Gef1p and Scd1p, the Two GDP-GTP exchange factors for Cdc42p, form a ring structure that shrinks during cytokinesis in Schizosaccharomyces pombe

Rho1 and Rgf1 establish a new actin-dependent signal to determine growth poles in yeast independently of microtubules and the Tea1-Tea4 complex

Cellular asymmetry begins with the selection of a discrete point on the cell surface that triggers Rho-GTPases activation and localized assembly of the cytoskeleton to establish new growth zones. The cylindrical shape of fission yeast is organized by microtubules (MT) that deliver the landmark Tea1-Tea4 complex at the cell tips to define the growth poles. However, only a few tea1Δ cells mistaken the direction of growth, indicating that they manage to detect their growth sites. Here, we show that Rgf1 (Rho1-GEF) and Tea4 are components of the same complex and that Rgf1 activity toward Rho1 is required for strengthen Tea4 at the cell tips. Moreover, in cells lacking Tea1, selection of the correct growth site depends on Rgf1 and on a correctly polarized actin cytoskeleton, both necessary for Rho1 activation at the pole. We propose an actin-dependent mechanism driven by Rgf1-Rho1 that marks the poles independently of MTs and the Tea1-Tea4 complex.

Metabolic stress-induced long ncRNA transcription governs the formation of meiotic DNA breaks in the fission yeast fbp1 gene

Meiotic recombination is a pivotal process that ensures faithful chromosome segregation and contributes to the generation of genetic diversity in offspring, which is initiated by the formation of double-strand breaks (DSBs). The distribution of meiotic DSBs is not uniform and is clustered at hotspots, which can be affected by environmental conditions. Here, we show that non-coding RNA (ncRNA) transcription creates meiotic DSBs through local chromatin remodeling in the fission yeast fbp1 gene. The fbp1 gene is activated upon glucose starvation stress, in which a cascade of ncRNA-transcription in the fbp1 upstream region converts the chromatin configuration into an open structure, leading to the subsequent binding of transcription factors. We examined the distribution of meiotic DSBs around the fbp1 upstream region in the presence and absence of glucose and observed several new DSBs after chromatin conversion under glucose starvation conditions. Moreover, these DSBs disappeared when cis-elements required for ncRNA transcription were mutated. These results indicate that ncRNA transcription creates meiotic DSBs in response to stress conditions in the fbp1 upstream region. This study addressed part of a long-standing unresolved mechanism underlying meiotic recombination plasticity in response to environmental fluctuation.

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.

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.

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.

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.

lncRNA transcription induces meiotic recombination through chromatin remodelling in fission yeast

Noncoding RNAs (ncRNAs) are involved in various biological processes, including gene expression, development, and disease. Here, we identify a novel consensus sequence of a cis-element involved in long ncRNA (lncRNA) transcription and demonstrate that lncRNA transcription from this cis-element activates meiotic recombination via chromatin remodeling. In the fission yeast fbp1 gene, glucose starvation induces a series of promoter-associated lncRNAs, referred to as metabolic-stress-induced lncRNAs (mlonRNAs), which contribute to chromatin remodeling and fbp1 activation. Translocation of the cis-element required for mlonRNA into a well-characterized meiotic recombination hotspot, ade6-M26, further stimulates transcription and meiotic recombination via local chromatin remodeling. The consensus sequence of this cis-element (mlon-box) overlaps with meiotic recombination sites in the fission yeast genome. At one such site, the SPBC24C6.09c upstream region, meiotic double-strand break (DSB) formation is induced in an mlon-box-dependent manner. Therefore, mlonRNA transcription plays a universal role in chromatin remodeling and the regulation of transcription and recombination.

Topoisomerase activity is linked to altered nucleosome positioning and transcriptional regulation in the fission yeast fbp1 gene

Chromatin structure, including nucleosome positioning, has a fundamental role in transcriptional regulation through influencing protein-DNA interactions. DNA topology is known to influence chromatin structure, and in doing so, can also alter transcription. However, detailed mechanism(s) linking transcriptional regulation events to chromatin structure that is regulated by changes in DNA topology remain to be well defined. Here we demonstrate that nucleosome positioning and transcriptional output from the fission yeast fbp1 and prp3 genes are altered by excess topoisomerase activity. Given that lncRNAs (long noncoding RNAs) are transcribed from the fbp1 upstream region and are important for fbp1 gene expression, we hypothesized that local changes in DNA topological state caused by topoisomerase activity could alter lncRNA and fbp1 transcription. In support of this, we found that topoisomerase overexpression caused destabilization of positioned nucleosomes within the fbp1 promoter region, which was accompanied by aberrant fbp1 transcription. Similarly, the direct recruitment of topoisomerase, but not a catalytically inactive form, to the promoter region of fbp1 caused local changes in nucleosome positioning that was also accompanied by altered fbp1 transcription. These data indicate that changes in DNA topological state induced by topoisomerase activity could lead to altered fbp1 transcription through modulating nucleosome positioning.

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.

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.

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.

Coordinating septum formation and the actomyosin ring during cytokinesis in Schizosaccharomyces pombe

During cytokinesis, animal and fungal cells form a membrane furrow via actomyosin ring constriction. Our understanding of actomyosin ring-driven cytokinesis stems extensively from the fission yeast model system. However, unlike animal cells, actomyosin ring constriction occurs simultaneously with septum formation in fungi. While the formation of an actomyosin ring is essential for cytokinesis in fission yeast, proper furrow formation also requires septum deposition. The molecular mechanisms of spatiotemporal coordination of septum deposition with actomyosin ring constriction are poorly understood. Although the role of the actomyosin ring as a mechanical structure driving furrow formation is better understood, its role as a spatiotemporal landmark for septum deposition is not widely discussed. Here we review and discuss the recent advances describing how the actomyosin ring spatiotemporally regulates membrane traffic to promote septum-driven cytokinesis in fission yeast. Finally, we explore emerging questions in cytokinesis, and discuss the role of extracellular matrix during cytokinesis in other organisms.

