Opposing roles for actin in Cdc42p polarization

Actin remodelling controls proteasome homeostasis upon stress

When cells are stressed, bulk translation is often downregulated to reduce energy demands while stress-response proteins are simultaneously upregulated. To promote proteasome assembly and activity and maintain cell viability upon TORC1 inhibition, 19S regulatory-particle assembly chaperones (RPACs) are selectively translated. However, the molecular mechanism for such selective translational upregulation is unclear. Here, using yeast, we discover that remodelling of the actin cytoskeleton is important for RPAC translation following TORC1 inhibition. mRNA of the RPAC ADC17 is associated with actin cables and is enriched at cortical actin patches under stress, dependent upon the early endocytic protein Ede1. ede1∆ cells failed to induce RPACs and proteasome assembly upon TORC1 inhibition. Conversely, artificially tethering ADC17 mRNA to cortical actin patches enhanced its translation upon stress. These findings suggest that actin-dense structures such as cortical actin patches may serve as a translation platform for a subset of stress-induced mRNAs including regulators of proteasome homeostasis.

Williams et al. report that, upon TORC1 inhibition in yeast, mRNA of the chaperone protein ADC17 is localized to cortical actin patches where its translation is enhanced upon stress.

Essential role of the endocytic site-associated protein Ecm25 in stress-induced cell elongation

How cells adopt a different morphology to cope with stress is not well understood. Here, we show that budding yeast Ecm25 associates with polarized endocytic sites and interacts with the polarity regulator Cdc42 and several late-stage endocytic proteins via distinct regions, including an actin filament-binding motif. Deletion of ECM25 does not affect Cdc42 activity or cause any strong defects in fluid-phase and clathrin-mediated endocytosis but completely abolishes hydroxyurea-induced cell elongation. This phenotype is accompanied by depolarization of the spatiotemporally coupled exo-endocytosis in the bud cortex while maintaining the overall mother-bud polarity. These data suggest that Ecm25 provides an essential link between the polarization signal and the endocytic machinery to enable adaptive morphogenesis under stress conditions.

How cells adopt a different morphology to cope with stress is not well understood. Duan et al. report that the budding yeast protein Ecm25 plays an essential role in stress-induced cell elongation by linking the polarity regulator Cdc42 to the late-stage endocytic machinery.

Control of Formin Distribution and Actin Cable Assembly by the E3 Ubiquitin Ligases Dma1 and Dma2

Formins are widespread actin-polymerizing proteins that play pivotal roles in a number of processes, such as cell polarity, morphogenesis, cytokinesis, and cell migration. In agreement with their crucial function, formins are prone to a variety of regulatory mechanisms that include autoinhibition, post-translational modifications, and interaction with formin modulators. Furthermore, activation and function of formins is intimately linked to their ability to interact with membranes. In the budding yeast Saccharomyces cerevisiae, the two formins Bni1 and Bnr1 play both separate and overlapping functions in the organization of the actin cytoskeleton. In addition, they are controlled by both common and different regulatory mechanisms. Here we show that proper localization of both formins requires the redundant E3 ubiquitin ligases Dma1 and Dma2, which were previously involved in spindle positioning and septin organization. In dma1 dma2 double mutants, formin distribution at polarity sites is impaired, thus causing defects in the organization of the actin cable network and hypersensitivity to the actin depolymerizer latrunculin B. Expression of a hyperactive variant of Bni1 (Bni1-V360D) rescues these defects and partially restores proper spindle positioning in the mutant, suggesting that the failure of dma1 dma2 mutant cells to position the spindle is partly due to faulty formin activity. Strikingly, Dma1/2 interact physically with both formins, while their ubiquitin-ligase activity is required for formin function and polarized localization. Thus, ubiquitylation of formin or a formin interactor(s) could promote formin binding to membrane and its ability to nucleate actin. Altogether, our data highlight a novel level of formin regulation that further expands our knowledge of the complex and multilayered controls of these key cytoskeleton organizers.

A Predictive Model for Yeast Cell Polarization in Pheromone Gradients

Budding yeast cells exist in two mating types, a and α, which use peptide pheromones to communicate with each other during mating. Mating depends on the ability of cells to polarize up pheromone gradients, but cells also respond to spatially uniform fields of pheromone by polarizing along a single axis. We used quantitative measurements of the response of a cells to α-factor to produce a predictive model of yeast polarization towards a pheromone gradient. We found that cells make a sharp transition between budding cycles and mating induced polarization and that they detect pheromone gradients accurately only over a narrow range of pheromone concentrations corresponding to this transition. We fit all the parameters of the mathematical model by using quantitative data on spontaneous polarization in uniform pheromone concentration. Once these parameters have been computed, and without any further fit, our model quantitatively predicts the yeast cell response to pheromone gradient providing an important step toward understanding how cells communicate with each other.

A Cellular System for Spatial Signal Decoding in Chemical Gradients

Directional cell growth requires that cells read and interpret shallow chemical gradients, but how the gradient directional information is identified remains elusive. We use single-cell analysis and mathematical modeling to define the cellular gradient decoding network in yeast. Our results demonstrate that the spatial information of the gradient signal is read locally within the polarity site complex using double-positive feedback between the GTPase Cdc42 and trafficking of the receptor Ste2. Spatial decoding critically depends on low Cdc42 activity, which is maintained by the MAPK Fus3 through sequestration of the Cdc42 activator Cdc24. Deregulated Cdc42 or Ste2 trafficking prevents gradient decoding and leads to mis-oriented growth. Our work discovers how a conserved set of components assembles a network integrating signal intensity and directionality to decode the spatial information contained in chemical gradients.

