N-acetylglucosamine (GlcNAc) functions in cell signaling

The amino sugar N-acetylglucosamine (GlcNAc) is well known for the important structural roles that it plays at the cell surface. It is a key component of bacterial cell wall peptidoglycan, fungal cell wall chitin, and the extracellular matrix of animal cells. Interestingly, recent studies have also identified new roles for GlcNAc in cell signaling. For example, GlcNAc stimulates the human fungal pathogen Candida albicans to undergo changes in morphogenesis and expression of virulence genes. Pathogenic E. coli respond to GlcNAc by altering the expression of fimbriae and CURLI fibers that promote biofilm formation and GlcNAc stimulates soil bacteria to undergo changes in morphogenesis and production of antibiotics. Studies with animal cells have revealed that GlcNAc influences cell signaling through the post-translational modification of proteins by glycosylation. O-linked attachment of GlcNAc to Ser and Thr residues regulates a variety of intracellular proteins, including transcription factors such as NFκB, c-myc and p53. In addition, the specificity of Notch family receptors for different ligands is altered by GlcNAc attachment to fucose residues in the extracellular domain. GlcNAc also impacts signal transduction by altering the degree of branching of N-linked glycans, which influences cell surface signaling proteins. These emerging roles of GlcNAc as an activator and mediator of cellular signaling in fungi, animals, and bacteria will be the focus of this review.

The Sur7 protein resides in punctate membrane subdomains and mediates spatial regulation of cell wall synthesis in Candida albicans

The eukaryotic plasma membrane is organized into distinct domains that contribute to its function. One new type of plasma membrane domain was identified by studies on the Sur7 protein, which was discovered in the yeast S. cerevisiae to localize into stable punctate patches known as MCC or eisosomes. Sur7 shares similarities with Claudin proteins that form tight junction domains in animal cells, suggesting common roles for these tetraspanning membrane proteins. Recent analysis of C. albicans revealed broad new roles for Sur7; a sur7Delta mutant mislocalized septins and actin and was defective in morphogenesis. Strikingly, cell wall synthesis was very abnormal, including long projections of chitin-rich cell wall into the cytoplasm. Some phenotypes of the sur7Delta mutant are similar to the effects of inhibiting cell wall beta-glucan synthesis. This suggests that the abnormal cell wall structures are related to the increased chitin synthesis commonly seen under cell wall stress conditions, which could be mediated in part by the altered septin localization. Altogether, these results identify new roles for Sur7 and MCC/eisosomes in plasma membrane organization and coordination of the different aspects of cell wall synthesis.

N-acetylglucosamine (GlcNAc) induction of hyphal morphogenesis and transcriptional responses in Candida albicans are not dependent on its metabolism

N-acetylglucosamine (GlcNAc) stimulates important signaling pathways in a wide range of organisms. In the human fungal pathogen Candida albicans, GlcNAc stimulates hyphal cell morphogenesis, virulence genes, and the genes needed to catabolize GlcNAc. Previous studies on the GlcNAc transporter (NGT1) indicated that GlcNAc has to be internalized to induce signaling. Therefore, the role of GlcNAc catabolism was examined by deleting the genes required to phosphorylate, deacetylate, and deaminate GlcNAc to convert it to fructose-6-PO(4) (HXK1, NAG1, and DAC1). As expected, the mutants failed to utilize GlcNAc. Surprisingly, GlcNAc inhibited the growth of the nag1Δ and dac1Δ mutants in the presence of other sugars, suggesting that excess GlcNAc-6-PO(4) is deleterious. Interestingly, both hxk1Δ and an hxk1Δ nag1Δ dac1Δ triple mutant could be efficiently stimulated by GlcNAc to form hyphae. These mutants could also be stimulated to express GlcNAc-regulated genes. Because GlcNAc must be phosphorylated by Hxk1 to be catabolized, and also for it to enter the anabolic pathways that form chitin, N-linked glycosylation, and glycosylphosphatidylinositol anchors, the mutant phenotypes indicate that GlcNAc metabolism is not needed to induce signaling in C. albicans. Thus, these studies in C. albicans reveal a novel role for GlcNAc in cell signaling that may also regulate critical pathways in other organisms.

