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At least 19 recordsLinked to original sources
PubMed1996

How fission yeast fission in the middle.

In fission yeast, we propose that the division plane may be positioned by the position of the premitotic nucleus, perhaps by a signal emanating from the nucleus. Gene products involved in the assembly of the ring and its temporal and spatial controls are beginning to be characterized. Some of these, such as mid1p, may be involved in signals that position the division site. In animal cells, the division site may be determined by analogous signals emanating from the mitotic asters (Rappaport, 1986). We speculate that the signals defined in fission yeast will help identify signals that determine the division plane in all kinds of eukaryotes.

Cell Division↗
PubMed2004

Comparative analysis of cytokinesis in budding yeast, fission yeast and animal cells.

Cytokinesis is a temporally and spatially regulated process through which the cellular constituents of the mother cell are partitioned into two daughter cells, permitting an increase in cell number. When cytokinesis occurs in a polarized cell it can create daughters with distinct fates. In eukaryotes, cytokinesis is carried out by the coordinated action of a cortical actomyosin contractile ring and targeted membrane deposition. Recent use of model organisms with facile genetics and improved light-microscopy methods has led to the identification and functional characterization of many proteins involved in cytokinesis. To date, this analysis indicates that some of the basic components involved in cytokinesis are conserved from yeast to humans, although their organization into functional machinery that drives cytokinesis and the associated regulatory mechanisms bear species-specific features. Here, we briefly review the current status of knowledge of cytokinesis in the budding yeast Saccharomyces cerevisiae, the fission yeast Schizosaccharomyces pombe and animal cells, in an attempt to highlight both the common and the unique features. Although these organisms diverged from a common ancestor about a billion years ago, there are eukaryotes that are far more divergent. To evaluate the overall evolutionary conservation of cytokinesis, it will be necessary to include representatives of these divergent branches. Nevertheless, the three species discussed here provide substantial mechanistic diversity.

Animals↗
PubMed2005

The kinesin Klp2 mediates polarization of interphase microtubules in fission yeast.

Fission yeast (Schizosaccharomyces pombe) cells grow longitudinally in a manner dependent on a polarized distribution of their interphase microtubules. We found that this distribution required sliding of microtubules toward the cell center along preexisting microtubules. This sliding was mediated by the minus end-directed kinesin motor Klp2, which helped microtubules to become properly organized with plus ends predominantly oriented toward the cell ends and minus ends toward the cell center. Thus, interphase microtubules in the fission yeast require motor activities for their proper organization.

Benzimidazoles↗
PubMed2006

Global roles of Ste11p, cell type, and pheromone in the control of gene expression during early sexual differentiation in fission yeast.

Fission yeast cells belong to one of two specialized cell types, M or P. Specific environmental conditions trigger sexual differentiation, which leads to an internal program starting with pheromone signaling between M and P cells, followed by mating, meiosis, and sporulation. The initial steps of this process are controlled by Ste11p, a master transcriptional regulator that activates the expression of cell type-specific genes (only expressed in either M or P cells) as well as genes expressed in both M and P cells. Pheromone signaling is activated by Ste11p-dependent transcription and, in turn, enhances some of this transcription in a positive feedback. To obtain a genomewide view of Ste11p target genes, their cell-type specificity, and their dependence on pheromone, we used DNA microarrays along with different genetic and environmental manipulations of fission yeast cells. We identified 78 Ste11p-dependent genes, 12 and 4 of which are only expressed in M and P cells, respectively. These genes show differing grades of pheromone dependencies for Ste11p-activated transcription, ranging from complete independence to complete dependence on pheromone. We systematically deleted all novel cell type-specific genes and characterized their phenotype during sexual differentiation. A comparison with a similar data set from the distantly related budding yeast reveals striking conservation in both number and types of the proteins that define cell types. Given the divergent mechanisms regulating cell type-specific gene expression, our results highlight the plasticity of regulatory circuits, which evolve to allow adaptation to changing environments and lifestyles.

Cell Differentiation↗
PubMed1998

Growth polarity transitions in a dimorphic fission yeast.

