MAPK-dependent control of mitotic progression in S. pombe

BACKGROUND

Mitogen-activated protein kinases (MAPKs) preserve cell homeostasis by transducing physicochemical fluctuations of the environment into multiple adaptive responses. These responses involve transcriptional rewiring and the regulation of cell cycle transitions, among others. However, how stress conditions impinge mitotic progression is largely unknown. The mitotic checkpoint is a surveillance mechanism that inhibits mitotic exit in situations of defective chromosome capture, thus preventing the generation of aneuploidies. In this study, we investigate the role of MAPK Pmk1 in the regulation of mitotic exit upon stress.

RESULTS

We show that Schizosaccharomyces pombe cells lacking Pmk1, the MAP kinase effector of the cell integrity pathway (CIP), are hypersensitive to microtubule damage and defective in maintaining a metaphase arrest. Epistasis analysis suggests that Pmk1 is involved in maintaining spindle assembly checkpoint (SAC) signaling, and its deletion is additive to the lack of core SAC components such as Mad2 and Mad3. Strikingly, pmk1Δ cells show up to twofold increased levels of the anaphase-promoting complex (APC/C) activator Cdc20Slp1 during unperturbed growth. We demonstrate that Pmk1 physically interacts with Cdc20Slp1 N-terminus through a canonical MAPK docking site. Most important, the Cdc20Slp1 pool is rapidly degraded in stressed cells undergoing mitosis through a mechanism that requires MAPK activity, Mad3, and the proteasome, thus resulting in a delayed mitotic exit.

CONCLUSIONS

Our data reveal a novel function of MAPK in preventing mitotic exit and activation of cytokinesis in response to stress. The regulation of Cdc20Slp1 turnover by MAPK Pmk1 provides a key mechanism by which the timing of mitotic exit can be adjusted relative to environmental conditions.

Effects of the microtubule nucleator Mto1 on chromosomal movement, DNA repair, and sister chromatid cohesion in fission yeast

Although the function of microtubules (MTs) in chromosomal segregation during mitosis is well characterized, much less is known about the role of MTs in chromosomal functions during interphase. In the fission yeast Schizosaccharomyces pombe, dynamic cytoplasmic MT bundles move chromosomes in an oscillatory manner during interphase via linkages through the nuclear envelope (NE) at the spindle pole body (SPB) and other sites. Mto1 is a cytoplasmic factor that mediates the nucleation and attachment of cytoplasmic MTs to the nucleus. Here, we test the function of these cytoplasmic MTs and Mto1 on DNA repair and recombination during interphase. We find that mto1Δ cells exhibit defects in DNA repair and homologous recombination (HR) and abnormal DNA repair factory dynamics. In these cells, sister chromatids are not properly paired, and binding of Rad21 cohesin subunit along chromosomal arms is reduced. Our findings suggest a model in which cytoplasmic MTs and Mto1 facilitate efficient DNA repair and HR by promoting dynamic chromosomal organization and cohesion in the nucleus.

Microtubule polymerase and processive plus-end tracking functions originate from distinct features within TOG domain arrays

XMAP215/Stu2/Alp14 accelerates tubulin polymerization while processively tracking microtubule (MT) plus ends via tumor overexpressed gene (TOG) domain arrays. It remains poorly understood how these functions arise from tubulin recruitment, mediated by the distinct TOG1 and TOG2 domains, or the assembly of these arrays into large square complexes. Here, we describe a relationship between MT plus-end tracking and polymerase functions revealing their distinct origin within TOG arrays. We study Alp14 mutants designed based on structural models, with defects in either tubulin recruitment or self-organization. Using in vivo live imaging in fission yeast and in vitro MT dynamics assays, we show that tubulins recruited by TOG1 and TOG2 serve concerted, yet distinct, roles in MT plus-end tracking and polymerase functions. TOG1 is critical for processive plus-end tracking, whereas TOG2 is critical for accelerating tubulin polymerization. Inactivating interfaces that stabilize square complexes lead to defects in both processive MT plus-end tracking and polymerase. Our studies suggest that a dynamic cycle between square and unfurled TOG array states gives rise to processive polymerase activity at MT plus ends.

