Cell Polarity in Yeast

The roles of yeast formins and their regulators Bud6 and Bil2 in the pheromone response

In response to pheromone Saccharomyces cerevisiae extend a mating projection. This process depends on the formation of polarized actin cables which direct secretion to the mating tip and translocate the nucleus for karyogamy. Here, we demonstrate that proper mating projection formation requires the formin Bni1, as well as the actin nucleation promoting activities of Bud6, but not the formin Bnr1. Further, Bni1 is required for pheromone gradient tracking. Our work also reveals unexpected new functions for Bil2 in the pheromone response. Previously we identified Bil2 as a direct inhibitor of Bnr1 during vegetative cell growth. Here, we show that Bil2 has Bnr1-independent functions in spatially focusing Bni1-GFP at mating projection tips, and in vitro Bil2 and its binding partner Bud6 organize Bni1 into clusters that nucleate actin assembly. bil2∆ cells also display entangled Bni1-generated actin cable arrays and defects in secretory vesicle transport and nuclear positioning. At low pheromone concentrations, bil2∆ cells are delayed in establishing a polarity axis, and at high concentrations they prematurely form a second and a third mating projection. Together, these results suggest that Bil2 promotes the proper formation and timing of mating projections by organizing Bni1 and maintaining a persistent axis of polarized growth.

Tools for live-cell imaging of cytoskeletal and nuclear behavior in the unconventional yeast, Aureobasidium pullulans

Aureobasidium pullulans is a ubiquitous fungus with a wide variety of morphologies and growth modes including "typical" single-budding yeast, and interestingly, larger multinucleate yeast than can make multiple buds in a single cell cycle. The study of A. pullulans promises to uncover novel cell biology, but currently tools are lacking to achieve this goal. Here, we describe initial components of a cell biology toolkit for A. pullulans, which is used to express and image fluorescent probes for nuclei as well as components of the cytoskeleton. These tools allowed live-cell imaging of the multinucleate and multibudding cycles, revealing highly synchronous mitoses in multinucleate yeast that occur in a semiopen manner with an intact but permeable nuclear envelope. These findings open the door to using this ubiquitous polyextremotolerant fungus as a model for evolutionary cell biology.

Patterning of the cell cortex by Rho GTPases

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

A Natural Small Molecule Mitigates Kidney Fibrosis by Targeting Cdc42-mediated GSK-3β/β-catenin Signaling

Kidney fibrosis is a common fate of chronic kidney diseases (CKDs), eventually leading to renal dysfunction. Yet, no effective treatment for this pathological process has been achieved. During the bioassay-guided chemical investigation of the medicinal plant Wikstroemia chamaedaphne, a daphne diterpenoid, daphnepedunin A (DA), is characterized as a promising anti-renal fibrotic lead. DA shows significant anti-kidney fibrosis effects in cultured renal fibroblasts and unilateral ureteral obstructed mice, being more potent than the clinical trial drug pirfenidone. Leveraging the thermal proteome profiling strategy, cell division cycle 42 (Cdc42) is identified as the direct target of DA. Mechanistically, DA targets to reduce Cdc42 activity and down-regulates its downstream phospho-protein kinase Cζ(p-PKCζ)/phospho-glycogen synthase kinase-3β (p-GSK-3β), thereby promoting β-catenin Ser33/37/Thr41 phosphorylation and ubiquitin-dependent proteolysis to block classical pro-fibrotic β-catenin signaling. These findings suggest that Cdc42 is a promising therapeutic target for kidney fibrosis, and highlight DA as a potent Cdc42 inhibitor for combating CKDs.

Study of Impacts of Two Types of Cellular Aging on the Yeast Bud Morphogenesis

Understanding the mechanisms of cellular aging processes is crucial for attempting to extend organismal lifespan and for studying age-related degenerative diseases. Yeast cells divide through budding, providing a classical biological model for studying cellular aging. With their powerful genetics, relatively short lifespan and well-established signaling pathways also found in animals, yeast cells offer valuable insights into the aging process. Recent experiments suggested the existence of two aging modes in yeast characterized by nucleolar and mitochondrial declines, respectively. In this study, by analyzing experimental data it was shown that cells evolving into those two aging modes behave differently when they are young. While buds grow linearly in both modes, cells that consistently generate spherical buds throughout their lifespan demonstrate greater efficacy in controlling bud size and growth rate at young ages. A three-dimensional chemical-mechanical model was developed and used to suggest and test hypothesized mechanisms of bud morphogenesis during aging. Experimentally calibrated simulations showed that tubular bud shape in one aging mode could be generated by locally inserting new materials at the bud tip guided by the polarized Cdc42 signal during the early stage of budding. Furthermore, the aspect ratio of the tubular bud could be stabilized during the late stage, as observed in experiments, through a reduction on the new cell surface material insertion or an expansion of the polarization site. Thus model simulations suggest the maintenance of new cell surface material insertion or chemical signal polarization could be weakened due to cellular aging in yeast and other cell types.

Turing pattern formation on the sphere is robust to the removal of a hole

The formation of buds on the cell membrane of budding yeast cells is thought to be driven by reactions and diffusion involving the protein Cdc42. These processes can be described by a coupled system of partial differential equations known as the Schnakenberg system. The Schnakenberg system is known to exhibit diffusion-driven pattern formation, thus providing a mechanism for bud formation. However, it is not known how the accumulation of bud scars on the cell membrane affect the ability of the Schnakenberg system to form patterns. We have approached this problem by modelling a bud scar on the cell membrane with a hole on the sphere. We have studied how the spectrum of the Laplace-Beltrami operator, which determines the resulting pattern, is affected by the size of the hole, and by numerically solving the Schnakenberg system on a sphere with a hole using the finite element method. Both theoretical predictions and numerical solutions show that pattern formation is robust to the introduction of a bud scar of considerable size, which lends credence to the hypothesis that bud formation is driven by diffusion-driven instability.

