Dual modes of cdc42 recycling fine-tune polarized morphogenesis

The biochemical mechanism of Rho GTPase membrane binding, activation and retention in activity patterning

Rho GTPases form plasma membrane-associated patterns that control the cytoskeleton during cell division, morphogenesis, migration, and wound repair. Their patterning involves transitions between inactive cytosolic and active membrane-bound states, regulated by guanine nucleotide exchange factors (GEFs), GTPase-activating proteins (GAPs), and guanine nucleotide dissociation inhibitors (GDIs). However, the relationships between these transitions and role of different regulators remain unclear. We developed a novel reconstitution approach to study Rho GTPase patterning with all major GTPase regulators in a biochemically defined system. We show that Rho GTPase dissociation from RhoGDI is rate-limiting for its membrane association. Rho GTPase activation occurs after membrane insertion, which is unaffected by GEF activity. Once activated, Rho GTPases are retained at the membrane through effector interactions, essential for their enrichment at activation sites. Thus, high cytosolic levels of RhoGDI-bound GTPases ensure a constant supply of inactive GTPases for the membrane, where GEF-mediated activation and effector binding stabilize them. These results delineate the route by which Rho GTPase patterns are established and define stage-dependent roles of its regulators.

RhoGDI1 regulates cell-cell junctions in polarized epithelial cells

Cell-cell contact formation of polarized epithelial cells is a multi-step process that involves the co-ordinated activities of Rho family small GTPases. Consistent with the central role of Rho GTPases, a number of Rho guanine nucleotide exchange factors (GEFs) and Rho GTPase-activating proteins (GAPs) have been identified at cell-cell junctions at various stages of junction maturation. As opposed to RhoGEFs and RhoGAPs, the role of Rho GDP dissociation inhibitors (GDIs) during cell-cell contact formation is poorly understood. Here, we have analyzed the role of RhoGDI1/ARHGDIA, a member of the RhoGDI family, during cell-cell contact formation of polarized epithelial cells. Depletion of RhoGDI1 delays the development of linear cell-cell junctions and the formation of barrier-forming tight junctions. In addition, RhoGDI1 depletion impairs the ability of cells to stop migration in response to cell collision and increases the migration velocity of collectively migrating cells. We also find that the cell adhesion receptor JAM-A promotes the recruitment of RhoGDI1 to cell-cell contacts. Our findings implicate RhoGDI1 in various processes involving the dynamic reorganization of cell-cell junctions.

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.

A positive feedback loop involving the Spa2 SHD domain contributes to focal polarization

Focal polarization is necessary for finely arranged cell-cell interactions. The yeast mating projection, with its punctate polarisome, is a good model system for this process. We explored the critical role of the polarisome scaffold protein Spa2 during yeast mating with a hypothesis motivated by mathematical modeling and tested by in vivo experiments. Our simulations predicted that two positive feedback loops generate focal polarization, including a novel feedback pathway involving the N-terminal domain of Spa2. We characterized the latter using loss-of-function and gain-of-function mutants. The N-terminal region contains a Spa2 Homology Domain (SHD) which is conserved from yeast to humans, and when mutated largely reproduced the spa2Δ phenotype. Our work together with published data show that the SHD domain recruits Msb3/4 that stimulates Sec4-mediated transport of Bud6 to the polarisome. There, Bud6 activates Bni1-catalyzed actin cable formation, recruiting more Spa2 and completing the positive feedback loop. We demonstrate that disrupting this loop at any point results in morphological defects. Gain-of-function perturbations partially restored focal polarization in a spa2 loss-of-function mutant without restoring localization of upstream components, thus supporting the pathway order. Thus, we have collected data consistent with a novel positive feedback loop that contributes to focal polarization during pheromone-induced polarization in yeast.

Mechanisms that ensure monogamous mating in Saccharomyces cerevisiae

Haploid cells of the budding yeast Saccharomyces cerevisiae communicate using secreted pheromones and mate to form diploid zygotes. Mating is monogamous, resulting in the fusion of precisely one cell of each mating type. Monogamous mating in crowded conditions, where cells have access to more than one potential partner, raises the question of how multiple-mating outcomes are prevented. Here we identify mutants capable of mating with multiple partners, revealing the mechanisms that ensure monogamous mating. Before fusion, cells develop polarity foci oriented toward potential partners. Competition between these polarity foci within each cell leads to disassembly of all but one focus, thus favoring a single fusion event. Fusion promotes the formation of heterodimeric complexes between subunits that are uniquely expressed in each mating type. One complex shuts off haploid-specific gene expression, and the other shuts off the ability to respond to pheromone. Zygotes able to form either complex remain monogamous, but zygotes lacking both can re-mate.

Coordinating cell polarization and morphogenesis through mechanical feedback

Many cellular processes require cell polarization to be maintained as the cell changes shape, grows or moves. Without feedback mechanisms relaying information about cell shape to the polarity molecular machinery, the coordination between cell polarization and morphogenesis, movement or growth would not be possible. Here we theoretically and computationally study the role of a genetically-encoded mechanical feedback (in the Cell Wall Integrity pathway) as a potential coordination mechanism between cell morphogenesis and polarity during budding yeast mating projection growth. We developed a coarse-grained continuum description of the coupled dynamics of cell polarization and morphogenesis as well as 3D stochastic simulations of the molecular polarization machinery in the evolving cell shape. Both theoretical approaches show that in the absence of mechanical feedback (or in the presence of weak feedback), cell polarity cannot be maintained at the projection tip during growth, with the polarization cap wandering off the projection tip, arresting morphogenesis. In contrast, for mechanical feedback strengths above a threshold, cells can robustly maintain cell polarization at the tip and simultaneously sustain mating projection growth. These results indicate that the mechanical feedback encoded in the Cell Wall Integrity pathway can provide important positional information to the molecular machinery in the cell, thereby enabling the coordination of cell polarization and morphogenesis.

