The S. pombe cdc16 gene is required both for maintenance of p34cdc2 kinase activity and regulation of septum formation: a link between mitosis and cytokinesis?

New insights into development from mitosis of a unicellular yeast

Studies in the fission yeast Schizosaccharomyces pombe have uncovered a new spindle checkpoint.

Conditional mutations in gamma-tubulin reveal its involvement in chromosome segregation and cytokinesis

gamma-Tubulin is a conserved essential protein required for assembly and function of the mitotic spindle in humans and yeast. For example, human gamma-tubulin can replace the gamma-tubulin gene in Schizosaccharomyces pombe. To understand the structural/functional domains of gamma-tubulin, we performed a systematic alanine-scanning mutagenesis of human gamma-tubulin (TUBG1) and studied phenotypes of each mutant allele in S. pombe. Our screen, both in the presence and absence of the endogenous S. pombe gamma-tubulin, resulted in 11 lethal mutations and 12 cold-sensitive mutations. Based on structural mapping onto a homology model of human gamma-tubulin generated by free energy minimization, all deleterious mutations are found in residues predicted to be located on the surface, some in positions to interact with alpha- and/or beta-tubulins in the microtubule lattice. As expected, one class of tubg1 mutations has either an abnormal assembly or loss of the mitotic spindle. Surprisingly, a subset of mutants with abnormal spindles does not arrest in M phase but proceeds through anaphase followed by abnormal cytokinesis. These studies reveal that in addition to its previously appreciated role in spindle microtubule nucleation, gamma-tubulin is involved in the coordination of postmetaphase events, anaphase, and cytokinesis.

Correct regulation of the septation initiation network in Schizosaccharomyces pombe requires the activities of par1 and par2

In Schizosaccharomyces pombe, the initiation of cytokinesis is regulated by a septation initiation network (SIN). We previously reported that deletion of par1 and par2, two S. pombe genes encoding B' regulatory subunits of protein phosphatase 2A, causes a multiseptation phenotype, very similar to that seen in hyperactive SIN mutants. In this study, we examined the genetic interactions between par deletions and mutations in the genes encoding components of SIN and found that deletion of par1 and par2 suppressed the morphological and viability defects caused by overproduction of Byr4p and rescued a loss-of-function allele of spg1. However, par deletions could not suppress any mutations in genes downstream of spg1 in the SIN pathway. We showed further that, in suppressing the lethality of a spg1 loss-of-function allele, the correct localization of Cdc7p to the spindle pole body (SPB), which is normally lost in spg1 mutant cells, was restored. The fact that par mutant cells themselves exhibited a symmetric localization of Cdc7p to SPBs indicated a hyperactivity of SIN in such cells. On the basis of our epistasis analyses and cytological studies, we concluded that par genes normally negatively regulate SIN at or upstream of cdc7, ensuring that multiple rounds of septation do not occur.

Timing is everything: regulation of mitotic exit and cytokinesis by the MEN and SIN

Proper completion of mitosis requires careful coordination of numerous cellular events. It is crucial, for example, that cells do not initiate spindle disassembly and cytokinesis until chromosomes have been properly segregated. Cells have developed numerous safeguards or checkpoints to delay exit from mitosis and initiation of the next cell cycle in response to defects in late mitosis. In this review, we discuss recent work on two homologous signaling pathways in budding and fission yeast, termed the mitotic exit network (MEN) and septation initiation network (SIN), respectively, that are essential for coordinating completion of mitosis and cytokinesis with other mitotic events.

Live analysis of lagging chromosomes during anaphase and their effect on spindle elongation rate in fission yeast

The fission yeast Schizosaccharomyces pombe is widely used as a model system for studies of the cell cycle and chromosome biology. To enhance these studies we have fused GFP to the chromodomain protein Swi6p, thus allowing nuclear and chromosome behaviour to be followed in living cells using time-lapse fluorescence microscopy. Like endogenous Swi6p, GFP-Swi6p localises to the nucleus and is concentrated at the heterochromatic centromeres and telomeres. The nucleus is highly dynamic during interphase: the clustered centromeres, in particular, are highly mobile. By expressing GFP-(α)2-tubulin and GFP-Swi6p in the same cells we observe that the clustered centromeres move in concert with the cytoplasmic microtubules, which is likely to reflect their association with the spindle pole body. Drug treatment indicates that this movement is dependent on intact cytoplasmic microtubules. We have also used GFP-Swi6p to investigate the properties of lagging chromosomes observed in mutants with defects in chromosome segregation. Lagging chromosomes display a variety of behaviours on anaphase spindles, most surprisingly, chromosomes appear to initiate microtubule interactions and move to the poles late in anaphase B. Interestingly, in cells displaying lagging chromosomes, the rate of spindle elongation is slowed by a factor of two. This suggests that cells are able to sense the presence of a lagging chromosome and slow anaphase B in order to allow it extra time to reach the pole. However, this mechanism is not dependent on the spindle checkpoint proteins Bub1p or Dma1p, raising the possibility that a novel checkpoint mechanism operates to retard spindle elongation if lagging chromosomes are detected. An alternative model is also discussed in which single defective kinetochores on lagging chromatids are able to interact simultaneously with microtubules emanating from both poles and affect spindle dynamics by counteracting the spindle elongation force.

The spike of S phase cyclin Cig2 expression at the G1-S border in fission yeast requires both APC and SCF ubiquitin ligases

We describe a novel set of oscillation mechanisms for the fission yeast S phase cyclin Cig2, which contains an authentic destruction box and is destroyed at anaphase via the APC/cyclosome (APC/C). Unlike the mitotic cyclin Cdc13, however, Cig2 mRNA and protein peak at the G1/S boundary and decline to low levels in G2 and M phases. We show here that SCF(Pop1, Pop2) plays a role in transcriptional periodicity, as pop mutations result in constitutive cig2(+) transcripts. The instability of Cig2 during G2 and M is independent of either the APC/C or Pop1/Pop2, but requires Skp1, a core component of SCF. These data indicate that the APC/C and SCF control Cig2 levels differentially at different stages of the cell cycle.

