Identification of yeast Rho1p GTPase as a regulatory subunit of 1,3-beta-glucan synthase

G1/S cyclin-dependent kinase regulates small GTPase Rho1p through phosphorylation of RhoGEF Tus1p in Saccharomyces cerevisiae

Rho1p is an essential small GTPase that plays a key role in the morphogenesis of Saccharomyces cerevisiae. We show here that the activation of Rho1p is regulated by a cyclin-dependent kinase (CDK). Rho1p is activated at the G1/S transition at the incipient-bud sites by the Cln2p (G1 cyclin) and Cdc28p (CDK) complex, in a process mediated by Tus1p, a guanine nucleotide exchange factor for Rho1p. Tus1p interacts physically with Cln2p/Cdc28p and is phosphorylated in a Cln2p/Cdc28p-dependent manner. CDK phosphorylation consensus sites in Tus1p are required for both Cln2p-dependent activation of Rho1p and polarized organization of the actin cytoskeleton. We propose that Cln2p/Cdc28p-dependent phosphorylation of Tus1p is required for appropriate temporal and spatial activation of Rho1p at the G1/S transition.

The cell wall: a carbohydrate armour for the fungal cell

The cell wall is composed of a polysaccharide-based three-dimensional network. Considered for a long time as an inert exoskeleton, the cell wall is now seen as a dynamic structure that is continuously changing as a result of the modification of culture conditions and environmental stresses. Although the cell wall composition varies among fungal species, chemogenomic comparative analysis have led to a better understanding of the genes and mechanisms involved in the construction of the common central core composed of branched beta1,3 glucan-chitin. Because of its essential biological role, unique biochemistry and structural organization and the absence in mammalian cells of most of its constitutive components, the cell wall is an attractive target for the development of new antifungal agents. Genomic as well as drug studies have shown that the death of the fungus can result from inhibition of cell wall polysaccharide synthases. To date, only beta1,3 glucan synthase inhibitors have been launched clinically and many more targets remain to be explored.

Regulation of glucose partitioning by PAS kinase and Ugp1 phosphorylation

The ability of cells to recognize and respond to specific metabolic deficiencies is required for all forms of life. We have uncovered a system in the yeast S. cerevisiae that, in response to a perceived deficiency in cell wall glucan, alters partitioning of glucose toward glucan synthesis and away from glycogen synthesis. The paralogous yeast PAS kinases Psk1 and Psk2 phosphorylate UDP-glucose pyrophosphorylase (Ugp1), the primary producer of UDP-glucose, the glucose donor for glucan biosynthesis. Unexpectedly, phosphorylation of Ugp1 does not affect its catalytic activity but instead alters the terminal destination of the UDP-glucose it generates. Phosphorylated Ugp1 is required for intensive glucan production, and inability to phosphorylate Ugp1 is associated with a weak cell wall, decreased glucan content, and increased glycogen content. We provide data indicating that phosphorylation by Psk1 or Psk2 targets Ugp1 to the cell periphery, where the UDP-glucose it produces is in proximity to the site of glucan synthesis. We propose that regulation of glucose partitioning by altered enzyme and substrate localization is a rapid and potent response to metabolic deficiency.

Susceptibility of Cryptococcus neoformans to photodynamic inactivation is associated with cell wall integrity

Photodynamic therapy is a rapidly developing antimicrobial technology which combines a nontoxic photoactivatable dye or photosensitizer with harmless visible light of the correct wavelength to excite the dye to its reactive triplet state to generate reactive oxygen species toxic to cells. In this report we present evidence that the fungal pathogen Cryptococcus neoformans is susceptible to photodynamic inactivation by use of a polycationic conjugate of polyethyleneimine and the photosensitizer chlorin(e6). A C. neoformans rom2 mutant, with a mutation involving a putative Rho1 guanyl nucleotide exchange factor that is part of the protein kinase C-cell wall integrity pathway, demonstrated a compromised cell wall and less (1,3)beta-d glucan than the wild-type strain and increased accumulation of PEI-ce6 as assessed by fluorescence uptake and confocal microscopy. Interestingly, C. neoformans rom2 was hypersusceptible to photodynamic inactivation and coincubation of wild-type C. neoformans strain KN99alpha with caspofungin-enhanced photoinactivation. These studies demonstrated that C. neoformans is sensitive to photodynamic therapy and illustrated the significance of cell wall integrity in microbial susceptibility to antimicrobial photodynamic inactivation.

The pharmacology and clinical use of caspofungin

Caspofungin was the first echinocandin to be licensed for the treatment of invasive fungal infections. Caspofungin has in vitro and in vivo activity against Candida spp. and Aspergillus spp., which constitute the majority of medically important opportunistic fungal pathogens. Caspofungin inhibits the synthesis of the 1,3-beta-glucan, with resultant osmotic instability and lysis. The pharmacology of caspofungin is relatively complex. Trafficking of drug into tissues is an important determinant of the shape of the concentration-time relationship. Caspofungin has demonstrated efficacy in experimental models of invasive candidiasis and aspergillosis, which reflect its activity in the treatment of oropharyngeal, esophageal and disseminated candidiasis, as well as salvage therapy for patients with invasive aspergillosis.

Resistance to echinocandin-class antifungal drugs

Invasive fungal infections cause morbidity and mortality in severely ill patients, and limited drug classes restrict treatment choices. The echinocandin drugs are the first new class of antifungal compounds that target the fungal cell wall by blocking beta-1,3-d-glucan synthase. Elevated MIC values with occasional treatment failure have been reported for strains of Candida. Yet, an uncertain correlation exists between clinical failure and elevated MIC values for the echinocandin drugs. Fungi display several adaptive physiological mechanisms that result in elevated MIC values. However, resistance to echinocandin drugs among clinical isolates is associated with amino acid substitutions in two "hot-spot" regions of Fks1, the major subunit of glucan synthase. The mutations, yielding highly elevated MIC values, are genetically dominant and confer cross-resistance to all echinocandin drugs. Prominent Fks1 mutations decrease the sensitivity of glucan synthase for drug by 1000-fold or more, and strains harboring such mutations may require a concomitant increase in drug to reduce fungal organ burdens in animal infection models. The Fks1-mediated resistance mechanism is conserved in a wide variety of Candida spp. and can account for intrinsic reduced susceptibility of certain species. Fks1 mutations confer resistance in both yeasts and moulds suggesting that this mechanism is pervasive in the fungal kingdom.