Targeting and signaling of Rho of plants guanosine triphosphatases require synergistic interaction between guanine nucleotide inhibitor and vesicular trafficking

Most eukaryotic cells are polarized. Common toolbox regulating cell polarization includes Rho guanosine triphosphatases (GTPases), in which spatiotemporal activation is regulated by a plethora of regulators. Rho of plants (ROPs) are the only Rho GTPases in plants. Although vesicular trafficking was hinted in the regulation of ROPs, it was unclear where vesicle-carried ROP starts, whether it is dynamically regulated, and which components participate in vesicle-mediated ROP targeting. In addition, although vesicle trafficking and guanine nucleotide inhibitor (GDI) pathways in Rho signaling have been extensively studied in yeast, it is unknown whether the two pathways interplay. Unclear are also cellular and developmental consequences of their interaction in multicellular organisms. Here, we show that the dynamic targeting of ROP through vesicles requires coat protein complex II and ADP-ribosylation factor 1-mediated post-Golgi trafficking. Trafficking of vesicle-carried ROPs between the plasma membrane and the trans-Golgi network is mediated through adaptor protein 1 and sterol-mediated endocytosis. Finally, we show that GDI and vesicle trafficking synergistically regulate cell polarization and ROP targeting, suggesting that the establishment and maintenance of cell polarity is regulated by an evolutionarily conserved mechanism.

Regulation of Cdc42 for polarized growth in budding yeast

The Rho GTPase Cdc42 is a central regulator of cell polarity in diverse cell types. The activity of Cdc42 is dynamically controlled in time and space to enable distinct polarization events, which generally occur along a single axis in response to spatial cues. Our understanding of the mechanisms underlying Cdc42 polarization has benefited largely from studies of the budding yeast Saccharomyces cerevisiae, a genetically tractable model organism. In budding yeast, Cdc42 activation occurs in two temporal steps in the G1 phase of the cell cycle to establish a proper growth site. Here, we review findings in budding yeast that reveal an intricate crosstalk among polarity proteins for biphasic Cdc42 regulation. The first step of Cdc42 activation may determine the axis of cell polarity, while the second step ensures robust Cdc42 polarization for growth. Biphasic Cdc42 polarization is likely to ensure the proper timing of events including the assembly and recognition of spatial landmarks and stepwise assembly of a new ring of septins, cytoskeletal GTP-binding proteins, at the incipient bud site. Biphasic activation of GTPases has also been observed in mammalian cells, suggesting that biphasic activation could be a general mechanism for signal-responsive cell polarization. Cdc42 activity is necessary for polarity establishment during normal cell division and development, but its activity has also been implicated in the promotion of aging. We also discuss negative polarity signaling and emerging concepts of Cdc42 signaling in cellular aging.

Intracellular organization in cell polarity - placing organelles into the polarity loop

Many studies have investigated the processes that support polarity establishment and maintenance in cells. On the one hand, polarity complexes at the cell cortex and their downstream signaling pathways have been assigned as major regulators of polarity. On the other hand, intracellular organelles and their polarized trafficking routes have emerged as important components of polarity. In this Review, we argue that rather than trying to identify the prime 'culprit', now it is time to consider all these players as a collective. We highlight that understanding the intimate coordination between the polarized cell cortex and the intracellular compass that is defined by organelle positioning is essential to capture the concept of polarity. After briefly reviewing how polarity emerges from a dynamic maintenance of cellular asymmetries, we highlight how intracellular organelles and their associated trafficking routes provide diverse feedback for dynamic cell polarity maintenance. We argue that the asymmetric organelle compass is an indispensable element of the polarity network.

A novel interplay between GEFs orchestrates Cdc42 activity during cell polarity and cytokinesis in fission yeast

Cdc42, a conserved regulator of cell polarity, is activated by two GEFs, Gef1 and Scd1, in fission yeast. Why the cell needs two GEFs is unclear, given that they are partially redundant and activate the same GTPase. Using the GEF localization pattern during cytokinesis as a paradigm, we report a novel interplay between Gef1 and Scd1 that spatially modulates Cdc42. We find that Gef1 promotes Scd1 localization to the division site during cytokinesis through recruitment of the scaffold protein Scd2, via a Cdc42 feedforward pathway. Similarly, during interphase Gef1 promotes Scd1 recruitment at the new end to enable the transition from monopolar to bipolar growth. Reciprocally, Scd1 restricts Gef1 localization to prevent ectopic Cdc42 activation during cytokinesis to promote cell separation, and to maintain cell shape during interphase. Our findings reveal an elegant regulatory pattern in which Gef1 primes Cdc42 activation at new sites to initiate Scd1-dependent polarized growth, while Scd1 restricts Gef1 to sites of polarization. We propose that crosstalk between GEFs is a conserved mechanism that orchestrates Cdc42 activation during complex cellular processes.This article has an associated First Person interview with the first author of the paper.

A novel interplay between GEFs orchestrates Cdc42 activity during cell polarity and cytokinesis in fission yeast

Cdc42, a conserved regulator of cell polarity, is activated by two GEFs, Gef1 and Scd1, in fission yeast. Why the cell needs two GEFs is unclear, given that they are partially redundant and activate the same GTPase. Using the GEF localization pattern during cytokinesis as a paradigm, we report a novel interplay between Gef1 and Scd1 that spatially modulates Cdc42. We find that Gef1 promotes Scd1 localization to the division site during cytokinesis through recruitment of the scaffold protein Scd2, via a Cdc42 feedforward pathway. Similarly, during interphase Gef1 promotes Scd1 recruitment at the new end to enable the transition from monopolar to bipolar growth. Reciprocally, Scd1 restricts Gef1 localization to prevent ectopic Cdc42 activation during cytokinesis to promote cell separation, and to maintain cell shape during interphase. Our findings reveal an elegant regulatory pattern in which Gef1 primes Cdc42 activation at new sites to initiate Scd1-dependent polarized growth, while Scd1 restricts Gef1 to sites of polarization. We propose that crosstalk between GEFs is a conserved mechanism that orchestrates Cdc42 activation during complex cellular processes.This article has an associated First Person interview with the first author of the paper.

