Spa2p interacts with cell polarity proteins and signaling components involved in yeast cell morphogenesis

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

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

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

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.

Identification of dimorphism-involved genes of Yarrowia lipolytica by means of microarray analysis

Fungal dimorphism is the capacity of certain species of fungi to grow in the form of budding yeasts or mycelium depending on the environmental conditions. This characteristic is a complex phenomenon that involves modifications of the molecular machinery in response to different environmental signals. Through the use of microarrays, in this work we identified genes involved in the early stages of the yeast-to-mycelium transition of Yarrowia lipolytica induced by a shift in pH of the medium. As controls, yeast and mycelium monomorphic mutants were used, identifying by this mean a total of 61 upregulated and 165 downregulated genes specifically involved in dimorphism. Determination of the putative function of these genes was accomplished by means of BLAST analyses which showed that they were involved mainly in processes such as remodeling and biogenesis of the cell wall, membrane trafficking and N- or O-glycosylation. Some of these genes were identified by homology with Saccharomyces cerevisiae genes, and found to play a role during the dimorphic transition in both systems.

Regulation of cell wall biogenesis in Saccharomyces cerevisiae: the cell wall integrity signaling pathway

The yeast cell wall is a strong, but elastic, structure that is essential not only for the maintenance of cell shape and integrity, but also for progression through the cell cycle. During growth and morphogenesis, and in response to environmental challenges, the cell wall is remodeled in a highly regulated and polarized manner, a process that is principally under the control of the cell wall integrity (CWI) signaling pathway. This pathway transmits wall stress signals from the cell surface to the Rho1 GTPase, which mobilizes a physiologic response through a variety of effectors. Activation of CWI signaling regulates the production of various carbohydrate polymers of the cell wall, as well as their polarized delivery to the site of cell wall remodeling. This review article centers on CWI signaling in Saccharomyces cerevisiae through the cell cycle and in response to cell wall stress. The interface of this signaling pathway with other pathways that contribute to the maintenance of cell wall integrity is also discussed.

Comparative live-cell imaging analyses of SPA-2, BUD-6 and BNI-1 in Neurospora crassa reveal novel features of the filamentous fungal polarisome

A key multiprotein complex involved in regulating the actin cytoskeleton and secretory machinery required for polarized growth in fungi, is the polarisome. Recognized core constituents in budding yeast are the proteins Spa2, Pea2, Aip3/Bud6, and the key effector Bni1. Multicellular fungi display a more complex polarized morphogenesis than yeasts, suggesting that the filamentous fungal polarisome might fulfill additional functions. In this study, we compared the subcellular organization and dynamics of the putative polarisome components BUD-6 and BNI-1 with those of the bona fide polarisome marker SPA-2 at various developmental stages of Neurospora crassa. All three proteins exhibited a yeast-like polarisome configuration during polarized germ tube growth, cell fusion, septal pore plugging and tip repolarization. However, the localization patterns of all three proteins showed spatiotemporally distinct characteristics during the establishment of new polar axes, septum formation and cytokinesis, and maintained hyphal tip growth. Most notably, in vegetative hyphal tips BUD-6 accumulated as a subapical cloud excluded from the Spitzenkörper (Spk), whereas BNI-1 and SPA-2 partially colocalized with the Spk and the tip apex. Novel roles during septal plugging and cytokinesis, connected to the reinitiation of tip growth upon physical injury and conidial maturation, were identified for BUD-6 and BNI-1, respectively. Phenotypic analyses of gene deletion mutants revealed additional functions for BUD-6 and BNI-1 in cell fusion regulation, and the maintenance of Spk integrity. Considered together, our findings reveal novel polarisome-independent functions of BUD-6 and BNI-1 in Neurospora, but also suggest that all three proteins cooperate at plugged septal pores, and their complex arrangement within the apical dome of mature hypha might represent a novel aspect of filamentous fungal polarisome architecture.

Analysis of the biological activity of a novel 24-membered macrolide JBIR-19 in Saccharomyces cerevisiae by the morphological imaging program CalMorph

To investigate the biological activity of a novel 24-membered macrolide compound, JBIR-19, isolated from the culture broth of the entomopathogenic fungus Metarhizium sp. fE61, morphological changes in yeast cells were examined using the automated image-processing program CalMorph. Principal components analysis was used to elucidate dynamic changes in the phenotypes, revealing two independent effects of JBIR-19 in yeast cells: bud elongation and increased size of the actin region. Using a fitness assay, we identified the genes required for robust growth in the presence of JBIR-19. Among these were CCW12, YLR111W, and DHH1, which are also involved in abnormal bud morphology. Based on these results and others, we predict intracellular targets of JBIR-19 and its functional interactions.

Yeast formin Bni1p has multiple localization regions that function in polarized growth and spindle orientation

There are four distinct localization domains in formin Bni1p of budding yeast. Analysis of the functions of the domains in the actin cytoskeleton and in spindle orientation reveals unexpected complexity in the mechanism of formin localization and function.