Saccharomyces cerevisiae BUB2 prevents mitotic exit in response to both spindle and kinetochore damage

The spindle assembly checkpoint-mediated mitotic arrest depends on proteins that signal the presence of one or more unattached kinetochores and prevents the onset of anaphase in the presence of kinetochore or spindle damage. In the presence of either damage, bub2 cells initiate a preanaphase delay but do not maintain it. Inappropriate sister chromatid separation in nocodazole-treated bub2 cells is prevented when mitotic exit is blocked using a conditional tem1(c) mutant, indicating that the preanaphase failure in bub2 cells is a consequence of events downstream of TEM1 in the mitotic exit pathway. Using a conditional bub2(tsd) mutant, we demonstrate that the continuous presence of Bub2 protein is required for maintaining spindle damage-induced arrest. BUB2 is not required to maintain a DNA damage checkpoint arrest, revealing a specificity for spindle assembly checkpoint function. In a yeast two-hybrid assay and in vitro, Bub2 protein interacts with the septin protein Cdc3, which is essential for cytokinesis. These data support the view that the spindle assembly checkpoint encompasses regulation of distinct mitotic steps, including a MAD2-directed block to anaphase initiation and a BUB2-directed block to TEM1-dependent exit.

A new genetic method for isolating functionally interacting genes: high plo1(+)-dependent mutants and their suppressors define genes in mitotic and septation pathways in fission yeast

We describe a general genetic method to identify genes encoding proteins that functionally interact with and/or are good candidates for downstream targets of a particular gene product. The screen identifies mutants whose growth depends on high levels of expression of that gene. We apply this to the plo1(+) gene that encodes a fission yeast homologue of the polo-like kinases. plo1(+) regulates both spindle formation and septation. We have isolated 17 high plo1(+)-dependent (pld) mutants that show defects in mitosis or septation. Three mutants show a mitotic arrest phenotype. Among the 14 pld mutants with septation defects, 12 mapped to known loci: cdc7, cdc15, cdc11 spg1, and sid2. One of the pld mutants, cdc7-PD1, was selected for suppressor analysis. As multicopy suppressors, we isolated four known genes involved in septation in fission yeast: spg1(+), sce3(+), cdc8(+), and rho1(+), and two previously uncharacterized genes, mpd1(+) and mpd2(+). mpd1(+) exhibits high homology to phosphatidylinositol 4-phosphate 5-kinase, while mpd2(+) resembles Saccharomyces cerevisiae SMY2; both proteins are involved in the regulation of actin-mediated processes. As chromosomal suppressors of cdc7-PD1, we isolated mutations of cdc16 that resulted in multiseptation without nuclear division. cdc16(+), dma1(+), byr3(+), byr4(+) and a truncated form of the cdc7 gene were isolated by complementation of one of these cdc16 mutations. These results demonstrate that screening for high dose-dependent mutants and their suppressors is an effective approach to identify functionally interacting genes.

Mis3 with a conserved RNA binding motif is essential for ribosome biogenesis and implicated in the start of cell growth and S phase checkpoint

BACKGROUND

In normal somatic cell cycle, growth and cell cycle are properly coupled. Although CDK (cyclin-dependent kinase) activity is known to be essential for cell cycle control, the mechanism to ensure the coupling has been little understood.

RESULTS

We here show that fission yeast Mis3, a novel evolutionarily highly conserved protein with the RNA-interacting KH motif, is essential for ribosome RNA processing, and implicated in initiating the cell growth. Growth arrest of mis3-224, a temperature sensitive mutant at the restrictive temperature, coincides with the early G2 block in the complete medium or the G1/S block in the release from nitrogen starvation, reflecting coupling of cell growth and division. Genetic interactions indicated that Mis3 shares functions with cell cycle regulators and RNA processing proteins, and is under the control of Dsk1 kinase and PP1 phosphatase. Mis3 is needed for the formation of 18S ribosome RNA, and may hence direct the level of proteins required for the coupling. One such candidate is Mik1 kinase. mis3-224 is sensitive to hydroxyurea, and the level of Mik1 protein increases during replication checkpoint in a manner dependent upon the presence of Mis3 and Cds1.

CONCLUSIONS

Mis3 is essential for ribosome biogenesis, supports S phase checkpoint, and is needed for the coupling between growth and cell cycle. Whether Mis3 interacts solely with ribosomal precursor RNA remains to be determined.

Relationship of actin, microtubules, and crosswall synthesis during septation in Aspergillus nidulans

Studies of cytokinesis in animal cells demonstrate that microtubules play an important role in signaling the position of the actin-containing contractile ring and subsequent formation of the cleavage furrow. Septation in several fungi closely resembles animal cell cytokinesis in that a circumferential ring of actin is visible at the incipient division site. However, this does not necessarily mean that division is contractile since actin may also serve to localize septal wall synthesis. In addition, several studies in fission yeast have suggested that microtubules are dispensable for actin ring formation. We have used synchronized cells and fluorescence microscopy to follow actin structures, nuclear division and septal wall synthesis during septation in Aspergillus nidulans. Our data suggest that actin first appears at the septum site as a circumferential ring and that it later broadens and invaginates, forming an hourglass-shaped structure coincident with septal cell wall synthesis. Depolymerization of microtubules early in septation prevents circumferential actin ring formation. Depolymerization of microtubules after circumferential actin ring formation blocks both the progression to invaginating bands and septal wall synthesis. In contrast to studies in yeast cells, our data suggest that microtubules are required for both the initiation and progression of septation in A. nidulans.

The S. pombe orthologue of the S. cerevisiae mob1 gene is essential and functions in signalling the onset of septum formation

We have isolated the Schizosaccharomyces pombe orthologue of the Saccharomyces cerevisiae MOB1 gene in a screen designed to enrich for septation mutants. The gene is essential, and cells lacking it display a phenotype typical of septation signalling network mutants. mob1p is located on both spindle pole bodies throughout mitosis. In addition it is also co-localised with the medial ring later in mitosis, and flanks the septum as the medial ring contracts. We also demonstrate that mob1p can be precipitated from cells in a complex with the septation regulating kinase sid2p.