Strategies for isolating constitutively active and dominant-negative pheromone receptor mutants in yeast

Mating pheromone receptors of the yeast Saccharomyces cerevisiae are useful models for the study of G protein-coupled receptors. The mating pheromone receptors, Ste2 and Ste3, are not essential for viability so they can be readily targeted for analysis by a variety of genetic approaches. This chapter will describe methods for identification of two kinds of mutants that have been very informative about the mechanisms of receptor signaling: constitutively active mutants and dominant-negative mutants. Interestingly, these distinct types of mutants have revealed complementary information. Constitutive signaling is caused by mutations that are thought to weaken interactions between the seven transmembrane domains (TMDs), whereas the dominant-negative mutants apparently stabilize contacts between TMDs and lock receptors in the off conformation. In support of these conclusions, certain combinations of constitutively active and dominant-negative mutants restore nearly normal signaling properties.

Recognition of yeast by murine macrophages requires mannan but not glucan

The first barrier against infection by Candida albicans involves fungal recognition and destruction by phagocytic cells of the innate immune system. It is well established that interactions between different phagocyte receptors and components of the fungal cell wall trigger phagocytosis and subsequent immune responses, but the fungal ligands mediating the initial stage of recognition have not been identified. Here, we describe a novel assay for fungal recognition and uptake by macrophages which monitors this early recognition step independently of other downstream events of phagocytosis. To analyze infection in live macrophages, we validated the neutrality of a codon-optimized red fluorescent protein (yEmRFP) biomarker in C. albicans; growth, hyphal formation, and virulence in infected mice and macrophages were unaffected by yEmRFP production. This permitted a new approach for studying phagocytosis by carrying out competition assays between red and green fluorescent yeast cells to measure the efficiency of yeast uptake by murine macrophages as a function of dimorphism or cell wall defects. These competition experiments demonstrate that, given a choice, macrophages display strong preferences for phagocytosis based on genus, species, and morphology. Candida glabrata and Saccharomyces cerevisiae are taken up by J774 macrophage cells more rapidly than C. albicans, and C. albicans yeast cells are favored over hyphal cells. Significantly, these preferences are mannan dependent. Mutations that affect mannan, but not those that affect glucan or chitin, reduce the uptake of yeast challenged with wild-type competitors by both J774 and primary murine macrophages. These results suggest that mannose side chains or mannosylated proteins are the ligands recognized by murine macrophages prior to fungal uptake.

Identification of amino acids at two dimer interface regions of the alpha-factor receptor (Ste2)

The yeast alpha-factor pheromone receptor (Ste2) belongs to the large superfamily of G protein-coupled receptors (GPCRs) that characteristically contain seven transmembrane domains (TMs). A wide range of GPCRs are thought to exist as dimers or oligomers. To identify the interface regions that mediate oligomerization of Ste2, a set of 73 different mutants with Cys residues substituted near the extracellular ends of the transmembrane domains were screened for the ability to form intermolecular disulfide bonds. Disulfide bonds formed between Cys residues at six positions in Ste2. Cys substituted for Val-45 formed disulfide bonds, indicating contact between residues at the extracellular end of TM1. Disulfide bonds also formed with Cys residues substituted for five different residues clustered near the extracellular end of TM4 (Val-183, Val-186, Lys-187, Met-189, and Ile-190). Binding of the alpha-factor ligand to Ste2 did not change the sites at which cross-linking occurred in these TMs, but it did increase the efficiency of dimer formation for the Ste2-V183C mutant. Interestingly, oligomers of the class A family of vertebrate GPCRs are also thought to form homomeric contacts at TM1 and TM4. These results support the conclusion that GPCRs form oligomers and not just dimers, since TM1 and TM4 are too far apart in the class A GPCRs to form contacts in the same dimer moiety. Similar dimer interface sites in Ste2 and class A receptors provide further evidence that many aspects of structure and function are highly conserved across the divergent GPCR superfamily.

The Sur7 protein regulates plasma membrane organization and prevents intracellular cell wall growth in Candida albicans

The Candida albicans plasma membrane plays important roles in cell growth and as a target for antifungal drugs. Analysis of Ca-Sur7 showed that this four transmembrane domain protein localized to stable punctate patches, similar to the plasma membrane subdomains known as eisosomes or MCC that were discovered in S. cerevisiae. The localization of Ca-Sur7 depended on sphingolipid synthesis. In contrast to S. cerevisiae, a C. albicans sur7Delta mutant displayed defects in endocytosis and morphogenesis. Septins and actin were mislocalized, and cell wall synthesis was very abnormal, including long projections of cell wall into the cytoplasm. Several phenotypes of the sur7Delta mutant are similar to the effects of inhibiting beta-glucan synthase, suggesting that the abnormal cell wall synthesis is related to activation of chitin synthase activity seen under stress conditions. These results expand the roles of eisosomes by demonstrating that Sur7 is needed for proper plasma membrane organization and cell wall synthesis. A conserved Cys motif in the first extracellular loop of fungal Sur7 proteins is similar to a characteristic motif of the claudin proteins that form tight junctions in animal cells, suggesting a common role for these tetraspanning membrane proteins in forming specialized plasma membrane domains.