The cell morphological diversity of Saccharomycotina yeasts

The ∼1 200 known species in subphylum Saccharomycotina are a highly diverse clade of unicellular fungi. During its lifecycle, a typical yeast exhibits multiple cell types with various morphologies; these morphologies vary across Saccharomycotina species. Here, we synthesize the evolutionary dimensions of variation in cellular morphology of yeasts across the subphylum, focusing on variation in cell shape, cell size, type of budding, and filament production. Examination of 332 representative species across the subphylum revealed that the most common budding cell shapes are ovoid, spherical, and ellipsoidal, and that their average length and width is 5.6 µm and 3.6 µm, respectively. 58.4% of yeast species examined can produce filamentous cells, and 87.3% of species reproduce asexually by multilateral budding, which does not require utilization of cell polarity for mitosis. Interestingly, ∼1.8% of species examined have not been observed to produce budding cells, but rather only produce filaments of septate hyphae and/or pseudohyphae. 76.9% of yeast species examined have sexual cycle descriptions, with most producing one to four ascospores that are most commonly hat-shaped (37.4%). Systematic description of yeast cellular morphological diversity and reconstruction of its evolution promises to enrich our understanding of the evolutionary cell biology of this major fungal lineage.

A precise balance of TETRASPANIN1/TORNADO2 activity is required for vascular proliferation and ground tissue patterning in Arabidopsis

The molecular mechanisms guiding oriented cell divisions in the root vascular tissues of Arabidopsis thaliana are still poorly characterised. By overlapping bulk and single-cell transcriptomic datasets, we unveiled TETRASPANIN1 (TET1) as a putative regulator in this process. TET1 is expressed in root vascular cells, and loss-of-function mutants contain fewer vascular cell files. We further generated and characterised a CRISPR deletion mutant and showed, unlike previously described mutants, that the full knock out is additionally missing endodermal cells in a stochastic way. Finally, we show that HA-tagged versions of TET1 are functional in contrast to fluorescent TET1 translational fusions. Immunostaining using HA-TET1 lines complementing the mutant phenotype suggested a dual plasma membrane and intracellular localisation in the root vasculature and a polar membrane localisation in the young cortex, endodermal and initial cells. Taken together, we show that TET1 is involved in both vascular proliferation and ground tissue patterning. Our initial results pave the way for future work to decipher its precise mode of action.

Transient septin sumoylation steers a Fir1-Skt5 protein complex between the split septin ring

Ubiquitylation and phosphorylation control composition and architecture of the cell separation machinery in yeast and other eukaryotes. The significance of septin sumoylation on cell separation remained an enigma. Septins form an hourglass structure at the bud neck of yeast cells that transforms into a split septin double ring during mitosis. We discovered that sumoylated septins recruit the cytokinesis checkpoint protein Fir1 to the peripheral side of the septin hourglass just before its transformation into the double-ring configuration. As this transition occurs, Fir1 is released from the septins and seamlessly relocates between the split septin rings through synchronized binding to the scaffold Spa2. Fir1 binds and carries the membrane-bound Skt5 on its route to the division plane where the Fir1-Skt5 complex serves as receptor for chitin synthase III.

Coarsening and wavelength selection far from equilibrium: A unifying framework based on singular perturbation theory

Intracellular protein patterns are described by (nearly) mass-conserving reaction-diffusion systems. While these patterns initially form out of a homogeneous steady state due to the well-understood Turing instability, no general theory exists for the dynamics of fully nonlinear patterns. We develop a unifying theory for nonlinear wavelength-selection dynamics in (nearly) mass-conserving two-component reaction-diffusion systems independent of the specific mathematical model chosen. Previous work has shown that these systems support an extremely broad band of stable wavelengths, but the mechanism by which a specific wavelength is selected has remained unclear. We show that an interrupted coarsening process selects the wavelength at the threshold to stability. Based on the physical intuition that coarsening is driven by competition for mass and interrupted by weak source terms that break strict mass conservation, we develop a singular perturbation theory for the stability of stationary patterns. The resulting closed-form analytical expressions enable us to quantitatively predict the coarsening dynamics and the final pattern wavelength. We find excellent agreement with numerical results throughout the diffusion- and reaction-limited regimes of the dynamics, including the crossover region. Further, we show how, in these limits, the two-component reaction-diffusion systems map to generalized Cahn-Hilliard and conserved Allen-Cahn dynamics, therefore providing a link to these two fundamental scalar field theories. The systematic understanding of the length-scale dynamics of fully nonlinear patterns in two-component systems provided here builds the basis to reveal the mechanisms underlying wavelength selection in multicomponent systems with potentially several conservation laws.

Membrane microdomains: Structural and signaling platforms for establishing membrane polarity

Cell polarity results from the asymmetric distribution of cellular structures, molecules, and functions. Polarity is a fundamental cellular trait that can determine the orientation of cell division, the formation of particular cell shapes, and ultimately the development of a multicellular body. To maintain the distinct asymmetric distribution of proteins and lipids in cellular membranes, plant cells have developed complex trafficking and regulatory mechanisms. Major advances have been made in our understanding of how membrane microdomains influence the asymmetric distribution of proteins and lipids. In this review, we first give an overview of cell polarity. Next, we discuss current knowledge concerning membrane microdomains and their roles as structural and signaling platforms to establish and maintain membrane polarity, with a special focus on the asymmetric distribution of proteins and lipids, and advanced microscopy techniques to observe and characterize membrane microdomains. Finally, we review recent advances regarding membrane trafficking in cell polarity establishment and how the balance between exocytosis and endocytosis affects membrane polarity.

A Protein-Protein Interaction Analysis Suggests a Wide Range of New Functions for the p21-Activated Kinase (PAK) Ste20

The p21-activated kinases (PAKs) are important signaling proteins. They contribute to a surprisingly wide range of cellular processes and play critical roles in a number of human diseases including cancer, neurological disorders and cardiac diseases. To get a better understanding of PAK functions, mechanisms and integration of various cellular activities, we screened for proteins that bind to the budding yeast PAK Ste20 as an example, using the split-ubiquitin technique. We identified 56 proteins, most of them not described previously as Ste20 interactors. The proteins fall into a small number of functional categories such as vesicle transport and translation. We analyzed the roles of Ste20 in glucose metabolism and gene expression further. Ste20 has a well-established role in the adaptation to changing environmental conditions through the stimulation of mitogen-activated protein kinase (MAPK) pathways which eventually leads to transcription factor activation. This includes filamentous growth, an adaptation to nutrient depletion. Here we show that Ste20 also induces filamentous growth through interaction with nuclear proteins such as Sac3, Ctk1 and Hmt1, key regulators of gene expression. Combining our observations and the data published by others, we suggest that Ste20 has several new and unexpected functions.