Formins are conserved proteins that assemble unbranched actin filaments in a regulated, localized manner. Budding yeast's two formins, Bni1p and Bnr1p, assemble actin cables necessary for polarized cell growth and organelle segregation. Here we define four regions in Bni1p that contribute to its localization to the bud and at the bud neck. The first (residues 1–333) requires dimerization for its localization and encompasses the Rho-binding domain. The second (residues 334–821) covers the Diaphanous inhibitory–dimerization–coiled coil domains, and the third is the Spa2p-binding domain. The fourth region encompasses the formin homology 1–formin homology 2–COOH region of the protein. These four regions can each localize to the bud cortex and bud neck at the right stage of the cell cycle independent of both F-actin and endogenous Bni1p. The first three regions contribute cumulatively to the proper localization of Bni1p, as revealed by the effects of progressive loss of these regions on the actin cytoskeleton and fidelity of spindle orientation. The fourth region contributes to the localization of Bni1p in tiny budded cells. Expression of mislocalized Bni1p constructs has a dominant-negative effect on both growth and nuclear segregation due to mislocalized actin assembly. These results define an unexpected complexity in the mechanism of formin localization and function.

Existence of a novel clathrin-independent endocytic pathway in yeast that depends on Rho1 and formin

Much like mammalian cells, yeast contain a Rho-dependent pathway for endocytosis in addition to canonical clathrin-dependent endocytosis.

Yeast is a powerful model organism for dissecting the temporal stages and choreography of the complex protein machinery during endocytosis. The only known mechanism for endocytosis in yeast is clathrin-mediated endocytosis, even though clathrin-independent endocytic pathways have been described in other eukaryotes. Here, we provide evidence for a clathrin-independent endocytic pathway in yeast. In cells lacking the clathrin-binding adaptor proteins Ent1, Ent2, Yap1801, and Yap1802, we identify a second endocytic pathway that depends on the GTPase Rho1, the downstream formin Bni1, and the Bni1 cofactors Bud6 and Spa2. This second pathway does not require components of the better-studied endocytic pathway, including clathrin and Arp2/3 complex activators. Thus, our results reveal the existence of a second pathway for endocytosis in yeast, which suggests similarities with the RhoA-dependent endocytic pathways of mammalian cells.

CDK-dependent phosphorylation of Mob2 is essential for hyphal development in Candida albicans

In yeast, CDKs and the NDR kinase Cbk1 are regulators of polarized growth. It is found that the CDK Cdc28 regulates the function of Cbk1 in response to hypha-inducing conditions by direct phosphorylation of Mob2, a conserved regulatory subunit of Cbk1.

Nuclear Dbf2-related (NDR) protein kinases are essential components of regulatory pathways involved in cell morphogenesis, cell cycle control, and viability in eukaryotic cells. For their activity and function, these kinases require interaction with Mob proteins. However, little is known about how the Mob proteins are regulated. In Candida albicans, the cyclin-dependent kinase (CDK) Cdc28 and the NDR kinase Cbk1 are required for hyphal growth. Here we demonstrate that Mob2, the Cbk1 activator, undergoes a Cdc28-dependent differential phosphorylation on hyphal induction. Mutations in the four CDK consensus sites in Mob2 to Ala significantly impaired hyphal development. The mutant cells produced short hyphae with enlarged tips that displayed an illicit activation of cell separation. We also show that Cdc28 phosphorylation of Mob2 is essential for the maintenance of polarisome components at hyphal tips but not at bud tips during yeast growth. Thus we have found a novel signaling pathway by which Cdc28 controls Cbk1 through the regulatory phosphorylation of Mob2, which is crucial for normal hyphal development.

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.

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.

Cellular ageing and the actin cytoskeleton

For some time the view that the actin cytoskeleton acts primarily as a scaffold, to be assembled in response to a signaling cascade as an end point in the pathway, has prevailed. However, it is now clear that the dynamic nature of the cytoskeleton is linked to downstream signaling events that further modulate cellular activity, and which can determine cell fate. Examples of this lie within the regulation of programmed cell death, the maintenance of homeostasis and the process of cellular ageing. In yeast the actin cytoskeleton has been shown to interact directly with signaling pathways known to be important in the regulation of both ageing and cell death. For example it has been discovered that the level of damage sustained by the actin cytoskeleton under conditions of oxidative stressoxidative stress is directly linked to apoptosis. Further evidence comes from the finding that actin based propulsion mechanisms are required for the inheritance of mitochondria and anti-ageing factors into newly formed cells. In addition to this actin is known to directly influence the formation of protein aggregations. In this chapter we will discuss these points and postulate as to their significance with respect to the maintenance of cellular homeostasis.

Septin structure and function in yeast and beyond

Septins are conserved GTP-binding proteins that assemble into hetero-oligomeric complexes and higher-order structures such as filaments, rings, hourglasses or gauzes. Septins are usually associated with a discrete region of the plasma membrane and function as a cell scaffold or diffusion barrier to effect cytokinesis, cell polarity, and many other functions. Recent structural studies of septin complexes have provided mechanistic insights into septin filament assembly, but key questions concerning the assembly, dynamics, and function of different septin structures remain to be answered.