The temperature-sensitive role of Cryptococcus neoformans ROM2 in cell morphogenesis

ROM2 is associated with Cryptococcus neoformans virulence. We examined additional roles of ROM2 in C. neoformans and found that ROM2 plays a role in several cell functions specifically at high temperature conditions. Morphologically rom2 mutant cells demonstrated a “tear”-like shape and clustered together. A sub-population of cells had a hyperelongated phenotype at restrictive growth conditions. Altered morphology was associated with defects in actin that was concentrated at the cell periphery and with abnormalities in microtubule organization. Interestingly, the ROM2 associated defects in cell morphology, location of nuclei, and actin and microtubule organization were not observed in cells grown at temperatures below 37°C. These results indicate that in C. neoformans, ROM2 is important at restrictive temperature conditions and is involved in several cell maintenance functions.

Up-regulation of the cell integrity pathway in saccharomyces cerevisiae suppresses temperature sensitivity of the pgs1Delta mutant

We have previously shown that mutants in the cardiolipin (CL) pathway exhibit temperature-sensitive growth defects that are not associated with mitochondrial dysfunction. The pgs1Delta mutant, lacking the first enzyme of the CL pathway, phosphatidylglycerolphosphate synthase (Pgs1p), has a defective cell wall due to decreased beta-1,3-glucan (Zhong, Q., Gvozdenovic-Jeremic, J., Webster, P., Zhou, J., and Greenberg, M. L. (2005) Mol. Biol. Cell 16, 665-675). Disruption of KRE5, a gene involved in cell wall biogenesis, restores beta-1,3-glucan synthesis and suppresses pgs1Delta temperature sensitivity. To gain insight into the mechanisms underlying the cell wall defect in pgs1Delta, we show in the current report that pgs1Delta cells have reduced glucan synthase activity and diminished levels of Fks1p, the glucan synthase catalytic subunit. In addition, activation of Slt2p, the downstream effector of the protein kinase C (PKC)-activated cell integrity pathway, was defective in pgs1Delta. The kre5W1166X suppressor restored Slt2p activation and dramatically increased (>10-fold) mRNA levels of FKS2, the alternate catalytic subunit of glucan synthase, partially restoring glucan synthase activity. Consistent with these results, up-regulation of PKC-Slt2 signaling and overexpression of FKS1 or FKS2 alleviated sensitivity of pgs1Delta to cell wall-perturbing agents and restored growth at elevated temperature. These findings demonstrate that functional Pgs1p is essential for cell wall biogenesis and activation of the PKC-Slt2 signaling pathway.

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.

Homologous subunits of 1,3-beta-glucan synthase are important for spore wall assembly in Saccharomyces cerevisiae

During sporulation in Saccharomyces cerevisiae, the four haploid nuclei are encapsulated within multilayered spore walls. Glucan, the major constituent of the spore wall, is synthesized by 1,3-beta-glucan synthase, which is composed of a putative catalytic subunit encoded by FKS1 and FKS2. Although another homolog, encoded by FKS3, was identified by homology searching, its function is unknown. In this report, we show that FKS2 and FKS3 are required for spore wall assembly. The ascospores of fks2 and fks3 mutants were enveloped by an abnormal spore wall with reduced resistance to diethyl ether, elevated temperatures, and ethanol. However, deletion of the FKS1 gene did not result in a defective spore wall. The construction of fusion genes that expressed Fks1p and Fks2p under the control of the FKS2 promoter revealed that asci transformed with FKS2p-driven Fks1p and Fks2p were resistant to elevated temperatures, which suggests that the expression of FKS2 plays an important role in spore wall assembly. The expression of FKS1p-driven Fks3p during vegetative growth did not affect 1,3-beta-glucan synthase activity in vitro but effectively suppressed the growth defect of the temperature-sensitive fks1 mutant by stabilizing Rho1p, which is a regulatory subunit of glucan synthase. Based on these results, we propose that FKS2 encodes the primary 1,3-beta-glucan synthase in sporulation and that FKS3 is required for normal spore wall formation because it affects the upstream regulation of 1,3-beta-glucan synthase.

The Hsp40 molecular chaperone Ydj1p, along with the protein kinase C pathway, affects cell-wall integrity in the yeast Saccharomyces cerevisiae

Molecular chaperones, such as Hsp40, regulate cellular processes by aiding in the folding, localization, and activation of multi-protein machines. To identify new targets of chaperone action, we performed a multi-copy suppressor screen for genes that improved the slow-growth defect of yeast lacking the YDJ1 chromosomal locus and expressing a defective Hsp40 chimera. Among the genes identified were MID2, which regulates cell-wall integrity, and PKC1, which encodes protein kinase C and is linked to cell-wall biogenesis. We found that ydj1delta yeast exhibit phenotypes consistent with cell-wall defects and that these phenotypes were improved by Mid2p or Pkc1p overexpression or by overexpression of activated downstream components in the PKC pathway. Yeast containing a thermosensitive allele in the gene encoding Hsp90 also exhibited cell-wall defects, and Mid2p or Pkc1p overexpression improved the growth of these cells at elevated temperatures. To determine the physiological basis for suppression of the ydj1delta growth defect, wild-type and ydj1delta yeast were examined by electron microscopy and we found that Mid2p overexpression thickened the mutant's cell wall. Together, these data provide the first direct link between cytoplasmic chaperone function and cell-wall integrity and suggest that chaperones orchestrate the complex biogenesis of this structure.

Substitution F659G in the Irr1p/Scc3p cohesin influences the cell wall of Saccharomyces cerevisiae

The sister chromatid cohesion complex of Saccharomyces cerevisiae is composed of proteins termed cohesins. The complex forms a ring structure that entraps sister DNAs, probably following replication. The mechanism of cohesion is universal and the proteins participating in this process are evolutionarily highly conserved. We investigated the Irr1p/Scc3p cohesin subunit, an under-studied protein. We show that the presence of a mutated copy of IRR1 gene, encoding the F658G substitution in Irr1p, changes the sensitivity of the heterozygous irr1-1/IRR1 diploid to cell wall-affecting compounds. Microscopic images indicate that chitin distribution in the mutant cell wall is affected, although the biochemical composition of the cell wall is not drastically changed. This observation suggests that irr1-1 mutation in heterozygous state may influence the cell wall integrity and indicates a possible link between mechanisms regulating the cell wall biosynthesis, nuclear migration and chromosome segregation.

Involvement of Rho-type GTPase in control of cell size in Saccharomyces cerevisiae

Maintaining specific cell size, which is important for many organisms, is achieved by coordinating cell growth and cell division. In the budding yeast Saccharomyces cerevisiae, the existence of two cell-size checkpoints is proposed: at the first checkpoint, cell size is monitored before budding at the G1/S transition, and at the second checkpoint, actin depolymerization occurring in the small bud is monitored before the G2/M transition. Morphological analyses have revealed that the small GTPase Rho1p participates in cell-size control at both the G1/S and the G2/M boundaries. One group of rho1 mutants (rho1A) underwent premature entry into mitosis, leading to the birth of abnormally small cells. In another group of rho1 mutants (rho1B), the mother cells failed to reach an appropriate size before budding, and expression of the G1 cyclin Cln2p began at an earlier phase of the cell cycle. Analyses of mutants defective in Rho1p effector proteins indicate that Skn7p, Fks1p and Mpk1p are involved in cell-size control. Thus, Rho1p and its downstream regulatory pathways are involved in controlling cell size in S. cerevisiae.

