Pob1 participates in the Cdc42 regulation of fission yeast actin cytoskeleton

Fission yeast Bgs1 glucan synthase participates in the control of growth polarity and membrane traffic

Rod-shaped fission yeast grows through cell wall expansion at poles and septum, synthesized by essential glucan synthases. Bgs1 synthesizes the linear β(1,3)glucan of primary septum at cytokinesis. Linear β(1,3)glucan is also present in the wall poles, suggesting additional Bgs1 roles in growth polarity. Our study reveals an essential collaboration between Bgs1 and Tea1-Tea4, but not other polarity factors, in controlling growth polarity. Simultaneous absence of Bgs1 function and Tea1-Tea4 causes complete loss of growth polarity, spread of other glucan synthases, and spherical cell formation, indicating this defect is specifically due to linear β(1,3)glucan absence. Furthermore, linear β(1,3)glucan absence induces actin patches delocalization and sterols spread, which are ultimately responsible for the growth polarity loss without Tea1-Tea4. This suggests strong similarities in Bgs1 functions controlling actin structures during cytokinesis and polarized growth. Collectively, our findings unveil that cell wall β(1,3)glucan regulates polarized growth, like the equivalent extracellular matrix in neuronal cells.

Generation and characterization of conditional yeast mutants affecting each of the 2 essential functions of the scaffolding proteins Boi1/2 and Bem1

Boi1 and Boi2 are closely related yeast scaffolding proteins, either of which can perform an essential function. Previous studies have suggested a role in cell polarity, interacting with lipids, components of the late secretory pathway, and actin nucleators. We report detailed studies of their localization, dynamics, and the generation and characterization of conditional mutants. Boi1/2 are present on the plasma membrane in dynamic patches, then at the bud neck during cytokinesis. These distributions are unaffected by perturbation of the actin cytoskeleton or the secretory pathway. We identify 2 critical aromatic residues, present in both Boi1 and Boi2, in the essential C-terminal Pleckstrin-Homology domain, that cause temperature-sensitive growth resulting in defects in polarized growth leading to cell lysis. The scaffolding protein, Bem1, colocalizes with Boi1 in patches at the growing bud, and at the bud neck, the latter requiring the N-terminal SH3 domain of Boi1p. Loss of function of Boi1-SH3 domain renders Bem1 essential, which can be fully replaced by a fusion of the SH3b and PB1 domains of Bem1. Thus, the 2 essential functions of the Boi1/2/Bem1 proteins can be satisfied by Bem1-SH3b-PB1 and Boi1-Pleckstrin-Homology. Generation and characterization of conditional mutations in the essential function of Bem1 reveal a slow onset of defects in polarized growth, which is difficult to define a specific initial defect. This study provides more details into the functions of Boi1/2 and their relationship with Bem1 and presents the generation of conditional mutants that will be useful for future genetic analysis.

Identification of ksg1 mutation showing long-lived phenotype in fission yeast

Fission yeast is a good model organism for the study of lifespan. To elucidate the mechanism, we screened for long-lived mutants. We found a nonsense mutation in the ksg1⁺ gene, which encodes an ortholog of mammalian PDK1 (phosphoinositide-dependent protein kinase). The mutation was in the PH domain of Ksg1 and caused defect in membrane localization and protein stability. Analysis of the ksg1 mutant revealed that the reduced amounts and/or activity of the Ksg1 protein are responsible for the increased lifespan. Ksg1 is essential for growth and known to phosphorylate multiple substrates, but the substrate responsible for the long-lived phenotype of ksg1 mutation is not yet known. Genetic analysis showed that deletion of pck2 suppressed the long-lived phenotype of ksg1 mutant, suggesting that Pck2 might be involved in the lifespan extension caused by ksg1 mutation.

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.

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.

