Dual modes of cdc42 recycling fine-tune polarized morphogenesis

Bem3: Filling the GAP between cell polarity and secretion

A highly conserved member of the Rho family of small GTPases, Cdc42 functions as the "master regulator of cell polarity." It has been reported that for proper establishment and maintenance of cell polarity, Cdc42 regulates and requires vesicle trafficking. Importantly, we recently discovered that in budding yeast, vesicle trafficking also controls the localization and function of Bem3, a GTPase activating protein for Cdc42. Specifically, we observed that Bem3 partitioned between the plasma membrane and an internal membrane-bound compartment. This Bem3-containing compartment was present during extended periods of apical growth, required actin tracks for trafficking to polarized sites and functioned as a recycling station that was positioned at the junction of endocytic and secretory pathways. Strikingly, many of these features are reminiscent of the Spitzenkörper, a dynamic structure involved in polarized growth during hyphal development in several filamentous fungi. Furthermore, Bem3 was not merely a passive cargo but actively recruited the secretory Rab GTPase Sec4 to this Spitzenkörper-like compartment. Importantly, this function of Bem3 was independent of its GAP activity. Our work demonstrates the existence of a complementary regulation between Bem3, a regulator of Cdc42 signaling and Sec4, a key component of the secretory machinery.

Model of fission yeast cell shape driven by membrane-bound growth factors and the cytoskeleton

Fission yeast serves as a model for how cellular polarization machinery consisting of signaling molecules and the actin and microtubule cytoskeleton regulates cell shape. In this work, we develop mathematical models to investigate how these cells maintain a tubular shape of approximately constant diameter. Many studies identify active Cdc42, found in a cap at the inner membrane of growing cell tips, as an important regulator of local cell wall remodeling, likely through control of exocyst tethering and the targeting of other polarity-enhancing structures. First, we show that a computational model with Cdc42-dependent local cell wall remodeling under turgor pressure predicts a relationship between spatial extent of growth signal and cell diameter that is in agreement with prior experiments. Second, we model the consequences of feedback between cell shape and distribution of Cdc42 growth signal at cell tips. We show that stability of cell diameter over successive cell divisions places restrictions on their mutual dependence. We argue that simple models where the spatial extent of the tip growth signal relies solely on geometrical alignment of confined microtubules might lead to unstable width regulation. Third, we study a computational model that combines a growth signal distributed over a characteristic length scale (as, for example, by a reaction-diffusion mechanism) with an axis-sensing microtubules system that places landmarks at positions where microtubule tips touch the cortex. A two-dimensional implementation of this model leads to stable cell diameter for a wide range of parameters. Changes to the parameters of this model reproduce straight, bent, and bulged cell shapes, and we discuss how this model is consistent with other observed cell shapes in mutants. Our work provides an initial quantitative framework for understanding the regulation of cell shape in fission yeast, and a scaffold for understanding this process on a more molecular level in the future.

Fission yeast is a rod-shaped organism that is studied, in part, as a model for how cells develop and regulate their shape. Despite extensive work identifying effects of genetic mutations and pharmacological treatments on the shape of these cells, there is a lack of mathematical and computational models examining how internal cell signals and the cytoskeleton organize to remodel the cell wall, direct growth at cell tips, and maintain tubular shape. In this work we describe how the spatial distribution of regulatory protein signal at growing cell tips relates to cell diameter. Further, we describe the consequences of this signal depending on the shape of the cell, namely its length and diameter. Finally, we propose a computational model for understanding growth and shape that includes an axis-sensing microtubule system, landmarks delivered to cell tips along those microtubules, and a growth zone signal that moves around but is attracted to the landmarks. This picture explains a large number of reported abnormal shapes in terms of only a few modular components.

Independence of symmetry breaking on Bem1-mediated autocatalytic activation of Cdc42

Rather than acting directly on Cdc42, Bem1 works in concert with the Cdc42 binding partner Rdi1 to relocalize Cdc42 to the cytosol during symmetry breaking in the absence of an intact actin cytoskeleton.

The ability to break symmetry and polarize through self-organization is a fundamental feature of cellular systems. A prevailing theory in yeast posits that symmetry breaking occurs via a positive feedback loop, wherein the adaptor protein Bem1 promotes local activation and accumulation of Cdc42 by directly tethering Cdc42^GTP with its guanine nucleotide exchange factor (GEF) Cdc24. In this paper, we find that neither Bem1 nor the ability of Bem1 to bind Cdc42^GTP is required for cell polarization. Instead, Bem1 functions primarily by boosting GEF activity, a role critical for polarization without actin filaments. In the absence of actin-based transport, polarization of Cdc42 is accomplished through Rdi1, the Cdc42 guanine nucleotide dissociation inhibitor. A mathematical model is constructed describing cell polarization as a product of distinct pathways controlling Cdc42 activation and protein localization. The model predicts a nonmonotonic dependence of cell polarization on the concentration of Rdi1 relative to that of Cdc42.

Interaction between bud-site selection and polarity-establishment machineries in budding yeast

Saccharomyces cerevisiae yeast cells polarize in order to form a single bud in each cell cycle. Distinct patterns of bud-site selection are observed in haploid and diploid cells. Genetic approaches have identified the molecular machinery responsible for positioning the bud site: during bud formation, specific locations are marked with immobile landmark proteins. In the next cell cycle, landmarks act through the Ras-family GTPase Rsr1 to promote local activation of the conserved Rho-family GTPase, Cdc42. Additional Cdc42 accumulates by positive feedback, creating a concentrated patch of GTP-Cdc42, which polarizes the cytoskeleton to promote bud emergence. Using time-lapse imaging and mathematical modelling, we examined the process of bud-site establishment. Imaging reveals unexpected effects of the bud-site-selection system on the dynamics of polarity establishment, raising new questions about how that system may operate. We found that polarity factors sometimes accumulate at more than one site among the landmark-specified locations, and we suggest that competition between clusters of polarity factors determines the final location of the Cdc42 cluster. Modelling indicated that temporally constant landmark-localized Rsr1 would weaken or block competition, yielding more than one polarity site. Instead, we suggest that polarity factors recruit Rsr1, effectively sequestering it from other locations and thereby terminating landmark activity.