Redundancy and the role of protein copy numbers in the cell polarization machinery of budding yeast

How can a self-organized cellular function evolve, adapt to perturbations, and acquire new sub-functions? To make progress in answering these basic questions of evolutionary cell biology, we analyze, as a concrete example, the cell polarity machinery of Saccharomyces cerevisiae. This cellular module exhibits an intriguing resilience: it remains operational under genetic perturbations and recovers quickly and reproducibly from the deletion of one of its key components. Using a combination of modeling, conceptual theory, and experiments, we propose that multiple, redundant self-organization mechanisms coexist within the protein network underlying cell polarization and are responsible for the module's resilience and adaptability. Based on our mechanistic understanding of polarity establishment, we hypothesize that scaffold proteins, by introducing new connections in the existing network, can increase the redundancy of mechanisms and thus increase the evolvability of other network components. Moreover, our work gives a perspective on how a complex, redundant cellular module might have evolved from a more rudimental ancestral form.

Particle-based simulations reveal two positive feedback loops allow relocation and stabilization of the polarity site during yeast mating

Many cells adjust the direction of polarized growth or migration in response to external directional cues. The yeast Saccharomyces cerevisiae orient their cell fronts (also called polarity sites) up pheromone gradients in the course of mating. However, the initial polarity site is often not oriented towards the eventual mating partner, and cells relocate the polarity site in an indecisive manner before developing a stable orientation. During this reorientation phase, the polarity site displays erratic assembly-disassembly behavior and moves around the cell cortex. The mechanisms underlying this dynamic behavior remain poorly understood. Particle-based simulations of the core polarity circuit revealed that molecular-level fluctuations are unlikely to overcome the strong positive feedback required for polarization and generate relocating polarity sites. Surprisingly, inclusion of a second pathway that promotes polarity site orientation generated relocating polarity sites with properties similar to those observed experimentally. This pathway forms a second positive feedback loop involving the recruitment of receptors to the cell membrane and couples polarity establishment to gradient sensing. This second positive feedback loop also allows cells to stabilize their polarity site once the site is aligned with the pheromone gradient.

Cell autonomous polarization by the planar cell polarity signaling pathway

Planar Cell Polarity (PCP) signaling polarizes epithelial cells in a plane orthogonal to their apical-basal axis. A core PCP signaling module both generates molecular asymmetry within cells and coordinates the direction of polarization between neighboring cells. Two subcomplexes of core proteins segregate to opposite sides of the cell, defining a polarity axis. Homodimers of the atypical cadherin Flamingo are thought to be the scaffold upon which these subcomplexes assemble and are required for intercellular polarity signaling. The central role for Flamingo homodimers in scaffolding and intercellular communication suggests that cells in which intercellular signaling via Flamingo is disabled should fail to polarize. We show that cells lacking Flamingo, or bearing a truncated Flamingo that cannot homodimerize do in fact polarize. Cell polarization requires both positive and negative feedback, and in a multicellular tissue, feedback might involve both intracellular and intercellular pathways. We identify positive and negative feedback pathways that operate cell autonomously to drive polarization.

Effects of environmental stress factors on the actin cytoskeleton of fungi and plants: Ionizing radiation and ROS

Actin is an abundant and multifaceted protein in eukaryotic cells that has been detected in the cytoplasm as well as in the nucleus. In cooperation with numerous interacting accessory-proteins, monomeric actin (G-actin) polymerizes into microfilaments (F-actin) which constitute ubiquitous subcellular higher order structures. Considering the extensive spatial dimensions and multifunctionality of actin superarrays, the present study analyses the issue if and to what extent environmental stress factors, specifically ionizing radiation (IR) and reactive oxygen species (ROS), affect the cellular actin-entity. In that context, this review particularly surveys IR-response of fungi and plants. It examines in detail which actin-related cellular constituents and molecular pathways are influenced by IR and related ROS. This comprehensive survey concludes that the general integrity of the total cellular actin cytoskeleton is a requirement for IR-tolerance. Actin's functions in genome organization and nuclear events like chromatin remodeling, DNA-repair, and transcription play a key role. Beyond that, it is highly significant that the macromolecular cytoplasmic and cortical actin-frameworks are affected by IR as well. In response to IR, actin-filament bundling proteins (fimbrins) are required to stabilize cables or patches. In addition, the actin-associated factors mediating cellular polarity are essential for IR-survivability. Moreover, it is concluded that a cellular homeostasis system comprising ROS, ROS-scavengers, NADPH-oxidases, and the actin cytoskeleton plays an essential role here. Consequently, besides the actin-fraction which controls crucial genome-integrity, also the portion which facilitates orderly cellular transport and polarized growth has to be maintained in order to survive IR.

Pleiotropy drives evolutionary repair of the responsiveness of polarized cell growth to environmental cues

The ability of cells to translate different extracellular cues into different intracellular responses is vital for their survival in unpredictable environments. In Saccharomyces cerevisiae, cell polarity is modulated in response to environmental signals which allows cells to adopt varying morphologies in different external conditions. The responsiveness of cell polarity to extracellular cues depends on the integration of the molecular network that regulates polarity establishment with networks that signal environmental changes. The coupling of molecular networks often leads to pleiotropic interactions that can make it difficult to determine whether the ability to respond to external signals emerges as an evolutionary response to environmental challenges or as a result of pleiotropic interactions between traits. Here, we study how the propensity of the polarity network of S. cerevisiae to evolve toward a state that is responsive to extracellular cues depends on the complexity of the environment. We show that the deletion of two genes, BEM3 and NRP1, disrupts the ability of the polarity network to respond to cues that signal the onset of the diauxic shift. By combining experimental evolution with whole-genome sequencing, we find that the restoration of the responsiveness to these cues correlates with mutations in genes involved in the sphingolipid synthesis pathway and that these mutations frequently settle in evolving populations irrespective of the complexity of the selective environment. We conclude that pleiotropic interactions make a significant contribution to the evolution of networks that are responsive to extracellular cues.

