Pom1 DYRK regulates localization of the Rga4 GAP to ensure bipolar activation of Cdc42 in fission yeast

Where to grow and where to go

Filamentous fungi grow as very elongated tubular cells that extend by membrane extension and cell-wall biosynthesis. Membrane and enzyme delivery depend on secretory vesicles that travel along microtubules, accumulate in a structure called the Spitzenkörper and then move along actin cables towards the apical membrane. Whereas vesicle fusion and membrane insertion are well studied, less is known about the mechanisms with which the zones of vesicle fusion and hence the growth zones are defined. One mechanism by which polarity is established and maintained is the polar localization of cell-end marker proteins (CEMPs). They form multi-protein complexes with formin as F-actin polymerase. CEMP delivery depends on microtubules, and hence CEMPs coordinate the microtubule with the actin cytoskeleton. Actin filaments capture microtubule ends, and this positive feedback loop quickly establishes active growth sites. However, CEMP complexes are self-limiting, because fusing vesicles disturb local growth zones and Ca2+ influx pulses lead to F-actin disassembly. This model emerged from studies in Schizosaccharomyces pombe and Aspergillus nidulans. Surprisingly, deletion of CEMP-coding genes is not lethal. S. pombe mutants form T-shaped cells and A. nidulans germlings grow less straight. In comparison, CEMP-mutants had a strong phenotype in Arthrobotrys flagrans, a nematode-trapping fungus, which produces ring-like trapping structures. CEMP-mutants fail to form adhesive rings and instead form sticks. CEMP overexpression caused a hyperbranching phenotype. Hence, CEMPs are involved in polarity maintenance and play critical roles during modulations of polarity. Here, we are going to discuss the functions of CEMPs and their connections to other polarity determinants.

Cell-end marker proteins are required for hyphal ring formation and size determination of traps in Arthrobotrys flagrans

Filamentous fungi grow by apical extension where secretory vesicles are transported long distances by microtubules and by actin prior to fusion with the cell membrane. Apical, membrane-bound cell-end marker proteins (CEMPs) organise the cytoskeletons and thereby the growth machinery. CEMPs have been characterised mainly in Schizosaccharomyces pombe and Aspergillus nidulans. Here, we studied the role of CEMPs in the nematode-trapping fungus Arthrobotrys flagrans. This predatory fungus forms ring-shaped adhesive traps to capture nematodes, such as Caenorhabditis elegans. Traps are morphologically and physiologically different from vegetative hyphae and are generated by hyphal turning and fusion of the trap tip cell with the basal hypha. The absence of the membrane-anchored CEMP receptor protein, TeaR, caused a reduction in ring size, whereas deletion of teaA or teaC largely prevented the formation of ring-shaped hyphae, and most traps appeared as adhesive sticks. Hence, compared to Schizosaccharomyces pombe and Aspergillus nidulans, loss of function of the CEMPs results in a severe morphological phenotype. The mutant strains also show changes in cell-to-cell communication and hyphal fusion, suggesting novel functions and interconnections with other signalling processes in the cell.

Reversal of metformin's anti-proliferative effect in fission yeast efr3 and dnm1 (DRP1) mutants with elongated mitochondria

Metformin is a well-tolerated drug frequently prescribed for managing type 2 diabetes. Extended metformin use has been linked to a significant decrease in cancer incidence across both diabetic and non-diabetic populations. Here we investigate the anti-proliferative effects of metformin on fission yeast S. pombe. Our findings demonstrate that metformin's inhibitory impact on cell proliferation is effective in the absence of AMP-activated protein kinase (AMPK). Using an unbiased genetic screen we identified the plasma membrane signalling scaffold Efr3, critical for phosphatidylinositol signalling and the generation of PI4Ps, as a key determinant of resistance to the anti-proliferative effect of metformin. Deletion of efr3 resulted in both AMPK-dependent and AMPK-independent resistance to metformin. We show that Efr3 does not influence cell proliferation by controlling Ras1 activity or its cellular localization in yeast. We observe that dnm1 (DRP1) mutants with elongated mitochondria are also resistant to the anti-proliferative effect of metformin and that metformin treatment promotes mitochondrial fusion. Metabolic measurements after prolonged metformin exposure demonstrated a reduction in respiration in both wild type and the efr3 deletion, however, that reduction is less pronounced in the efr3 deletion, which also contained elongated mitochondria. It is likely that mitochondrial fusion enhances yeast fitness in response to metformin exposure. Together we provide a new perspective on the cellular response to metformin.

Quantitative analysis of septin Cdc10 & Cdc3-associated proteome during stress response in the fungal pathogen Cryptococcus neoformans

Cryptococcus neoformans is a pathogenic basidiomycetous yeast that primarily infects immunocompromised individuals. Fatal outcome of cryptococcosis depends on the ability of C. neoformans to sense and adapt to 37°C. A complex of conserved filament forming GTPases, called septins, composed of Cdc3, Cdc10, Cdc11, and Cdc12, assembles at the mother-bud neck in C. neoformans. Septins Cdc3 and Cdc12 are essential for proliferation of C. neoformans at 37°C and for virulence in the Galleria mellonella model of infection, presumably due to their requirement for septin complex formation, and the involvement in cytokinesis. However, how exactly Cdc3, and Cdc12 contribute to C. neoformans growth at 37°C remains unknown. Based on studies investigating roles of septins in Saccharomyces cerevisiae, septin complex at the mother-bud neck of C. neoformans is predicted to interact with proteins involved in cell cycle control, morphogenesis, and cytokinesis, but the septin-associated proteome in C. neoformans has not been investigated. Here, we utilized tandem mass spectrometry to define C. neoformans proteins that associate with either Cdc3 or Cdc10 at ∼25°C or after the shift to 37°C. Our findings unveil a diverse array of septin-associated proteins, highlighting potential roles of septins in cell division, and stress response. Two proteins, identified as associated with both Cdc3 and Cdc10, the actin-binding protein profilin, which was detected at both temperatures, and ATP-binding multi-drug transporter Afr1, which was detected exclusively at 37°C, were further confirmed by co-immunoprecipitation. We also confirmed that association of Cdc3 with Afr1 was enhanced at 37°C. Upon shift to 37°C, septins Cdc3 and Cdc10 exhibited altered localization and Cdc3 partially co-localized with Afr1. In addition, we also investigated changes to levels of individual C. neoformans proteins upon shift from ∼25 to 37°C in exponentially grown culture and when cells entered stationary phase at ∼25°C. Our study reveals changes to C. neoformans proteome associated with heat and nutrient deprivation stresses and provides a landscape of septin-associated C. neoformans proteome, which will facilitate elucidating the biology of septins and mechanisms of stress response in this fungal pathogen.

Cdc42 mobility and membrane flows regulate fission yeast cell shape and survival

Polarized exocytosis induced by local Cdc42 GTPase activity results in membrane flows that deplete low-mobility membrane-associated proteins. A reaction-diffusion particle model comprising Cdc42 positive feedback activation, hydrolysis by GTPase-activating proteins (GAPs), and flow-induced displacement by exo/endocytosis shows that flow-induced depletion of low mobility GAPs promotes polarization. We modified Cdc42 mobility in Schizosaccharomyces pombe by replacing its prenylation site with 1, 2 or 3 repeats of the Rit C-terminal membrane-binding domain (ritC), yielding alleles with progressively lower mobility and increased flow-coupling. While Cdc42-1ritC cells are viable and polarized, Cdc42-2ritC polarize poorly and Cdc42-3ritC are inviable, in agreement with model's predictions. Deletion of Cdc42 GAPs restores viability to Cdc42-3ritC cells, verifying the model's prediction that GAP deletion increases Cdc42 activity at the expense of polarization. Our work demonstrates how membrane flows are an integral part of Cdc42-driven pattern formation and require Cdc42-GTP to turn over faster than the surface on which it forms.

