AMPK phosphorylation by Ssp1 is required for proper sexual differentiation in fission yeast

Calcium signaling positively regulates cellulase translation and secretion in a Clr-2-overexpressing, catabolically derepressed strain of Penicillium funiculosum

BACKGROUND

Low-cost cellulase production is vital to sustainable second-generation biorefineries. The catabolically derepressed strain of Penicillium funiculosum NCIM1228 (PfMig188 or ∆Mig1) secretes a superior set of cellulolytic enzymes, that are most suitable for 2G biorefineries. At a 3% (w/w) load, the ∆Mig1 secretome can release > 80% of fermentable sugars from lignocellulose at a 15% (w/v) biomass load, irrespective of the type of biomass and pretreatment. The robustness of the secretome can be further increased by improving the cellulase production capacity of the fungal strain.

RESULTS

We began by identifying the transcription factor responsible for cellulase production in NCIM1228. An advanced RNA-seq screen identified three genes, clr-2, ctf1a and ctf1b; the genes were cloned under their native promoters and transformed into NCIM1228. Of the three, clr-2 overexpression led to twofold higher cellulase production than the parent strain and was thus identified as the transcriptional activator of cellulase in NCIM1228. Next, we overexpressed clr-2 in ∆Mig1 and expected an exponential increase in cellulolytic attributes accredited to the reinforced activation mechanisms, conjoint with diminished negative regulation. Although clr-2 overexpression increased the transcript levels of cellulase genes in ∆Mig1, there was no increase in cellulase yield. Even a further increase in the transcript levels of clr-2 via a stronger promoter was ineffective. However, when the CaCO3 concentration was increased to 5 g/l in the growth medium, we achieved a 1.5-fold higher activity of 6.4 FPU/ml in the ∆Mig1 strain with clr-2 overexpression. Enthused by the calcium effect, a transcriptomic screen for genes encoding Ca2+-activated kinase identified ssp1, whose overexpression could further increase cellulase yield to ~ 7.5 FPU/ml. Investigation of the mechanism revealed that calcium signaling exclusively enhances the translation and secretion of cellulase in Penicillium funiculosum.

CONCLUSIONS

Our study identifies for the first time that cellulose activates two discrete signaling events to govern cellulase transcription and posttranscriptional processes (translation, processing and secretion) in P. funiculosum NCIM1228. Whereas Clr-2, the transcriptional activator of cellulase, governs transcription, calcium signaling specifically activates cellulase translation and secretion.

Molecular Mechanisms Underlying Ca2+/Calmodulin-Dependent Protein Kinase Kinase Signal Transduction

Ca²⁺/calmodulin-dependent protein kinase kinase (CaMKK) is the activating kinase for multiple downstream kinases, including CaM-kinase I (CaMKI), CaM-kinase IV (CaMKIV), protein kinase B (PKB/Akt), and 5′AMP-kinase (AMPK), through the phosphorylation of their activation-loop Thr residues in response to increasing the intracellular Ca²⁺ concentration, as CaMKK itself is a Ca²⁺/CaM-dependent enzyme. The CaMKK-mediated kinase cascade plays important roles in a number of Ca²⁺-dependent pathways, such as neuronal morphogenesis and plasticity, transcriptional activation, autophagy, and metabolic regulation, as well as in pathophysiological pathways, including cancer progression, metabolic syndrome, and mental disorders. This review focuses on the molecular mechanism underlying CaMKK-mediated signal transduction in normal and pathophysiological conditions. We summarize the current knowledge of the structural, functional, and physiological properties of the regulatory kinase, CaMKK, and the development and application of its pharmacological inhibitors.

