The effect of myosin antibody on the division of starfish blastomeres

Antiserum against starfish egg myosin was produced in rabbits. Antibody specificity to myosin was demonstrated by Ouchterlony's immunodiffusion test and by immunoelectrophoresis in the presence of sodium dodecylsulfate (SDS). The latter technique showed that the antibody binds to both heavy and light chains of egg myosin. Furthermore, the antibody reacted with starfish sperm mysosin and starfish adult muscle myosin at both the heavy and light chains. It did not react with bovine platelet mysosin or rabbit skeletal muscle myosin in Ouchterlony's test; however, a weak reaction was observed in the presence of SDS between the antibody and these myosin heavy chains. Ca- and Mg-ATPase activities of egg myosin were not affected by the antibody, but it did inhibit actin-activated ATPase activity of egg myosin. Microinjection of the antibody into blastomeres of starfish eggs at the two-cell stage was carried out. Anti-egg myosin gamma-globulin inhibited the subsequent cleavages at an amount of more than 0.3 ng when injected at interphase. The inhibition was reduced when the injection was carried out near the initiation of cleavage. At the onset of the second cleavage the antibody was not inhibitory; however, an appropriate amount inhibited the third cleavage. Although the disappearance of the nuclear membrane was observed in the presence of the antibody, the formation of the mitotic apparatus was more or less disturbed. However the formation of daughter nuclei seemed to be scarcely affected by the antibody except that the distance between the nuclei was significantly smaller than normal.

Homologous recombination at c-fyn locus of mouse embryonic stem cells with use of diphtheria toxin A-fragment gene in negative selection

In attempting to produce a mutant mouse with embryonic stem cells, the critical step is the efficient isolation of homologous recombinants; the frequency of the homologous recombination is usually low and the potency of the cells to differentiate into germ cells is unstable in culture. Here, we report an efficacious method for such isolation in which the diphtheria toxin A-fragment gene is used to negatively select nonhomologous recombinants. In contrast to the use of the herpes simplex virus thymidine kinase gene, the selection can be made singly by the neomycin analog G418 without using a drug such as ganciclovir, a nucleoside analog. At the c-fyn locus, the diphtheria-toxin negative selection enriched the recombinants about 10-fold, and half of the cells integrating with the neomycin phosphotransferase gene were homologous recombinants.

A rho-like protein is involved in the organisation of the contractile ring in dividing sand dollar eggs

Sand dollar eggs were microinjected with botulinum C3 exoenzyme, an ADP-ribosyltransferase from Clostridium botulinum that specifically ADP-ribosylates and inactivates rho proteins. C3 exoenzyme microinjected during nuclear division interfered with subsequent cleavage furrow formation. No actin filaments were detected in the equatorial cortical layer of these eggs by rhodamine-phalloidin staining. When microinjected into furrowing eggs, C3 exoenzyme rapidly disrupted the contractile ring actin filaments and caused regression of the cleavage furrows. C3 exoenzyme had no apparent effect on nuclear division, however, and multinucleated embryos developed from the microinjected eggs. By contrast, C3 exoenzyme did not affect the organisation of cortical actin filaments immediately after fertilisation. Only one protein (molecular weight 22,000) was ADP-ribosylated by C3 exoenzyme in the isolated cleavage furrow. This protein co-migrated with ADP-ribosylated rhoA derived from human platelets when analysed by two-dimensional gel electrophoresis. These results strongly suggest that a rho-like, small GTP-binding protein is selectively involved in the organisation and maintenance of the contractile ring.

Three-dimensional arrangement of F-actin in the contractile ring of fission yeast

The contractile ring, which is required for cytokinesis in animal and yeast cells, consists mainly of actin filaments. Here, we investigate the directionality of the filaments in fission yeast using myosin S1 decoration and electron microscopy. The contractile ring is composed of around 1,000 to 2,000 filaments each around 0.6 mum in length. During the early stages of cytokinesis, the ring consists of two semicircular populations of parallel filaments of opposite directionality. At later stages, before contraction, the ring filaments show mixed directionality. We consider that the ring is initially assembled from a single site in the division plane and that filaments subsequently rearrange before contraction initiates.