Sid4p is required to localize components of the septation initiation pathway to the spindle pole body in fission yeast

A mutation in the Schizosaccharomyces pombe sid4(+) (septation initiation defective) gene was isolated in a screen for mutants defective in cytokinesis. We have cloned sid4(+) and have found that sid4(+) encodes a previously unknown 76.4-kDa protein that localizes to the spindle pole body (SPB) throughout the cell cycle. Sid4p is required for SPB localization of key regulators of septation initiation, including the GTPase Spg1p, the protein kinase Cdc7p, and the GTPase-activating protein Byr4p. An N-terminally truncated Sid4p mutant does not localize to SPBs and when overproduced acts as a dominant-negative mutant by titrating endogenous Sid4p and Spg1p from the SPB. Conversely, the Sid4p N-terminal 153 amino acids are sufficient for SPB localization. Biochemical studies demonstrate that Sid4p interacts with itself, and yeast two-hybrid analysis shows that its self-interaction domain lies within the C-terminal half of the protein. Our data indicate that Sid4p SPB localization is a prerequisite for the execution of the Spg1p signaling cascade.

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.

Cytokinesis in fission yeast: a myosin pas de deux

Cytokinesis in the fission yeast, Schizosaccharomyces pombe consists of two distinct but overlapping events: the assembly and constriction of a cytokinetic actomyosin ring (CAR) and the formation of a cross wall or septum. These two processes must be spatially and temporally coordinated both with each other and with other cell cycle events, most notably spindle formation and anaphase chromosome segregation. In fission yeast, the CAR contains two unusual type II myosins, Myo2, encoded by the gene myo2(+), and Myp2, encoded by myp2(+). The relationship of these two proteins to each other and their relative contribution to CAR assembly and contraction is largely unknown. Here we review what is known about the role of each myosin in cytokinesis and present some new information concerning their regulation and possible physical interaction.

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.

Controlling the end of the cell cycle

The past year has seen significant advances in our understanding of how the events which occur at the end of mitosis, such as cytokinesis and the inactivation of mitotic cyclin dependent kinases are triggered, and also how they are prevented from occurring prematurely or inappropriately. This control is achieved through a combination of temporally ordered proteolytic events and changes in the subcellular localisation of proteins. These studies have also revealed that the nucleolus and spindle pole bodies play a key role in this regulation.

The S. pombe zfs1 gene is required to prevent septation if mitotic progression is inhibited

Schizosaccharomyces pombe cdc16p is required to limit the cell to forming a single division septum per cell cycle; the heat-sensitive loss-of-function mutant cdc16-116 completes mitosis, and then undergoes multiple rounds of septum formation without cell cleavage. cdc16p is a homologue of Saccharomyces cerevisiae BUB2p, and has also been implicated in the spindle assembly checkpoint function in S. pombe. To identify other proteins involved in regulating septum formation, we have screened for multicopy suppressors of the cdc16-116 mutation. In this paper, we describe one of these suppressors, zfs1. The null allele (zfs1-D1) is viable. However, at low temperatures it divides at a reduced size, while at higher temperatures, it partially suppresses heat sensitive mutants in genes signalling the onset of septum formation. Zfs1-D1 cells show an increased rate of chromosome loss during exponential growth. Moreover, if assembly of the spindle is prevented, zfs1-D1 cells do not arrest normally, but the activity of cdc2p kinase decays, and cells form a division septum without completing a normal mitosis. We conclude that zfs1 function is required to prevent septum formation and exit from mitosis if the mitotic spindle is not assembled. The suppression of cdc16-116 by zfs1 is independent of dma1 function and the spindle assembly checkpoint genes mad2 and mph1. The genetic interactions of zfs1 with genes regulating septum formation suggest that it may be a modulator of the signal transduction network controlling the onset of septum formation and exit from mitosis.

Asymmetry of the spindle pole bodies and spg1p GAP segregation during mitosis in fission yeast

In the fission yeast Schizosaccharomyces pombe, the onset of septum formation is induced by a signal transduction network involving several protein kinases and a GTPase switch. One of the roles of the spg1p GTPase is to localise the cdc7p protein kinase to the poles of the mitotic spindle, from where the onset of septation is thought to be signalled at the end of mitosis. Immunofluorescence studies have shown that cdc7p is located on both spindle pole bodies early in mitosis, but only on one during the later stages of anaphase. This is mediated by inactivation of spg1p on one pole before the other. The GAP for spg1p is a complex of two proteins, cdc16p and byr4p. Localisation of cdc16p and byr4p by indirect immunofluorescence during the mitotic cell cycle showed that both proteins are present on the spindle pole body in interphase cells. During mitosis, byr4p is seen first on both poles of the spindle, then on only one. This occurs prior to cdc7p becoming asymmetric. In contrast, the signal due to cdc16p decreases to a low level during early mitosis, before being seen strongly on the same pole as byr4p. Double staining indicates that this is the opposite pole to that which retains cdc7p in late anaphase. Examination of the effect of inactivating cdc16p at various stages of the cell cycle suggests that cdc16p, together with cdc2p plays a role in restraining septum formation during interphase. The asymmetric inactivation of spg1p is mediated by recruitment of the cdc16p-byr4p GAP to one of the poles of the spindle before the other, and the asymmetry of the spindle pole bodies may be established early during mitosis. Moreover, the spindle pole bodies appear to be non-equivalent even after division has been completed.

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

Genetic analysis of 20 sterile mutants prone to form hyphae revealed 11 novel ste genes (sep6 to sep16) of Schizosaccharomyces pombe. None of the mutants was completely mycelial. Most mutants formed branching hyphae and showed normal septation. Aberrant septal structures and actin distribution were seen only at 36 degrees C. sep9-307, sep14-576 and sep15-598 showed genetic interactions with sep1-1, a mutation in a forkhead transcription factor homologue. Additional genetic interactions were detected between sep6-194, sep15-598 and cdc16-116, a mutant allele of an anaphase modulator of p34cdc2. sep9-307 and sep15-598 caused dikaryosis in wee1- background. In mating and sporulation tests, sep6-, sep7-, sep9-, sep10-, sep11- and sep15- proved to be defective in conjugation only, whereas sep8-, sep13- and sep16- were also defective in meiosis-sporulation. sep12- and sep14- were only partially sterile. All mutants could produce M-factor but sep8-, sep11-, sep15- and sep16- were defective in P-factor production. The mutations in sep8, sep11 and sep16 suppressed the pat1-114-driven meiosis. All mutants were sensitive to the presence of higher concentrations of chloride in the medium and to short heat shocks. The diversity of the mutant phenotypes and the pleiotropic effects of the mutations suggest that these sep genes might act in, or interact with, a multiple overlapping network of regulatory modules.