Scanning mutagenesis of regions in the Galpha protein Gpa1 that are predicted to interact with yeast mating pheromone receptors

The mechanism by which receptors activate heterotrimeric G proteins was examined by scanning mutagenesis of the Saccharomyces cerevisiae pheromone-responsive Galpha protein (Gpa1). The juxtaposition of high-resolution structures for rhodopsin and its cognate G protein transducin predicted that at least six regions of Galpha are in close proximity to the receptor. Mutagenesis was targeted to residues in these domains in Gpa1, which included four loop regions (beta2-beta3, alpha2-beta4, alpha3-beta5, and alpha4-beta6) as well as the N and C termini. The mutants displayed a range of phenotypes from nonsignaling to constitutive activation of the pheromone pathway. The constitutive activity of some mutants could be explained by decreased production of Gpa1, which permits unregulated signaling by Gbetagamma. However, the constitutive activity caused by the F344C and E335C mutations in the alpha2-beta4 loop and F378C in the alpha3-beta5 loop was not due to decreased protein levels, and was apparently due to defects in sequestering Gbetagamma. The strongest loss of the function mutant, which was not detectably induced by a pheromone, was caused by a K314C substitution in the beta2-beta3 loop. Several other mutations caused weak signaling phenotypes. Altogether, these results suggest that residues in different interface regions of Galpha contribute to activation of signaling.

Functional expression of mammalian receptors and membrane channels in different cells

In native tissues, the majority of medically important membrane proteins is only present at low concentrations, making their overexpression in recombinant systems a prerequisite for structural studies. Here, we explore the commonly used eukaryotic expression systems-yeast, baculovirus/insect cells (Sf9) and Semliki Forest Virus (SFV)/mammalian cells-for the expression of seven different eukaryotic membrane proteins from a variety of protein families. The expression levels, quality, biological activity, localization and solubility of all expressed proteins are compared in order to identify the advantages of one system over the other. SFV-transfected mammalian cell lines provide the closest to native environment for the expression of mammalian membrane proteins, and they exhibited the best overall performance. But depending on the protein, baculovirus-infected Sf9 cells performed almost as well as mammalian cells. The lowest expression levels for the proteins tested here were obtained in yeast.

Identification of an N-acetylglucosamine transporter that mediates hyphal induction in Candida albicans

The sugar N-acetylglucosamine (GlcNAc) plays an important role in nutrient sensing and cellular regulation in a wide range of organisms from bacteria to humans. In the fungal pathogen Candida albicans, GlcNAc induces a morphological transition from budding to hyphal growth. Proteomic comparison of plasma membrane proteins from buds and from hyphae induced by GlcNAc identified a novel hyphal protein (Ngt1) with similarity to the major facilitator superfamily of transporters. An Ngt1-GFP fusion was detected in the plasma membrane after induction with GlcNAc, but not other related sugars. Ngt1-GFP was also induced by macrophage phagocytosis, suggesting a role for the GlcNAc response in signaling entry into phagolysosomes. NGT1 is needed for efficient GlcNAc uptake and for the ability to induce hyphae at low GlcNAc concentrations. High concentrations of GlcNAc could bypass the need for NGT1 to induce hyphae, indicating that elevated intracellular levels of GlcNAc induce hyphal formation. Expression of NGT1 in Saccharomyces cerevisiae promoted GlcNAc uptake, indicating that Ngt1 acts directly as a GlcNAc transporter. Transport mediated by Ngt1 was specific, as other sugars could not compete for the uptake of GlcNAc. Thus, Ngt1 represents the first eukaryotic GlcNAc transporter to be discovered. The presence of NGT1 homologues in the genome sequences of a wide range of eukaryotes from yeast to mammals suggests that they may also function in the cellular processes regulated by GlcNAc, including those that underlie important diseases such as cancer and diabetes.

