Isolation and characterization of pos mutants defective in correct positioning of septum in Schizosaccharomyces pombe

How to scaffold the contractile ring for a safe cytokinesis - lessons from Anillin-related proteins

The ingression of a cleavage furrow separates the two daughter cells at the end of cell division. In many organisms this furrow ingression is driven by the assembly and contraction of actomyosin filaments, forming a contractile ring. To achieve a successful cytokinesis, these actomyosin filaments need to be assembled in an organized manner. For this purpose, a network of cytoskeletal proteins is built at the cleavage site to act as a scaffold for actomyosin filaments and to connect them to the plasma membrane. The Drosophila melanogaster protein Anillin, and its related proteins in other organisms, has a pivotal role in the organization of this scaffold in many species, ranging from yeast to humans. Recent studies indicate that Anillin-related proteins interact not only with the structural components of the contractile ring, but also with the signalling factors that control their dynamics. In addition, Drosophila Anillin connects the actomyosin ring to the spindle microtubules through its interaction with the RacGAP component of the centralspindlin complex. Here I review the structures and functions of Anillin and Anillin-related proteins in various model systems, and aim to highlight both the common and distinctive features of these essential organizers of the molecular machinery that drives furrow ingression.

Mad1p, a component of the spindle assembly checkpoint in fission yeast, suppresses a novel septation-defective mutant,sun1, in a cell-division cycle

To enhance our understanding of the cytokinesis, we have carried out a genetic screen for temperature-sensitive Schizosaccharomyces pombe mutants that show defects in septum formation and cell division. Here we present the isolation and characterization of a new temperature-sensitive mutant, sun1 (septum uncontrolled), which undergoes uncontrolled septation during cell-division cycle at restrictive temperature (37 degrees C). In sun1 mutant, the actin ring and septum are positioned at random locations and angles, and the nuclear division cycle continues. These observations suggest that the sun1 gene product is required for the proper placement of the actin ring as well as precise septation. In a screen for the sun1(+) gene to complement the sun1 mutant, we have isolated a mad1(+) (mitotic arrest deficient) gene, which encodes a component of the spindle checkpoint in the cell-division cycle. Analysis of crossing the sun1 cell with the mad1(+) null mutant indicates that mad1(+) suppresses the sun1 mutant defective in controlled septation in a cell-division cycle.

The Scw1 RNA-Binding Domain Protein Regulates Septation and Cell-Wall Structure in Fission Yeast

Loss of the nonessential RNA-binding domain protein, Scw1, increases resistance to cell-wall-degrading enzymes in fission yeast. Surprisingly, scw1 null mutations also suppress the lethality of mutations (cdc11-136, cdc7-24, cdc14-118, sid1-239, sid2-250, sid3-106, sid4-A1, and mob1-1) at all levels of the sid pathway. This pathway forms part of the septation initiation network (SIN), which regulates the onset of septum formation and ensures the proper coupling of mitosis to cytokinesis. In contrast, scw1- mutations do not suppress ts alleles of the rng genes, cdc12 or cdc15. These mutations also prevent the formation of a septum and in addition block assembly and/or function of the contractile acto-myosin ring. sid mutants exhibit a hyper-sensitivity to cell-wall-degrading enzymes that is suppressed by loss of Scw1. Furthermore, scw1--mediated rescue of sid mutants is abolished in the presence of calcofluor white, a compound that interferes with cell-wall synthesis. These data suggest that Scw1 acts in opposition to the SIN as a negative regulator of cell-wall/septum deposition. Unlike components of the SIN, Scw1 is predominantly a cytoplasmic protein and is not localized to the spindle pole body.

Cytokinesis in eukaryotes

Cytokinesis is the final event of the cell division cycle, and its completion results in irreversible partition of a mother cell into two daughter cells. Cytokinesis was one of the first cell cycle events observed by simple cell biological techniques; however, molecular characterization of cytokinesis has been slowed by its particular resistance to in vitro biochemical approaches. In recent years, the use of genetic model organisms has greatly advanced our molecular understanding of cytokinesis. While the outcome of cytokinesis is conserved in all dividing organisms, the mechanism of division varies across the major eukaryotic kingdoms. Yeasts and animals, for instance, use a contractile ring that ingresses to the cell middle in order to divide, while plant cells build new cell wall outward to the cortex. As would be expected, there is considerable conservation of molecules involved in cytokinesis between yeast and animal cells, while at first glance, plant cells seem quite different. However, in recent years, it has become clear that some aspects of division are conserved between plant, yeast, and animal cells. In this review we discuss the major recent advances in defining cytokinesis, focusing on deciding where to divide, building the division apparatus, and dividing. In addition, we discuss the complex problem of coordinating the division cycle with the nuclear cycle, which has recently become an area of intense research. In conclusion, we discuss how certain cells have utilized cytokinesis to direct development.