Pak1 kinase controls cell shape through ribonucleoprotein granules

Fission yeast cells maintain a rod shape due to conserved signaling pathways that organize the cytoskeleton for polarized growth. We discovered a mechanism linking the conserved protein kinase Pak1 with cell shape through the RNA-binding protein Sts5. Pak1 (also called Shk1 and Orb2) prevents Sts5 association with P bodies by directly phosphorylating its intrinsically disordered region (IDR). Pak1 and the cell polarity kinase Orb6 both phosphorylate the Sts5 IDR but at distinct residues. Mutations preventing phosphorylation in the Sts5 IDR cause increased P body formation and defects in cell shape and polarity. Unexpectedly, when cells encounter glucose starvation, PKA signaling triggers Pak1 recruitment to stress granules with Sts5. Through retargeting experiments, we reveal that Pak1 localizes to stress granules to promote rapid dissolution of Sts5 upon glucose addition. Our work reveals a new role for Pak1 in regulating cell shape through ribonucleoprotein granules during normal and stressed growth conditions.

Overproduction of plant nuclear export signals enhances diamide tolerance in Schizosaccharomyces pombe

The nuclear export signal (NES) endows a protein nuclear export ability. Surprisingly, our previous study shows that just the NES peptide of Schizosaccharomyces pombe Oxs1 (SpOxs1^NES) can confer diamide tolerance by competing with transcription factor Pap1 for nuclear transport. This finding intrigued us to test the function of NESs from heterologous organisms. The Arabidopsis thaliana zinc finger transcription factor OXIDATIVE STRESS 2 (AtOXS2) is a nucleocytoplasmic shuttling protein and nearly all OXS2 members from maize and rice contain an NES. In this study, we find that the plant OXS2 members and their C-terminus (AT3 peptide) can confer diamide tolerance due to their NESs, and amino acids in non-conserved as well as conserved positions are necessary for the diamide tolerance. As in SpOxs1^NES, the enhanced tolerance to diamide in fission yeast depends on Pap1. Like SpOxs1^NES, OXS2 family NESs appear to compete for nuclear transport of the Pap1-like Arabidopsis protein bZIP10, as when overproduced in Arabidopsis protoplasts, bZIP10 is retained in the nucleus.

Annotation of AMP-activated protein kinase genes and its comparative transcriptional analysis between high and low lipid producing strains of Mucor circinelloides

BACKGROUND

AMP-activated protein kinase (AMPK) is an important regulator for lipid accumulation, potentially known to have an inhibitory role in lipid synthesis. It inactivates acetyl-CoA carboxylase (ACC), an important regulatory enzyme required for lipid synthesis. However, in Mucor circinelloides, AMPK and its association with lipid accumulation has not been studied yet.

OBJECTIVES

To identify AMPK genes in M. circinelloides and to compare their expression levels in high and low lipid-producing strains of M. circinelloides to predict the possible roles of AMPK in lipid metabolism and to select candidate genes for further studies to enhance lipid accumulation.

RESULTS

Two genes for α-subunit, one for β-subunit and six for γ-subunit were identified and annotated. Bioinformatic analysis confirmed the presence of typical conserved domains in these genes. Furthermore, transcriptional profiling displayed marked differences in expression kinetics of subunits among the selected strains. The expression of AMPK genes decreased rapidly in WJ11, high lipid producer strain during the lipid accumulation phase while contrasting profile of expression was observed in CBS 277.49, low lipid producer strain.

CONCLUSION

The present study has shown the association of AMPK genes with lipid metabolism at the transcriptional level. The involvement of Snf-α1, Snf-α2, Snf-β, Snf-γ1, Snf-γ4, Snf-γ5 subunits were shown to be more pronounced and could potentially be further explored in future studies.

Evolution of TOR-SnRK dynamics in green plants and its integration with phytohormone signaling networks

The target of rapamycin (TOR)-sucrose non-fermenting 1 (SNF1)-related protein kinase 1 (SnRK1) signaling is an ancient regulatory mechanism that originated in eukaryotes to regulate nutrient-dependent growth. Although the TOR-SnRK1 signaling cascade shows highly conserved functions among eukaryotes, studies in the past two decades have identified many important plant-specific innovations in this pathway. Plants also possess SnRK2 and SnRK3 kinases, which originated from the ancient SnRK1-related kinases and have specialized roles in controlling growth, stress responses and nutrient homeostasis in plants. Recently, an integrative picture has started to emerge in which different SnRKs and TOR kinase are highly interconnected to control nutrient and stress responses of plants. Further, these kinases are intimately involved with phytohormone signaling networks that originated at different stages of plant evolution. In this review, we highlight the evolution and divergence of TOR-SnRK signaling components in plants and their communication with each other as well as phytohormone signaling to fine-tune growth and stress responses in plants.