Reprogramming Cdr2-Dependent Geometry-Based Cell Size Control in Fission Yeast

How cell size is determined and maintained remains unclear, even in simple model organisms. In proliferating cells, cell size is regulated by coordinating growth and division through sizer, adder, or timer mechanisms or through some combination [1, 2]. Currently, the best-characterized example of sizer behavior is in fission yeast, Schizosaccharomyces pombe, which enters mitosis at a minimal cell size threshold. The peripheral membrane kinase Cdr2 localizes in clusters (nodes) on the medial plasma membrane and promotes mitotic entry [3]. Here, we show that the Cdr2 nodal density, which scales with cell size, is used by the cell to sense and control its size. By analyzing cells of different widths, we first show that cdr2+ cells divide at a fixed cell surface area. However, division in the cdr2Δ mutant is more closely specified by cell volume, suggesting that Cdr2 is essential for area sensing and supporting the existence of a Cdr2-independent secondary sizer mechanism more closely based on volume. To investigate how Cdr2 nodes may sense area, we derive a minimal mathematical model that incorporates the cytoplasmic kinase Ssp1 as a Cdr2 activator. The model predicts that a cdr2 mutant in an Ssp1 phosphorylation site (cdr2-T166A) [4] should form nodes whose density registers cell length. We confirm this prediction experimentally and find that thin cells now follow this new scaling by dividing at constant length instead of area. This work supports the role of Cdr2 as a sizer factor and highlights the importance of studying geometrical aspects of size control.

Proteome-wide search for PP2A substrates in fission yeast

PP2A (protein phosphatase 2A) is a major phosphatase in eukaryotic cells that plays an essential role in many processes. PP2A mutations in Schizosaccharomyces pombe result in defects of cell cycle control, cytokinesis and morphogenesis. Which PP2A substrates are responsible for these changes is not known. In this work, we searched for PP2A substrates in S. pombe using two approaches, 2D-DIGE analysis of PP2A complex mutants and identification of PP2A interacting proteins. In both cases, we used MS to identify proteins of interest. In the DIGE experiment, we compared proteomes of wild-type S. pombe, deletion of pta2, the phosphoactivator of the PP2A catalytic subunit, and pab1-4, a mutant of B-type PP2A regulatory subunit. A total of 1742 protein spots were reproducibly resolved by 2D-DIGE and 51 spots demonstrated significant changes between PP2A mutants and the wild-type control. MS analysis of these spots identified 27 proteins that include key regulators of glycerol synthesis, carbon metabolism, amino acid biosyntesis, vitamin production, and protein folding. Importantly, we independently identified a subset of these proteins as PP2A binding partners by affinity precipitation, suggesting they may be direct targets of PP2A. We have validated our approach by demonstrating that phosphorylation of Gpd1, a key enzyme in glycerol biogenesis, is regulated by PP2A and that ability of cells to respond to osmotic stress by synthesizing glycerol is compromised in the PP2A mutants. Our work contributes to a better understanding of PP2A function and identifies potential PP2A substrates.

Cortical regulation of cell size by a sizer cdr2p

Cells can, in principle, control their size by growing to a specified size before commencing cell division. How any cell actually senses its own size remains poorly understood. The fission yeast Schizosaccharomyces pombe are rod-shaped cells that grow to ∼14 µm in length before entering mitosis. In this study, we provide evidence that these cells sense their surface area as part of this size control mechanism. We show that cells enter mitosis at a certain surface area, as opposed to a certain volume or length. A peripheral membrane protein kinase cdr2p has properties of a dose-dependent ‘sizer’ that controls mitotic entry. As cells grow, the local cdr2p concentration in nodes at the medial cortex accumulates as a measure of cell surface area. Our findings, which challenge a previously proposed pom1p gradient model, lead to a new model in which cells sense their size by using cdr2p to probe the surface area over the whole cell and relay this information to the medial cortex.

DOI:http://dx.doi.org/10.7554/eLife.02040.001

Although different types of cells come in a variety of shapes and sizes, most cells are able to maintain a fairly consistent size and shape as they grow and divide. For example, the rod-shaped cells of the fission yeast S. pombe grow to be 14 microns long before dividing in the middle to form two new cells. This prevents any single cell becoming too large or small.