Negative control of cytokinesis by stress-activated MAPK signaling

Mitogen-activated protein kinase (MAPK) signalling pathways regulate multiple cellular functions in eukaryotic organisms in response to environmental cues, including the dynamic remodeling of the actin cytoskeleton. The fission yeast S. pombe is an optimal model to investigate the conserved regulatory mechanisms of cytokinesis, which relies in an actomyosin-based contractile ring (CAR) that prompts the physical separation of daughter cells during cellular division. Our group has recently shown that p38 MAPK ortholog Sty1, the core component of the stress-activated pathway (SAPK), negatively modulates CAR assembly and integrity in S. pombe during actin cytoskeletal damage induced with Latrunculin A and in response to environmental stress. This response involves downregulation of protein levels of the formin For3, which assembles actin filaments for cables and the CAR, likely through an ubiquitin-mediated degradation mechanism. Contrariwise, Sty1 function positively reinforces CAR assembly during stress in the close relative dimorphic fission yeast S. japonicus. The opposite effect of SAPK signaling on CAR integrity may represent an evolutionary refined adaptation to cope with the marked differences in cytokinesis onset in both fission yeast species.

Roles of the PH, coiled-coil and SAM domains of the yeast polarity protein Boi2 in polarity-site localization and function in polarized growth

Boi1 and Boi2 are paralogous proteins essential for bud formation in budding yeast. So far, the domains that target Boi1/Boi2 to the polarity sites and function in bud formation are not well understood. Here, we report that a coiled-coil domain of Boi2 cooperates with the adjacent PH domain to confer Boi2's bud-cortex localization and major function in cell growth. The PH domain portion of the PH-CC bi-domain interacts with the Rho GTPases Cdc42 and Rho3 and both interactions are independent of the GTP/GDP-bound state of each GTPase. Interestingly, high-copy RHO3 and BOI2 but not CDC42 suppressed the growth defect of RGA1-C538 overexpression and the sec15-1 mutant and this BOI2 function depends on RHO3, suggesting that Boi2 may function in the Rho3 pathway. The SAM domain of Boi2 plays an essential role in high-copy suppression of the two mutants as well as in the early bud-neck localization of Boi2. The SAM domain and the CC domain also interact homotypically. They are likely involved in the formation of Boi2-containing protein complex. Our results provide new insights in the localization and function of Boi2 and highlight the importance of the PH-CC bi-domain and the SAM domain in Boi2's localization and function.

The cell polarity proteins Boi1 and Boi2 direct an actin nucleation complex to sites of exocytosis in Saccharomyces cerevisiae

Owing to the local enrichment of factors that influence its dynamics and organization, the actin cytoskeleton displays different shapes and functions within the same cell. In yeast cells, post-Golgi vesicles ride on long actin cables to the bud tip. The proteins Boi1 and Boi2 (Boi1/2) participate in tethering and docking these vesicles to the plasma membrane. Here, we show in Saccharomyces cerevisiae that Boi1/2 also recruit nucleation and elongation factors to form actin filaments at sites of exocytosis. Disrupting the connection between Boi1/2 and the nucleation factor Bud6 impairs filament formation, reduces the directed movement of the vesicles to the tip and shortens the vesicles' tethering time at the cortex. Transplanting Boi1 from the bud tip to the peroxisomal membrane partially redirects the actin cytoskeleton and the vesicular flow towards the peroxisome, and creates an alternative, rudimentary vesicle-docking zone. We conclude that Boi1/2, through interactions with Bud6 and Bni1, induce the formation of a cortical actin structure that receives and aligns incoming vesicles before fusion with the membrane.

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.

