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Dannel McCollum

Publications and source records attributed to Dannel McCollum.

18 recordsLinked to original sources
PubMed2006

S. pombe FEAR protein orthologs are not required for release of Clp1/Flp1 phosphatase from the nucleolus during mitosis.

Cdc14 family phosphatases are highly conserved regulators of cell-cycle progression. Two of the best studied members of this family are budding yeast Cdc14p and its fission yeast homolog Clp1p/Flp1p. The function of both Saccharomyces cerevisiae Cdc14p and Schizosaccharomyces pombe Clp1p/Flp1p are controlled in part by their regulated sequestration and release from the nucleolus. In the budding yeast S. cerevisiae a set of proteins collectively termed the FEAR network promote nucleolar and telomeric DNA segregation by triggering the release of the conserved Cdc14 phosphatase from the nucleolus. Here we show that FEAR homologs in S. pombe do not promote release of the Cdc14 homolog Clp1p/Flp1p from the nucleolus, and that Clp1p/Flp1p is not required for nucleolar and telomeric DNA segregation suggesting that this aspect of Cdc14 regulation and function may not be universally conserved.

Active Transport, Cell Nucleus↗
PubMed2006

A role for the septation initiation network in septum assembly revealed by genetic analysis of sid2-250 suppressors.

In the fission yeast Schizosaccharomyces pombe the septation initiation network (SIN) is required for stabilization of the actomyosin ring in late mitosis as well as for ring constriction and septum deposition. In a genetic screen for suppressors of the SIN mutant sid2-250, we isolated a mutation, ace2-35, in the transcription factor Ace2p. Both ace2Delta and ace2-35 show defects in cell separation, and both can rescue the growth defects of some SIN mutants at low restrictive temperatures, where the SIN single mutants lyse at the time of cytokinesis. By detailed analysis of the formation and constriction of the actomyosin ring and septum in the sid2-250 mutant at low restrictive temperatures, we show that the lysis phenotype of the sid2-250 mutant is likely due to a weak cell wall and septum combined with enzymatic activity of septum-degrading enzymes. Consistent with the recent findings that Ace2p controls transcription of genes involved in cell separation, we show that disruption of some of these genes can also rescue sid2-250 mutants. Consistent with SIN mutants having defects in septum formation, many SIN mutants can be rescued at the low restrictive temperature by the osmotic stabilizer sorbitol. The small GTPase Rho1 is known to promote cell wall formation, and we find that Rho1p expressed from a multi-copy plasmid can also rescue sid2-250 at the low restrictive temperature. Together these results suggest that the SIN has a role in promoting proper cell wall formation at the division septa.

Actomyosin↗
PubMed2005

Cytokinesis: breaking the ties that bind.

It has been unclear how cells complete cell division and resolve membrane connections to bring about cell separation. Recent work has shown that targeted secretion to the midbody is required to complete cell division.

Animals↗
PubMed2005

Distinct nuclear and cytoplasmic functions of the S. pombe Cdc14-like phosphatase Clp1p/Flp1p and a role for nuclear shuttling in its regulation.

Cdc14-like phosphatases regulate a variety of cell cycle events by dephosphorylating CDK sites. Their cell cycle-dependent changes in localization may be important to carry out distinct functions. Work in budding and fission yeast suggested that Cdc14-like phosphatases are inhibited by nucleolar sequestration. In S. cerevisiae, Cdc14p is released from the nucleolus by the FEAR network and Cdk1, whereas the S. pombe CDC14-like phosphatase Clp1p (also known as Flp1p) is released at mitotic entry by an unknown mechanism. The mitotic exit network (MEN) in S. cerevisiae and its homologous network, the septation initiation network (SIN), in S. pombe act through an unknown mechanism to keep the phosphatase out of the nucleolus in late mitosis. SIN-dependent cytoplasmic maintenance of Clp1p is thought to be essential for the cytokinesis checkpoint, which blocks further rounds of nuclear division until cytokinesis is completed. By targeting Clp1p to the nucleus or the cytoplasm, we demonstrate distinct functions for these pools of Clp1p in chromosome segregation and cytokinesis, respectively. Our results further suggest that the SIN does not keep Clp1p out of the nucleolus by regulating nucleolar affinity, as proposed for S. cerevisiae Cdc14p, but instead, Clp1p may be regulated by nuclear import/export.