Arp2/3-dependent endocytosis ensures Cdc42 oscillations by removing Pak1-mediated negative feedback

The GTPase Cdc42 regulates polarized growth in most eukaryotes. In the bipolar yeast Schizosaccharomyces pombe, Cdc42 activation cycles periodically at sites of polarized growth. These periodic cycles are caused by alternating positive feedback and time-delayed negative feedback loops. At each polarized end, negative feedback is established when active Cdc42 recruits the Pak1 kinase to prevent further Cdc42 activation. It is unclear how Cdc42 activation returns to each end after Pak1-dependent negative feedback. We find that disrupting branched actin-mediated endocytosis disables Cdc42 reactivation at the cell ends. Using experimental and mathematical approaches, we show that endocytosis-dependent Pak1 removal from the cell ends allows the Cdc42 activator Scd1 to return to that end to enable reactivation of Cdc42. Moreover, we show that Pak1 elicits its own removal via activation of endocytosis. These findings provide a deeper insight into the self-organization of Cdc42 regulation and reveal previously unknown feedback with endocytosis in the establishment of cell polarity.

Patterning of the cell cortex by Rho GTPases

The Rho GTPases - RHOA, RAC1 and CDC42 - are small GTP binding proteins that regulate basic biological processes such as cell locomotion, cell division and morphogenesis by promoting cytoskeleton-based changes in the cell cortex. This regulation results from active (GTP-bound) Rho GTPases stimulating target proteins that, in turn, promote actin assembly and myosin 2-based contraction to organize the cortex. This basic regulatory scheme, well supported by in vitro studies, led to the natural assumption that Rho GTPases function in vivo in an essentially linear matter, with a given process being initiated by GTPase activation and terminated by GTPase inactivation. However, a growing body of evidence based on live cell imaging, modelling and experimental manipulation indicates that Rho GTPase activation and inactivation are often tightly coupled in space and time via signalling circuits and networks based on positive and negative feedback. In this Review, we present and discuss this evidence, and we address one of the fundamental consequences of coupled activation and inactivation: the ability of the Rho GTPases to self-organize, that is, direct their own transition from states of low order to states of high order. We discuss how Rho GTPase self-organization results in the formation of diverse spatiotemporal cortical patterns such as static clusters, oscillatory pulses, travelling wave trains and ring-like waves. Finally, we discuss the advantages of Rho GTPase self-organization and pattern formation for cell function.

The Arp2/3 complex promotes periodic removal of Pak1-mediated negative feedback to facilitate anticorrelated Cdc42 oscillations

The conserved GTPase Cdc42 is a major regulator of polarized growth in most eukaryotes. Cdc42 periodically cycles between active and inactive states at sites of polarized growth. These periodic cycles are caused by positive feedback and time-delayed negative feedback loops. In the bipolar yeast S. pombe, both growing ends must regulate Cdc42 activity. At each cell end, Cdc42 activity recruits the Pak1 kinase which prevents further Cdc42 activation thus establishing negative feedback. It is unclear how Cdc42 activation returns to the end after Pak1-dependent negative feedback. Using genetic and chemical perturbations, we find that disrupting branched actin-mediated endocytosis disables Cdc42 reactivation at the cell ends. With our experimental data and mathematical models, we show that endocytosis-dependent Pak1 removal from the cell ends allows the Cdc42 activator Scd1 to return to that end to enable reactivation of Cdc42. Moreover, we show that Pak1 elicits its own removal via activation of endocytosis. In agreement with these observations, our model and experimental data show that in each oscillatory cycle, Cdc42 activation increases followed by an increase in Pak1 recruitment at that end. These findings provide a deeper insight into the self-organization of Cdc42 regulation and reveal previously unknown feedback with endocytosis in the establishment of cell polarity.

Cdc42 mobility and membrane flows regulate fission yeast cell shape and survival

Local Cdc42 GTPase activation promotes polarized exocytosis, resulting in membrane flows that deplete low-mobility membrane-associated proteins from the growth region. To investigate the self-organizing properties of the Cdc42 secretion-polarization system under membrane flow, we developed a reaction-diffusion particle model. The model includes positive feedback activation of Cdc42, hydrolysis by GTPase-activating proteins (GAPs), and flow-induced displacement by exo/endocytosis. Simulations show how polarization relies on flow-induced depletion of low mobility GAPs. To probe the role of Cdc42 mobility in the fission yeast Schizosaccharomyces pombe, we changed its membrane binding properties by replacing its prenylation site with 1, 2 or 3 repeats of the Rit1 C terminal membrane binding domain (ritC), yielding alleles with progressively lower unbinding and diffusion rates. Concordant modelling predictions and experimental observations show that lower Cdc42 mobility results in lower Cdc42 activation level and wider patches. Indeed, while Cdc42-1ritC cells are viable and polarized, Cdc42-2ritC polarize poorly and Cdc42-3ritC is inviable. The model further predicts that GAP depletion increases Cdc42 activity at the expense of loss of polarization. Experiments confirm this prediction, as deletion of Cdc42 GAPs restores viability to Cdc42-3ritC cells. Our combined experimental and modelling studies demonstrate how membrane flows are an integral part of Cdc42-driven pattern formation.

Cdc42 prevents precocious Rho1 activation during cytokinesis in a Pak1-dependent manner

During cytokinesis, a series of coordinated events partition a dividing cell. Accurate regulation of cytokinesis is essential for proliferation and genome integrity. In fission yeast, these coordinated events ensure that the actomyosin ring and septum start ingressing only after chromosome segregation. How cytokinetic events are coordinated remains unclear. The GTPase Cdc42 promotes recruitment of certain cell wall-building enzymes whereas the GTPase Rho1 activates these enzymes. We show that Cdc42 prevents early Rho1 activation during fission yeast cytokinesis. Using an active Rho probe, we find that although the Rho1 activators Rgf1 and Rgf3 localize to the division site in early anaphase, Rho1 is not activated until late anaphase, just before the onset of ring constriction. We find that loss of Cdc42 activation enables precocious Rho1 activation in early anaphase. Furthermore, we provide functional and genetic evidence that Cdc42-dependent Rho1 inhibition is mediated by the Cdc42 target Pak1 kinase. Our work proposes a mechanism of Rho1 regulation by active Cdc42 to coordinate timely septum formation and cytokinesis fidelity.

Stress-induced cell depolarization through the MAP kinase-Cdc42 axis

General stress responses, which sense environmental or endogenous signals, aim at promoting cell survival and fitness during adverse conditions. In eukaryotes, mitogen-activated protein (MAP) kinase-driven cascades trigger a shift in the cell's gene expression program as a cellular adaptation to stress. Here, we review another aspect of activated MAP kinase cascades reported in fission yeast: the transient inhibition of cell polarity in response to oxidative stress. The phosphorylation by a stress-activated MAP kinase of regulators of the GTPase cell division cycle 42 (Cdc42) causes a transient inhibition of polarized cell growth. The formation of growth sites depends on limiting and essential polarity components. We summarize here some processes in which inhibition of Cdc42 may be a general mechanism to regulate polarized growth also under physiological conditions.

