Eleven novel sep genes of Schizosaccharomyces pombe required for efficient cell separation and sexual differentiation

Disruption of the Schizosaccharomyces japonicus lig4 Disturbs Several Cellular Processes and Leads to a Pleiotropic Phenotype

Gene targeting is a commonly used method to reveal the function of genes. Although it is an attractive tool for molecular studies, it can frequently be a challenge because its efficiency can be low and it requires the screening of a large number of transformants. Generally, these problems originate from the elevated level of ectopic integration caused by non-homologous DNA end joining (NHEJ). To eliminate this problem, NHEJ-related genes are frequently deleted or disrupted. Although these manipulations can improve gene targeting, the phenotype of the mutant strains raised the question of whether mutations have side effects. The aim of this study was to disrupt the lig4 gene in the dimorphic fission yeast, S. japonicus, and investigate the phenotypic changes of the mutant strain. The mutant cells have shown various phenotypic changes, such as increased sporulation on complete medium, decreased hyphal growth, faster chronological aging, and higher sensitivity to heat shock, UV light, and caffeine. In addition, higher flocculation capacity has been observed, especially at lower sugar concentrations. These changes were supported by transcriptional profiling. Many genes belonging to metabolic and transport processes, cell division, or signaling had altered mRNA levels compared to the control strain. Although the disruption improved the gene targeting, we assume that the lig4 inactivation can cause unexpected physiological side effects, and we have to be very careful with the manipulations of the NHEJ-related genes. To reveal the exact mechanisms behind these changes, further investigations are required.

Expression pattern and phenotypic characterization of the mutant strain reveals target genes and processes regulated by pka1 in the dimorphic fission yeast Schizosaccharomyces japonicus

The cAMP cascade plays an important role in several biological processes. Thus, study of its molecular details can contribute to a better understanding of these processes, treatment of diseases, or even finding antifungal drug targets. To gain further information about the PKA pathway, and its evolutionarily conserved and species-specific features, the central regulator pka1 gene, which encodes the cAMP-dependent protein kinase catalytic subunit, was studied in the less known haplontic, dimorphic fission yeast Schizosaccharomyces japonicus. Namely, this species belongs to a highly divergent phylogenetic branch of fungi. Furthermore, S. japonicus had only a single copy pka1 gene in contrast to the budding yeasts. Therefore, the pka1 deleted mutant was created, whose RNA sequencing and phenotypic studies revealed that the Pka1 regulated at least 373 genes, among them further kinases, phosphatases and transcriptional regulators. It regulated elongation of hyphae, cell size, aging and stress response. Furthermore, half of the pka1 target genes seemed to be conserved in Schizosaccharomyces pombe and S. japonicus. However, there were oppositely regulated genes in the two closely related species. The target genes suggest that this single gene must be able to fulfill all the functions of TPK1-3 of Saccharomyces cerevisiae. Thus, our results shed light on certain similarities and differences of the PKA pathway of S. japonicus compared to the budding yeasts and confirmed the multifunctionality of the pka1 gene, but further experiments are needed to prove its involvement in the metabolic processes and transport.

A new transcription factor for mitosis: in Schizosaccharomyces pombe, the RFX transcription factor Sak1 works with forkhead factors to regulate mitotic expression

Mitotic genes are one of the most strongly oscillating groups of genes in the eukaryotic cell cycle. Understanding the regulation of mitotic gene expression is a key issue in cell cycle control but is poorly understood in most organisms. Here, we find a new mitotic transcription factor, Sak1, in the fission yeast Schizosaccharomyces pombe. Sak1 belongs to the RFX family of transcription factors, which have not previously been connected to cell cycle control. Sak1 binds upstream of mitotic genes in close proximity to Fkh2, a forkhead transcription factor previously implicated in regulation of mitotic genes. We show that Sak1 is the major activator of mitotic gene expression and also confirm the role of Fkh2 as the opposing repressor. Sep1, another forkhead transcription factor, is an activator for a small subset of mitotic genes involved in septation. From yeasts to humans, forkhead transcription factors are involved in mitotic gene expression and it will be interesting to see whether RFX transcription factors may also be involved in other organisms.