Extraction of active RhoGTPases by RhoGDI regulates spatiotemporal patterning of RhoGTPases

The RhoGTPases are characterized as membrane-associated molecular switches that cycle between active, GTP-bound and inactive, GDP-bound states. However, 90-95% of RhoGTPases are maintained in a soluble form by RhoGDI, which is generally viewed as a passive shuttle for inactive RhoGTPases. Our current understanding of RhoGTPase:RhoGDI dynamics has been limited by two experimental challenges: direct visualization of the RhoGTPases in vivo and reconstitution of the cycle in vitro. We developed methods to directly image vertebrate RhoGTPases in vivo or on lipid bilayers in vitro. Using these methods, we identified pools of active and inactive RhoGTPase associated with the membrane, found that RhoGDI can extract both inactive and active RhoGTPases, and found that extraction of active RhoGTPase contributes to their spatial regulation around cell wounds. These results indicate that RhoGDI directly contributes to the spatiotemporal patterning of RhoGTPases by removing active RhoGTPases from the plasma membrane.

Regulation of Cdc42 and its effectors in epithelial morphogenesis

Cdc42 - a member of the small Rho GTPase family - regulates cell polarity across organisms from yeast to humans. It is an essential regulator of polarized morphogenesis in epithelial cells, through coordination of apical membrane morphogenesis, lumen formation and junction maturation. In parallel, work in yeast and Caenorhabditis elegans has provided important clues as to how this molecular switch can generate and regulate polarity through localized activation or inhibition, and cytoskeleton regulation. Recent studies have revealed how important and complex these regulations can be during epithelial morphogenesis. This complexity is mirrored by the fact that Cdc42 can exert its function through many effector proteins. In epithelial cells, these include atypical PKC (aPKC, also known as PKC-3), the P21-activated kinase (PAK) family, myotonic dystrophy-related Cdc42 binding kinase beta (MRCKβ, also known as CDC42BPB) and neural Wiskott-Aldrich syndrome protein (N-WASp, also known as WASL). Here, we review how the spatial regulation of Cdc42 promotes polarity and polarized morphogenesis of the plasma membrane, with a focus on the epithelial cell type.

RhoGDI2 positively regulates the Rho GTPases activation in response to the β2 outside-in signaling in T cells adhesion and migration on ICAM-1

Cytoskeletal reorganization driven by Rho GTPases plays a crucial role in the migration of T cells, which are key regulators of immunity. The molecular mechanisms that control actin cytoskeleton remodeling during T cell movement have only partially been clarified as the function of many modulators has not been evaluated in these cells. Here, we report a new function of RhoGDI2 by showing that this protein positively regulates Rho GTPase activation during T cell adhesion and migration. RhoGDI2 knockdown significantly reduced T cell adhesion and migration. Furthermore, RhoGDI2 knockdown decreased the activation of Rac1 and Cdc42, 2 members of Rho GTPases, and the remodeling of the actin cytoskeleton. Upon P-selectin glycoprotein ligand-1 engagement, RhoGDI2 was phosphorylated at Y24 and Y153 by kinases related to β2 integrin outside-in signaling, Src, c-Abl, and Syk, resulting in the accumulation of RhoGDI2 at the cell membrane. Subsequent phosphorylation of S31 induced the opening of RhoGDI2 and the release of Rho GTPases, whereas phosphorylation of Y153 might promote the activation of Rho GTPases by recruiting Vav1. Moreover, the disruption of lipid rafts with methyl-β-cyclodextrin blocked the interaction between integrins and RhoGDI2, reducing the level of phosphorylated RhoGDI2 and the activation of downstream Rho GTPases. Based on these observations, RhoGDI2 is a target of intergrin outside-in signaling that activates Rho GTPases during T cell adhesion and migration, and RhoGDI2-mediated signal transduction is based on the lipid rafts integrity.

Self-organization principles of intracellular pattern formation

Dynamic patterning of specific proteins is essential for the spatio-temporal regulation of many important intracellular processes in prokaryotes, eukaryotes and multicellular organisms. The emergence of patterns generated by interactions of diffusing proteins is a paradigmatic example for self-organization. In this article, we review quantitative models for intracellular Min protein patterns in Escherichia coli, Cdc42 polarization in Saccharomyces cerevisiae and the bipolar PAR protein patterns found in Caenorhabditis elegans. By analysing the molecular processes driving these systems we derive a theoretical perspective on general principles underlying self-organized pattern formation. We argue that intracellular pattern formation is not captured by concepts such as ‘activators’, ‘inhibitors’ or ‘substrate depletion’. Instead, intracellular pattern formation is based on the redistribution of proteins by cytosolic diffusion, and the cycling of proteins between distinct conformational states. Therefore, mass-conserving reaction–diffusion equations provide the most appropriate framework to study intracellular pattern formation. We conclude that directed transport, e.g. cytosolic diffusion along an actively maintained cytosolic gradient, is the key process underlying pattern formation. Thus the basic principle of self-organization is the establishment and maintenance of directed transport by intracellular protein dynamics.

This article is part of the theme issue ‘Self-organization in cell biology’.

Minimal in vitro systems shed light on cell polarity

Cell polarity - the morphological and functional differentiation of cellular compartments in a directional manner - is required for processes such as orientation of cell division, directed cellular growth and motility. How the interplay of components within the complexity of a cell leads to cell polarity is still heavily debated. In this Review, we focus on one specific aspect of cell polarity: the non-uniform accumulation of proteins on the cell membrane. In cells, this is achieved through reaction-diffusion and/or cytoskeleton-based mechanisms. In reaction-diffusion systems, components are transformed into each other by chemical reactions and are moving through space by diffusion. In cytoskeleton-based processes, cellular components (i.e. proteins) are actively transported by microtubules (MTs) and actin filaments to specific locations in the cell. We examine how minimal systems - in vitro reconstitutions of a particular cellular function with a minimal number of components - are designed, how they contribute to our understanding of cell polarity (i.e. protein accumulation), and how they complement in vivo investigations. We start by discussing the Min protein system from Escherichia coli, which represents a reaction-diffusion system with a well-established minimal system. This is followed by a discussion of MT-based directed transport for cell polarity markers as an example of a cytoskeleton-based mechanism. To conclude, we discuss, as an example, the interplay of reaction-diffusion and cytoskeleton-based mechanisms during polarity establishment in budding yeast.