Candida albicans Vrp1 is required for polarized morphogenesis and interacts with Wal1 and Myo5

Recently, a link between endocytosis and hyphal morphogenesis has been identified in Candida albicans via the Wiskott–Aldrich syndrome gene homologue WAL1. To get a more detailed mechanistic understanding of this link we have investigated a potentially conserved interaction between Wal1 and the C. albicans WASP-interacting protein (WIP) homologue encoded by VRP1. Deletion of both alleles of VRP1 results in strong hyphal growth defects under serum inducing conditions but filamentation can be observed on Spider medium. Mutant vrp1 cells show a delay in endocytosis – measured as the uptake and delivery of the lipophilic dye FM4-64 into small endocytic vesicles – compared to the wild-type. Vacuolar morphology was found to be fragmented in a subset of cells and the cortical actin cytoskeleton was depolarized in vrp1 daughter cells. The morphology of the vrp1 null mutant could be complemented by reintegration of the wild-type VRP1 gene at the BUD3 locus. Using the yeast two-hybrid system we could demonstrate an interaction between the C-terminal part of Vrp1 and the N-terminal part of Wal1, which contains the WH1 domain. Furthermore, we found that Myo5 has several potential interaction sites on Vrp1. This suggests that a Wal1–Vrp1–Myo5 complex plays an important role in endocytosis and the polarized localization of the cortical actin cytoskeleton to promote polarized hyphal growth in C. albicans.

Multicopy suppression screen in a Saccharomyces cerevisiae strain lacking the Rab GTPase-activating protein Msb3p

The yeast proteins, Msb3p and Msb4p, are two Ypt/Rab-specific GTPase-activating proteins sharing redundant functions in exocytosis, organization of the actin cytoskeleton, and budding site selection. To see if Msb3p might play an additional, specific role, we first tested the sensitivities of msb3 and msb4 mutant strains to different drugs and then screened a genomic library for multicopy suppressors of msb3 sensitivity to CdCl(2) or to the calcium channel blocker diltiazem hydrochloride. Three genes (ADH1, RNT1, and SUI1) were found to suppress the CdCl(2) sensitivity of the msb3 strain and three others (YAP6, ZEO1, and SLM1) its diltiazem-HCl sensitivity. The results suggest a possible involvement of Msb3p in calcineurin-mediated signalling.

Spitzenkorper, exocyst, and polarisome components in Candida albicans hyphae show different patterns of localization and have distinct dynamic properties

During the extreme polarized growth of fungal hyphae, secretory vesicles are thought to accumulate in a subapical region called the Spitzenkörper. The human fungal pathogen Candida albicans can grow in a budding yeast or hyphal form. When it grows as hyphae, Mlc1 accumulates in a subapical spot suggestive of a Spitzenkörper-like structure, while the polarisome components Spa2 and Bud6 localize to a surface crescent. Here we show that the vesicle-associated protein Sec4 also localizes to a spot, confirming that secretory vesicles accumulate in the putative C. albicans Spitzenkörper. In contrast, exocyst components localize to a surface crescent. Using a combination of fluorescence recovery after photobleaching (FRAP) and fluorescence loss in photobleaching (FLIP) experiments and cytochalasin A to disrupt actin cables, we showed that Spitzenkörper-located proteins are highly dynamic. In contrast, exocyst and polarisome components are stably located at the cell surface. It is thought that in Saccharomyces cerevisiae exocyst components are transported to the cell surface on secretory vesicles along actin cables. If each vesicle carried its own complement of exocyst components, then it would be expected that exocyst components would be as dynamic as Sec4 and would have the same pattern of localization. This is not what we observe in C. albicans. We propose a model in which a stream of vesicles arrives at the tip and accumulates in the Spitzenkörper before onward delivery to the plasma membrane mediated by exocyst and polarisome components that are more stable residents of the cell surface.

Activation of the mitogen-activated protein kinase, Slt2p, at bud tips blocks a late stage of endoplasmic reticulum inheritance in Saccharomyces cerevisiae

We explore the role of components that act both upstream and downstream of Slt2p in the Ptc1p-dependent regulation of ER inheritance and mitochondrial inheritance. Our findings are that Ptc1p is needed to inactivate the pool of Slt2p associated with the bud tip to promote the cortical distribution of the ER in daughter cells.

Inheritance of the endoplasmic reticulum (ER) requires Ptc1p, a type 2C protein phosphatase of Saccharomyces cerevisiae. Genetic analysis indicates that Ptc1p is needed to inactivate the cell wall integrity (CWI) MAP kinase, Slt2p. Here we show that under normal growth conditions, Ptc1p inactivates Slt2p just as ER tubules begin to spread from the bud tip along the cortex. In ptc1Δ cells, the propagation of cortical ER from the bud tip to the periphery of the bud is delayed by hyperactivation of Slt2p. The pool of Slt2p that controls ER inheritance requires the CWI pathway scaffold, Spa2p, for its retention at the bud tip, and a mutation within Slt2p that prevents its association with the bud tip blocks its role in ER inheritance. These results imply that Slt2p inhibits a late step in ER inheritance by phosphorylating a target at the tip of daughter cells. The PI4P5-kinase, Mss4p, is an upstream activator of this pool of Slt2p. Ptc1p-dependant inactivation of Slt2p is also needed for mitochondrial inheritance; however, in this case, the relevant pool of Slt2p is not at the bud tip.