NPFXD-mediated endocytosis is required for polarity and function of a yeast cell wall stress sensor

The actin-associated protein Sla1p, through its SHD1 domain, acts as an adaptor for the NPFX(1,2)D endocytic targeting signal in yeast. Here we report that Wsc1p, a cell wall stress sensor, depends on this signal-adaptor pair for endocytosis. Mutation of NPFDD in Wsc1p or expression of Sla1p lacking SHD1 blocked Wsc1p internalization. By live cell imaging, endocytically defective Wsc1p was not concentrated at sites of endocytosis. Polarized distribution of Wsc1p to regions of cell growth was lost in the absence of endocytosis. Mutations in genes necessary for endosome to Golgi traffic caused redistribution of Wsc1p from the cell surface to internal compartments, indicative of recycling. Inhibition of Wsc1p endocytosis caused defects in polarized deposition of the cell wall and increased sensitivity to perturbation of cell wall synthesis. Our results reveal that the NPFX(1,2)D-Sla1p system is responsible for directing Wsc1p into an endocytosis and recycling pathway necessary to maintain yeast cell wall polarity. The dynamic localization of Wsc1p, a sensor of the extracellular wall in yeast, resembles polarized distribution of certain extracellular matrix-sensing integrins through endocytic recycling.

The echinocandins: comparison of their pharmacokinetics, pharmacodynamics and clinical applications

Caspofungin, micafungin and anidulafungin are three drugs of the echinocandin class of antifungals available for intravenous treatment of invasive candidiasis and aspergillosis. They exhibit high in vitro and in vivo activities against Candida spp. and Aspergillus spp. In various clinical studies investigating candidemia and invasive candidiasis, Candida esophagitis, and fever in neutropenia, the clinical efficacy of the echinocandin tested was similar to that of established antifungals. Antifungal activity against strains no longer susceptible to conventional antifungal agents, such as fluconazole and amphotericin B suggests that echinocandins can be used as salvage therapy in life-threatening fungal infections. There is no cross-resistance to other antifungals. Excellent safety and tolerability of treatment with caspofungin has been documented over a total of 4.3 million patient days. Echinocandins are poor substrates of the cytochrome P450 enzyme family and can be safely co-administered with most drugs without the need for dosage adaptation. No dose reduction is required in renal impairment. A reduction in the daily maintenance dose has been recommended for caspofungin, but not for micafungin and anidulafungin in patients presenting with mild to moderate hepatic failure.

The structure and synthesis of the fungal cell wall

The fungal cell wall is a dynamic structure that protects the cell from changes in osmotic pressure and other environmental stresses, while allowing the fungal cell to interact with its environment. The structure and biosynthesis of a fungal cell wall is unique to the fungi, and is therefore an excellent target for the development of anti-fungal drugs. The structure of the fungal cell wall and the drugs that target its biosynthesis are reviewed. Based on studies in a number of fungi, the cell wall has been shown to be primarily composed of chitin, glucans, mannans and glycoproteins. The biosynthesis of the various components of the fungal cell wall and the importance of the components in the formation of a functional cell wall, as revealed through mutational analyses, are discussed. There is strong evidence that the chitin, glucans and glycoproteins are covalently cross-linked together and that the cross-linking is a dynamic process that occurs extracellularly.

The Cdc34/SCF ubiquitination complex mediates Saccharomyces cerevisiae cell wall integrity

To identify novel functions for the Cdc34/SCF ubiquitination complex, we analyzed genomewide transcriptional profiles of cdc53-1 and cdc34-2 Saccharomyces cerevisiae mutants. This analysis revealed altered expression for several gene families, including genes involved in the regulation of cell wall organization and biosynthesis. This led us to uncover a role for the Cdc34/SCF complex in the regulation of cell wall integrity. In support of this, cdc53-1 and cdc34-2 mutants exhibit phenotypes characteristic of cell wall integrity mutants, such as SDS sensitivity and temperature-sensitive suppression by osmotic stabilizers. Examination of these mutants revealed defects in their induction of Slt2 phosphorylation, indicating defects in Pkc1-Slt2 MAPK signaling. Consistent with this, synthetic genetic interactions were observed between the genes encoding the Cdc34/SCF complex and key components of the Pck1-Slt2 MAPK pathway. Further analysis revealed that Cdc34/SCF mutants have reduced levels of active Rho1, suggesting that these defects stem from the deregulated activity of the Rho1 GTPase. Altering the activity of Rho1 via manipulation of the Rho1-GAPs LRG1 or SAC7 affected Cdc34/SCF mutant growth. Strikingly, however, deletion of LRG1 rescued the growth defects associated with Cdc34/SCF mutants, whereas deletion of SAC7 enhanced these defects. Given the differential roles that these GAPs play in the regulation of Rho1, these observations indicate the importance of coordinating Cdc34/SCF activity with specific Rho1 functions.

Functional complementation of a yeast knockout strain by Schistosoma mansoni Rho1 GTPase in the presence of caffeine, an agent that affects mutants defective in the protein kinase C signal transduction pathway

In a previous study, the Schistosoma mansoni Rho1 protein was able to complement Rho1 null mutant Saccharomyces cerevisiae cells at restrictive temperatures and under osmotic stress (low calcium concentration) better than the human homologue (RhoA). It is known that under osmotic stress, the S. cerevisiae Rho1 triggers two distinct pathways: activation of the membrane 1,3-beta-glucan synthase enzymatic complex and activation of the protein kinase C1 signal transduction pathway, promoting the transcription of response genes. In the present work the SmRho1 protein and its mutants smrho1E97P, smrho1L101T, and smrho1E97P, L101T were used to try to clarify the basis for the differential complementation of Rho1 knockout yeast strain by the human and S. mansoni genes. Experiments of functional complementation in the presence of caffeine and in the presence of the osmotic regulator sorbitol were conducted. SmRho1 and its mutants showed a differential complementation of the yeast cells in the presence of caffeine, since smrho1E97P and smrho1E97P, L101T mutants showed a delay in the growth when compared to the yeast complemented with the wild type SmRho1. However, in the presence of sorbitol and caffeine the wild type SmRho1 and mutants showed a similar complementation phenotype, as they allowed yeast growth in all caffeine concentrations tested.