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.

A Cdc42 GEF, Gef1, through endocytosis organizes F-BAR Cdc15 along the actomyosin ring and promotes concentric furrowing

During cytokinesis, fission yeast coordinates actomyosin ring constriction with septum ingression, resulting in concentric furrow formation by a poorly defined mechanism. We report that Schizosaccharomyces pombe cells lacking the Cdc42 activator Gef1, combined with an activated allele of the formin, Cdc12, display non-concentric furrowing. Non-concentrically furrowing cells display uneven distribution of the scaffold Cdc15 along the ring. This suggests that, after ring assembly, uniform Cdc15 distribution along the ring enables proper furrow formation. We find that, after assembly, Cdc15 is recruited to the ring in an Arp2/3 complex-dependent manner and is decreased in the activated cdc12 mutant. Cdc15 at cortical endocytic patches shows increased levels and extended lifetimes in gef1 and activated cdc12 mutants. We hypothesize endocytosis helps recruit Cdc15 to assembled rings; uneven Cdc15 distribution at the ring occurs when endocytic patches contain increased Cdc15 levels and the patch-association rate is slow. Based on this, we developed a mathematical model that captures experimentally observed Cdc15 distributions along the ring. We propose that, at the ring, Gef1 and endocytic events promote uniform Cdc15 organization to enable proper septum ingression and concentric furrow formation.

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.

Local and global Cdc42 guanine nucleotide exchange factors for fission yeast cell polarity are coordinated by microtubules and the Tea1-Tea4-Pom1 axis

The conserved Rho-family GTPase Cdc42 plays a central role in eukaryotic cell polarity. The rod-shaped fission yeast Schizosaccharomyces pombe has two Cdc42 guanine nucleotide exchange factors (GEFs), Scd1 and Gef1, but little is known about how they are coordinated in polarized growth. Although the microtubule cytoskeleton is normally not required for polarity maintenance in fission yeast, we show here that when scd1 function is compromised, disruption of microtubules or the polarity landmark proteins Tea1, Tea4 or Pom1 leads to disruption of polarized growth. Instead, cells adopt an isotropic-like pattern of growth, which we term PORTLI growth. Surprisingly, PORTLI growth is caused by spatially inappropriate activity of Gef1. Although most Cdc42 GEFs are membrane associated, we find that Gef1 is a broadly distributed cytosolic protein rather than a membrane-associated protein at cell tips like Scd1. Microtubules and the Tea1–Tea4–Pom1 axis counteract inappropriate Gef1 activity by regulating the localization of the Cdc42 GTPase-activating protein Rga4. Our results suggest a new model of fission yeast cell polarity regulation, involving coordination of ‘local’ (Scd1) and ‘global’ (Gef1) Cdc42 GEFs via microtubules and microtubule-dependent polarity landmarks.

Highlighted Article: Cell polarity in fission yeast is regulated by two different Cdc42 guanine nucleotide exchange factors, coordinated by the microtubule-dependent landmark system.

Cdc42 activation state affects its localization and protein levels in fission yeast

Rho GTPases control polarized cell growth and are well-known regulators of exocytic and endocytic processes. Cdc42 is an essential GTPase, conserved from yeast to humans, that is critical for cell polarization. Cdc42 is negatively regulated by the GTPase-activating proteins (GAPs) and the GDP dissociation inhibitors (GDIs), and positively regulated by guanine nucleotide exchange factors (GEFs). Cdc42 GTPase can be found in a GTP- or GDP-bound state, which determines the ability to bind downstream effector proteins and activate signalling pathways. Only GTP-bound Cdc42 is active. In this study we have analysed the localization of the different nucleotide-bound states of Cdc42 in Schizosaccharomyces pombe: the wild-type Cdc42 protein that cycles between an active and inactive form, the Cdc42G12V form that is permanently bound to GTP and the Cdc42T17N form that is constitutively inactive. Our results indicate that Cdc42 localizes to several membrane compartments in the cell and this localization is mediated by its C-terminal prenylation. Constitutively active Cdc42 localizes mainly to the plasma membrane and concentrates at the growing tips where it is considerably less dynamic than wild-type or GDP-bound Cdc42. Additionally we show that the activation state of Cdc42 also participates in the regulation of its protein levels mediated by endocytosis and by the exocyst complex.

Unique spatiotemporal activation pattern of Cdc42 by Gef1 and Scd1 promotes different events during cytokinesis

The Rho-family GTPase Cdc42 regulates cell polarity and localizes to the cell division site. Cdc42 is activated by guanine nucleotide exchange factors (GEFs). We report that Cdc42 promotes cytokinesis via a unique spatiotemporal activation pattern due to the distinct action of its GEFs, Gef1 and Scd1, in fission yeast. Before cytokinetic ring constriction, Cdc42 activation, is Gef1 dependent, and after ring constriction, it is Scd1 dependent. Gef1 localizes to the actomyosin ring immediately after ring assembly and promotes timely onset of ring constriction. Gef1 is required for proper actin organization during cytokinesis, distribution of type V myosin Myo52 to the division site, and timely recruitment of septum protein Bgs1. In contrast, Scd1 localizes to the broader region of ingressing membrane during cytokinetic furrowing. Scd1 promotes normal septum formation, andscd1Δcells display aberrant septa with reduced Bgs1 localization. Thus we define unique roles of the GEFs Gef1 and Scd1 in the regulation of distinct events during cytokinesis. Gef1 localizes first to the cytokinetic ring and promotes timely constriction, whereas Scd1 localizes later to the ingressing membrane and promotes septum formation. Our findings are consistent with reports that complexity in GTPase signaling patterns enables exquisite precision over the control of cellular processes.