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

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

The shared role of the Rsr1 GTPase and Gic1/Gic2 in Cdc42 polarization

The Cdc42 GTPase plays a central role in polarity development in many species. In budding yeast, Cdc42 is essential for polarized growth at the proper site and also for spontaneous cell polarization in the absence of spatial cues. Cdc42 polarization is critical for multiple events in the G1 phase prior to bud emergence, including bud-site assembly, polarization of the actin cytoskeleton, and septin filament assembly to form a ring at the new bud site. Yet the mechanism by which Cdc42 polarizes is not fully understood. Here we report that biphasic Cdc42 polarization in the G1 phase is coupled to stepwise assembly of the septin ring for bud emergence. We show that the Rsr1 GTPase shares a partially redundant role with Gic1 and Gic2, two related Cdc42 effectors, in the first phase of Cdc42 polarization in haploid cells. We propose that the first phase of Cdc42 polarization is mediated by positive feedback loops that function in parallel-one involving Rsr1 via local activation of Cdc42 in response to spatial cues and another involving Gic1 or Gic2 via reduction of diffusion of active Cdc42.

Many roads to symmetry breaking: molecular mechanisms and theoretical models of yeast cell polarity

Mathematical modeling has been instrumental in identifying common principles of cell polarity across diverse systems. These principles include positive feedback loops that are required to destabilize a spatially uniform state of the cell. The conserved small G-protein Cdc42 is a master regulator of eukaryotic cellular polarization. Here we discuss recent developments in studies of Cdc42 polarization in budding and fission yeasts and demonstrate that models describing symmetry-breaking polarization can be classified into six minimal classes based on the structure of positive feedback loops that activate and localize Cdc42. Owing to their generic system-independent nature, these model classes are also likely to be relevant for the G-protein–based symmetry-breaking systems of higher eukaryotes. We review experimental evidence pro et contra different theoretically plausible models and conclude that several parallel and non–mutually exclusive mechanisms are likely involved in cellular polarization of yeasts. This potential redundancy needs to be taken into consideration when interpreting the results of recent cell-rewiring studies.

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.

A Two-Tiered Mechanism Enables Localized Cdc42 Signaling during Enterocyte Polarization

Signaling by the small GTPase Cdc42 governs a diverse set of cellular processes that contribute to tissue morphogenesis. Since these processes often require highly localized signaling, Cdc42 activity must be clustered in order to prevent ectopic signaling. During cell polarization, apical Cdc42 signaling directs the positioning of the nascent apical membrane. However, the molecular mechanisms that drive Cdc42 clustering during polarity establishment are largely unknown. Here, we demonstrate that during cell polarization localized Cdc42 signaling is enabled via activity-dependent control of Cdc42 mobility. By performing photoconversion experiments, we show that inactive Cdc42-GDP is 30-fold more mobile than active Cdc42-GTP. This switch in apical mobility originates from a dual mechanism involving RhoGDI-mediated membrane dissociation of Cdc42-GDP and Tuba-mediated immobilization of Cdc42-GTP. Interference with either mechanism affects Cdc42 clustering and as a consequence impairs Cdc42-mediated apical membrane clustering. We therefore identify a molecular network, comprised of Cdc42, the guanine nucleotide exchange factor (GEF) Tuba, and RhoGDI, that enables differential diffusion of inactive and active Cdc42 and is required to establish localized Cdc42 signaling during enterocyte polarization.

Polarity establishment by Cdc42: Key roles for positive feedback and differential mobility

Cell polarity is fundamental to the function of most cells. The evolutionarily conserved molecular machinery that controls cell polarity is centered on a family of GTPases related to Cdc42. Cdc42 becomes activated and concentrated at polarity sites, but studies in yeast model systems led to controversy on the mechanisms of polarization. Here we review recent studies that have clarified how Cdc42 becomes polarized in yeast. On one hand, findings that appeared to support a key role for the actin cytoskeleton and vesicle traffic in polarity establishment now appear to reflect the action of stress response pathways induced by cytoskeletal perturbations. On the other hand, new findings strongly support hypotheses on the polarization mechanism whose origins date back to the mathematician Alan Turing. The key features of the polarity establishment mechanism in yeasts include a positive feedback pathway in which active Cdc42 recruits a Cdc42 activator to polarity sites, and differential mobility of polarity "activators" and "substrates."

Spatial and temporal signal processing and decision making by MAPK pathways

Mitogen-activated protein kinase (MAPK) pathways are conserved from yeast to man and regulate a variety of cellular processes, including proliferation and differentiation. Recent developments show how MAPK pathways perform exquisite spatial and temporal signal processing and underscores the importance of studying the dynamics of signaling pathways to understand their physiological response. The importance of dynamic mechanisms that process input signals into graded downstream responses has been demonstrated in the pheromone-induced and osmotic stress-induced MAPK pathways in yeast and in the mammalian extracellular signal-regulated kinase MAPK pathway. Particularly, recent studies in the yeast pheromone response have shown how positive feedback generates switches, negative feedback enables gradient detection, and coherent feedforward regulation underlies cellular memory. More generally, a new wave of quantitative single-cell studies has begun to elucidate how signaling dynamics determine cell physiology and represents a paradigm shift from descriptive to predictive biology.

Dynamics of cell wall elasticity pattern shapes the cell during yeast mating morphogenesis

The cell wall defines cell shape and maintains integrity of fungi and plants. When exposed to mating pheromone, Saccharomyces cerevisiae grows a mating projection and alters in morphology from spherical to shmoo form. Although structural and compositional alterations of the cell wall accompany shape transitions, their impact on cell wall elasticity is unknown. In a combined theoretical and experimental approach using finite-element modelling and atomic force microscopy (AFM), we investigated the influence of spatially and temporally varying material properties on mating morphogenesis. Time-resolved elasticity maps of shmooing yeast acquired with AFM in vivo revealed distinct patterns, with soft material at the emerging mating projection and stiff material at the tip. The observed cell wall softening in the protrusion region is necessary for the formation of the characteristic shmoo shape, and results in wider and longer mating projections. The approach is generally applicable to tip-growing fungi and plants cells.