The Cdc42 GAP Rga6 promotes monopolar outgrowth of spores

The molecular mechanisms underlying the establishment of the monopolar growth of fission yeast spores have been less characterized. Here, we report that the Cdc42 GTPase-activating protein (GAP) Rga6 is required for promoting monopolar growth during spore germination. The absence of Rga6 increases the number of spores that grow in a bipolar fashion. Rga6 decorates the non-growing cortical region, binds phosphatidylinositol 4,5-bisphosphate, and colocalizes with the phosphatidylinositol 4,5-bisphosphate-binding protein Opy1. Overexpression of Opy1 diminishes the cortical localization of Rga6. The characteristic localization of Rga6 on the cell cortex depends on the C-terminal PBR region of Rga6. Moreover, engineered chimera composed of the Rga6 C-terminal PBR region fused to the GAP domain of Rga3 or Rga4 are sufficient to rescue the spore growth phenotype caused by the absence of Rga6. Hence, our work establishes a paradigm in which the lipid composition of the plasma membrane directs polarized cell growth by specifying the cortical localization of a GAP protein.

Condensation of the fusion focus by the intrinsically disordered region of the formin Fus1 is essential for cell-cell fusion

Secretory vesicle clusters transported on actin filaments by myosin V motors for local secretion underlie various cellular processes, such as neurotransmitter release at neuronal synapses,1 hyphal steering in filamentous fungi,2,3 and local cell wall digestion preceding the fusion of yeast gametes.4 During fission yeast Schizosaccharomyces pombe gamete fusion, the actin fusion focus assembled by the formin Fus1 concentrates secretory vesicles carrying cell wall digestive enzymes.5,6,7 The position and coalescence of the vesicle focus are controlled by local signaling and actin-binding proteins to prevent inappropriate cell wall digestion that would cause lysis,6,8,9,10 but the mechanisms of focusing have been elusive. Here, we show that the regulatory N terminus of Fus1 contains an intrinsically disordered region (IDR) that mediates Fus1 condensation in vivo and forms dense assemblies that exclude ribosomes. Fus1 lacking its IDR fails to concentrate in a tight focus and causes cell lysis during attempted cell fusion. Remarkably, the replacement of Fus1 IDR with a heterologous low-complexity region that forms molecular condensates fully restores Fus1 focusing and function. By contrast, the replacement of Fus1 IDR with a domain that forms more stable oligomers restores focusing but poorly supports cell fusion, suggesting that condensation is tuned to yield a selectively permeable structure. We propose that condensation of actin structures by an IDR may be a general mechanism for actin network organization and the selective local concentration of secretory vesicles.

Cdc42 GTPase activating proteins Rga4 and Rga6 coordinate septum synthesis and membrane trafficking at the division plane during cytokinesis

Fission yeast cytokinesis is driven by simultaneous septum synthesis, membrane furrowing and actomyosin ring constriction. The septum consists of a primary septum flanked by secondary septa. First, delivery of the glucan synthase Bgs1 and membrane vesicles initiate primary septum synthesis and furrowing. Next, Bgs4 is delivered for secondary septum formation. It is unclear how septum synthesis is coordinated with membrane furrowing. Cdc42 promotes delivery of Bgs1 but not Bgs4. We find that after primary septum initiation, Cdc42 inactivators Rga4 and Rga6 localize to the division site. In rga4Δrga6Δ mutants, Cdc42 activity is enhanced during late cytokinesis and cells take longer to separate. Electron micrographs of the division site in these mutants exhibit malformed septum with irregular membrane structures. These mutants have a larger division plane with enhanced Bgs1 delivery but fail to enhance accumulation of Bgs4 and several exocytic proteins. Additionally, these mutants show endocytic defects at the division site. This suggests that Cdc42 regulates primary septum formation and only certain membrane trafficking events. As cytokinesis progresses Rga4 and Rga6 localize to the division site to decrease Cdc42 activity to allow coupling of Cdc42-independent membrane trafficking events with septum formation for proper septum morphology.

Ubiquitination of CLIP-170 family protein restrains polarized growth upon DNA replication stress

Microtubules play a crucial role during the establishment and maintenance of cell polarity. In fission yeast cells, the microtubule plus-end tracking proteins (+TIPs) (including the CLIP-170 homologue Tip1) regulate microtubule dynamics and also transport polarity factors to the cell cortex. Here, we show that the E3 ubiquitin ligase Dma1 plays an unexpected role in controlling polarized growth through ubiquitinating Tip1. Dma1 colocalizes with Tip1 to cortical sites at cell ends, and is required for ubiquitination of Tip1. Although the absence of dma1⁺ does not cause apparent polar growth defects in vegetatively growing cells, Dma1-mediated Tip1 ubiquitination is required to restrain polar growth upon DNA replication stress. This mechanism is distinct from the previously recognized calcineurin-dependent inhibition of polarized growth. In this work, we establish a link between Dma1-mediated Tip1 ubiquitination and DNA replication or DNA damage checkpoint-dependent inhibition of polarized growth in fission yeast.

The number of cytokinesis nodes in mitotic fission yeast scales with cell size

Cytokinesis nodes are assemblies of stoichiometric ratios of proteins associated with the plasma membrane, which serve as precursors for the contractile ring during cytokinesis by fission yeast. The total number of nodes is uncertain, because of the limitations of the methods used previously. Here, we used the ~140 nm resolution of Airyscan super-resolution microscopy to measure the fluorescence intensity of small, single cytokinesis nodes marked with Blt1-mEGFP in live fission yeast cells early in mitosis. The ratio of the total Blt1-mEGFP fluorescence in the broad band of cytokinesis nodes to the average fluorescence of a single node gives about 190 single cytokinesis nodes in wild-type fission yeast cells early in mitosis. Most, but not all of these nodes condense into a contractile ring. The number of cytokinesis nodes scales with cell size in four strains tested, although large diameter rga4Δ mutant cells form somewhat fewer cytokinesis nodes than expected from the overall trend. The Pom1 kinase restricts cytokinesis nodes from the ends of cells, but the surface density of Pom1 on the plasma membrane around the equators of cells is similar with a wide range of node numbers, so Pom1 does not control cytokinesis node number. However, when the concentrations of either kinase Pom1 or kinase Cdr2 were varied with the nmt1 promoter, the numbers of cytokinesis nodes increased above a baseline of about ~190 with the total cellular concentration of either kinase.

Fission yeast polycystin Pkd2p promotes cell size expansion and antagonizes the Hippo-related SIN pathway

Polycystins are conserved mechanosensitive channels whose mutations lead to the common human renal disorder autosomal dominant polycystic kidney disease (ADPKD). Previously, we discovered that the plasma membrane-localized fission yeast polycystin homolog Pkd2p is an essential protein required for cytokinesis; however, its role remains unclear. Here, we isolated a novel temperature-sensitive pkd2 mutant, pkd2-B42. Among the strong growth defects of this mutant, the most striking was that many mutant cells often lost a significant portion of their volume in just 5 min followed by a gradual recovery, a process that we termed 'deflation'. Unlike cell lysis, deflation did not result in plasma membrane rupture and occurred independently of cell cycle progression. The tip extension of pkd2-B42 cells was 80% slower than that of wild-type cells, and their turgor pressure was 50% lower. Both pkd2-B42 and the hypomorphic depletion mutant pkd2-81KD partially rescued mutants of the septation initiation network (SIN), a yeast Hippo-related signaling pathway, by preventing cell lysis, enhancing septum formation and doubling the number of Sid2p and Mob1p molecules at the spindle pole bodies. We conclude that Pkd2p promotes cell size expansion during interphase by regulating turgor pressure and antagonizes the SIN during cytokinesis. This article has an associated First Person interview with the first author of the paper.