ER-PM Contacts Restrict Exocytic Sites for Polarized Morphogenesis

Spatial control of exocytosis underlies polarized cell morphogenesis. In rod-shaped fission yeast, exocytic vesicles are conveyed along the actin cytoskeleton by myosin V motors toward growing cell ends [1, 2], the major sites for exocytosis. However, actomyosin-based vesicle delivery is dispensable for polarized secretion and cylindrical cell shape of fission yeast [3]. Thus, additional mechanisms should function in the spatial confinement of exocytosis. Here we report a novel role of endoplasmic reticulum (ER)-plasma membrane (PM) contacts in restricting exocytic sites for polarized fission yeast morphogenesis. We show that fission yeast cells deficient in both ER-PM contacts and actomyosin-based secretory vesicle transport display aberrant globular cell shape due to delocalized exocytosis. By artificially manipulating the strength and extent of ER-PM contacts in wild-type and mutant cells that exhibit induced ectopic exocytosis, we demonstrate that exocytosis and ER-PM contact formation are spatially incompatible. Furthermore, extensive ER-PM junctions at the non-growing lateral cell cortex prevent the PM from exocytic vesicle tethering and hence attenuate growth potential at cell sides. We thus propose that ER-PM contacts function as a new morphogenetic module by limiting exocytosis to growing cell tips in fission yeast. A similar mechanism could apply to other cell types with prominent ER-PM contacts.

Evolutionary dynamics in the fungal polarization network, a mechanistic perspective

Polarity establishment underlies proper cell cycle completion across virtually all organisms. Much progress has been made in generating an understanding of the structural and functional components of this process, especially in model species. Here we focus on the evolutionary dynamics of the fungal polarization protein network in order to determine general components and mechanistic principles, species- or lineage-specific adaptations and the evolvability of the network. The currently available genomic and proteomic screens in a variety of fungal species have shown three main characteristics: (1) certain proteins, processes and functions are conserved throughout the fungal clade; (2) orthologous functions can never be assumed, as various cases have been observed of homologous loci with dissimilar functions; (3) species have, typically, various species- or lineage-specific proteins incorporated in their polarization network. Further large-scale comparative and experimental studies, including those on non-model species representing the great fungal diversity, are needed to gain a better understanding of the evolutionary dynamics and generalities of the polarization network in fungi.

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.

Internetwork competition for monomers governs actin cytoskeleton organization

Cells precisely control the formation of dynamic actin cytoskeleton networks to coordinate fundamental processes, including motility, division, endocytosis and polarization. To support these functions, actin filament networks must be assembled, maintained and disassembled at the correct time and place, and with proper filament organization and dynamics. Regulation of the extent of filament network assembly and of filament network organization has been largely attributed to the coordinated activation of actin assembly factors through signalling cascades. Here, we discuss an intriguing model in which actin monomer availability is limiting and competition between homeostatic actin cytoskeletal networks for actin monomers is an additional crucial regulatory mechanism that influences the density and size of different actin networks, thereby contributing to the organization of the cellular actin cytoskeleton.

Sterol-Rich Membrane Domains Define Fission Yeast Cell Polarity

Cell polarization is crucial for the functioning of all organisms. The cytoskeleton is central to the process but its role in symmetry breaking is poorly understood. We study cell polarization when fission yeast cells exit starvation. We show that the basis of polarity generation is de novo sterol biosynthesis, cell surface delivery of sterols, and their recruitment to the cell poles. This involves four phases occurring independent of the polarity factor cdc42p. Initially, multiple, randomly distributed sterol-rich membrane (SRM) domains form at the plasma membrane, independent of the cytoskeleton and cell growth. These domains provide platforms on which the growth and polarity machinery assembles. SRM domains are then polarized by the microtubule-dependent polarity factor tea1p, which prepares for monopolar growth initiation and later switching to bipolar growth. SRM polarization requires F-actin but not the F-actin organizing polarity factors for3p and bud6p. We conclude that SRMs are key to cell polarization.