A similar phenomenon has been observed in other types of cells, so it is clear that cells must be able to measure their own size, and then use that information to trigger cell division. A number of proteins that regulate cell size and cell division in fission yeast have now been identified. These proteins form a pathway in which a protein called pom1p inhibits another protein, cdr2p, which in turn causes a third protein, cdk1p, to start the process of cell division. However, the details of the measurement process and the property that the cells are actually measuring—surface area, volume, mass or something else—remain mysterious.

Pan et al. have now used imaging techniques and mathematical modeling to probe the distribution and movements of proteins in fission yeast cells. Their results do not support a previous model in which the cell uses the gradient of pom1p as a ruler to measure cell length. Rather, Pan et al. propose a new model in which the level of cdr2p is used to sense the size of the cell. Individual molecules of cdr2p come together to from clusters called nodes on the cell membrane. As the cell grows larger, more and more cdr2p proteins accumulate in these nodes, which are found in a band around the middle of the cell. When the cells reaches a critical cell size, the increased concentration of cdr2p at these nodes may help to trigger the start of cell division.

By examining cells that grow at different rates, Pan et al. show that the rate of accumulation of cdr2p in the nodes depends on how big the cells are, rather than on the length of time that has elapsed. Analysis of fission yeast cells of different shapes shows that cell division starts when the surface area of the cell grows to a certain value, as opposed to starting when the volume or length reach a given value.

Pan et al. also show that cdr2p binds to all parts of the cell membrane, not just to the nodes near the middle, and go on to provide a simple mathematical model showing how this property can allow cells to measure their surface area. However, as Pan et al. point out, this is probably just one component of a larger mechanism that tells cells when they need to divide.

DOI:http://dx.doi.org/10.7554/eLife.02040.002

Cell migration and division in amoeboid-like fission yeast

Yeast cells are non-motile and are encased in a cell wall that supports high internal turgor pressure. The cell wall is also essential for cellular morphogenesis and cell division. Here, we report unexpected morphogenetic changes in a Schizosaccharomyces pombe mutant defective in cell wall biogenesis. These cells form dynamic cytoplasmic protrusions caused by internal turgor pressure and also exhibit amoeboid-like cell migration resulting from repeated protrusive cycles. The cytokinetic ring responsible for cell division in wild-type yeast often fails in these cells; however, they were still able to divide using a ring-independent alternative mechanism relying on extrusion of the cell body through a hole in the cell wall. This mechanism of cell division may resemble an ancestral mode of division in the absence of cytokinetic machinery. Our findings highlight how a single gene change can lead to the emergence of different modes of cell growth, migration and division.

A Lallzyme MMX-based rapid method for fission yeast protoplast preparation

Fungal cells including yeasts are surrounded by cell wall that counteracts turgor pressure and prevents cell lysis. Many yeast experiments, including genetic manipulation of sterile strains, morphogenesis studies, nucleic acid isolation and many others, require mechanical breakage or enzymatic removal of the cell wall. Some of these experiments require the generation of live cells lacking cell walls, called protoplasts, that can be maintained in osmostabilized medium. Enzymatic digestion of cell wall proteoglycans is a commonly used method of protoplast preparation. Currently existing protocols for fission yeast cell wall digestion are time consuming and not very efficient. We developed a new rapid method for fission yeast protoplast preparation that relies on digesting cell walls with Lallzyme MMX commercial enzyme mix, which produces protoplasts from all cells in less than 10 min. We demonstrate that these protoplasts can be utilized in three commonly used fission yeast protocols. Thus, we provide the fission yeast community with a robust and efficient plasmid extraction method, a new protocol for diploid generation and an assay for protoplast recovery that should be useful for studies of morphogenesis. Our method is potentially applicable to other yeasts and fungi.

Fission yeast Alp14 is a dose-dependent plus end-tracking microtubule polymerase

Alp14, a XMAP215 orthologue in fission yeast, is a microtubule (MT) polymerase. It tracks growing MT plus ends and regulates the polymerization state of tubulin by cycling between a tubulin dimer–bound cytoplasmic state and a MT polymerase state that promotes rapid MT assembly.