An actin-depolymerizing protein (depactin) from starfish oocytes: properties and interaction with actin

Physico-chemical properties and interaction with actin of an actin-depolymerizing protein from mature starfish oocytes were studied. This protein, which is called depactin, exists in a monomeric form under physiological conditions. Its molecular weight is approximately 20,000 for the native protein and approximately 17,000 for denatured protein. The Glu + Asp/Lys + Arg molar ratio of this protein is 1.55. The apparent pl of the denatured depactin is approximately 6. The extent of actin polymerization is reduced by the presence of depactin; however, the rate of polymerization seems to be accelerated as measured spectrophotometrically at 238nm. This effect is interpreted to indicate that depactin cut the newly formed filaments into small fragments, thereby increasing the number of the filament ends to which monomers are added. The apparent critical concentration of actin for polymerization, as determined by viscometry or flow birefringence measurement, is increased by the presence of depactin in a concentration-dependent manner. Raising the pH of the solution does not reverse the action of depactin. The molar ratio of actin and depactin, which interact with each other, is estimated to be 1:1 by means of a cross-linking experiment using a water-soluble carbodiimide. Depactin binds to a DNase I-Sepharose column via actin and is selectively eluted with 0.6 M KCl or 0.6 M Kl. The association constant between actin and depactin is estimated, using the column, to be 2-3 X 10(6) M-1. The content of depactin in the high-speed supernatant of the oocyte extract is determined to be 1%; this can act upon approximately 63% of the actin in the supernatant.

Evidence that myosin does not contribute to force production in chromosome movement

Antibody against cytoplasmic myosin, when microinjected into actively dividing cells, provides a physiological test for the role of actin and myosin in chromosome movement. Anti-Asterias egg myosin, characterized by Mabuchi and Okuno (1977, J. Cell Biol., 74:251), completely and specifically inhibits the actin activated Mg++ -ATPase of myosin in vitro and, when microinjected, inhibits cytokinesis in vivo. Here, we demonstrate that microinjected antibody has no observable effect on the rate or extent of anaphase chromosome movements. Neither central spindle elongation nor chromosomal fiber shortening is affected by doses up to eightfold higher than those require to uniformly inhibit cytokinesis in all injected cells. We calculate that such doses are sufficient to completely inhibit myosin ATPase activity in these cells. Cells injected with buffer alone, with myosin-absorbed antibody, or with nonimmune gamma-globulin, proceed normally through both mitosis and cytokinesis. Control gamma-globulin, labeled with fluorescein, diffuses to homogeneity throughout the cytoplasm in 2-4 min and remains uniformly distributed. Antibody is not excluded from the spindle region. Prometaphase chromosome movements, fertilization, pronuclear migration, and pronuclear fusion are also unaffected by microinjected antimyosin. These experiments demonstrate that antimyosin blocks the actomyosin interaction thought to be responsible for force production in cytokinesis but has no effect on mitotic or meiotic chromosome motion. They provide direct physiological evidence that myosin is not involved in force production for chromosome movement.

Myosin-II reorganization during mitosis is controlled temporally by its dephosphorylation and spatially by Mid1 in fission yeast

Cytokinesis in many eukaryotes requires an actomyosin contractile ring. Here, we show that in fission yeast the myosin-II heavy chain Myo2 initially accumulates at the division site via its COOH-terminal 134 amino acids independently of F-actin. The COOH-terminal region can access to the division site at early G2, whereas intact Myo2 does so at early mitosis. Ser1444 in the Myo2 COOH-terminal region is a phosphorylation site that is dephosphorylated during early mitosis. Myo2 S1444A prematurely accumulates at the future division site and promotes formation of an F-actin ring even during interphase. The accumulation of Myo2 requires the anillin homologue Mid1 that functions in proper ring placement. Myo2 interacts with Mid1 in cell lysates, and this interaction is inhibited by an S1444D mutation in Myo2. Our results suggest that dephosphorylation of Myo2 liberates the COOH-terminal region from an intramolecular inhibition. Subsequently, dephosphorylated Myo2 is anchored by Mid1 at the medial cortex and promotes the ring assembly in cooperation with F-actin.