The Cdc42 GTPase activating protein Rga6 promotes the cortical localization of Septin

Septins are a family of filament-forming GTP-binding proteins that regulate fundamental cellular activities such as cytokinesis and cell polarity. In general, Septin filaments function as barriers and scaffolds on the cell cortex. However, little is known about the mechanism that governs the recruitment and localization of the Septin complex to the cell cortex. Here, we identified the Cdc42 GTPase activating protein Rga6 as a key protein involved in promoting the localization of the Septin complex to the cell cortex in the fission yeast Schizosaccharomyces pombe. Rga6 interacts with the Septin complex and partially colocalizes with the Septin complex on the cell cortex. Live-cell microscopic analysis further showed Septin enrichment at the cortical regions adjacent to the growing cell tip. The Septin enrichment likely plays a crucial role in confining active Cdc42 to the growing cell tip. Hence, our findings support a model that Rga6 regulates polarized cell growth partly through promoting targeted localization of the Septin complex on the cell cortex.

Dyrk1a from Gene Function in Development and Physiology to Dosage Correction across Life Span in Down Syndrome

Down syndrome is the main cause of intellectual disabilities with a large set of comorbidities from developmental origins but also that appeared across life span. Investigation of the genetic overdosage found in Down syndrome, due to the trisomy of human chromosome 21, has pointed to one main driver gene, the Dual-specificity tyrosine-regulated kinase 1A (Dyrk1a). Dyrk1a is a murine homolog of the drosophila minibrain gene. It has been found to be involved in many biological processes during development and in adulthood. Further analysis showed its haploinsufficiency in mental retardation disease 7 and its involvement in Alzheimer's disease. DYRK1A plays a role in major developmental steps of brain development, controlling the proliferation of neural progenitors, the migration of neurons, their dendritogenesis and the function of the synapse. Several strategies targeting the overdosage of DYRK1A in DS with specific kinase inhibitors have showed promising evidence that DS cognitive conditions can be alleviated. Nevertheless, providing conditions for proper temporal treatment and to tackle the neurodevelopmental and the neurodegenerative aspects of DS across life span is still an open question.

Stress-dependent inhibition of polarized cell growth through unbalancing the GEF/GAP regulation of Cdc42

Cdc42 GTPase rules cell polarity and growth in fission yeast. It is negatively and positively regulated by GTPase-activating proteins (GAPs) and guanine nucleotide exchange factors (GEFs), respectively. Active Cdc42-GTP localizes to the poles, where it associates with numerous proteins constituting the polarity module. However, little is known about its downregulation. We describe here that oxidative stress causes Sty1-kinase-dependent Cdc42 inactivation at cell poles. Both the amount of active Cdc42 at tips and cell length inversely correlate with Sty1 activity, explaining the elongated morphology of Δsty1 cells. We have created stress-blinded cell poles either by eliminating two Cdc42 GAPs or through the constitutive tethering of Gef1 to cell tips, and we biochemically demonstrate that the GAPs Rga3/6 and the GEF Gef1 are direct substrates of Sty1. We propose that phosphorylation of Rga3/6 and Gef1 mediates the Sty1-dependent inhibition of Cdc42 at cell tips, halting polarized growth during stress adaptation.

Sequestration of the exocytic SNARE Psy1 into multiprotein nodes reinforces polarized morphogenesis in fission yeast

Polarized morphogenesis is achieved by targeting or inhibiting growth in distinct regions. Rod-shaped fission yeast cells grow exclusively at their ends by restricting exocytosis and secretion to these sites. This growth pattern implies the existence of mechanisms that prevent exocytosis and growth along nongrowing cell sides. We previously identified a set of 50-100 megadalton-sized node structures along the sides of fission yeast cells that contained the interacting proteins Skb1 and Slf1. Here, we show that Skb1-Slf1 nodes contain the syntaxin-like soluble N-ethylmaleimide-sensitive factor attachment protein receptor Psy1, which mediates exocytosis in fission yeast. Psy1 localizes in a diffuse pattern at cell tips, where it likely promotes exocytosis and growth, but is sequestered in Skb1-Slf1 nodes at cell sides where growth does not occur. Mutations that prevent node assembly or inhibit Psy1 localization to nodes lead to aberrant exocytosis at cell sides and increased cell width. Genetic results indicate that this Psy1 node mechanism acts in parallel to actin cables and Cdc42 regulation. Our work suggests that sequestration of syntaxin-like Psy1 at nongrowing regions of the cell cortex reinforces cell morphology by restricting exocytosis to proper sites of polarized growth.

Cdc42 reactivation at growth sites is regulated by local cell-cycle-dependent loss of its GTPase-activating protein Rga4 in fission yeast

In fission yeast, polarized cell growth stops during division and resumes after cytokinesis completes and cells separate. It is unclear how growth reactivation is timed to occur immediately after cell separation. We uncoupled these sequential events by delaying cytokinesis with a temporary Latrunculin A treatment. Mitotic cells recovering from treatment initiate end growth during septation, displaying a polar elongation simultaneous with septation (PrESS) phenotype. PrESS cell ends reactivate Cdc42, a major regulator of polarized growth, during septation, but at a fixed time after anaphase B. A candidate screen implicates Rga4, a negative regulator of Cdc42, in this process. We show that Rga4 appears punctate at the cell sides during G2, but is diffuse during mitosis, extending to the ends. Although the Morphogenesis Orb6 (MOR) pathway is known to promote cell separation and growth by activating protein synthesis, we find that, for polarized growth, removal of Rga4 from the ends is also necessary. Therefore, we propose that growth resumes after division once the MOR pathway is activated and the ends lose Rga4 in a cell-cycle-dependent manner.

Cell patterning by secretion-induced plasma membrane flows

Cells self-organize using reaction-diffusion and fluid-flow principles. Whether bulk membrane flows contribute to cell patterning has not been established. Here, using mathematical modeling, optogenetics, and synthetic probes, we show that polarized exocytosis causes lateral membrane flows away from regions of membrane insertion. Plasma membrane–associated proteins with sufficiently low diffusion and/or detachment rates couple to the flows and deplete from areas of exocytosis. In rod-shaped fission yeast cells, zones of Cdc42 GTPase activity driving polarized exocytosis are limited by GTPase activating proteins (GAPs). We show that membrane flows pattern the GAP Rga4 distribution and that coupling of a synthetic GAP to membrane flows is sufficient to establish the rod shape. Thus, membrane flows induced by Cdc42-dependent exocytosis form a negative feedback restricting the zone of Cdc42 activity.