Cryptococcus neoformans sexual reproduction is controlled by a quorum sensing peptide

Bacterial quorum sensing is a well-characterized communication system that governs a large variety of collective behaviours. By comparison, quorum sensing regulation in eukaryotic microbes remains poorly understood, especially its functional role in eukaryote-specific behaviours, such as sexual reproduction. Cryptococcus neoformans is a prevalent fungal pathogen that has two defined sexual cycles (bisexual and unisexual) and is a model organism for studying sexual reproduction in fungi. Here, we show that the quorum sensing peptide Qsp1 serves as an important signalling molecule for both forms of sexual reproduction. Qsp1 orchestrates various differentiation and molecular processes, including meiosis, the hallmark of sexual reproduction. It activates bisexual mating, at least in part through the control of pheromone, a signal necessary for bisexual activation. Notably, Qsp1 also plays a major role in the intercellular regulation of unisexual initiation and coordination, in which pheromone is not strictly required. Through a multi-layered genetic screening approach, we identified the atypical zinc finger regulator Cqs2 as an important component of the Qsp1 signalling cascade during both bisexual and unisexual reproduction. The absence of Cqs2 eliminates the Qsp1-stimulated mating response. Together, these findings extend the range of behaviours governed by quorum sensing to sexual development and meiosis.

TORC1 and TORC2 converge to regulate the SAGA co-activator in response to nutrient availability

Gene expression regulation is essential for cells to adapt to changes in their environment. Co-activator complexes have well-established roles in transcriptional regulation, but less is known about how they sense and respond to signaling cues. We have previously shown that, in fission yeast, one such co-activator, the SAGA complex, controls gene expression and the switch from proliferation to differentiation in response to nutrient availability. Here, using a combination of genetic, biochemical, and proteomic approaches, we show that SAGA responds to nutrients through the differential phosphorylation of its Taf12 component, downstream of both the TORC1 and TORC2 pathways. Taf12 phosphorylation increases early upon starvation and is controlled by the opposing activities of the PP2A phosphatase, which is activated by TORC1, and the TORC2-activated Gad8^AKT kinase. Mutational analyses suggest that Taf12 phosphorylation prevents cells from committing to differentiation until starvation reaches a critical level. Overall, our work reveals that SAGA is a direct target of nutrient-sensing pathways and has uncovered a mechanism by which TORC1 and TORC2 converge to control gene expression and cell fate decisions.

Epe1 contributes to activation of AMPK by promoting phosphorylation of AMPK alpha subunit, Ssp2

AMP-activated protein kinase (AMPK) is a pivotal cellular energy sensor. It is activated by stresses that cause depletion of energy and initiates adaptive responses by regulating metabolism balance. AMPK forms αβγ heterotrimer. In fission yeast, activation of AMPK mainly depends on the phosphorylation of AMPKα subunit Ssp2 at Thr¹⁸⁹ by upstream kinase Ssp1. However, not much is known about the regulation of this process. In this study, we identified Epe1 as a novel positive regulator of AMPK. Epe1, a jmjC-domain-containing protein, is best-known as a negative regulator of heterochromatin spreading. Although the novel role of Epe1 in regulation of AMPK relies on predicted iron- and 2-oxyglutarate-binding residues inside jmjC domain, it seems to be irrelevant to inhibition of heterochromatin spreading. Epe1 is associated with Ssp2 directly and promotes phosphorylation of Ssp2 upon various environmental stresses, including low-glucose, high-sodium, high-pH and oxidative conditions. Similar to Epe1, Jmj1 and Msc1 also contribute to phosphorylation of Ssp2. Deletion of epe1⁺ impairs downstream events following phosphorylation of Ssp2, including nuclear translocation of Ssp2, sexual differentiation and inhibition of fatty acid synthesis. Our study reveals a novel way in which a jmjC-domain-containing protein regulates adaptive response by directly binding to a principal sensor.