Bem3, a Cdc42 GTPase-activating protein, traffics to an intracellular compartment and recruits the secretory Rab GTPase Sec4 to endomembranes

Cell polarity is essential for many cellular functions including division and cell-fate determination. Although RhoGTPase signaling and vesicle trafficking are both required for the establishment of cell polarity, the mechanisms by which they are coordinated are unclear. Here, we demonstrate that the yeast RhoGAP (GTPase activating protein), Bem3, is targeted to sites of polarized growth by the endocytic and recycling pathways. Specifically, deletion of SLA2 or RCY1 led to mislocalization of Bem3 to depolarized puncta and accumulation in intracellular compartments, respectively. Bem3 partitioned between the plasma membrane and an intracellular membrane-bound compartment. These Bem3-positive structures were polarized towards sites of bud emergence and were mostly observed during the pre-mitotic phase of apical growth. Cell biological and biochemical approaches demonstrated that this intracellular Bem3 compartment contained markers for both the endocytic and secretory pathways, which were reminiscent of the Spitzenkörper present in the hyphal tips of growing fungi. Importantly, Bem3 was not a passive cargo, but recruited the secretory Rab protein, Sec4, to the Bem3-containing compartments. Moreover, Bem3 deletion resulted in less efficient localization of Sec4 to bud tips during early stages of bud emergence. Surprisingly, these effects of Bem3 on Sec4 were independent of its GAP activity, but depended on its ability to efficiently bind endomembranes. This work unveils unsuspected and important details of the relationship between vesicle traffic and elements of the cell polarity machinery: (1) Bem3, a cell polarity and peripherally associated membrane protein, relies on vesicle trafficking to maintain its proper localization; and (2) in turn, Bem3 influences secretory vesicle trafficking.

Non-uniform membrane diffusion enables steady-state cell polarization via vesicular trafficking

Actin-based vesicular trafficking of Cdc42, leading to a polarized concentration of the GTPase, has been implicated in cell polarization, but it was recently debated whether this mechanism allows stable maintenance of cell polarity. Here we show that endocytosis and exocytosis are spatially segregated in the polar plasma membrane, with sites of exocytosis correlating with microdomains of higher concentration and slower diffusion of Cdc42 compared with surrounding regions. Numerical simulations using experimentally obtained diffusion coefficients and trafficking geometry revealed that non-uniform membrane diffusion of Cdc42 in fact enables temporally sustained cell polarity. We show further that phosphatidylserine, a phospholipid recently found to be crucial for cell polarity, is enriched in Cdc42 microdomains. Weakening a potential interaction between phosphatidylserine and Cdc42 enhances Cdc42 diffusion in the microdomains but impedes the strength of polarization. These findings demonstrate a critical role for membrane microdomains in vesicular trafficking-mediated cell polarity.

Polarization of diploid daughter cells directed by spatial cues and GTP hydrolysis of Cdc42 budding yeast

Cell polarization occurs along a single axis that is generally determined by a spatial cue. Cells of the budding yeast exhibit a characteristic pattern of budding, which depends on cell-type-specific cortical markers, reflecting a genetic programming for the site of cell polarization. The Cdc42 GTPase plays a key role in cell polarization in various cell types. Although previous studies in budding yeast suggested positive feedback loops whereby Cdc42 becomes polarized, these mechanisms do not include spatial cues, neglecting the normal patterns of budding. Here we combine live-cell imaging and mathematical modeling to understand how diploid daughter cells establish polarity preferentially at the pole distal to the previous division site. Live-cell imaging shows that daughter cells of diploids exhibit dynamic polarization of Cdc42-GTP, which localizes to the bud tip until the M phase, to the division site at cytokinesis, and then to the distal pole in the next G1 phase. The strong bias toward distal budding of daughter cells requires the distal-pole tag Bud8 and Rga1, a GTPase activating protein for Cdc42, which inhibits budding at the cytokinesis site. Unexpectedly, we also find that over 50% of daughter cells lacking Rga1 exhibit persistent Cdc42-GTP polarization at the bud tip and the distal pole, revealing an additional role of Rga1 in spatiotemporal regulation of Cdc42 and thus in the pattern of polarized growth. Mathematical modeling indeed reveals robust Cdc42-GTP clustering at the distal pole in diploid daughter cells despite random perturbation of the landmark cues. Moreover, modeling predicts different dynamics of Cdc42-GTP polarization when the landmark level and the initial level of Cdc42-GTP at the division site are perturbed by noise added in the model.

Endocytosis puts exocytosis in pole position

Researchers find that endocytosis confines exocytosis to narrow zone during cell polarization.

Robust polarity establishment occurs via an endocytosis-based cortical corralling mechanism

Cell polarity can be established via the spatial coordination of the opposing membrane trafficking activities of endocytosis and exocytosis.

Formation of a stable polarity axis underlies numerous biological processes. Here, using high-resolution imaging and complementary mathematical modeling we find that cell polarity can be established via the spatial coordination of opposing membrane trafficking activities: endocytosis and exocytosis. During polarity establishment in budding yeast, these antagonistic processes become apposed. Endocytic vesicles corral a central exocytic zone, tightening it to a vertex that establishes the polarity axis for the ensuing cell cycle. Concomitantly, the endocytic system reaches an equilibrium where internalization events occur at a constant frequency. Endocytic mutants that failed to initiate periodic internalization events within the corral displayed wide, unstable polarity axes. These results, predicted by in silico modeling and verified by high resolution in vivo studies, identify a requirement for endocytic corralling during robust polarity establishment.