Eng2, a new player involved in feedback loop regulation of Cdc42 activity in fission yeast

Cell polarity and morphogenesis are regulated by the small GTPase Cdc42. Even though major advances have been done in the field during the last years, the molecular details leading to its activation in particular cellular contexts are not completely understood. In fission yeast, the β(1,3)-glucanase Eng2 is a "moonlighting protein" with a dual function, acting as a hydrolase during spore dehiscence, and as component of the endocytic machinery in vegetative cells. Here, we report that Eng2 plays a role in Cdc42 activation during polarized growth through its interaction with the scaffold protein Scd2, which brings Cdc42 together with its guanine nucleotide exchange factor (GEF) Scd1. eng2Δ mutant cells have defects in activation of the bipolar growth (NETO), remaining monopolar during all the cell cycle. In the absence of Eng2 the accumulation of Scd1 and Scd2 at the poles is reduced, the levels of Cdc42 activation decrease, and the Cdc42 oscillatory behavior, associated with bipolar growth in wild type cells, is altered. Furthermore, overexpression of Eng2 partially rescues the growth and polarity defects of a cdc42-L160S mutant. Altogether, our work unveils a new factor regulating the activity of Cdc42, which could potentially link the polarity and endocytic machineries.

Cdc42 GTPase Activating Proteins (GAPs) Regulate Generational Inheritance of Cell Polarity and Cell Shape in Fission Yeast

The highly conserved small GTPase Cdc42 regulates polarized cell growth and morphogenesis from yeast to humans. We previously reported that Cdc42 activation exhibits oscillatory dynamics at cell tips of Schizosaccharomyces pombe cells. Mathematical modeling suggests that this dynamic behavior enables a variety of symmetric and asymmetric Cdc42 activation distributions to coexist in cell populations. For individual wild-type cells, however, Cdc42 distribution is initially asymmetrical and becomes more symmetrical as cell volume increases, enabling bipolar growth activation. To explore whether different patterns of Cdc42 activation are possible in vivo, we examined S. pombe rga4∆ mutant cells, lacking the Cdc42 GTPase-activating protein (GAP) Rga4. We found that monopolar rga4∆ mother cells divide asymmetrically leading to the emergence of both symmetric and asymmetric Cdc42 distributions in rga4∆ daughter cells. Motivated by different hypotheses that can mathematically reproduce the unequal fate of daughter cells, we used genetic screening to identify mutants that alter the rga4∆ phenotype. We found that the unequal distribution of active Cdc42 GTPase is consistent with an unequal inheritance of another Cdc42 GAP, Rga6, in the two daughter cells. Our findings highlight the crucial role of Cdc42 GAP localization in maintaining consistent Cdc42 activation and growth patterns across generations.

The Multiple Functions of Rho GTPases in Fission Yeasts

The Rho family of GTPases represents highly conserved molecular switches involved in a plethora of physiological processes. Fission yeast Schizosaccharomyces pombe has become a fundamental model organism to study the functions of Rho GTPases over the past few decades. In recent years, another fission yeast species, Schizosaccharomyces japonicus, has come into focus offering insight into evolutionary changes within the genus. Both fission yeasts contain only six Rho-type GTPases that are spatiotemporally controlled by multiple guanine-nucleotide exchange factors (GEFs) and GTPase-activating proteins (GAPs), and whose intricate regulation in response to external cues is starting to be uncovered. In the present review, we will outline and discuss the current knowledge and recent advances on how the fission yeasts Rho family GTPases regulate essential physiological processes such as morphogenesis and polarity, cellular integrity, cytokinesis and cellular differentiation.

Stress-Activated Protein Kinase Signalling Regulates Mycoparasitic Hyphal-Hyphal Interactions in Trichoderma atroviride

Trichoderma atroviride is a mycoparasitic fungus used as biological control agent against fungal plant pathogens. The recognition and appropriate morphogenetic responses to prey-derived signals are essential for successful mycoparasitism. We established microcolony confrontation assays using T. atroviride strains expressing cell division cycle 42 (Cdc42) and Ras-related C3 botulinum toxin substrate 1 (Rac1) interactive binding (CRIB) reporters to analyse morphogenetic changes and the dynamic displacement of localized GTPase activity during polarized tip growth. Microscopic analyses showed that Trichoderma experiences significant polarity stress when approaching its fungal preys. The perception of prey-derived signals is integrated via the guanosine triphosphatase (GTPase) and mitogen-activated protein kinase (MAPK) signalling network, and deletion of the MAP kinases Trichoderma MAPK 1 (Tmk1) and Tmk3 affected T. atroviride tip polarization, chemotropic growth, and contact-induced morphogenesis so severely that the establishment of mycoparasitism was highly inefficient to impossible. The responses varied depending on the prey species and the interaction stage, reflecting the high selectivity of the signalling process. Our data suggest that Tmk3 affects the polarity-stress adaptation process especially during the pre-contact phase, whereas Tmk1 regulates contact-induced morphogenesis at the early-contact phase. Neither Tmk1 nor Tmk3 loss-of-function could be fully compensated within the GTPase/MAPK signalling network underscoring the crucial importance of a sensitive polarized tip growth apparatus for successful mycoparasitism.

Wake-up alarm: virtual time-lapse gene expression landscape illuminates mechanisms underlying dormancy breaking of germinating spores

Dormancy breaking is a common physiological phenomenon that is shared by eukaryotes. Germination of spores in fungi is one of the most representative cases of dormancy breaking. Understanding the mechanisms of spore germination is therefore fundamental to basic studies on the control of cell proliferation and differentiation, as well as agricultural applications and medical investigation of fungal pathogenesis. In fission yeast, spores are generated as a consequence of sexual differentiation under nutrient starvation, remaining dormant until further nourishment, but little is known about how dormant spores germinate in response to environmental change. In a breakthrough, methods for single-cell-based gene expression profiling have recently been introduced. Several mRNA expression profiles were assembled from single spore cells during dormancy or germination. Single-cell RNA-seq profiles were aligned sequentially according to their similarities. The alignment of transcriptomes visualised how gene expression varies over time upon dormancy breaking. In this review, we revisit knowledge from previous studies on germination, select candidate genes that may be involved in germination, and query their expression from the temporal transcriptomic dataset so that studies on S. pombe germination can be extended further.

Fission yeast Pak1 phosphorylates anillin-like Mid1 for spatial control of cytokinesis

Protein kinases direct polarized growth by regulating the cytoskeleton in time and space and could play similar roles in cell division. We found that the Cdc42-activated polarity kinase Pak1 colocalizes with the assembling contractile actomyosin ring (CAR) and remains at the division site during septation. Mutations in pak1 led to defects in CAR assembly and genetic interactions with cytokinesis mutants. Through a phosphoproteomic screen, we identified novel Pak1 substrates that function in polarized growth and cytokinesis. For cytokinesis, we found that Pak1 regulates the localization of its substrates Mid1 and Cdc15 to the CAR. Mechanistically, Pak1 phosphorylates the Mid1 N-terminus to promote its association with cortical nodes that act as CAR precursors. Defects in Pak1-Mid1 signaling lead to misplaced and defective division planes, but these phenotypes can be rescued by synthetic tethering of Mid1 to cortical nodes. Our work defines a new signaling mechanism driven by a cell polarity kinase that promotes CAR assembly in the correct time and place.

An image analysis method to survey the dynamics of polar protein abundance in the regulation of tip growth

Tip growth is critical for the lifestyle of many walled cells. In yeast and fungi, this process is typically associated with the polarized deposition of conserved tip factors, including landmarks, Rho GTPases, cytoskeleton regulators, and membrane and cell wall remodelers. Because tip growth speeds may vary extensively between life cycles or species, we asked whether the local amount of specific polar elements could determine or limit tip growth speeds. Using the model fission yeast, we developed a quantitative image analysis pipeline to dynamically correlate single tip elongation speeds and polar protein abundance in large data sets. We found that polarity landmarks are typically diluted by growth. In contrast, tip growth speed is positively correlated with the local amount of factors related to actin, secretion or cell wall remodeling, but, surprisingly, exhibits long saturation plateaus above certain concentrations of those factors. Similar saturation observed for Spitzenkörper components in much faster growing fungal hyphae suggests that elements independent of canonical surface remodelers may limit single tip growth. This work provides standardized methods and resources to decipher the complex mechanisms that control cell growth.This article has an associated First Person interview with Sarah Taheraly, joint first author of the paper.