Molecular basis and design principles of switchable front-rear polarity and directional migration in Myxococcus xanthus

During cell migration, front-rear polarity is spatiotemporally regulated; however, the underlying design of regulatory interactions varies. In rod-shaped Myxococcus xanthus cells, a spatial toggle switch dynamically regulates front-rear polarity. The polarity module establishes front-rear polarity by guaranteeing front pole-localization of the small GTPase MglA. Conversely, the Frz chemosensory system, by acting on the polarity module, causes polarity inversions. MglA localization depends on the RomR/RomX GEF and MglB/RomY GAP complexes that localize asymmetrically to the poles by unknown mechanisms. Here, we show that RomR and the MglB and MglC roadblock domain proteins generate a positive feedback by forming a RomR/MglC/MglB complex, thereby establishing the rear pole with high GAP activity that is non-permissive to MglA. MglA at the front engages in negative feedback that breaks the RomR/MglC/MglB positive feedback allosterically, thus ensuring low GAP activity at this pole. These findings unravel the design principles of a system for switchable front-rear polarity.

Silencing Notch4 promotes tumorigenesis and inhibits metastasis of triple-negative breast cancer via Nanog and Cdc42

Elucidation of individual Notch protein biology in specific cancer is crucial to develop safe, effective, and tumor-selective Notch-targeting therapeutic reagents for clinical use [1]. Here, we explored the Notch4 function in triple-negative breast cancer (TNBC). We found that silencing Notch4 enhanced tumorigenic ability in TNBC cells via upregulating Nanog expression, a pluripotency factor of embryonic stem cells. Intriguingly, silencing Notch4 in TNBC cells suppressed metastasis via downregulating Cdc42 expression, a key molecular for cell polarity formation. Notably, downregulation of Cdc42 expression affected Vimentin distribution, but not Vimentin expression to inhibit EMT shift. Collectively, our results show that silencing Notch4 enhances tumorigenesis and inhibits metastasis in TNBC, indicating that targeting Notch4 may not be a potential strategy for drug discovery in TNBC.

TRIM56 acts through the IQGAP1-CDC42 signaling axis to promote glioma cell migration and invasion

Diffuse invasion is an important factor leading to treatment resistance and a poor prognosis in gliomas. Herein, we found that expression of the tripartite motif containing 56 (TRIM56), a RING-finger domain containing E3 ubiquitin ligase, was markedly higher in glioma than in normal brain tissue, and was significantly correlated with malignant phenotypes and a poor prognosis. In vitro and in vivo experimental studies revealed that TRIM56 promoted the migration and invasion of glioma cells. Mechanistically, TRIM56 was transcriptionally regulated by SP1 and promoted the K48-K63-linked poly-ubiquitination transition of IQGAP1 at Lys-1230 by interacting with it, which in turn promoted CDC42 activation. This mechanism was confirmed to mediate glioma migration and invasion. In conclusion, our study provides insights into the mechanisms through which TRIM56 promotes glioma motility, i.e., by regulating IQGAP1 ubiquitination to promote CDC42 activation, which might be clinically targeted for the treatment of glioma.

The role of anillin/Mid1p during medial division and cytokinesis: from fission yeast to cancer cells

Cytokinesis is the final stage of cell division cycle when cellular constituents are separated to produce two daughter cells. This process is driven by the formation and constriction of a contractile ring. Progression of these events is controlled by mechanisms and proteins that are evolutionary conserved in eukaryotes from fungi to humans. Genetic and molecular studies in different model organisms identified essential cytokinesis genes, with several conserved proteins, including the anillin/Mid1p proteins, constituting the core cytokinetic machinery. The fission yeast Schizosaccharomyces pombe represents a well-established model organism to study eukaryotic cell cycle regulation. Cytokinesis in fission yeast and mammalian cells depends on the placement, assembly, maturation, and constriction of a medially located actin-myosin contractile ring (ACR). Here, we review aspects of the ACR assembly and cytokinesis process in fission yeast and consider the regulation of such events in mammalian cells. First, we briefly describe the role of anillin during mammalian ACR assembly and cytokinesis. Second, we describe different aspects of the anillin-like protein Mid1p regulation during the S. pombe cell cycle, including its structure, function, and phospho-regulation. Third, we briefly discuss Mid1pindependent ACR assembly in S. pombe. Fourth, we highlight emerging studies demonstrating the roles of anillin in human tumourigenesis introducing anillin as a potential drug target for cancer treatment. Collectively, we provide an overview of the current understanding of medial division and cytokinesis in S. pombe and suggest the implications of these observations in other eukaryotic organisms, including humans.

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

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

Polarization of brown algal zygotes

Brown algae are a group of multicellular, heterokont algae that have convergently evolved developmental complexity that rivals that of embryophytes, animals or fungi. Early in development, brown algal zygotes establish a basal and an apical pole, which will become respectively the basal system (holdfast) and the apical system (thallus) of the adult alga. Brown algae are interesting models for understanding the establishment of cell polarity in a broad evolutionary context, because they exhibit a large diversity of life cycles, reproductive strategies and, importantly, their zygotes are produced in large quantities free of parental tissue, with symmetry breaking and asymmetric division taking place in a highly synchronous manner. This review describes the current knowledge about the establishment of the apical-basal axis in the model brown seaweeds Ectocarpus, Dictyota, Fucus and Saccharina, highlighting the advantages and specific interests of each system. Ectocarpus is a genetic model system that allows access to the molecular basis of early development and life-cycle control over apical-basal polarity. The oogamous brown alga Fucus, together with emerging comparative models Dictyota and Saccharina, emphasize the diversity of strategies of symmetry breaking in determining a cell polarity vector in brown algae. A comparison with symmetry-breaking mechanisms in land plants, animals and fungi, reveals that the one-step zygote polarisation of Fucus compares well to Saccharomyces budding and Arabidopsis stomata development, while the two-phased symmetry breaking in the Dictyota zygote compares to Schizosaccharomyces fission, the Caenorhabditis anterior-posterior zygote polarisation and Arabidopsis prolate pollen polarisation. The apical-basal patterning in Saccharina zygotes on the other hand, may be seen as analogous to that of land plants. Overall, brown algae have the potential to bring exciting new information on how a single cell gives rise to an entire complex body plan.