Identification of the first Oomycete annexin as a (1→3)-β-d-glucan synthase activator

(1-->3)-beta-D-Glucans are major components of the cell walls of Oomycetes and as such they play an essential role in the morphogenesis and growth of these microorganisms. Despite the biological importance of (1-->3)-beta-D-glucans, their mechanisms of biosynthesis are poorly understood. Previous studies on (1-->3)-beta-D-glucan synthases from Saprolegnia monoica have shown that three protein bands of an apparent molecular weight of 34, 48 and 50 kDa co-purify with enzyme activity. However, none of the corresponding proteins have been identified. Here we have identified, purified, sequenced and characterized a protein from the 34 kDa band and clearly shown that it has all the biochemical properties of proteins from the annexin family. In addition, we have unequivocally demonstrated that the purified protein is an activator of (1-->3)-beta-D-glucan synthase. This represents a new type of function for proteins belonging to the annexin family. Two other proteins from the 48 and 50 kDa bands were identified as ATP synthase subunits, which most likely arise from contaminations by mitochondria during membrane preparation. The results, which are discussed in relation with the possible regulation mechanisms of (1-->3)-beta-D-glucan synthases, represent a first step towards a better understanding of cell wall polysaccharide biosynthesis in Oomycetes.

KlRHO1 and KlPKC1 are essential for cell integrity signalling in Kluyveromyces lactis

Cell integrity in yeasts is ensured by a rigid cell wall whose synthesis is triggered by a MAP kinase-mediated signal-transduction cascade. Upstream regulatory components of this pathway inSaccharomyces cerevisiaeinvolve a single protein kinase C, which is regulated by interaction with the small GTPase Rho1. Here, two genes were isolated which encode these proteins fromKluyveromyces lactis(KlPKC1andKlRHO1). Sequencing showed ORFs which encode proteins of 1161 and 208 amino acids, respectively. The deduced proteins shared 59 and 85 % overall amino acid identities, respectively, with their homologues fromS. cerevisiae. Null mutants in both genes were non-viable, as shown by tetrad analyses of the heterozygous diploid strains. Overexpression of theKlRHO1gene under the control of theScGAL1promoter severely impaired growth in bothS. cerevisiaeandK. lactis. On the other hand, a similar construct withKlPKC1did not show a pronounced phenotype. Two-hybrid analyses showed interaction between Rho1 and Pkc1 for theK. lactisproteins and theirS. cerevisiaehomologues. A green fluorescent protein (GFP) fusion to the C-terminal end of KlPkc1 located the protein to patches in the growing bud, and at certain stages of the division process also to the bud neck. N-terminal GFP fusions to KlRho1 localized mainly to the cell surface (presumably the cytoplasmic side of the plasma membrane) and to the vacuole, with some indications of traffic from the former to the latter. Thus, KlPkc1 and KlRho1 have been shown to serve vital functions inK. lactis, to interact in cell integrity signalling and to traffic between the plasma membrane and the vacuole.

A RHOse by any other name: a comparative analysis of animal and plant Rho GTPases

Rho GTPases are molecular switches that act as key regulators of a many cellular processes, including cell movement, morphogenesis, host defense, cell division and gene expression. Rho GTPases are found in all eukaryotic kingdoms. Plants lack clear homologs to conventional Rho GTPases found in yeast and animals; instead, they have over time developed a unique subfamily, ROPs, also known as RAC. The origin of ROP-like proteins appears to precede the appearance of land plants. This review aims to discuss the evolution of ROP/RAC and to compare plant ROP and animal Rho GTPases, focusing on similarities and differences in regulation of the GTPases and their downstream effectors.

The plasma membrane proteome of Saccharomyces cerevisiae and its response to the antifungal calcofluor

Calcofluor is an antifungal compound known to induce structural perturbations of the cell wall by interfering with the synthesis of chitin microfibril. Proteins from a stripped plasma membrane fraction were solubilized with the neutral and non-denaturing detergent, the n-dodecyl beta-D-maltoside. Proteins were then resolved using a recently described ion-exchange chromatography (IEC)/lithium dodecyl sulfate (LDS)-PAGE procedure. Nearly 90 proteins were identified and clustered, based on their pI, molecular weight, abundance and/or hydrophobicity. This method was then applied to profile the plasma membrane response to calcofluor. The LDS-PAGE patterns obtained from whole plasma membrane proteins were similar for the non-treated and calcofluor-treated samples. However, IEC/LDS-PAGE analysis revealed subtle changes in the expression of several proteins of low abundance, in response to calcofluor. These proteins include Pil1p and Lsp1p, two sphingolipid long-chain base-responsive inhibitors of protein kinases involved in signaling pathways for cell wall integrity and Rho1p, a small GTPase. It was recently hypothesized that Pil1p and Lsp1p could associate with, and regulate, the plasma membrane beta-1-3-glucan synthase, responsible for the synthesis of another major microfibril for yeast cell wall. Results are discussed with respect to both calcofluor effects on the plasma membrane proteins and the power of the IEC/LDS-PAGE procedure in the search for new potential therapeutics targets.

Cell wall assembly in Saccharomyces cerevisiae

An extracellular matrix composed of a layered meshwork of beta-glucans, chitin, and mannoproteins encapsulates cells of the yeast Saccharomyces cerevisiae. This organelle determines cellular morphology and plays a critical role in maintaining cell integrity during cell growth and division, under stress conditions, upon cell fusion in mating, and in the durable ascospore cell wall. Here we assess recent progress in understanding the molecular biology and biochemistry of cell wall synthesis and its remodeling in S. cerevisiae. We then review the regulatory dynamics of cell wall assembly, an area where functional genomics offers new insights into the integration of cell wall growth and morphogenesis with a polarized secretory system that is under cell cycle and cell type program controls.

Molecular organization of the cell wall of Candida albicans and its relation to pathogenicity

Candida albicans is one of the most important opportunistic pathogenic fungi. Weakening of the defense mechanisms of the host, and the ability of the microorganism to adapt to the environment prevailing in the host tissues, turn the fungus from a rather harmless saprophyte into an aggressive pathogen. The disease, candidiasis, ranges from light superficial infections to deep processes that endanger the life of the patient. In the establishment of the pathogenic process, the cell wall of C. albicans (as in other pathogenic fungi) plays an important role. It is the outer structure that protects the fungus from the host defense mechanisms and initiates the direct contact with the host cells by adhering to their surface. The wall also contains important antigens and other compounds that affect the homeostatic equilibrium of the host in favor of the parasite. In this review, we discuss our present knowledge of the structure of the cell wall of C. albicans, the synthesis of its different components, and the mechanisms involved in their organization to give rise to a coherent composite. Furthermore, special emphasis has been placed on two further aspects: how the composition and structure of C. albicans cell wall compare with those from other fungi, and establishing the role of some specific wall components in pathogenesis. From the data presented here, it becomes clear that the composition, structure and synthesis of the cell wall of C. albicans display both subtle and important differences with the wall of different saprophytic fungi, and that some of these differences are of utmost importance for its pathogenic behavior.