Rga6 is a Fission Yeast Rho GAP Involved in Cdc42 Regulation of Polarized Growth

Active Cdc42 is essential for the establishment of polarized growth. This GTPase is negatively regulated by the GTPase-activating proteins (GAPs), which are important for the spatial specificity of Cdc42 function. Rga4 is the only GAP described as negative regulator of fission yeast Cdc42. We report here that Rga6 is another fission yeast Cdc42 GAP which shares some functions with Rga4. Cells lacking Rga6 are viable but slightly shorter and broader than wild type, and cells lacking Rga6 and Rga4 simultaneously are rounded. In these cells, active Cdc42 is observed all around the membrane. These additive effects indicate that both GAPs collaborate in the spatial regulation of active Cdc42. Rga6 localizes to the plasma membrane forming clusters different from those formed by Rga4. A polybasic region at the Rga6 C-terminus is responsible for its membrane localization. Rga6-GFP fluorescence decreases considerably at the growing tips, and this decrease is dependent on the actin cables. Notably, in the absence of Rga6 the amplitude of active Cdc42 oscillations at the tips decreases, and less GTP-Cdc42 accumulates at the new end of the cells. We propose here that Rga6 collaborates with Rga4 to spatially restrict active Cdc42 at the cell tips and maintain cell dimensions.

Balance between exocytosis and endocytosis determines the efficacy of sterol-targeting antibiotics

Antifungals targeting membrane ergosterol are longstanding, yet indispensable drugs in clinical use. However, the mechanisms by which the cellular membrane domains recognized by these antibiotics are generated remain largely unknown. Here, we demonstrate that the balance between endocytosis and exocytosis in membrane trafficking is a critical factor in the action of sterol-targeting antibiotics. When fission yeast cells were treated with manumycin A, cellular binding and the action of the antifungals filipin, amphotericin B, and theonellamides, all of which are ergosterol-binders, were abolished. Additionally, manumycin A treatment attenuated Cdc42 activity and inhibited exocytosis, while endocytosis was only moderately suppressed. Similar defects in membrane trafficking could be reproduced by heat shock and genetic perturbation, which also abolished the action of the antibiotics. We propose that exocytosis and endocytosis respectively supply and internalize the specific plasma membrane domains recognized by sterol-targeting antibiotics.

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 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.

Symmetry breaking in spore germination relies on an interplay between polar cap stability and spore wall mechanics

The morphogenesis of single cells depends on their ability to coordinate surface mechanics and polarity. During germination, spores of many species develop a polar tube that hatches out of a rigid outer spore wall (OSW) in a process termed outgrowth. However, how these awakening cells reorganize to stabilize this first growth axis remains unknown. Here, using quantitative experiments and modeling, we reveal the mechanisms underlying outgrowth in fission yeast. We find that, following an isotropic growth phase during which a single polarity cap wanders around the surface, outgrowth occurs when spores have doubled their volume, concomitantly with the stabilization of the cap and a singular rupture in the OSW. This rupture happens when OSW mechanical stress exceeds a threshold, releases the constraints of the OSW on growth, and stabilizes polarity. Thus, outgrowth exemplifies a self-organizing morphogenetic process in which reinforcements between growth and polarity coordinate mechanics and internal organization.

The yeast actin cytoskeleton

The actin cytoskeleton is a complex network of dynamic polymers, which plays an important role in various fundamental cellular processes, including maintenance of cell shape, polarity, cell division, cell migration, endocytosis, vesicular trafficking, and mechanosensation. Precise spatiotemporal assembly and disassembly of actin structures is regulated by the coordinated activity of about 100 highly conserved accessory proteins, which nucleate, elongate, cross-link, and sever actin filaments. Both in vivo studies in a wide range of organisms from yeast to metazoans and in vitro studies of purified proteins have helped shape the current understanding of actin dynamics and function. Molecular genetics, genome-wide functional analysis, sophisticated real-time imaging, and ultrastructural studies in concert with biochemical analysis have made yeast an attractive model to understand the actin cytoskeleton, its molecular dynamics, and physiological function. Studies of the yeast actin cytoskeleton have contributed substantially in defining the universal mechanism regulating actin assembly and disassembly in eukaryotes. Here, we review some of the important insights generated by the study of actin cytoskeleton in two important yeast models the budding yeast Saccharomyces cerevisiae and the fission yeast Schizosaccharomyces pombe.

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.