The final cut: cell polarity meets cytokinesis at the bud neck in S. cerevisiae

Cell division is a fundamental but complex process that gives rise to two daughter cells. It includes an ordered set of events, altogether called "the cell cycle", that culminate with cytokinesis, the final stage of mitosis leading to the physical separation of the two daughter cells. Symmetric cell division equally partitions cellular components between the two daughter cells, which are therefore identical to one another and often share the same fate. In many cases, however, cell division is asymmetrical and generates two daughter cells that differ in specific protein inheritance, cell size, or developmental potential. The budding yeast Saccharomyces cerevisiae has proven to be an excellent system to investigate the molecular mechanisms governing asymmetric cell division and cytokinesis. Budding yeast is highly polarized during the cell cycle and divides asymmetrically, producing two cells with distinct sizes and fates. Many components of the machinery establishing cell polarization during budding are relocalized to the division site (i.e., the bud neck) for cytokinesis. In this review we recapitulate how budding yeast cells undergo polarized processes at the bud neck for cell division.

Parallel Actin-Independent Recycling Pathways Polarize Cdc42 in Budding Yeast

The highly conserved Rho-family GTPase Cdc42 is an essential regulator of polarity in many different cell types. During polarity establishment, Cdc42 becomes concentrated at a cortical site, where it interacts with downstream effectors to orient the cytoskeleton along the front-back axis. To concentrate Cdc42, loss of Cdc42 by diffusion must be balanced by recycling to the front. In Saccharomyces cerevisiae, the guanine nucleotide dissociation inhibitor (GDI) Rdi1 recycles Cdc42 through the cytoplasm. Loss of Rdi1 slowed but did not eliminate Cdc42 accumulation at the front, suggesting the existence of other recycling pathways. One proposed pathway involves actin-directed trafficking of vesicles carrying Cdc42 to the front. However, we found no role for F-actin in Cdc42 concentration, even in rdi1Δ cells. Instead, Cdc42 was still able to exchange between the membrane and cytoplasm in rdi1Δ cells, albeit at a reduced rate. Membrane-cytoplasm exchange of GDP-Cdc42 was faster than that of GTP-Cdc42, and computational modeling indicated that such exchange would suffice to promote polarization. We also uncovered a novel role for the Cdc42-directed GTPase-activating protein (GAP) Bem2 in Cdc42 polarization. Bem2 was known to act in series with Rdi1 to promote recycling of Cdc42, but we found that rdi1Δ bem2Δ mutants were synthetically lethal, suggesting that they also act in parallel. We suggest that GAP activity cooperates with the GDI to counteract the dissipative effect of a previously unappreciated pathway whereby GTP-Cdc42 escapes from the polarity site through the cytoplasm.

Role of Polarized G Protein Signaling in Tracking Pheromone Gradients

Yeast cells track gradients of pheromones to locate mating partners. Intuition suggests that uniform distribution of pheromone receptors over the cell surface would yield optimal gradient sensing. However, yeast cells display polarized receptors. The benefit of such polarization was unknown. During gradient tracking, cell growth is directed by a patch of polarity regulators that wanders around the cortex. Patch movement is sensitive to pheromone dose, with wandering reduced on the up-gradient side of the cell, resulting in net growth in that direction. Mathematical modeling suggests that active receptors and associated G proteins lag behind the polarity patch and act as an effective drag on patch movement. In vivo, the polarity patch is trailed by a G protein-rich domain, and this polarized distribution of G proteins is required to constrain patch wandering. Our findings explain why G protein polarization is beneficial and illuminate a novel mechanism for gradient tracking.

Polarity establishment requires localized activation of Cdc42

Positive feedback by localized activation of Cdc42 drives polarity establishment in the budding yeast Saccharomyces cerevisiae.

Establishment of cell polarity in animal and fungal cells involves localization of the conserved Rho-family guanosine triphosphatase, Cdc42, to the cortical region destined to become the “front” of the cell. The high local concentration of active Cdc42 promotes cytoskeletal polarization through various effectors. Cdc42 accumulation at the front is thought to involve positive feedback, and studies in the budding yeast Saccharomyces cerevisiae have suggested distinct positive feedback mechanisms. One class of mechanisms involves localized activation of Cdc42 at the front, whereas another class involves localized delivery of Cdc42 to the front. Here we show that Cdc42 activation must be localized for successful polarity establishment, supporting local activation rather than local delivery as the dominant mechanism in this system.

Arabidopsis RhoGDIs Are Critical for Cellular Homeostasis of Pollen Tubes

Rhos of plants (ROPs) play a key role in plant cell morphogenesis, especially in tip-growing pollen tubes and root hairs, by regulating an array of intracellular activities such as dynamic polymerization of actin microfilaments. ROPs are regulated by guanine nucleotide exchange factors (RopGEFs), GTPase activating proteins (RopGAPs), and guanine nucleotide dissociation inhibitors (RhoGDIs). RopGEFs and RopGAPs play evolutionarily conserved function in ROP signaling. By contrast, although plant RhoGDIs regulate the membrane extraction and cytoplasmic sequestration of ROPs, less clear are their positive roles in ROP signaling as do their yeast and metazoan counterparts. We report here that functional loss of all three Arabidopsis (Arabidopsis thaliana) GDIs (tri-gdi) significantly reduced male transmission due to impaired pollen tube growth in vitro and in vivo. We demonstrate that ROPs were ectopically activated at the lateral plasma membrane of the tri-gdi pollen tubes. However, total ROPs were reduced posttranslationally in the tri-gdi mutant, resulting in overall dampened ROP signaling. Indeed, a ROP5 mutant that was unable to interact with GDIs failed to induce growth, indicating the importance of the ROP-GDI interaction for ROP signaling. Functional loss of GDIs impaired cellular homeostasis, resulting in excess apical accumulation of wall components in pollen tubes, similar to that resulting from ectopic phosphatidylinositol 4,5-bisphosphate signaling. GDIs and phosphatidylinositol 4,5-bisphosphate may antagonistically coordinate to maintain cellular homeostasis during pollen tube growth. Our results thus demonstrate a more complex role of GDIs in ROP-mediated pollen tube growth.