Actin, Membrane Trafficking and the Control of Prion Induction, Propagation and Transmission in Yeast

The model eukaryotic yeast Saccharomyces cerevisiae has proven a useful model system in which prion biogenesis and elimination are studied. Several yeast prions exist in budding yeast and a number of studies now suggest that these alternate protein conformations may play important roles in the cell. During the last few years cellular factors affecting prion induction, propagation and elimination have been identified. Amongst these, proteins involved in the regulation of the actin cytoskeleton and dynamic membrane processes such as endocytosis have been found to play a critical role not only in facilitating de novo prion formation but also in prion propagation. Here we briefly review prion formation and maintenance with special attention given to the cellular processes that require the functionality of the actin cytoskeleton.

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.

Combinatorial genetic analysis of a network of actin disassembly-promoting factors

The patterning of actin cytoskeleton structures in vivo is a product of spatially and temporally regulated polymer assembly balanced by polymer disassembly. While in recent years our understanding of actin assembly mechanisms has grown immensely, our knowledge of actin disassembly machinery and mechanisms has remained comparatively sparse. Saccharomyces cerevisiae is an ideal system to tackle this problem, both because of its amenabilities to genetic manipulation and live‐cell imaging and because only a single gene encodes each of the core disassembly factors: cofilin (COF1), Srv2/CAP (SRV2), Aip1 (AIP1), GMF (GMF1/AIM7), coronin (CRN1), and twinfilin (TWF1). Among these six factors, only the functions of cofilin are essential and have been well defined. Here, we investigated the functions of the nonessential actin disassembly factors by performing genetic and live‐cell imaging analyses on a combinatorial set of isogenic single, double, triple, and quadruple mutants in S. cerevisiae. Our results show that each disassembly factor makes an important contribution to cell viability, actin organization, and endocytosis. Further, our data reveal new relationships among these factors, providing insights into how they work together to orchestrate actin turnover. Finally, we observe specific combinations of mutations that are lethal, e.g., srv2Δ aip1Δ and srv2Δ crn1Δ twf1Δ, demonstrating that while cofilin is essential, it is not sufficient in vivo, and that combinations of the other disassembly factors perform vital functions. © 2015 The Authors. Cytoskeleton Published by Wiley Periodicals, Inc.

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.

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.

Asymmetric cell division in polyploid giant cancer cells and low eukaryotic cells

Asymmetric cell division is critical for generating cell diversity in low eukaryotic organisms. We previously have reported that polyploid giant cancer cells (PGCCs) induced by cobalt chloride demonstrate the ability to use an evolutionarily conserved process for renewal and fast reproduction, which is normally confined to simpler organisms. The budding yeast, Saccharomyces cerevisiae, which reproduces by asymmetric cell division, has long been a model for asymmetric cell division studies. PGCCs produce daughter cells asymmetrically in a manner similar to yeast, in that both use budding for cell polarization and cytokinesis. Here, we review the results of recent studies and discuss the similarities in the budding process between yeast and PGCCs.

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.

Insight into actin organization and function in cytokinesis from analysis of fission yeast mutants

Actin is a key cytoskeletal protein with multiple roles in cellular processes such as polarized growth, cytokinesis, endocytosis, and cell migration. Actin is present in all eukaryotes as highly dynamic filamentous structures, such as linear cables and branched filaments. Detailed investigation of the molecular role of actin in various processes has been hampered due to the multifunctionality of the protein and the lack of alleles defective in specific processes. The actin cytoskeleton of the fission yeast, Schizosaccharomyces pombe, has been extensively characterized and contains structures analogous to those in other cell types. In this study, primarily with the view to uncover actin function in cytokinesis, we generated a large bank of fission yeast actin mutants that affect the organization of distinct actin structures and/or discrete physiological functions of actin. Our screen identified 17 mutants with specific defects in cytokinesis. Some of these cytokinesis mutants helped in dissecting the function of specific actin structures during ring assembly. Further genetic analysis of some of these actin mutants revealed multiple genetic interactions with mutants previously known to affect the actomyosin ring assembly. We also characterize a mutant allele of actin that is suppressed upon overexpression of Cdc8p-tropomyosin, underscoring the utility of this mutant bank. Another 22 mutant alleles, defective in polarized growth and/or other functions of actin obtained from this screen, are also described in this article. This mutant bank should be a valuable resource to study the physiological and biochemical functions of actin.

Signaling regulated endocytosis and exocytosis lead to mating pheromone concentration dependent morphologies in yeast

Polarized cell morphogenesis requires actin cytoskeleton rearrangement for polarized transport of proteins, organelles and secretory vesicles, which fundamentally underlies cell differentiation and behavior. During yeast mating, Saccharomyces cerevisiae responds to extracellular pheromone gradients by extending polarized projections, which are likely maintained through vesicle transport to (exocytosis) and from (endocytosis) the membrane. We experimentally demonstrate that the projection morphology is pheromone concentration-dependent, and propose the underlying mechanism through mathematical modeling. The inclusion of membrane flux and dynamically evolving cell boundary into our yeast mating signaling model shows good agreement with experimental measurements, and provides a plausible explanation for pheromone-induced cell morphology.