Spontaneous cell polarization: Feedback control of Cdc42 GTPase breaks cellular symmetry

Spontaneous polarization without spatial cues, or symmetry breaking, is a fundamental problem of spatial organization in biological systems. This question has been extensively studied using yeast models, which revealed the central role of the small GTPase switch Cdc42. Active Cdc42-GTP forms a coherent patch at the cell cortex, thought to result from amplification of a small initial stochastic inhomogeneity through positive feedback mechanisms, which induces cell polarization. Here, I review and discuss the mechanisms of Cdc42 activity self-amplification and dynamic turnover. A robust Cdc42 patch is formed through the combined effects of Cdc42 activity promoting its own activation and active Cdc42-GTP displaying reduced membrane detachment and lateral diffusion compared to inactive Cdc42-GDP. I argue the role of the actin cytoskeleton in symmetry breaking is not primarily to transport Cdc42 to the active site. Finally, negative feedback and competition mechanisms serve to control the number of polarization sites.

Spontaneous Cdc42 polarization independent of GDI-mediated extraction and actin-based trafficking

The small Rho-family GTPase Cdc42 is critical for cell polarization and polarizes spontaneously in absence of upstream spatial cues. Spontaneous polarization is thought to require dynamic Cdc42 recycling through Guanine nucleotide Dissociation Inhibitor (GDI)-mediated membrane extraction and vesicle trafficking. Here, we describe a functional fluorescent Cdc42 allele in fission yeast, which demonstrates Cdc42 dynamics and polarization independent of these pathways. Furthermore, an engineered Cdc42 allele targeted to the membrane independently of these recycling pathways by an amphipathic helix is viable and polarizes spontaneously to multiple sites in fission and budding yeasts. We show that Cdc42 is highly mobile at the membrane and accumulates at sites of activity, where it displays slower mobility. By contrast, a near-immobile transmembrane domain-containing Cdc42 allele supports viability and polarized activity, but does not accumulate at sites of activity. We propose that Cdc42 activation, enhanced by positive feedback, leads to its local accumulation by capture of fast-diffusing inactive molecules.

This study of fission yeast reveals that the active and inactive forms of the small GTPase Cdc42 have different rates of lateral diffusion in the membrane, providing insights into how it becomes spontaneously polarized, thereby determining the polarity of the cell.

Cell polarization is a critical feature of most cells that underlies their functional organization. A central polarity factor called Cdc42, a small GTPase targeted to the plasma membrane by prenylation, promotes cell polarization in its active GTP-bound form. Cdc42 is a key polarity factor because it accumulates at presumptive sites of polarity, which previous work suggested involves Cdc42 recycling on and off the plasma membrane. In addition, its activity can spontaneously polarize cells in a single location by self-enhancing positive feedback mechanisms, even in the absence of any pre-localized landmarks. In this study, we constructed the first functional fluorescently tagged allele of Cdc42 that replaces the endogenous genomic copy in Schizosaccharomyces pombe. This allowed measurements of Cdc42 dynamics at the plasma membrane by live microscopy. Unexpectedly, this approach revealed that Cdc42 primarily moves through lateral diffusion, rather than on and off the plasma membrane. Engineered Cdc42 alleles with alternative membrane-targeting mechanisms demonstrated that Cdc42 activity, indeed, polarizes in the absence of known pathways that recycle Cdc42 on and off the membrane. We further show that the active form, Cdc42-GTP, is less mobile than Cdc42-GDP. We thus propose that Cdc42 polarization occurs as a consequence of its local activation—either through self-enhanced feedback or in response to upstream cues—by a reduction in the active Cdc42 diffusion rate.

Oscillatory AAA+ ATPase Knk1 constitutes a novel morphogenetic pathway in fission yeast

Cellular morphogenesis relies partly on cell polarization by the cytoskeleton. In the fission yeast Schizosaccharomyces pombe, it is well established that microtubules (MTs) deliver the spatial cue Tea1, a kelch repeat protein, to the tip regions to direct the growth machinery at the cell tips driving the linear extension of the rod-shaped organism to maintain a straight long axis. Here, we report the characterization of Knk1 (kink), a previously unidentified member of the superfamily of ATPases associated with various cellular activities (AAA(+)), whose deletion causes a unique morphological defect characterized by the formation of kinks close to cell tips. Through genetic analysis, we place Knk1 into a novel pathway controlling cell shape independently of MTs and Tea1. Knk1 localizes at cell tips. Its localization is mediated by the Knk1 N terminus and is enhanced upon ATP binding to the C-terminal ATPase domain. Furthermore, Knk1 tip recruitment is regulated by SRC-like adaptor 2 (Sla2) and cell division cycle 42 (Cdc42) independently of Sla2's role in endocytosis. Finally, we discovered that Knk1 shows an anticorrelated oscillatory behavior between the two cell tips at a periodicity that is different from the reported oscillatory Cdc42 dynamics.

The Rho-GEF Gef3 interacts with the septin complex and activates the GTPase Rho4 during fission yeast cytokinesis

Rho GTPases, activated by Rho guanine nucleotide exchange factors (GEFs), are conserved molecular switches for signal transductions that regulate diverse cellular processes, including cell polarization and cytokinesis. The fission yeast Schizosaccharomyces pombe has six Rho GTPases (Cdc42 and Rho1-Rho5) and seven Rho GEFs (Scd1, Rgf1-Rgf3, and Gef1-Gef3). The GEFs for Rho2-Rho5 have not been unequivocally assigned. In particular, Gef3, the smallest Rho GEF, was barely studied. Here we show that Gef3 colocalizes with septins at the cell equator. Gef3 physically interacts with septins and anillin Mid2 and depends on them to localize. Gef3 coprecipitates with GDP-bound Rho4 in vitro and accelerates nucleotide exchange of Rho4, suggesting that Gef3 is a GEF for Rho4. Consistently, Gef3 and Rho4 are in the same genetic pathways to regulate septum formation and/or cell separation. In gef3∆ cells, the localizations of two potential Rho4 effectors--glucanases Eng1 and Agn1--are abnormal, and active Rho4 level is reduced, indicating that Gef3 is involved in Rho4 activation in vivo. Moreover, overexpression of active Rho4 or Eng1 rescues the septation defects of mutants containing gef3∆. Together our data support that Gef3 interacts with the septin complex and activates Rho4 GTPase as a Rho GEF for septation in fission yeast.