How to bake a brain: yeast as a model neuron

More than 30 years ago Dan Koshland published an inspirational essay presenting the bacterium as a model neuron (Koshland, Trends Neurosci 6:133-137, 1983). In the article he argued that there are several similarities between neurons and bacterial cells in "how signals are processed within a cell or how this processing machinery can be modified to produce plasticity". He then explored the bacterial chemosensory system to emphasize its attributes that are analogous to information processing in neurons. In this review, we wish to expand Koshland's original idea by adding the yeast cell to the list of useful models of a neuron. The fact that yeasts and neurons are specialized versions of the eukaryotic cell sharing all principal components sets the stage for a grand evolutionary tinkering where these components are employed in qualitatively different tasks, but following analogous molecular logic. By way of example, we argue that evolutionarily conserved key components involved in polarization processes (from budding or mating in Saccharomyces cervisiae to neurite outgrowth or spinogenesis in neurons) are shared between yeast and neurons. This orthologous conservation of modules makes S. cervisiae an excellent model organism to investigate neurobiological questions. We substantiate this claim by providing examples of yeast models used for studying neurological diseases.

Requirement of Phosphoinositides Containing Stearic Acid To Control Cell Polarity

Phosphoinositides (PIPs) are present in very small amounts but are essential for cell signaling, morphogenesis, and polarity. By mass spectrometry, we demonstrated that some PIPs with stearic acyl chains were strongly disturbed in a psi1Δ Saccharomyces cerevisiae yeast strain deficient in the specific incorporation of a stearoyl chain at the sn-1 position of phosphatidylinositol. The absence of PIPs containing stearic acid induced disturbances in intracellular trafficking, although the total amount of PIPs was not diminished. Changes in PIPs also induced alterations in the budding pattern and defects in actin cytoskeleton organization (cables and patches). Moreover, when the PSI1 gene was impaired, a high proportion of cells with bipolar cortical actin patches that occurred concomitantly with the bipolar localization of Cdc42p was specifically found among diploid cells. This bipolar cortical actin phenotype, never previously described, was also detected in a bud9Δ/bud9Δ strain. Very interestingly, overexpression of PSI1 reversed this phenotype.

Role of competition between polarity sites in establishing a unique front

Polarity establishment in many cells is thought to occur via positive feedback that reinforces even tiny asymmetries in polarity protein distribution. Cdc42 and related GTPases are activated and accumulate in a patch of the cortex that defines the front of the cell. Positive feedback enables spontaneous polarization triggered by stochastic fluctuations, but as such fluctuations can occur at multiple locations, how do cells ensure that they make only one front? In polarizing cells of the model yeast Saccharomyces cerevisiae, positive feedback can trigger growth of several Cdc42 clusters at the same time, but this multi-cluster stage rapidly evolves to a single-cluster state, which then promotes bud emergence. By manipulating polarity protein dynamics, we show that resolution of multi-cluster intermediates occurs through a greedy competition between clusters to recruit and retain polarity proteins from a shared intracellular pool.

Evolutionary adaptation after crippling cell polarization follows reproducible trajectories

Cells are organized by functional modules, which typically contain components whose removal severely compromises the module's function. Despite their importance, these components are not absolutely conserved between parts of the tree of life, suggesting that cells can evolve to perform the same biological functions with different proteins. We evolved Saccharomyces cerevisiae for 1000 generations without the important polarity gene BEM1. Initially the bem1∆ lineages rapidly increase in fitness and then slowly reach >90% of the fitness of their BEM1 ancestors at the end of the evolution. Sequencing their genomes and monitoring polarization reveals a common evolutionary trajectory, with a fixed sequence of adaptive mutations, each improving cell polarization by inactivating proteins. Our results show that organisms can be evolutionarily robust to physiologically destructive perturbations and suggest that recovery by gene inactivation can lead to rapid divergence in the parts list for cell biologically important functions.

DOI:http://dx.doi.org/10.7554/eLife.09638.001

Cells use the genetic instructions provided by genes in particular combinations called ‘modules’ to perform particular jobs. Very different organisms can share many of the same modules because certain abilities are fundamental to the survival of all cells and so they have been retained over the course of evolution. That said, these modules may not necessarily involve the same genes because it is often possible to achieve the same result using different components.

One way to study how those modules can diversify is to deliberately disrupt one of the genes in a module, and observe how the organism and its descendants respond over many generations. Other genes in these organisms may acquire genetic mutations that enable the genes to take on the role of the missing protein. However, the removal of a single component can be detrimental to the survival of the organisms or may affect many different processes. This can make it difficult to understand what is going on.

A gene called BEM1 is crucial for yeast cells to establish polarity, that is, to allow the different sides of a cell to become distinct from one another. This activity is essential for the yeast to replicate itself. Previous studies have shown that the BEM1 gene had a different role in other species of fungi, which suggests that yeast may have other genes that previously assumed the role that BEM1 does now. In this study, Laan et al. removed BEM1 from yeast and allowed the population of mutant cells to evolve for a thousand generations. The approach differs from previous studies because Laan et al. deliberately selected for yeast that had acquired multiple genetic mutations that can together almost fully compensate for the loss of BEM1.

Initially, the mutant cells grew very slowly, were abnormal in shape and likely to burst open. However, by the end of the experiment, the cells were able to grow almost as well as the original yeast cells had before the gene deletion. Genetic analysis revealed that the deletion of BEM1 triggers the inactivation of other genes that are also involved in the regulation of polarity, which largely restored the ability of the disrupted polarity module to work. This restoration follows a ‘reproducible trajectory’, as the same genes were switched off in the same order in different populations of yeast that were studied at the same time.

The work is an example of reproducible evolution, whereby a specific order of changes to gene activity repeatedly enables cells with severe defects in important processes to adapt and restore a gene module, using whatever components they have left. The next challenge will be to understand how the particular roles of important modules affect their adaptability.