Active Transport, Cell Nucleus↗
PubMed2005

Centrosomes in cellular regulation.

Centrosomes, spindle pole bodies, and related structures in other organisms are a morphologically diverse group of organelles that share a common ability to nucleate and organize microtubules and are thus referred to as microtubule organizing centers or MTOCs. Features associated with MTOCs include organization of mitotic spindles, formation of primary cilia, progression through cytokinesis, and self-duplication once per cell cycle. Centrosomes bind more than 100 regulatory proteins, whose identities suggest roles in a multitude of cellular functions. In fact, recent work has shown that MTOCs are required for several regulatory functions including cell cycle transitions, cellular responses to stress, and organization of signal transduction pathways. These new liaisons between MTOCs and cellular regulation are the focus of this review. Elucidation of these and other previously unappreciated centrosome functions promises to yield exciting scientific discovery for some time to come.

Animals↗
PubMed2004

Cytokinesis: the central spindle takes center stage.

The central spindle plays a key role in cytokinesis. Recent studies have shed new light on how assembly of the central spindle is regulated, and also support a role for both the central spindle and astral microtubules in cytokinesis in animal cells.

Animals↗
PubMed2004

The Clp1p/Flp1p phosphatase ensures completion of cytokinesis in response to minor perturbation of the cell division machinery in Schizosaccharomyces pombe.

Fission yeast mutants defective in actomyosin ring formation and function exhibit a prolonged G2 delay following cytokinesis failure. This G2 delay depends on the SIN, a signaling network essential for cytokinesis, and the non-essential Cdc14p family phosphatase, Clp1p/Flp1p and has been proposed to signify a cytokinesis checkpoint mechanism. However, the physiological relevance of this proposed Clp1p/Flp1p-dependent checkpoint is unclear because all previous studies were carried out using mutations in essential actomyosin ring components under fully restrictive conditions and thus these cells would have died regardless of the presence of the checkpoint. Here we show that delays in cytokinesis caused by minor perturbations to different components of the cytokinetic machinery, which normally cause only mild defects, become lethal when Clp1p/Flp1p is inactivated. In addition, we show that Clp1p/Flp1p does not function simply to inhibit further rounds of nuclear division, but also allows damaged actomyosin rings to be maintained to facilitate completion of cell division. Ectopic activation of the SIN significantly bypasses the requirement of Clp1p/Flp1p for G2 delay as well as for completion of cytokinesis. We conclude that the Clp1p/Flp1p-dependent cytokinesis checkpoint provides a previously unrecognized cell survival advantage when the cell division apparatus is mildly perturbed.

Actomyosin↗
PubMed2004

A novel phosphatidylinositol(3,4,5)P3 pathway in fission yeast.

The mammalian tumor suppressor, phosphatase and tensin homologue deleted on chromosome 10 (PTEN), inhibits cell growth and survival by dephosphorylating phosphatidylinositol-(3,4,5)-trisphosphate (PI[3,4,5]P3). We have found a homologue of PTEN in the fission yeast, Schizosaccharomyces pombe (ptn1). This was an unexpected finding because yeast (S. pombe and Saccharomyces cerevisiae) lack the class I phosphoinositide 3-kinases that generate PI(3,4,5)P3 in higher eukaryotes. Indeed, PI(3,4,5)P3 has not been detected in yeast. Surprisingly, upon deletion of ptn1 in S. pombe, PI(3,4,5)P3 became detectable at levels comparable to those in mammalian cells, indicating that a pathway exists for synthesis of this lipid and that the S. pombe ptn1, like mammalian PTEN, suppresses PI(3,4,5)P3 levels. By examining various mutants, we show that synthesis of PI(3,4,5)P3 in S. pombe requires the class III phosphoinositide 3-kinase, vps34p, and the phosphatidylinositol-4-phosphate 5-kinase, its3p, but does not require the phosphatidylinositol-3-phosphate 5-kinase, fab1p. These studies suggest that a pathway for PI(3,4,5)P3 synthesis downstream of a class III phosphoinositide 3-kinase evolved before the appearance of class I phosphoinositide 3-kinases.

Evolution, Molecular↗
PubMed2004

The S. pombe Cdc14-like phosphatase Clp1p regulates chromosome biorientation and interacts with Aurora kinase.