Hsp70-Hsp40 chaperone complex functions in controlling polarized growth by repressing Hsf1-driven heat stress-associated transcription

How the molecular mechanisms of stress response are integrated at the cellular level remains obscure. Here we show that the cellular polarity machinery in the fission yeast Schizosaccharomyces pombe undergoes dynamic adaptation to thermal stress resulting in a period of decreased Cdc42 activity and altered, monopolar growth. Cells where the heat stress-associated transcription was genetically upregulated exhibit similar growth patterning in the absence of temperature insults. We identify the Ssa2-Mas5/Hsp70-Hsp40 chaperone complex as repressor of the heat shock transcription factor Hsf1. Cells lacking this chaperone activity constitutively activate the heat-stress-associated transcriptional program. Interestingly, they also exhibit intermittent monopolar growth within a physiological temperature range and are unable to adapt to heat stress. We propose that by negatively regulating the heat stress-associated transcription, the Ssa2-Mas5 chaperone system could optimize cellular growth under different temperature regiments.

Heat stress, caused by fluctuations in ambient temperature, occurs frequently in nature. How organisms adapt and maintain regular patterns of growth over a range of environmental conditions remain poorly understood. Our work in the simple unicellular yeast Schizosaccharomyces pombe suggests that the heat stress-associated transcription must be repressed by the evolutionary conserved Hsp70-Hsp40 chaperone complex to allow cells to adapt the polarized growth machinery to elevated temperature.

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.

Myosin Vs organize actin cables in fission yeast

Myosin V motors are believed to contribute to cell polarization by carrying cargoes along actin tracks. In Schizosaccharomyces pombe, Myosin Vs transport secretory vesicles along actin cables, which are dynamic actin bundles assembled by the formin For3 at cell poles. How these flexible structures are able to extend longitudinally in the cell through the dense cytoplasm is unknown. Here we show that in myosin V (myo52 myo51) null cells, actin cables are curled, bundled, and fail to extend into the cell interior. They also exhibit reduced retrograde flow, suggesting that formin-mediated actin assembly is impaired. Myo52 may contribute to actin cable organization by delivering actin regulators to cell poles, as myoV defects are partially suppressed by diverting cargoes toward cell tips onto microtubules with a kinesin 7-Myo52 tail chimera. In addition, Myo52 motor activity may pull on cables to provide the tension necessary for their extension and efficient assembly, as artificially tethering actin cables to the nuclear envelope via a Myo52 motor domain restores actin cable extension and retrograde flow in myoV mutants. Together these in vivo data reveal elements of a self-organizing system in which the motors shape their own tracks by transporting cargoes and exerting physical pulling forces.

Fission yeast: in shape to divide

How are cell morphogenesis and cell cycle coordinated? The fission yeast is a rod-shaped unicellular organism widely used to study how a cell self-organizes in space and time. Here, we discuss recent advances in understanding how the cell acquires and maintains its regular rod shape and uses it to control cell division. The cellular body plan is established by microtubules, which mark antipodal growth zones and medial division. In turn, cellular dimensions are defined by the small GTPase Cdc42 and downstream regulators of vesicle trafficking. Yeast cells then repetitively use their simple rod shape to orchestrate the position and timing of cell division.

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.

Fission yeast sec3 bridges the exocyst complex to the actin cytoskeleton

The exocyst complex tethers post-Golgi secretory vesicles to the plasma membrane prior to docking and fusion. In this study, we identify Sec3, the missing component of the Schizosaccharomyces pombe exocyst complex (SpSec3). SpSec3 shares many properties with its orthologs, and its mutants are rescued by human Sec3/EXOC1. Although involved in exocytosis, SpSec3 does not appear to mark the site of exocyst complex assembly at the plasma membrane. It does, however, mark the sites of actin cytoskeleton recruitment and controls the organization of all three yeast actin structures: the actin cables, endocytic actin patches and actomyosin ring. Specifically, SpSec3 physically interacts with For3 and sec3 mutants have no actin cables as a result of a failure to polarize this nucleating formin. SpSec3 also interacts with actin patch components and sec3 mutants have depolarized actin patches of reduced endocytic capacity. Finally, the constriction and disassembly of the cytokinetic actomyosin ring is compromised in these sec3 mutant cells. We propose that a role of SpSec3 is to spatially couple actin machineries and their independently polarized regulators. As a consequence of its dual role in secretion and actin organization, Sec3 appears as a major co-ordinator of cell morphology in fission yeast.