Anillin-related protein Mid1p coordinates the assembly of the cytokinetic contractile ring in fission yeast

Anillin-like protein Mid1p coordinates contractile ring assembly in fission yeast by restricting precursors in nodes around the equator. Without Mid1p, contractile ring assembly is slow and unreliable because ring precursors are separated in nodes (Blt1p, Cdc12p) or strands (myosin-II, Rng2p, Cdc15p, actin filaments) scattered widely in the cortex.

In fission yeast cells cortical nodes containing the protein Blt1p and several kinases appear early in G2, mature into cytokinetic nodes by adding anillin Mid1p, myosin-II, formin Cdc12p, and other proteins, and condense into a contractile ring by movements that depend on actin and myosin-II. Previous studies concluded that cells without Mid1p lack cytokinetic nodes and assemble rings unreliably from myosin-II strands but left open questions. Why do strands form outside the equatorial region? Why is ring assembly unreliable without Mid1p? We found in Δmid1 cells that Cdc12p accumulates in cytokinetic nodes scattered in the cortex and produces actin filaments that associate with myosin-II, Rng2p, and Cdc15p to form strands located between the nodes. Strands incorporate nodes, and in ∼67% of cells, strands slowly close into rings that constrict without the normal ∼25-min maturation period. Ring assembly is unreliable and slow without Mid1p because the scattered Cdc12p nodes generate strands spread widely beyond the equator, and growing strands depend on random encounters to merge with other strands into a ring. We conclude that orderly assembly of the contractile ring in wild-type cells depends on Mid1p to recruit myosin-II, Rng2p, and Cdc15p to nodes and to place cytokinetic nodes around the cell equator.

A novel fission yeast mei4 mutant that allows efficient synchronization of telomere dispersal and the first meiotic division

The progression of meiosis is controlled by a number of gene-expression systems in the fission yeast Schizosaccharomyces pombe. A forkhead-type transcription factor Mei4 activates a number of genes essential for progression from the middle to late stages of meiosis, which include meiosis I, meiosis II and sporulation. The mei4-deletion mutant (mei4Δ) arrests after meiotic prophase and does not enter meiosis I. To further analyse the Mei4 function, we isolated novel temperature-sensitive mei4 alleles. The two alleles isolated in the initial screen turned out to contain a substitution at N136 in the forkhead DNA-binding domain. Among site-directed mutants that carried a point mutation at this position, the mei4-N136A mutant showed the most severe temperature sensitivity. The mei4-N136A mutant arrested before meiosis I at the restrictive temperature, as did the mei4Δ mutant. In fission yeast, the telomeres are clustered at the spindle pole body (SPB) in meiotic prophase and disperse from it at the onset of meiosis I. The mei4Δ mutant was found to arrest with its telomeres clustered at the SPB, demonstrating a role for Mei4 in telomere dispersion. The mei4-N136A mutant also arrested with clustered telomeres at the restrictive temperature, and the clustering was synchronously resolved after a temperature down-shift, indicating that mei4-N136A is a reversible allele. Hence, the mei4-N136A mutant will be a unique tool to synchronize the meiotic cell cycle from meiosis I onwards and may facilitate analyses of cellular activities occurring during meiosis I.

Red1 promotes the elimination of meiosis-specific mRNAs in vegetatively growing fission yeast

Meiosis-specific mRNAs are transcribed in vegetative fission yeast, and these meiotic mRNAs are selectively removed from mitotic cells to suppress meiosis. This RNA elimination system requires degradation signal sequences called determinant of selective removal (DSR), an RNA-binding protein Mmi1, polyadenylation factors, and the nuclear exosome. However, the detailed mechanism by which meiotic mRNAs are selectively degraded in mitosis but not meiosis is not understood fully. Here we report that Red1, a novel protein, is essential for elimination of meiotic mRNAs from mitotic cells. A red1 deletion results in the accumulation of a large number of meiotic mRNAs in mitotic cells. Red1 interacts with Mmi1, Pla1, the canonical poly(A) polymerase, and Rrp6, a subunit of the nuclear exosome, and promotes the destabilization of DSR-containing mRNAs. Moreover, Red1 forms nuclear bodies in mitotic cells, and these foci are disassembled during meiosis. These results demonstrate that Red1 is involved in DSR-directed RNA decay to prevent ectopic expression of meiotic mRNAs in vegetative cells.