Budding yeast Bub2 is localized at spindle pole bodies and activates the mitotic checkpoint via a different pathway from Mad2

The mitotic checkpoint blocks cell cycle progression before anaphase in case of mistakes in the alignment of chromosomes on the mitotic spindle. In budding yeast, the Mad1, 2, 3, and Bub1, 2, 3 proteins mediate this arrest. Vertebrate homologues of Mad1, 2, 3, and Bub1, 3 bind to unattached kinetochores and prevent progression through mitosis by inhibiting Cdc20/APC-mediated proteolysis of anaphase inhibitors, like Pds1 and B-type cyclins. We investigated the role of Bub2 in budding yeast mitotic checkpoint. The following observations indicate that Bub2 and Mad1, 2 probably activate the checkpoint via different pathways: (a) unlike the other Mad and Bub proteins, Bub2 localizes at the spindle pole body (SPB) throughout the cell cycle; (b) the effect of concomitant lack of Mad1 or Mad2 and Bub2 is additive, since nocodazole-treated mad1 bub2 and mad2 bub2 double mutants rereplicate DNA more rapidly and efficiently than either single mutant; (c) cell cycle progression of bub2 cells in the presence of nocodazole requires the Cdc26 APC subunit, which, conversely, is not required for mad2 cells in the same conditions. Altogether, our data suggest that activation of the mitotic checkpoint blocks progression through mitosis by independent and partially redundant mechanisms.

Bifurcation of the mitotic checkpoint pathway in budding yeast

The coordination of mitotic events is ensured through the spindle assembly checkpoint. BFA1 is required for this checkpoint in budding yeast because its disruption abolishes the mitotic arrest when spindle assembly is inhibited. Analysis of the genetic interaction of BFA1 with known mitotic checkpoint genes suggest that Bfa1 functions in the same pathway with Bub2 but not with Mad1 or Mad2. Both Bfa1 and Bub2 localize to spindle poles, and overexpression of Bfa1 arrests the cell cycle in anaphase. These findings suggest a bifurcation of the spindle assembly checkpoint: whereas one branch of the pathway, consisting of Mad1-3, Bub1 and 3, and Mps1, may prevent premature disjunction of sister chromosomes, the other, consisting of Bfa1 and Bub2, may function at spindle poles to prevent cytokinesis before the completion of chromosome segregation.

Regions of Byr4, a regulator of septation in fission yeast, that bind Spg1 or Cdc16 and form a two-component GTPase-activating protein with Cdc16

In the fission yeast Schizosaccharomyces pombe, septation and constriction of the actomyosin ring for cell division are positively regulated by the Spg1 GTPase, a member of the Ras superfamily. Spg1 is negatively regulated by Byr4 and Cdc16, which together form a two-component GTPase-activating protein for the Spg1 GTPase. To better understand how Byr4 regulates septation, Byr4 mutants were tested for in vitro functions. This analysis revealed that Byr4 contained one Cdc16-binding site and four Spg1-binding sites (SBS), designated SBS1-SBS4. Although mutants with a single SBS bound Spg1 and inhibited GTP dissociation, the equilibrium binding affinity of these mutants was 28-280-fold weaker than Byr4. Because some Byr4 mutants with multiple SBSs bound Spg1 tighter than the corresponding mutants with a single SBS, multiple SBSs probably interact to cause the high affinity binding of Byr4 to Spg1. A region of Byr4 that bound Spg1, SBS4, and the region that bound Cdc16, Cdc16-binding site, was necessary and sufficient to form Cdc16-dependent Spg1GAP activity that was similar to that of wild-type Byr4 with Cdc16.

Ibd1p, a possible spindle pole body associated protein, regulates nuclear division and bud separation in Saccharomyces cerevisiae

The proper spatial and temporal coordination of mitosis and cytokinesis is essential for maintaining genomic integrity. We describe the identification and characterization of the Saccharomyces cerevisiae IBD1 gene, which encodes a novel protein that regulates the proper nuclear division and bud separation. IBD1 was identified by the limited homology to byr4, a dosage-dependent regulator of cytokinesis in Schizosaccharomyces pombe. IBD1 is not an essential gene, and the knock-out cells show no growth defects except for the reduced mating efficiency [1]. However, upon ectopic expression from an inducible promoter, IBD1 is lethal to the cell and leads to abnormal nuclear division and bud separation. In detail, approximately 90% of the IBD1 overexpressing cells arrest at large bud stages with dividing or divided nuclei. In some IBD1 overexpressing cells, spindle elongation and chromosome separation occur within the mother cell, leading to anucleated and binucleate daughter cells. The anucleated cell can not bud, but the binucleate cell proceeds through another cell cycle(s) to produce a cell with multiple nuclei and multiple buds. Observations of the F-actin and chitin rings in the IBD1 overexpressing cells reveal that these cells lose the polarity for bud site selection and growth or attain the hyper-polarity for growth. Consistent with the phenotypes, the IBD1 overexpressing cells contain a broad range of DNA content, from 2 to 4 N or more. A functional Ibd1p-GFP fusion protein localizes to a single dot at the nuclear DNA boundary in the divided nuclei or to double dots in dividing nuclei, suggesting its localization on the spindle pole body (SPB). The cross-species expressions of IBD1 in S. pombe and byr4 in S. cerevisiae cause defects in shape, implicating the presence of a conserved mechanism for the control of cytokinesis in eukaryotes. We propose that Ibd1p is an SPB associated protein that links proper nuclear division to cytokinesis and bud separation.