The S. pombe Cdc14-related phosphatase Clp1p/Flp1p regulates G2/M transition by antagonizing CDK activity and is essential for coordinating the nuclear division cycle with cytokinesis through the cytokinesis checkpoint. At the G2/M transition, Clp1p/Flp1p is released from the nucleolus and SPB and distributes throughout the nucleus to the spindle and the contractile ring. This early relocalization is analogous to vertebrate Cdc14 homologs and stands in contrast to S. cerevisiae Cdc14p, which is not released from the nucleolus until metaphase/anaphase transition. Here, we report that Clp1p/Flp1p localizes to kinetochores in prometaphase and functions in chromosome segregation, since deletion of clp1/flp1 causes cosegregation of sister chromatids, when sister kinetochores are prone to mono-orientation. Genetic, cytological, and biochemical experiments suggest that Clp1p/Flp1p functions together with Aurora kinase at kinetochores. Together, these results suggest that Clp1p/Flp1p has a role in repairing mono-orientation of sister kinetochores.

Aurora Kinases↗
PubMed2004

Initiation of cytokinesis is controlled through multiple modes of regulation of the Sid2p-Mob1p kinase complex.

The Sid2p-Mob1p kinase complex is an important component of the septation initiation network (SIN) in the fission yeast Schizosaccharomyces pombe. However, regulation of this complex is still elusive. Here we show that Mob1p is required not only for the subcellular localization of Sid2p but also for its kinase activity. We identified a region at the amino terminus of Sid2p that is required for Mob1p binding and spindle pole body (SPB) localization. Deletion of this region abolishes Mob1p binding and diminishes SPB localization, whereas this region alone is sufficient to associate with Mob1p and SPBs. We further show that a similar region of the N terminus of the Sid2p-related protein kinase Orb6p binds to the Mob1p-related protein Mob2p, suggesting that this may be a conserved mode of interaction for this family of kinases. Phosphorylation of Ser402 and especially Thr578 is important for Sid2p function. Sid2p with a mutation of Thr578 to Ala (T578A) can no longer rescue sid2-250 mutant cells, and this results in reduction of Mob1p binding. Sid2p mutants mimicking phosphorylation at this site (T578D and T578E) can rescue sid2-250 cells, enhance Sid2p kinase activity, and partially rescue growth defects of upstream sin mutants. Interestingly, Sid2p, but not Mob1p, is self-associated. Our experiments suggest that self-associated Sid2p is inactive. This self-association is mediated by a region that overlaps with Mob1p and SPB binding sites. Overexpression of Mob1p is able to disrupt the self-association of Sid2p. Taken together, our results suggest that Sid2p kinase may utilize multiple modes of regulation including self-association, Mob1p binding, and phosphorylation to achieve its full activity at an appropriate time and place in the cell.

Amino Acid Sequence↗
PubMed2003

Mob2p interacts with the protein kinase Orb6p to promote coordination of cell polarity with cell cycle progression.

The molecular mechanisms that temporally and spatially coordinate cell morphogenesis with the cell cycle remain poorly understood. Here we describe the characterization of fission yeast Mob2p, a novel protein required for regulating cell polarity and cell cycle control. Deletion of mob2 is lethal and causes cells to become spherical, with depolarized actin and microtubule cytoskeletons. A decrease in Mob2p protein level results in a defect in the activation of bipolar growth. This phenotype is identical to that of mutants defective in the orb6 protein kinase gene, and we find that Mob2p physically interacts with Orb6p. In addition, overexpression of Mob2p, like that of Orb6p, results in a delay in the onset of mitosis. Mob2p localizes to the cell periphery and cytoplasm throughout the cell cycle and to the division site during late anaphase and telophase. Mob2p is unable to localize to the cell middle in mutants defective in actomyosin ring and septum formation. Our results suggest that Mob2p, along with Orb6p, is required for coordinating polarized cell growth during interphase with the onset of mitosis.

Actins↗
PubMed2003

Scw1p antagonizes the septation initiation network to regulate septum formation and cell separation in the fission yeast Schizosaccharomyces pombe.