Negative feedback enhances robustness in the yeast polarity establishment circuit

Many cells undergo symmetry-breaking polarization toward a randomly oriented "front" in the absence of spatial cues. In budding yeast, such polarization involves a positive feedback loop that enables amplification of stochastically arising clusters of polarity factors. Previous mathematical modeling suggested that, if more than one cluster were amplified, the clusters would compete for limiting resources and the largest would "win," explaining why yeast cells always make one and only one bud. Here, using imaging with improved spatiotemporal resolution, we show the transient coexistence of multiple clusters during polarity establishment, as predicted by the model. Unexpectedly, we also find that initial polarity factor clustering is oscillatory, revealing the presence of a negative feedback loop that disperses the factors. Mathematical modeling predicts that negative feedback would confer robustness to the polarity circuit and make the kinetics of competition between polarity factor clusters relatively insensitive to polarity factor concentration. These predictions are confirmed experimentally.

Cell polarization and cytokinesis in budding yeast

Asymmetric cell division, which includes cell polarization and cytokinesis, is essential for generating cell diversity during development. The budding yeast Saccharomyces cerevisiae reproduces by asymmetric cell division, and has thus served as an attractive model for unraveling the general principles of eukaryotic cell polarization and cytokinesis. Polarity development requires G-protein signaling, cytoskeletal polarization, and exocytosis, whereas cytokinesis requires concerted actions of a contractile actomyosin ring and targeted membrane deposition. In this chapter, we discuss the mechanics and spatial control of polarity development and cytokinesis, emphasizing the key concepts, mechanisms, and emerging questions in the field.

Arp2/3 mediates early endosome dynamics necessary for the maintenance of PAR asymmetry in Caenorhabditis elegans

The widely conserved Arp2/3 complex regulates branched actin dynamics that are necessary for a variety of cellular processes. In Caenorhabditis elegans, the actin cytoskeleton has been extensively characterized in its role in establishing PAR asymmetry; however, the contributions of actin to the maintenance of polarity before the onset of mitosis are less clear. Endocytic recycling has emerged as a key mechanism in the dynamic stabilization of cellular polarity, and the large GTPase dynamin participates in the stabilization of cortical polarity during maintenance phase via endocytosis in C. elegans. Here we show that disruption of Arp2/3 function affects the formation and localization of short cortical actin filaments and foci, endocytic regulators, and polarity proteins during maintenance phase. We detect actin associated with events similar to early endosomal fission, movement of endosomes into the cytoplasm, and endosomal movement from the cytoplasm to the plasma membrane, suggesting the involvement of actin in regulating processes at the early endosome. We also observe aberrant accumulations of PAR-6 cytoplasmic puncta near the centrosome along with early endosomes. We propose a model in which Arp2/3 affects the efficiency of rapid endocytic recycling of polarity cues that ultimately contributes to their stable maintenance.

Morphogenesis and the cell cycle

Studies of the processes leading to the construction of a bud and its separation from the mother cell in Saccharomyces cerevisiae have provided foundational paradigms for the mechanisms of polarity establishment, cytoskeletal organization, and cytokinesis. Here we review our current understanding of how these morphogenetic events occur and how they are controlled by the cell-cycle-regulatory cyclin-CDK system. In addition, defects in morphogenesis provide signals that feed back on the cyclin-CDK system, and we review what is known regarding regulation of cell-cycle progression in response to such defects, primarily acting through the kinase Swe1p. The bidirectional communication between morphogenesis and the cell cycle is crucial for successful proliferation, and its study has illuminated many elegant and often unexpected regulatory mechanisms. Despite considerable progress, however, many of the most puzzling mysteries in this field remain to be resolved.

Exo-endocytic trafficking and the septin-based diffusion barrier are required for the maintenance of Cdc42p polarization during budding yeast asymmetric growth

Cdc42p plays a central role in asymmetric cell growth in yeast by controlling actin organization and vesicular trafficking. However, how Cdc42p is maintained specifically at the daughter cell plasma membrane during asymmetric cell growth is unclear. We have analyzed Cdc42p localization in yeast mutants defective in various stages of membrane trafficking by fluorescence microscopy and biochemical fractionation. We found that two separate exocytic pathways mediate Cdc42p delivery to the daughter cell. Defects in one of these pathways result in Cdc42p being rerouted through the other. In particular, the pathway involving trafficking through endosomes may couple Cdc42p endocytosis from, and subsequent redelivery to, the plasma membrane to maintain Cdc42p polarization at the daughter cell. Although the endo-exocytotic coupling is necessary for Cdc42p polarization, it is not sufficient to prevent the lateral diffusion of Cdc42p along the cell cortex. A barrier function conferred by septins is required to counteract the dispersal of Cdc42p and maintain its localization in the daughter cell but has no effect on the initial polarization of Cdc42p at the presumptive budding site before symmetry breaking. Collectively, membrane trafficking and septins function synergistically to maintain the dynamic polarization of Cdc42p during asymmetric growth in yeast.