F-actin ring formation and the role of F-actin cables in the fission yeastSchizosaccharomyces pombe

Cells of the fission yeast Schizosaccharomyces pombe divide by the contraction of the F-actin ring formed at the medial region of the cell. We investigated the process of F-actin ring formation in detail using optical sectioning and three-dimensional reconstruction fluorescence microscopy. In wild-type cells, formation of an aster-like structure composed of F-actin cables and accumulation of F-actin cables were recognized at the medial cortex of the cell during prophase to metaphase. The formation of the aster-like structure seemed to initiate from branching of the longitudinal F-actin cables at a site near the spindle pole bodies, which had been duplicated but not yet separated. A single cable extended from the aster and encircled the cell at the equator to form a primary F-actin ring during metaphase. During anaphase,the accumulated F-actin cables were linked to the primary F-actin ring, and then all of these structures seemed to be packed to form the F-actin ring. These observations suggest that formation of the aster-like structure and the accumulation of the F-actin cables at the medial region of the cell during metaphase may be required to initiate the F-actin ring formation. In the nda3 mutant, which has a mutation in ß-tubulin and has been thought to be arrested at prophase, an F-actin ring with accumulated F-actin cables similar to that of anaphase wild-type cells was formed at a restrictive temperature. Immediately after shifting to a permissive temperature, this structure changed into a tightly packed ring. This suggests that the F-actin ring formation progresses beyond prophase in the nda3 cells once the cells enter prophase. We further examined F-actin structures in both cdc12 and cdc15 early cytokinesis mutants. As a result,Cdc12 seemed to be required for the primary F-actin ring formation during prophase, whereas Cdc15 may be involved in both packing the F-actin cables to form the F-actin ring and rearrangement of the F-actin after anaphase. In spg1, cdc7 and sid2 septum initiation mutants, the F-actin ring seemed to be formed in order.

The small GTPase Rho3 and the diaphanous/formin For3 function in polarized cell growth in fission yeast

We identified a novel Rho gene rho3(+) and studied its interaction with diaphanous/formin for3(+) in the fission yeast Schizosaccharomyces pombe. Both rho3 null cells and for3 null cells showed defects in organization of not only actin cytoskeleton but also cytoplasmic microtubules (MTs). rho3 for3 double null cells had defects that were more severe than each single null cell: polarized growth was deficient in the double null cells. Function of For3 needed the highly conserved FH1 and FH2 domains, an N-terminal region containing a Rho-binding domain, and the C-terminal region. For3 bound to active forms of both Rho3 and Cdc42 but not to that of Rho1. For3 was localized as dots to the ends of interphase cells and to the mid-region in dividing cells. This localization was probably dependent on its interaction with Rho proteins. Overexpression of For3 produced huge swollen cells containing depolarized F-actin patches and thick cytoplasmic MT bundles. In addition, overexpression of a constitutively active Rho3Q71L induced a strong defect in cytokinesis. In conclusion, we propose that the Rho3-For3 signaling system functions in the polarized cell growth of fission yeast by controlling both actin cytoskeleton and MTs.

Molecular mechanism of myosin-II assembly at the division site in Schizosaccharomyces pombe