Complexity and self-organization in the evolution of cell polarization

Cellular life exhibits order and complexity, which typically increase over the course of evolution. Cell polarization is a well-studied example of an ordering process that breaks the internal symmetry of a cell by establishing a preferential axis. Like many cellular processes, polarization is driven by self-organization, meaning that the macroscopic pattern emerges as a consequence of microscopic molecular interactions at the biophysical level. However, the role of self-organization in the evolution of complex protein networks remains obscure. In this Review, we provide an overview of the evolution of polarization as a self-organizing process, focusing on the model species Saccharomyces cerevisiae and its fungal relatives. Moreover, we use this model system to discuss how self-organization might relate to evolutionary change, offering a shift in perspective on evolution at the microscopic scale.

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.

Arginylation Regulates Cytoskeleton Organization and Cell Division and Affects Mitochondria in Fission Yeast

Protein arginylation mediated by arginyltransferase Ate1 is a posttranslational modification of emerging importance implicated in the regulation of mammalian embryogenesis, the cardiovascular system, tissue morphogenesis, cell migration, neurodegeneration, cancer, and aging. Ate1 deletion results in embryonic lethality in mice but does not affect yeast viability, making yeast an ideal system to study the molecular pathways regulated by arginylation. Here, we conducted a global analysis of cytoskeleton-related arginylation-dependent phenotypes in Schizosaccharomyces pombe, a fission yeast species that shares many fundamental features of higher eukaryotic cells. Our studies revealed roles of Ate1 in cell division, cell polarization, organelle transport, and interphase cytoskeleton organization and dynamics. We also found a role of Ate1 in mitochondria morphology and maintenance. Furthermore, targeted mass spectrometry analysis of the total Sc. pombe arginylome identified a number of arginylated proteins, including those that play direct roles in these processes; lack of their arginylation may be responsible for ate1-knockout phenotypes. Our work outlines global biological processes potentially regulated by arginylation and paves the way to unraveling the functions of protein arginylation that are conserved at multiple levels of evolution and potentially constitute the primary role of this modification in vivo.

Separable mechanisms drive local and global polarity establishment in the Caenorhabditiselegans intestinal epithelium

Apico-basolateral polarization is essential for epithelial cells to function as selective barriers and transporters, and to provide mechanical resilience to organs. Epithelial polarity is established locally, within individual cells to establish distinct apical, junctional and basolateral domains, and globally, within a tissue where cells coordinately orient their apico-basolateral axes. Using live imaging of endogenously tagged proteins and tissue-specific protein depletion in the Caenorhabditiselegans embryonic intestine, we found that local and global polarity establishment are temporally and genetically separable. Local polarity is initiated prior to global polarity and is robust to perturbation. PAR-3 is required for global polarization across the intestine but local polarity can arise in its absence, as small groups of cells eventually established polarized domains in PAR-3-depleted intestines in a HMR-1 (E-cadherin)-dependent manner. Despite the role of PAR-3 in localizing PKC-3 to the apical surface, we additionally found that PAR-3 and PKC-3/aPKC have distinct roles in the establishment and maintenance of local and global polarity. Taken together, our results indicate that different mechanisms are required for local and global polarity establishment in vivo.

The Cdc42 GAP Rga6 promotes monopolar outgrowth of spores

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

Phosphorylation of RGS regulates MAP kinase localization and promotes completion of cytokinesis

Phosphorylation of the RGS Sst2 alters its subcellular distribution, MAPK localization, and interaction with Kel1, which promotes coordination of polarized growth with completion of cytokinesis.

Yeast use the G-protein–coupled receptor signaling pathway to detect and track the mating pheromone. The G-protein–coupled receptor pathway is inhibited by the regulator of G-protein signaling (RGS) Sst2 which induces Gα GTPase activity and inactivation of downstream signaling. G-protein signaling activates the MAPK Fus3, which phosphorylates the RGS; however, the role of this modification is unknown. We found that pheromone-induced RGS phosphorylation peaks early; the phospho-state of RGS controls its localization and influences MAPK spatial distribution. Surprisingly, phosphorylation of the RGS promotes completion of cytokinesis before pheromone-induced growth. Completion of cytokinesis in the presence of pheromone is promoted by the kelch-repeat protein, Kel1 and antagonized by the formin Bni1. We found that RGS complexes with Kel1 and prefers the unphosphorylatable RGS mutant. We also found overexpression of unphosphorylatable RGS exacerbates cytokinetic defects, whereas they are rescued by overexpression of Kel1. These data lead us to a model where Kel1 promotes completion of cytokinesis before pheromone-induced polarity but is inhibited by unphosphorylated RGS binding.

Plant cell division from the perspective of polarity

The orientation of cell division is a major determinant of plant morphogenesis. In spite of considerable efforts over the past decades, the precise mechanism of division plane selection remains elusive. The majority of studies on the topic have addressed division orientation from either a predominantly developmental or a cell biological perspective. Thus, mechanistic insights into the links between developmental and cellular factors affecting division orientation are particularly lacking. Here, I review recent progress in the understanding of cell division orientation in the embryo and primary root meristem of Arabidopsis from both developmental and cell biological standpoints. I offer a view of multilevel polarity as a central aspect of cell division: on the one hand, the division plane is a readout of tissue- and organism-wide polarities; on the other hand, the cortical division zone can be seen as a transient polar subcellular plasma membrane domain. Finally, I argue that a polarity-focused conceptual framework and the integration of developmental and cell biological approaches hold great promise to unravel the mechanistic basis of plant cell division orientation in the near future.