The Rgd1p Rho GTPase-activating protein and the Mid2p cell wall sensor are required at low pH for protein kinase C pathway activation and cell survival in Saccharomyces cerevisiae

The protein kinase C (PKC) pathway is involved in the maintenance of cell shape and cell integrity in Saccharomyces cerevisiae. Here, we show that this pathway mediates tolerance to low pH and that the Bck1 and Slt2 proteins belonging to the mitogen-activated protein kinase cascade are essential for cell survival at low pH. The PKC pathway is activated during acidification of the extracellular environment, and this activation depends mainly on the Mid2p cell wall sensor. Rgd1p, which encodes a Rho GTPase-activating protein for the small G proteins Rho3p and Rho4p, also plays a role in low-pH response. The rgd1Delta strain is sensitive to low pH, and Rgd1p activates the PKC pathway in an acidic environment. Inactivation of both genes in the double mutant rgd1Delta mid2Delta strain renders yeast cells unable to survive at low pH as in bck1Delta and slt2Delta strains. Our data provide evidence for the existence of two distinct ways, one involving Mid2p and the other involving Rgd1p, with both converging to the cell integrity pathway to mediate low-pH tolerance in Saccharomyces cerevisiae. Nevertheless, even if Rgd1p acts on the PKC pathway, it seems that its mediating action on low-pH tolerance is not limited to this pathway. As the Mid2p amount plays a role in rgd1Delta sensitivity to low pH, Mid2p seems to act more like a molecular rheostat, controlling the level of PKC pathway activity and thus allowing phenotypical expression of RGD1 inactivation.

Disruption of LRG1 inhibits mother-daughter separation in Saccharomyces cerevisiae

LRG1, previously characterized as a Rho-GAP that regulates beta-1,3-glucan synthesis, was identified using a filtration screen designed to isolate genes that promote cell aggregation in Sigma1278b Saccharomyces cerevisiae. Disruption of LRG1 in haploid cells resulted in enhanced invasive growth and a strain-specific 'clustered' phenotype that is a consequence of failed separation of mother and daughter cells. Genetic analysis revealed that clustering required functional ROM2, FKS1 and STE12 but not FLO8 or FLO11. Additionally, this phenotype required sufficient nitrogen. beta-1,3-Glucan content was elevated in lrg1 haploids and these cells possessed a thicker cell wall and septum than wild-type strains. Additional glucan at the bud site inhibited separation of lrg1 mutants as the clustered phenotype was repressed by expression of the glucanases ENG1 or EGT2.

Toward a molecular interpretation of the surface stress theory for yeast morphogenesis

The surface stress theory was proposed more than twenty years ago to explain morphogenesis of walled organisms. This theory makes simple assumptions on the force that drives microbial growth and how a cell's response to this force generates shape. This classic formulation may now be explained in more detailed molecular terms due to recent advances in the study of yeast morphogenesis with respect to the mechanism of cell polarization, the fine tuning of polarized growth to allocate necessary components to proper locations, and the local and global responses to turgor that provide control over the location and duration of growth.

New fungal metabolite geranylgeranyltransferase inhibitors with antifungal activity

Geranylgeranyltransferase I (GGTase I) catalyzes the post-translational transfer of lyophilic diterpenoid geranylgeranyl to the cysteine residue of proteins terminating with a CaaX motif such as Rho1p and Cdc42p. It has been shown that GGTase I activity is essential for viability of Saccharomyces cerevisiae and hence its inhibition is a potential antifungal target. From natural product screening, a number of azaphilones including one novel analog were isolated as broad-spectrum inhibitors of GGTase I. Isolation, structure elucidation, GGTase I inhibitory activities and antifungal activities of these compounds are described.

Deficiency in mitochondrial anionic phospholipid synthesis impairs cell wall biogenesis

Cardiolipin (CL) is the signature lipid of the mitochondrial membrane and plays a key role in mitochondrial physiology and cell viability. The importance of CL is underscored by the finding that the severe genetic disorder Barth syndrome results from defective CL composition and acylation. Disruption of PGS1, which encodes the enzyme that catalyses the committed step of CL synthesis, results in loss of the mitochondrial anionic phospholipids phosphatidylglycerol and CL. The pgs1Δ mutant exhibits severe growth defects at 37°C. To understand the essential functions of mitochondrial anionic lipids at elevated temperatures, we isolated suppressors of pgs1Δ that grew at 37°C. The present review summarizes our analysis of suppression of pgs1Δ growth defects by a mutant that has a loss-of-function mutation in KRE5, a gene involved in cell wall biogenesis.

A microbial TRP-like polycystic-kidney-disease-related ion channel gene

Ion channel genes have been discovered in many microbial organisms. We have investigated a microbial TRP (transient receptor potential) ion channel gene which has most similarity to polycystic-kidney-disease-related ion channel genes. We have shown that this gene (pkd2) is essential for cellular viability, and is involved in cell growth and cell wall synthesis. Expression of this gene increases following damage to the cell wall. This fission yeast pkd2 gene, orthologues of which are found in all eukaryotic cells, appears to be a key signalling component in the regulation of cell shape and cell wall synthesis in yeast through an interaction with a Rho1-GTPase. A model for the mode of action of this Schizosaccharomyces pombe protein in a Ca2+ signalling pathway is hypothesized.

Cell wall integrity signaling in Saccharomyces cerevisiae

The yeast cell wall is a highly dynamic structure that is responsible for protecting the cell from rapid changes in external osmotic potential. The wall is also critical for cell expansion during growth and morphogenesis. This review discusses recent advances in understanding the various signal transduction pathways that allow cells to monitor the state of the cell wall and respond to environmental challenges to this structure. The cell wall integrity signaling pathway controlled by the small G-protein Rho1 is principally responsible for orchestrating changes to the cell wall periodically through the cell cycle and in response to various forms of cell wall stress. This signaling pathway acts through direct control of wall biosynthetic enzymes, transcriptional regulation of cell wall-related genes, and polarization of the actin cytoskeleton. However, additional signaling pathways interface both with the cell wall integrity signaling pathway and with the actin cytoskeleton to coordinate polarized secretion with cell wall expansion. These include Ca(2+) signaling, phosphatidylinositide signaling at the plasma membrane, sphingoid base signaling through the Pkh1 and -2 protein kinases, Tor kinase signaling, and pathways controlled by the Rho3, Rho4, and Cdc42 G-proteins.