Schizosaccharomyces pombe cells divide by virtue of the F-actin-based contractile ring (F-actin ring). Two myosin-II heavy chains, Myo2 and Myp2/Myo3, have been localized to the F-actin ring. Here, we investigated the mechanism of myosin-II assembly at the division site in S. pombe cells. First, we showed that Cdc4, an EF-hand protein, appears to be a common myosin light chain associated with both Myo2 and Myo3. Loss of function of both Myo2 and Myo3 caused a defect in F-actin assembly at the division site, like the phenotype of cdc4 null cells. It is suggested that Myo2, Myo3 and Cdc4 function in a cooperative manner in the formation of the F-actin ring during mitosis. Next, we investigated the dynamics of myosin-II during mitosis in S. pombe cells. In early mitosis when accumulation of F-actin cables in the medial region was not yet observed, Myo2 was detected primarily as dots widely located in the medial cortex. Myo2 fibers also became visible following the appearance of the dots. The Myo2 dots and fibers then fused with each other to form a medial cortical network. Some Myo2 dots appeared to be localized with F-actin cables which are also accumulated in the medial region. Finally these structures were packed into a thin contractile ring. In mutant cells that cannot form the F-actin ring such as cdc3(ts), cdc8(ts) and cdc12(ts), Myo2 was able to accumulate as dots in the medial cortex, whereas no accumulation of Myo2 dots was detected in cdc4(ts) cells. Moreover, disruption of F-actin in the cell by applying latrunculin-A did not affect the accumulation of Myo2 dots, suggesting that F-actin is not required for their accumulation. A truncated Myo2 which lacks putative Cdc4-binding sites (Myo2dIQs) was able to rescue myo2 null cells, myo3 null cells, cdc4(ts) mutant cells and cdc4 null cells. The Myo2dIQs could assemble into a normal-shaped ring in these cells. Therefore, its assembly at the division site does not require the function of either Cdc4 or Myo3.

Actin-depolymerizing protein Adf1 is required for formation and maintenance of the contractile ring during cytokinesis in fission yeast

The role of the actin-depolymerizing factor (ADF)/cofilin-family protein Adf1 in cytokinesis of fission yeast cells was studied. Adf1 was required for accumulation of actin at the division site by depolymerizing actin at the cell ends, assembly of the contractile ring through severing actin filaments, and maintenance of the contractile ring once formed. Genetic and cytological analyses suggested that it collaborates with profilin and capping protein in the mitotic reorganization of the actin cytoskeleton. Furthermore, it was unexpectedly found that Adf1 and myosin-II also collaborate in assembling the contractile ring. Tropomyosin was shown to antagonize the function of Adf1 in the contractile ring. We propose that formation and maintenance of the contractile ring are achieved by a balanced collaboration of these proteins.

Cleavage furrow isolated from newt eggs: contraction, organization of the actin filaments, and protein components of the furrow

The cleavage-furrow region was isolated surgically from newt eggs at the early stage of the first cleavage. The isolated furrow contracted in the presence of ATP at a Ca2+ concentration of 10 or 100 nM, although the speed was less than that of the furrow in vivo. Cytochalasin B, cytochalasin D, phalloidin, p-chloromercuribenzoate, and N-ethyl-maleimide interfered with the contraction, but colchicine did not. The furrow contained bundles of actin filaments of opposite polarities oriented parallel to the long axis of the furrow; these bundles may be the main component of the contractile arc. From electron microscopic observation of thin sections of the furrow, it was suggested that the actin bundles of the contractile arc were organized from preexisting cortical filaments that were connected to the plasma membrane by granular materials at their barbed ends. Contractile-arc actin filaments were revealed to be crosslinked by thin strands by the rapid freezing/deep etching-replication technique. Two-dimensional polyacrylamide gel electrophoresis showed that several proteins found in the furrow cortex are absent from the cortical layer before the cleavage furrow is formed.

Identification of two type V myosins in fission yeast, one of which functions in polarized cell growth and moves rapidly in the cell

We characterized the novel Schizosaccharomyces pombe genes myo4(+) and myo5(+), both of which encode myosin-V heavy chains. Disruption of myo4 caused a defect in cell growth and led to an abnormal accumulation of secretory vesicles throughout the cytoplasm. The mutant cells were rounder than normal, although the sites for cell polarization were still established. Elongation of the cell ends and completion of septation required more time than in wild-type cells, indicating that Myo4 functions in polarized growth both at the cell ends and during septation. Consistent with this conclusion, Myo4 was localized around the growing cell ends, the medial F-actin ring, and the septum as a cluster of dot structures. In living cells, the dots of green fluorescent protein-tagged Myo4 moved rapidly around these regions. The localization and movement of Myo4 were dependent on both F-actin cables and its motor activity but seemed to be independent of microtubules. Moreover, the motor activity of Myo4 was essential for its function. These results suggest that Myo4 is involved in polarized cell growth by moving with a secretory vesicle along the F-actin cables around the sites for polarization. In contrast, the phenotype of myo5 null cells was indistinguishable from that of wild-type cells. This and other data suggest that Myo5 has a role distinct from that of Myo4.