Cdc42p regulation of the yeast formin Bni1p mediated by the effector Gic2p

Regulation of the formin Bni1p by Cdc42p in yeast does not require direct interaction between Bni1p and Cdc42p. The Cdc42p effector Gic2p can bind both Bni1p and GTP-Cdc42p, providing a novel regulatory input.

Actin filaments are dynamically reorganized to accommodate ever-changing cellular needs for intracellular transport, morphogenesis, and migration. Formins, a major family of actin nucleators, are believed to function as direct effectors of Rho GTPases, such as the polarity regulator Cdc42p. However, the presence of extensive redundancy has made it difficult to assess the in vivo significance of the low-affinity Rho GTPase–formin interaction and specifically whether Cdc42p polarizes the actin cytoskeleton via direct formin binding. Here we exploit a synthetically rewired budding yeast strain to eliminate the redundancy, making regulation of the formin Bni1p by Cdc42p essential for viability. Surprisingly, we find that direct Cdc42p–Bni1p interaction is dispensable for Bni1p regulation. Alternative paths linking Cdc42p and Bni1p via “polarisome” components Spa2p and Bud6p are also collectively dispensable. We identify a novel regulatory input to Bni1p acting through the Cdc42p effector, Gic2p. This pathway is sufficient to localize Bni1p to the sites of Cdc42p action and promotes a polarized actin organization in both rewired and wild-type contexts. We suggest that an indirect mechanism linking Rho GTPases and formins via Rho effectors may provide finer spatiotemporal control for the formin-nucleated actin cytoskeleton.

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.

Cdc42 regulates multiple membrane traffic events in fission yeast

Fission yeast Cdc42 regulates polarized growth and is involved in For3 formin activation and actin cable assembly. We show here that a thermosensitive strain carrying the cdc42L160S allele has membrane traffic defects independent of the actin cable defects. This strain has decreased acid phosphatase (AP) secretion, intracellular accumulation of vesicles and fragmentation of vacuoles. In addition, the exocyst is not localized to the tips of these cells. Overproduction of the scaffold protein Pob1 suppressed cdc42L160S thermosensitive growth and restored exocyst localization and AP secretion. The GTPase Rho3 also suppressed cdc42L160S thermosensitivity, restored exocyst localization and AP secretion. However, Rho3 did not restore the actin cables in these cells as Pob1 does. Similarly, overexpression of psy1(+) , coding a syntaxin (t-SNARE) homolog, or of ypt2(+) , coding an SEC4 homolog in fission yeast, rescued growth at high temperature but did not restore actin cables, nor the exocyst-polarized localization. cdc42L160S cells also have defects in vacuole formation that were rescued by Pob1, Rho3 and Psy1. All together, we propose that Cdc42 and the scaffold Pob1 are required for membrane trafficking and fusion, contributing to polarized secretion, endosome recycling, vacuole formation and growth.

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.