The Mediator complex protein Med31 is required for embryonic growth and cell proliferation during mammalian development

During development, the mammalian embryo must integrate signals to control growth and proliferation. A failure in the ability to respond to mitogenic stimuli can cause embryonic growth restriction. We have identified a mouse mutant, l11Jus15, from a mutagenesis screen that exhibits growth defects and late-gestation lethality. Here we demonstrate that this phenotype results from a mutation in the Mediator complex gene Med31, which causes degradation of Med31 protein. The Med31 mutant phenotype is not similar to other Mediator complex mouse mutants, and target genes of other Mediator proteins are expressed normally in Med31 mutants, suggesting that Med31 has distinct target genes required for mammalian development. Med31 mutant embryos have fewer proliferating cells than controls, especially in regions that expand rapidly during development such as the forelimb buds. Likewise, embryonic fibroblast cells cultured from mutant embryos have a severe proliferation defect, as well as reduced levels of the cell cycle protein Cdc2. Med31 mutants have normal limb bud patterning but defective or delayed chondrogenesis due to a lack of Sox9 and Col2a1 expression. As the Mediator complex is a transcriptional co-activator, our results suggest that Med31 functions to promote the transcription of genes required for embryonic growth and cell proliferation.

The pleiotropic cell separation mutation spl1-1 is a nucleotide substitution in the internal promoter of the proline tRNACGG gene of Schizosaccharomyces pombe

spl1-1 was originally identified as a spontaneous mutation genetically interacting with sep1-1 and cdc4-8 in producing multinucleate syncytia. This study shows that it is allelic with the proline-tRNA(CGG) gene SPATRNAPRO.02. Its nucleotide sequence contains a C-->T substitution in the region corresponding to the B-box of the putative intragenic promoter and the TpsiC loop of the mature tRNA. The substitution drastically reduces the transcription efficiency of the gene and pleiotropically affects numerous cellular processes. spl1-1 cells are temperature sensitive, osmosensitive, bend at higher temperatures, have extended G2 phase and are defective in cell separation (septum cleavage). The proline-tRNA(TGG) gene SPATRNAPRO.01 can partially suppress the spl1-1 mutation when introduced into the cells on a multicopy plasmid. The effect of a mutation in a tRNA gene on cell separation brings a new element into the complexity of the regulation of cell division and its co-ordination with other cellular processes in Schizosaccharomyces pombe.

The S. pombe SAGA complex controls the switch from proliferation to sexual differentiation through the opposing roles of its subunits Gcn5 and Spt8

The SAGA complex is a conserved multifunctional coactivator known to play broad roles in eukaryotic transcription. To gain new insights into its functions, we performed biochemical and genetic analyses of SAGA in the fission yeast, Schizosaccharomyces pombe. Purification of the S. pombe SAGA complex showed that its subunit composition is identical to that of Saccharomyces cerevisiae. Analysis of S. pombe SAGA mutants revealed that SAGA has two opposing roles regulating sexual differentiation. First, in nutrient-rich conditions, the SAGA histone acetyltransferase Gcn5 represses ste11(+), which encodes the master regulator of the mating pathway. In contrast, the SAGA subunit Spt8 is required for the induction of ste11(+) upon nutrient starvation. Chromatin immunoprecipitation experiments suggest that these regulatory effects are direct, as SAGA is physically associated with the ste11(+) promoter independent of nutrient levels. Genetic tests suggest that nutrient levels do cause a switch in SAGA function, as spt8Delta suppresses gcn5Delta with respect to ste11(+) derepression in rich medium, whereas the opposite relationship, gcn5Delta suppression of spt8Delta, occurs during starvation. Thus, SAGA plays distinct roles in the control of the switch from proliferation to differentiation in S. pombe through the dynamic and opposing activities of Gcn5 and Spt8.