Kinetochores and the checkpoint mechanism that monitors for defects in the chromosome segregation machinery

Whether we consider the division of the simplest unicellular organisms into two daughter cells or the generation of haploid gametes by the most complex eukaryotes, no two processes secure the continuance of life more than the proper replication and segregation of the genetic material. The cell cycle, marked in part by the periodic rise and fall of cyclin-dependent kinase (CDK) activities, is the means by which these two processes are separated. DNA damage and mistakes in chromosome segregation are costly, so nature has further devised elaborate checkpoint mechanisms that halt cell cycle progression, allowing time for repairs or corrections. In this article, we review the mitotic checkpoint mechanism that responds to defects in the chromosome segregation machinery and arrests cells in mitosis prior to anaphase onset. At opposite ends of this pathway are the kinetochore, where many checkpoint proteins reside, and the anaphase-promoting complex (APC), the metaphase-to-interphase transition regulator. Throughout this review we focus on budding yeast but reference parallel processes found in other organisms.

The spindle checkpoint

Prior to sister-chromatid separation, the spindle checkpoint inhibits cell-cycle progression in response to a signal generated by mitotic spindle damage or by chromosomes that have not attached to microtubules. Recent work has shown that the spindle checkpoint inhibits cell-cycle progression by direct binding of components of the spindle checkpoint pathway to components of a specialized ubiquitin-conjugating system that is responsible for triggering sister-chromatid separation.

Byr4 and Cdc16 form a two-component GTPase-activating protein for the Spg1 GTPase that controls septation in fission yeast

BACKGROUND

Spatial and temporal control of cytokinesis ensures the accurate transmission of genetic material and the correct development of multicellular organisms. An excellent model system in which to study cytokinesis is Schizosaccharomyces pombe because there are similarities between cytokinesis in S. pombe and mammals and because genes involved in S. pombe cytokinesis have been characterized. In particular, formation of the septum is positively regulated by the Spg1 GTPase and its effector, the Cdc7 kinase. Septation is negatively regulated by Cdc16, a protein similar to GTPase-activating proteins (GAPs) for Ypt GTPases, and by Byr4, a protein of unknown biochemical function. This study investigates the relationship between Byr4, Cdc16, and Spg1.

RESULTS

Genetic interactions were observed between byr4, cdc16, and spg1 mutants. Byr4 bound to Cdc16 and Spg1 in yeast two-hybrid assays and in coprecipitations in vitro and in yeast. Byr4 inhibited the dissociation and hydrolysis of GTP bound to Spg1, but when Byr4 and Cdc16 were combined together they displayed Spg1GAP activity in vitro; Cdc16 alone had no detectable GAP activity. The binding of Byr4 to Spg1 and the Byr4-Cdc16 Spg1GAP activity were specific because Byr4 and Cdc16 did not bind to or affect the GTPase activities of the seven known S pombe Ypt family GTPase.

CONCLUSIONS

Byr4 and Cdc16 form a two-component GAP for the Spg1 GTPase. Byr4 and Cdc16 appear to negatively regulate septation in S. pombe by modulating the nucleotide state of Spg1 possibly in a spatially or temporally controlled manner.

Genetic control of fission yeast cell wall synthesis: the genes involved in wall biogenesis and their interactions in Schizosaccharomyces pombe

The fungal cell wall is an essential structure which protects cells from various environmental stresses such as hyper- or hypo-osmosis, and endows them with specific morphology in response to their life or cell division cycle. In addition, the cell wall has a variety of enzymatic activities per se, which are required for nutritional uptake, secretion, and cell adhesion including mating processes. In addition to these cytological interests, clinical demands to clarify the regulatory mechanisms of cell wall synthesis have been increasing, since the cell wall is a unique and effective target of antifungal agents. However, the molecular mechanisms are poorly understood at present, although the role of several signal transduction pathways have recently been implicated in regulation. In this review, the author focuses on genes and their interactions which are involved in fission yeast cell wall biogenesis.

Cytochalasin D interferes with contractile actin ring and septum formation in Schizosaccharomyces japonicus var. versatilis

The cells of Schizosaccharomyces japonicus var. versatilis responded to the presence of cytochalasin D (CD), an inhibitor of actin polymerization, by the disappearance of contractile actin rings (ARs) that had already formed and by inhibition of new ring formation. Actin cables disappeared. Actin patches remained preserved and became co-localized with regions of actual cell wall formation (at cell poles and at the site of septum development). Removal of the AR arrested formation of the primary septum and led to the production of aberrant septum protrusions in that region. Nuclear division was accomplished in the presence of CD but new ARs were not produced. The wall (septum) material was deposited in the form of a wide band at the inner surface of the lateral cell wall in the cell centre. This layer showed a thin fibrillar structure. The removal of CD resulted in rapid formation of new ARs in the equatorial region of the cells. This implies that the signal for AR localization was not abolished either by CD effects or by removal of an AR already formed. Some of the newly developed ARs showed atypical localization and orientation. In addition, redundant, subcortically situated actin bundles were produced. The removal of CD was quickly followed by the development of primary septa co-localized with ARs. Wall protrusions occurred co-localized with the redundant actin bundles. If these were completed in a circle, redundant septa developed. The AR is a mechanism which, in time and space, triggers cytokinesis by building a septum sequentially dependent on the AR. Aberrant septa were not capable of separating daughter cells. However, non-separated daughter cells subsequently gave rise to normal cells.

NB4S, a member of the TBC1 domain family of genes, is truncated as a result of a constitutional t(1;10)(p22;q21) chromosome translocation in a patient with stage 4S neuroblastoma

Molecular cloning of the breakpoints of a t(1;10)(p22q21) constitutional translocation breakpoint in a patient with stage 4S neuroblastoma has identified two genes which are fused in-frame to generate a novel gene. The 1p22 gene, which we have called NB4S , encodes a 7.5 kb transcript with an 810 amino acid open reading frame and is expressed in a wide variety of tissues. NB4S has >88% homology with the mouse EVI -5 gene within the coding region and shows strong homology over a 200 amino acid region with TBC1 box motif genes involved in cell growth and differentiation. The C-teminal end of the protein contains a number of coiled coil domains, indicating a possible protein-protein binding function. The chromosome 10 breakpoint interrupts a novel transcript (TRNG10) which could only be detected in tumor cells. This transcript has no exon/intron structure or significant open reading frame, suggesting that it is a structural RNA which is transcribed but not translated. The chromosome rearrangement creates a fusion gene product which combines the TBC1 motif of NB4S with a polyadenylation signal from TRNG10 , potentially generating a truncated protein with oncogenic properties.