Rga4, a Rho-GAP from fission yeast: Finding specificity within promiscuity

Regulation by signaling molecules of pathways involved in determining cell size and shape is fundamental to understand morphogenesis. In eukaryotic cells, Rho GTPases modulate cellular events by acting as molecular switches. GTPase Activating Proteins (GAPs) control the fine-tuning of Rho GTPase activity as downregulators that promote their inactive state. We use Schizosaccharomyces pombe as a model to unveil key mechanisms underlying processes of general significance. Rga4, one of the nine RhoGAPs present in the fission yeast, is a key factor in the control of cell polarity and morphogenesis by negatively regulating the activity of the essential Rho GTPase Cdc42. We have demonstrated that Rga4 is also a GAP for Rho2 GTPase, which acts upstream of the Pmk1 cell integrity MAP kinase pathway and positively regulates cell integrity and cell separation. Our findings suggest that Rga4 control of both Cdc42 and Rho2 function is rather independent, thus providing a good example of regulatory specificity. Additionally, we describe multiple GAPs that can downregulate Pmk1 activity in a Rho2-dependent and independent fashion. These studies corroborate the existence of a sophisticated regulatory network by which different RhoGAPs modulate differentially the activity of Rho GTPases, and the existence of different inputs for the Pmk1 cell integrity MAP kinase pathway.

Pob1 ensures cylindrical cell shape by coupling two distinct rho signaling events during secretory vesicle targeting

Proper cell morphogenesis requires the co-ordination of cell polarity, cytoskeletal organization and vesicle trafficking. The Schizosaccharomyces pombe mutant pob1-664 has a curious lemon-like shape, the basis of which is not understood. Here, we found abundant vesicle accumulation in these cells, suggesting that Pob1 plays a role in vesicle trafficking. We identified Rho3 as a multicopy suppressor of this phenotype. Because Rho3 function is related to For3, an actin-polymerizing protein, and Sec8, a component of the exocyst complex, we analyzed their functional relationship with Pob1. Pob1 was essential for the formation of actin cables (by interacting with For3) and for the polarized localization of Sec8. Although neither For3 nor Sec8 is essential for polarized growth, their simultaneous disruption prevented tip growth and yielded a lemon-like cell morphology similar to pob1-664. Thus, Pob1 may ensure cylindrical cell shape of S. pombe by coupling actin-mediated vesicle transport and exocyst-mediated vesicle tethering during secretory vesicle targeting.

Actin cables and the exocyst form two independent morphogenesis pathways in the fission yeast

In fission yeast, long-range transport and vesicle tethering by the exocyst are individually dispensable but together essential for cell morphogenesis. Both pathways function downstream of Cdc42. The exocyst localizes to growing cell tips independently of the cytoskeleton and instead depends on PIP₂.

Cell morphogenesis depends on polarized exocytosis. One widely held model posits that long-range transport and exocyst-dependent tethering of exocytic vesicles at the plasma membrane sequentially drive this process. Here, we describe that disruption of either actin-based long-range transport and microtubules or the exocyst did not abolish polarized growth in rod-shaped fission yeast cells. However, disruption of both actin cables and exocyst led to isotropic growth. Exocytic vesicles localized to cell tips in single mutants but were dispersed in double mutants. In contrast, a marker for active Cdc42, a major polarity landmark, localized to discreet cortical sites even in double mutants. Localization and photobleaching studies show that the exocyst subunits Sec6 and Sec8 localize to cell tips largely independently of the actin cytoskeleton, but in a cdc42 and phospholipid phosphatidylinositol 4,5-bisphosphate (PIP₂)–dependent manner. Thus in fission yeast long-range cytoskeletal transport and PIP₂-dependent exocyst represent parallel morphogenetic modules downstream of Cdc42, raising the possibility of similar mechanisms in other cell types.

Three's company: the fission yeast actin cytoskeleton

How the actin cytoskeleton assembles into different structures to drive diverse cellular processes is a fundamental cell biological question. In addition to orchestrating the appropriate combination of regulators and actin-binding proteins, different actin-based structures must insulate themselves from one another to maintain specificity within a crowded cytoplasm. Actin specification is particularly challenging in complex eukaryotes where a multitude of protein isoforms and actin structures operate within the same cell. Fission yeast Schizosaccharomyces pombe possesses a single actin isoform that functions in three distinct structures throughout the cell cycle. In this review we explore recent studies in fission yeast that help unravel how different actin structures operate in cells.