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.

Singularity in polarization: rewiring yeast cells to make two buds

For budding yeast to ensure formation of only one bud, cells must polarize toward one, and only one, site. Polarity establishment involves the Rho family GTPase Cdc42, which concentrates at polarization sites via a positive feedback loop. To assess whether singularity is linked to the specific Cdc42 feedback loop, we disabled the yeast cell's endogenous amplification mechanism and synthetically rewired the cells to employ a different positive feedback loop. Rewired cells violated singularity, occasionally making two buds. Even cells that made only one bud sometimes initiated two clusters of Cdc42, but then one cluster became dominant. Mathematical modeling indicated that, given sufficient time, competition between clusters would promote singularity. In rewired cells, competition occurred slowly and sometimes failed to develop a single "winning" cluster before budding. Slowing competition in normal cells also allowed occasional formation of two buds, suggesting that singularity is enforced by rapid competition between Cdc42 clusters.

Synthetic morphology using alternative inputs

Designing the shape and size of a cell is an interesting challenge for synthetic biology. Prolonged exposure to the mating pheromone α-factor induces an unusual morphology in yeast cells: multiple mating projections. The goal of this work was to reproduce the multiple projections phenotype in the absence of α-factor using a gain-of-function approach termed “Alternative Inputs (AIs)”. An alternative input is defined as any genetic manipulation that can activate the signaling pathway instead of the natural input. Interestingly, none of the alternative inputs were sufficient to produce multiple projections although some produced a single projection. Then, we extended our search by creating all combinations of alternative inputs and deletions that were summarized in an AIs-Deletions matrix. We found a genetic manipulation (AI-Ste5p ste2Δ) that enhanced the formation of multiple projections. Following up this lead, we demonstrated that AI-Ste4p and AI-Ste5p were sufficient to produce multiple projections when combined. Further, we showed that overexpression of a membrane-targeted form of Ste5p alone could also induce multiple projections. Thus, we successfully re-engineered the multiple projections mating morphology using alternative inputs without α-factor.

Comprehensive and quantitative analysis of yeast deletion mutants defective in apical and isotropic bud growth

To obtain a comprehensive understanding of the budding phase transition, 4,711 Saccharomyces cerevisiae haploid nonessential gene deletion mutants were screened with the image processing program CalMorph, and 35 mutants with a round bud and 173 mutants with an elongated bud were statistically identified. We classified round and elongated bud mutants based on factors thought to affect the duration of the apical bud growth phase. Two round bud mutants (arc18 and sac6) were found to be defective in apical actin patch localization. Several elongated bud mutants demonstrated a delay of cell cycle progression at the apical growth phase, suggesting that these mutants have a defect in the control of cell cycle progression.

The yeast AAA+ chaperone Hsp104 is part of a network that links the actin cytoskeleton with the inheritance of damaged proteins

The yeast AAA(+) chaperone Hsp104 is essential for the development of thermotolerance and for the inheritance of prions. Recently, Hsp104, together with the actin cytoskeleton, has been implicated in the asymmetric distribution of carbonylated proteins. Here, we investigated the interplay between Hsp104 and actin by using a dominant-negative variant of Hsp104 (HAP/ClpP) that degrades substrate proteins instead of remodeling them. Coexpression of HAP/ClpP causes defects in morphology and the actin cytoskeleton. Taking a candidate approach, we identified Spa2, a member of the polarisome complex, as an Hsp104 substrate. Furthermore, we provided genetic evidence that links Spa2 and Hsp104 to Hof1, a member of the cytokinesis machinery. Spa2 and Hof1 knockout cells are affected in the asymmetric distribution of damaged proteins, suggesting that Hsp104, Spa2, and Hof1 are members of a network controlling the inheritance of carbonylated proteins.

Cell polarity determinants establish asymmetry in MEN signaling

Components of the mitotic exit network (MEN), a signaling pathway that triggers exit from mitosis, localize to the spindle pole body (SPB) that migrates into the daughter cell during anaphase but are largely absent from the SPB that remains in the mother cell. Through the analysis of one of the determinants of this asymmetry, Bfa1, we find that the machinery responsible for establishing cell polarity and cytoplasmic microtubules collaborate to establish MEN asymmetry. In cells defective in the Cdc42 signaling pathway or the formin Bni1, Bfa1 localizes to both SPBs. The quantitative analysis of Bfa1 localization further shows that Bfa1 can associate with both SPBs in a transient and highly dynamic fashion, but the protein is stabilized on the SPB that migrates into the daughter cell during anaphase through microtubule-bud cortex interactions. Our results indicate that mother-daughter cell asymmetry determinants establish MEN signaling asymmetry through microtubule-bud cortex interactions.