The putative exchange factor Gef3p interacts with Rho3p GTPase and the septin ring during cytokinesis in fission yeast

The small GTP-binding proteins of the Rho family and its regulatory proteins play a central role in cytokinetic actomyosin ring assembly and cytokinesis. Here we show that the fission yeast guanine nucleotide exchange factor Gef3p interacts with Rho3p at the division site. Gef3p contains a putative DH homology domain and a BAR/IMD-like domain. The protein localized to the division site late in mitosis, where it formed a ring that did not constrict with actomyosin ring (cytokinetic actomyosin ring) invagination; instead, it split into a double ring that resembled the septin ring. Gef3p co-localized with septins and Mid2p and required septins and Mid2p for its localization. Gef3p interacts physically with the GTP-bound form of Rho3p. Although Gef3p is not essential for cell separation, the simultaneous disruption of gef3(+) and Rho3p-interacting proteins, such as Sec8p, an exocyst component, Apm1p, a subunit of the clathrin adaptor complex or For3p, an actin-polymerizing protein, yielded cells with strong defects in septation and polarity respectively. Our results suggest that interactions between septins and Rho-GEFs provide a new targeting mechanism for GTPases in cytokinesis, in this case probably contributing to Rho3p function in vesicle tethering and vesicle trafficking in the later steps of cell separation.

Cdc42 regulates polarized growth and cell integrity in fission yeast

Polarized cell growth requires a well-orchestrated number of events, namely selection of growth site, organization of cytoskeleton elements and delivery of new material to the growth region. The small Rho GTPase Cdc42 has emerged as a major organizer of polarized growth through its participation in many of these events. In the present short review, we focus on the regulation of Cdc42 activity and localization as well as how it controls downstream events necessary for polarized cell growth in Schizosaccharomyces pombe. Owing to the high level of similarity of the polarity pathways, analogies between fission yeast and other model systems can be useful to decipher how cells can actively define their shape by polarized growth.

Yeast as a model for Ras signalling

For centuries yeast species have been popular hosts for classical biotechnology processes, such as baking, brewing, and wine making, and more recently for recombinant proteins production, thanks to the advantages of unicellular organisms (i.e., ease of genetic manipulation and rapid growth) together with the ability to perform eukaryotic posttranslational modifications. Moreover, yeast cells have been used for few decades as a tool for identifying the genes and pathways involved in basic cellular processes such as the cell cycle, aging, and stress response. In the budding yeast S. cerevisiae the Ras/cAMP/PKA pathway is directly involved in the regulation of metabolism, cell growth, stress resistance, and proliferation in response to the availability of nutrients and in the adaptation to glucose, controlling cytosolic cAMP levels and consequently the cAMP-dependent protein kinase (PKA) activity. Moreover, Ras signalling has been identified in several pathogenic yeasts as a key controller for virulence, due to its involvement in yeast morphogenesis. Nowadays, yeasts are still useful for Ras-like proteins investigation, both as model organisms and as a test tube to study variants of heterologous Ras-like proteins.

The coordination of cell growth during fission yeast mating requires Ras1-GTP hydrolysis

The spatial and temporal control of polarity is fundamental to the survival of all organisms. Cells define their polarity using highly conserved mechanisms that frequently rely upon the action of small GTPases, such as Ras and Cdc42. Schizosaccharomyces pombe is an ideal system with which to study the control of cell polarity since it grows from defined tips using Cdc42-mediated actin remodeling. Here we have investigated the importance of Ras1-GTPase activity for the coordination of polarized cell growth during fission yeast mating. Following pheromone stimulation, Ras1 regulates both a MAPK cascade and the activity of Cdc42 to enable uni-directional cell growth towards a potential mating partner. Like all GTPases, when bound to GTP, Ras1 adopts an active conformation returning to an inactive state upon GTP-hydrolysis, a process accelerated through interaction with negative regulators such as GAPs. Here we show that, at low levels of pheromone stimulation, loss of negative regulation of Ras1 increases signal transduction via the MAPK cascade. However, at the higher concentrations observed during mating, hyperactive Ras1 mutations promote cell death. We demonstrate that these cells die due to their failure to coordinate active Cdc42 into a single growth zone resulting in disorganized actin deposition and unsustainable elongation from multiple tips. These results provide a striking demonstration that the deactivation stage of Ras signaling is fundamentally important in modulating cell polarity.

Cooperation between Rho-GEF Gef2 and its binding partner Nod1 in the regulation of fission yeast cytokinesis

Previous results showed that putative Rho-GEF Gef2 regulates division-site positioning during early cytokinesis in fission yeast. Here Nod1 is identified as a binding partner of Gef2. The two proteins form a complex to regulate division-site positioning and contractile-ring maintenance. In addition, Gef2 binds to GTPases Rho1, Rho4, and Rho5 in vitro.

