The Spa2-related protein, Sph1p, is important for polarized growth in yeast

Spitzenkörper assembly mechanisms reveal conserved features of fungal and metazoan polarity scaffolds

The Spitzenkörper (SPK) constitutes a collection of secretory vesicles and polarity-related proteins intimately associated with polarized growth of fungal hyphae. Many SPK-localized proteins are known, but their assembly and dynamics remain poorly understood. Here, we identify protein-protein interaction cascades leading to assembly of two SPK scaffolds and recruitment of diverse effectors in Neurospora crassa. Both scaffolds are transported to the SPK by the myosin V motor (MYO-5), with the coiled-coil protein SPZ-1 acting as cargo adaptor. Neither scaffold appears to be required for accumulation of SPK secretory vesicles. One scaffold consists of Leashin-2 (LAH-2), which is required for SPK localization of the signalling kinase COT-1 and the glycolysis enzyme GPI-1. The other scaffold comprises a complex of Janus-1 (JNS-1) and the polarisome protein SPA-2. Via its Spa homology domain (SHD), SPA-2 recruits a calponin domain-containing F-actin effector (CCP-1). The SHD NMR structure reveals a conserved surface groove required for effector binding. Similarities between SPA-2/JNS-1 and the metazoan GIT/PIX complex identify foundational features of the cell polarity apparatus that predate the fungal-metazoan divergence.

The Spitzenkörper (SPK) is a polarized accumulation of proteins and secretory vesicles associated with tip growth of fungal hyphae. Here, Zheng et al. study SPK assembly and dynamics, identify SPK protein scaffolds and associated proteins, and reveal similarities with other scaffolds from metazoans.

Cell polarity protein Spa2 coordinates Chs2 incorporation at the division site in budding yeast

Deposition of additional plasma membrane and cargoes during cytokinesis in eukaryotic cells must be coordinated with actomyosin ring contraction, plasma membrane ingression and extracellular matrix remodelling. The process by which the secretory pathway promotes specific incorporation of key factors into the cytokinetic machinery is poorly understood. Here, we show that cell polarity protein Spa2 interacts with actomyosin ring components during cytokinesis. Spa2 directly binds to cytokinetic factors Cyk3 and Hof1. The lethal effects of deleting the SPA2 gene in the absence of either Cyk3 or Hof1 can be suppressed by expression of the hypermorphic allele of the essential chitin synthase II (Chs2), a transmembrane protein transported on secretory vesicles that makes the primary septum during cytokinesis. Spa2 also interacts directly with the chitin synthase Chs2. Interestingly, artificial incorporation of Chs2 into the cytokinetic machinery allows the localisation of Spa2 at the site of division. In addition, increased Spa2 protein levels promote Chs2 incorporation at the site of division and primary septum formation. Our data indicate that Spa2 is recruited to the cleavage site to co-operate with the secretory vesicle system and particular actomyosin ring components to promote the incorporation of Chs2 into the so-called 'ingression progression complexes' during cytokinesis in budding yeast.

Different polarisome components play distinct roles in Slt2p-regulated cortical ER inheritance in Saccharomyces cerevisiae

Slt2p kinase activity controls cortical ER inheritance by regulating the association of the ER with the actin-based cytoskeleton. The polarisome complex affects ER inheritance through its effects on Slt2p, with different components playing distinct roles: some are required for Slt2p retention at the bud tip, whereas others affect Slt2p activation.

Ptc1p, a type 2C protein phosphatase, is required for a late step in cortical endoplasmic reticulum (cER) inheritance in Saccharomyces cerevisiae. In ptc1Δ cells, ER tubules migrate from the mother cell and contact the bud tip, yet fail to spread around the bud cortex. This defect results from the failure to inactivate a bud tip–associated pool of the cell wall integrity mitogen-activated protein kinase, Slt2p. Here we report that the polarisome complex affects cER inheritance through its effects on Slt2p, with different components playing distinct roles: Spa2p and Pea2p are required for Slt2p retention at the bud tip, whereas Bni1p, Bud6p, and Sph1p affect the level of Slt2p activation. Depolymerization of actin relieves the ptc1Δ cER inheritance defect, suggesting that in this mutant the ER becomes trapped on the cytoskeleton. Loss of Sec3p also blocks ER inheritance, and, as in ptc1Δ cells, this block is accompanied by activation of Slt2p and is reversed by depolymerization of actin. Our results point to a common mechanism for the regulation of ER inheritance in which Slt2p activity at the bud tip controls the association of the ER with the actin-based cytoskeleton.