Cbf11 and Cbf12, the fission yeast CSL proteins, play opposing roles in cell adhesion and coordination of cell and nuclear division

The CSL (CBF1/RBP-Jkappa/Suppressor of Hairless/LAG-1) family is comprised of transcription factors essential for metazoan development, mostly due to their involvement in the Notch receptor signaling pathway. Recently, we identified two novel classes of CSL genes in the genomes of several fungal species, organisms lacking the Notch pathway. In this study, we characterized experimentally cbf11+ and cbf12+, the two CSL genes of Schizosaccharomyces pombe, in order to elucidate the CSL function in fungi. We provide evidence supporting their identity as genuine CSL genes. Both cbf11+ and cbf12+ are non-essential; they have distinct expression profiles and code for nuclear proteins with transcription activation potential. Significantly, we demonstrated that Cbf11 recognizes specifically the canonical CSL response element GTGA/GGAA in vitro. The deletion of cbf11+ is associated with growth phenotypes and altered colony morphology. Furthermore, we found that Cbf11 and Cbf12 play opposite roles in cell adhesion, nuclear and cell division and their coordination. Disturbed balance of the two CSL proteins leads to cell separation defects (sep phenotype), cut phenotype, and high-frequency diploidization in heterothallic strains. Our data show that CSL proteins operate in an organism predating the Notch pathway, which should be of relevance to the understanding of (Notch-independent) CSL functions in metazoans.

Stress-regulated kinase pathways in the recovery of tip growth and microtubule dynamics following osmotic stress in S. pombe

The cell-integrity and stress-response MAP kinase pathways (CIP and SRP, respectively) are stimulated by various environmental stresses. Ssp1 kinase modulates actin dynamics and is rapidly recruited to the plasma membrane following osmotic stress. Here, we show that osmotic stress arrested tip growth, induced the deposition of abnormal cell-wall deposits at tips and led to disassociation of F-actin foci from cell tips together with a reduction in the amount of F-actin in these foci. Osmotic stress also ;froze' the dynamics of interphase microtubule bundles, with microtubules remaining static for approximately 38 minutes (at 30 degrees C) before fragmenting upon return to dynamic behaviour. The timing with which microtubules resumed dynamic behaviour relied upon SRP activation of Atf1-mediated transcription, but not on either CIP or Ssp1 signalling. Analysis of the recovery of tip growth showed that: (1) the timing of recovery was controlled by SRP-stimulated Atf1 transcription; (2) re-establishment of polarized tip growth was absolutely dependent upon SRP and partially dependent upon Ssp1 signalling; and (3) selection of the site for polarized tip extension required Ssp1 and the SRP-associated polarity factor Wsh3 (also known as Tea4). CIP signalling did not impact upon any aspect of recovery. The normal kinetics of tip growth following osmotic stress of plo1.S402A/E mutants established that SRP control over the resumption of tip growth after osmotic stress is distinct from its control of tip growth following heat or gravitational stresses.

Two conserved modules of Schizosaccharomyces pombe Mediator regulate distinct cellular pathways

Mediator is an evolutionary conserved coregulator complex required for transcription of almost all RNA polymerase II-dependent genes. The Schizosaccharomyces pombe Mediator consists of two dissociable components—a core complex organized into a head and middle domain as well as the Cdk8 regulatory subcomplex. In this work we describe a functional characterization of the S. pombe Mediator. We report the identification of the S. pombe Med20 head subunit and the isolation of ts alleles of the core head subunit encoding med17⁺. Biochemical analysis of med8^ts, med17^ts, Δmed18, Δmed20 and Δmed27 alleles revealed a stepwise head domain molecular architecture. Phenotypical analysis of Cdk8 and head module alleles including expression profiling classified the Mediator mutant alleles into one of two groups. Cdk8 module mutants flocculate due to overexpression of adhesive cell-surface proteins. Head domain-associated mutants display a hyphal growth phenotype due to defective expression of factors required for cell separation regulated by transcription factor Ace2. Comparison with Saccharomyces cerevisiae Mediator expression data reveals that these functionally distinct modules are conserved between S. pombe and S. cerevisiae.