Non-random distribution of vacuoles in Schizosaccharomyces pombe

A central question in eukaryotic cell biology asks, during cell division, how is the growth and distribution of organelles regulated to ensure each daughter cell receives an appropriate amount. For vacuoles in budding yeast, there are well described organelle-to-cell size scaling trends as well as inheritance mechanisms involving highly coordinated movements. It is unclear whether such mechanisms are necessary in the symmetrically dividing fission yeast, Schizosaccharomyces pombe, in which random partitioning may be utilized to distribute vacuoles to daughter cells. To address the increasing need for high-throughput analysis, we are augmenting existing semi-automated image processing by developing fully automated machine learning methods for locating vacuoles and segmenting fission yeast cells from brightfield and fluorescence micrographs. All strains studied show qualitative correlations in vacuole-to-cell size scaling trends, i.e. vacuole volume, surface area, and number all increase with cell size. Furthermore, increasing vacuole number was found to be a consistent mechanism for the increase in total vacuole size in the cell. Vacuoles are not distributed evenly throughout the cell with respect to available cytoplasm. Rather, vacuoles show distinct peaks in distribution close to the nucleus, and this preferential localization was confirmed in mutants in which nucleus position is perturbed. Disruption of microtubules leads to quantitative changes in both vacuole size scaling trends and distribution patterns, indicating the microtubule cytoskeleton is a key mechanism for maintaining vacuole structure.

Activation of Cdc42 GTPase upon CRY2-Induced Cortical Recruitment Is Antagonized by GAPs in Fission Yeast

The small GTPase Cdc42 is critical for cell polarization in eukaryotic cells. In rod-shaped fission yeast Schizosaccharomyces pombe cells, active GTP-bound Cdc42 promotes polarized growth at cell poles, while inactive Cdc42-GDP localizes ubiquitously also along cell sides. Zones of Cdc42 activity are maintained by positive feedback amplification involving the formation of a complex between Cdc42-GTP, the scaffold Scd2, and the guanine nucleotide exchange factor (GEF) Scd1, which promotes the activation of more Cdc42. Here, we use the CRY2-CIB1 optogenetic system to recruit and cluster a cytosolic Cdc42 variant at the plasma membrane and show that this leads to its moderate activation also on cell sides. Surprisingly, Scd2, which binds Cdc42-GTP, is still recruited to CRY2-Cdc42 clusters at cell sides in individual deletion of the GEFs Scd1 or Gef1. We show that activated Cdc42 clusters at cell sides are able to recruit Scd1, dependent on the scaffold Scd2. However, Cdc42 activity is not amplified by positive feedback and does not lead to morphogenetic changes, due to antagonistic activity of the GTPase activating protein Rga4. Thus, the cell architecture is robust to moderate activation of Cdc42 at cell sides.

Stress-activated MAPK signaling controls fission yeast actomyosin ring integrity by modulating formin For3 levels

Cytokinesis, which enables the physical separation of daughter cells once mitosis has been completed, is executed in fungal and animal cells by a contractile actin- and myosin-based ring (CAR). In the fission yeast Schizosaccharomyces pombe, the formin For3 nucleates actin cables and also co-operates for CAR assembly during cytokinesis. Mitogen-activated protein kinases (MAPKs) regulate essential adaptive responses in eukaryotic organisms to environmental changes. We show that the stress-activated protein kinase pathway (SAPK) and its effector, MAPK Sty1, downregulates CAR assembly in S. pombe when its integrity becomes compromised during cytoskeletal damage and stress by reducing For3 levels. Accurate control of For3 levels by the SAPK pathway may thus represent a novel regulatory mechanism of cytokinesis outcome in response to environmental cues. Conversely, SAPK signaling favors CAR assembly and integrity in its close relative Schizosaccharomyces japonicus, revealing a remarkable evolutionary divergence of this response within the fission yeast clade.

Fission Yeast Polarization: Modeling Cdc42 Oscillations, Symmetry Breaking, and Zones of Activation and Inhibition

Cells polarize for growth, motion, or mating through regulation of membrane-bound small GTPases between active GTP-bound and inactive GDP-bound forms. Activators (GEFs, GTP exchange factors) and inhibitors (GAPs, GTPase activating proteins) provide positive and negative feedbacks. We show that a reaction-diffusion model on a curved surface accounts for key features of polarization of model organism fission yeast. The model implements Cdc42 membrane diffusion using measured values for diffusion coefficients and dissociation rates and assumes a limiting GEF pool (proteins Gef1 and Scd1), as in prior models for budding yeast. The model includes two types of GAPs, one representing tip-localized GAPs, such as Rga3; and one representing side-localized GAPs, such as Rga4 and Rga6, that we assume switch between fast and slow diffusing states. After adjustment of unknown rate constants, the model reproduces active Cdc42 zones at cell tips and the pattern of GEF and GAP localization at cell tips and sides. The model reproduces observed tip-to-tip oscillations with periods of the order of several minutes, as well as asymmetric to symmetric oscillations transitions (corresponding to NETO "new end take off"), assuming the limiting GEF amount increases with cell size.

Optogenetics reveals Cdc42 local activation by scaffold-mediated positive feedback and Ras GTPase

Local activity of the small GTPase Cdc42 is critical for cell polarization. Whereas scaffold-mediated positive feedback was proposed to break symmetry of budding yeast cells and produce a single zone of Cdc42 activity, the existence of similar regulation has not been probed in other organisms. Here, we address this problem using rod-shaped cells of fission yeast Schizosaccharomyces pombe, which exhibit zones of active Cdc42-GTP at both cell poles. We implemented the CRY2-CIB1 optogenetic system for acute light-dependent protein recruitment to the plasma membrane, which allowed to directly demonstrate positive feedback. Indeed, optogenetic recruitment of constitutively active Cdc42 leads to co-recruitment of the guanine nucleotide exchange factor (GEF) Scd1 and endogenous Cdc42, in a manner dependent on the scaffold protein Scd2. We show that Scd2 function is dispensable when the positive feedback operates through an engineered interaction between the GEF and a Cdc42 effector, the p21-activated kinase 1 (Pak1). Remarkably, this rewired positive feedback confers viability and allows cells to form 2 zones of active Cdc42 even when otherwise essential Cdc42 activators are lacking. These cells further revealed that the small GTPase Ras1 plays a role in both localizing the GEF Scd1 and promoting its activity, which potentiates the positive feedback. We conclude that scaffold-mediated positive feedback, gated by Ras activity, confers robust polarization for rod-shape formation.

The small GTPase Cdc42 is a key regulator of cell polarization. This study uses optogenetic and genetic strategies to show that Cdc42 is under positive feedback regulation potentiated by Ras GTPase activity.