DOI:http://dx.doi.org/10.7554/eLife.09638.002

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

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

Regulation of Cdc42 polarization by the Rsr1 GTPase and Rga1, a Cdc42 GTPase-activating protein, in budding yeast

Cdc42 plays a central role in establishing polarity in yeast and animals, yet how polarization of Cdc42 is achieved in response to spatial cues is poorly understood. Using live-cell imaging, we found distinct dynamics of Cdc42 polarization in haploid budding yeast in correlation with two temporal steps of the G1 phase. The position at which the Cdc42-GTP cluster develops changes rapidly around the division site during the first step but becomes stabilized in the second step, suggesting that an axis of polarized growth is determined in mid G1. Cdc42 polarization in the first step and its proper positioning depend on Rsr1 and its GTPase-activating protein (GAP) Bud2. Interestingly, Rga1, a Cdc42 GAP, exhibits transient localization to a site near the bud neck and to the division site during cytokinesis and G1, and this temporal change of Rga1 distribution is necessary for determination of a proper growth site. Mathematical modeling suggests that a proper axis of Cdc42 polarization in haploid cells might be established through a biphasic mechanism involving sequential positive feedback and transient negative feedback.

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

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

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

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

Imaging methodologies for systems biology

Systems biology has recently achieved significant success in the understanding of complex interconnected phenomena such as cell polarity and migration. In this context, the definition of systems biology has come to encompass the integration of quantitative measurements with sophisticated modeling approaches. This article will review recent progress in live cell imaging technologies that have expanded the possibilities of quantitative in vivo measurements, particularly in regards to molecule counting and quantitative measurements of protein concentration and dynamics. These methods have gained and continue to gain popularity with the biological community. In general, we will discuss three broad categories: protein interactions, protein quantitation, and protein dynamics.

Vesicle trafficking from a lipid perspective: Lipid regulation of exocytosis in Saccharomyces cerevisiae

The protein cargo transported by specific types of vesicles largely defines the different secretory trafficking pathways operating within cells. However, mole per mole the most abundant cargo contained within transport vesicles is not protein, but lipid. Taking a "lipid-centric" point-of-view, we examine the importance of lipid signaling, membrane lipid organization and lipid metabolism for vesicle transport during exocytosis in budding yeast. In fact, the essential requirement for some exocytosis regulatory proteins can be bypassed by making simple manipulations of the lipids involved. During polarized exocytosis the sequential steps required to generate post-Golgi vesicles and target them to the plasma membrane (PM) involves the interplay of several types of lipids that are coordinately linked through PI4P metabolism and signaling. In turn, PI4P levels are regulated by PI4P kinases, the Sac1p PI4P phosphatase and the yeast Osh proteins, which are homologs of mammalian oxysterol-binding protein (OSBP). Together these regulators integrate the transitional steps required for vesicle maturation directly through changes in lipid composition and organization.

RhoGDI: A rheostat for the Rho switch

Regulation of the Rho switch has been typically centered on their main regulators, RhoGEFs and RhoGAPs. On the side, RhoGDI proteins have been considered mostly as passive regulators devoid of catalytic activity simply holding Rho proteins in the cytosol. In the May issue of Nature Cell Biology,1 we describe a novel evolutionary conserved function for RhoGDI1 as a chaperoning protein which prevents degradation of prenylated Rho GTPases. The limited amount of RhoGDI1 in cells generates a competitive balance between GTPases in order to prevent their degradation. Therefore, this creates a crosstalk regulatory mechanism of Rho proteins, whereby the level of one Rho protein can affect both the level and activity of the others. For example, overexpression of a single GTPase will promote the degradation and inactivation of all endogenous Rho proteins bound to GDI. These results suggest that some of the conclusions drawn from studies that manipulate Rho protein levels may need to be reevaluated. Here, we discuss some of the consequences of this mechanism on the regulation of Rho proteins, the dissociation of Rho-RhoGDI complexes by GDF and whether this regulation might be extended to other GTPases of the Ras superfamily.

Quantitative analysis of membrane trafficking in regulation of Cdc42 polarity

Vesicle delivery of Cdc42 has been proposed as an important mechanism for generating and maintaining Cdc42 polarity at the plasma membrane. This mechanism requires the density of Cdc42 on secretory vesicles to be equal to or higher than the plasma membrane polarity cap. Using a novel method to estimate Cdc42 levels on post-Golgi secretory vesicles in intact yeast cells, we: (1) determined that endocytosis plays an important role in Cdc42's association with secretory vesicles (2) found that a GFP-tag placed on the N-terminus of Cdc42 negatively impacts this vesicle association and (3) quantified the surface densities of Cdc42 on post-Golgi vesicles which revealed that the vesicle density of Cdc42 is three times more dilute than that at the polarity cap. This work suggests that the immediate consequence of secretory vesicle fusion with the plasma membrane polarity cap is to dilute the local Cdc42 surface density. This provides strong support for the model in which vesicle trafficking acts to negatively regulate Cdc42 polarity on the cell surface while also providing a means to recycle Cdc42 between the cell surface and internal membrane locations.

Role of Cdc42 dynamics in the control of fission yeast cell polarization

Cell polarization is fundamental to many cellular processes, including cell differentiation, cell motility and cell fate determination. A key regulatory enzyme in the control of cell morphogenesis is the conserved Rho GTPase Cdc42, which breaks symmetry via self-amplifying positive-feedback mechanisms. Additional mechanisms of control, including competition between different sites of polarized cell growth and time-delayed negative feedback, define a cellular-level system that promotes Cdc42 oscillatory dynamics and modulates activated Cdc42 intracellular distribution.

Recent applications of fluorescence correlation spectroscopy in live systems

Fluorescence correlation spectroscopy (FCS) is a widely used technique in biophysics and has helped address many questions in the life sciences. It provides important advantages compared to other fluorescence and biophysical methods. Its single molecule sensitivity allows measuring proteins within biological samples at physiological concentrations without the need of overexpression. It provides quantitative data on concentrations, diffusion coefficients, molecular transport and interactions even in live organisms. And its reliance on simple fluorescence intensity and its fluctuations makes it widely applicable. In this review we focus on applications of FCS in live samples, with an emphasis on work in the last 5 years, in the hope to provide an overview of the present capabilities of FCS to address biologically relevant questions.