Genomic expression patterns in cell separation mutants of Schizosaccharomyces pombe defective in the genes sep10 ( + ) and sep15 ( + ) coding for the Mediator subunits Med31 and Med8

Cell division is controlled by a complex network involving regulated transcription of genes and postranslational modification of proteins. The aim of this study is to demonstrate that the Mediator complex, a general regulator of transcription, is involved in the regulation of the second phase (cell separation) of cell division of the fission yeast Schizosaccharomyces pombe. In previous studies we have found that the fission yeast cell separation genes sep10 ( + ) and sep15 ( + ) code for proteins (Med31 and Med8) associated with the Mediator complex. Here, we show by genome-wide gene expression profiling of mutants defective in these genes that both Med8 and Med31 control large, partially overlapping sets of genes scattered over the entire genome and involved in diverse biological functions. Six cell separation genes controlled by the transcription factors Sep1 and Ace2 are among the target genes. Since neither sep1 ( + ) nor ace2 ( + ) is affected in the mutant cells, we propose that the Med8 and Med31 proteins act as coactivators of the Sep1-Ace2-dependent cell separation genes. The results also indicate that the subunits of Mediator may contribute to the coordination of cellular processes by fine-tuning of the expression of larger sets of genes.

TFIIS elongation factor and Mediator act in conjunction during transcription initiation in vivo

The transcription initiation and elongation steps of protein-coding genes usually rely on unrelated protein complexes. However, the TFIIS elongation factor is implicated in both processes. We found that, in the absence of the Med31 Mediator subunit, yeast cells required the TFIIS polymerase II (Pol II)-binding domain but not its RNA cleavage stimulatory activity that is associated with its elongation function. We also found that the TFIIS Pol II-interacting domain was needed for the full recruitment of Pol II to several promoters in the absence of Med31. This work demonstrated that, in addition to its thoroughly characterized role in transcription elongation, TFIIS is implicated through its Pol II-binding domain in the formation or stabilization of the transcription initiation complex in vivo.

The fission yeast Rpb4 subunit of RNA polymerase II plays a specialized role in cell separation

RNA polymerase II is a complex of 12 subunits, Rpb1 to Rpb12, whose specific roles are only partly understood. Rpb4 is essential in mammals and fission yeast, but not in budding yeast. To learn more about the roles of Rpb4, we expressed the rpb4 gene under the control of regulatable promoters of different strength in fission yeast. We demonstrate that below a critical level of transcription, Rpb4 affects cellular growth proportional to its expression levels: cells expressing lower levels of rpb4 grew slower compared to cells expressing higher levels. Lowered rpb4 expression did not affect cell survival under several stress conditions, but it caused specific defects in cell separation similar to sep mutants. Microarray analysis revealed that lowered rpb4 expression causes a global reduction in gene expression, but the transcript levels of a distinct subset of genes were particularly responsive to changes in rpb4 expression. These genes show some overlap with those regulated by the Sep1-Ace2 transcriptional cascade required for cell separation. Most notably, the gene expression signature of cells with lowered rpb4 expression was highly similar to those of mcs6, pmh1, sep10 and sep15 mutants. Mcs6 and Pmh1 encode orthologs of metazoan TFIIH-associated cyclin-dependent kinase (CDK)-activating kinase (Cdk7-cyclin H-Mat1), while Sep10 and Sep15 encode mediator components. Our results suggest that Rpb4, along with some other general transcription factors, plays a specialized role in a transcriptional pathway that controls the cell cycle-regulated transcription of a specific subset of genes involved in cell division.

Characterisation of two novel fork-head gene homologues of Schizosaccharomyces pombe: their involvement in cell cycle and sexual differentiation

The fork-head type transcription factors are a class of regulators that function in a broad spectrum of cellular and developmental processes in many species ranging from yeasts to human. Previous data on yeast fork-head genes suggested roles for these regulators in the control of cell division, sexual differentiation and development. The genome of Schizosaccharomyces pombe has four genes that code for proteins containing fork-head domains (FKH), two of which have been characterised. Here we describe the remaining two genes, fhl1 and fkh2, that code for proteins containing fork-head-associated domains (FHA) besides their FKHs. Neither of them is essential for viability, although the deletion of either fhl1 (putative homologue of Saccharomyces cerevisiae FHL1) or fkh2 (similar to FKH1 and FKH2 of S. cerevisiae) reduced the growth rate and caused an extension of cell length due to delayed G2-to-M transition. Occasionally, multiseptate cells were also produced, indicating the involvement of fhl1 and fkh2 in efficient septum cleavage. The fkh2Delta cells were slightly more sensitive than the wild-type cells to certain environmental stresses, showed reduced fertility and occasional deficiencies in meiosis II, indicating that fkh2 might also act in stress response and sexual differentiation.