Fission yeast cells grow by extension at the ends (poles) and divide by transverse fission. It has previously been reported that Schizosaccharomyces japonicus var. japonicus can switch to unipolar, filamentous growth. Here it is shown that the yeast-to-mycelium transition is a gradual process involving a changeover to unipolar growth associated with asymmetric divisions, the development of large polarly located vacuoles, the modifications of the actin and microtubular cytoskeleton and the repression of cell separation after division. High concentrations of glucose in the medium or supplementation of the medium with caffeine or cAMP support the bipolar yeast phase, inhibit the transition to the mycelial phase and induce the conversion of hyphae to yeasts. These effects suggest that cAMP may be involved in the regulation of dimorphism. Temperatures below 18 degrees C or over 35 degrees C are restrictive for the mycelial phase and provoke a return to yeast phase.

Caffeine↗
PubMed1999

The pub1 E3 ubiquitin ligase negatively regulates leucine uptake in response to NH(4)(+) in fission yeast.

Fission yeast strains auxotrophic for leucine are unable to proliferate in normally supplemented minimal media adjusted to pH 6. 4 or above. High-pH sensitivity can be suppressed by the loss of Pub1, an E3 ubiquitin ligase, or by the replacement of NH(4)(+) with a non-repressing source of nitrogen such as L-proline. In this report we show pub1 to be required for the rapid down-regulation of leucine uptake observed in response to the addition of NH(4)(+) to the growth media. Furthermore, we corroborate earlier results demonstrating the transport of leucine to be negatively influenced by high extracellular pH. pub1 is homologous to the budding yeast nitrogen permease inactivator, NPI1/RSP5, which mediates the ubiquitination and subsequent destruction of NH(4)(+)-sensitive permeases. The high-pH sensitivity of cells auxotrophic for leucine thus seems to reflect an inability of NH(4)(+)-insensitive permeases to transport sufficient leucine under conditions where the proton gradient driving nutrient transport is low, and NH(4)(+)-sensitive permeases have been destroyed. Intriguingly, the partial suppression of both high pH sensitivity, and the inactivating effect of NH(4)(+) on leucine transport, seen in pub1-1 point mutants, becomes as complete as seen in pub1Delta backgrounds when cells have concomitantly lost the function of the spc1 stress-activated MAPK.

Cell Division↗
PubMed1992

Developmental choices in mating-type interconversion in fission yeast.

Fission yeast cells follow a specific pattern of mating (cell) type switching in single cell pedigrees. Asymmetric cell divisions producing sisters of different developmental fates result from inheritance of specific parental DNA strands according to the classical model of semiconservative replication and segregation.

Gene Expression Regulation, Fungal↗
PubMed2003

A mutation in the gene involved in sister chromatid separation causes a defect in nuclear mRNA export in fission yeast.

Fission yeast ptr4-1 is one of the mRNA transport mutants that accumulate poly(A)(+) RNA in the nuclei at the nonpermissive temperature. We cloned the ptr4(+) gene and found that it is identical with the cut1(+) gene essential for chromosome segregation during mitosis. ptr4/cut1 has no defects in nucleocytoplasmic transport of a protein, indicative of a specific blockage of mRNA export by this mutation. A mutant of Cut2p cooperating with Cut1p in sister chromatid separation also showed defective mRNA export at the nonpermissive temperature. Our results suggest a novel linkage between the cell division cycle and nuclear mRNA export in eukaryotic cells.

Cell Nucleus↗
PubMed1998

RNA-assisted nuclear transport of the meiotic regulator Mei2p in fission yeast.

Fission yeast Mei2p is an RNA-binding protein required for both premeiotic DNA synthesis and meiosis I. Mei2p binds to a polyadenylated RNA molecule, meiRNA, loss of which blocks meiosis I. Mei2p forms a dot in meiotic prophase nuclei. Here, we show that meiRNA is required for the nuclear localization of Mei2p and is detectable in the dot. However, Mei2p carrying a nuclear localization signal can produce a nuclear dot and promote meiosis I in the absence of meiRNA. Mei2p expressed in cultured mammalian cells stays in the cytoplasm, but it accumulates in the nucleolus if meiRNA is coexpressed. These results indicate that meiRNA contributes to the promotion of meiosis I exclusively as a cofactor that assists nuclear transport of Mei2p.