Neighbor overlap is enriched in the yeast interaction network: analysis and implications

The yeast protein-protein interaction network has been shown to have distinct topological features such as a scale free degree distribution and a high level of clustering. Here we analyze an additional feature which is called Neighbor Overlap. This feature reflects the number of shared neighbors between a pair of proteins. We show that Neighbor Overlap is enriched in the yeast protein-protein interaction network compared with control networks carefully designed to match the characteristics of the yeast network in terms of degree distribution and clustering coefficient. Our analysis also reveals that pairs of proteins with high Neighbor Overlap have higher sequence similarity, more similar GO annotations and stronger genetic interactions than pairs with low ones. Finally, we demonstrate that pairs of proteins with redundant functions tend to have high Neighbor Overlap. We suggest that a combination of three mechanisms is the basis for this feature: The abundance of protein complexes, selection for backup of function, and the need to allow functional variation.

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.

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.

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.

Uncovering signal transduction networks from high-throughput data by integer linear programming

Signal transduction is an important process that transmits signals from the outside of a cell to the inside to mediate sophisticated biological responses. Effective computational models to unravel such a process by taking advantage of high-throughput genomic and proteomic data are needed to understand the essential mechanisms underlying the signaling pathways. In this article, we propose a novel method for uncovering signal transduction networks (STNs) by integrating protein interaction with gene expression data. Specifically, we formulate STN identification problem as an integer linear programming (ILP) model, which can be actually solved by a relaxed linear programming algorithm and is flexible for handling various prior information without any restriction on the network structures. The numerical results on yeast MAPK signaling pathways demonstrate that the proposed ILP model is able to uncover STNs or pathways in an efficient and accurate manner. In particular, the prediction results are found to be in high agreement with current biological knowledge and available information in literature. In addition, the proposed model is simple to be interpreted and easy to be implemented even for a large-scale system.

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.

Scaffold proteins of MAP-kinase modules

Mitogen-activated protein kinases (MAPKs) regulate critical signaling pathways involved in cell proliferation, differentiation and apoptosis. Recent studies have shown that a novel class of scaffold proteins mediates the structural and functional organization of the three-tier MAPK module. By linking the MAP3K, MAP2K and MAPK into a multienzyme complex, these MAPK-specific scaffold proteins provide an insulated physical conduit through which signals from the respective MAPK can be transmitted to the appropriate spatiotemporal cellular loci. Scaffold proteins play a determinant role in modulating the signaling strength of their cognate MAPK module by regulating the signal amplitude and duration. The scaffold proteins themselves are finely regulated resulting in dynamic intra- and inter-molecular interactions that can modulate the signaling outputs of MAPK modules. This review focuses on defining the diverse mechanisms by which these scaffold proteins interact with their respective MAPK modules and the role of such interactions in the spatiotemporal organization as well as context-specific signaling of the different MAPK modules.

The yeast actin cytoskeleton: from cellular function to biochemical mechanism

All cells undergo rapid remodeling of their actin networks to regulate such critical processes as endocytosis, cytokinesis, cell polarity, and cell morphogenesis. These events are driven by the coordinated activities of a set of 20 to 30 highly conserved actin-associated proteins, in addition to many cell-specific actin-associated proteins and numerous upstream signaling molecules. The combined activities of these factors control with exquisite precision the spatial and temporal assembly of actin structures and ensure dynamic turnover of actin structures such that cells can rapidly alter their cytoskeletons in response to internal and external cues. One of the most exciting principles to emerge from the last decade of research on actin is that the assembly of architecturally diverse actin structures is governed by highly conserved machinery and mechanisms. With this realization, it has become apparent that pioneering efforts in budding yeast have contributed substantially to defining the universal mechanisms regulating actin dynamics in eukaryotes. In this review, we first describe the filamentous actin structures found in Saccharomyces cerevisiae (patches, cables, and rings) and their physiological functions, and then we discuss in detail the specific roles of actin-associated proteins and their biochemical mechanisms of action.

Functional characterization of Aspergillus nidulans homologues of Saccharomyces cerevisiae Spa2 and Bud6

The importance of polarized growth for fungi has elicited significant effort directed at better understanding underlying mechanisms of polarization, with a focus on yeast systems. At sites of tip growth, multiple protein complexes assemble and coordinate to ensure that incoming building material reaches the appropriate destination sites, and polarized growth is maintained. One of these complexes is the polarisome that consists of Spa2, Bud6, Pea2, and Bni1 in Saccharomyces cerevisiae. Filamentous hyphae differ in their development and life style from yeasts and likely regulate polarized growth in a different way. This is expected to reflect on the composition and presence of protein complexes that assemble at the hyphal tip. In this study we searched for polarisome homologues in the model filamentous fungus Aspergillus nidulans and characterized the S. cerevisiae Spa2 and Bud6 homologues, SpaA and BudA. Compared to the S. cerevisiae Spa2, SpaA lacks domain II but has three additional domains that are conserved within filamentous fungi. Gene replacement strains and localization studies show that SpaA functions exclusively at the hyphal tip, while BudA functions at sites of septum formation and possibly at hyphal tips. We show that SpaA is not required for the assembly or maintenance of the Spitzenkörper. We propose that the core function of the polarisome in polarized growth is maintained but with different contributions of polarisome components to the process.