A bipartite, low-affinity roadblock domain-containing GAP complex regulates bacterial front-rear polarity

The Ras-like GTPase MglA is a key regulator of front-rear polarity in the rod-shaped Myxococcus xanthus cells. MglA-GTP localizes to the leading cell pole and stimulates assembly of the two machineries for type IV pili-dependent motility and gliding motility. MglA-GTP localization is spatially constrained by its cognate GEF, the RomR/RomX complex, and GAP, the MglB Roadblock-domain protein. Paradoxically, RomR/RomX and MglB localize similarly with low and high concentrations at the leading and lagging poles, respectively. Yet, GEF activity dominates at the leading and GAP activity at the lagging pole by unknown mechanisms. Here, we identify RomY and show that it stimulates MglB GAP activity. The MglB/RomY interaction is low affinity, restricting formation of the bipartite MglB/RomY GAP complex almost exclusively to the lagging pole with the high MglB concentration. Our data support a model wherein RomY, by forming a low-affinity complex with MglB, ensures that the high MglB/RomY GAP activity is confined to the lagging pole where it dominates and outcompetes the GEF activity of the RomR/RomX complex. Thereby, MglA-GTP localization is constrained to the leading pole establishing front-rear polarity.

Bacterial cells are spatially highly organized with proteins localizing to distinct subcellular locations. This spatial organization, or cell polarity, is important for many cellular processes including motility. The rod-shaped M. xanthus cells move with defined leading and lagging cell poles. This front-rear polarity is brought about by the polarity module, which consists of the small Ras-like GTPase MglA, its GEF (the RomR/RomX complex) and its GAP (MglB). Specifically, MglA-GTP localizes to the leading pole and stimulates assembly of the motility machineries. MglA-GTP localization, in turn, is spatially constrained by its GEF and GAP. Paradoxically, the RomR/RomX GEF and MglB GAP localize similarly with low and high concentrations at the leading and lagging poles, respectively. Yet, GEF activity dominates at the leading and GAP activity at the lagging pole. Here, we identify RomY and show that it stimulates MglB GAP activity. Interestingly, the MglB/RomY interaction is low affinity. Consequently, MglB/RomY complex formation almost exclusively occurs at the lagging cell pole with the high MglB concentration. Thus, the key to precisely stimulating MglB GAP activity only at the lagging pole is that the MglB/RomY interaction is low-affinity, ultimately restricting MglA-GTP to the leading pole.

Mechanism of commitment to a mating partner in Saccharomyces cerevisiae

Many cells detect and follow gradients of chemical signals to perform their functions. Yeast cells use gradients of extracellular pheromones to locate mating partners, providing a tractable model to understand how cells decode the spatial information in gradients. To mate, yeast cells must orient polarity toward the mating partner. Polarity sites are mobile, exploring the cell cortex until they reach the proper position, where they stop moving and "commit" to the partner. A simple model to explain commitment posits that a high concentration of pheromone is only detected upon alignment of partner cells' polarity sites, and causes polarity site movement to stop. Here we explore how yeast cells respond to partners that make different amounts of pheromone. Commitment was surprisingly robust to varying pheromone levels, ruling out the simple model. We also tested whether adaptive pathways were responsible for the robustness of commitment, but our results show that cells lacking those pathways were still able to accommodate changes in pheromone. To explain this robustness, we suggest that the steep pheromone gradients near each mating partner's polarity site trap the polarity site in place.

Polarity proteins: Shaping dendritic spines and memory

The morphogenesis and plasticity of dendritic spines are associated with synaptic strength, learning, and memory. Dendritic spines are highly compartmentalized structures, which makes proteins involved in cellular polarization and membrane compartmentalization likely candidates regulating their formation and maintenance. Indeed, recent studies suggest polarity proteins help form and maintain dendritic spines by compartmentalizing the spine neck and head. Here, we review emerging evidence that polarity proteins regulate dendritic spine plasticity and stability through the cytoskeleton, scaffolding molecules, and signaling molecules. We specifically analyze various polarity complexes known to contribute to different forms of cell polarization processes and examine the essential conceptual context linking these groups of polarity proteins to dendritic spine morphogenesis, plasticity, and cognitive functions.

Biomechanical regulation of planar cell polarity in endothelial cells

Cell polarity refers to the uneven distribution of certain cytoplasmic components in a cell with a spatial order. The planar cell polarity (PCP), the cell aligns perpendicular to the polar plane, in endothelial cells (ECs) has become a research hot spot. The planar polarity of ECs has a positive significance on the regulation of cardiovascular dysfunction, pathological angiogenesis, and ischemic stroke. The endothelial polarity is stimulated and regulated by biomechanical force. Mechanical stimuli promote endothelial polarization and make ECs produce PCP to maintain the normal physiological and biochemical functions. Here, we overview recent advances in understanding the interplay and mechanism between PCP and ECs function involved in mechanical forces, with a focus on PCP signaling pathways and organelles in regulating the polarity of ECs. And then showed the related diseases caused by ECs polarity dysfunction. This study provides new ideas and therapeutic targets for the treatment of endothelial PCP-related diseases.

Pheromone Guidance of Polarity Site Movement in Yeast

Cells’ ability to track chemical gradients is integral to many biological phenomena, including fertilization, development, accessing nutrients, and combating infection. Mating of the yeast Saccharomyces cerevisiae provides a tractable model to understand how cells interpret the spatial information in chemical gradients. Mating yeast of the two different mating types secrete distinct peptide pheromones, called a-factor and α-factor, to communicate with potential partners. Spatial gradients of pheromones are decoded to guide mobile polarity sites so that polarity sites in mating partners align towards each other, as a prerequisite for cell-cell fusion and zygote formation. In ascomycetes including S. cerevisiae, one pheromone is prenylated (a-factor) while the other is not (α-factor). The difference in physical properties between the pheromones, combined with associated differences in mechanisms of secretion and extracellular pheromone metabolism, suggested that the pheromones might differ in the spatial information that they convey to potential mating partners. However, as mating appears to be isogamous in this species, it is not clear why any such signaling difference would be advantageous. Here we report assays that directly track movement of the polarity site in each partner as a way to understand the spatial information conveyed by each pheromone. Our findings suggest that both pheromones convey very similar information. We speculate that the different pheromones were advantageous in ancestral species with asymmetric mating systems and may represent an evolutionary vestige in yeasts that mate isogamously.