Structure and function in the budding yeast nucleus

Budding yeast, like other eukaryotes, carries its genetic information on chromosomes that are sequestered from other cellular constituents by a double membrane, which forms the nucleus. An elaborate molecular machinery forms large pores that span the double membrane and regulate the traffic of macromolecules into and out of the nucleus. In multicellular eukaryotes, an intermediate filament meshwork formed of lamin proteins bridges from pore to pore and helps the nucleus reform after mitosis. Yeast, however, lacks lamins, and the nuclear envelope is not disrupted during yeast mitosis. The mitotic spindle nucleates from the nucleoplasmic face of the spindle pole body, which is embedded in the nuclear envelope. Surprisingly, the kinetochores remain attached to short microtubules throughout interphase, influencing the position of centromeres in the interphase nucleus, and telomeres are found clustered in foci at the nuclear periphery. In addition to this chromosomal organization, the yeast nucleus is functionally compartmentalized to allow efficient gene expression, repression, RNA processing, genomic replication, and repair. The formation of functional subcompartments is achieved in the nucleus without intranuclear membranes and depends instead on sequence elements, protein-protein interactions, specific anchorage sites at the nuclear envelope or at pores, and long-range contacts between specific chromosomal loci, such as telomeres. Here we review the spatial organization of the budding yeast nucleus, the proteins involved in forming nuclear subcompartments, and evidence suggesting that the spatial organization of the nucleus is important for nuclear function.

Computational analysis of Rho GTPase cycling

The Rho family of GTPases control actin organization during diverse cellular responses (migration, cytokinesis and endocytosis). Although the primary members of this family (RhoA, Rac and Cdc42) have different downstream effects on actin remodeling, the basic mechanism involves targeting to the plasma membrane and activation by GTP binding. Our hypothesis is that the details of GTPase cycling between membrane and cytosol are key to the differential upstream regulation of these biochemical switches. Accordingly, we developed a modeling framework to analyze experimental data for these systems. This analysis can reveal details of GDI-mediated cycling and help distinguish between GDI-dependent and -independent mechanisms, including vesicle trafficking and direct association-dissociation of GTPase with membrane molecules. Analysis of experimental data for Rac membrane cycling reveals that the lower apparent affinity of GDI for RacGTP compared to RacGDP can be fully explained by the faster dissociation of the latter from the membrane. Non-dimensional steady-state solutions for membrane fraction of GTPase are presented in multidimensional charts. This methodology is then used to analyze glucose stimulated Rac cycling in pancreatic β-cells. The charts are used to illustrate the effects of GEFs/GAPs and regulated affinities between GTPases and membrane and/or GDI on the amount of membrane bound GTPase. In a similar fashion, the charts can be used as a guide in assessing how targeted modifications may compensate for altered GTPase-GDI balance in disease scenarios.

Among the functions of the small GTPases Rac, RhoA and Cdc42 are the regulation of protein traffic, insulin secretion, cell shape, survival and motility. The last two are important steps for tumor growth and metastasis. The function of these proteins relies on their expression levels, proper membrane localization and activation. In addition, all three proteins compete for the same protein ‘GDI’, which modulates their cycling. These proteins are ubiquitous in mammalian cells, but also studied in simpler in vitro systems and cultured yeast. Here we show, using a series of computational analyses, that for each of these experimental systems the dominant pathway for membrane cycling of GTPases seems to differ. This means that the researcher interested in the physiological function of any of those proteins must make sure that the experimental system is appropriate. We present a methodology to identify the dominant pathways by measuring the apparent membrane dissociation rate of the protein as a function of GDI concentration. We provide charts generated from parametric scans. This analysis is then applied to the Rac-dependent insulin secretion pathway in pancreatic ß-cells, revealing that direct signaling between Rac and the membrane is an essential mechanism that emerges from the data.

Membrane trafficking in podocyte health and disease

Podocytes are highly specialized epithelial cells localized in the kidney glomerulus. The distinct cell signaling events and unique cytoskeletal architecture tailor podocytes to withstand changes in hydrostatic pressure during glomerular filtration. Alteration of glomerular filtration leads to kidney disease and frequently manifests with proteinuria. It has been increasingly recognized that cell signaling and cytoskeletal dynamics are coupled more tightly to membrane trafficking than previously thought. Membrane trafficking coordinates the cross-talk between protein networks and signaling cascades in a spatially and temporally organized fashion and may be viewed as a communication highway between the cell exterior and interior. Membrane trafficking involves transport of cargo from the plasma membrane to the cell interior (i.e., endocytosis) followed by cargo trafficking to lysosomes for degradation or to the plasma membrane for recycling. Yet, recent studies indicate that the conventional classification does not fully reflect the complex and versatile nature of membrane trafficking. While the increasing complexity of elaborate protein scaffolds and signaling cascades is being recognized in podocytes, the role of membrane trafficking is less well understood. This review will focus on the role of membrane trafficking in podocyte health and disease.

Septin-containing barriers control the differential inheritance of cytoplasmic elements

Fusion of haploid cells of Saccharomyces cerevisiae generates zygotes. We observe that the zygote midzone includes a septin annulus and differentially affects redistribution of supramolecular complexes and organelles. Redistribution across the midzone of supramolecular complexes (polysomes and Sup35p-GFP [PSI+]) is unexpectedly delayed relative to soluble proteins; however, in [psi-] × [PSI+] crosses, all buds eventually receive Sup35p-GFP [PSI+]. Encounter between parental mitochondria is further delayed until septins relocate to the bud site, where they are required for repolarization of the actin cytoskeleton. This delay allows rationalization of the longstanding observation that terminal zygotic buds preferentially inherit a single mitochondrial genotype. The rate of redistribution of complexes and organelles determines whether their inheritance will be uniform.