Actin-mediated delivery of astral microtubules instructs Kar9p asymmetric loading to the bud-ward spindle pole

We show that Kar9p polarity is instructed by a feedback loop that requires astral microtubules, actin cables, and Myo2p-based transport to enforce Kar9p loading to the bud-ward pole. This novel mechanism also provides the basis for a model unifying Kar9p polarity and the stereotyped pattern of spindle pole inheritance known to occur in yeast.

In Saccharomyces cerevisiae, Kar9p, one player in spindle alignment, guides the bud-ward spindle pole by linking astral microtubule plus ends to Myo2p-based transport along actin cables generated by the formins Bni1p and Bnr1p and the polarity determinant Bud6p. Initially, Kar9p labels both poles but progressively singles out the bud-ward pole. Here, we show that this polarization requires cell polarity determinants, actin cables, and microtubules. Indeed, in a bud6Δ bni1Δ mutant or upon direct depolymerization of actin cables Kar9p symmetry increased. Furthermore, symmetry was selectively induced by myo2 alleles, preventing Kar9p binding to the Myo2p cargo domain. Kar9p polarity was rebuilt after transient disruption of microtubules, dependent on cell polarity and actin cables. Symmetry breaking also occurred after transient depolymerization of actin cables, with Kar9p increasing at the spindle pole engaging in repeated cycles of Kar9p-mediated transport. Kar9p returning to the spindle pole on shrinking astral microtubules may contribute toward this bias. Thus, Myo2p transport along actin cables may support a feedback loop by which delivery of astral microtubule plus ends sustains Kar9p polarized recruitment to the bud-ward spindle pole. Our findings also explain the link between Kar9p polarity and the choice setting aside the old spindle pole for daughter-bound fate.

Polarity and endocytosis: reciprocal regulation

The establishment and maintenance of polarized plasma membrane domains is essential for cellular function and proper development of organisms. The molecules and pathways involved in determining cell polarity are remarkably well conserved between animal species. Historically, exocytic mechanisms have received primary emphasis among trafficking routes responsible for cell polarization. Accumulating evidence now reveals that endocytosis plays an equally important role in the proper localization of key polarity proteins. Intriguingly, some polarity proteins can also regulate the endocytic machinery. Here, we review emerging evidence for the reciprocal regulation between polarity proteins and endocytic pathways, and discuss possible models for how these distinct processes could interact to create separate cellular domains.

Polarization of the yeast pheromone receptor requires its internalization but not actin-dependent secretion

The data presented in this paper suggest that pheromone-induced receptor phosphorylation and internalization, but not actin-dependent directed secretion, are required to establish receptor polarity.

In the best understood models of eukaryotic directional sensing, chemotactic cells maintain a uniform distribution of surface receptors even when responding to chemical gradients. The yeast pheromone receptor is also uniformly distributed on the plasma membrane of vegetative cells, but pheromone induces its polarization into “crescents” that cap the future mating projection. Here, we find that in pheromone-treated cells, receptor crescents are visible before detectable polarization of actin cables and that the receptor can polarize in the absence of actin-dependent directed secretion. Receptor internalization, in contrast, seems to be essential for the generation of receptor polarity, and mutations that deregulate this process confer dramatic defects in directional sensing. We also show that pheromone induces the internalization and subsequent polarization of the mating-specific Gα and Gβ proteins and that the changes in G protein localization depend on receptor internalization and receptor–Gα coupling. Our data suggest that the polarization of the receptor and its G protein precedes actin polarization and is important for gradient sensing. We propose that the establishment of receptor/G protein polarity depends on a novel mechanism involving differential internalization and that this serves to amplify the shallow gradient of activated receptor across the cell.

Rho GTPases: regulation of cell polarity and growth in yeasts

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

Initial polarized bud growth by endocytic recycling in the absence of actin cable-dependent vesicle transport in yeast

Budding yeast mutants in assembly of actin cables, which are thought to be the only actin structures essential for budding, still could form a small bud. Mutations in actin patch endocytic machineries/endocytic recycling factors inhibited this budding, suggesting a mechanism that promotes polarized growth by local recycling of endocytic vesicles.

The assembly of filamentous actin is essential for polarized bud growth in budding yeast. Actin cables, which are assembled by the formins Bni1p and Bnr1p, are thought to be the only actin structures that are essential for budding. However, we found that formin or tropomyosin mutants, which lack actin cables, are still able to form a small bud. Additional mutations in components for cortical actin patches, which are assembled by the Arp2/3 complex to play a pivotal role in endocytic vesicle formation, inhibited this budding. Genes involved in endocytic recycling were also required for small-bud formation in actin cable-less mutants. These results suggest that budding yeast possesses a mechanism that promotes polarized growth by local recycling of endocytic vesicles. Interestingly, the type V myosin Myo2p, which was thought to use only actin cables to track, also contributed to budding in the absence of actin cables. These results suggest that some actin network may serve as the track for Myo2p-driven vesicle transport in the absence of actin cables or that Myo2p can function independent of actin filaments. Our results also show that polarity regulators including Cdc42p were still polarized in mutants defective in both actin cables and cortical actin patches, suggesting that the actin cytoskeleton does not play a major role in cortical assembly of polarity regulators in budding yeast.