Transient activation of fission yeast AMPK is required for cell proliferation during osmotic stress

Transient activation of the cellular energy sensor AMPK during osmotic stress requires its energy-sensing subunit. Cellular ATP levels decrease during osmotic stress, which triggers energy stress, which in turn requires dynamic activation of AMPK.

The heterotrimeric kinase AMPK acts as an energy sensor to coordinate cell metabolism with environmental status in species from yeast through humans. Low intracellular ATP leads to AMPK activation through phosphorylation of the activation loop within the catalytic subunit. Other environmental stresses also activate AMPK, but it is unclear whether cellular energy status affects AMPK activation under these conditions. Fission yeast AMPK catalytic subunit Ssp2 is phosphorylated at Thr-189 by the upstream kinase Ssp1 in low-glucose conditions, similar to other systems. Here we find that hyperosmotic stress induces strong phosphorylation of Ssp2-T189 by Ssp1. Ssp2-pT189 during osmotic stress is transient and leads to transient regulation of AMPK targets, unlike sustained activation by low glucose. Cells lacking this activation mechanism fail to proliferate after hyperosmotic stress. Activation during osmotic stress requires energy sensing by AMPK heterotrimer, and osmotic stress leads to decreased intracellular ATP levels. We observed mitochondrial fission during osmotic stress, but blocking fission did not affect AMPK activation. Stress-activated kinases Sty1 and Pmk1 did not promote AMPK activation but contributed to subsequent inactivation. Our results show that osmotic stress induces transient energy stress, and AMPK activation allows cells to manage this energy stress for proliferation in new osmotic states.

Phosphatases Generate Signal Specificity Downstream of Ssp1 Kinase in Fission Yeast

AMPK-related protein kinases (ARKs) coordinate cell growth, proliferation, and migration with environmental status. It is unclear how specific ARKs are activated at specific times. In the fission yeast Schizosaccharomyces pombe, the CaMKK-like protein kinase Ssp1 promotes cell cycle progression by activating the ARK Cdr2 according to cell growth signals. Here, we demonstrate that Ssp1 activates a second ARK, Ssp2/AMPKα, for cell proliferation in low environmental glucose. Ssp1 activates these two related targets by the same biochemical mechanism: direct phosphorylation of a conserved residue in the activation loop (Cdr2-T166 and Ssp2-T189). Despite a shared upstream kinase and similar phosphorylation sites, Cdr2 and Ssp2 have distinct regulatory input cues and distinct functional outputs. We investigated this specificity and found that distinct protein phosphatases counteract Ssp1 activity toward its different substrates. We identified the PP6 family phosphatase Ppe1 as the primary phosphatase for Ssp2-T189 dephosphorylation. The phosphatase inhibitor Sds23 acts upstream of PP6 to regulate Ssp2-T189 phosphorylation in a manner that depends on energy but not on the intact AMPK heterotrimer. In contrast, Cdr2-T166 phosphorylation is regulated by protein phosphatase 2A but not by the Sds23-PP6 pathway. Thus, our study provides a phosphatase-driven mechanism to induce specific physiological responses downstream of a master protein kinase.

Genetic interactions and functional analyses of the fission yeast gsk3 and amk2 single and double mutants defective in TORC1-dependent processes

The Target of Rapamycin (TOR) signalling network plays important roles in aging and disease. The AMP-activated protein kinase (AMPK) and the Gsk3 kinase inhibit TOR during stress. We performed genetic interaction screens using synthetic genetic arrays (SGA) with gsk3 and amk2 as query mutants, the latter encoding the regulatory subunit of AMPK. We identified 69 negative and 82 positive common genetic interactors, with functions related to cellular growth and stress. The 120 gsk3-specific negative interactors included genes functioning in translation and ribosomes. The 215 amk2-specific negative interactors included genes functioning in chromatin silencing and DNA damage repair. Both amk2- and gsk3-specific interactors were enriched in phenotype categories related to abnormal cell size and shape. We also performed SGA screen with the amk2 gsk3 double mutant as a query. Mutants sensitive to 5-fluorouracil, an anticancer drug are under-represented within the 305 positive interactors specific for the amk2 gsk3 query. The triple-mutant SGA screen showed higher number of negative interactions than the double mutant SGA screens and uncovered additional genetic network information. These results reveal common and specialized roles of AMPK and Gsk3 in mediating TOR-dependent processes, indicating that AMPK and Gsk3 act in parallel to inhibit TOR function in fission yeast.