Impairment of the TFIIH-associated CDK-activating kinase selectively affects cell cycle-regulated gene expression in fission yeast

The fission yeast Mcs6-Mcs2-Pmh1 complex, homologous to metazoan Cdk7-cyclin H-Mat1, has dual functions in cell division and transcription: as a partially redundant cyclin-dependent kinase (CDK)-activating kinase (CAK) that phosphorylates the major cell cycle CDK, Cdc2, on Thr-167; and as the RNA polymerase (Pol) II carboxyl-terminal domain (CTD) kinase associated with transcription factor (TF) IIH. We analyzed conditional mutants of mcs6 and pmh1, which activate Cdc2 normally but cannot complete cell division at restrictive temperature and arrest with decreased CTD phosphorylation. Transcriptional profiling by microarray hybridization revealed only modest effects on global gene expression: a one-third reduction in a severe mcs6 mutant after prolonged incubation at 36 degrees C. In contrast, a small subset of transcripts ( approximately 5%) decreased by more than twofold after Mcs6 complex function was compromised. The signature of repressed genes overlapped significantly with those of cell separation mutants sep10 and sep15. Sep10, a component of the Pol II Mediator complex, becomes essential in mcs6 or pmh1 mutant backgrounds. Moreover, transcripts dependent on the forkhead transcription factor Sep1, which are expressed coordinately during mitosis, were repressed in Mcs6 complex mutants, and Mcs6 also interacts genetically with Sep1. Thus, the Mcs6 complex, a direct activator of Cdc2, also influences the cell cycle transcriptional program, possibly through its TFIIH-associated kinase function.

The Soh1/MED31 Protein Is an Ancient Component of Schizosaccharomyces pombe and Saccharomyces cerevisiae Mediator

We here demonstrated that the Soh1/MED31 protein is a stable component of Mediator complex isolated from Schizosaccharomyces pombe and Saccharomyces cerevisiae. Bioinformatic analysis traces the Soh1/MED31 family of Mediator subunits to the point of major eukaryotic divergence, before the appearance of the canonical heptapeptide repeat structure of the RNA polymerase II C-terminal domain.

The endo-beta-1,3-glucanase eng1p is required for dissolution of the primary septum during cell separation in Schizosaccharomyces pombe

Schizosaccharomyces pombe cells divide by medial fission throughout contraction of an actomyosin ring and deposition of a multilayered division septum that must be cleaved to release the two daughter cells. Although many studies have focused on the actomoysin ring and septum assembly, little information is available concerning the mechanism of cell separation. Here we describe the characterization of eng1+, a new gene that encodes a protein with detectable endo-beta-1,3-glucanase activity and whose deletion is not lethal to the cells but does interfere in their separation. Electron microscopic observation of mutant cells indicated that this defect is mainly due to the failure of the cells to degrade the primary septum, a structure rich in beta-1,3-glucans, that separates the two sisters cells. Expression of eng1+ varies during the cell cycle, maximum expression being observed before septation, and the protein localizes to a ring-like structure that surrounds the septum region during cell separation. This suggests that it could also be involved in the cleavage of the cylinder of the cell wall that covers the division septum. The expression of eng1+ during vegetative growth is regulated by a C2H2 zinc-finger protein (encoded by the SPAC6G10.12c ORF), which shows significant sequence similarity to the Saccharomyces cerevisiae ScAce2p, especially in the zinc-finger region. Mutants lacking this transcriptional regulator (which we have named ace2+) show a severe cell separation defect, hyphal growth being observed. Thus, ace2p may regulate the expression of the eng1+ gene together with that of other genes whose products are also involved in cell separation.

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.

Regulation of cell separation in the dimorphic fungus Ustilago maydis

During its haploid phase the dimorphic fungus Ustilago maydis grows vegetatively by budding. We have identified two genes, don1 and don3, which control the separation of mother and daughter cells. Mutant cells form tree-like clusters in liquid culture and grow as ring-like (donut-shaped) colonies on solid medium. In wild-type U. maydis cells, two distinct septa are formed during cytokinesis and delimit a fragmentation zone. Cells defective for either don1 or don3 display only a single septum and fail to complete cell separation. don1 encodes a guanine nucleotide exchange factor (GEF) of the Dbl family specific for Rho/Rac GTPases. Don3 belongs to the germinal-centre-kinase (GC) subfamily of Ste20-like protein kinases. We have isolated the U. maydis homologues of the small GTP binding proteins Rho2, Rho3, Rac1 and Cdc42. Out of these, only Cdc42 interacts specifically with Don1 and Don3 in the yeast two-hybrid system. We propose that Don1 and Don3 regulate the initiation of the secondary septum, which is required for proper cell separation.