Coccidioides posadasii contains a single 1,3-beta-glucan synthase gene that appears to be essential for growth

1,3-beta-Glucan synthase is responsible for the synthesis of beta-glucan, an essential cell wall structural component in most fungi. We sought to determine whether Coccidioides posadasii possesses genes homologous to known fungal FKS genes that encode the catalytic subunit of 1,3-beta-glucan synthase. A single gene, designated FKS1, was identified, and examination of its predicted protein product showed a high degree of conservation with Fks proteins from other filamentous fungi. FKS1 is expressed at similar levels in mycelia and early spherulating cultures, and expression decreases as the spherules mature. We used Agrobacterium-mediated transformation to create strains that harbor DeltaFKS1::hygB, a null allele of FKS1, and hypothesize that Fks1p function is essential, due to our inability to purify this allele away from a complementing wild-type FKS1 allele in a heterokaryotic strain. The heterokaryon appears normal with respect to growth rate and arthroconidium production; however, microscopic examination of strains with DeltaFKS1::hygB alleles revealed abnormal swelling of hyphal elements.

The pleckstrin homology domain proteins Slm1 and Slm2 are required for actin cytoskeleton organization in yeast and bind phosphatidylinositol-4,5-bisphosphate and TORC2

Phosphatidylinositol-4,5-bisphosphate [PtdIns(4,5)P(2)] is a key second messenger that regulates actin and membrane dynamics, as well as other cellular processes. Many of the effects of PtdIns(4,5)P(2) are mediated by binding to effector proteins that contain a pleckstrin homology (PH) domain. Here, we identify two novel effectors of PtdIns(4,5)P(2) in the budding yeast Saccharomyces cerevisiae: the PH domain containing protein Slm1 and its homolog Slm2. Slm1 and Slm2 serve redundant roles essential for cell growth and actin cytoskeleton polarization. Slm1 and Slm2 bind PtdIns(4,5)P(2) through their PH domains. In addition, Slm1 and Slm2 physically interact with Avo2 and Bit61, two components of the TORC2 signaling complex, which mediates Tor2 signaling to the actin cytoskeleton. Together, these interactions coordinately regulate Slm1 targeting to the plasma membrane. Our results thus identify two novel effectors of PtdIns(4,5)P(2) regulating cell growth and actin organization and suggest that Slm1 and Slm2 integrate inputs from the PtdIns(4,5)P(2) and TORC2 to modulate polarized actin assembly and growth.

Involvement of actin and polarisome in morphological change during spore germination of Saccharomyces cerevisiae

We studied the morphological changes of Saccharomyces cerevisiae ascospores during germination. Initiation of germination is followed by polarization of actin patches, maintaining their localization to the site of cell surface growth. Loss of polarisome components, Spa2p, Pea2p, Bud6p or Bni1p, results in depolarization of actin patches. Green fluorescent protein-fused polarisome components exhibit the polarized localization, implying that polarisome is involved in the polarized outgrowth during germination. At the late stage of germination, we found that actin patches temporally depolarize before bud emergence. The observation that loss of Cla4p extends the polarized growth period suggests that Cla4p is involved in the actin-depolization step. Actin polarization in the initial stage is accelerated by overexpression of Ras2p, whereas hyperpolarization is continuously observed by overexpression of Rho1p. Thus, yeast spore germination is a morphological event that is regulated by a number of factors implicated in mitotic bud morphogenesis.

Lrg1p Is a Rho1 GTPase-activating protein required for efficient cell fusion in yeast

To identify additional cell fusion genes in Saccharomyces cerevisiae, we performed a high-copy suppressor screen of fus2Delta. Higher dosage of three genes, BEM1, LRG1, and FUS1, partially suppressed the fus2Delta cell fusion defect. BEM1 and FUS1 were high-copy suppressors of many cell-fusion-defective mutations, whereas LRG1 suppressed only fus2Delta and rvs161Delta. Lrg1p contains a Rho-GAP homologous region. Complete deletion of LRG1, as well as deletion of the Rho-GAP coding region, caused decreased rates of cell fusion and diploid formation comparable to that of fus2Delta. Furthermore, lrg1Delta caused a more severe mating defect in combination with other cell fusion mutations. Consistent with an involvement in cell fusion, Lrg1p localized to the tip of the mating projection. Lrg1p-GAP domain strongly and specifically stimulated the GTPase activity of Rho1p, a regulator of beta(1-3)-glucan synthase in vitro. beta(1-3)-glucan deposition was increased in lrg1Delta strains and mislocalized to the tip of the mating projection in fus2Delta strains. High-copy LRG1 suppressed the mislocalization of beta(1-3) glucan in fus2Delta strains. We conclude that Lrg1p is a Rho1p-GAP involved in cell fusion and speculate that it acts to locally inhibit cell wall synthesis to aid in the close apposition of the plasma membranes of mating cells.

SKG1, a suppressor gene of synthetic lethality ofkex2?gas1? mutations, encodes a novel membrane protein that affects cell wall composition

The fungal GAS1-related genes encode GPI-anchored beta-1,3-glucanosyltransferase, and their loss causes a defect in the assembly of the cell wall. The KEX2 gene encodes a processing protease in the late Golgi compartment and its loss also results in defects in the cell wall. Simultaneous mutations of these genes are lethal in Saccharomyces cerevisiae. To understand the basis of this synthetic lethality, we screened for multicopy suppressors and identified 13 SKG (suppressor of kex2 gas1 synthetic lethality) genes. SKG1 encodes a transmembrane protein that localizes on the inner surface of the plasma membrane at the bud and in the daughter cell. The multicopy SKG1 increases the sensitivity of cells to zymolyase, and the skg1Delta null mutation increases resistance to it. This zymolyase susceptibility corresponds to an increase of alkali-soluble beta-1,3-glucan and a decrease of chitin in the cell wall. Thus SKG1 encodes a novel protein that affects the cell wall polymer composition in the growing region of the cell.

Analysis of beta-1,3-glucan assembly in Saccharomyces cerevisiae using a synthetic interaction network and altered sensitivity to caspofungin

Large-scale screening of genetic and chemical-genetic interactions was used to examine the assembly and regulation of beta-1,3-glucan in Saccharomyces cerevisiae. Using the set of deletion mutants in approximately 4600 nonessential genes, we scored synthetic interactions with genes encoding subunits of the beta-1,3-glucan synthase (FKS1, FKS2), the glucan synthesis regulator (SMI1/KNR4), and a beta-1,3-glucanosyltransferase (GAS1). In the resulting network, FKS1, FKS2, GAS1, and SMI1 are connected to 135 genes in 195 interactions, with 26 of these genes also interacting with CHS3 encoding chitin synthase III. A network core of 51 genes is multiply connected with 112 interactions. Thirty-two of these core genes are known to be involved in cell wall assembly and polarized growth, and 8 genes of unknown function are candidates for involvement in these processes. In parallel, we screened the yeast deletion mutant collection for altered sensitivity to the glucan synthase inhibitor, caspofungin. Deletions in 52 genes led to caspofungin hypersensitivity and those in 39 genes to resistance. Integration of the glucan interaction network with the caspofungin data indicates an overlapping set of genes involved in FKS2 regulation, compensatory chitin synthesis, protein mannosylation, and the PKC1-dependent cell integrity pathway.