Accessibility of cysteine residues substituted into the cytoplasmic regions of the alpha-factor receptor identifies the intracellular residues that are available for G protein interaction

The yeast alpha-factor pheromone receptor (Ste2) belongs to the family of G protein-coupled receptors (GPCRs) that contain seven transmembrane domains. To define the residues that are accessible to the cytoplasmic G protein, Cys scanning mutagenesis was carried out in which each of the residues that span the intracellular loops and the cytoplasmic end of transmembrane domain 7 was substituted with Cys. The 90 different Cys-substituted residues were then assayed for reactivity with MTSEA-biotin [[2-[(biotinoyl)amino]ethyl]methanethiosulfonate], which reacts with solvent-accessible sulfhydryl groups. As part of these studies we show that adding free Cys to stop the MTSEA-biotin reactions has potential pitfalls in that Cys can rapidly undergo disulfide exchange with the biotinylated receptor proteins at pH >or=7. The central regions of the intracellular loops of Ste2 were all highly accessible to MTSEA-biotin. Residues near the ends of the loops typically exhibited a drop in the level of reactivity over a consecutive series of residues that was inferred to be the membrane boundary. Interestingly, these boundary residues were enriched in hydrophobic residues, suggesting that they may form a hydrophobic pocket for interaction with the G protein. Comparison with accessibility data from a previous study of the extracellular side of Ste2 indicates that the transmembrane domains vary in length, consistent with some transmembrane domains being tilted relative to the plane of the membrane as they are in rhodopsin. Altogether, these results define the residues that are accessible to the G protein and provide an important structural framework for the interpretation of the role of Ste2 residues that function in G protein activation.

DEP-domain-mediated regulation of GPCR signaling responses

G protein-coupled receptors (GPCRs) mediate cellular responses to a variety of stimuli, but how specific responses are regulated has been elusive, as the types of GPCRs vastly outnumber the classes of G protein heterotrimers available to initiate downstream signaling. In our analysis of signaling proteins containing DEP domains ( approximately 90 residue sequence motifs first recognized in fly Dishevelled, worm EGL-10, and mammalian Pleckstrin), we find that DEP domains are responsible for specific recognition of GPCRs. We examined the yeast regulator of G protein signaling (RGS) protein Sst2 and demonstrate that the DEP domains in Sst2 mediate binding to its cognate GPCR (Ste2). DEP-domain-mediated tethering promotes downregulation by placing the RGS protein in proximity to its substrate (receptor-activated Galpha subunit). Sst2 docks to the Ste2 cytosolic tail, but only its unphosphorylated state, allowing for release and recycling of this regulator upon receptor desensitization and internalization. DEP-domain-mediated targeting of effectors and regulators to specific GPCRs provides a means to dictate the nature, duration, and specificity of the response.

Cell cycle dynamics and quorum sensing in Candida albicans chlamydospores are distinct from budding and hyphal growth

The regulation of morphogenesis in the human fungal pathogen Candida albicans is under investigation to better understand how the switch between budding and hyphal growth is linked to virulence. Therefore, in this study we examined the ability of C. albicans to undergo a distinct type of morphogenesis to form large thick-walled chlamydospores whose role in infection is unclear, but they act as a resting form in other species. During chlamydospore morphogenesis, cells switch to filamentous growth and then develop elongated suspensor cells that give rise to chlamydospores. These filamentous cells were distinct from true hyphae in that they were wider and were not inhibited by the quorum-sensing factor farnesol. Instead, farnesol increased chlamydospore production, indicating that quorum sensing can also have a positive role. Nuclear division did not occur across the necks of chlamydospores, as it does in budding. Interestingly, nuclei divided within the suspensor cells, and then one daughter nucleus subsequently migrated into the chlamydospore. Septins were not detected near mitotic nuclei but were localized at chlamydospore necks. At later stages, septins localized throughout the chlamydospore plasma membrane and appeared to form long filamentous structures. Deletion of the CDC10 or CDC11 septins caused greater curvature of cells growing in a filamentous manner and morphological defects in suspensor cells and chlamydospores. These studies identify aspects of chlamydospore morphogenesis that are distinct from bud and hyphal morphogenesis.