Multifunctional cytokinesis genes in Schizosaccharomyces pombe

The proper division of cells is essential for the production of viable daughter cells. In plants and fungi, the dividing cell produces a cross-wall or septum that bisects the cytoplasm. For separation of the daughter cells, the septum has to be cleaved. To study the regulation of this process, we isolated mutants defective in septum cleavage. The mutants showed highly pleiotropic phenotypes and defined 17 novel genes. The deduced amino acid sequences of the products of the cloned genes exhibited homologies to various transcription regulators of other organisms. The homologies and the pleiotropic effects of the mutations on sexual development, stress response, mitotic stability, septum initiation and septum placement indicated that these genes affect cell separation indirectly, through multifunctional regulatory modules.

Human BIN3 complements the F-actin localization defects caused by loss of Hob3p, the fission yeast homolog of Rvs161p

The BAR adaptor proteins encoded by the RVS167 and RVS161 genes from Saccharomyces cerevisiae form a complex that regulates actin, endocytosis, and viability following starvation or osmotic stress. In this study, we identified a human homolog of RVS161, termed BIN3 (bridging integrator-3), and a Schizosaccharomyces pombe homolog of RVS161, termed hob3+ (homolog of Bin3). In human tissues, the BIN3 gene was expressed ubiquitously except for brain. S. pombe cells lacking Hob3p were often multinucleate and characterized by increased amounts of calcofluor-stained material and mislocalized F-actin. For example, while wild-type cells localized F-actin to cell ends during interphase, hob3Delta mutants had F-actin patches distributed randomly around the cell. In addition, medial F-actin rings were rarely found in hob3Delta mutants. Notably, in contrast to S. cerevisiae rvs161Delta mutants, hob3Delta mutants showed no measurable defects in endocytosis or response to osmotic stress, yet hob3+ complemented the osmosensitivity of a rvs161Delta mutant. BIN3 failed to rescue the osmosensitivity of rvs161Delta, but the actin localization defects of hob3Delta mutants were completely rescued by BIN3 and partially rescued by RVS161. These findings suggest that hob3+ and BIN3 regulate F-actin localization, like RVS161, but that other roles for this gene have diverged somewhat during evolution.

Role of cell shape in determination of the division plane in Schizosaccharomyces pombe: random orientation of septa in spherical cells

The establishment of growth polarity in Schizosaccharomyces pombe cells is a combined function of the cytoplasmic cytoskeleton and the shape of the cell wall inherited from the mother cell. The septum that divides the cylindrical cell into two siblings is formed midway between the growing poles and perpendicularly to the axis that connects them. Since the daughter cells also extend at their ends and form their septa at right angles to the longitudinal axis, their septal (division) planes lie parallel to those of the mother cell. To gain a better understanding of how this regularity is ensured, we investigated septation in spherical cells that do not inherit morphologically predetermined cell ends to establish poles for growth. We studied four mutants (defining four novel genes), over 95% of whose cells displayed a completely spherical morphology and a deficiency in mating and showed a random distribution of cytoplasmic microtubules, Tea1p, and F-actin, indicating that the cytoplasmic cytoskeleton was poorly polarized or apolar. Septum positioning was examined by visualizing septa and division scars by calcofluor staining and by the analysis of electron microscopic images. Freeze-substitution, freeze-etching, and scanning electron microscopy were used. We found that the elongated bipolar shape is not essential for the determination of a division plane that can separate the postmitotic nuclei. However, it seems to be necessary for the maintenance of the parallel orientation of septa over the generations. In the spherical cells, the division scars and septa usually lie at angles to each other on the cell surface. We hypothesize that the shape of the cell indirectly affects the positioning of the septum by directing the extension of the spindle.