Regulation of cell polarity by interactions of Msb3 and Msb4 with Cdc42 and polarisome components

In Saccharomyces cerevisiae, polarized growth depends on interactions between the actin cytoskeleton and the secretory machinery. Here we show that the Rab GTPase-activating proteins (GAPs) Msb3 and Msb4 interact directly with Spa2, a scaffold protein of the "polarisome" that also interacts with the formin Bni1. Spa2 is required for the polarized localization of Msb3 and Msb4 at the bud tip. We also show that Msb3 and Msb4 bind specifically to Cdc42-GDP and Rho1-GDP in vitro and that Msb3 and Rho GDP dissociation inhibitor act independently but oppositely on Cdc42. Finally, we show that Msb3 and Msb4 are involved in Bni1-nucleated actin assembly in vivo. These results suggest that Msb3 and Msb4 regulate polarized growth by multiple mechanisms, directly regulating exocytosis through their GAP activity toward Sec4 and potentially coordinating the functions of Cdc42, Rho1, and Bni1 in the polarisome through their binding to these GTPases. A functional equivalent of the polarisome probably exists in other fungi and mammals.

Maximal polar growth potential depends on the polarisome component AgSpa2 in the filamentous fungus Ashbya gossypii

We used actin staining and videomicroscopy to analyze the development from a spore to a young mycelium in the filamentous ascomycete Ashbya gossypii. The development starts with an initial isotropic growth phase followed by the emergence of germ tubes. The initial tip growth speed of 6-10 microm/h increases during early stages of development. This increase is transiently interrupted in response to the establishment of lateral branches or septa. The hyphal tip growth speed finally reaches a maximum of up to 200 micro/h, and the tips of these mature hyphae have the ability to split into two equally fast-growing hyphae. A search for A. gossypii homologs of polarisome components of the yeast Saccharomyces cerevisiae revealed a remarkable size difference between Spa2p of both organisms, with AgSpa2p being double as long as ScSpa2p due to an extended internal domain. AgSpa2 colocalizes with sites of polarized actin. Using time-lapse videomicroscopy, we show that AgSpa2p-GFP polarization is established at sites of branch initiation and then permanently maintained at hyphal tips. Polarization at sites of septation is transient. During apical branching the existing AgSpa2p-GFP polarization is symmetrically divided. To investigate the function of AgSpa2p, we generated two AgSPA2 mutants, a partial deletion of the internal domain alone, and a complete deletion. The mutations had an impact on the maximal hyphal tip growth speed, on the hyphal diameter, and on the branching pattern. We suggest that AgSpa2p is required for the determination of the area of growth at the hyphal tip and that the extended internal domain plays an important role in this process.

CaSPA2 is important for polarity establishment and maintenance in Candida albicans

Saccharomyces cerevisiae Spa2p is a component of polarisome that controls cell polarity. Here, we have characterized the role of its homologue, CaSpa2p, in the polarized growth in Candida albicans. During yeast growth, GFP-tagged CaSpa2p localized to distinct growth sites in a cell cycle-dependent manner, while during hyphal growth it persistently localized to hyphal tips throughout the cell cycle. Persistent tip localization of the protein was also observed in Catup1Delta and Canrg1Delta, mutants constitutive for filamentous growth. Caspa2Delta exhibited defects in polarity establishment and maintenance, such as random budding and failure to confine growth to a small surface area leading to round cells with wide, elongated bud necks and markedly thicker hyphae. It was also defective in nuclear positioning, presumably a result of defective interactions between cytoplasmic microtubules with certain polarity determinants. The highly conserved SHD-I and SHD-V domains were found to be important and responsible for different aspects of CaSpa2p function. Caspa2Delta exhibited no virulence in the mouse systemic candidiasis model. Because of the existence of distinct growth forms and the easy control of the switch between them in vitro, C. albicans may serve as a useful model in cell polarity research.

Activation of a putative MAP kinase in pollen is stimulated by the self-incompatibility (SI) response

Mitogen-activated protein kinases (MAPKs) operate downstream of receptor-ligand interactions, playing a pivotal role in responses to extracellular signals. The self-incompatibility (SI) response in Papaver rhoeas L. triggers a Ca2+-dependent signalling cascade resulting in inhibition of incompatible pollen. We have investigated the possible involvement of MAPKs in SI. We report the enhanced activation of a 56 kDa protein kinase (p56) in SI-induced pollen and provide evidence that p56 has MAPK activity. This provides an important advance in our understanding of the SI response. We believe this is the first direct biochemical demonstration of activation of a MAPK during SI.