Identification of novel temperature-sensitive lethal alleles in essential beta-tubulin and nonessential alpha 2-tubulin genes as fission yeast polarity mutants

We have screened for temperature-sensitive (ts) fission yeast mutants with altered polarity (alp1-15). Genetic analysis indicates that alp2 is allelic to atb2 (one of two alpha-tubulin genes) and alp12 to nda3 (the single beta-tubulin gene). atb2(+) is nonessential, and the ts atb2 mutations we have isolated are dominant as expected. We sequenced two alleles of ts atb2 and one allele of ts nda3. In the ts atb2 mutants, the mutated residues (G246D and C356Y) are found at the longitudinal interface between alpha/beta-heterodimers, whereas in ts nda3 the mutated residue (Y422H) is situated in the domain located on the outer surface of the microtubule. The ts nda3 mutant is highly sensitive to altered gene dosage of atb2(+); overexpression of atb2(+) lowers the restrictive temperature, and, conversely, deletion rescues ts. Phenotypic analysis shows that contrary to undergoing mitotic arrest with high viability via the spindle assembly checkpoint as expected, ts nda3 mutants execute cytokinesis and septation and lose viability. Therefore, it appears that the ts nda3 mutant becomes temperature lethal because of irreversible progression through the cell cycle in the absence of activating the spindle assembly checkpoint pathway.

Mph1, a member of the Mps1-like family of dual specificity protein kinases, is required for the spindle checkpoint in S. pombe

The spindle assembly checkpoint pathway is not essential for normal mitosis but ensures accurate nuclear division by blocking the metaphase to anaphase transition in response to a defective spindle. Here, we report the isolation of a new spindle checkpoint gene, mph1 (Mps1p-like pombe homolog), in the fission yeast Schizosaccharomyces pombe, that is required for checkpoint activation in response to spindle defects. mph1 functions upstream of mad2, a previously characterized component of the spindle checkpoint. Overexpression of mph1, like overexpression of mad2, mimics activation of the checkpoint and imposes a metaphase arrest. mph1 protein shares sequence similarity with Mps1p, a dual specificity kinase that functions in the spindle checkpoint of the budding yeast Saccharomyces cerevisiae. Complementation analysis demonstrates that mph1 and Mps1p are functionally related. They differ in that Mps1p, but not mph1, has an additional essential role in spindle pole body duplication. We propose that mph1 is the MPS1 equivalent in the spindle checkpoint pathway but not in the SPB duplication pathway. Overexpression of mad2 does not require mph1 to impose a metaphase arrest, which indicates a mechanism of spindle checkpoint activation other than mph1/Mps1p kinase-dependent phosphorylation. In the same screen which led to the isolation of mad2 and mph1, we also isolated dph1, a cDNA that encodes a protein 46% identical to an S. cerevisiae SPB duplication protein, Dsk2p. Our initial characterization indicates that S.p. dph1 and S.c. DSK2 are functionally similar. Together these results suggest that the budding and fission yeasts share common elements for SPB duplication, despite differences in SPB structure and the timing of SPB duplication relative to mitotic entry.

The fission yeast microtubule cytoskeleton

The Schizosaccharomyces pombe genome sequencing project (http://www.sanger.ac.uk/Projects/S_pombe/) is nearly complete, and this is likely to generate interest in fission yeast as a model system beyond its traditional strongholds in the study of the cell cycle and sexual differentiation. In many fields S. pombe will offer a useful complement to the more widely studied Saccharomyces cerevisiae, but in some areas the impact of S. pombe may well rival or exceed that of this budding yeast in terms of relevance to higher systems. Because of the considerable differences from the S. cerevisiae microtubule cytoskeleton, studying microtubules in S. pombe is likely to enhance the contribution of model systems to our understanding of the principles and practices of microtubule organisation in eukaryotes in general.

Onset of chromosome segregation at the metaphase to anaphase transition of the cell cycle

Chromosome segregation is one of the most important acts in the life of the cell. Unequal inheritance of chromosomes (aneuploidy) is a cause of a number of disorders, particularly in humans, even though eukaryotic cells can arrest or delay the transition from metaphase to anaphase if an event critical to the completion of metaphase is impaired. In this report, we review recent advances in our knowledge of how the complex process of chromosome segregation is coupled with cell cycle progression, and starts at onset of anaphase with sister chromatids separation of the replicated chromosomes.

Cell cycle arrest in cdc20 mutants of Saccharomyces cerevisiae is independent of Ndc10p and kinetochore function but requires a subset of spindle checkpoint genes

The spindle checkpoint ensures accurate chromosome segregation by inhibiting anaphase onset in response to altered microtubule function and impaired kinetochore function. In this study, we report that the ability of the anti-microtubule drug nocodazole to inhibit cell cycle progression in Saccharomyces cerevisiae depends on the function of the kinetochore protein encoded by NDC10. We examined the role of the spindle checkpoint in the arrest in cdc20 mutants that arrest prior to anaphase with an aberrant spindle. The arrest in cdc20 defective cells is dependent on the BUB2 checkpoint and independent of the BUB1, BUB3, and MAD spindle checkpoint genes. We show that the lesion recognized by Bub2p is not excess microtubules, and the cdc20 arrest is independent of kinetochore function. We show that Cdc20p is not required for cyclin proteolysis at two points in the cell cycle, suggesting that CDC20 is distinct from genes encoding integral proteins of the anaphase promoting complex.

Regulation of septum formation in Aspergillus nidulans by a DNA damage checkpoint pathway

In Aspergillus nidulans, germinating conidia undergo multiple rounds of nuclear division before the formation of the first septum. Previous characterization of temperature-sensitive sepB and sepJ mutations showed that although they block septation, they also cause moderate defects in chromosomal DNA metabolism. Results presented here demonstrate that a variety of other perturbations of chromosomal DNA metabolism also delay septum formation, suggesting that this is a general cellular response to the presence of sublethal DNA damage. Genetic evidence is provided that suggests that high levels of cyclin-dependent kinase (cdk) activity are required for septation in A. nidulans. Consistent with this notion, the inhibition of septum formation triggered by defects in chromosomal DNA metabolism depends upon Tyr-15 phosphorylation of the mitotic cdk p34nimX. Moreover, this response also requires elements of the DNA damage checkpoint pathway. A model is proposed that suggests that the DNA damage checkpoint response represents one of multiple sensory inputs that modulates p34nimX activity to control the timing of septum formation.