Rsr1 focuses Cdc42 activity at hyphal tips and promotes maintenance of hyphal development in Candida albicans

The extremely elongated morphology of fungal hyphae is dependent on the cell's ability to assemble and maintain polarized growth machinery over multiple cell cycles. The different morphologies of the fungus Candida albicans make it an excellent model organism in which to study the spatiotemporal requirements for constitutive polarized growth and the generation of different cell shapes. In C. albicans, deletion of the landmark protein Rsr1 causes defects in morphogenesis that are not predicted from study of the orthologous protein in the related yeast Saccharomyces cerevisiae, thus suggesting that Rsr1 has expanded functions during polarized growth in C. albicans. Here, we show that Rsr1 activity localizes to hyphal tips by the differential localization of the Rsr1 GTPase-activating protein (GAP), Bud2, and guanine nucleotide exchange factor (GEF), Bud5. In addition, we find that Rsr1 is needed to maintain the focused localization of hyphal polarity structures and proteins, including Bem1, a marker of the active GTP-bound form of the Rho GTPase, Cdc42. Further, our results indicate that tip-localized Cdc42 clusters are associated with the cell's ability to express a hyphal transcriptional program and that the ability to generate a focused Cdc42 cluster in early hyphae (germ tubes) is needed to maintain hyphal morphogenesis over time. We propose that in C. albicans, Rsr1 "fine-tunes" the distribution of Cdc42 activity and that self-organizing (Rsr1-independent) mechanisms of polarized growth are not sufficient to generate narrow cell shapes or to provide feedback to the transcriptional program during hyphal morphogenesis.

Vesicle trafficking: ROP-RIC roundabout

Mechanisms governing dynamic protein recycling include small GTPases that activate/inactivate their partner proteins to affect cytoskeletal dynamics, and thereby polar growth, asymmetric cell shape and physiological responses to external stimuli. Three recent studies illustrate the control of PIN endocytosis by ROP-RIC activity in leaf pavement cells and root cells.

Tracking shallow chemical gradients by actin-driven wandering of the polarization site

BACKGROUND

Many cells are remarkably proficient at tracking very shallow chemical gradients, despite considerable noise from stochastic receptor-ligand interactions. Motile cells appear to undergo a biased random walk: spatial noise in receptor activity may determine the instantaneous direction, but because noise is spatially unbiased, it is filtered out by time averaging, resulting in net movement upgradient. How nonmotile cells might filter out noise is unknown.

RESULTS

Using yeast chemotropic mating as a model, we demonstrate that a polarized patch of polarity regulators "wanders" along the cortex during gradient tracking. Computational and experimental findings suggest that actin-directed membrane traffic contributes to wandering by diluting local polarity factors. The pheromone gradient appears to bias wandering via interactions between receptor-activated Gβγ and polarity regulators. Artificially blocking patch wandering impairs gradient tracking.

CONCLUSIONS

We suggest that the polarity patch undergoes an intracellular biased random walk that enables noise filtering by time averaging, allowing nonmotile cells to track shallow gradients.

Cell polarity: quantitative modeling as a tool in cell biology

Among a number of innovative approaches that have modernized cell biology, modeling has a prominent yet unusual place. One popular view is that we progress linearly, from conceptual to ever more detailed models. We review recent discoveries of cell polarity mechanisms, in which modeling played an important role, to demonstrate that the experiment-theory feedback loop requires diverse models characterized by varying levels of biological detail and mathematical complexity. We argue that a quantitative model is a tool that has to fit an experimental study, and the model's value should be judged not by how complex and detailed it is, but by what could be learned from it.

Protein disulfide isomerase is required for platelet-derived growth factor-induced vascular smooth muscle cell migration, Nox1 NADPH oxidase expression, and RhoGTPase activation

Vascular Smooth Muscle Cell (VSMC) migration into vessel neointima is a therapeutic target for atherosclerosis and postinjury restenosis. Nox1 NADPH oxidase-derived oxidants synergize with growth factors to support VSMC migration. We previously described the interaction between NADPH oxidases and the endoplasmic reticulum redox chaperone protein disulfide isomerase (PDI) in many cell types. However, physiological implications, as well as mechanisms of such association, are yet unclear. We show here that platelet-derived growth factor (PDGF) promoted subcellular redistribution of PDI concomitant to Nox1-dependent reactive oxygen species production and that siRNA-mediated PDI silencing inhibited such reactive oxygen species production, while nearly totally suppressing the increase in Nox1 expression, with no change in Nox4. Furthermore, PDI silencing inhibited PDGF-induced VSMC migration assessed by distinct methods, whereas PDI overexpression increased spontaneous basal VSMC migration. To address possible mechanisms of PDI effects, we searched for PDI interactome by systems biology analysis of physical protein-protein interaction networks, which indicated convergence with small GTPases and their regulator RhoGDI. PDI silencing decreased PDGF-induced Rac1 and RhoA activities, without changing their expression. PDI co-immunoprecipitated with RhoGDI at base line, whereas such association was decreased after PDGF. Also, PDI co-immunoprecipitated with Rac1 and RhoA in a PDGF-independent way and displayed detectable spots of perinuclear co-localization with Rac1 and RhoGDI. Moreover, PDI silencing promoted strong cytoskeletal changes: disorganization of stress fibers, decreased number of focal adhesions, and reduced number of RhoGDI-containing vesicular recycling adhesion structures. Overall, these data suggest that PDI is required to support Nox1/redox and GTPase-dependent VSMC migration.

Ras and Rho small G proteins: insights from the Schizophyllum commune genome sequence and comparisons to other fungi

Unlike in animal cells and yeasts, the Ras and Rho small G proteins and their regulators have not received extensive research attention in the case of the filamentous fungi. In an effort to begin to rectify this deficiency, the genome sequence of the basidiomycete mushroom Schizophyllum commune was searched for all known components of the Ras and Rho signalling pathways. The results of this study should provide an impetus for further detailed investigations into their role in polarized hyphal growth, sexual reproduction and fruiting body development. These processes have long been the targets for genetic and cell biological research in this fungus.

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.

Drosophila CK1-γ, gilgamesh, controls PCP-mediated morphogenesis through regulation of vesicle trafficking

CK1-γ/gilgamesh spatially limits the planar cell polarity–regulated process of trichome formation in Drosophila through its effect on polarized vesicle recycling.