The alpha-factor receptor C-terminus is important for mating projection formation and orientation in Saccharomyces cerevisiae

Successful mating of MATa Saccharomyces cerevisiae cells is dependent on Ste2p, the alpha-factor receptor. Besides receiving the pheromone signal and transducing it through the G-protein coupled MAP kinase pathway, Ste2p is active in the establishment and orientation of the mating projection. We investigated the role of the carboxyl terminus of the receptor in mating projection formation and orientation using a spatial gradient assay. Cells carrying the ste2-T326 mutation, truncating 105 of the 135 amino acids in the receptor tail including a motif necessary for its ligand-mediated internalization, display slow onset of projection formation, abnormal shmoo morphology, and reduced ability to orient the mating projection toward a pheromone source. This reduction was due to the increased loss of mating projection orientation in a pheromone gradient. Cells with a mutated endocytosis motif were defective in reorientation in a pheromone gradient. ste2-Delta296 cells, which carry a complete truncation of the Ste2p tail, exhibit a severe defect in projection formation, and those projections that do form are unable to orient in a pheromone gradient. These results suggest a complex role for the Ste2p carboxy-terminal tail in the formation, orientation, and directional adjustment of the mating projection, and that endocytosis of the receptor is important for this process. In addition, mutations in RSR1/BUD1 and SPA2, genes necessary for budding polarity, exhibited little or no defect in formation or orientation of mating projections. We conclude that mating projection orientation depends upon the carboxyl terminus of the pheromone receptor and not the directional machinery used in budding.

Spa2p functions as a scaffold-like protein to recruit the Mpk1p MAP kinase module to sites of polarized growth

Scaffold proteins play a major role in regulating MAP kinase pathways. In yeast, the Mpk1p-MAP kinase pathway functions to maintain the integrity of the cytoskeleton and the cell wall. In this module, the MEKK Bck1p functions upstream of the MEKs Mkk1p and Mkk2p, which in turn activate the MAP kinase Mpk1p. Mpk1p regulates several nuclear targets, including the transcription factors Rlm1p and SBF, and the two HMG1-like proteins NHP6A and NHP6B. Here we show that Mpk1p constitutively shuttles between the nucleus and the cytoplasm, and both Mpk1p and Mkk1p localize to sites of polarized growth in a Spa2p-dependent manner. Spa2p belongs to a group of proteins that includes Bni1p, Bud6p, and Pea2p, which are involved in the dynamic organization of the actin cytoskeleton during polarized growth. FRAP analysis shows that Spa2p-GFP is stably anchored at bud tips, whereas Mpk1p binds transiently. Spa2p interacts with Mkk1p and Mpk1p, and membrane bound Spa2p is sufficient to recruit Mkk1p and Mpk1p but not other MAP kinases to the cell cortex. Taken together, these results suggest that Spa2p functions as a scaffold-like protein for the cell wall integrity pathway during polarized growth.

Cdc42 regulation of kinase activity and signaling by the yeast p21-activated kinase Ste20

The Saccharomyces cerevisiae kinase Ste20 is a member of the p21-activated kinase (PAK) family with several functions, including pheromone-responsive signal transduction. While PAKs are usually activated by small G proteins and Ste20 binds Cdc42, the role of Cdc42-Ste20 binding has been controversial, largely because Ste20 lacking its entire Cdc42-binding (CRIB) domain retains kinase activity and pheromone response. Here we show that, unlike CRIB deletion, point mutations in the Ste20 CRIB domain that disrupt Cdc42 binding also disrupt pheromone signaling. We also found that Ste20 kinase activity is stimulated by GTP-bound Cdc42 in vivo and this effect is blocked by the CRIB point mutations. Moreover, the Ste20 CRIB and kinase domains bind each other, and mutations that disrupt this interaction cause hyperactive kinase activity and bypass the requirement for Cdc42 binding. These observations demonstrate that the Ste20 CRIB domain is autoinhibitory and that this negative effect is antagonized by Cdc42 to promote Ste20 kinase activity and signaling. Parallel results were observed for filamentation pathway signaling, suggesting that the requirement for Cdc42-Ste20 interaction is not qualitatively different between the mating and filamentation pathways. While necessary for pheromone signaling, the role of the Cdc42-Ste20 interaction does not require regulation by pheromone or the pheromone-activated G beta gamma complex, because the CRIB point mutations also disrupt signaling by activated forms of the kinase cascade scaffold protein Ste5. In total, our observations indicate that Cdc42 converts Ste20 to an active form, while pathway stimuli regulate the ability of this active Ste20 to trigger signaling through a particular pathway.

Role of a ubiquitin-like modification in polarized morphogenesis

Type I ubiquitin-like proteins constitute a family of protein modifiers. Here we report the identification of a posttranslational protein modifier from Saccharomyces cerevisiae, Hub1. Overexpression of Hub1 resulted in enhanced conjugate formation when its carboxyl-terminal residue was deleted, suggesting that mature Hub1 may be produced by proteolytic processing. In vivo targets of Hub1 conjugation included cell polarity factors Sph1 and Hbt1. In the hub1Delta mutant, the subcellular localization of both Hbt1 and Sph1 was disrupted, and cell polarization during the formation of mating projections was defective. Consistent with these polarization defects, the hub1Delta mutant was deficient in mating.