Asymmetric segregation on spindle poles of the Schizosaccharomyces pombe septum-inducing protein kinase Cdc7p

Schizosaccharomyces pombe divides by means of a centrally placed division septum. The initiation of septation must be tightly coordinated with events in mitosis, as premature formation of the septum can lethally cut the undivided nucleus. The Spg1p GTPase and the Cdc7p kinase, with which it interacts, play a central role in signaling the initiation of septum formation. Loss-of-function mutations in either gene prevent septation, whereas inappropriate activation of Spg1p can induce septum formation from G1 or G2 interphase cells. Increased expression of either gene leads to multiple rounds of septation without cell cleavage, emphasizing the need for precise cell cycle regulation of their activity. To understand the mechanisms underlying this regulation, we have investigated whether these key initiators of septum formation are controlled by changes in their activity and/or location during mitosis and cytokinesis. We demonstrate that Spg1p localizes to the spindle pole body in interphase and to both spindle poles during mitosis. In contrast, Cdc7p shows no discrete localization during interphase, but early in mitosis it associates with both spindle pole bodies and, as the spindle extends, is seen on only one pole of the spindle during anaphase B. Spg1p activity is required for localization of Cdc7p in vivo but not for its kinase activity in vitro. Staining with an antiserum that recognizes preferentially GDP-Spg1p indicates that activated GTP-Spg1p predominates during mitosis when Cdc7p is associated with the spindle pole body. Furthermore, staining with this antibody shows that asymmetric distribution of Cdc7p may be mediated by inactivation of Spg1p on one spindle pole. Deregulated septation in mutant cells correlates with segregation of Cdc7p to both spindle poles.

Coupling cell division and cell death to microtubule dynamics

The mitotic spindle is a self-organizing structure that is constructed primarily from microtubules. Among the most important spindle microtubules are those that bind to kinetochores and form the fibers along which chromosomes move. Chemotherapeutics such as taxol and the vinca alkaloids perturb kinetochore-microtubule attachment and disrupt chromosome segregation. This activates a checkpoint pathway that delays cell cycle progression and induces programmed cell death. Recent work has identified at least four mammalian spindle assembly checkpoint proteins.

Identification of a second myosin-II in Schizosaccharomyces pombe: Myp2p is conditionally required for cytokinesis

As in many eukaryotic cells, fission yeast cytokinesis depends on the assembly of an actin ring. We cloned myp2(+), a myosin-II in Schizosaccharomyces pombe, conditionally required for cytokinesis. myp2(+), the second myosin-II identified in S. pombe, does not completely overlap in function with myo2(+). The catalytic domain of Myp2p is highly homologous to known myosin-IIs, and phylogenetic analysis places Myp2p in the myosin-II family. The Myp2p sequence contains well-conserved ATP- and actin-binding motifs, as well as two IQ motifs. However, the tail sequence is unusual, since it is predicted to form two long coiled-coils separated by a stretch of sequence containing 19 prolines. Disruption of myp2(+) is not lethal but under nutrient limiting conditions cells lacking myp2(+) function are multiseptated, elongated, and branched, indicative of a defect in cytokinesis. The presence of salt enhances these morphological defects. Additionally, Deltamyp2 cells are cold sensitive in high salt, failing to form colonies at 17 degrees C. Thus, myp2(+) is required under conditions of stress, possibly linking extracellular growth conditions to efficient cytokinesis and cell growth. GFP-Myp2p localizes to a ring in the middle of late mitotic cells, consistent with a role in cytokinesis. Additionally, we constructed double mutants of Deltamyp2 with temperature-sensitive mutant strains defective in cytokinesis. We observed synthetic lethal interactions between Deltamyp2 and three alleles of cdc11ts, as well as more modest synthetic interactions with cdc14ts and cdc16ts, implicating myp2(+) function for efficient cytokinesis under normal conditions.

PSTPIP: a tyrosine phosphorylated cleavage furrow-associated protein that is a substrate for a PEST tyrosine phosphatase

We have investigated proteins which interact with the PEST-type protein tyrosine phosphatase, PTP hematopoietic stem cell fraction (HSCF), using the yeast two-hybrid system. This resulted in the identification of proline, serine, threonine phosphatase interacting protein (PSTPIP), a novel member of the actin- associated protein family that is homologous to Schizosaccharomyces pombe CDC15p, a phosphorylated protein involved with the assembly of the actin ring in the cytokinetic cleavage furrow. The binding of PTP HSCF to PSTPIP was induced by a novel interaction between the putative coiled-coil region of PSTPIP and the COOH-terminal, proline-rich region of the phosphatase. PSTPIP is tyrosine phosphorylated both endogenously and in v-Src transfected COS cells, and cotransfection of dominant-negative PTP HSCF results in hyperphosphorylation of PSTPIP. This dominant-negative effect is dependent upon the inclusion of the COOH-terminal, proline-rich PSTPIP-binding region of the phosphatase. Confocal microscopy analysis of endogenous PSTPIP revealed colocalization with the cortical actin cytoskeleton, lamellipodia, and actin-rich cytokinetic cleavage furrow. Overexpression of PSTPIP in 3T3 cells resulted in the formation of extended filopodia, consistent with a role for this protein in actin reorganization. Finally, overexpression of mammalian PSTPIP in exponentially growing S. pombe results in a dominant-negative inhibition of cytokinesis. PSTPIP is therefore a novel actin-associated protein, potentially involved with cytokinesis, whose tyrosine phosphorylation is regulated by PTP HSCF.

Mis6, a fission yeast inner centromere protein, acts during G1/S and forms specialized chromatin required for equal segregation

Disorder in sister chromatid separation can lead to genome instability and cancer. A temperature-sensitive S. pombe mis6-302 frequently loses a minichromosome at 26 degrees C and abolishes equal segregation of regular chromosomes at 36 degrees C. The mis6+ gene is essential for viability, and its deletion results in missegregation identical to mis6-302. Mis6 acts before or at the onset of S phase, and mitotic missegregation defects are produced only after the passage of G1/S at 36 degrees C. Mis6 locates at the centromeres throughout the cell cycle. In the mutant, positioning of the centromeres becomes abnormal, and specialized chromatin in the inner centromeres, which give the smear micrococcal nuclease pattern in wild type, is disrupted. The ability to establish correct biorientation of sister centromeres in metaphase cells requires the Mis6-containing chromatin and originates during the passage of G1/S.