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.

Contractile ring formation in Xenopus egg and fission yeast

How actin filaments (F-actin) and myosin II (myosin) assemble to form the contractile ring was investigated with fission yeast and Xenopus egg. In fission yeast cells, an aster-like structure composed of F-actin cables is formed at the medial cortex of the cell during prophase to metaphase, and a single F-actin cable(s) extends from this structure, which seems to be a structural basis of the contractile ring. In early mitosis, myosin localizes as dots in the medial cortex independently of F-actin. Then they fuse with each other and are packed into a thin contractile ring. At the growing ends of the cleavage furrow of Xenopus eggs, F-actin at first assembles to form patches. Next they fuse with each other to form short F-actin bundles. The short bundles then form long bundles. Myosin seems to be transported by the cortical movement to the growing end and assembles there as spots earlier than F-actin. Actin polymerization into the patches is likely to occur after accumulation of myosin. The myosin spots and the F-actin patches are simultaneously reorganized to form the contractile ring bundles. The idea that a Ca signal triggers cleavage furrow formation was tested with Xenopus eggs during the first cleavage. We could not detect any Ca signals such as a Ca wave, Ca puffs or even Ca blips at the growing end of the cleavage furrow. Furthermore, cleavages are not affected by Ca-chelators injected into the eggs at concentrations sufficient to suppress the Ca waves. Thus we conclude that formation of the contractile ring is not induced by a Ca signal at the growing end of the cleavage furrow.

Analysis of mid1p, a protein required for placement of the cell division site, reveals a link between the nucleus and the cell surface in fission yeast

mid1 is required for the proper placement of the contractile actin ring for cytokinesis at a medial site overlying the nucleus. Here we find that mid1 protein (mid1p) shuttles between the nucleus and a cortical medial broad band during interphase and early mitosis. The position of this broad band, which overlies the nucleus, is linked to nuclear position even in cells with displaced or multiple nuclei. We identified and created mutations in an NLS and in two crm1-dependent NES sequences in mid1p. NES mutations caused mid1p accumulation in the nucleus and loss of function. An NLS mutations greatly reduced nuclear localization but did not perturb cytoplasmic localization or function. mid1p localization to the medial broad band was also not dependent on mid1p PH domain or microtubule and actin cytoskeletons. Overexpression of mid1p produced ectopic cell growth at this band during interphase and abnormal karmellae-like nuclear membrane structures. In plo1-1, mid1p formed a medial broad band but did not incorporate into a tight ring, suggesting that polo kinase plo1p is required for activation of mid1p function. Thus, the mid1p broad band defines a compartment at the medial cell surface, whose localization is linked to the position of the nucleus, and whose function may be to position the plane of cell division.

Tying the knot: linking cytokinesis to the nuclear cycle

For the survival of both the parent and the progeny, it is imperative that the process of their physical division (cytokinesis) be precisely coordinated with progression through the mitotic cell cycle. Recent studies in the budding yeast Saccharomyces cerevisiae and the fission yeast Schizosaccharomyces pombe are beginning to unravel the nature of the links between cytokinesis and the nuclear division cycle. The cyclin-dependent kinases and a novel surveillance mechanism that monitors cytokinesis and/or morphogenesis appear to play important regulatory roles in forging these links. It is becoming increasingly clear that the inactivation of the mitosis-promoting cyclin-dependent kinase, which marks the completion of the nuclear division cycle, is essential for actomyosin ring constriction and division septum assembly in both yeasts. Additionally, the spindle pole bodies are emerging as important transient locale for proteins that might play a key role in coupling the completion of mitosis to the onset of cytokinesis.