Ssp1 CaMKK: A Sensor of Actin Polarization That Controls Mitotic Commitment through Srk1 in Schizosaccharomyces pombe

Background

Calcium/calmodulin-dependent protein kinase kinase (CaMKK) is required for diverse cellular functions. Mammalian CaMKK activates CaMKs and also the evolutionarily-conserved AMP-activated protein kinase (AMPK). The fission yeast Schizosaccharomyces pombe CaMKK, Ssp1, is required for tolerance to limited glucose through the AMPK, Ssp2, and for the integration of cell growth and division through the SAD kinase Cdr2.

Results

Here we report that Ssp1 controls the G2/M transition by regulating the activity of the CaMK Srk1. We show that inhibition of Cdc25 by Srk1 is regulated by Ssp1; and also that restoring growth polarity and actin localization of ssp1-deleted cells by removing the actin-monomer-binding protein, twinfilin, is sufficient to suppress the ssp1 phenotype.

Conclusions

These findings demonstrate that entry into mitosis is mediated by a network of proteins, including the Ssp1 and Srk1 kinases. Ssp1 connects the network of components that ensures proper polarity and cell size with the network of proteins that regulates Cdk1-cyclin B activity, in which Srk1 plays an inhibitory role.

The opposing actions of target of rapamycin and AMP-activated protein kinase in cell growth control

Cell growth is a highly regulated, plastic process. Its control involves balancing positive regulation of anabolic processes with negative regulation of catabolic processes. Although target of rapamycin (TOR) is a major promoter of growth in response to nutrients and growth factors, AMP-activated protein kinase (AMPK) suppresses anabolic processes in response to energy stress. Both TOR and AMPK are conserved throughout eukaryotic evolution. Here, we review the fundamentally important roles of these two kinases in the regulation of cell growth with particular emphasis on their mutually antagonistic signaling.

TORC1 Regulates Developmental Responses to Nitrogen Stress via Regulation of the GATA Transcription Factor Gaf1

The TOR (target of rapamycin [sirolimus]) is a universally conserved kinase that couples nutrient availability to cell growth. TOR complex 1 (TORC1) in Schizosaccharomyces pombe positively regulates growth in response to nitrogen availability while suppressing cellular responses to nitrogen stress. Here we report the identification of the GATA transcription factor Gaf1 as a positive regulator of the nitrogen stress-induced gene isp7⁺, via three canonical GATA motifs. We show that under nitrogen-rich conditions, TORC1 positively regulates the phosphorylation and cytoplasmic retention of Gaf1 via the PP2A-like phosphatase Ppe1. Under nitrogen stress conditions when TORC1 is inactivated, Gaf1 becomes dephosphorylated and enters the nucleus. Gaf1 was recently shown to negatively regulate the transcription induction of ste11⁺, a major regulator of sexual development. Our findings support a model of a two-faceted role of Gaf1 during nitrogen stress. Gaf1 positively regulates genes that are induced early in the response to nitrogen stress, while inhibiting later responses, such as sexual development. Taking these results together, we identify Gaf1 as a novel target for TORC1 signaling and a step-like mechanism to modulate the nitrogen stress response.

TOR complex 1 (TORC1) is an evolutionary conserved protein complex that positively regulates growth and proliferation, while inhibiting starvation responses. In fission yeast, the activity of TORC1 is downregulated in response to nitrogen starvation, and cells reprogram their transcriptional profile and prepare for sexual development. We identify Gaf1, a GATA-like transcription factor that regulates transcription and sexual development in response to starvation, as a downstream target for TORC1 signaling. Under nitrogen-rich conditions, TORC1 positively regulates the phosphorylation and cytoplasmic retention of Gaf1 via the PP2A-like phosphatase Ppe1. Under nitrogen stress conditions when TORC1 is inactivated, Gaf1 becomes dephosphorylated and enters the nucleus. Budding yeast TORC1 regulates GATA transcription factors via the phosphatase Sit4, a structural homologue of Ppe1. Thus, the TORC1-GATA transcription module appears to be conserved in evolution and may also be found in higher eukaryotes.