The Spg1p GTPase is an essential, dosage-dependent inducer of septum formation in Schizosaccharomyces pombe

The spg1 gene (septum-promoting GTPase) was cloned as a multicopy suppressor of a dominant-negative mutant of the Cdc7p kinase. It encodes a small GTPase of the Ras superfamily. spg1 is an essential gene. Null or heat-sensitive alleles do not make a division septum, but growth, S-phase, and mitosis continue in the absence of cell division, producing elongated, multinucleate cells. Increased expression of Spg1p induces septum formation in G2, S-phase, and pre-Start G1-arrested cells. This requires the activity of Cdc7p kinase, but not p34(cdc2). Increased expression of Cdc7p bypasses the requirement for Spg1p. Spg1p and Cdc7p can be coimmunoprecipitated from cell extracts, and interact in the two-hybrid system. These data indicate that Spg1p is a key element in controlling the onset of septum formation in Schizosaccharomyces pombe, and that it acts through the Cdc7p kinase.

The fission yeast dma1 gene is a component of the spindle assembly checkpoint, required to prevent septum formation and premature exit from mitosis if spindle function is compromised

Premature initiation of cytokinesis can lead to loss of chromosomes, and 'cutting' of the nucleus. Therefore, the proper spatial and temporal co-ordination of mitosis and cytokinesis is essential for maintaining the integrity of the genome. The fission yeast cdc16 gene is implicated both in the spindle assembly checkpoint and control of septum formation. To identify other proteins involved in these controls, we have isolated multicopy suppressors of the cdc16-116 mutation, and the characterization of one of these, dma1 (defective in mitotic arrest), is presented here. dma1 is not an essential gene, but in a dma1 null background (dma1-D1) the function of the spindle assembly checkpoint is compromised. If assembly of the spindle is prevented, dma1-D1 cells do not arrest, the activity of cdc2 kinase decays and cells form a division septum without completing a normal mitosis. dma1-D1 cells also show an increased rate of chromosome loss during exponential growth. Upon ectopic expression from an inducible promoter, dma1p delays progress through mitosis and inhibits septum formation, giving rise to elongated, multinucleate cells. We propose that dma1 is a component of the spindle assembly checkpoint, required to prevent septum formation and premature exit from mitosis if spindle function is impaired.

Protein phosphatase 2A regulates MPF activity and sister chromatid cohesion in budding yeast

BACKGROUND

Mitosis is regulated by MPF (maturation promoting factor), the active form of Cdc2/28-cyclin B complexes. Increasing levels of cyclin B abundance and the loss of inhibitory phosphates from Cdc2/28 drives cells into mitosis, whereas cyclin B destruction inactivates MPF and drives cells out of mitosis. Cells with defective spindles are arrested in mitosis by the spindle-assembly checkpoint, which prevents the destruction of mitotic cyclins and the inactivation of MPF. We have investigated the relationship between the spindle-assembly checkpoint, cyclin destruction, inhibitory phosphorylation of Cdc2/28, and exit from mitosis.

RESULTS

The previously characterized budding yeast mad mutants lack the spindle-assembly checkpoint. Spindle depolymerization does not arrest them in mitosis because they cannot stabilize cyclin B. In contrast, a newly isolated mutant in the budding yeast CDC55 gene, which encodes a protein phosphatase 2A (PP2A) regulatory subunit, shows a different checkpoint defect. In the presence of a defective spindle, these cells separate their sister chromatids and leave mitosis without inducing cyclin B destruction. Despite the persistence of B-type cyclins, cdc55 mutant cells inactivate MPF. Two experiments show that this inactivation is due to inhibitory phosphorylation on Cdc28: phosphotyrosine accumulates on Cdc28 in cdc55 delta cells whose spindles have been depolymerized, and a cdc28 mutant that lacks inhibitory phosphorylation sites on Cdc28 allows spindle defects to arrest cdc55 mutants in mitosis with active MPF and unseparated sister chromatids.

CONCLUSIONS

We conclude that perturbations of protein phosphatase activity allow MPF to be inactivated by inhibitory phosphorylation instead of by cyclin destruction. Under these conditions, sister chromatid separation appears to be regulated by MPF activity rather than by protein degradation. We discuss the role of PP2A and Cdc28 phosphorylation in cell-cycle control, and the possibility that the novel mitotic exit pathway plays a role in adaptation to prolonged activation of the spindle-assembly checkpoint.

The spindle assembly checkpoint

The spindle assembly checkpoint monitors proper chromosome attachment to spindle microtubules and is conserved from yeast to humans. Checkpoint components reside on kinetochores of chromosomes and show changes in phosphorylation and localization as cells proceed through mitosis. Adaptation to prolonged checkpoint arrest can occur by inhibitory phosphorylation of Cdc2.

The dmf1/mid1 gene is essential for correct positioning of the division septum in fission yeast

Little is known about the mechanisms that establish the position of the division plane in eukaryotic cells. Wild-type fission yeast cells divide by forming a septum in the middle of the cell at the end of mitosis. Dmf1 mutants complete mitosis and initiate septum formation, but the septa that form are positioned at random locations and angles in the cell, rather than in the middle. We have cloned the dmf1 gene as a suppressor of the cdc7-24 mutant. The dmf1 mutant is allelic with mid1. The gene encodes a novel protein containing a putative nuclear localization signal, and a carboxy-terminal PH domain. In wild-type cells, Dmf1p is nuclear during interphase, and relocates to form a medial ring at the cell cortex coincident with the onset of mitosis. This relocalization occurs before formation of the actin ring and is associated with increased phosphorylation of Dmf1p. The Dmf1p ring can be formed in the absence of an actin ring, but depends on some of the genes required for actin ring formation. When the septum is completed and the cells separate, Dmf1p staining is once again nuclear. These data implicate Dmf1p as an important element in assuring correct placement of the division septum in Schizosaccharomyces pombe cells.