Isolation and characterization of par1(+) and par2(+): two Schizosaccharomyces pombe genes encoding B' subunits of protein phosphatase 2A

Protein phosphatase 2A (PP2A) is one of the major serine/threonine phosphatases found in eukaryotic cells. We cloned two genes, par1(+) and par2(+), encoding distinct B' subunits of PP2A in fission yeast. They share 52% identity at the amino acid sequence level. Neither gene is essential but together they are required for normal septum positioning and cytokinesis, for growth at both high and low temperature, and for growth under a number of stressful conditions. Immunofluorescence microscopy revealed that Par2p has a cell-cycle-related localization pattern, being localized at cell ends during interphase and forming a medial ring in cells that are undergoing septation and cytokinesis. Our analyses also indicate that Par1p is more abundant than Par2p in the cell. Cross-organism studies showed that both par1(+) and par2(+) could complement the rts1Delta allele in Saccharomyces cerevisiae, albeit to different extents, in spite of the fact that neither contains a serine/threonine-rich N-terminal domain like that found in the S. cerevisiae homolog Rts1p. Thus, while Schizosaccharomyces pombe is more similar to higher eukaryotes with respect to its complement of B'-encoding genes, the function of those proteins is conserved relative to that of Rts1p.

LvsA, a protein related to the mouse beige protein, is required for cytokinesis in Dictyostelium

We isolated a Dictyostelium cytokinesis mutant with a defect in a novel locus called large volume sphere A (lvsA). lvsA mutants exhibit an unusual phenotype when attempting to undergo cytokinesis in suspension culture. Early in cytokinesis, they initiate furrow formation with concomitant myosin II localization at the cleavage furrow. However, the furrow is later disrupted by a bulge that forms in the middle of the cell. This bulge is bounded by furrows on both sides, which are often enriched in myosin II. The bulge can increase and decrease in size multiple times as the cell attempts to divide. Interestingly, this phenotype is similar to the cytokinesis failure of Dictyostelium clathrin heavy-chain mutants. Furthermore, both cell lines cap ConA receptors but form only a C-shaped loose cap. Unlike clathrin mutants, lvsA mutants are not defective in endocytosis or development. The LvsA protein shares several domains in common with the molecules beige and Chediak-Higashi syndrome proteins that are important for lysosomal membrane traffic. Thus, on the basis of the sequence analysis of the LvsA protein and the phenotype of the lvsA mutants, we postulate that LvsA plays an important role in a membrane-processing pathway that is essential for cytokinesis.

Cytokinesis in eukaryotes: a mechanistic comparison

Cytokinesis is a crucial but poorly understood process of cell proliferation. Recently, molecular genetic analyses of fungal cytokinesis have led to an appreciation of contractile mechanisms in simple eukaryotes, and studies in animal and plant cells have led to new insights into the role of microtubules in the cleavage process. These findings suggest that fundamental mechanisms of cytokinesis may be highly conserved among eukaryotic organisms.

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.

On growth and form: control of cell morphogenesis in fission yeast

In the past year, we have gained considerable insight into the process of cell morphogenesis and the establishment of positional information in fission yeast. The highlights include a better understanding of the role of the microtubule cytoskeleton in the control of cell shape, as well as the identification of novel genes essential for the establishment of cell polarity and for the positioning of the site of cell division.

Pom1p, a fission yeast protein kinase that provides positional information for both polarized growth and cytokinesis

Schizosaccharomyces pombe cells have a well-defined pattern of polarized growth at the cell ends during interphase and divide symmetrically into two equal-sized daughter cells. We identified a gene, pom1, that provides positional information for both growth and division in S. pombe. pom1 mutants form functioning growth zones and division septa but show several abnormalities: (1) After division, cells initiate growth with equal frequencies from either the old or the new end; (2) most cells never switch to bipolar growth but instead grow exclusively at the randomly chosen end; (3) some cells mislocalize their growth axis altogether, leading to the formation of angled and branched cells; and (4) many cells misplace and/or misorient their septa, leading to asymmetric cell division. pom1 encodes a putative protein kinase that is concentrated at the new cell end during interphase, at both cell ends during mitosis, and at the septation site after mitosis. Small amounts of Pom1p are also found at the old cell end during interphase and associated with the actin ring during mitosis. Pom1p localization to the cell ends is independent of actin but requires microtubules and Tea1p. pom1 mutations are synthetically lethal with several other mutations that affect cytokinesis and/or the actin or microtubule cytoskeleton. Thus, Pom1p may position the growth and cytokinesis machineries by interaction with both the actin and microtubule cytoskeletons.