Cellular morphogenesis, including polarized outgrowth, promotes tissue shape and function. Polarized vesicle trafficking has emerged as a fundamental mechanism by which protein and membrane can be targeted to discrete subcellular domains to promote localized protrusions. Frizzled (Fz)/planar cell polarity (PCP) signaling orchestrates cytoskeletal polarization and drives morphogenetic changes in such contexts as the vertebrate body axis and external Drosophila melanogaster tissues. Although regulation of Fz/PCP signaling via vesicle trafficking has been identified, the interplay between the vesicle trafficking machinery and downstream terminal PCP-directed processes is less established. In this paper, we show that Drosophila CK1-γ/gilgamesh (gish) regulates the PCP-associated process of trichome formation through effects on Rab11-mediated vesicle recycling. Although the core Fz/PCP proteins dictate prehair formation broadly, CK1-γ/gish restricts nucleation to a single site. Moreover, CK1-γ/gish works in parallel with the Fz/PCP effector multiple wing hairs, which restricts prehair formation along the perpendicular axis to Gish. Our findings suggest that polarized Rab11-mediated vesicle trafficking regulated by CK1-γ is required for PCP-directed processes.

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.

Quantitative analysis of prenylated RhoA interaction with its chaperone, RhoGDI

Small GTPases of the Rho family regulate cytoskeleton remodeling, cell polarity, and transcription, as well as the cell cycle, in eukaryotic cells. Membrane delivery and recycling of the Rho GTPases is mediated by Rho GDP dissociation inhibitor (RhoGDI), which forms a stable complex with prenylated Rho GTPases. We analyzed the interaction of RhoGDI with the active and inactive forms of prenylated and unprenylated RhoA. We demonstrate that RhoGDI binds the prenylated form of RhoA·GDP with unexpectedly high affinity (K(d) = 5 pm). The very long half-life of the complex is reduced 25-fold on RhoA activation, with a concomitant reduction in affinity (K(d) = 3 nm). The 2.8-Å structure of the RhoA·guanosine 5'-[β,γ-imido] triphosphate (GMPPNP)·RhoGDI complex demonstrated that complex formation forces the activated RhoA into a GDP-bound conformation in the absence of nucleotide hydrolysis. We demonstrate that membrane extraction of Rho GTPase by RhoGDI is a thermodynamically favored passive process that operates through a series of progressively tighter intermediates, much like the one that is mediated by RabGDI.

Oscillatory dynamics of Cdc42 GTPase in the control of polarized growth

Cells promote polarized growth by activation of Rho-family protein Cdc42 at the cell membrane. We combined experiments and modeling to study bipolar growth initiation in fission yeast. Concentrations of a fluorescent marker for active Cdc42, Cdc42 protein, Cdc42-activator Scd1, and scaffold protein Scd2 exhibited anticorrelated fluctuations and oscillations with a 5-minute average period at polarized cell tips. These dynamics indicate competition for active Cdc42 or its regulators and the presence of positive and delayed negative feedbacks. Cdc42 oscillations and spatial distribution were sensitive to the amounts of Cdc42-activator Gef1 and to the activity of Cdc42-dependent kinase Pak1, a negative regulator. Feedbacks regulating Cdc42 oscillations and spatial self-organization appear to provide a flexible mechanism for fission yeast cells to explore polarization states and to control their morphology.

Mechanistic mathematical model of polarity in yeast

The establishment of cell polarity involves positive-feedback mechanisms that concentrate polarity regulators, including the conserved GTPase Cdc42p, at the "front" of the polarized cell. Previous studies in yeast suggested the presence of two parallel positive-feedback loops, one operating as a diffusion-based system, and the other involving actin-directed trafficking of Cdc42p on vesicles. F-actin (and hence directed vesicle traffic) speeds fluorescence recovery of Cdc42p after photobleaching, suggesting that vesicle traffic of Cdc42p contributes to polarization. We present a mathematical modeling framework that combines previously developed mechanistic reaction-diffusion and vesicle-trafficking models. Surprisingly, the combined model recapitulated the observed effect of vesicle traffic on Cdc42p dynamics even when the vesicles did not carry significant amounts of Cdc42p. Vesicle traffic reduced the concentration of Cdc42p at the front, so that fluorescence recovery mediated by Cdc42p flux from the cytoplasm took less time to replenish the bleached pool. Simulations in which Cdc42p was concentrated into vesicles or depleted from vesicles yielded almost identical predictions, because Cdc42p flux from the cytoplasm was dominant. These findings indicate that vesicle-mediated delivery of Cdc42p is not required to explain the observed Cdc42p dynamics, and raise the question of whether such Cdc42p traffic actually contributes to polarity establishment.

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.

ROP GTPase-dependent actin microfilaments promote PIN1 polarization by localized inhibition of clathrin-dependent endocytosis

Cell polarization via asymmetrical distribution of structures or molecules is essential for diverse cellular functions and development of organisms, but how polarity is developmentally controlled has been poorly understood. In plants, the asymmetrical distribution of the PIN-FORMED (PIN) proteins involved in the cellular efflux of the quintessential phytohormone auxin plays a central role in developmental patterning, morphogenesis, and differential growth. Recently we showed that auxin promotes cell interdigitation by activating the Rho family ROP GTPases in leaf epidermal pavement cells. Here we found that auxin activation of the ROP2 signaling pathway regulates the asymmetric distribution of PIN1 by inhibiting its endocytosis. ROP2 inhibits PIN1 endocytosis via the accumulation of cortical actin microfilaments induced by the ROP2 effector protein RIC4. Our findings suggest a link between the developmental auxin signal and polar PIN1 distribution via Rho-dependent cytoskeletal reorganization and reveal the conservation of a design principle for cell polarization that is based on Rho GTPase-mediated inhibition of endocytosis.

The epsin protein family: coordinators of endocytosis and signaling

The epsins are a conserved family of endocytic adaptors essential for cell viability in yeast and for embryo development in higher eukaryotes. Epsins function as adaptors by recognizing ubiquitinated cargo and as endocytic accessory proteins by contributing to endocytic network stability/regulation and membrane bending. Importantly, epsins play a critical role in signaling by contributing to epidermal growth factor receptor downregulation and the activation of notch and RhoGTPase pathways. In this review, we present an overview of the epsins and emphasize their functional importance as coordinators of endocytosis and signaling.