Signal transduction cascades regulating fungal development and virulence

Cellular differentiation, mating, and filamentous growth are regulated in many fungi by environmental and nutritional signals. For example, in response to nitrogen limitation, diploid cells of the yeast Saccharomyces cerevisiae undergo a dimorphic transition to filamentous growth referred to as pseudohyphal differentiation. Yeast filamentous growth is regulated, in part, by two conserved signal transduction cascades: a mitogen-activated protein kinase cascade and a G-protein regulated cyclic AMP signaling pathway. Related signaling cascades play an analogous role in regulating mating and virulence in the plant fungal pathogen Ustilago maydis and the human fungal pathogens Cryptococcus neoformans and Candida albicans. We review here studies on the signaling cascades that regulate development of these and other fungi. This analysis illustrates both how the model yeast S. cerevisiae can serve as a paradigm for signaling in other organisms and also how studies in other fungi provide insights into conserved signaling pathways that operate in many divergent organisms.

Human ERK1 induces filamentous growth and cell wall remodeling pathways in Saccharomyces cerevisiae

Expression of an activated extracellular signal-regulated kinase 1 (ERK1) construct in yeast cells was used to examine the conservation of function among mitogen-activated protein (MAP) kinases. Sequence alignment of the human MAP kinase ERK1 with all Saccharomyces cerevisiae kinases reveals a particularly strong kinship with Kss1p (invasive growth promoting MAP kinase), Fus3p (pheromone response MAP/ERK kinase), and Mpk1p (cell wall remodeling MAP kinase). A fusion protein of constitutively active human MAP/ERK kinase 1 (MEK) and human ERK1 was introduced under regulated expression into yeast cells. The fusion protein (MEK/ERK) induced a filamentation response element promoter and led to a growth retardation effect concomitant with a morphological change resulting in elongated cells, bipolar budding, and multicell chains. Induction of filamentous growth was also observed for diploid cells following MEK/ERK expression in liquid culture. Neither haploids nor diploids, however, showed marked penetration of agar medium. These effects could be triggered by either moderate MEK/ERK expression at 37 degrees C or by high level MEK/ERK expression at 30 degrees C. The combination of high level MEK/ERK expression and 37 degrees C resulted in cell death. The deleterious effects of MEK/ERK expression and high temperature were significantly mitigated by 1 m sorbitol, which also enhanced the filamentous phenotype. MEK/ERK was able to constitutively activate a cell wall maintenance reporter gene, suggesting misregulation of this pathway. In contrast, MEK/ERK effectively blocked expression from a pheromone-responsive element promoter and inhibited mating. These results are consistent with MEK/ERK promoting filamentous growth and altering the cell wall through its ability to partially mimic Kss1p and stimulate a pathway normally controlled by Mpk1p, while appearing to inhibit the normal functioning of the structurally related yeast MAP kinase Fus3p.

Polarized growth controls cell shape and bipolar bud site selection in Saccharomyces cerevisiae

We examined the relationship between polarized growth and division site selection, two fundamental processes important for proper development of eukaryotes. Diploid Saccharomyces cerevisiae cells exhibit an ellipsoidal shape and a specific division pattern (a bipolar budding pattern). We found that the polarity genes SPA2, PEA2, BUD6, and BNI1 participate in a crucial step of bud morphogenesis, apical growth. Deleting these genes results in round cells and diminishes bud elongation in mutants that exhibit pronounced apical growth. Examination of distribution of the polarized secretion marker Sec4 demonstrates that spa2Delta, pea2Delta, bud6Delta, and bni1Delta mutants fail to concentrate Sec4 at the bud tip during apical growth and at the division site during repolarization just prior to cytokinesis. Moreover, cell surface expansion is not confined to the distal tip of the bud in these mutants. In addition, we found that the p21-activated kinase homologue Ste20 is also important for both apical growth and bipolar bud site selection. We further examined how the duration of polarized growth affects bipolar bud site selection by using mutations in cell cycle regulators that control the timing of growth phases. The grr1Delta mutation enhances apical growth by stabilizing G(1) cyclins and increases the distal-pole budding in diploids. Prolonging polarized growth phases by disrupting the G(2)/M cyclin gene CLB2 enhances the accuracy of bud site selection in wild-type, spa2Delta, and ste20Delta cells, whereas shortening the polarized growth phases by deleting SWE1 decreases the fidelity of bipolar budding. This study reports the identification of components required for apical growth and demonstrates the critical role of polarized growth in bipolar bud site selection. We propose that apical growth and repolarization at the site of cytokinesis are crucial for establishing spatial cues used by diploid yeast cells to position division planes.

