp2 also shows genetic interactions with several, previously characterized septation genes, strengthening the conclusion that it plays a role in normal fission yeast septation.

Involvement of an actomyosin contractile ring in Saccharomyces cerevisiae cytokinesis

In Saccharomyces cerevisiae, the mother cell and bud are connected by a narrow neck. The mechanism by which this neck is closed during cytokinesis has been unclear. Here we report on the role of a contractile actomyosin ring in this process. Myo1p (the only type II myosin in S. cerevisiae) forms a ring at the presumptive bud site shortly before bud emergence. Myo1p ring formation depends on the septins but not on F-actin, and preexisting Myo1p rings are stable when F-actin is depolymerized. The Myo1p ring remains in the mother-bud neck until the end of anaphase, when a ring of F-actin forms in association with it. The actomyosin ring then contracts to a point and disappears. In the absence of F-actin, the Myo1p ring does not contract. After ring contraction, cortical actin patches congregate at the mother-bud neck, and septum formation and cell separation rapidly ensue. Strains deleted for MYO1 are viable; they fail to form the actin ring but show apparently normal congregation of actin patches at the neck. Some myo1Delta strains divide nearly as efficiently as wild type; other myo1Delta strains divide less efficiently, but it is unclear whether the primary defect is in cytokinesis, septum formation, or cell separation. Even cells lacking F-actin can divide, although in this case division is considerably delayed. Thus, the contractile actomyosin ring is not essential for cytokinesis in S. cerevisiae. In its absence, cytokinesis can still be completed by a process (possibly localized cell-wall synthesis leading to septum formation) that appears to require septin function and to be facilitated by F-actin.

Characterization of the p22 subunit of dynactin reveals the localization of cytoplasmic dynein and dynactin to the midbody of dividing cells

Dynactin, a multisubunit complex that binds to the microtubule motor cytoplasmic dynein, may provide a link between dynein and its cargo. Many subunits of dynactin have been characterized, elucidating the multifunctional nature of this complex. Using a dynein affinity column, p22, the smallest dynactin subunit, was isolated and microsequenced. The peptide sequences were used to clone a full-length human cDNA. Database searches with the predicted amino acid sequence of p22 indicate that this polypeptide is novel. We have characterized p22 as an integral component of dynactin by biochemical and immunocytochemical methods. Affinity chromatography experiments indicate that p22 binds directly to the p150(Glued) subunit of dynactin. Immunocytochemistry with antibodies to p22 demonstrates that this polypeptide localizes to punctate cytoplasmic structures and to the centrosome during interphase, and to kinetochores and to spindle poles throughout mitosis. Antibodies to p22, as well as to other dynactin subunits, also revealed a novel localization for dynactin to the cleavage furrow and to the midbodies of dividing cells; cytoplasmic dynein was also localized to these structures. We therefore propose that dynein/dynactin complexes may have a novel function during cytokinesis.

Spindle self-organization and cytokinesis during male meiosis in asterless mutants of Drosophila melanogaster

While Drosophila female meiosis is anastral, both meiotic divisions in Drosophila males exhibit prominent asters. We have identified a gene we call asterless (asl) that is required for aster formation during male meiosis. Ultrastructural analysis showed that asl mutants have morphologically normal centrioles. However, immunostaining with antibodies directed either to gamma tubulin or centrosomin revealed that these proteins do not accumulate in the centrosomes, as occurs in wild-type. Thus, asl appears to specify a function required for the assembly of centrosomal material around the centrioles. Despite the absence of asters, meiotic cells of asl mutants manage to develop an anastral spindle. Microtubules grow from multiple sites around the chromosomes, and then focus into a peculiar bipolar spindle that mediates chromosome segregation, although in a highly irregular way. Surprisingly, asl spermatocytes eventually form a morphologically normal ana-telophase central spindle that has full ability to stimulate cytokinesis. These findings challenge the classical view on central spindle assembly, arguing for a self-organization of this structure from either preexisting or newly formed microtubules. In addition, these findings strongly suggest that the asters are not required for signaling cytokinesis.

Mlc1p is a light chain for the unconventional myosin Myo2p in Saccharomyces cerevisiae

In Saccharomyces cerevisiae, the unconventional myosin Myo2p is of fundamental importance in polarized growth. We explore the role of the neck region and its associated light chains in regulating Myo2p function. Surprisingly, we find that precise deletion of the six IQ sites in the neck region results in a myosin, Myo2-Delta6IQp, that can support the growth of a yeast strain at 90% the rate of a wild-type isogenic strain. We exploit this mutant in a characterization of the light chains of Myo2p. First, we demonstrate that the localization of calmodulin to sites of polarized growth largely depends on the IQ sites in the neck of Myo2p. Second, we demonstrate that a previously uncharacterized protein, Mlc1p, is a myosin light chain of Myo2p. MLC1 (YGL106w) is an essential gene that exhibits haploinsufficiency. Reduced levels of MYO2 overcome the haploinsufficiency of MLC1. The mutant MYO2-Delta6IQ is able to suppress haploinsufficiency but not deletion of MLC1. We used a modified gel overlay assay to demonstrate a direct interaction between Mlc1p and the neck of Myo2p. Overexpression of MYO2 is toxic, causing a severe decrease in growth rate. When MYO2 is overexpressed, Myo2p is fourfold less stable than in a wild-type strain. High copies of MLC1 completely overcome the growth defects and increase the stability of Myo2p. Our results suggest that Mlc1p is responsible for stabilizing this myosin by binding to the neck region.

pavarotti encodes a kinesin-like protein required to organize the central spindle and contractile ring for cytokinesis

Mutations in the Drosophila gene pavarotti result in the formation of abnormally large cells in the embryonic nervous system. In mitotic cycle 16, cells of pav mutant embryos undergo normal anaphase but then develop an abnormal telophase spindle and fail to undertake cytokinesis. We show that the septin Peanut, actin, and the actin-associated protein Anillin, do not become correctly localized in pav mutants. pav encodes a kinesin-like protein, PAV-KLP, related to the mammalian MKLP-1. In cellularized embryos, the protein is localized to centrosomes early in mitosis, and to the midbody region of the spindle in late anaphase and telophase. We show that Polo kinase associates with PAV-KLP with which it shows an overlapping pattern of subcellular localization during the mitotic cycle and this distribution is disrupted in pav mutants. We suggest that PAV-KLP is required both to establish the structure of the telophase spindle to provide a framework for the assembly of the contractile ring, and to mobilize mitotic regulator proteins.