Cytokinesis is the last step of the cell-division cycle, which requires precise spatial and temporal regulation to ensure genetic stability. Rho guanine nucleotide exchange factors (Rho GEFs) and Rho GTPases are among the key regulators of cytokinesis. We previously found that putative Rho-GEF Gef2 coordinates with Polo kinase Plo1 to control the medial cortical localization of anillin-like protein Mid1 in fission yeast. Here we show that an adaptor protein, Nod1, colocalizes with Gef2 in the contractile ring and its precursor cortical nodes. Like gef2∆, nod1∆ has strong genetic interactions with various cytokinesis mutants involved in division-site positioning, suggesting a role of Nod1 in early cytokinesis. We find that Nod1 and Gef2 interact through the C-termini, which is important for their localization. The contractile-ring localization of Nod1 and Gef2 also depends on the interaction between Nod1 and the F-BAR protein Cdc15, where the Nod1/Gef2 complex plays a role in contractile-ring maintenance and affects the septation initiation network. Moreover, Gef2 binds to purified GTPases Rho1, Rho4, and Rho5 in vitro. Taken together, our data indicate that Nod1 and Gef2 function cooperatively in a protein complex to regulate fission yeast cytokinesis.

Cdc42 regulation of polarized traffic in fission yeast

Cdc42 is a key factor in the control of cell polarity and morphogenesis. Fission yeast Cdc42 regulates formin activation and actin cable assembly. Cdc42 is also required for exocyst function, contributing to polarized secretion. Additionally, Cdc42 participates in membrane trafficking, endosome recycling, and vacuole formation. We show here how Cdc42 is required for the correct transport/recycling to the plasma membrane of the glucan synthases Bgs1 and Bgs4, responsible of cell wall biosynthesis and polarized growth at the cell tips.

Regulation of fission yeast morphogenesis by PP2A activator pta2

Cell polarization is key for the function of most eukaryotic cells, and regulates cell shape, migration and tissue architecture. Fission yeast, Schizosaccharomyces pombe cells are cylindrical and polarize cell growth to one or both cell tips dependent on the cell cycle stage. Whereas microtubule cytoskeleton contributes to the positioning of the growth sites by delivering polarity factors to the cell ends, the Cdc42 GTPase polarizes secretion via actin-dependent delivery and tethering of secretory vesicles to plasma membrane. How growth is restricted to cell tips and how re-initiation of tip growth is regulated in the cell cycle remains poorly understood. In this work we investigated the function of protein phosphatase type 2A (PP2A) in S. pombe morphogenesis by deleting the evolutionary conserved PTPA-type regulatory subunit that we named pta2. pta2-deleted cells showed morphological defects and altered growth pattern. Consistent with this, actin patches and active Cdc42 were mislocalized in the pta2 deletion. These defects were additive to the lack of Cdc42-GAP Rga4. pta2Δ cells show upregulated Cdc42 activity and pta2 interacts genetically with polarisome components Tea1, Tea4 and For3 leading to complete loss of cell polarity and rounded morphology. Thus, regulation of polarity by PP2A requires the polarisome and involves Pta2-dependent control of Cdc42 activity.

Roles of putative Rho-GEF Gef2 in division-site positioning and contractile-ring function in fission yeast cytokinesis

How Rho-GEFs and Rho GTPases regulate division-site selection during cytokinesis in fission yeast is unknown. The Rho-GEF Gef2 interacts with the anillin Mid1 to regulate contractile-ring positioning and assembly in coordination with the polo kinase Plo1. In addition, Gef2 is involved in contractile-ring stability and disassembly.

Cytokinesis is crucial for integrating genome inheritance and cell functions. In multicellular organisms, Rho-guanine nucleotide exchange factors (GEFs) and Rho GTPases are key regulators of division-plane specification and contractile-ring formation during cytokinesis, but how they regulate early steps of cytokinesis in fission yeast remains largely unknown. Here we show that putative Rho-GEF Gef2 and Polo kinase Plo1 coordinate to control the medial cortical localization and function of anillin-related protein Mid1. The division-site positioning defects of gef2∆ plo1-ts18 double mutant can be partially rescued by increasing Mid1 levels. We find that Gef2 physically interacts with the Mid1 N-terminus and modulates Mid1 cortical binding. Gef2 localization to cortical nodes and the contractile ring depends on its last 145 residues, and the DBL-homology domain is important for its function in cytokinesis. Our data suggest the interaction between Rho-GEFs and anillins is an important step in the signaling pathways during cytokinesis. In addition, Gef2 also regulates contractile-ring function late in cytokinesis and may negatively regulate the septation initiation network. Collectively, we propose that Gef2 facilitates and stabilizes Mid1 binding to the medial cortex, where the localized Mid1 specifies the division site and induces contractile-ring assembly.

De novo growth zone formation from fission yeast spheroplasts

Eukaryotic cells often form polarized growth zones in response to internal or external cues. To understand the establishment of growth zones with specific dimensions we used fission yeast, which grows as a rod-shaped cell of near-constant width from growth zones located at the cell tips. Removing the cell wall creates a round spheroplast with a disorganized cytoskeleton and depolarized growth proteins. As spheroplasts recover, new growth zones form that resemble normal growing cell tips in shape and width, and polarized growth resumes. Regulators of the GTPase Cdc42, which control width in exponentially growing cells, also control spheroplast growth zone width. During recovery the Cdc42 scaffold Scd2 forms a polarized patch in the rounded spheroplast, demonstrating that a growth zone protein can organize independent of cell shape. Rga4, a Cdc42 GTPase activating protein (GAP) that is excluded from cell tips, is initially distributed throughout the spheroplast membrane, but is excluded from the growth zone after a stable patch of Scd2 forms. These results provide evidence that growth zones with normal width and protein localization can form de novo through sequential organization of cellular domains, and that the size of these growth zones is genetically controlled, independent of preexisting cell shape.