Schizosaccharomyces pombe rho2p GTPase regulates cell wall alpha-glucan biosynthesis through the protein kinase pck2p

Schizosaccharomyces pombe rho1(+) and rho2(+) genes are involved in the control of cell morphogenesis, cell integrity, and polarization of the actin cytoskeleton. Although both GTPases interact with each of the two S. pombe protein kinase C homologues, Pck1p and Pck2p, their functions are distinct from each other. It is known that Rho1p regulates (1,3)beta-D-glucan synthesis both directly and through Pck2p. In this paper, we have investigated Rho2p signaling and show that pck2 delta and rho2 delta strains display similar defects with regard to cell wall integrity, indicating that they might be in the same signaling pathway. We also show that Rho2 GTPase regulates the synthesis of alpha-D-glucan, the other main structural polymer of the S. pombe cell wall, primarily through Pck2p. Although overexpression of rho2(+) in wild-type or pck1 delta cells is lethal and causes morphological alterations, actin depolarization, and an increase in alpha-D-glucan biosynthesis, all of these effects are suppressed in a pck2 delta strain. In addition, genetic interactions suggest that Rho2p and Pck2p are important for the regulation of Mok1p, the major (1-3)alpha-D-glucan synthase. Thus, a rho2 delta mutation, like pck2 delta, is synthetically lethal with mok1-664, and the mutant partially fails to localize Mok1p to the growing areas. Moreover, overexpression of mok1(+) in rho2 delta cells causes a lethal phenotype that is completely different from that of mok1(+) overexpression in wild-type cells, and the increase in alpha-glucan is considerably lower. Taken together, all of these results indicate the presence of a signaling pathway regulating alpha-glucan biosynthesis in which the Rho2p GTPase activates Pck2p, and this kinase in turn controls Mok1p.

Interactions among a fimbrin, a capping protein, and an actin-depolymerizing factor in organization of the fission yeast actin cytoskeleton

We report studies of the fission yeast fimbrin-like protein Fim1, which contains two EF-hand domains and two actin-binding domains (ABD1 and ABD2). Fim1 is a component of both F-actin patches and the F-actin ring, but not of F-actin cables. Fim1 cross-links F-actin in vitro, but a Fim1 protein lacking either EF-hand domains (Fim1A12) or both the EF-hand domains and ABD1 (Fim1A2) has no actin cross-linking activity. Overexpression of Fim1 induced the formation of F-actin patches throughout the cell cortex, whereas the F-actin patches disappear in cells overexpressing Fim1A12 or Fim1A2. Thus, the actin cross-linking activity of Fim1 is probably important for the formation of F-actin patches. The overexpression of Fim1 also excluded the actin-depolymerizing factor Adf1 from the F-actin patches and inhibited the turnover of actin in these structures. Thus, Fim1 may function in stabilizing the F-actin patches. We also isolated the gene encoding Acp1, a subunit of the heterodimeric F-actin capping protein. fim1 acp1 double null cells showed more severe defects in the organization of the actin cytoskeleton than those seen in each single mutant. Thus, Fim1 and Acp1 may function in a similar manner in the organization of the actin cytoskeleton. Finally, genetic studies suggested that Fim1 may function in cytokinesis in cooperation with Cdc15 (PSTPIP) and Rng2 (IQGAP), respectively.