Nitrogen regulates AMPK to control TORC1 signaling

BACKGROUND

Cell growth and cell-cycle progression are tightly coordinated to enable cells to adjust their size (timing of division) to the demands of proliferation in varying nutritional environments. In fission yeast, nitrogen stress results in sustained proliferation at a reduced size.

RESULTS

Here, we show that cells can sense nitrogen stress to reduce target of rapamycin complex-1 (TORC1) activity. Nitrogen-stress-induced TORC1 inhibition differs from amino-acid-dependent control of TORC1 and requires the Ssp2 (AMPKα) kinase, the Tsc1/2 complex, and Rhb1 GTPase. Importantly, the β and γ regulatory subunits of AMPK are not required to control cell division in response to nitrogen stress, providing evidence for a nitrogen-sensing mechanism that is independent of changes in intracellular ATP/AMP levels. The CaMKK homolog Ssp1 is constitutively required for phosphorylation of the AMPKα(Ssp2) T loop. However, we find that a second homolog CaMKK(Ppk34) is specifically required to stimulate AMPKα(Ssp2) activation in response to nitrogen stress. Finally, ammonia also controls mTORC1 activity in human cells; mTORC1 is activated upon the addition of ammonium to glutamine-starved Hep3B cancer cells.

CONCLUSIONS

The alternative nitrogen source ammonia can simulate TORC1 activity to support growth and division under challenging nutrient settings, a situation often seen in cancer.

Negative feedback regulation of calcineurin-dependent Prz1 transcription factor by the CaMKK-CaMK1 axis in fission yeast

Calcium signals trigger the translocation of the Prz1 transcription factor from the cytoplasm to the nucleus. The process is regulated by the calcium-activated phosphatase calcineurin, which activates Prz1 thereby maintaining active transcription during calcium signalling. When calcium signalling ceases, Prz1 is inactivated by phosphorylation and exported to the cytoplasm. In budding yeast and mammalian cells, different kinases have been reported to counter calcineurin activity and regulate nuclear export. Here, we show that the Ca(2+)/calmodulin-dependent kinase Cmk1 is first phosphorylated and activated by the newly identified kinase CaMKK2 homologue, Ckk2, in response to Ca(2+). Then, active Cmk1 binds, phosphorylates and inactivates Prz1 transcription activity whilst at the same time cmk1 expression is enhanced by Prz1 in response to Ca(2+). Furthermore, Cdc25 phosphatase is also phosphorylated by Cmk1, inducing cell cycle arrest in response to an increase in Ca(2+). Moreover, cmk1 deletion shows a high tolerance to chronic exposure to Ca(2+), due to the lack of cell cycle inhibition and elevated Prz1 activity. This work reveals that Cmk1 kinase activated by the newly identified Ckk2 counteracts calcineurin function by negatively regulating Prz1 activity which in turn is involved in activating cmk1 gene transcription. These results are the first insights into Cmk1 and Ckk2 function in Schizosaccharomyces pombe.

Glucose activates TORC2-Gad8 protein via positive regulation of the cAMP/cAMP-dependent protein kinase A (PKA) pathway and negative regulation of the Pmk1 protein-mitogen-activated protein kinase pathway