Polarization of cell growth in yeast

The actin cytoskeleton provides the structural basis for cell polarity in Saccharomyces cerevisiae as well as most other eukaryotes. In Part I of this two-part commentary, presented in the previous issue of Journal of Cell Science, we discussed the basis by which yeast establishes and maintains different states of polarity through Ρ GTPases and cyclin-dependent protein kinase signaling. Here we discuss how, in response to those signals, the actin cytoskeleton guides growth of the yeast cell. A polarized array of actin cables at the cell cortex is the primary structural determinant of polarity. Motors such as class V myosins use this array to transport secretory vesicles, mRNA and organelles towards growth sites, where they are anchored by a cap of cytoskeletal and regulatory proteins. Cortical actin patches enhance and maintain this polarity, probably through endocytic recycling, which allows reuse of materials and prevents continued growth at old sites. The dynamic arrangement of targeting and recycling provides flexibility for the precise control of morphogenesis.

Sbe2p and sbe22p, two homologous Golgi proteins involved in yeast cell wall formation

The cell wall of fungal cells is important for cell integrity and cell morphogenesis and protects against harmful environmental conditions. The yeast cell wall is a complex structure consisting mainly of mannoproteins, glucan, and chitin. The molecular mechanisms by which the cell wall components are synthesized and transported to the cell surface are poorly understood. We have identified and characterized two homologous yeast proteins, Sbe2p and Sbe22p, through their suppression of a chs5 spa2 mutant strain defective in chitin synthesis and cell morphogenesis. Although sbe2 and sbe22 null mutants are viable, sbe2 sbe22 cells display several phenotypes indicative of defects in cell integrity and cell wall structure. First, sbe2 sbe22 cells display a sorbitol-remediable lysis defect at 37 degrees C and are hypersensitive to SDS and calcofluor. Second, electron microscopic analysis reveals that sbe2 sbe22 cells have an aberrant cell wall structure with a reduced mannoprotein layer. Finally, immunofluorescence experiments reveal that in small-budded cells, sbe2 sbe22 mutants mislocalize Chs3p, a protein involved in chitin synthesis. In addition, sbe2 sbe22 diploids have a bud-site selection defect, displaying a random budding pattern. A Sbe2p-GFP fusion protein localizes to cytoplasmic patches, and Sbe2p cofractionates with Golgi proteins. Deletion of CHS5, which encodes a Golgi protein involved in the transport of Chs3p to the cell periphery, is lethal in combination with disruption of SBE2 and SBE22. Thus, we suggest a model in which Sbe2p and Sbe22p are involved in the transport of cell wall components from the Golgi apparatus to the cell surface periphery in a pathway independent of Chs5p.

Polarization of cell growth in yeast. I. Establishment and maintenance of polarity states

The ability to polarize is a fundamental property of cells. The yeast Saccharomyces cerevisiae has proven to be a fertile ground for dissecting the molecular mechanisms that regulate cell polarity during growth. Here we discuss the signaling pathways that regulate polarity. In the second installment of this two-part commentary, which appears in the next issue of Journal of Cell Science, we discuss how the actin cytoskeleton responds to these signals and guides the polarity of essentially all events in the yeast cell cycle. During the cell cycle, yeast cells assume alternative states of polarized growth, which range from tightly focused apical growth to non-focused isotropic growth. RhoGTPases, and in particular Cdc42p, are essential to guiding this polarity. The distribution of Cdc42p at the cell cortex establishes cell polarity. Cyclin-dependent protein kinase, Ras, and heterotrimeric G proteins all modulate yeast cell polarity in part by altering the distribution of Cdc42p. In turn, Cdc42p generates feedback signals to these molecules in order to establish stable polarity states and coordinate cytoskeletal organization with the cell cycle. Given that many of these signaling pathways are present in both fungi and animals, they are probably ancient and conserved mechanisms for regulating polarity.

The secretory pathway mediates localization of the cell polarity regulator Aip3p/Bud6p

Aip3p/Bud6p is a regulator of cell and cytoskeletal polarity in Saccharomyces cerevisiae that was previously identified as an actin-interacting protein. Actin-interacting protein 3 (Aip3p) localizes at the cell cortex where cytoskeleton assembly must be achieved to execute polarized cell growth, and deletion of AIP3 causes gross defects in cell and cytoskeletal polarity. We have discovered that Aip3p localization is mediated by the secretory pathway. Mutations in early- or late-acting components of the secretory apparatus lead to Aip3p mislocalization. Biochemical data show that a pool of Aip3p is associated with post-Golgi secretory vesicles. An investigation of the sequences within Aip3p necessary for Aip3p localization has identified a sequence within the N terminus of Aip3p that is sufficient for directing Aip3p localization. Replacement of the N terminus of Aip3p with a homologous region from a Schizosaccharomyces pombe protein allows for normal Aip3p localization, indicating that the secretory pathway-mediated Aip3p localization pathway is conserved. Delivery of Aip3p also requires the type V myosin motor Myo2p and its regulatory light-chain calmodulin. These data suggest that one function of calmodulin is to activate Myo2p's activity in the secretory pathway; this function is likely the polarized movement of late secretory vesicles and associated Aip3p on actin cables.