Characterization of Bud3 domains sufficient for bud neck targeting in S. cerevisiae

The cytoskeleton serves a diverse set of functions in both multi- and unicellular organisms, including movement, transport, morphology, cell division and cell signalling. The septin family of cytoskeletal proteins are found within all fungi and metazoans and can generate three-dimensional scaffolds in vivo that promote membrane curvature, serve as physical barriers and coordinate cell cycle checkpoints. In budding yeast, the septins organize into polymerized filaments that decorate the division site between mother and daughter cells during mitosis; assembly of this structure at the ‘bud neck’ is critical for completion of cytokinesis and execution of numerous other cellular events. One such pathway includes bud site selection and the recruitment of proteins such as Bud4 and Bud3 that are responsible for promoting an axial budding pattern in haploid yeast. While Bud4 appears to be recruited to the septins independently of the presence of Bud3, it is likely that Bud3 can localize to the bud neck using both Bud4-dependent and Bud4-independent mechanisms. Furthermore, it remains unclear which precise domain or domains within Bud3 is/are both necessary and sufficient for optimal association at the septin structure. In this study, we examined the localization of GFP-Bud3 constructs in otherwise wild-type (WT) haploid yeast cells expressing Cdc10-mCherry using fluorescence microscopy; we tested a collection of N- and C-terminal truncations and fusions of separate Bud3 protein elements to identify the smallest domain(s) responsible for bud neck localization. We found that the coordinate action of the central amphipathic helix (residues 847–865) and a partially conserved C-terminal motif (residues 1172–1273) was sufficient to promote bud neck recruitment in the presence of endogenous Bud3. This domain is considerably smaller than the previously characterized C-terminal portion required to physically interact with Bud4 (1221–1636) and utilizes a similar mechanism of pairing membrane association, with a separate localization domain, similar to other non-septin proteins targeted to the division site during cell division.

How to assemble a scale-invariant gradient

Intracellular protein gradients serve a variety of functions, such as the establishment of cell polarity or to provide positional information for gene expression in developing embryos. Given that cell size in a population can vary considerably, for the protein gradients to work properly they often have to be scaled to the size of the cell. Here, we examine a model of protein gradient formation within a cell that relies on cytoplasmic diffusion and cortical transport of proteins toward a cell pole. We show that the shape of the protein gradient is determined solely by the cell geometry. Furthermore, we show that the length scale over which the protein concentration in the gradient varies is determined by the linear dimensions of the cell, independent of the diffusion constant or the transport speed. This gradient provides scale-invariant positional information within a cell, which can be used for assembly of intracellular structures whose size is scaled to the linear dimensions of the cell, such as the cytokinetic ring and actin cables in budding yeast cells.

A role for cell polarity in lifespan and mitochondrial quality control in the budding yeast Saccharomyces cerevisiae

Babies are born young, largely independent of the age of their mothers. Mother-daughter age asymmetry in yeast is achieved, in part, by inheritance of higher-functioning mitochondria by buds and retention of some high-functioning mitochondria in mother cells. The mitochondrial F box protein, Mfb1p, tethers mitochondria at both poles in a cell cycle-regulated manner: it localizes to and anchors mitochondria at the mother cell tip throughout the cell cycle and at the bud tip before cytokinesis. Here, we report that cell polarity and polarized localization of Mfb1p decline with age in Saccharomyces cerevisiae. Moreover, deletion of genes (BUD1, BUD2, and BUD5) that mediate symmetry breaking during establishment of cell polarity and asymmetric yeast cell division cause depolarized Mfb1p localization and defects in mitochondrial distribution and quality control. Our results support a role for the polarity machinery in lifespan through modulating Mfb1 function in asymmetric inheritance of mitochondria during yeast cell division.

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Budding polarity declines with age

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Polarization of a mitochondrial tether, Mfb1p, within mother cells declines with age

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Defects in budding polarity disrupt Mfb1p polarization and mitochondrial distribution

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Polarity defects affect Mfb1p-mediated mitochondrial quality and lifespan control

Persistent cell migration emerges from a coupling between protrusion dynamics and polarized trafficking

Migrating cells present a variety of paths, from random to highly directional ones. While random movement can be explained by basal intrinsic activity, persistent movement requires stable polarization. Here, we quantitatively address emergence of persistent migration in (hTERT)–immortalizedRPE1 (retinal pigment epithelial) cells over long timescales. By live cell imaging and dynamic micropatterning, we demonstrate that the Nucleus-Golgi axis aligns with direction of migration leading to efficient cell movement. We show that polarized trafficking is directed toward protrusions with a 20-min delay, and that migration becomes random after disrupting internal cell organization. Eventually, we prove that localized optogenetic Cdc42 activation orients the Nucleus-Golgi axis. Our work suggests that polarized trafficking stabilizes the protrusive activity of the cell, while protrusive activity orients this polarity axis, leading to persistent cell migration. Using a minimal physical model, we show that this feedback is sufficient to recapitulate the quantitative properties of cell migration in the timescale of hours.

Chemotropism and Cell-Cell Fusion in Fungi

Fungi exhibit an enormous variety of morphologies, including yeast colonies, hyphal mycelia, and elaborate fruiting bodies. This diversity arises through a combination of polar growth, cell division, and cell fusion. Because fungal cells are nonmotile and surrounded by a protective cell wall that is essential for cell integrity, potential fusion partners must grow toward each other until they touch and then degrade the intervening cell walls without impacting cell integrity. Here, we review recent progress on understanding how fungi overcome these challenges. Extracellular chemoattractants, including small peptide pheromones, mediate communication between potential fusion partners, promoting the local activation of core cell polarity regulators to orient polar growth and cell wall degradation. However, in crowded environments, pheromone gradients can be complex and potentially confusing, raising the question of how cells can effectively find their partners. Recent findings suggest that the cell polarity circuit exhibits searching behavior that can respond to pheromone cues through a remarkably flexible and effective strategy called exploratory polarization.