Animals↗
PubMed2001

Phosphorylation of Mei2 and Ste11 by Pat1 kinase inhibits sexual differentiation via ubiquitin proteolysis and 14-3-3 protein in fission yeast.

Fission yeast Pat1 kinase inhibits sexual differentiation by phosphorylating the meiotic inducer Mei2 and the transcription factor Ste11. Here, we show how Pat1 downregulates these proteins. Mei2 is degraded via a ubiquitin-proteasome pathway in a phosphorylation-dependent fashion. The E2 Ubc2 and the E3 Ubr1 are required for this proteolysis. In addition, Pat1 negatively regulates Ste11 via Rad24/14-3-3, thereby repressing mei2+ transcription. The Pat1 phosphorylation sites of Ste11 match the consensus recognition sequence for 14-3-3. Rad24 binds preferentially to phosphorylated Ste11, and this binding results in inhibition of the transcriptional activation capacity of Ste11. Overall, therefore, these results show that Pat1 coordinates concerted molecular mechanisms that govern the sexual differentiation developmental decision.

14-3-3 Proteins↗
PubMed2002

Recruitment of cohesin to heterochromatic regions by Swi6/HP1 in fission yeast.

Fission yeast centromeres, like those of higher eukaryotes, are composed of repeated DNA structures and associated heterochromatin protein complexes, that have a critical function in the faithful segregation of chromosomes during cell division. Cohesin protein complexes, which are essential for sister-chromatid cohesion and proper chromosome segregation, are enriched at centromeric repeats. We have identified a functional and physical link between heterochromatin and cohesin. We find that the preferential localization of cohesins at the centromeric repeats is dependent on Swi6, a conserved heterochromatin protein that is required for proper kinetochore function. Cohesin is also enriched at the mating-type heterochromatic region in a manner that depends on Swi6 and is required to preserve the genomic integrity of this locus. We provide evidence that a cohesin subunit Psc3 interacts with Swi6 and its mouse homologue HP1. These data define a conserved function of Swi6/HP1 in recruitment of cohesin to heterochromatic regions, promoting the proper segregation of chromosomes.

Animals↗
PubMed2005

Profilin-mediated competition between capping protein and formin Cdc12p during cytokinesis in fission yeast.

Fission yeast capping protein SpCP is a heterodimer of two subunits (Acp1p and Acp2p) that binds actin filament barbed ends. Neither acp1 nor acp2 is required for viability, but cells lacking either or both subunits have cytokinesis defects under stressful conditions, including elevated temperature, osmotic stress, or in combination with numerous mild mutations in genes important for cytokinesis. Defects arise as the contractile ring constricts and disassembles, resulting in delays in cell separation. Genetic and biochemical interactions show that the cytokinesis formin Cdc12p competes with capping protein for actin filament barbed ends in cells. Deletion of acp2 partly suppresses cytokinesis defects in temperature-sensitive cdc12-112 cells and mild overexpression of capping protein kills cdc12-112 cells. Biochemically, profilin has opposite effects on filaments capped with Cdc12p and capping protein. Profilin depolymerizes actin filaments capped by capping protein but allows filaments capped by Cdc12p to grow at their barbed ends. Once associated with a barbed end, either Cdc12p or capping protein prevents the other from influencing polymerization at that end. Given that capping protein arrives at the division site 20 min later than Cdc12p, capping protein may slowly replace Cdc12p on filament barbed ends in preparation for filament disassembly during ring constriction.

Actin Depolymerizing Factors↗
PubMed1997

p56(chk1) protein kinase is required for the DNA replication checkpoint at 37 degrees C in fission yeast.