Noise reduction in the intracellular pom1p gradient by a dynamic clustering mechanism

Chemical gradients can generate pattern formation in biological systems. In the fission yeast Schizosaccharomyces pombe, a cortical gradient of pom1p (a DYRK-type protein kinase) functions to position sites of cytokinesis and cell polarity and to control cell length. Here, using quantitative imaging, fluorescence correlation spectroscopy, and mathematical modeling, we study how its gradient distribution is formed. Pom1p gradients exhibit large cell-to-cell variability, as well as dynamic fluctuations in each individual gradient. Our data lead to a two-state model for gradient formation in which pom1p molecules associate with the plasma membrane at cell tips and then diffuse on the membrane while aggregating into and fragmenting from clusters, before disassociating from the membrane. In contrast to a classical one-component gradient, this two-state gradient buffers against cell-to-cell variations in protein concentration. This buffering mechanism, together with time averaging to reduce intrinsic noise, allows the pom1p gradient to specify positional information in a robust manner.

Flippase-mediated phospholipid asymmetry promotes fast Cdc42 recycling in dynamic maintenance of cell polarity

Lipid asymmetry at the plasma membrane is essential for such processes as cell polarity, cytokinesis and phagocytosis. Here we find that a lipid flippase complex, composed of Lem3, Dnf1 or Dnf2, has a role in the dynamic recycling of the Cdc42 GTPase, a key regulator of cell polarity, in yeast. By using quantitative microscopy methods, we show that the flippase complex is required for fast dissociation of Cdc42 from the polar cortex by the guanine nucleotide dissociation inhibitor. A loss of flippase activity, or pharmacological blockage of the inward flipping of phosphatidylethanolamine, a phospholipid with a neutral head group, disrupts Cdc42 polarity maintained by guanine nucleotide dissociation inhibitor-mediated recycling. Phosphatidylethanolamine flipping may reduce the charge interaction between a Cdc42 carboxy-terminal cationic region with the plasma membrane inner leaflet, enriched for the negatively charged lipid phosphatidylserine. Using a reconstituted system with supported lipid bilayers, we show that the relative composition of phosphatidylethanolamine versus phosphatidylserine directly modulates Cdc42 extraction from the membrane by guanine nucleotide dissociation inhibitor.

Transport of influenza virus neuraminidase (NA) to host cell surface is regulated by ARHGAP21 and Cdc42 proteins

Influenza virus neuraminidase (NA) is transported to the virus assembly site at the plasma membrane and is a major viral envelope component that plays a critical role in the release of progeny virions and in determination of host range restriction. However, little is known about the host factors that are involved in regulating the intracellular and cell surface transport of NA. Here we identified the Cdc42-specific GAP, ARHGAP21 differentially expressed in host cells infected with influenza A virus using cDNA microarray analysis. Furthermore, we have investigated the involvement of Rho family GTPases in NA transport to the cell surface. We found that expression of constitutively active or inactive mutants of RhoA or Rac1 did not significantly affect the amount of NA that reached the cell surface. However, expression of constitutively active Cdc42 or depletion of ARHGAP21 promoted the transport of NA to the plasma membranes. By contrast, cells expressing shRNA targeting Cdc42 or overexpressing ARHGAP21 exhibited a significant decrease in the amount of cell surface-localized NA. Importantly, silencing Cdc42 reduced influenza A virus replication, whereas silencing ARHGAP21 increased the virus replication. Together, our results reveal that ARHGAP21- and Cdc42-based signaling regulates the NA transport and thereby impacts virus replication.

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.

De novo growth zone formation from fission yeast spheroplasts

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

Symmetry breaking and the establishment of cell polarity in budding yeast

Cell polarity is typically oriented by external cues such as cell-cell contacts, chemoattractants, or morphogen gradients. In the absence of such cues, however, many cells can spontaneously polarize in a random direction, suggesting the existence of an internal polarity-generating mechanism whose direction can be spatially biased by external cues. Spontaneous 'symmetry-breaking' polarization is likely to involve an autocatalytic process set off by small random fluctuations. Here we review recent work on the nature of the autocatalytic process in budding yeast and on the question of why polarized cells only develop a single 'front'.

Phosphatidylserine promotes polar Cdc42 localization

The establishment and maintenance of cell polarity requires targeted recruitment of polarity regulators to the plasma membrane. Phosphatidylserine is now shown to have a key role in polarization of yeast cells and the localization of the central polarity regulator Cdc42.

A density-dependent switch drives stochastic clustering and polarization of signaling molecules

Positive feedback plays a key role in the ability of signaling molecules to form highly localized clusters in the membrane or cytosol of cells. Such clustering can occur in the absence of localizing mechanisms such as pre-existing spatial cues, diffusional barriers, or molecular cross-linking. What prevents positive feedback from amplifying inevitable biological noise when an un-clustered "off" state is desired? And, what limits the spread of clusters when an "on" state is desired? Here, we show that a minimal positive feedback circuit provides the general principle for both suppressing and amplifying noise: below a critical density of signaling molecules, clustering switches off; above this threshold, highly localized clusters are recurrently generated. Clustering occurs only in the stochastic regime, suggesting that finite sizes of molecular populations cannot be ignored in signal transduction networks. The emergence of a dominant cluster for finite numbers of molecules is partly a phenomenon of random sampling, analogous to the fixation or loss of neutral mutations in finite populations. We refer to our model as the "neutral drift polarity model." Regulating the density of signaling molecules provides a simple mechanism for a positive feedback circuit to robustly switch between clustered and un-clustered states. The intrinsic ability of positive feedback both to create and suppress clustering is a general mechanism that could operate within diverse biological networks to create dynamic spatial organization.

Polarized growth in fungi: symmetry breaking and hyphal formation

Cell shape is a critical determinant for function. The baker's yeast Saccharomyces cerevisiae changes shape in response to its environment, growing by budding in rich nutrients, forming invasive pseudohyphal filaments in nutrient poor conditions and pear shaped shmoos for growth towards a partner during mating. The human opportunistic pathogen Candida albicans can switch from budding to hyphal growth, in response to numerous environmental stimuli to colonize and invade its host. Hyphal growth, typical of filamentous fungi, is not observed in S. cerevisiae. A number of internal cues regulate when and where yeast cells break symmetry leading to polarized growth and ultimately distinct cell shapes. This review discusses how cells break symmetry using the yeast S. cerevisiae paradigm and how polarized growth is initiated and maintained to result in dramatic morphological changes during C. albicans hyphal growth.