Membrane organization and dynamics in cell polarity

The establishment and maintenance of cell polarity is important to a wide range of biological processes ranging from chemotaxis to embryogenesis. An essential feature of cell polarity is the asymmetric organization of proteins and lipids in the plasma membrane. In this article, we discuss how polarity regulators such as small GTP-binding proteins and phospholipids spatially and kinetically control vesicular trafficking and membrane organization. Conversely, we discuss how membrane trafficking contributes to cell polarization through delivery of polarity determinants and regulators to the plasma membrane.

Phosphoinositides, exocytosis and polarity in yeast: all about actin?

Cell polarity is necessary for cell division, morphogenesis and motility in eukaryotes, and is determined by dynamic control of the cytoskeleton and secretory pathway to promote directional growth. In yeast, three essential and tightly-regulated processes orchestrate polarization and facilitate bud growth. These processes include phosphoinositide (PI) signaling, Rho GTPase regulation of the actin cytoskeleton, and exocytosis. As yet, the interplay between these different processes is unclear, and two main models (Spatial Landmark and Allosteric Local Activation) have been proposed for Rho GTPase control of polarization in yeast. Here, we summarize the inter-relationship between these growth processes and present a more unified model, the Exocytic Signal model, which proposes that exocytosis and actin regulation are fully integrated events mediated by PI signaling.

Rebuilding cytoskeleton roads: active-transport-induced polarization of cells

Many cellular processes require a polarization axis which generally initially emerges as an inhomogeneous distribution of molecular markers in the cell. We present a simple analytical model of a general mechanism of cell polarization taking into account the positive feedback due to the coupled dynamics of molecular markers and cytoskeleton filaments. We find that the geometry of the organization of cytoskeleton filaments, nucleated on the membrane (e.g., cortical actin) or from a center in the cytoplasm (e.g., microtubule asters), dictates whether the system is capable of spontaneous polarization or polarizes only in response to external asymmetric signals. Our model also captures the main features of recent experiments of cell polarization in two considerably different biological systems, namely, mating budding yeast and neuron growth cones.

SLA2 mutations cause SWE1-mediated cell cycle phenotypes in Candida albicans and Saccharomyces cerevisiae

The early endocytic patch protein Sla2 is important for morphogenesis and growth rates in Saccharomyces cerevisiae and Candida albicans, but the mechanism that connects these processes is not clear. Here we report that growth defects in cells lacking CaSLA2 or ScSLA2 are associated with a cell cycle delay that is influenced by Swe1, a morphogenesis checkpoint kinase. To establish how Swe1 monitors Sla2 function, we compared actin organization and cell cycle dynamics in strains lacking other components of early endocytic patches (Sla1 and Abp1) with those in strains lacking Sla2. Only sla2 strains had defects in actin cables, a known trigger of the morphogenesis checkpoint, yet all three strains exhibited Swe1-dependent phenotypes. Thus, Swe1 appears to monitor actin patch in addition to actin cable function. Furthermore, Swe1 contributed to virulence in a mouse model of disseminated candidiasis, implying a role for the morphogenesis checkpoint during the pathogenesis of C. albicans infections.

Symmetry breaking in the life cycle of the budding yeast

The budding yeast Saccharomyces cerevisiae has been an invaluable model system for the study of the establishment of cellular asymmetry and growth polarity in response to specific physiological cues. A large body of experimental observations has shown that yeast cells are able to break symmetry and establish polarity through two coupled and partially redundant intrinsic mechanisms, even in the absence of any pre-existing external asymmetry. One of these mechanisms is dependent upon interplay between the actin cytoskeleton and the Rho family GTPase Cdc42, whereas the other relies on a Cdc42 GTPase signaling network. Integral to these mechanisms appear to be positive feedback loops capable of amplifying small and stochastic asymmetries. Spatial cues, such as bud scars and pheromone gradients, orient cell polarity by modulating the regulation of the Cdc42 GTPase cycle, thereby biasing the site of asymmetry amplification.

Symmetry breaking: scaffold plays matchmaker for polarity signaling proteins

Many cell types can spontaneously polarize even in the absence of specific positional cues. In budding yeast, this symmetry-breaking polarization depends on a scaffold protein called Bem1p. A recent study defines Bem1p's molecular function during symmetry breaking.

Dissecting the involvement of formins in Bud6p-mediated cortical capture of microtubules in S. cerevisiae

In S. cerevisiae, spindle orientation is linked to the inheritance of the `old' spindle pole by the bud. A player in this asymmetric commitment, Bud6p, promotes cortical capture of astral microtubules. Additionally, Bud6p stimulates actin cable formation though the formin Bni1p. A relationship with the second formin, Bnr1p, is unclear. Another player is Kar9p, a protein that guides microtubules along actin cables organised by formins. Here, we ask whether formins mediate Bud6p-dependent microtubule capture beyond any links to Kar9p and actin. We found that both formins control Bud6p localisation. bni1 mutations advanced recruitment of Bud6p at the bud neck, ahead of spindle assembly, whereas bnr1Δ reduced Bud6p association with the bud neck. Accordingly, bni1 or bnr1 mutations redirected microtubule capture to or away from the bud neck, respectively. Furthermore, a Bni1p truncation that can form actin cables independently of Bud6p could not bypass a bud6Δ for microtubule capture. Conversely, Bud61-565p, a truncation insufficient for correct actin organisation via formins, supported microtubule capture. Finally, Bud6p or Bud61-565p associated with microtubules in vitro. Thus, surprisingly, Bud6p may promote microtubule capture independently of its links to actin organisation, whereas formins would contribute to the program of Bud6p-dependent microtubule-cortex interactions by controlling Bud6p localisation.