Fission yeast p56(chk1) kinase is known to be involved in the DNA damage checkpoint but not to be required for cell cycle arrest following exposure to the DNA replication inhibitor hydroxyurea (HU). For this reason, p56(chk1) is considered not to be necessary for the DNA replication checkpoint which acts through the inhibitory phosphorylation of p34(cdc2) kinase activity. In a search for Schizosaccharomyces pombe mutants that abolish the S phase cell cycle arrest of a thermosensitive DNA polymerase delta strain at 37 degrees C, we isolated two chk1 alleles. These alleles are proficient for the DNA damage checkpoint, but induce mitotic catastrophe in several S phase thermosensitive mutants. We show that the mitotic catastrophe correlates with a decreased level of tyrosine phosphorylation of p34(cdc2). In addition, we found that the deletion of chk1 and the chk1 alleles abolish the cell cycle arrest and induce mitotic catastrophe in cells exposed to HU, if the cells are grown at 37 degrees C. These findings suggest that chk1 is important for the maintenance of the DNA replication checkpoint in S phase thermosensitive mutants and that the p56(chk1) kinase must possess a novel function that prevents premature activation of p34(cdc2) kinase under conditions of impaired DNA replication at 37 degrees C.

Checkpoint Kinase 1↗
PubMed2002

Spindle-kinetochore attachment requires the combined action of Kin I-like Klp5/6 and Alp14/Dis1-MAPs in fission yeast.

Fission yeast Klp5 and Klp6 belong to the microtubule-destabilizing Kin I family. In klp5 mutants, spindle checkpoint proteins Mad2 and Bub1 are recruited to mitotic kinetochores for a prolonged duration, indicating that these kinetochores are unattached. Further analysis shows that there are kinetochores to which only Bub1, but not Mad2, localizes. These kinetochores are likely to have been captured, yet lack tension. Thus Klp5 and Klp6 play a role in a spindle- kinetochore interaction at dual steps, capture and generation of tension. The TOG/XMAP215 family, Alp14 and Dis1 are known to stabilize microtubules and be required for the bivalent attachment of the kinetochore to the spindle. Despite apparent opposing activities towards microtubule stability, Klp5/Klp6 and Alp14/Dis1 share an essential function, as either dis1klp or alp14klp mutants are synthetically lethal, like alp14dis1. Defective phenotypes are similar to each other, characteristic of attachment defects and chromosome mis-segregation. Furthermore Alp14 is of significance for kinetochore localization of Klp5. We propose that Klp5/Klp6 and Alp14/Dis1 play a collaborative role in bipolar spindle formation during prometaphase through producing spindle dynamism.

Chromosomes, Fungal↗
PubMed2003

Replication checkpoint protein Mrc1 is regulated by Rad3 and Tel1 in fission yeast.

Fission yeast Mrc1 (mediator of replication checkpoint 1) is an adaptor checkpoint protein required for Rad3-dependent activation of the checkpoint kinase Cds1 in response to arrest of replication forks. Here we report studies on the regulation of Mrc1 by phosphorylation. Replication arrest induced by hydroxyurea (HU) induces Mrc1 phosphorylation that is detected by a change in Mrc1 electrophoretic mobility. Phosphorylation is maintained in cds1Delta, rad3Delta, and tel1Delta single mutants but eliminated in a rad3Delta tel1Delta double mutant. Mrc1 has two clusters of S/TQ motifs that are potential Rad3/Tel1 phosphorylation sites. Mutation of six S/TQ motifs in these two clusters strongly impairs Mrc1 phosphorylation. Two motifs located at S604 and T645 are vital for HU resistance. The T645A mutation strongly impairs a Cds1-Mrc1 yeast two-hybrid interaction that is dependent on a functional forkhead-associated (FHA) domain in Cds1, indicating that phosphorylation of T645 mediates Mrc1's association with Cds1. Consistent with this model, the T645 region of Mrc1 effectively substitutes for the T11 region of Cds1 that is thought to be phosphorylated by Rad3 and to mediate FHA-dependent oligomerization of Cds1. The S/TQ cluster that includes S604 is needed for Mrc1's increased association with chromatin in replication-arrested cells. These data indicate that Rad3 and Tel1 regulate Mrc1 through differential phosphorylation to control Cds1.