Lipid raft polarization contributes to hyphal growth in Candida albicans

The polarization of sterol- and sphingolipid-enriched domains (lipid rafts) has been linked to morphogenesis and cell movement in diverse cell types. In the yeast Saccharomyces cerevisiae, a dramatic polarization of sterol-rich domains to the shmoo tip was observed in pheromone-induced cells (M. Bagnat and K. Simons, Proc. Natl. Acad. Sci. USA 99:14183-14188, 2002). We therefore examined whether plasma membrane lipid polarization contributes to the ability of the fungal pathogen Candida albicans to grow in a highly polarized manner to form hyphae. Interestingly, staining with filipin revealed that membrane sterols were highly polarized to the leading edge of growth during all stages of hyphal growth. Budding and pseudohyphal cells did not display polarized staining. Filipin staining was also enriched at septation sites in hyphae, where colocalization with septin proteins was observed, suggesting a role for the septins in forming a boundary domain. Actin appeared to play a role in sterol polarization and hyphal morphogenesis in that both were disrupted by low concentrations of latrunculin A that did not prevent budding. Furthermore, blocking either sphingolipid biosynthesis with myriocin or sterol biosynthesis with ketoconazole resulted in a loss of ergosterol polarization and caused abnormal hyphal morphogenesis, suggesting that lipid rafts are involved. Since hyphal growth is required for the full virulence of C. albicans, these results suggest that membrane polarization may contribute to the pathogenesis of this organism.

Comparison of class A and D G protein-coupled receptors: common features in structure and activation

All G protein-coupled receptors (GPCRs) share a common seven TM helix architecture and the ability to activate heterotrimeric G proteins. Nevertheless, these receptors have widely divergent sequences with no significant homology. We present a detailed structure-function comparison of the very divergent Class A and D receptors to address whether there is a common activation mechanism across the GPCR superfamily. The Class A and D receptors are represented by the vertebrate visual pigment rhodopsin and the yeast alpha-factor pheromone receptor Ste2, respectively. Conserved amino acids within each specific receptor class and amino acids where mutation alters receptor function were located in the structures of rhodopsin and Ste2 to assess whether there are functionally equivalent positions or regions within these receptors. We find several general similarities that are quite striking. First, strongly polar amino acids mediate helix interactions. Their mutation generally leads to loss of function or constitutive activity. Second, small and weakly polar amino acids facilitate tight helix packing. Third, proline is essential at similar positions in transmembrane helices 6 and 7 of both receptors. Mapping the specific location of the conserved amino acids and sites of constitutively active mutations identified conserved microdomains on transmembrane helices H3, H6, and H7, suggesting that there are underlying similarities in the mechanism of the widely divergent Class A and Class D receptors.

Successful expression of a functional yeast G-protein-coupled receptor (Ste2) in mammalian cells

G-protein-coupled receptors (GPCRs) are membrane-embedded cell signaling devices transducing ligand binding to activation of heterotrimeric G-proteins, providing a paradigm for signaling for yeast and mammals alike. Probing the extent to which yeast GPCRs may couple to mammalian G-proteins has been problematic. In the current work, we explored conditions that enable the cell-surface expression of a yeast alpha-factor pheromone receptor (Ste2). When expressed in human HEK293 cells, Ste2 is shown to bind its ligand alpha-factor, to be functional and catalyze activation of the mitogen-activated protein kinase cascade, and to demonstrate agonist-induced internalization. In response to agonist Ste2 as maintained intracellularly for several hours and avoids the degradation process observed for Ste2 in yeast cells. This is the first successful demonstration of the ability to express a functional yeast GPCR in mammalian cells.

Identification of residues that contribute to receptor activation through the analysis of compensatory mutations in the G protein-coupled alpha-factor receptor

The alpha-factor receptor (Ste2p) stimulates mating of the yeast Saccharomyces cerevisiae. Ste2p belongs to the large family of G protein-coupled receptors that are characterized by seven transmembrane alpha-helices. Receptor activation is thought to involve changes in the packing of the transmembrane helix bundle. To identify residues that contribute to Ste2p activation, second-site suppressor mutations were isolated that restored function to defective receptors carrying either an F204S or Y266C substitution which affect residues at the extracellular ends of transmembrane domains 5 and 6, respectively. Thirty-five different suppressor mutations were identified. On their own, these mutations caused a range of phenotypes, including hypersensitivity, constitutive activity, altered ligand binding, and loss of function. The majority of the mutations affected residues in the transmembrane segments that are predicted to face the helix bundle. Many of the suppressor mutations caused constitutive receptor activity, suggesting they improved receptor function by partially restoring the balance between the active and inactive states. Analysis of mutations in transmembrane domain 7 implicated residues Ala281 and Thr282 in receptor activation. The A281T and T282A mutants were supersensitive to S. cerevisiae alpha-factor, but were defective in responding to a variant of alpha-factor produced by another species, Saccharomyces kluyveri. The A281T mutant also displayed 8.7-fold enhanced basal signaling. Interestingly, Ala281 and Thr282 are situated in approximately the same position as Lys296 in rhodopsin, which is covalently linked to retinal. These results suggest that transmembrane domain 7 plays a role in receptor activation in a wide range of G protein-coupled receptors from yeast to humans.