The target of rapamycin (TOR) kinase belongs to the highly conserved eukaryotic family of phosphatidylinositol 3-kinase-related kinases. TOR proteins are found at the core of two evolutionary conserved complexes, known as TORC1 and TORC2. In fission yeast, TORC2 is dispensable for proliferation under optimal growth conditions but is required for starvation and stress responses. TORC2 has been implicated in a wide variety of functions; however, the signals that regulate TORC2 activity have so far remained obscure. TORC2 has one known direct substrate, the AGC kinase Gad8, which is related to AKT in human cells. Gad8 is phosphorylated by TORC2 at Ser-546 (equivalent to AKT Ser-473), leading to its activation. Here, we show that glucose is necessary and sufficient to induce Gad8 Ser-546 phosphorylation in vivo and Gad8 kinase activity in vitro. The glucose signal that activates TORC2-Gad8 is mediated via the cAMP/PKA pathway, a major glucose-sensing pathway. By contrast, Pmk1, similar to human extracellular signal-regulated kinases and a major stress-induced mitogen activated protein kinase (MAPK) in fission yeast, inhibits TORC2-dependent Gad8 phosphorylation and activation. Inhibition of TORC2-Gad8 also occurs in response to ionic or osmotic stress, in a manner dependent on the cAMP/PKA and Pmk1-MAPK signaling pathways. Our findings highlight the significance of glucose availability in regulation of TORC2-Gad8 and indicate a novel link between the cAMP/PKA, Pmk1/MAPK, and TORC2-Gad8 signaling.

Does a shift to limited glucose activate checkpoint control in fission yeast?

Here we review cell cycle control in the fission yeast, Schizosaccharomyces pombe, in response to an abrupt reduction of glucose concentration in culture media. S. pombe arrests cell cycle progression when transferred from media containing 2.0% glucose to media containing 0.1%. After a delay, S. pombe resumes cell division at a surprisingly fast rate, comparable to that observed in 2% glucose. We found that a number of genes, including zinc-finger transcription factor Scr1, CaMKK-like protein kinase Ssp1, and glucose transporter Ght5, enable rapid cell division in low glucose. In this article, we examine whether cell cycle checkpoint-like control operates during the delay and after resumption of cell division in limited-glucose. Using microarray analysis and genetic screening, we identified several candidate genes that may be involved in controlling this low-glucose adaptation.

Dueling kinases regulate cell size at division through the SAD kinase Cdr2

Cell size control requires mechanisms that integrate cell growth and division. Key to this integration in fission yeast is the SAD family kinase Cdr2, which organizes a set of cortical nodes in the cell middle to promote mitotic entry through Wee1 and Cdk1. Cdr2 is inhibited by a spatial gradient of the DYRK kinase Pom1 emanating from cell tips in a cell-size-dependent manner, but how the Pom1 gradient inhibits Cdr2 activity during cell growth is unknown. Here, we show that Pom1 acts to prevent activation of Cdr2 kinase activity by the CaMKK Ssp1. We found that Ssp1 activates Cdr2 through phosphorylation of a conserved threonine residue (Thr166) in the activation loop of the Cdr2 N-terminal kinase domain both in vitro and in cells. The levels of this activating phosphorylation increased with cell-cycle progression, and genetic epistasis demonstrated that Ssp1 promotes mitotic entry through Cdr2. Intriguingly, Pom1 phosophorylated the C-terminal domain of Cdr2, and this modification reduced Cdr2-T166 phosphorylation by Ssp1. These findings show how activation of the conserved mitotic inducer Cdr2 is integrated with an inhibitory spatial gradient to ensure proper cell size control at mitosis.

Genetic and physical interaction of Ssp1 CaMKK and Rad24 14-3-3 during low pH and osmotic stress in fission yeast

The Ssp1 calmodulin kinase kinase (CaMKK) is necessary for stress-induced re-organization of the actin cytoskeleton and initiation of growth at the new cell end following division in Schizosaccharomyces pombe. In addition, it regulates AMP-activated kinase and functions in low glucose tolerance. ssp1⁻ cells undergo mitotic delay at elevated temperatures and G₂ arrest in the presence of additional stressors. Following hyperosmotic stress, Ssp1-GFP forms transient foci which accumulate at the cell membrane and form a band around the cell circumference, but not co-localizing with actin patches. Hyperosmolarity-induced localization to the cell membrane occurs concomitantly with a reduction of its interaction with the 14-3-3 protein Rad24, but not Rad25 which remains bound to Ssp1. The loss of rad24 in ssp1⁻ cells reduces the severity of hyperosmotic stress response and relieves mitotic delay. Conversely, overexpression of rad24 exacerbates stress response and concomitant cell elongation. rad24⁻ does not impair stress-induced localization of Ssp1 to the cell membrane, however this response is almost completely absent in cells overexpressing rad24.