Beyond polymer polarity: how the cytoskeleton builds a polarized cell

Cell polarity relies on the asymmetric organization of cellular components and structures. Actin and microtubules are well suited to provide the structural basis for cell polarization because of their inherent structural polarity along the polymer lattices and intrinsic dynamics that allow them to respond rapidly to polarity cues. In general, the actin cytoskeleton drives the symmetry-breaking process that enables the establishment of a polarized distribution of regulatory molecules, whereas microtubules build on this asymmetry and maintain the stability of the polarized organization. Crosstalk coordinates the functions of the two cytoskeletal systems.

Swf1p, a member of the DHHC-CRD family of palmitoyltransferases, regulates the actin cytoskeleton and polarized secretion independently of its DHHC motif

Rho and Rab family GTPases play a key role in cytoskeletal organization and vesicular trafficking, but the exact mechanisms by which these GTPases regulate polarized cell growth are incompletely understood. A previous screen for genes that interact with CDC42, which encodes a Rho GTPase, found SWF1/PSL10. Here, we show Swf1p, a member of the DHHC-CRD family of palmitoyltransferases, localizes to actin cables and cortical actin patches in Saccharomyces cerevisiae. Deletion of SWF1 results in misorganization of the actin cytoskeleton and decreased stability of actin filaments in vivo. Cdc42p localization depends upon Swf1p primarily after bud emergence. Importantly, we revealed that the actin regulating activity of Swf1p is independent of its DHHC motif. A swf1 mutant, in which alanine substituted for the cysteine required for the palmitoylation activity of DHHC-CRD proteins, displayed wild-type actin organization and Cdc42p localization. Bgl2p-marked exocytosis was found wild type in this mutant, although invertase secretion was impaired. These data indicate Swf1p has at least two distinct functions, one of which regulates actin organization and Bgl2p-marked secretion. This report is the first to link the function of a DHHC-CRD protein to Cdc42p and the regulation of the actin cytoskeleton.

Polarized growth in the absence of F-actin in Saccharomyces cerevisiae exiting quiescence

BACKGROUND

Polarity establishment and maintenance are crucial for morphogenesis and development. In budding yeast, these two intricate processes involve the superposition of regulatory loops between polarity landmarks, RHO GTPases, actin-mediated vesicles transport and endocytosis. Deciphering the chronology and the significance of each molecular step of polarized growth is therefore very challenging.

PRINCIPAL FINDINGS

We have taken advantage of the fact that yeast quiescent cells display actin bodies, a non polarized actin structure, to evaluate the role of F-actin in bud emergence. Here we show that upon exit from quiescence, actin cables are not required for the first steps of polarized growth. We further show that polarized growth can occur in the absence of actin patch-mediated endocytosis. We finally establish, using latrunculin-A, that the first steps of polarized growth do not require any F-actin containing structures. Yet, these structures are required for the formation of a bona fide daughter cell and cell cycle completion. We propose that upon exit from quiescence in the absence of F-actin, secretory vesicles randomly reach the plasma membrane but preferentially dock and fuse where polarity cues are localized, this being sufficient to trigger polarized growth.

Interference with actin dynamics is superior to disturbance of microtubule function in the inhibition of human ovarian cancer cell motility

Cellular movement is mainly orchestrated by the actin and microtubule cytoskeleton in which Rho GTPases closely collaborate. We studied whether cytoskeleton-interfering agents at subtoxic and 50% growth-inhibiting concentrations affect motility of five unselected human ovarian cancer cell lines. Cisplatin and doxorubicin as control cytotoxic agents were not effective, the microtubule-targeting agents docetaxel, epothilone B and vinblastine only marginally inhibited cell motility, while the actin-targeting agent cytochalasin D was most potent in hampering both cell migration and invasion. Disturbance of microtubule dynamics by docetaxel did not importantly affect the cellular structures of beta-tubulin and F-actin. In contrast, hindrance of actin dynamics by cytochalasin D resulted in loss of lamellipodial extensions, induced thick layers of F-actin and disorder in cellular organization. In OVCAR-3 cells the activity of Rac1 was only slightly diminished by docetaxel, but clearly reduced by cytochalasin D. In conclusion, targeting the actin cytoskeleton might provide a means to prevent metastasis formation.