Spatial control of Cdc42 activation determines cell width in fission yeast

The fission yeast Schizosaccharomyces pombe is a rod-shaped cell that grows by linear extension at the cell tips, with a nearly constant width throughout the cell cycle. This simple geometry makes it an ideal system for studying the control of cellular dimensions. In this study, we carried out a near-genome-wide screen for mutants wider than wild-type cells. We found 11 deletion mutants that were wider; seven of the deleted genes are implicated in the control of the small GTPase Cdc42, including the Cdc42 guanine nucleotide exchange factor (GEF) Scd1 and the Cdc42 GTPase-activating protein (GAP) Rga4. Deletions of rga4 and scd1 had additive effects on cell width, and the proteins localized independently of one another, with Rga4 located at the cell sides and Scd1 at the cell tips. Activated Cdc42 localization is altered in rga4Δ, scd1Δ, and scd2Δ mutants. Delocalization and ectopic retargeting experiments showed that the localizations of Rga4 and Scd1 are crucial for their roles in determining cell width. We propose that the GAP Rga4 and the GEF Scd1 establish a gradient of activated Cdc42 within the cellular tip plasma membrane, and it is this gradient that determines cell growth-zone size and normal cell width.

Shaping fission yeast with microtubules

For cell morphogenesis, the cell must establish distinct spatial domains at specified locations at the cell surface. Here, we review the molecular mechanisms of cell polarity in the fission yeast Schizosaccharomyces pombe. These are simple rod-shaped cells that form cortical domains at cell tips for cell growth and at the cell middle for cytokinesis. In both cases, microtubule-based systems help to shape the cell by breaking symmetry, providing endogenous spatial cues to position these sites. The plus ends of dynamic microtubules deliver polarity factors to the cell tips, leading to local activation of the GTPase cdc42p and the actin assembly machinery. Microtubule bundles contribute to positioning the division plane through the nucleus and the cytokinesis factor mid1p. Recent advances illustrate how the spatial and temporal regulation of cell polarization integrates many elements, including historical landmarks, positive and negative controls, and competition between pathways.

Rho GTPases: regulation of cell polarity and growth in yeasts

Eukaryotic cells display a wide range of morphologies important for cellular function and development. A particular cell shape is made via the generation of asymmetry in the organization of cytoskeletal elements, usually leading to actin localization at sites of growth. The Rho family of GTPases is present in all eukaryotic cells, from yeast to mammals, and their role as key regulators in the signalling pathways that control actin organization and morphogenetic processes is well known. In the present review we will discuss the role of Rho GTPases as regulators of yeasts' polarized growth, their mechanism of activation and signalling pathways in Saccharomyces cerevisiae and Schizosaccharomyces pombe. These two model yeasts have been very useful in the study of the molecular mechanisms responsible for cell polarity. As in other organisms with cell walls, yeast's polarized growth is closely related to cell-wall biosynthesis, and Rho GTPases are critical modulators of this process. They provide the co-ordinated regulation of cell-wall biosynthetic enzymes and actin organization required to maintain cell integrity during vegetative growth.

Pob1 participates in the Cdc42 regulation of fission yeast actin cytoskeleton

Rho GTPases regulate the actin cytoskeleton in all eukaryotes. Fission yeast Cdc42 is involved in actin cable assembly and formin For3 regulation. We isolated cdc42-879 as a thermosensitive strain with actin cable and For3 localization defects. In a multicopy suppressor screening, we identified pob1(+) as suppressor of cdc42-879 thermosensitivity. Pob1 overexpression also partially restores actin cables and localization of For3 in the mutant strain. Pob1 interacts with Cdc42 and this GTPase regulates Pob1 localization and/or stability. The C-terminal pleckstrin homology (PH) domain of Pob1 is required for Cdc42 binding. Pob1 also binds to For3 through its N-terminal sterile alpha motif (SAM) domain and contributes to the formin localization at the cell tips. The previously described pob1-664 mutant strain (Mol. Biol. Cell. 10, 2745-2757, 1999), which carries a mutation in the PH domain, as well as pob1 mutant strains in which Pob1 lacks the N-terminal region (pob1DeltaN) or the SAM domain (pob1DeltaSAM), have cytoskeletal defects similar to that of cdc42-879 cells. Expression of constitutively active For3DAD* partially restores actin organization in cdc42-879, pob1-664, pob1DeltaN, and pob1DeltaSAM. Therefore, we propose that Pob1 is required for For3 localization to the tips and facilitates Cdc42-mediated relief of For3 autoinhibition to stimulate actin cable formation.

Microtubule-dependent cell morphogenesis in the fission yeast

In many systems, microtubules contribute spatial information to cell morphogenesis, for instance in cell migration and division. In rod-shaped fission yeast cells, microtubules control cell morphogenesis by transporting polarity factors, namely the Tea1-Tea4 complex, to cell tips. This complex then recruits the DYRK kinase Pom1 to cell ends. Interestingly, recent work has shown that these proteins also provide long-range spatial cues to position the division site in the middle of the cell and temporal signals to coordinate cell length with the cell cycle. Here I review how these microtubule-associated proteins form polar morphogenesis centers that control and integrate both spatial and temporal aspects of cell morphogenesis.

Schizosaccharomyces pombe Pxl1 is a paxillin homologue that modulates Rho1 activity and participates in cytokinesis

Schizosaccharomyces pombe Rho GTPases regulate actin cytoskeleton organization and cell integrity. We studied the fission yeast gene SPBC4F6.12 based on its ability to suppress the thermosensitivity of cdc42-1625 mutant strain. This gene, named pxl1(+), encodes a protein with three LIM domains that is similar to paxillin. Pxl1 does not interact with Cdc42 but it interacts with Rho1, and it negatively regulates this GTPase. Fission yeast Pxl1 forms a contractile ring in the cell division region and deletion of pxl1(+) causes a delay in cell-cell separation, suggesting that it has a function in cytokinesis. Pxl1 N-terminal region is required and sufficient for its localization to the medial ring, whereas the LIM domains are necessary for its function. Pxl1 localization requires actin polymerization and the actomyosin ring, but it is independent of the septation initiation network (SIN) function. Moreover, Pxl1 colocalizes and interacts with Myo2, and Cdc15, suggesting that it is part of the actomyosin ring. Here, we show that in cells lacking Pxl1, the myosin ring is not correctly assembled and that actomyosin ring contraction is delayed. Together, these data suggest that Pxl1 modulates Rho1 GTPase signaling and plays a role in the formation and contraction of the actomyosin ring during cytokinesis.