Base Sequence↗
PubMed2006

Sterol regulatory element binding protein is a principal regulator of anaerobic gene expression in fission yeast.

Fission yeast sterol regulatory element binding protein (SREBP), called Sre1p, functions in an oxygen-sensing pathway to allow adaptation to fluctuating oxygen concentrations. The Sre1p-Scp1p complex responds to oxygen-dependent sterol synthesis as an indirect measure of oxygen availability. To examine the role of Sre1p in anaerobic gene expression in Schizosaccharomyces pombe, we performed transcriptional profiling experiments after a shift to anaerobic conditions for 1.5 h. Of the 4,940 genes analyzed, expression levels of 521 (10.5%) and 686 (13.9%) genes were significantly increased and decreased, respectively, under anaerobic conditions. Sre1p controlled 68% of genes induced > or = 2-fold. Oxygen-requiring biosynthetic pathways for ergosterol, heme, sphingolipid, and ubiquinone were primary targets of Sre1p. Induction of glycolytic genes and repression of mitochondrial oxidative phosphorylation genes largely did not require Sre1p. Using chromatin immunoprecipitation, we demonstrated that Sre1p acts directly at target gene promoters and stimulates its own transcription under anaerobic conditions. sre1+ promoter analysis identified two DNA elements that are both necessary and sufficient for oxygen-dependent, Sre1p-dependent transcription. Interestingly, these elements are homologous to sterol regulatory elements bound by mammalian SREBP, highlighting the evolutionary conservation between Sre1p and SREBP. We conclude that Sre1p is a principal activator of anaerobic gene expression, upregulating genes required for nonrespiratory oxygen consumption.

Anaerobiosis↗
PubMed2002

A novel chk1-dependent G1/M checkpoint in fission yeast.

Fission yeast cells with a temperature-sensitive Orp1 protein, a component of the origin recognition complex, cannot perform DNA replication at the restrictive temperature. Seventy percent of orp1-4 cells arrest with a 1C DNA content, whereas 30% proceed to mitosis ('cut'). The arrest depends upon the checkpoint Rad proteins and, surprisingly, the Chk1 protein, which is thought to act only from late S phase. The arrested cells maintain a 1C DNA content, as judged by flow cytometry, and the early origin ars3001 has not been initiated, as judged by 2D gel analysis. We show that in G1-arrested orp1-4 cells, Wee1 phosphorylates and inactivates Cdc2. Activation of Chk1 occurs earlier than Cdc2 phosphorylation, indicating a novel role for Chk1, namely to induce and/or maintain Cdc2 phosphorylation upon checkpoint activation in G1. We also show that commitment to cutting occurs already in early G1 phase.

CDC2 Protein Kinase↗
PubMed1999

Mitotic control in the absence of cdc25 mitotic inducer in fission yeast.

Fission yeast cells tolerate the total absence of the cdc25 mitotic inducer in two cases, either in cdc2-3w or in wee1 genetic backgrounds. In the cdc2-3w cdc25Delta double mutant, the rate-limiting step leading to mitosis is reaching a critical size. However, the size control of this mutant operates in late G2, which is different from wild-type (WT) cells. This fact suggests that in WT the rate-limiting molecular process during the G2 timer is the Tyr15 dephosphorylation of cdc2, for which the cdc25 phosphatase (together with its back-up, pyp3) is dependent. In the wee1-50 cdc25Delta mutant, the population splits into different clusters, all lacking mitotic size control. This strain maintains size homeostasis by a novel method, which is random movement of the cells from one cluster to another in the successive generations. These cells should normally have a 'minimal cycle', a 'timer' with short G1 and G2 phases. However, very often the cells abort mitosis, possibly at an early event and return back to early G2, thus lengthening their cycles. The inability of these cells to start anaphase might be caused by the absence of the main mitotic regulators (wee1 and cdc25) and the improper regulation of their back-up copies (mik1 and pyp3, respectively).

Cell Cycle Proteins↗