Piperazine Propanol Derivative as a Novel Antifungal Targeting 1,3-.BETA.-D-Glucan Synthase

1,3-beta-D-Glucan synthase, which synthesizes a main component of fungal cell wall, is one of the promising targets for antifungal agents. In order to identify novel chemical classes of 1,3-beta-D-glucan synthase inhibitors, we screened a chemical library monitoring inhibition of the Candida albicans 1,3-beta-D-glucan synthase activity. The piperazine propanol derivative GSI578 [(2,6-difluoro-phenyl)-carbamic acid 3-(4-benzothiazol-2-yl-piperazine-1-yl)-propyl ester] was identified as a potent inhibitor against 1,3-beta-D-glucan synthase with an IC50 value of 0.16 microM. GSI578 exhibited in vitro antifungal activity against pathogenic fungi including C. albicans and Aspergillus fumigatus. Temperature-sensitive mutations of the FKS1 gene in the Deltafks2 background of Saccharomyces cerevisiae, where FKS1 and FKS2 encode putative catalytic subunits of 1,3-beta-D-glucan synthase, altered sensitivity to GSI578. This suggests that the antifungal activity of the piperazine propanol derivative has an effect on 1,3-beta-D-glucan synthase inhibition. Results of our initial evaluation suggest that the piperazine propanol derivative is a novel chemical structure of the class of antifungals which inhibit fungal cell growth by inhibiting fungal 1,3-beta-D-glucan synthase.

Loss of function of KRE5 suppresses temperature sensitivity of mutants lacking mitochondrial anionic lipids

Disruption of PGS1, which encodes the enzyme that catalyzes the committed step of cardiolipin (CL) synthesis, results in loss of the mitochondrial anionic phospholipids phosphatidylglycerol (PG) and CL. The pgs1Delta mutant exhibits severe growth defects at 37 degrees C. To understand the essential functions of mitochondrial anionic lipids at elevated temperatures, we isolated suppressors of pgs1Delta that grew at 37 degrees C. One of the suppressors has a loss of function mutation in KRE5, which is involved in cell wall biogenesis. The cell wall of pgs1Delta contained markedly reduced beta-1,3-glucan, which was restored in the suppressor. Stabilization of the cell wall with osmotic support alleviated the cell wall defects of pgs1Delta and suppressed the temperature sensitivity of all CL-deficient mutants. Evidence is presented suggesting that the previously reported inability of pgs1Delta to grow in the presence of ethidium bromide was due to defective cell wall integrity, not from "petite lethality." These findings demonstrated that mitochondrial anionic lipids are required for cellular functions that are essential in cell wall biogenesis, the maintenance of cell integrity, and survival at elevated temperature.

The Ras/protein kinase A pathway acts in parallel with the Mob2/Cbk1 pathway to effect cell cycle progression and proper bud site selection

In Saccharomyces cerevisiae, Ras proteins connect nutrient availability to cell growth through regulation of protein kinase A (PKA) activity. Ras proteins also have PKA-independent functions in mitosis and actin repolarization. We have found that mutations in MOB2 or CBK1 confer a slow-growth phenotype in a ras2Delta background. The slow-growth phenotype of mob2Delta ras2Delta cells results from a G1 delay that is accompanied by an increase in size, suggesting a G1/S role for Ras not previously described. In addition, mob2Delta strains have imprecise bud site selection, a defect exacerbated by deletion of RAS2. Mob2 and Cbk1 act to properly localize Ace2, a transcription factor that directs daughter cell-specific transcription of several genes. The growth and budding phenotypes of the double-deletion strains are Ace2 independent but are suppressed by overexpression of the PKA catalytic subunit, Tpk1. From these observations, we conclude that the PKA pathway and Mob2/Cbk1 act in parallel to determine bud site selection and promote cell cycle progression.

Anin vitroassay for (1 → 6)-β-D-glucan synthesis inSaccharomyces cerevisiae

(1 --> 6)-beta-D-glucan is a key cell wall component of Saccharomyces cerevisiae and Candida albicans. Many genes are known to affect the levels or structure of this glucan, but their roles and a molecular description of the synthesis of (1 --> 6)-beta-D-glucan remain to be established and a method to measure (1 --> 6)-beta-D-glucan synthase activity in vitro would provide an enabling tool. Here, conditions for the detection of in vitro synthesis of this polymer are described. Crude membrane preparations from S. cerevisiae were isolated, and incubated in the presence of UDP-glucose and GTP. With anti-(1 --> 6)-beta-D-glucan-specific antibodies, a time-dependent increase in the amount of this glucan was demonstrated in a dot-blot assay, or through an inhibition enzyme immunoassay. Antibody specificity was validated by competition experiments using pustulan, a (1 --> 6)-beta-D-glucan, laminarin, a (1 --> 3)-beta-D-glucan, yeast mannan and glycogen. The identity of the reaction product was also demonstrated by its sensitivity to a recombinant (1 --> 6)-beta-D-glucanase. Extracts from mutants in 10 genes with a wide range of altered cell wall (1 --> 6)-beta-D-glucan levels were assayed for in vitro synthesis of the polymer. A strong correlation of in vitro synthase activity with in vivo glucan levels was found, providing genetic support for the specificity of the assay. The basis for the GTP-dependence of the synthase reaction was studied. Extracts from rho2, rho3, rho4 and rho5 null mutants had wild-type in vitro activity. In contrast, Rho1p overproduction led to increased in vitro synthesis, implicating Rho1p in the regulation of (1 --> 6)-beta-D-glucan synthesis.

The PAK family kinase Cla4 is required for budding and morphogenesis in Ustilago maydis

The phytopathogenic basidiomycete Ustilago maydis displays a dimorphic switch between budding growth of haploid cells and filamentous growth of the dikaryon. In a screen for mutants affected in morphogenesis and cytokinesis, we identified the serine/threonine protein kinase Cla4, a member of the family of p21-activated kinases (PAKs). Cells, in which cla4 has been deleted, are viable but they are unable to bud properly. Instead, cla4 mutant cells grow as branched septate hyphae and divide by contraction and fission at septal cross walls. Delocalized deposition of chitinous cell wall material along the cell surface is observed in cla4 mutant cells. Deletion of the Cdc42/Rac1 interaction domain (CRIB) results in a constitutive active Cla4 kinase, whose expression is lethal for the cell. cla4 mutant cells are unable to induce pathogenic development in plants and to display filamentous growth in a mating reaction, although they are still able to secrete pheromone and to undergo cell fusion with wild-type cells. We propose that Cla4 is involved in the regulation of cell polarity during budding and filamentation.