A knockout screen for protein kinases required for the proper meiotic segregation of chromosomes in the fission yeast Schizosaccharomyces pombe

The reduction of chromosome number during meiosis is achieved by two successive rounds of chromosome segregation after just single round of DNA replication. To identify novel proteins required for the proper segregation of chromosomes during meiosis, we analyzed the consequences of deleting Schizosaccharomyces pombe genes predicted to encode protein kinases that are not essential for cell viability. We show that Mph1, a member of the Mps1 family of spindle assembly checkpoint kinases, is required to prevent meiosis I homolog non-disjunction. We also provide evidence for a novel function of Spo4, the fission yeast ortholog of Dbf4-dependent Cdc7 kinase, in regulating the length of anaphase II spindles. In the absence of Spo4, abnormally elongated anaphase II spindles frequently overlap and thus destroy the linear order of nuclei in the ascus. Our observation that the spo4Δ mutant phenotype can be partially suppressed by inhibiting Cdc2-as suggests that dysregulation of the activity of this cyclin-dependent kinase may cause abnormal elongation of anaphase II spindles in spo4Δ mutant cells.

Linkers of cell polarity and cell cycle regulation in the fission yeast protein interaction network

The study of gene and protein interaction networks has improved our understanding of the multiple, systemic levels of regulation found in eukaryotic and prokaryotic organisms. Here we carry out a large-scale analysis of the protein-protein interaction (PPI) network of fission yeast (Schizosaccharomyces pombe) and establish a method to identify ‘linker’ proteins that bridge diverse cellular processes - integrating Gene Ontology and PPI data with network theory measures. We test the method on a highly characterized subset of the genome consisting of proteins controlling the cell cycle, cell polarity and cytokinesis and identify proteins likely to play a key role in controlling the temporal changes in the localization of the polarity machinery. Experimental inspection of one such factor, the polarity-regulating RNB protein Sts5, confirms the prediction that it has a cell cycle dependent regulation. Detailed bibliographic inspection of other predicted ‘linkers’ also confirms the predictive power of the method. As the method is robust to network perturbations and can successfully predict linker proteins, it provides a powerful tool to study the interplay between different cellular processes.

Analysis of protein interaction networks has been of use as a means to grapple with the complexity of the interactome of biological organisms. So far, network based approaches have only been used in a limited number of organisms due to the lack of high-throughput experiments. In this study, we investigate by graph theoretical network analysis approaches the protein-protein interaction network of fission yeast, and present a new network measure, linkerity, that predicts the ability of certain proteins to function as bridges between diverse cellular processes. We apply this linkerity measure to a highly conserved and coupled subset of the fission yeast network, consisting of the proteins that regulate cell cycle, polarized cell growth, and cell division. In depth literature analysis confirms that several proteins identified as linkers of cell polarity regulation are indeed also associated with cell cycle and/or cell division control. Similarly, experimental testing confirms that a mostly uncharacterized polarity regulator identified by the method as an important linker is regulated by the cell cycle, as predicted.

Environmental control of cell size at division

Tight coupling between cell growth and cell cycle progression allows cells to adjust their size to the demands of proliferation in varying nutrient environments. Target of rapamycin (TOR) signalling pathways co-ordinate cell growth with cell cycle progression in response to altered nutritional availability. To increase cell size the active TOR Complex 1 (TORC1) promotes cell growth to delay mitosis and cell division, whereas under limited nutrients TORC1 activity is decreased to reduce cell size. It remains unclear why cell size is subject to such tight control. Recent evidence suggests that in addition to modulating cell size, changes in nutrient availability also alter nuclear:cytoplasmic (N/C) ratios and may therefore compromise optimal cellular physiology. This could explain why cells increase their size when conditions are favourable, despite being competent to survive at a smaller size if necessary.