Invasive hyphal growth in Wangiella dermatitidis is induced by stab inoculation and shows dependence upon melanin biosynthesis

Stab inoculation of agar medium with yeasts of the human pathogen Wangiella dermatitidis resulted in induction of invasive hyphae. Mechanical penetration of agar was indicated by the observation that an increase in medium gel strength slowed the rate of substrate invasion. A melanized wild-type strain (8656) exhibited much faster invasive growth through 2-8% agar than three melanin-deficient mutants. Inhibition of melanin synthesis in strain 8656 using tricyclazole resulted in a decrease in its rate of invasive growth, while scytalone restored melanin synthesis in the albino mel3 strain and boosted its rate of invasive growth. Earlier research established that cellular melanization is also associated with invasive hyphal growth in the mouse brain, and infections with strain 8656 are invariably lethal. Together, these in vitro and in vivo data indicate that biomechanical characteristics of fungi may be important determinants of virulence and disease progression in human and animal mycoses.

The MAPKKK Ste11 regulates vegetative growth through a kinase cascade of shared signaling components

In haploid Saccharomyces cerevisiae, the mating and invasive growth (IG) pathways use the same mitogen-activated protein kinase kinase kinase kinase (MAPKKKK, Ste20), MAPKKK (Ste11), MAPKK (Ste7), and transcription factor (Ste12) to promote either G(1) arrest and fusion or foraging in response to distinct stimuli. This exquisite specificity is the result of pathway-specific receptors, G proteins, scaffold protein, and MAPKs. It is currently not thought that the shared signaling components function under the basal conditions of vegetative growth. We tested this hypothesis by searching for mutations that cause lethality when the STE11 gene is deleted. Strikingly, we found that Ste11, together with Ste20, Ste7, Ste12, and the IG MAPK Kss1, functions in a third pathway that promotes vegetative growth and is essential in an och1 mutant that does not synthesize mannoproteins. We term this pathway the STE vegetative growth (SVG) pathway. The SVG pathway functions, in part, to promote cell wall integrity in parallel with the protein kinase C pathway. During vegetative growth, the SVG pathway is inhibited by the mating MAPK Fus3. By contrast, the SVG pathway is constitutively activated in an och1 mutant, suggesting that it senses intracellular changes arising from the loss of mannoproteins. We predict that general proliferative functions may also exist for other MAPK cascades thought only to perform specialized functions.

Regulatory networks controlling Candida albicans morphogenesis

Candida albicans undergoes reversible morphogenetic transitions between budding, pseudohyphal and hyphal growth forms that promote the virulence of this pathogenic fungus. The regulatory networks that control morphogenesis are being elucidated; however, the primary signals that trigger morphogenesis remain obscure, and the physiological outputs of these networks are complex.

POG1, a novel yeast gene, promotes recovery from pheromone arrest via the G1 cyclin CLN2

In the absence of a successful mating, pheromone-arrested Saccharomyces cerevisiae cells reenter the mitotic cycle through a recovery process that involves downregulation of the mating mitogen-activated protein kinase (MAPK) cascade. We have isolated a novel gene, POG1, whose promotion of recovery parallels that of the MAPK phosphatase Msg5. POG1 confers alpha-factor resistance when overexpressed and enhances alpha-factor sensitivity when deleted in the background of an msg5 mutant. Overexpression of POG1 inhibits alpha-factor-induced G1 arrest and transcriptional repression of the CLN1 and CLN2 genes. The block in transcriptional repression occurs at SCB/MCB promoter elements by a mechanism that requires Bck1 but not Cln3. Genetic tests strongly argue that POG1 promotes recovery through upregulation of the CLN2 gene and that the resulting Cln2 protein promotes recovery primarily through an effect on Ste20, an activator of the mating MAPK cascade. A pog1 cln3 double mutant displays synthetic mutant phenotypes shared by cell-wall integrity and actin cytoskeleton mutants, with no synthetic defect in the expression of CLN1 or CLN2. These and other results suggest that POG1 may regulate additional genes during vegetative growth and recovery.

Cell polarity and morphogenesis in budding yeast

Eukaryotic cells respond to intracellular and extracellular cues to direct asymmetric cell growth and division. The yeast Saccharomyces cerevisiae undergoes polarized growth at several times during budding and mating and is a useful model organism for studying asymmetric growth and division. In recent years, many regulatory and cytoskeletal components important for directing and executing growth have been identified, and molecular mechanisms have been elucidated in yeast. Key signaling pathways that regulate polarization during the cell cycle and mating response have been described. Since many of the components important for polarized cell growth are conserved in other organisms, the basic mechanisms mediating polarized cell growth are likely to be universal among eukaryotes.