Identification of Myo3, a second type-II myosin heavy chain in the fission yeast Schizosaccharomyces pombe

We cloned the myo3+ gene of Schizosaccharomyces pombe which encodes a type-II myosin heavy chain. myo3 null cells showed a defect in cytokinesis under certain conditions. Overproduction of Myo3 also showed a defect in cytokinesis. Double mutant analysis indicated that Myo3 genetically interacts with Cdc8 tropomyosin and actin. Myo3 may be implicated in cytokinesis and stabilization of F-actin cables. Moreover, the function of Myo2 can be replaced by overexpressed Myo3. We observed a modest synthetic interaction between Myo2 and Myo3. Thus, Myo2 and Myo3 seem to cooperate in the formation of the F-actin ring in S. pombe.

Alpha-actinin from sea urchin eggs: biochemical properties, interaction with actin, and distribution in the cell during fertilization and cleavage

A protein similar to alpha-actinin has been isolated from unfertilized sea urchin eggs. This protein co-precipitated with actin from an egg extract as actin bundles. Its apparent molecular weight was estimated to be approximately 95,000 on an SDS gel: it co-migrated with skeletal-muscle alpha-actinin. This protein also co-eluted with skeletal muscle alpha-actinin from a gel filtration column giving a Stokes radius of 7.7 nm, and its amino acid composition was very similar to that of alpha-actinins. It reacted weakly but significantly with antibodies against chicken skeletal muscle alpha-actinin. We designated this protein as sea urchin egg alpha-actinin. The appearance of sea urchin egg alpha-actinin as revealed by electron microscopy using the low-angle rotary shadowing technique was also similar to that of skeletal muscle alpha-actinin. This protein was able to cross-link actin filaments side by side to form large bundles. The action of sea urchin egg alpha-actinin on the actin filaments was studied by viscometry at a low-shear rate. It gelled the F-actin solution at a molar ratio to actin of more than 1:20, at pH 6-7.5, and at Ca ion concentration less than 1 microM. The effect was abolished by the presence of tropomyosin. Distribution of this protein in the egg during fertilization and cleavage was investigated by means of microinjection of the rhodamine-labeled protein in the living eggs. This protein showed a uniform distribution in the cytoplasm in the unfertilized eggs. Upon fertilization, however, it was concentrated in the cell cortex, including the fertilization cone. At cleavage, it seemed to be concentrated in the cleavage furrow region.

The small GTP-binding protein Rho1 is a multifunctional protein that regulates actin localization, cell polarity, and septum formation in the fission yeast Schizosaccharomyces pombe

BACKGROUND

The small GTP-binding protein Rho has been shown to regulate the formation of the actin cytoskeleton in animal cells. We have previously isolated two rho genes, rho1+ and rho2+, from the fission yeast Schizosaccharomyces pombe in order to investigate the function of Rho using genetic techniques. In this paper, we report the cellular function of Rho1.

RESULTS

We found that Rho1 is essential for cell viability and cell polarity using gene disruption and by exogenous expression of botulinum C3 ADP-ribosyltransferase. In cells expressing either a constitutively active Rho1 or a dominant-negative Rho1, actin patches were delocalized. Both the cell wall and secondary septum were thick and stratified in cells expressing the constitutively active Rho1, while the cell wall of cells expressing the dominant-negative Rho1 seemed to be loosely organized. Furthermore, inactivation of Rho1 is apparently required for the separation of daughter cells. Cell fractionation studies suggested that Rho1 is predominantly membrane-bound. Moreover, we observed that Rho1 is localized to the cell periphery and to the septum.

CONCLUSIONS

Rho1 is involved in actin patch localization, the control of cell polarity, the regulation of septation, and cell wall synthesis.