Afi1p functions as an Arf3p polarization-specific docking factor for development of polarity

ADP-ribosylation factors (Arfs) are highly conserved small GTPases and are critical components of vesicle trafficking. Yeast Arf3p, despite its similarity to mammalian Arf6, is not required for endocytosis but is involved in polarity development. In this study, we identified an Arf3p interacting protein 1 (Afi1p), which, through its N-terminal conserved region, specifically interacts with GTP-bound Arf3p. Afi1p is distributed asymmetrically at the plasma membrane and is required for polarized distribution of Arf3p but not of an Arf3p guanine nucleotide-exchange factor, Yel1p. However, Afi1p is not required for targeting of Arf3p or Yel1p to the plasma membrane. Like arf3 mutant yeast, afi1 mutant yeast exhibited an abnormal budding pattern and partially delayed actin patch polarization. An Afi1p, (38)KLGP4A-Afi1p, mutated at the Arf3p-binding region, loses its ability to interact with Arf3p and maintain the polarized distribution of Arf3p. Although (38)KLGP4A-Afi1p still possessed a proper polarized distribution, it lost its ability to rescue actin patch polarization in afi1 mutant cells. Our findings demonstrate that Afi1p functions as an Arf3p polarization-specific adapter and participates in development of polarity.

Quantitative analysis of robustness and fragility in biological networks based on feedback dynamics

MOTIVATION

It has been widely reported that biological networks are robust against perturbations such as mutations. On the contrary, it has also been known that biological networks are often fragile against unexpected mutations. There is a growing interest in these intriguing observations and the underlying design principle that causes such robust but fragile characteristics of biological networks. For relatively small networks, a feedback loop has been considered as an important motif for realizing the robustness. It is still, however, not clear how a number of coupled feedback loops actually affect the robustness of large complex biological networks. In particular, the relationship between fragility and feedback loops has not yet been investigated till now.

RESULTS

Through extensive computational experiments, we found that networks with a larger number of positive feedback loops and a smaller number of negative feedback loops are likely to be more robust against perturbations. Moreover, we found that the nodes of a robust network subject to perturbations are mostly involved with a smaller number of feedback loops compared with the other nodes not usually subject to perturbations. This topological characteristic eventually makes the robust network fragile against unexpected mutations at the nodes not previously exposed to perturbations.

Transbilayer phospholipid flipping regulates Cdc42p signaling during polarized cell growth via Rga GTPase-activating proteins

An important problem in polarized morphogenesis is how polarized transport of membrane vesicles is spatiotemporally regulated. Here, we report that a local change in the transbilayer phospholipid distribution of the plasma membrane regulates the axis of polarized growth. Type 4 P-type ATPases Lem3p-Dnf1p and -Dnf2p are putative heteromeric phospholipid flippases in budding yeast that are localized to polarized sites on the plasma membrane. The lem3Delta mutant exhibits prolonged apical growth due to a defect in the switch to isotropic bud growth. In lem3Delta cells, the small GTPase Cdc42p remains polarized at the bud tip where phosphatidylethanolamine remains exposed on the outer leaflet. Intriguingly, phosphatidylethanolamine and phosphatidylserine stimulate GTPase-activating protein (GAP) activity of Rga1p and Rga2p toward Cdc42p, whereas PI(4,5)P(2) inhibits it. We propose that a redistribution of phospholipids to the inner leaflet of the plasma membrane triggers the dispersal of Cdc42p from the apical growth site, through activation of GAPs.

Tight junction and polarity interaction in the transporting epithelial phenotype

Development of tight junctions and cell polarity in epithelial cells requires a complex cellular machinery to execute an internal program in response to ambient cues. Tight junctions, a product of this machinery, can act as gates of the paracellular pathway, fences that keep the identity of plasma membrane domains, bridges that communicate neighboring cells. The polarization internal program and machinery are conserved in yeast, worms, flies and mammals, and in cell types as different as epithelia, neurons and lymphocytes. Polarization and tight junctions are dynamic features that change during development, in response to physiological and pharmacological challenges and in pathological situations like infection.

mRNAs encoding polarity and exocytosis factors are cotransported with the cortical endoplasmic reticulum to the incipient bud in Saccharomyces cerevisiae

Polarized growth in the budding yeast Saccharomyces cerevisiae depends upon the asymmetric localization and enrichment of polarity and secretion factors at the membrane prior to budding. We examined how these factors (i.e., Cdc42, Sec4, and Sro7) reach the bud site and found that their respective mRNAs localize to the tip of the incipient bud prior to nuclear division. Asymmetric mRNA localization depends upon factors that facilitate ASH1 mRNA localization (e.g., the 3' untranslated region, She proteins 1 to 5, Puf6, actin cytoskeleton, and a physical association with She2). mRNA placement precedes protein enrichment and subsequent bud emergence, implying that mRNA localization contributes to polarization. Correspondingly, mRNAs encoding proteins which are not asymmetrically distributed (i.e., Snc1, Mso1, Tub1, Pex3, and Oxa1) are not polarized. Finally, mutations which affect cortical endoplasmic reticulum (ER) entry and anchoring in the bud (myo4Delta, sec3Delta, and srp101) also affect asymmetric mRNA localization. Bud-localized mRNAs, including ASH1, were found to cofractionate with ER microsomes in a She2- and Sec3-dependent manner; thus, asymmetric mRNA transport and cortical ER inheritance are connected processes in yeast.