The 'invisible hand': regulation of RHO GTPases by RHOGDIs

The 'invisible hand' is a term originally coined by Adam Smith in The Theory of Moral Sentiments to describe the forces of self-interest, competition and supply and demand that regulate the resources in society. This metaphor continues to be used by economists to describe the self-regulating nature of a market economy. The same metaphor can be used to describe the RHO-specific guanine nucleotide dissociation inhibitor (RHOGDI) family, which operates in the background, as an invisible hand, using similar forces to regulate the RHO GTPase cycle.

The mating projections of Saccharomyces cerevisiae and Candida albicans show key characteristics of hyphal growth

Fungi can grow in a variety of growth forms: yeast, pseudohyphae and hyphae. The human fungal pathogen Candida albicans can grow in all three of these forms. In this fungus, hyphal growth is distinguished by the presence of a Spitzenkörper-like structure at the hyphal tip and a band of septin bars around the base of newly evaginated germ tubes. The budding yeast Saccharomyces cerevisiae grows as yeast and pseudohyphae, but is not normally considered to show hyphal growth. We show here that in mating projections of both C. albicans and S. cerevisiae a Spitzenkörper-like structure is present at the growing tip and a band of septin bars is present at the base. Furthermore, in S. cerevisiae mating projections, Spa2 and Bni1 form a cap to the 3-dimensional ball of FM4-64 staining, exactly as previously observed in C. albicans hyphae, suggesting that the putative Spitzenkörper may be a distinct structure from the polarisome. Taken together this work shows that mating projections of both S. cerevisiae and C. albicans show the key characteristics of hyphal growth.

Functional analysis of RhoGDI inhibitory activity on vacuole membrane fusion

RhoGDIs (Rho GDP-dissociation inhibitors) are the natural inhibitors of Rho GTPases. They interfere with Rho protein function by either blocking upstream activation or association with downstream signalling molecules. RhoGDIs can also extract membrane-bound Rho GTPases to form soluble cytosolic complexes. We have shown previously that purified yeast RhoGDI Rdi1p, can inhibit vacuole membrane fusion in vitro. In the present paper we functionally dissect Rdi1p to discover its mode of regulating membrane fusion. Overexpression of Rdi1p in vivo profoundly affected cell morphology including increased actin patches in mother cells indicative of polarity defects, delayed ALP (alkaline phosphatase) sorting and the presence of highly fragmented vacuoles indicative of membrane fusion defects. These defects were not caused by the loss of typical transport and fusion proteins, but rather were linked to the reduction of membrane localization and activation of Cdc42p and Rho1p. Subcellular fractionation showed that Rdi1p is predominantly a cytosolic monomer, free of bound Rho GTPases. Overexpression of endogenous Rdi1p, or the addition of exogenous Rdi1p, generated stable cytosolic complexes. Rdi1p structure-function analysis showed that membrane association via the C-terminal β-sheet domain was required for the functional inhibition of membrane fusion. Furthermore, Rdi1p inhibited membrane fusion through the binding of Rho GTPases independent from its extraction activity.

Modular coherence of protein dynamics in yeast cell polarity system

In this study, we investigated on a systems level how complex protein interactions underlying cell polarity in yeast determine the dynamic association of proteins with the polar cortical domain (PCD) where they localize and perform morphogenetic functions. We constructed a network of physical interactions among >100 proteins localized to the PCD. This network was further divided into five robust modules correlating with distinct subprocesses associated with cell polarity. Based on this reconstructed network, we proposed a simple model that approximates a PCD protein's molecular residence time as the sum of the characteristic time constants of the functional modules with which it interacts, weighted by the number of edges forming these interactions. Regression analyses showed excellent fitting of the model with experimentally measured residence times for a large subset of the PCD proteins. The model is able to predict residence times using small training sets. Our analysis also revealed a scaffold protein that imposes a local constraint of dynamics for certain interacting proteins.

Cortical actin dynamics driven by formins and myosin V

Cell morphogenesis requires complex and rapid reorganization of the actin cytoskeleton. The budding yeast Saccharomyces cerevisiae is an invaluable model system for studying molecular mechanisms driving actin dynamics. Actin cables in yeast are formin-generated linear actin arrays that serve as tracks for directed intracellular transport by type V myosins. Cables are constantly reorganized throughout the cell cycle but the molecular basis for such dynamics remains poorly understood. By combining total internal reflection microscopy, quantitative image analyses and genetic manipulations we identify kinetically distinct subpopulations of cables that are differentially driven by formins and myosin. Bni1 drives elongation of randomly oriented actin cables in unpolarized cells, whereas both formins Bnr1 and Bni1 mediate slower polymerization of cables in polarized cells. Type V myosin Myo2 surprisingly acts as a motor for translational cable motility along the cell cortex. During polarization, cells change from fast to slow cable dynamics through spatio-temporal regulation of Bni1, Bnr1 and Myo2. In summary, we identify molecular mechanisms for the regulation of cable dynamics and suggest that fast actin reorganization is necessary for fidelity of cell polarization.

A comparison of mathematical models for polarization of single eukaryotic cells in response to guided cues

Polarization, a primary step in the response of an individual eukaryotic cell to a spatial stimulus, has attracted numerous theoretical treatments complementing experimental studies in a variety of cell types. While the phenomenon itself is universal, details differ across cell types, and across classes of models that have been proposed. Most models address how symmetry breaking leads to polarization, some in abstract settings, others based on specific biochemistry. Here, we compare polarization in response to a stimulus (e.g., a chemoattractant) in cells typically used in experiments (yeast, amoebae, leukocytes, keratocytes, fibroblasts, and neurons), and, in parallel, responses of several prototypical models to typical stimulation protocols. We find that the diversity of cell behaviors is reflected by a diversity of models, and that some, but not all models, can account for amplification of stimulus, maintenance of polarity, adaptation, sensitivity to new signals, and robustness.