Growth-speed-correlated localization of exocyst and polarisome components in growth zones of Ashbya gossypii hyphal tips

We use the fungus Ashbya gossypii to investigate how its polar growth machinery is organized to achieve sustained hyphal growth. In slowly elongating hyphae exocyst, cell polarity and polarisome proteins permanently localize as cortical cap at hyphal tips, thus defining the zone of secretory vesicle fusion. In tenfold faster growing hyphae, this zone is only slightly enlarged demonstrating a capacity of hyphal growth zones to increase rates of vesicle processing to reach higher speeds. Concomitant with this increase, vesicles accumulate as spheroid associated with the tip cortex, indicating that a Spitzenkörper forms in fast hyphae. We also found spheroid-like accumulations for the exocyst components AgSec3, AgSec5, AgExo70 and the polarisome components AgSpa2, AgBni1 and AgPea2 (but not AgBud6 or cell polarity factors such as AgCdc42 or AgBem1). The localization of AgSpa2, AgPea2 and AgBni1 depend on each other but only marginally on AgBud6, as concluded from a set of deletions. Our data define three conditions to achieve fast growth at hyphal tips: permanent presence of the polarity machinery in a confined cortical area, organized accumulation of vesicles and a subset of polarity components close to this area, and spatial separation of the zones of exocytosis (tip front) and endocytosis (tip rim).

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.

Spa2 is required for morphogenesis but it is dispensable for pathogenicity in the phytopathogenic fungus Ustilago maydis

The increasing evidence linking regulation of polar growth and pathogenicity in fungi has elicited a significant effort devoted to produce a better understanding of mechanisms determining polarization in pathogenic fungi. Here we characterize in the phytopathogenic basidiomycete Ustilago maydis, the Spa2 protein, a well-known component of polarisome, firstly described in Saccharomyces cerevisiae. U. maydis display a dimorphic switch between budding growth of hapoid cells and filamentous growth of the dikaryon. During yeast growth, a GFP-tagged Spa2 protein localized to distinct growth sites in a cell cycle-specific manner, while during hyphal growth is persistently located to hyphal tips. Deletion of spa2 gene produces rounder budding cells and thicker filaments than wild-type cells, suggesting a role of Spa2 for the determination of the growth area in U. maydis. We also address the connections between Spa2 and the actin- and microtubule-cytoskeleton. We found that the absence of Spa2 does not affect cytoskeleton organization and strikingly, interference with actin filament or microtubule formation does not affect the polar localization of Spa2. In contrast, defects in the small GTPase Rac1 seems to affect the ability of Spa2 to locate to precise sites at the tip cell. Finally, to our surprise, we found that cells defectives in Spa2 function were as pathogenic as wild-type cells.

Dynamic localization of yeast Fus2p to an expanding ring at the cell fusion junction during mating

Fus2p is a pheromone-induced protein associated with the amphiphysin homologue Rvs161p, which is required for cell fusion during mating in Saccharomyces cerevisiae. We constructed a functional Fus2p–green fluorescent protein (GFP), which exhibits highly dynamic localization patterns in pheromone-responding cells (shmoos): diffuse nuclear, mobile cytoplasmic dots and stable cortical patches concentrated at the shmoo tip. In mitotic cells, Fus2p-GFP is nuclear but becomes cytoplasmic as cells form shmoos, dependent on the Fus3p protein kinase and high levels of pheromone signaling. The rapid cytoplasmic movement of Fus2p-GFP dots requires Rvs161p and polymerized actin and is aberrant in mutants with compromised actin organization, which suggests that the Fus2p dots are transported along actin cables, possibly in association with vesicles. Maintenance of Fus2p-GFP patches at the shmoo tip cortex is jointly dependent on actin and a membrane protein, Fus1p, which suggests that Fus1p is an anchor for Fus2p. In zygotes, Fus2p-GFP forms a dilating ring at the cell junction, returning to the nucleus at the completion of cell fusion.

Spatial controls for growth zone formation during the fission yeast cell cycle

Because of its regular shape, fission yeast is becoming an increasingly important organism in the study of cellular morphogenesis. Genetic experiments with mutants and drug treatment studies with wild-type cells have revealed the importance of microtubules in controlling new growth zone formation. It is believed that microtubules exert this role by delivering to cell ends a 'dynamic landmark' protein, tea1p, which promotes actin polymerization and growth zone formation. Here we present a simple model for fission yeast morphogenesis that describes the interplay between these two cytoskeletal elements. An essential assumption of the model is that actin polymerization is a self-reinforcing process: filamentous actin promotes its own formation from globular actin subunits via regulatory molecules. In our model, microtubules stimulate actin polymerization by delivering a component of the autocatalytic actin-assembly feedback loop (not by delivering a de novo inducer of actin polymerization). We show that the model captures all the characteristic features of polarized growth in fission yeast during normal mitotic cycles. We categorize the types of growth patterns that can exist in the model and show that they correspond to the major classes of morphogenetic mutants (monopolar, orb, banana and tea). Based on these results, we propose that fission yeast cells have specific size ranges in which they can exhibit two or more different stable patterns of growth.