Cleavage furrow: timing of emergence of contractile ring actin filaments and establishment of the contractile ring by filament bundling in sea urchin eggs

Cleavage furrow formation at the first cell division of sea urchin and sand dollar eggs was investigated in detail by fluorescence staining of actin filaments with rhodamine-phalloidin of either whole eggs or isolated egg cortices. Cortical actin filaments were clustered at anaphase and then the clusters became fibrillar at the end of anaphase. The timing when the contractile ring actin filaments appear was precisely determined in the course of mitosis: accumulation of the contractile ring actin filaments at the equatorial cell cortex is first noticed at the beginning of telophase (shortly before furrow formation), when the chromosomal vesicles are fusing with each other. The accumulated actin filaments were not well organized at the early stage but were organized into parallel bundles as the furrowing progressed. The bundles were finally fused into a tightly packed filament belt. Wheat germ agglutinin (WGA)-binding sites were distributed on the surface of the egg in a manner similar to the actin filaments after anaphase. The WGA-binding sites became accumulated in the contractile ring together with the contractile ring actin filaments, indicating an intimate relationship between these sites and actin filament-anchoring sites on the plasma membrane. Myosin also appeared in the contractile ring together with the actin filaments. The ‘cleavage stimulus’, a signal hypothesized by Rappaport (reviewed by R. Rappaport (1986) Int. Rev. Cytol. 105, 245–281) was suggested to induce aggregation or bundling of the actin filaments in the cortical layer.

The S. pombe rlc1 gene encodes a putative myosin regulatory light chain that binds the type II myosins myo3p and myo2p

In order to identify additional components important for cell division in the fission yeast Schizosaccharomyces pombe we have screened a bank of conditional cold-sensitive mutants for cytokinesis defects. One of these mutants showed a delay in cell cleavage, and strong genetic interactions with other genes implicated in medial ring formation. Cloning of the corresponding gene indicates that it encodes a protein with significant homology to the regulatory light chain of non-muscle myosins. We have named the gene rlc1 (regulatory light chain 1). The gene is not essential for division, but null mutants display a cell cleavage defect and form an aberrant F-actin ring. Two myosin-II heavy chains have been identified in fission yeast: Co-immunoprecipitation experiments indicate that rlc1p associates more strongly with myo3p than myo2p.

Determinants of myosin II cortical localization during cytokinesis

Myosin II is an essential component of the contractile ring that divides the cell during cytokinesis. Previous work showed that regulatory light chain (RLC) phosphorylation is required for localization of myosin at the cellular equator. However, the molecular mechanisms that concentrate myosin at the site of furrow formation remain unclear. By analyzing the spatiotemporal dynamics of mutant myosin subunits in Drosophila S2 cells, we show that myosin accumulates at the equator through stabilization of interactions between the cortex and myosin filaments and that the motor domain is dispensable for localization. Filament stabilization is tightly controlled by RLC phosphorylation. However, we show that regulatory mechanisms other than RLC phosphorylation contribute to myosin accumulation at three different stages: (1) turnover of thick filaments throughout the cell cycle, (2) myosin heavy chain-based control of myosin assembly at the metaphase-anaphase transition, and (3) redistribution and/or activation of myosin binding sites at the equator during anaphase. Surprisingly, the third event can occur to a degree in a Rho-independent fashion, gathering preassembled filaments to the equatorial zone via cortical flow. We conclude that multiple regulatory pathways cooperate to control myosin localization during mitosis and cytokinesis to ensure that this essential biological process is as robust as possible.

Subcellular localization and possible function of actin, tropomyosin and actin-related protein 3 (Arp3) in the fission yeast Schizosaccharomyces pombe

We investigated subcellular localizations and interactions of actin and two actin cytoskeleton-related proteins, Cdc8 tropomyosin and actin-related protein 3, Arp3, in the fission yeast Schizosaccharomyces pombe, using specific antibodies and by gene disruption. Actin was localized to the medial microfilamentous ring in the region of the septum during cytokinesis and to cortical patches by immunoelectron microscopy. F-actin cables were detected throughout the cell cycle by fluorescent staining with Bodipy-phallacidin. Cables were often linked to the patches and to the medial ring during its formation. Tropomyosin was localized to the medial ring and the cables. It was also distributed in the cell as patches, although co-localization with F-actin was not frequent. In cdc8ts mutant cells, F-actin cables were not observed although the F-actin patches were detected and cell polarity was maintained. These observations suggest that the F-actin cables may be involved in the formation of the medial ring, and that tropomyosin plays an important role in organizing both the ring and the cable, but is not involved in the F-actin patch formation or maintenance of cell polarity. Binding of Arp3 to actin was revealed by immunoprecipitation as well as by DNase I column chromatography. Arp3 seemed to form a complex with several proteins in the cell extracts, as previously reported for other organisms. Contrary to a previous report (McCollum et al., EMBO J. 15, 6438-6446, 1996), Arp3 was found to be concentrated in the medial region from early anaphase to late cytokinesis. Following arp3 gene disruption, F-actin patches were delocalized throughout the cell and cells did not undergo polarized growth, suggesting that Arp3 influences the proper localization of the actin patches in the cell and thereby controls the polarized growth of the cell.