Fluorescent proteins illuminate the structure and function of the hyphal tip apparatus

Fungal hyphae show extreme polarized growth at the tip. Electron microscope studies have revealed a apical body called the Spitzenkörper that is thought to drive polarized growth. Studies of polarized growth in S. cerevisiae have identified the protein components of the polarized growth machinery, that are conserved in other fungi. Fusion of these proteins to GFP and its variants has for the first time allowed the localization of these proteins in real time to the hyphal tip without the need for drastic fixation procedures. Such studies showed that vesicle-associated proteins localize to the Spitzenkörper and identified a second compartment located at the tip surface composed of exocyst and other proteins that mediate the fusion of secretory vesicles with the plasma membrane.

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.

Modeling vesicle traffic reveals unexpected consequences for Cdc42p-mediated polarity establishment

BACKGROUND

Polarization in yeast has been proposed to involve a positive feedback loop whereby the polarity regulator Cdc42p orients actin cables, which deliver vesicles carrying Cdc42p to the polarization site. Previous mathematical models treating Cdc42p traffic as a membrane-free flux suggested that directed traffic would polarize Cdc42p, but it remained unclear whether Cdc42p would become polarized without the membrane-free simplifying assumption.

RESULTS

We present mathematical models that explicitly consider stochastic vesicle traffic via exocytosis and endocytosis, providing several new insights. Our findings suggest that endocytic cargo influences the timing of vesicle internalization in yeast. Moreover, our models provide quantitative support for the view that integral membrane cargo proteins would become polarized by directed vesicle traffic given the experimentally determined rates of vesicle traffic and diffusion. However, such traffic cannot effectively polarize the more rapidly diffusing Cdc42p in the model without making additional assumptions that seem implausible and lack experimental support.

CONCLUSIONS

Our findings suggest that actin-directed vesicle traffic would perturb, rather than reinforce, polarization in yeast.

ASYMPTOTIC AND BIFURCATION ANALYSIS OF WAVE-PINNING IN A REACTION-DIFFUSION MODEL FOR CELL POLARIZATION

We describe and analyze a bistable reaction-diffusion (RD) model for two interconverting chemical species that exhibits a phenomenon of wave-pinning: a wave of activation of one of the species is initiated at one end of the domain, moves into the domain, decelerates, and eventually stops inside the domain, forming a stationary front. The second ("inactive") species is depleted in this process. This behavior arises in a model for chemical polarization of a cell by Rho GTPases in response to stimulation. The initially spatially homogeneous concentration profile (representative of a resting cell) develops into an asymmetric stationary front profile (typical of a polarized cell). Wave-pinning here is based on three properties: (1) mass conservation in a finite domain, (2) nonlinear reaction kinetics allowing for multiple stable steady states, and (3) a sufficiently large difference in diffusion of the two species. Using matched asymptotic analysis, we explain the mathematical basis of wave-pinning, and predict the speed and pinned position of the wave. An analysis of the bifurcation of the pinned front solution reveals how the wave-pinning regime depends on parameters such as rates of diffusion and total mass of the species. We describe two ways in which the pinned solution can be lost depending on the details of the reaction kinetics: a saddle-node or a pitchfork bifurcation.

Cdc42 localization and cell polarity depend on membrane traffic

Cell polarity is essential for cell division, cell differentiation, and most differentiated cell functions including cell migration. The small G protein Cdc42 controls cell polarity in a wide variety of cellular contexts. Although restricted localization of active Cdc42 seems to be important for its distinct functions, mechanisms responsible for the concentration of active Cdc42 at precise cortical sites are not fully understood. In this study, we show that during directed cell migration, Cdc42 accumulation at the cell leading edge relies on membrane traffic. Cdc42 and its exchange factor βPIX localize to intracytosplasmic vesicles. Inhibition of Arf6-dependent membrane trafficking alters the dynamics of Cdc42-positive vesicles and abolishes the polarized recruitment of Cdc42 and βPIX to the leading edge. Furthermore, we show that Arf6-dependent membrane dynamics is also required for polarized recruitment of Rac and the Par6-aPKC polarity complex and for cell polarization. Our results demonstrate influence of membrane dynamics on the localization and activation of Cdc42 and consequently on directed cell migration.

Rac1 dynamics in the human opportunistic fungal pathogen Candida albicans

The small Rho G-protein Rac1 is highly conserved from fungi to humans, with approximately 65% overall sequence identity in Candida albicans. As observed with human Rac1, we show that C. albicans Rac1 can accumulate in the nucleus, and fluorescence recovery after photobleaching (FRAP) together with fluorescence loss in photobleaching (FLIP) studies indicate that this Rho G-protein undergoes nucleo-cytoplasmic shuttling. Analyses of different chimeras revealed that nuclear accumulation of C. albicans Rac1 requires the NLS-motifs at its carboxyl-terminus, which are blocked by prenylation of the adjacent cysteine residue. Furthermore, we show that C. albicans Rac1 dynamics, both at the plasma membrane and in the nucleus, are dependent on its activation state and in particular that the inactive form accumulates faster in the nucleus. Heterologous expression of human Rac1 in C. albicans also results in nuclear accumulation, yet accumulation is more rapid than that of C. albicans Rac1. Taken together our results indicate that Rac1 nuclear accumulation is an inherent property of this G-protein and suggest that the requirements for its nucleo-cytoplasmic shuttling are conserved from fungi to humans.

Cellular responses to extracellular guidance cues

Extracellular guidance cues have a key role in orchestrating cell behaviour. They can take many forms, including soluble and cell-bound ligands (proteins, lipids, peptides or small molecules) and insoluble matrix substrates, but to act as guidance cues, they must be presented to the cell in a spatially restricted manner. Cells that recognize such cues respond by activating intracellular signal transduction pathways in a spatially restricted manner and convert the extracellular information into intracellular polarity. Although extracellular cues influence a broad range of cell polarity decisions, such as mitotic spindle orientation during asymmetric cell division, or the establishment of apical-basal polarity in epithelia, this review will focus specifically on guidance cues that promote cell migration (chemotaxis), or localized cell shape changes (chemotropism).