The polarisome component SpaA localises to hyphal tips of Aspergillus niger and is important for polar growth

Hyphal tip growth is a key feature of filamentous fungi, however, the molecular mechanism(s) that regulate cell polarity are poorly understood. On the other hand, much more is known about polarised growth in the yeast Saccharomyces cerevisiae. Here, the proteins Spa2p, Bni1p, Bud6p and Pea2p form a protein complex named the polarisome known to be important for the assurance of polar growth. We searched the genome of Aspergillus niger and identified homologues for Spa2p, Bni1p, Bud6p but not for Pea2p. We characterised the function of the Spa2p homologue SpaA by determining its cellular localisation and by constructing deletion and overexpressing mutant strains. SpaA was found to be localised exclusively at the hyphal tip, suggesting that SpaA can be used as marker for polarisation. Deletion and overexpression of spaA resulted in reduced growth rate, increased hyphal diameter and polarity defects, indicating that one of the functions of SpaA is to ensure polarity maintenance. In addition, we could show that SpaA is able to complement the defective haploid invasive growth phenotype of a S. cerevisiae SPA2 null mutant. We suggest that the function of SpaA is to ensure maximal polar growth rate in A. niger.

Inheritance of cortical ER in yeast is required for normal septin organization

How cells monitor the distribution of organelles is largely unknown. In budding yeast, the largest subdomain of the endoplasmic reticulum (ER) is a network of cortical ER (cER) that adheres to the plasma membrane. Delivery of cER from mother cells to buds, which is termed cER inheritance, occurs as an orderly process early in budding. We find that cER inheritance is defective in cells lacking Scs2, a yeast homologue of the integral ER membrane protein VAP (vesicle-associated membrane protein–associated protein) conserved in all eukaryotes. Scs2 and human VAP both target yeast bud tips, suggesting a conserved action of VAP in attaching ER to sites of polarized growth. In addition, the loss of either Scs2 or Ice2 (another protein involved in cER inheritance) perturbs septin assembly at the bud neck. This perturbation leads to a delay in the transition through G2, activating the Saccharomyces wee1 kinase (Swe1) and the morphogenesis checkpoint. Thus, we identify a mechanism involved in sensing the distribution of ER.

Apical sterol-rich membranes are essential for localizing cell end markers that determine growth directionality in the filamentous fungus Aspergillus nidulans

In filamentous fungi, hyphal extension depends on the continuous delivery of vesicles to the growing tip. Here, we describe the identification of two cell end marker proteins, TeaA and TeaR, in Aspergillus nidulans, corresponding to Tea1 and Mod5 in Schizosaccharomyces pombe. Deletion of teaA or teaR caused zig-zag-growing and meandering hyphae, respectively. The Kelch-repeat protein TeaA, the putatively prenylated TeaR protein, and the formin SepA were highly concentrated in the Spitzenkörper, a vesicle transit station at the tip, and localized along the tip membrane. TeaA localization at tips depended on microtubules, and TeaA was required for microtuble convergence in the hyphal apex. The CENP-E family kinesin KipA was necessary for proper localization of TeaA and TeaR, but not for their transportation. TeaA and TeaR localization were interdependent. TeaA interacted in vivo with TeaR, and TeaA colocalized with SepA. Sterol-rich membrane domains localized at the tip in teaA and teaR mutants like in wild type, and filipin treatment caused mislocalization of both proteins. This suggests that sterol-rich membrane domains determine cell end factor destinations and thereby polarized growth.

Spatial regulation of Fus3 MAP kinase activity through a reaction-diffusion mechanism in yeast pheromone signalling

Signal transduction through mitogen-activated protein kinase (MAPK) cascades is thought to occur through the assembly of macromolecular complexes. We quantified the abundance of complexes in the cytoplasm among the MAPKs Ste11, Ste7, Fus3 and the scaffold protein Ste5 in yeast pheromone signalling using fluorescence cross-correlation spectroscopy (FCCS). Significant complex concentrations were observed that remained unchanged on pheromone stimulation, demonstrating that global changes in complex abundances do not contribute to the transmission of signal through the cytoplasm. On the other hand, investigation of the distribution of active Fus3 (Fus3(PP)) across the cytoplasm using fluorescence lifetime imaging microscopy (FLIM) revealed a gradient of Fus3(PP) activity emanating from the tip of the mating projection. Spatial partitioning of Fus3 activating kinases to this site and deactivating phosphatases in the cytoplasm maintain this Fus3(PP)-activity distribution. Propagation of signalling from the shmoo is, therefore, spatially constrained by a gradient-generating reaction-diffusion mechanism.