A Ras1-Cdc24 signal transduction pathway mediates thermotolerance in the fungal pathogen Cryptococcus neoformans

Pathogenic microorganisms must precisely regulate morphogenesis to survive and proliferate within an infected host. This regulation is often controlled by conserved signal transduction pathways that direct morphological changes in varied species. One such pathway, whose components include Ras proteins and the PAK kinase Ste20, allows the human fungal pathogen Cryptococcus neoformans to grow at high temperature. Previously, we found that Ras1 signalling is required for differentiation, thermotolerance and pathogenesis in C. neoformans. We show here that the guanine nucleotide exchange factor Cdc24 is a Ras1 effector in C. neoformans to mediate the ability of this fungus to grow at high temperature and to cause disease. In addition, we provide evidence that the Ras1-Cdc24 signalling cascade functions specifically through one of the three Cdc42/Rac1 homologues in C. neoformans. In conclusion, our studies illustrate how components of conserved signalling cascades can be specialized for different downstream functions, such as pathogenesis.

Hob3p, the fission yeast ortholog of human BIN3, localizes Cdc42p to the division site and regulates cytokinesis

Cdc42 GTPase is required for polarization in eukaryotic cells, but its spatial regulation is poorly understood. In Schizosaccharomyces pombe, Cdc42p is activated by Scd1p and Gef1p, two guanine-nucleotide exchange factors. Two-hybrid screening identified Hob3p as a Gef1p binding partner. Hob3p is a BAR domain-containing protein ortholog of human Bin3. Hob3p also interacts directly with Cdc42p independently of Gef1p. Hob3p, Cdc42p and Gef1p form a complex, and Hob3p facilitates Gef1p-Cdc42p interaction and activation. Hob3p forms a ring in the division area, similar to that of Gef1p. This localization requires actin polymerization and Cdc15p but is independent of the septation initiation network. Hob3p is required for the concentration of Cdc42p to the division area. The actomyosin ring contraction is slower in hob3Delta than in wild-type cells, and this contributes to its cytokinesis defect. Moreover, this report extends previous evidence that human Bin3 suppresses the cytokinesis phenotype of hob3Delta cells, showing that Bin3 can partially recover the GTP-Cdc42p level and its localization. These results suggest that Hob3p is required to recruit and activate Cdc42p at the cell division site and that this function might be conserved in other eukaryotes.

Role of Rho GTPases and Rho-GEFs in the regulation of cell shape and integrity in fission yeast

The Rho family of GTPases are highly conserved molecular switches that control some of the most fundamental processes of cell biology, including morphogenesis, vesicular transport, cell division and motility. Guanine nucleotide-exchange factors (GEFs) are directly responsible for the activation of Rho-family GTPases in response to extracellular stimuli. In fission yeast, there are seven Dbl-related GEFs and they activate six Rho-type GTPases within a particular spatio-temporal context. The failure to do so might have consequences reflected in aberrant phenotypes and in some cases lead to cell death. In this review, we briefly summarize the role of Rho GTPases and Rho-GEFs in the establishment and maintenance of cell polarity and cell integrity in Schizosaccharomyces pombe.

Rho1-GEFs Rgf1 and Rgf2 are involved in formation of cell wall and septum, while Rgf3 is involved in cytokinesis in fission yeast

The Rho GTPase acts as a binary molecular switch by converting between a GDP-bound inactive and a GTP-bound active conformational state. The guanine nucleotide exchange factors (GEFs) are critical activators of Rho. Rho1 has been shown to regulate actin cytoskeleton and cell wall synthesis in the fission yeast Schizosaccharomyces pombe. Here we studied function of fission yeast RhoGEFs, Rgf1, Rgf2, and Rgf3. It was shown that these proteins have similar molecular structures, and function as GEFs for Rho1. Disruption of either rgf1 or rgf2 did not show a serious effect on the cell. On the other hand, disruption of rgf3 caused severe defects in contractile ring formation, F-actin patch localization, and septation during cytokinesis. Rgf1 and Rgf2 were localized to the cell ends during interphase and the septum. Rgf3 formed a ring at the division site, which was located outside the contractile ring and inside the septum where Rho1 was accumulated. In summary, Rgf1 and Rgf2 show functional redundancy, and roles of these RhoGEFs are likely to be different from that of Rgf3. Rho1 is likely to be activated by Rgf3 at the division site, and involved in contractile ring formation and/or maintenance and septation.

Rgf1p is a specific Rho1-GEF that coordinates cell polarization with cell wall biogenesis in fission yeast

Rho1p regulates cell integrity by controlling the actin cytoskeleton and cell wall synthesis. We have identified a new GEF, designated Rgf1p, which specifically regulates Rho1p during polarized growth. The phenotype of rgf1 null cells was very similar to that seen after depletion of Rho1p, 30% of cells being lysed. In addition, rgf1(+) deletion caused hypersensitivity to the antifungal drug Caspofungin and defects in the establishment of bipolar growth. rho1(+), but none of the other GTPases of the Rho-family, suppressed the rgf1Delta phenotypes. Moreover, deletion of rgf1(+) suppressed the severe growth defect in rga1(+) null mutants (a Rho1-GAP, negative regulator). Rgf1p and Rho1p coimmunoprecipitated and overexpression of rgf1(+) specifically increased the GTP-bound Rho1p; it caused changes in cell morphology, and a large increase in beta(1,3)-glucan synthase activity. These effects were similar to those elicited when the hyperactive rho1-G15V allele was expressed. A genetic relationship was observed between Rgf1p, Bgs4p (beta[1,3]-glucan synthase), and Pck1p (protein kinase C [PKC] homologue); Bgs4p and Pck1p suppressed the hypersensitivity to Caspofungin in rgf1Delta mutants. Rgf1p localized to the growing ends and the septum, where Rho1, Pck1p, and Bgs4p are known to function. Our results suggest that Rgf1p probably activates the Rho functions necessary for coordinating actin deposition with cell wall biosynthesis during bipolar growth, allowing the cells to remodel their wall without risk of rupture.