Pxl1p, a paxillin-like protein in Saccharomyces cerevisiae, may coordinate Cdc42p and Rho1p functions during polarized growth

Rho-family GTPases Cdc42p and Rho1p play critical roles in the budding process of the yeast Saccharomyces cerevisiae. However, it is not clear how the functions of these GTPases are coordinated temporally and spatially during this process. Based on its ability to suppress cdc42-Ts mutants when overexpressed, a novel gene PXL1 was identified. Pxl1p resembles mammalian paxillin, which is involved in integrating various signaling events at focal adhesion. Both proteins share amino acid sequence homology and structural organization. When expressed in yeast, chicken paxillin localizes to the sites of polarized growth as Pxl1p does. In addition, the LIM domains in both proteins are the primary determinant for targeting the proteins to the cortical sites in their native cells. These data strongly suggest that Pxl1p is the "ancient paxillin" in yeast. Deletion of PXL1 does not produce any obvious phenotype. However, Pxl1p directly binds to Rho1p-GDP in vitro, and inhibits the growth of rho1-2 and rho1-3 mutants in a dosage-dependent manner. The opposite effects of overexpressed Pxl1p on cdc42 and rho1 mutants suggest that the functions of Cdc42p and Rho1p may be coordinately regulated during budding and that Pxl1p may be involved in this coordination.

Negative Regulation of Phosphatidylinositol 4,5-Bisphosphate Levels by the INP51-associated Proteins TAX4 and IRS4

Phosphatidylinositol 4,5-bisphosphate (PI(4,5)P(2)) is an important second messenger in signaling pathways in organisms ranging from yeast to mammals, but the regulation of PI(4,5)P(2) levels remains unclear. Here we present evidence that PI(4,5)P(2) levels in Saccharomyces cerevisiae are down-regulated by the homologous and functionally redundant proteins TAX4 and IRS4. The EPS15 homology domain-containing proteins TAX4 and IRS4 bind and activate the PI(4,5)P 5-phosphatase INP51 via an Asn-Pro-Phe motif in INP51. Furthermore, the INP51-TAX4/IRS4 complex negatively regulates the cell integrity pathway. Thus, TAX4 and IRS4 are novel regulators of PI(4,5)P(2) and PI(4,5)P(2)-dependent signaling. The interaction between TAX4/IRS4 and INP51 is analogous to the association of EPS15 with the 5-phosphatase synaptojanin 1 in mammalian cells, suggesting that EPS15 is an activator of synaptojanin 1.

Knocking on the heaven's wall: pathogenesis of and resistance to biotrophic fungi at the cell wall

New findings challenge the traditional view of the plant cell wall as passive structural barrier to invasion by fungal microorganisms. A surveillance system for cell wall integrity appears to sense perturbation of the cell wall structure upon fungal attack and is interconnected with known plant defence signalling pathways. Biotrophic fungi might manipulate this surveillance system for the establishment of biotrophy. The attempts of fungi to invade also induce a sub-cellular polarisation in attacked cells, which activates an ancient vesicle-associated resistance response that possibly enables the focal transport of regulatory cargo and the secretion of toxic cargo. The underlying resistance machinery might have been subverted by biotrophic fungi for pathogenesis.

Receptor internalization in yeast requires the Tor2-Rho1 signaling pathway

Efficient internalization of proteins from the cell surface is essential for regulating cell growth and differentiation. In a screen for yeast mutants defective in ligand-stimulated internalization of the alpha-factor receptor, we identified a mutant allele of TOR2, tor2G2128R. Tor proteins are known to function in translation initiation and nutrient sensing and are required for cell cycle progression through G1. Yeast Tor2 has an additional role in regulating the integrity of the cell wall by activating the Rho1 guanine nucleotide exchange factor Rom2. The endocytic defect in tor2G2128R cells is due to disruption of this Tor2 unique function. Other proteins important for cell integrity, Rom2 and the cell integrity sensor Wsc1, are also required for efficient endocytosis. A rho1 mutant specifically defective in activation of the glucan synthase Fks1/2 does not internalize alpha-factor efficiently, and fks1Delta cells exhibit a similar phenotype. Removal of the cell wall does not inhibit internalization, suggesting that the function of Rho1 and Fks1 in endocytosis is not through cell wall synthesis or structural integrity. These findings reveal a novel function for the Tor2-Rho1 pathway in controlling endocytosis in yeast, a function that is mediated in part through the plasma membrane protein Fks1.

Citrafungins A and B, Two New Fungal Metabolite Inhibitors of GGTase I with Antifungal Activity

[structure: see text] Screening of natural products extracts led to the discovery of citrafungins A and B, two new fungal metabolites of the alkylcitrate family that are inhibitors of GGTase I of various pathogenic fungal species with IC(50) values of 2.5-15 microM. These compounds exhibited antifungal activities with MIC values of 0.40-55 microM. The isolation, structure elucidation, relative and absolute stereochemistry, and biological activities of citrafungins are described.

FKS1 mutations responsible for selective resistance of Saccharomyces cerevisiae to the novel 1,3-beta-glucan synthase inhibitor arborcandin C

Arborcandin C is a novel antibiotic with potent antifungal activity that inhibits 1,3-beta-glucan synthase in fungi. We examined spontaneous Saccharomyces cerevisiae mutants which are selectively resistant to arborcandin C and revealed that a single amino acid replacement in Fks1p of Asn(470) with Lys or of Leu(642) with Ser confers selective resistance on Fks1p mutants.

Feedback from the wall

The ability of cells to perceive changes in the composition and mechanical properties of their cell wall is crucial for plants to achieve coordinated growth and development. Evidence is accumulating to show that the plant cell wall, like its yeast counterpart, is capable of triggering multiple signalling pathways. The components of the cell wall that are responsible for initiating these signal responses remain unknown; however, recent technological advances in cell wall analysis may now facilitate the identification of these components and accelerate the characterisation of changes that occur in cell wall mutants.

The yeasts Rho1p and Pkc1p regulate the transport of chitin synthase III (Chs3p) from internal stores to the plasma membrane

During cell stress, Saccharomyces cerevisiae increases the synthesis of chitin and glucans to strengthen and repair the cell wall. In this study, we show that under conditions of cell stress, the steady-state localization of chitin synthase III (Chs3p) shifts from internal stores (chitosomes) to the plasma membrane (PM). This redistribution occurs rapidly and requires the activators of the cell wall stress response signaling pathway, the G protein Rho1p, and the protein kinase Pkc1p, but not the cell integrity response mitogen-activated protein kinase cascade. Furthermore, expression of activated forms of Rho1p or Pkc1p, in the absence of cell stress, is sufficient to redistribute Chs3p to the PM. In cells deficient for both the clathrin adaptor complex 1 and Chs6p, where Chs3p is transported to the PM by an alternative bypass pathway, cell wall stress did not cause mobilization of Chs3p, suggesting that Rho1p/Pkc1p regulate Chs3p exit from the trans-Golgi network. The mobilization of an intracellular reservoir of Chs3p presents a novel opportunity to investigate the genetic basis of regulated vesicular traffic.