Retrophosphorylation of Mkk1 and Mkk2 MAPKKs by the Slt2 MAPK in the yeast cell integrity pathway

In Saccharomyces cerevisiae, a variety of stresses and aggressions to the cell wall stimulate the activation of the cell wall integrity MAPK pathway, which triggers the expression of a series of genes important for the maintenance of cell wall homeostasis. This MAPK module lies downstream of the Rho1 small GTPase and protein kinase C Pkc1 and consists of MAPKKK Bck1, MAPKKs Mkk1 and Mkk2, and the Slt2 MAPK. In agreement with previous reports suggesting that Mkk1 and Mkk2 were functionally redundant, we show here that both Mkk1 and Mkk2 alone or even chimerical proteins constructed by interchanging their catalytic and regulatory domains are able to efficiently maintain signal transduction through the pathway. Both Mkk1 and Mkk2 are phosphorylated in vivo concomitant to activation of the cell integrity pathway. Interestingly, hyperphosphorylation of the MEKs required not only the upstream components of the pathway, but also a catalytically competent Slt2 MAPK downstream. Active Slt2 purified from yeast extracts was able to phosphorylate Mkk1 and Mkk2 in vitro. We have mapped Ser(50) as a direct phosphorylation target for Slt2 in Mkk2. However, substitution of all (Ser/Thr)-Pro canonical MAPK target sites with alanine did not totally abrogate Slt2-dependent Mkk2 phosphorylation. Mutation or deletion of a conserved MAPK-docking site at the N-terminal extension of Mkk2 precluded its interaction with Slt2 and negatively affected retrophosphorylation. Our data show that the cell wall integrity MAPKKs are targets for their downstream MAPK, suggesting the existence of complex feedback regulatory mechanisms at this level.

Function and regulation in MAPK signaling pathways: lessons learned from the yeast Saccharomyces cerevisiae

Signaling pathways that activate different mitogen-activated protein kinases (MAPKs) elicit many of the responses that are evoked in cells by changes in certain environmental conditions and upon exposure to a variety of hormonal and other stimuli. These pathways were first elucidated in the unicellular eukaryote Saccharomyces cerevisiae (budding yeast). Studies of MAPK pathways in this organism continue to be especially informative in revealing the molecular mechanisms by which MAPK cascades operate, propagate signals, modulate cellular processes, and are controlled by regulatory factors both internal to and external to the pathways. Here we highlight recent advances and new insights about MAPK-based signaling that have been made through studies in yeast, which provide lessons directly applicable to, and that enhance our understanding of, MAPK-mediated signaling in mammalian cells.

Rot1 plays an antagonistic role to Clb2 in actin cytoskeleton dynamics throughout the cell cycle

ROT1 is an essential gene whose inactivation causes defects in cell cycle progression and morphogenesis in budding yeast. Rot1 affects the actin cytoskeleton during the cell cycle at two levels. First, it is required for the maintenance of apical growth during bud growth. Second, Rot1 is necessary to polarize actin cytoskeleton to the neck region at the end of mitosis; because of this defect, rot1 cells do not properly form a septum to complete cell division. The inability to polarize the actin cytoskeleton at the end of mitosis is not due to a defect in the recruitment of the polarisome scaffold protein Spa2 or the actin cytoskeleton regulators Cdc42 and Cdc24 in the neck region. Previous results indicate a connection between Rot1 and the cyclin Clb2. In fact, overexpression of CLB2 is toxic when ROT1 is partially inactivated, and reciprocally, deletion of CLB2 suppresses the lethality of the rot1 mutant, which indicates a functional antagonism between Clb2 and Rot1. Several genetic interactions suggest a link between Rot1 and the ubiquitin-proteasome system and we show that the Clb2 cyclin is not properly degraded in rot1 cells.

Afr1p has a role in regulating the localization of Mpk1p at the shmoo tip inSaccharomyces cerevisiae

Afr1p functions to promote adaptation to pheromone-induced growth arrest and morphogenesis. We show here that Afr1p regulates polarized localization of the Mpk1p MAP kinase in shmooing cells. Deletion of AFR1 results in mislocalization of Mpk1p although the scaffold protein Spa2p localizes normally at shmoo tip, and overexpression of Spa2 cannot rescue this defect, indicating Afr1p in required for Spa2p to recruit Mpk1 to the site of polarized growth during mating. Overexpression of SPA2 partially suppresses the morphogenetic defect of afr1Delta cells upon alpha-factor induction, suggesting the two proteins function in the same genetic pathway with Spa2p acts downstream of Afr1p.

The Saccharomyces cerevisiae Arf3 protein is involved in actin cable and cortical patch formation

We show that Arf3p, a member of the ADP ribosylation family, is involved in the organization of actin cables and cortical patches in Saccharomyces cerevisiae. Profilin-deficient cells (pfy1Delta) have severe growth defects and lack actin cables. Overexpression of ARF3 restores actin cables and corrects growth defects in these cells. Cells deficient for the cortical patch proteins Las17p and Vrp1p have growth defects and a random cortical patch distribution. Overexpression of ARF3 in las17Delta and in vrp1Delta cells partially corrects growth defects and restores the polarized distribution of cortical patches. The N-terminal glycine, a myristoylation site in Arf3p, is necessary for its suppressor activity. arf3Delta cells show a random budding pattern. Overexpression of BNI1, GEA2 or SYP1, three genes involved in actin cytoskeleton formation, restores the normal axial budding pattern of arf3Delta cells. BUD6 is a polarity gene and GEA2 is involved in retrograde transport and the organization of the actin cytoskeleton. We have identified genetic interactions between ARF3 and BUD6, and between ARF3 and GEA2. Both double mutant strains have actin cytoskeleton defects. Our results support a role for ARF3 in cell polarity and the organization of the actin cytoskeleton.