A toolbox of stable integration vectors in the fission yeast Schizosaccharomyces pombe

Schizosaccharomyces pombe is a widely used model organism to study many aspects of eukaryotic cell physiology. Its popularity as an experimental system partially stems from the ease of genetic manipulations, where the innate homology-targeted repair is exploited to precisely edit the genome. While vectors to incorporate exogenous sequences into the chromosomes are available, most are poorly characterized. Here, we show that commonly used fission yeast vectors, which upon integration produce repetitive genomic regions, give rise to unstable genomic loci. We overcome this problem by designing a new series of stable integration vectors (SIVs) that target four different prototrophy genes. SIVs produce non-repetitive, stable genomic loci and integrate predominantly as single copy. Additionally, we develop a set of complementary auxotrophic alleles that preclude false-positive integration events. We expand the vector series to include antibiotic resistance markers, promoters, fluorescent tags and terminators, and build a highly modular toolbox to introduce heterologous sequences. Finally, as proof of concept, we generate a large set of ready-to-use, fluorescent probes to mark organelles and cellular processes with a wide range of applications in fission yeast research.This article has an associated First Person interview with the first author of the paper.

External signal-mediated polarized growth in fungi

As the majority of fungi are nonmotile, polarized growth in response to an external signal enables them to search for nutrients and mating partners, and hence is crucial for survival and proliferation. Although the mechanisms underlying polarization in response to external signals has commonalities with polarization during mitotic division, during budding, and fission growth, the importance of diverse feedback loops regulating external signal-mediated polarized growth is likely to be distinct and uniquely adapted to a dynamic environment. Here, we highlight recent advances in our understanding of the mechanisms that are crucial for polarity in response to external signals in fungi, with particular focus on the roles of membrane traffic, small GTPases, and lipids, as well as the interplay between cell shape and cell growth.

F-BAR Cdc15 Promotes Cdc42 Activation During Cytokinesis and Cell Polarization in Schizosaccharomyces pombe

Cdc42, a Rho-family GTPase, is a master regulator of cell polarity. Recently, it has been shown that Cdc42 also facilitates proper cytokinesis in the fission yeast Schizosaccharomyces pombe Cdc42 is activated by two partially redundant GEFs, Gef1 and Scd1. Although both GEFs activate Cdc42, their deletion mutants display distinct phenotypes, indicating that they are differentially regulated by an unknown mechanism. During cytokinesis, Gef1 localizes to the division site and activates Cdc42 to initiate ring constriction and septum ingression. Here, we report that the F-BAR protein Cdc15 promotes Gef1 localization to its functional sites. We show that cdc15 promotes Gef1 association with cortical puncta at the incipient division site to activate Cdc42 during ring assembly. Moreover, cdc15 phospho-mutants phenocopy the polarity phenotypes of gef1 mutants. In a hypermorphic cdc15 mutant, Gef1 localizes precociously to the division site and is readily detected at the cortical patches and the cell cortex. Correspondingly, the hypermorphic cdc15 mutant shows increased bipolarity during interphase and precocious Cdc42 activation at the division site during cytokinesis. Finally, loss of gef1 in hypermorphic cdc15 mutants abrogates the increased bipolarity and precocious Cdc42 activation phenotype. We did not see any change in the localization of the other GEF Scd1 in a Cdc15-dependent manner. Our data indicate that Cdc15 facilitates Cdc42 activation at the division site during cytokinesis at the cell cortex to promote bipolarity and this is mediated by promoting Gef1 localization to these sites.

Rho Family GTPases in Fission Yeast Cytokinesis

During cytokinesis, actomyosin ring constriction drives furrow formation. In animal cells, Rho GTPases drive this process through the positioning and assembly of the actomyosin ring, and through extracellular matrix remodeling within the furrow. In the fission yeast S. pombe, actomyosin ring constriction and septum formation are concurrent processes. While S. pombe is the primary source from which the mechanics of ring assembly and constriction stem, much less is known about the regulation of Rho GTPases that control these processes. Of the six Rho GTPases encoded in S. pombe, only Rho1, the RhoA homologue, has been shown to be essential for cytokinesis. While Rho3, Rho4, and Cdc42 have defined roles in cytokinesis, Rho2 and Rho5 play minor to no roles in this process. Here we review the roles of the Rho GTPases during cytokinesis, with a focus on their regulation, and discuss whether crosstalk between GTPases, as has been reported in other organisms, exists during cytokinesis in S. pombe.

Modulation of TOR complex 2 signaling by the stress-activated MAPK pathway in fission yeast

Sin1 is a substrate-binding subunit of target of rapamycin complex 2 (TORC2), an evolutionarily conserved protein kinase complex. In fission yeast, Sin1 has also been identified as a protein that interacts with Spc1 (also known as Sty1) in the stress-activated protein kinase (SAPK) pathway. Therefore, this study examined the relationship between TORC2 and Spc1 signaling. We found that the common docking (CD) domain of Spc1 interacts with a cluster of basic amino acid residues in Sin1. Although diminished TORC2 activity in the absence of the functional Spc1 cascade suggests positive regulation of TORC2 by Spc1, such regulation appears to be independent of the Sin1-Spc1 interaction. Hyperosmotic stress transiently inhibits TORC2, and its swift recovery is dependent on Spc1, the transcription factor Atf1, and the glycelrol-3-phosphate dehydrogenase Gpd1, whose expression is induced upon osmostress by the Spc1-Atf1 pathway. Thus, cellular adaptation to osmostress seems important for TORC2 reactivation, though Spc1 and Atf1 contribute to TORC2 activation also in the absence of osmostress. These results indicate coordinated actions of the SAPK and TORC2 pathways, both of which are essential for fission yeast cells to survive environmental stress.

Autoactivation of small GTPases by the GEF-effector positive feedback modules

Small GTPases are organizers of a plethora of cellular processes. The time and place of their activation are tightly controlled by the localization and activation of their regulators, guanine-nucleotide exchange factors (GEFs) and GTPase-activating proteins (GAPs). Remarkably, in some systems, the upstream regulators of GTPases are also found downstream of their activity. Resulting feedback loops can generate complex spatiotemporal dynamics of GTPases with important functional consequences. Here we discuss the concept of positive autoregulation of small GTPases by the GEF-effector feedback modules and survey recent developments in this exciting area of cell biology.

Conserved NDR/LATS kinase controls RAS GTPase activity to regulate cell growth and chronological lifespan

Adaptation to the nutritional environment is critical for all cells. RAS GTPase is a highly conserved GTP-binding protein with crucial functions for cell growth and differentiation in response to environmental conditions. Here, we describe a novel mechanism connecting RAS GTPase to nutrient availability in fission yeast. We report that the conserved NDR/LATS kinase Orb6 responds to nutritional cues and regulates Ras1 GTPase activity. Orb6 increases the protein levels of an Ras1 GTPase activator, the guanine nucleotide exchange factor Efc25, by phosphorylating Sts5, a protein bound to efc25 mRNA. By manipulating the extent of Orb6-mediated Sts5 assembly into RNP granules, we can modulate Efc25 protein levels, Ras1 GTPase activity, and, as a result, cell growth and cell survival. Thus, we conclude that the Orb6-Sts5-Ras1 regulatory axis plays a crucial role in promoting cell adaptation, balancing the opposing demands of promoting cell growth and extending chronological lifespan.