Cooperation between the septins and the actomyosin ring and role of a cell-integrity pathway during cell division in fission yeast

A major question about cytokinesis concerns the role of the septin proteins, which localize to the division site in all animal and fungal cells but are essential for cytokinesis only in some cell types. For example, in Schizosaccharomyces pombe, four septins localize to the division site, but deletion of the four genes produces only a modest delay in cell separation. To ask if the S. pombe septins function redundantly in cytokinesis, we conducted a synthetic-lethal screen in a septin-deficient strain and identified seven mutations. One mutation affects Cdc4, a myosin light chain that is an essential component of the cytokinetic actomyosin ring. Five others cause frequent cell lysis during cell separation and map to two loci. These mutations and their dosage suppressors define a signaling pathway (including Rho1 and a novel arrestin) for repairing cell-wall damage. The seventh mutation affects the poorly understood RNA-binding protein Scw1 and severely delays cell separation when combined either with a septin mutation or with a mutation affecting the septin-interacting, anillin-like protein Mid2, suggesting that Scw1 functions in a pathway parallel to that of the septins. Taken together, our results suggest that the S. pombe septins participate redundantly in one or more pathways that cooperate with the actomyosin ring during cytokinesis and that a septin defect causes septum defects that can be repaired effectively only when the cell-integrity pathway is intact.

Rho and anillin-dependent control of mDia2 localization and function in cytokinesis

Diaphanous-related formin, mDia, is an actin nucleation/polymerization factor functioning downstream of the small GTPase Rho. We found that, in addition to the Rho GTPase-mediated activation, the interaction between mDia2 and anillin is required for the localization and function of mDia2 in cytokinesis.

Diaphanous-related formin, mDia, is an actin nucleation/polymerization factor functioning downstream of the small GTPase Rho. Although Rho is critically involved in cytokinesis, it remains elusive how Rho effectors and other regulators of cytoskeletons work together to accomplish this process. Here we focused on mDia2, an mDia isoform involved in cytokinesis of NIH 3T3 cells, and analyzed mechanisms of its localization in cytokinesis. We found that targeting of mDia2 to the cleavage furrow requires not only its binding to RhoA but also its diaphanous-inhibitory domain (DID). We then performed pulldown assays using a fragment containing the latter domain as a bait and identified anillin as a novel mDia2 interaction partner. The anillin-binding is competitive with the diaphanous autoregulatory domain (DAD) of mDia2 in its autoinhibitory interaction. A series of RNA interference and functional rescue experiments has revealed that, in addition to the Rho GTPase-mediated activation, the interaction between mDia2 and anillin is required for the localization and function of mDia2 in cytokinesis.

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.

Rga4 modulates the activity of the fission yeast cell integrity MAPK pathway by acting as a Rho2 GTPase-activating protein

Rho GTPase-activating proteins (GAPs) are responsible for the inactivation of Rho GTPases, which are involved in the regulation of critical biological responses in eukaryotic cells, ranging from cell cycle control to cellular morphogenesis. The genome of fission yeast Schizosaccharomyces pombe contains six genes coding for putative Rho GTPases, whereas nine genes code for predicted Rho GAPs (Rga1 to Rga9). One of them, Rga4, has been recently described as a Cdc42 GAP, involved in the control of cell diameter and symmetry in fission yeast. In this work we show that Rga4 is also a Rho2 GAP that negatively modulates the activity of the cell integrity pathway and its main effector, MAPK Pmk1. The DYRK-type protein kinase Pom1, which regulates both the localization and phosphorylation state of Rga4, is also a negative regulator of the Pmk1 pathway, but this control is not dependent upon the Rga4 role as a Rho2-GAP. Hence, two subsets of Rga4 negatively regulate Cdc42 and Rho2 functions in a specific and unrelated way. Finally, we show that Rga7, another Rho2 GAP, down-regulates the Pmk1 pathway in addition to Rga4. These results reinforce the notion of the existence of complex mechanisms determining the selectivity of Rho GAPs toward Rho GTPases and their functions.