Cell wall remodeling at the fission yeast cell division site requires the Rho-GEF Rgf3p

Cytokinesis in Schizosaccharomyces pombe is accompanied by several stages of cell wall remodeling at the division site. Coincident with actomyosin ring constriction, primary and secondary septa are deposited and then the primary septum is degraded to release daughter cells from one another. These steps require the activities of glucan synthases and glucanases, respectively, which must be coordinated with one another to prevent cell lysis. The lad1-1 mutation undergoes cell lysis specifically at cell division owing to the absence of the Rgf3p Rho1-guanine nucleotide exchange factor (GEF) at the division site. Electron microscopic analysis indicates that lysis occurs only as the primary septum begins to be degraded. Overproduction of either Rho1p or the previously uncharacterized Rab-GTPase-activating protein (GAP) involved in secretion, Gyp10p, suppresses lad1-1 lethality. Rgf3p is periodically produced in an Ace2p-dependent manner and localizes to the medial region of the cell early in mitosis, a pattern of expression distinct from the highly related Rho-GEF, Rgf1p. Although rgf1+ is not an essential gene, it is synthetically lethal with rgf2-deleted cells whereas no negative genetic interactions were detected between rgf2-deleted cells and lad1-1. Our data suggest that the three closely related fission yeast Rho-GEF molecules perform two distinct essential functions. Rgf3p appears necessary to stimulate Rho1p-mediated activation of a glucan synthase crucial after septation for proper new cell-end formation.

Second thoughts on septation by the fission yeast, Schizosaccharomyces pombe: pull vs. push mechanisms with an appendix--dimensional modelling of the flat and variable septa

The correlation of contraction by an actomyosin band with the closing of the septum of dividing cells of the fission yeast, Schizosaccharomyces pombe, cannot suggest cause-and-effect because contraction would be apparent whether the membrane enveloping the centripetally closing septum were pulled or were pushed. Thus the common observation of contraction is not critical. Diagrams of published electron micrographs of dividing wild-type fission yeasts illustrate variable (tilted) septal images that are counterintuitive to a pull model. Circumference calculations based on those images suggest that some variable forms might be only 6% closed even though their two-dimensional profiles would be 50% closed, if they were not tilted. Development of multiseptate forms of cdc4-8 and cdc4-377 temperature sensitive mutants incubated at their restrictive temperature was followed. These multiseptate forms are shown to have functional (functional in terms of generating divided uninucleate cytoplasts) but grotesque septa which are formed in the absence of actomyosin bands. By contrast, the myosin of the plant phragmoplast is not properly oriented for contractility, and Dictyostelium (attached cells) and Saccharomyces (mutants) have been shown to divide in the absence of myosin II, just as S. pombe does (above). Hence contractility, the essence of a pull model for septum closure, would seem to be non-essential. Other, non-contractile mechanisms of myosin are emphasized, and a push model becomes a rational default hypothesis. The essence of push models is that their synthesis/assembly mechanisms are driving force sufficient for septum closure.

Schizosaccharomyces pombe Rgf3p is a specific Rho1 GEF that regulates cell wall beta-glucan biosynthesis through the GTPase Rho1p

Rho1p regulates cell integrity by controlling the actin cytoskeleton and cell-wall synthesis. Here, we describe the cloning and characterization of rgf3+, a member of the Rho family of guanine nucleotide exchange factors (Rho GEFs). The rgf3+ gene was cloned by complementation of a mutant (ehs2-1) hypersensitive to drugs that interfere with cell-wall biosynthesis. The rgf3+ gene was found to be essential for cell viability and depletion of Rgf3p afforded phenotypes similar to those obtained following depletion of Rho1p. However, the cell death caused by Rgf3p depletion could be rescued by the presence of 1.2 M sorbitol, whereas depletion of Rho1 was lethal under the same conditions. We show that Rgf3p is a specific Rho1-GEF. The hypersensitivity to drugs affecting the cell wall of the ehs2-1 mutant was suppressed by overexpression of rho1+ but not by any of the other GTPases of the Rho family. Rgf3p interacted with the GDP-bound form of Rho1p and promoted the GDP-GTP exchange. In addition, we show that overexpression of Rgf3p produces multiseptated cells and increases beta-1,3-glucan synthase activity and the amount of cell wall beta-1,3-glucan. Rgf3p localized to the septum and the mRNA level was regulated in a cell-cycle-dependent manner peaking during septation. Our results suggest that Rgf3p acts as a positive activator of Rho1p, probably activating the Rho functions that coordinate cell-wall biosynthesis to maintain cell integrity during septation.

Role of guanine nucleotide exchange factors for Rho family GTPases in the regulation of cell morphology and actin cytoskeleton in fission yeast

Rho GTPases regulate fundamental processes including cell morphology and migration in various organisms. Guanine nucleotide exchange factor (GEF) has a crucial role in activating small GTPase by exchange GDP for GTP. In fission yeast Schizosaccharomyces pombe, six members of the Rho small GTPase family were identified and reported to be involved in cell morphology and polarized cell growth. We identified seven genes encoding Rho GEF domain from genome sequence and analyzed. Overexpressions of identified genes in cell lead to change of morphology, suggesting that all of them are involved in the regulation of cell morphology. Although all of null mutants were viable, two of seven null cells had morphology defects and five of seven displayed altered actin cytoskeleton arrangements. Most of the double mutants were viable and biochemical analysis revealed that each of GEFs bound to several small G proteins. These data suggest that identified Rho GEFs are involved in the regulation of cell morphology and share signals via small GTPase Rho family.