Functional interaction between Cdc42 and the stress MAPK signaling pathway during the regulation of fission yeast polarized growth

Cell polarization can be defined as the generation and maintenance of directional cellular organization. The spatial distribution and protein or lipid composition of the cell are not symmetric but organized in specialized domains which allow cells to grow and acquire a certain shape that is closely linked to their physiological function. The establishment and maintenance of polarized growth requires the coordination of diverse processes including cytoskeletal dynamics, membrane trafficking, and signaling cascade regulation. Some of the major players involved in the selection and maintenance of sites for polarized growth are Rho GTPases, which recognize the polarization site and transmit the signal to regulatory proteins of the cytoskeleton. Additionally, cytoskeletal organization, polarized secretion, and endocytosis are controlled by signaling pathways including those mediated by mitogen-activated protein kinases (MAPKs). Rho GTPases and the MAPK signaling pathways are strongly conserved from yeast to mammals, suggesting that the basic mechanisms of polarized growth have been maintained throughout evolution. For this reason, the study of how polarized growth is established and regulated in simple organisms such as the fission yeast Schizosaccharomyces pombe has contributed to broaden our knowledge about these processes in multicellular organisms. We review here the function of the Cdc42 GTPase and the stress activated MAPK (SAPK) signaling pathways during fission yeast polarized growth, and discuss the relevance of the crosstalk between both pathways.

Spore Germination Requires Ferrichrome Biosynthesis and the Siderophore Transporter Str1 in Schizosaccharomyces pombe

Spore germination is a process whereby spores exit dormancy to become competent for mitotic cell division. In Schizosaccharomyces pombe, one critical step of germination is the formation of a germ tube that hatches out the spore wall in a stage called outgrowth. Here, we show that iron deficiency blocks the outgrowth of germinating spores. The siderophore synthetase Sib1 and the ornithine N5-oxygenase Sib2 participate in ferrichrome biosynthesis, whereas Str1 functions as a ferrichrome transporter. Expression profiles of sib1+ , sib2+ , and str1+ transcripts reveal that they are induced shortly after induction of germination and their expression remains upregulated throughout the germination program under low-iron conditions. sib1Δ sib2Δ mutant spores are unable to form a germ tube under iron-poor conditions. Supplementation with exogenous ferrichrome suppresses this phenotype when str1+ is present. Str1 localizes at the contour of swollen spores 4 hr after induction of germination. At the onset of outgrowth, localization of Str1 changes and it moves away from the mother spore to primarily localize at the periphery of the new daughter cell. Two conserved Tyr residues (Tyr553 and Tyr567) are predicted to be located in the last extracellular loop region of Str1. Results show that these amino acid residues are critical to ensure timely completion of the outgrowth phase of spores in response to exogenous ferrichrome. Taken together, the results reveal the essential requirement of ferrichrome biosynthesis to promote outgrowth, as well as the necessity to take up ferrichrome from an external source via Str1 when ferrichrome biosynthesis is blocked.

Fission Yeast NDR/LATS Kinase Orb6 Regulates Exocytosis via Phosphorylation of the Exocyst Complex

NDR/LATS kinases regulate multiple aspects of cell polarity and morphogenesis from yeast to mammals. Fission yeast NDR/LATS kinase Orb6 has been proposed to control cell polarity by regulating the Cdc42 guanine nucleotide exchange factor Gef1. Here, we show that Orb6 regulates polarity largely independently of Gef1 and that Orb6 positively regulates exocytosis. Through Orb6 inhibition in vivo and quantitative global phosphoproteomics, we identify Orb6 targets, including proteins involved in membrane trafficking. We confirm Sec3 and Sec5, conserved components of the exocyst complex, as substrates of Orb6 both in vivo and in vitro, and we show that Orb6 kinase activity is important for exocyst localization to cell tips and for exocyst activity during septum dissolution after cytokinesis. We further find that Orb6 phosphorylation of Sec3 contributes to exocyst function in concert with exocyst protein Exo70. We propose that Orb6 contributes to polarized growth by regulating membrane trafficking at multiple levels.

Cellular geometry scaling ensures robust division site positioning

Cells of a specific cell type may divide within a certain size range. Yet, functionally optimal cellular organization is typically maintained across different cell sizes, a phenomenon known as scaling. The mechanisms underlying scaling and its physiological significance remain elusive. Here we approach this problem by interfering with scaling in the rod-shaped fission yeast Schizosaccharomyces japonicus that relies on cellular geometry cues to position the division site. We show that S. japonicus uses the Cdc42 polarity module to adjust its geometry to changes in the cell size. When scaling is prevented resulting in abnormal cellular length-to-width aspect ratio, cells exhibit severe division site placement defects. We further show that despite the generally accepted view, a similar scaling phenomenon can occur in the sister species, Schizosaccharomyces pombe. Our results demonstrate that scaling is required for normal cell function and delineate possible rules for cellular geometry maintenance in populations of proliferating cells.

Cells divide within a given size range and can scale across differing cell sizes but mechanisms and function remain unclear. Here the authors show, despite the current dogma of fission yeast maintaining constant width, some fission yeast can scale their width and length, impacting the positioning of the cell division site.

Differential GAP requirement for Cdc42-GTP polarization during proliferation and sexual reproduction

The formation of a local zone of Cdc42 GTPase activity, which governs cell polarization in many cell types, requires not only local activation but also switch-off mechanisms. In this study, we identify Rga3, a paralog of Rga4, as a novel Cdc42 GTPase-activating protein (GAP) in the fission yeast Schizosaccharomyces pombe Contrary to Rga4, Rga3 localizes with Cdc42-GTP to sites of polarity. Rga3 is dispensable for cell polarization during mitotic growth, but it limits the lifetime of unstable Cdc42-GTP patches that underlie cell pairing during sexual reproduction, masking a partly compensatory patch-wandering motion. In consequence, cells lacking rga3 hyperpolarize and lose out in mating competition. Rga3 synergizes with the Cdc42 GAPs Rga4 and Rga6 to restrict Cdc42-GTP zone sizes during mitotic growth. Surprisingly, triple-mutant cells, which are almost fully round, retain pheromone-dependent dynamic polarization of Cdc42-GTP, extend a polarized projection, and mate. Thus, the requirement for Cdc42-GTP hydrolysis by GAPs is distinct during polarization by intrinsic or extrinsic cues.

UmTea1, a Kelch and BAR domain-containing protein, acts at the cell cortex to regulate cell morphogenesis in the dimorphic fungus Ustilago maydis

The spatial organization of a cell is crucial for distribution of cell components and for cell morphogenesis in all organisms. Ustilago maydis, a basidiomycete fungus, has a yeast-like and a filamentous form. The former buds once per cell cycle at one of the cell poles, and can use the same site repeatedly or choose a new site at the same pole or opposite pole. The filamentous form consists of a long apical cell with short septate basal compartments lacking cytoplasm. It grows at the apex and can reverse growth forming a new growth zone at the basal end. We are interested in understanding how these different morphologies are generated. Here we present identification and characterization of U. maydis Tea1, a homologue of the fission yeast cell end marker Tea1. We demonstrate that UmTea1, a Kelch domain protein, interacts with itself and is an important determinant of the site of polarized growth: tea1 mutants bud simultaneously from both cell poles and form bifurcate buds. UmTea1 also regulates septum positioning, cell wall deposition, cell and neck width, coordination of nuclear division and cell separation, and localization of sterol-rich membrane domains. Some of these functions are shared with UmTea4, another cell end marker. We show that Tea1::GFP localizes to sites of polarized or potential polarized growth and to the septation site in the yeast-like form. Additionally, localization of Tea1::GFP as rings along the filament suggests that the filament undergoes septation. We hypothesize that Tea1 may act as a scaffold for the assembly of proteins that determine the site of polarized growth.