MAP kinase pathways in the yeast Saccharomyces cerevisiae

A cascade of three protein kinases known as a mitogen-activated protein kinase (MAPK) cascade is commonly found as part of the signaling pathways in eukaryotic cells. Almost two decades of genetic and biochemical experimentation plus the recently completed DNA sequence of the Saccharomyces cerevisiae genome have revealed just five functionally distinct MAPK cascades in this yeast. Sexual conjugation, cell growth, and adaptation to stress, for example, all require MAPK-mediated cellular responses. A primary function of these cascades appears to be the regulation of gene expression in response to extracellular signals or as part of specific developmental processes. In addition, the MAPK cascades often appear to regulate the cell cycle and vice versa. Despite the success of the gene hunter era in revealing these pathways, there are still many significant gaps in our knowledge of the molecular mechanisms for activation of these cascades and how the cascades regulate cell function. For example, comparison of different yeast signaling pathways reveals a surprising variety of different types of upstream signaling proteins that function to activate a MAPK cascade, yet how the upstream proteins actually activate the cascade remains unclear. We also know that the yeast MAPK pathways regulate each other and interact with other signaling pathways to produce a coordinated pattern of gene expression, but the molecular mechanisms of this cross talk are poorly understood. This review is therefore an attempt to present the current knowledge of MAPK pathways in yeast and some directions for future research in this area.

Structural organization of MAP-kinase signaling modules by scaffold proteins in yeast and mammals

MAP-kinase signaling pathways are activated by multiple extracellular stimuli. The specificity of activation and function of MAP-kinase signaling modules is determined, in part, by scaffold proteins that create multienzyme complexes. In Saccharomyces cerevisiae, two MAP-kinase-scaffold proteins have been identified. Recent studies of mammalian cells have also led to the identification of putative scaffold proteins. These scaffold proteins appear to facilitate MAP-kinase activation, in response to specific physiological stimuli, and to insulate the bound MAP-kinase module against activation by irrelevant stimuli. Scaffold proteins are therefore critical components of MAP-kinase modules and ensure signaling specificity.

Polarity and division site specification in yeast

A significant component of polarization in budding yeast involves the regulated restructuring of the actin cytoskeleton in response to defined cellular signals. Recent evidence suggests that such cytoskeletal organization arises through the action of large protein complexes that form in response to signals from small GTP-binding proteins, such as Cdc42, Rho, and Ras. These actin-organizing complexes may be fairly diverse, but generally consist of one or more central scaffold proteins, such as those of the formin class, that bind to signaling molecules and recruit actin-binding proteins to bring about desired polarizing events.

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

The yeast protein Spa2p localizes to growth sites and is important for polarized morphogenesis during budding, mating, and pseudohyphal growth. To better understand the role of Spa2p in polarized growth, we analyzed regions of the protein important for its function and proteins that interact with Spa2p. Spa2p interacts with Pea2p and Bud6p (Aip3p) as determined by the two-hybrid system; all of these proteins exhibit similar localization patterns, and spa2Delta, pea2Delta, and bud6Delta mutants display similar phenotypes, suggesting that these three proteins are involved in the same biological processes. Coimmunoprecipitation experiments demonstrate that Spa2p and Pea2p are tightly associated with each other in vivo. Velocity sedimentation experiments suggest that a significant portion of Spa2p, Pea2p, and Bud6p cosediment, raising the possibility that these proteins form a large, 12S multiprotein complex. Bud6p has been shown previously to interact with actin, suggesting that the 12S complex functions to regulate the actin cytoskeleton. Deletion analysis revealed that multiple regions of Spa2p are involved in its localization to growth sites. One of the regions involved in Spa2p stability and localization interacts with Pea2p; this region contains a conserved domain, SHD-II. Although a portion of Spa2p is sufficient for localization of itself and Pea2p to growth sites, only the full-length protein is capable of complementing spa2 mutant defects, suggesting that other regions are required for Spa2p function. By using the two-hybrid system, Spa2p and Bud6p were also found to interact with components of two mitogen-activated protein kinase (MAPK) pathways important for polarized cell growth. Spa2p interacts with Ste11p (MAPK kinase [MEK] kinase) and Ste7p (MEK) of the mating signaling pathway as well as with the MEKs Mkk1p and Mkk2p of the Slt2p (Mpk1p) MAPK pathway; for both Mkk1p and Ste7p, the Spa2p-interacting region was mapped to the N-terminal putative regulatory domain. Bud6p interacts with Ste11p. The MEK-interacting region of Spa2p corresponds to the highly conserved SHD-I domain, which is shown to be important for mating and MAPK signaling. spa2 mutants exhibit reduced levels of pheromone signaling and an elevated level of Slt2p kinase activity. We thus propose that Spa2p, Pea2p, and Bud6p function together, perhaps as a complex, to promote polarized morphogenesis through regulation of the actin cytoskeleton and signaling pathways.