SUMO Modification of Septin-interacting Proteins in Candida albicans

The initiation of bud and hyphal growth in the opportunistic fungal pathogen Candida albicans both involve polarized morphogenesis. However, there are many differences including the function of the septin proteins, a family of proteins involved in membrane organization in a wide range of organisms. Septins form a characteristic ring on the inner surface of the plasma membrane at the bud neck, whereas the septins are diffusely localized across emerging hyphal tips. In addition, septin rings are maintained at sites of septum formation in hyphae rather than being disassembled immediately after cytokinesis. The possibility that C. albicans septins are regulated by the small ubiquitin-like protein SUMO was examined in this study because the Saccharomyces cerevisiae septins were shown previously to be modified by SUMO (Smt3p). However, SUMO conjugation to septins was not detected during budding or hyphal morphogenesis in C. albicans. These results are supported by the lack of conserved SUMO consensus motifs between septins from the two organisms even after adjusting the predicted Cdc3p and Cdc12p septin sequences to account for mRNA splicing in C. albicans. Interestingly, a homolog of the Smt3p SUMO was identified in the C. albicans genome, and an epitope tagged version of Smt3p was conjugated to a variety of proteins. Immunofluorescence analysis showed prominent Smt3p SUMO localization at bud necks and sites of septum formation in hyphae similar to the septins. However, Smt3p was primarily detected on the mother cell side of the septin ring. A subset of these Smt3p-modified proteins co-immunoprecipitated with the septin Cdc11p. These results indicate that septin-associated proteins and not the septins themselves are the key target of SUMO modification at the bud neck in C. albicans.

A microdomain formed by the extracellular ends of the transmembrane domains promotes activation of the G protein-coupled alpha-factor receptor

The alpha-factor receptor (Ste2p) that promotes mating in Saccharomyces cerevisiae is similar to other G protein-coupled receptors (GPCRs) in that it contains seven transmembrane domains. Previous studies suggested that the extracellular ends of the transmembrane domains are important for Ste2p function, so a systematic scanning mutagenesis was carried out in which 46 residues near the ends of transmembrane domains 1, 2, 3, 4, and 7 were replaced with cysteine. These mutants complement mutations constructed previously near the ends of transmembrane domains 5 and 6 to analyze all the extracellular ends. Eight new mutants created in this study were partially defective in signaling (V45C, N46C, T50C, A52C, L102C, N105C, L277C, and A281C). Treatment with 2-([biotinoyl] amino) ethyl methanethiosulfonate, a thiol-specific reagent that reacts with accessible cysteine residues but not membrane-embedded cysteines, identified a drop in the level of reactivity over a consecutive series of residues that was inferred to be the membrane boundary. An unusual prolonged zone of intermediate reactivity near the extracellular end of transmembrane domain 2 suggests that this region may adopt a special structure. Interestingly, residues implicated in ligand binding were mainly accessible, whereas residues involved in the subsequent step of promoting receptor activation were mainly inaccessible. These results define a receptor microdomain that provides an important framework for interpreting the mechanisms by which functionally important residues contribute to ligand binding and activation of Ste2p and other GPCRs.

Candida albicans septin mutants are defective for invasive growth and virulence

Hyphal growth of Candida albicans is implicated as an important virulence factor for this opportunistic human pathogen. Septin proteins, a family of cytoskeletal elements that regulate membrane events and are important for proper morphogenesis of C. albicans, were examined for their role in tissue invasion and virulence in the mouse model of systemic infection. In vitro, septin mutants are only mildly defective for hyphal growth in liquid culture but display pronounced defects for invasive growth into agar. In vivo, the septin mutants were found to exhibit attenuated virulence. However, mice infected with the mutants displayed high fungal burdens in their kidneys without obvious symptoms of disease. Histological examination of infected kidneys revealed defects in organ invasion for the cdc10 Delta and cdc11 Delta deletion mutants, which displayed both reduced tissue penetration and noninvasive fungal masses. Thus, the septin proteins are necessary for invasive growth, which appears to be more important to the successful pathogenesis of C. albicans than hyphal growth alone.