XMAP215/Dis1 proteins are conserved tubulin-binding TOG-domain proteins that regulate microtubule (MT) plus-end dynamics. Here we show that Alp14, a XMAP215 orthologue in fission yeast, Schizosaccharomyces pombe, has properties of a MT polymerase. In vivo, Alp14 localizes to growing MT plus ends in a manner independent of Mal3 (EB1). alp14-null mutants display short interphase MTs with twofold slower assembly rate and frequent pauses. Alp14 is a homodimer that binds a single tubulin dimer. In vitro, purified Alp14 molecules track growing MT plus ends and accelerate MT assembly threefold. TOG-domain mutants demonstrate that tubulin binding is critical for function and plus end localization. Overexpression of Alp14 or only its TOG domains causes complete MT loss in vivo, and high Alp14 concentration inhibits MT assembly in vitro. These inhibitory effects may arise from Alp14 sequestration of tubulin and effects on the MT. Our studies suggest that Alp14 regulates the polymerization state of tubulin by cycling between a tubulin dimer–bound cytoplasmic state and a MT polymerase state that promotes rapid MT assembly.

Polar growth in the infectious hyphae of the phytopathogen ustilago maydis depends on a virulence-specific cyclin

The maize smut fungus Ustilago maydis switches from yeast to hyphal growth to infect maize (Zea mays) plants. This switching is promoted by mating of compatible cells and seems to be required for plant penetration. Although many genes distinctively expressed during this dimorphic switch have been identified and shown to be essential for the infection process, none seems to be explicitly required for polar growth control. Here, we report the characterization of pcl12, encoding a cyclin that interacts specifically with Cdk5, an essential cyclin-dependent kinase with regulatory roles in morphogenesis in U. maydis. Pcl12 fulfills the requirements to be a virulence-specific regulator of polar growth in U. maydis. First, pcl12 expression is induced during the pathogenic development. Secondly, Pcl12 is sufficient to induce hyperpolarized growth in U. maydis cells, as haploid cells overexpressing pcl12 in axenic conditions produce filaments that were morphologically indistinguishable from those produced during the infection process. Finally, cells defective in pcl12 showed impaired polar growth during the formation of the b-dependent filament, the induction of the conjugation tubes, or the formation of a promycelium in spore germination. However, in spite of this pivotal role during morphogenesis, pcl12 mutants were virulent. We discuss the implications of these results for the role of polar growth during the infection process.

Biz1, a zinc finger protein required for plant invasion by Ustilago maydis, regulates the levels of a mitotic cyclin

Plant invasion by pathogenic fungi involves regulated growth and highly organized fungal morphological changes. For instance, when the smut fungus Ustilago maydis infects maize (Zea mays), its dikaryotic infective filament is cell cycle arrested, and appressoria are differentiated prior to plant penetration. Once the filament enters the plant, the cell cycle block is released and fungal cells begin proliferation, suggesting a tight interaction between plant invasion and the cell cycle and morphogenesis control systems. We describe a novel factor, Biz1 (b-dependent zinc finger protein), which has two Cys(2)His(2) zinc finger domains and nuclear localization, suggesting a transcriptional regulatory function. The deletion of biz1 shows no detectable phenotypic alterations during axenic growth. However, mutant cells show a severe reduction in appressoria formation and plant penetration, and those hyphae that invade the plant arrest their pathogenic development directly after plant penetration. biz1 is induced via the b-mating-type locus, the key control instance for pathogenic development. The gene is expressed at high levels throughout pathogenic development, which induces a G2 cell cycle arrest that is a direct consequence of the downregulation of the mitotic cyclin Clb1. Our data support a model in which Biz1 is involved in cell cycle arrest preceding plant penetration as well as in the induction of appressoria.

Pathocycles: Ustilago maydis as a model to study the relationships between cell cycle and virulence in pathogenic fungi

Activation of virulence in pathogenic fungi often involves differentiation processes that need the reset of the cell cycle and induction of a new morphogenetic program. Therefore, the fungal capability to modify its cell cycle constitutes an important determinant in carrying out a successful infection. The dimorphic fungus Ustilago maydis is the causative agent of corn smut disease and has lately become a highly attractive model in addressing fundamental questions about development in pathogenic fungi. The different morphological and genetic changes of U. maydis cells during the pathogenic process advocate an accurate control of the cell cycle in these transitions. This is why this model pathogen deserves attention as a powerful tool in analyzing the relationships between cell cycle, morphogenesis, and pathogenicity. The aim of this review is to summarize recent advances in the unveiling of cell cycle regulation in U. maydis. We also discuss the connection between cell cycle and virulence and how cell cycle control is an important downstream target in the fungus-plant interaction.