Cytokinesis in the fission yeast Schizosaccharomyces pombe is regulated by a signaling pathway termed the septation initiation network (SIN). The SIN is essential for initiation of actomyosin ring constriction and septum formation. In a screen to search for mutations that can rescue the sid2-250 SIN mutant, we obtained scw1-18. Both the scw1-18 mutant and the scw1 deletion mutant (scw1Delta mutant), have defects in cell separation. Both the scw1-18 and scw1Delta mutations rescue the growth defects of not just the sid2-250 mutant but also the other temperature-sensitive SIN mutants. Other cytokinesis mutants, such as those defective for actomyosin ring formation, are not rescued by scw1Delta. scw1Delta does not seem to rescue the SIN by restoring SIN signaling defects. However, scw1Delta may function downstream of the SIN to promote septum formation, since scw1Delta can rescue the septum formation defects of the cps1-191beta-1,3-glucan synthase mutant, which is required for synthesis of the primary septum.

Amino Acid Sequence↗
PubMed2002

Cell cycle: new functions for Cdc14 family phosphatases.

The Cdc14 phosphatase was identified by its requirement for mitotic exit in budding yeast. Cdc14 homologs exist throughout the eukaryotic kingdom, but it was unclear whether their function would also be conserved. Recent analyses in fission yeast, humans and now C. elegans suggest numerous other functions for this family of proteins.

Cell Cycle↗
PubMed2002

Cytokinesis: myosin spots the ring.

Faithful actomyosin ring assembly is pivotal for successful cell division. The mechanisms by which the actomyosin ring is assembled at the correct time and place remain unclear. Recent studies in fission yeast have shown that a myosin II-containing spot may be a novel progenitor structure essential for actomyosin ring assembly.

Cell Division↗
PubMed2002

Dma1 prevents mitotic exit and cytokinesis by inhibiting the septation initiation network (SIN).

In the fission yeast Schizosaccharomyces pombe, the septation initiation network (SIN) triggers cytokinesis after mitosis. We investigated the relationship between Dma1p, a spindle checkpoint protein and cytokinesis inhibitor, and the SIN. Deletion of dma1 inactivates the spindle checkpoint and allows precocious SIN activation, while overexpressing Dma1p reduces SIN signaling. Dma1p seems to function by inhibiting the SIN activator, Plo1p kinase, since dma1 overexpression and deletion phenotypes suggest that Dma1p antagonizes Plo1p localization. Furthermore, failure to maintain high cyclin-dependent kinase (CDK) activity during spindle checkpoint activation in dma1 deletion cells requires Plo1p. Dma1p itself localizes to spindle pole bodies through interaction with Sid4p. Our observations suggest that Dma1p functions to prevent mitotic exit and cytokinesis during spindle checkpoint arrest by inhibiting SIN signaling.

Cell Cycle Proteins↗
PubMed2002

The multiprotein exocyst complex is essential for cell separation in Schizosaccharomyces pombe.

Schizosaccharomyces pombe cells divide by medial fission through the use of an actomyosin-based contractile ring. A mulitlayered division septum is assembled in concert with ring constriction. Finally, cleavage of the inner layer of the division septum results in the liberation of daughter cells. Although numerous studies have focused on actomyosin ring and division septum assembly, little information is available on the mechanism of cell separation. Here we describe a mutant, sec8-1, that is defective in cell separation but not in other aspects of cytokinesis. sec8-1 mutants accumulate about 100-nm vesicles and have reduced secretion of acid phosphatase, suggesting that they are defective in exocytosis. Sec8p is a component of the exocyst complex. Using biochemical methods, we show that Sec8p physically interacts with other members of the exocyst complex, including Sec6p, Sec10p, and Exo70p. These exocyst proteins localize to regions of active exocytosis-at the growing ends of interphase cells and in the medial region of cells undergoing cytokinesis-in an F-actin-dependent and exocytosis-independent manner. Analysis of a number of mutations in various exocyst components has established that these components are essential for cell viability. Interestingly, all exocyst mutants analyzed appear to be able to elongate and to assemble division septa but are defective for cell separation. We therefore propose that the fission yeast exocyst is involved in targeting of enzymes responsible for septum cleavage. We further propose that cell elongation and division septum assembly can continue with minimal levels of exocyst function.

Actomyosin↗
PubMed2002

Cytokinesis in eukaryotes.

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

Actomyosin↗