Aromatic residues at the extracellular ends of transmembrane domains 5 and 6 promote ligand activation of the G protein-coupled alpha-factor receptor

The alpha-factor receptor (STE2) stimulates a G protein signaling pathway that promotes mating of the yeast Saccharomyces cerevisiae. Previous random mutagenesis studies implicated residues in the regions near the extracellular ends of the transmembrane domains in ligand activation. In this study, systematic Cys scanning mutagenesis across the ends of transmembrane domains 5 and 6 identified two residues, Phe(204) and Tyr(266), that were important for receptor signaling. These residues play a specific role in responding to alpha-factor since the F204C and Y266C substituted receptors responded to an alternative agonist (novobiocin). To better define the structure of this region, the Cys-substituted mutant receptors were assayed for reactivity with a thiol-specific probe that does not react with membrane-imbedded residues. A drop in reactivity coincided with residues likely to be buried in the membrane. Interestingly, both Phe(204) and Tyr(266) are located very near the interface region. However, these assays predict that Phe(204) is accessible at the surface of the receptor, consistent with the strong defect in binding alpha-factor caused by mutating this residue. In contrast, Tyr(266) was not accessible. This correlates with the ability of Y266C mutant receptors to bind alpha-factor and suggests that this residue is involved in the subsequent triggering of receptor activation. These results highlight the role of aromatic residues near the ends of the transmembrane segments in the alpha-factor receptor, and suggest that similar aromatic residues may play an important role in other G protein-coupled receptors.

Iqg1p links spatial and secretion landmarks to polarity and cytokinesis

Cytokinesis requires the polarization of the actin cytoskeleton, the secretion machinery, and the correct positioning of the division axis. Budding yeast cells commit to their cytokinesis plane by choosing a bud site and polarizing their growth. Iqg1p (Cyk1p) was previously implicated in cytokinesis (Epp and Chant, 1997; Lippincott and Li, 1998; Osman and Cerione, 1998), as well as in the establishment of polarity and protein trafficking (Osman and Cerione, 1998). To better understand how Iqg1p influences these processes, we performed a two-hybrid screen and identified the spatial landmark Bud4p as a binding partner. Iqg1p can be coimmunoprecipitated with Bud4p, and Bud4p requires Iqg1p for its proper localization. Iqg1p also appears to specify axial bud-site selection and mediates the proper localization of the septin, Cdc12p, as well as binds and helps localize the secretion landmark, Sec3p. The double mutants iqg1Deltasec3Delta and bud4Deltasec3Delta display defects in polarity, budding pattern and cytokinesis, and electron microscopic studies reveal that these cells have aberrant septal deposition. Taken together, these findings suggest that Iqg1p recruits landmark proteins to form a targeting patch that coordinates axial budding with cytokinesis.

Septin function in Candida albicans morphogenesis

The septin proteins function in the formation of septa, mating projections, and spores in Saccharomyces cerevisiae, as well as in cell division and other processes in animal cells. Candida albicans septins were examined in this study for their roles in morphogenesis of this multimorphic, opportunistically pathogenic fungus, which can range from round budding yeast to elongated hyphae. C. albicans green fluorescent protein labeled septin proteins localized to a tight ring at the bud and pseudohyphae necks and as a more diffuse array in emerging germ tubes of hyphae. Deletion analysis demonstrated that the C. albicans homologs of the S. cerevisiae CDC3 and CDC12 septins are essential for viability. In contrast, the C. albicans cdc10Delta and cdc11Delta mutants were viable but displayed conditional defects in cytokinesis, localization of cell wall chitin, and bud morphology. The mutant phenotypes were not identical, however, indicating that these septins carry out distinct functions. The viable septin mutants could be stimulated to undergo hyphal morphogenesis but formed hyphae with abnormal curvature, and they differed from wild type in the selection of sites for subsequent rounds of hyphal formation. The cdc11Delta mutants were also defective for invasive growth when embedded in agar. These results further extend the known roles of the septins by demonstrating that they are essential for the proper morphogenesis of C. albicans during both budding and filamentous growth.