Stable preanaphase spindle positioning requires Bud6p and an apparent interaction between the spindle pole bodies and the neck

Faithful partitioning of genetic material during cell division requires accurate spatial and temporal positioning of nuclei within dividing cells. In Saccharomyces cerevisiae, nuclear positioning is regulated by an elegant interplay between components of the actin and microtubule cytoskeletons. Regulators of this process include Bud6p (also referred to as the actin-interacting protein Aip3p) and Kar9p, which function to promote contacts between cytoplasmic microtubule ends and actin-delimited cortical attachment points. Here, we present the previously undetected association of Bud6p with the cytoplasmic face of yeast spindle pole bodies, the functional equivalent of metazoan centrosomes. Cells lacking Bud6p show exaggerated movements of the nucleus between mother and daughter cells and display reduced amounts of time a given spindle pole body spends in close association with the neck region of budding cells. Furthermore, overexpression of BUD6 greatly enhances interactions between the spindle pole body and mother-bud neck in a spindle alignment-defective dynactin mutant. These results suggest that association of either spindle pole body with neck components, rather than simply entry of a spindle pole body into the daughter cell, provides a positive signal for the progression of mitosis. We propose that Bud6p, through its localization at both spindle pole bodies and at the mother-bud neck, supports this positive signal and provides a regulatory mechanism to prevent excessive oscillations of preanaphase nuclei, thus reducing the likelihood of mitotic delays and nuclear missegregation.

Yeast formins Bni1 and Bnr1 utilize different modes of cortical interaction during the assembly of actin cables

The budding yeast formins Bni1 and Bnr1 control the assembly of actin cables. These formins exhibit distinct patterns of localization and polymerize two different populations of cables: Bni1 in the bud and Bnr1 in the mother cell. We generated a functional Bni1-3GFP that improved the visualization of Bni1 in vivo at endogenous levels. Bni1 exists as speckles in the cytoplasm, some of which colocalize on actin cables. These Bni1 speckles display linear, retrograde-directed movements. Loss of polymerized actin or specifically actin cables abolished retrograde movement, and resulted in depletion of Bni1 speckles from the cytoplasm, with enhanced targeting of Bni1 to the bud tip. Mutations that impair the actin assembly activity of Bni1 abolished the movement of Bni1 speckles, even when actin cables were present. In contrast, Bnr1-GFP or 3GFP-Bnr1 did not detectably associate with actin cables and was not observed as cytoplasmic speckles. Finally, fluorescence recovery after photobleaching demonstrated that Bni1 was very dynamic, exchanging between polarized sites and the cytoplasm, whereas Bnr1 was confined to the bud neck and did not exchange with a cytoplasmic pool. In summary, our results indicate that formins can have distinct modes of cortical interaction during actin cable assembly.

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.

Requirement for the polarisome and formin function in Ssk2p-mediated actin recovery from osmotic stress in Saccharomyces cerevisiae

Osmotic stress induces activation of an adaptive mitogen-activated protein kinase pathway in concert with disassembly of the actin cytoskeleton by a mechanism that is not understood. We have previously shown that the conserved actin-interacting MAP kinase kinase kinase Ssk2p/MEKK4, a member of the high-osmolarity glycerol (HOG) MAPK pathway of Saccharomyces cerevisiae, mediates recovery of the actin cytoskeleton following osmotic stress. In this study, we have employed in vitro kinase assays to show that Ssk2p kinase activity is activated for the actin recovery pathway via a noncanonical, Ssk1p-independent mechanism. Our work also shows that Ssk2p requires the polarisome proteins Bud6p and Pea2p to promote efficient, polarized actin reassembly but that this requirement can be bypassed by overexpression of Ssk2p. Formin (BNI1 or BNR1) and tropomyosin functions are also required for actin recovery but, unlike for Bud6p and Pea2p, these requirements cannot be bypassed by overexpression of Ssk2p. These results suggest that Ssk2p acts downstream of Bud6p and Pea2p and upstream of tropomyosin to drive actin recovery, possibly by upregulating the actin nucleation activity of the formins.

The MEK kinases MEKK4/Ssk2p facilitate complexity in the stress signaling responses of diverse systems

The mammalian JNK/p38 MAP kinase kinase kinase MEKK4 and the Saccharomyces cerevisiae Ssk2p are highly homologous. MEKK4 can replace all of the known functions of Ssk2p in yeast, including functioning in the high osmolarity glycerol (HOG) MAPK pathway and the recently described actin recovery pathway. MEKK4 and Ssk2p share a number of conserved domains and appear to be activated by a similar mechanism. Binding of an activating protein to the N-terminal region alleviates auto-inhibition and causes the kinase to auto-phosphorylate, resulting in activation. In this review we will examine the role of the MAP kinase kinase kinase isoform Ssk2p/MEKK4 in the adaptation of both yeast and mammalian systems to specific external stimuli. Recent work has provided a wealth of information about the activation, regulation, and functions of these MEKK kinases to extra-cellular signals. We will also highlight evidence supporting a role for MEKK4 in mediating actin recovery following osmotic shock in mammalian cells.