Genes that cause aberrant cell morphology by overexpression in fission yeast: a role of a small GTP-binding protein Rho2 in cell morphogenesis

To identify the genes involved in cell morphogenesis in Schizosaccharomyces pombe, we screened for the genes that cause aberrant cell morphology by overexpression. The isolated genes were classified on the basis of morphology conferred. One of the genes causing a rounded morphology was identified as the rho2+ gene encoding a small GTP-binding protein. The overexpression of rho2+ resulted in a randomized distribution of cortical F-actin and formation of a thick cell wall. Analyses using cdc mutants suggested that the overexpression of rho2+ prevents the establishment of growth polarity in G1. The rho2+ gene was not essential, but among cells deleted for rho2+, those with an irregular shape were observed. The disruptant also showed a defect in cell wall integrity. An HA-Rho2 expressed in the cell was suggested to be present as a membrane-bound form by a cell fractionation experiment. A GFP-Rho2 was localized at the growing end(s) of the cell and the septation site. The localization of GFP-Rho2 during interphase was partially dependent on sts5+. These results indicate that Rho2 is involved in cell morphogenesis, control of cell wall integrity, control of growth polarity, and maintenance of growth direction. Analysis of functional overlapping between Rho2 and Rho1 revealed that their functions are distinct from each other, with partial overlapping.

Reorganization of actin cytoskeleton at the growing end of the cleavage furrow of Xenopus egg during cytokinesis

We studied reorganization of actin-myosin cytoskeleton at the growing ends of the cleavage furrow of Xenopus eggs in order to understand how the contractile ring is formed during cytokinesis. Reorganization of F-actin structures during the furrow formation was demonstrated by rhodamine-phalloidin staining of the cleavage furrow and by time-lapse scanning with laser scanning microscopy of F-actin structures in the cleavage furrow of live eggs to which rhodamine-G-actin had been injected. Actin filaments assemble to form small clusters that we call ‘F-actin patches’ at the growing end of the furrow. In live recordings, we observed emergence and rapid growth of F-actin patches in the furrow region. These patches then align in tandem, elongate and fuse with each other to form short F-actin bundles. The short bundles then form long F-actin bundles that compose the contractile ring. During the furrow formation, a cortical movement towards the division plane occurs at the growing ends of the furrow, as shown by monitoring wheatgerm agglutinin-conjugated fluorescent beads attached to the egg surface. As a result, wheatgerm agglutinin-binding sites accumulate and form ‘bleb-like’ structures on the surface of the furrow region. The F-actin patch forms and grows underneath this structure. The slope of F-actin accumulation in the interior region of the furrow exceeds that of accumulation of the cortex transported by the cortical movement. In addition, rhodamine-G-actin microinjected at the growing end is immediately incorporated into the F-actin patches. These data, together with the rapid growth of F-actin patches in the live image, suggest that actin polymerization occurs in the contractile ring formation. Distribution of myosin II in the cleavage furrow was also examined by immunofluorescence microscopy. Myosin II assembles as spots at the growing end underneath the bleb-like structure. It was suggested that myosin is transported and accumulates as spots by way of the cortical movement. F-actin accumulates at the position of the myosin spot a little later as the F-actin patches. The myosin spots and the F-actin patches are then simultaneously reorganized to form the contractile ring bundles