Control of the G-protein cascade dynamics by GDP dissociation inhibitors

A network of the Rho family GTPases, which cycle between inactive GDP-bound and active GTP-bound states, controls key cellular processes, including proliferation and migration. Activating and deactivating GTPase transitions are controlled by guanine nucleotide exchange factors (GEFs), GTPase activating proteins (GAPs) and GDP dissociation inhibitors (GDIs) that sequester GTPases from the membrane to the cytoplasm. Here we show that a cascade of two Rho family GTPases, RhoA and Rac1, regulated by RhoGDI1, exhibits distinct modes of the dynamic behavior, including abrupt, bistable switches, excitable overshoot transitions and oscillations. The RhoGDI1 abundance and signal-induced changes in the RhoGDI1 affinity for GTPases control these different dynamics, enabling transitions from a single stable steady state to bistability, to excitable pulses and to sustained oscillations of GTPase activities. These RhoGDI1-controlled dynamic modes of RhoA and Rac1 activities form the basis of cell migration behaviors, including protrusion-retraction cycles at the leading edge of migrating cells.

Cdc42 GTPase dynamics control directional growth responses

Polarized cells reorient their direction of growth in response to environmental cues. In the fungus Candida albicans, the Rho-family small GTPase, Cdc42, is essential for polarized hyphal growth and Ca(2+) influx is required for the tropic responses of hyphae to environmental cues, but the regulatory link between these systems is unclear. In this study, the interaction between Ca(2+) influx and Cdc42 polarity-complex dynamics was investigated using hyphal galvanotropic and thigmotropic responses as reporter systems. During polarity establishment in an applied electric field, cathodal emergence of hyphae was lost when either of the two Cdc42 apical recycling pathways was disrupted by deletion of Rdi1, a guanine nucleotide dissociation inhibitor, or Bnr1, a formin, but was completely restored by extracellular Ca(2+). Loss of the Cdc42 GTPase activating proteins, Rga2 and Bem3, also abolished cathodal polarization, but this was not rescued by Ca(2+). Expression of GTP-locked Cdc42 reversed the polarity of hypha emergence from cathodal to anodal, an effect augmented by Ca(2+). The cathodal directional cue therefore requires Cdc42 GTP hydrolysis. Ca(2+) influx amplifies Cdc42-mediated directional growth signals, in part by augmenting Cdc42 apical trafficking. The Ca(2+)-binding EF-hand motif in Cdc24, the Cdc42 activator, was essential for growth in yeast cells but not in established hyphae. The Cdc24 EF-hand motif is therefore essential for polarity establishment but not for polarity maintenance.

Establishment of a robust single axis of cell polarity by coupling multiple positive feedback loops

Establishment of cell polarity—or symmetry breaking—relies on local accumulation of polarity regulators. Although simple positive feedback is sufficient to drive symmetry breaking, it is highly sensitive to stochastic fluctuations typical for living cells. Here, by integrating mathematical modelling with quantitative experimental validations, we show that in the yeast Saccharomyces cerevisiae a combination of actin- and guanine nucleotide dissociation inhibitor-dependent recycling of the central polarity regulator Cdc42 is needed to establish robust cell polarity at a single site during yeast budding. The guanine nucleotide dissociation inhibitor pathway consistently generates a single-polarization site, but requires Cdc42 to cycle rapidly between its active and inactive form, and is therefore sensitive to perturbations of the GTPase cycle. Conversely, actin-mediated recycling of Cdc42 induces robust symmetry breaking but cannot restrict polarization to a single site. Our results demonstrate how cells optimize symmetry breaking through coupling between multiple feedback loops.

A positive feedback loop which results in localized accumulation of the small GTPase Cdc42 generates cell polarity in budding yeast; however, such loops are inherently susceptible to noise. Here the authors demonstrate how two pathways that mediate Cdc42 recycling work together to ensure the robustness of symmetry breaking.

GDI-mediated cell polarization in yeast provides precise spatial and temporal control of Cdc42 signaling

Cell polarization is a prerequisite for essential processes such as cell migration, proliferation or differentiation. The yeast Saccharomyces cerevisiae under control of the GTPase Cdc42 is able to polarize without the help of cytoskeletal structures and spatial cues through a pathway depending on its guanine nucleotide dissociation inhibitor (GDI) Rdi1. To develop a fundamental understanding of yeast polarization we establish a detailed mechanistic model of GDI-mediated polarization. We show that GDI-mediated polarization provides precise spatial and temporal control of Cdc42 signaling and give experimental evidence for our findings. Cell cycle induced changes of Cdc42 regulation enhance positive feedback loops of active Cdc42 production, and thereby allow simultaneous switch-like regulation of focused polarity and Cdc42 activation. This regulation drives the direct formation of a unique polarity cluster with characteristic narrowing dynamics, as opposed to the previously proposed competition between transient clusters. As the key components of the studied system are conserved among eukaryotes, we expect our findings also to apply to cell polarization in other organisms.

Cell polarization is a fundamental cellular process that defines a single orientation axis within prokaryotic or eukaryotic cells and is a prerequisite for developmental processes such as cell migration, proliferation or differentiation. In the yeast Saccharomyces cerevisiae cell polarization determines the position of a new growth or bud site. Although many studies have focused on identifying polarity regulators and their interactions, the fundamental mechanisms and features of cell polarity still remain controversial. Here, we develop a detailed mathematical model of diffusion-driven cell polarization, which we verify experimentally. We show that this polarization mechanism provides precise spatial and temporal control of signals, which determine the place of a new growth site. Changes induced by the cell cycle allow simultaneous switch-like regulation of polarization and activation of the GTPase Cdc42, the central polarity regulator which initiates formation of a new bud. This regulation drives direct formation of a unique Cdc42 cluster with characteristic narrowing dynamics and robustly narrow spatial focus. Hence, our analysis reveals fundamental design principles that allow cell polarization to reliably initiate developmental processes at a specific time and place. As the key components of the studied system are conserved among eukaryotes, we expect our findings also to apply to cell polarization in other organisms.