Endocytosis optimizes the dynamic localization of membrane proteins that regulate cortical polarity

Diverse cell types require the ability to maintain dynamically polarized membrane-protein distributions through balancing transport and diffusion. However, design principles underlying dynamically maintained cortical polarity are not well understood. Here we constructed a mathematical model for characterizing the morphology of dynamically polarized protein distributions. We developed analytical approaches for measuring all model parameters from single-cell experiments. We applied our methods to a well-characterized system for studying polarized membrane proteins: budding yeast cells expressing activated Cdc42. We found that a balance of diffusion, directed transport, and endocytosis was sufficient for accurately describing polarization morphologies. Surprisingly, the model predicts that polarized regions are defined with a precision that is nearly optimal for measured endocytosis rates and that polarity can be dynamically stabilized through positive feedback with directed transport. Our approach provides a step toward understanding how biological systems shape spatially precise, unambiguous cortical polarity domains using dynamic processes.

Central roles of small GTPases in the development of cell polarity in yeast and beyond

SUMMARY

The establishment of cell polarity is critical for the development of many organisms and for the function of many cell types. A large number of studies of diverse organisms from yeast to humans indicate that the conserved, small-molecular-weight GTPases function as key signaling proteins involved in cell polarization. The budding yeast Saccharomyces cerevisiae is a particularly attractive model because it displays pronounced cell polarity in response to intracellular and extracellular cues. Cells of S. cerevisiae undergo polarized growth during various phases of their life cycle, such as during vegetative growth, mating between haploid cells of opposite mating types, and filamentous growth upon deprivation of nutrition such as nitrogen. Substantial progress has been made in deciphering the molecular basis of cell polarity in budding yeast. In particular, it becomes increasingly clear how small GTPases regulate polarized cytoskeletal organization, cell wall assembly, and exocytosis at the molecular level and how these GTPases are regulated. In this review, we discuss the key signaling pathways that regulate cell polarization during the mitotic cell cycle and during mating.

Coupled positive and negative feedback circuits form an essential building block of cellular signaling pathways

Cellular circuits have positive and negative feedback loops that allow them to respond properly to noisy external stimuli. It is intriguing that such feedback loops exist in many cases in a particular form of coupled positive and negative feedback loops with different time delays. As a result of our mathematical simulations and investigations into various experimental evidences, we found that such coupled feedback circuits can rapidly turn on a reaction to a proper stimulus, robustly maintain its status, and immediately turn off the reaction when the stimulus disappears. In other words, coupled feedback loops enable cellular systems to produce perfect responses to noisy stimuli with respect to signal duration and amplitude. This suggests that coupled positive and negative feedback loops form essential signal transduction motifs in cellular signaling systems.

Rho GTPase activity zones and transient contractile arrays

The Rho GTPases-Rho, Rac and Cdc42-act as molecular switches, cycling between an active GTP-bound state and an inactive GDP-bound state, to regulate the actin cytoskeleton. It has recently become apparent that the Rho GTPases can be activated in subcellular zones that appear semi-stable, yet are dynamically maintained. These Rho GTPase activity zones are associated with a variety of fundamental biological processes including symmetric and asymmetric cytokinesis and cellular wound repair. Here we review the basic features of Rho GTPase activity zones, suggest that these zones represent a fundamental signaling mechanism, and discuss the implications of zone properties from the perspective of both their function and how they are likely to be controlled.

Homologues of Oxysterol-Binding Proteins Affect Cdc42p- and Rho1p-Mediated Cell Polarization in Saccharomyces cerevisiae

Polarized cell growth requires the establishment of an axis of growth along which secretion can be targeted to a specific site on the cell cortex. How polarity establishment and secretion are choreographed is not fully understood, though Rho GTPase- and Rab GTPase-mediated signaling is required. Superimposed on this regulation are the functions of specific lipids and their cognate binding proteins. In a screen for Saccharomyces cerevisiae genes that interact with Rho family CDC42 to promote polarity establishment, we identified KES1/OSH4, which encodes a homologue of mammalian oxysterol-binding protein (OSBP). Other yeast OSH genes (OSBP homologues) had comparable genetic interactions with CDC42, implicating OSH genes in the regulation of CDC42-dependent polarity establishment. We found that the OSH gene family (OSH1-OSH7) promotes cell polarization by maintaining the proper localization of septins, the Rho GTPases Cdc42p and Rho1p, and the Rab GTPase Sec4p. Disruption of all OSH gene function caused specific defects in polarized exocytosis, indicating that the Osh proteins are collectively required for a secretory pathway implicated in the maintenance of polarized growth.

Epsin N-terminal homology domains perform an essential function regulating Cdc42 through binding Cdc42 GTPase-activating proteins

Epsins are endocytic proteins with a structured epsin N-terminal homology (ENTH) domain that binds phosphoinositides and a poorly structured C-terminal region that interacts with ubiquitin and endocytic machinery, including clathrin and endocytic scaffolding proteins. Yeast has two redundant genes encoding epsins, ENT1 and ENT2; deleting both genes is lethal. We demonstrate that the ENTH domain is both necessary and sufficient for viability of ent1Deltaent2Delta cells. Mutational analysis of the ENTH domain revealed a surface patch that is essential for viability and that binds guanine nucleotide triphosphatase-activating proteins for Cdc42, a critical regulator of cell polarity in all eukaryotes. Furthermore, the epsins contribute to regulation of specific Cdc42 signaling pathways in yeast cells. These data support a model in which the epsins function as spatial and temporal coordinators of endocytosis and cell polarity.