Tandem C2 domains mediate dynamic organelle targeting of a DOCK family guanine nucleotide exchange factor

Multicellular organisms use DOCK family guanine nucleotide exchange factors to activate Rac/Rho-of-Plants small GTPases and coordinate cell shape change. In developing tissues, DOCK signals integrate cell-cell interactions with cytoskeleton remodeling, and the GEFs cluster reversibly at specific organelle surfaces to orchestrate cytoskeletal reorganization. The domain organizations among DOCK orthologs are diverse, and the mechanisms of localization control are poorly understood. Here we use combinations of transgene complementation and live cell imaging assays to uncover an evolutionarily conserved and essential localization determinant in the DOCK-GEF named SPIKE1. The SPIKE1-DHR3 domain is sufficient for organelle association in vivo, and displays a complicated lipid binding selectivity for both phospholipid head groups and fatty acid chain saturation. SPIKE1-DHR3 is predicted to adopt a C2-domain structure and functions as part of tandem C2 array that enables reversible clustering at the cell apex. This work provides mechanistic insight into how DOCK GEFs sense compositional and biophysical membrane properties at the interface of two organelle systems.

Orientation of Cell Polarity by Chemical Gradients

Accurate decoding of spatial chemical landscapes is critical for many cell functions. Eukaryotic cells decode local chemical gradients to orient growth or movement in productive directions. Recent work on yeast model systems, whose gradient sensing pathways display much less complexity than those in animal cells, has suggested new paradigms for how these very small cells successfully exploit information in noisy and dynamic pheromone gradients to identify their mates. Pheromone receptors regulate a polarity circuit centered on the conserved Rho-family GTPase, Cdc42. The polarity circuit contains both positive and negative feedback pathways, allowing spontaneous symmetry breaking and also polarity site disassembly and relocation. Cdc42 orients the actin cytoskeleton, leading to focused vesicle traffic that promotes movement of the polarity site and also reshapes the cortical distribution of receptors at the cell surface. In this article, we review the advances from work on yeasts and compare them with the excitable signaling pathways that have been revealed in chemotactic animal cells. Expected final online publication date for the Annual Review of Biophysics, Volume 51 is May 2022. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Lattice-based Monte Carlo simulation of the effects of nutrient concentration and magnetic field exposure on yeast colony growth and morphology

Yeasts exist in communities that expand over space and time to form complex structures and patterns. We developed a lattice-based framework to perform spatial-temporal Monte Carlo simulations of budding yeast colonies exposed to different nutrient and magnetic field conditions. The budding patterns of haploid and diploid yeast cells were incorporated into the framework, as well as the filamentous growth that occurs in yeast colonies under nutrient limiting conditions. Simulation of the framework predicted that magnetic fields decrease colony growth rate, solidity, and roundness. Magnetic field simulations further predicted that colony elongation and boundary fluctuations increase in a nutrient- and ploidy-dependent manner. These in-silico predictions are an important step towards understanding the effects of the physico-chemical environment on microbial colonies and for informing bioelectromagnetic experiments on yeast colony biofilms and fungal pathogens.

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-Specific GTPase-Activating Protein Rga1 Squelches Crosstalk between the High-Osmolarity Glycerol (HOG) and Mating Pheromone Response MAPK Pathways

Eukaryotes utilize distinct mitogen/messenger-activated protein kinase (MAPK) pathways to evoke appropriate responses when confronted with different stimuli. In yeast, hyperosmotic stress activates MAPK Hog1, whereas mating pheromones activate MAPK Fus3 (and MAPK Kss1). Because these pathways share several upstream components, including the small guanosine-5'-triphosphate phosphohydrolase (GTPase) cell-division-cycle-42 (Cdc42), mechanisms must exist to prevent inadvertent cross-pathway activation. Hog1 activity is required to prevent crosstalk to Fus3 and Kss1. To identify other factors required to maintain signaling fidelity during hypertonic stress, we devised an unbiased genetic selection for mutants unable to prevent such crosstalk even when active Hog1 is present. We repeatedly isolated truncated alleles of RGA1, a Cdc42-specific GTPase-activating protein (GAP), each lacking its C-terminal catalytic domain, that permit activation of the mating MAPKs under hyperosmotic conditions despite Hog1 being present. We show that Rga1 down-regulates Cdc42 within the high-osmolarity glycerol (HOG) pathway, but not the mating pathway. Because induction of mating pathway output via crosstalk from the HOG pathway takes significantly longer than induction of HOG pathway output, our findings suggest that, under normal conditions, Rga1 contributes to signal insulation by limiting availability of the GTP-bound Cdc42 pool generated by hypertonic stress. Thus, Rga1 action contributes to squelching crosstalk by imposing a type of “kinetic proofreading”. Although Rga1 is a Hog1 substrate in vitro, we eliminated the possibility that its direct Hog1-mediated phosphorylation is necessary for its function in vivo. Instead, we found first that, like its paralog Rga2, Rga1 is subject to inhibitory phosphorylation by the S. cerevisiae cyclin-dependent protein kinase 1 (Cdk1) ortholog Cdc28 and that hyperosmotic shock stimulates its dephosphorylation and thus Rga1 activation. Second, we found that Hog1 promotes Rga1 activation by blocking its Cdk1-mediated phosphorylation, thereby allowing its phosphoprotein phosphatase 2A (PP2A)-mediated dephosphorylation. These findings shed light on why Hog1 activity is required to prevent crosstalk from the HOG pathway to the mating pheromone response pathway.

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

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

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.

Pattern Formation and Complexity in Single Cells

In the context of animal or plant development, we tend to think of cells as small, simple, building blocks, such that complex patterns or shapes can only be constructed from large numbers of cells, with cells in different parts of the organism taking on different fates. However, cells themselves are far from simple, and often take on complex shapes with a remarkable degree of intracellular patterning. How do these patterns arise? As in embryogenesis, the development of structure inside a cell can be broken down into a number of basic processes. For each part of the cell, morphogenetic processes create internal structures such as organelles, which might correspond to organs at the level of a whole organism. Given that mechanisms exist to generate parts, patterning processes are required to ensure that the parts are distributed in the correct arrangement relative to the rest of the cell. Such patterning processes make reference to global polarity axes, requiring mechanisms for axiation which, in turn, require processes to break symmetry. These fundamental processes of symmetry breaking, axiation, patterning, and morphogenesis have been extensively studied in developmental biology but less so at the subcellular level. This review will focus on developmental processes that give eukaryotic cells their complex structures, with a focus on cytoskeletal organization in free-living cells, ciliates in particular, in which these processes are most readily apparent.

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.