Budding yeast morphogenesis: signalling, cytoskeleton and cell cycle

Evaluating single-cell variability in proteasomal decay

Gene expression is a stochastic process that leads to variability in mRNA and protein abundances even within an isogenic population of cells grown in the same environment. This variation, often called gene-expression noise, has typically been attributed to transcriptional and translational processes while ignoring the contributions of protein decay variability across cells. Here we estimate the single-cell protein decay rates of two degron GFPs in Saccharomyces cerevisiae using time-lapse microscopy. We find substantial cell-to-cell variability in the decay rates of the degron GFPs. We evaluate cellular features that explain the variability in the proteasomal decay and find that the amount of 20s catalytic beta subunit of the proteasome marginally explains the observed variability in the degron GFP half-lives. We propose alternate hypotheses that might explain the observed variability in the decay of the two degron GFPs. Overall, our study highlights the importance of studying the kinetics of the decay process at single-cell resolution and that decay rates vary at the single-cell level, and that the decay process is stochastic. A complex model of decay dynamics must be included when modeling stochastic gene expression to estimate gene expression noise.

Lattice-based Monte Carlo simulation of the effects of nutrient concentration and magnetic field exposure on yeast colony growth and morphology

Yeasts exist in communities that expand over space and time to form complex structures and patterns. We developed a lattice-based framework to perform spatial-temporal Monte Carlo simulations of budding yeast colonies exposed to different nutrient and magnetic field conditions. The budding patterns of haploid and diploid yeast cells were incorporated into the framework, as well as the filamentous growth that occurs in yeast colonies under nutrient limiting conditions. Simulation of the framework predicted that magnetic fields decrease colony growth rate, solidity, and roundness. Magnetic field simulations further predicted that colony elongation and boundary fluctuations increase in a nutrient- and ploidy-dependent manner. These in-silico predictions are an important step towards understanding the effects of the physico-chemical environment on microbial colonies and for informing bioelectromagnetic experiments on yeast colony biofilms and fungal pathogens.

Calcium Binding Protein Ncs1 Is Calcineurin Regulated in Cryptococcus neoformans and Essential for Cell Division and Virulence

Intracellular calcium (Ca2+) is crucial for signal transduction in Cryptococcus neoformans, the major cause of fatal fungal meningitis. The calcineurin pathway is the only Ca2+-requiring signaling cascade implicated in cryptococcal stress adaptation and virulence, with Ca2+ binding mediated by the EF-hand domains of the Ca2+ sensor protein calmodulin. In this study, we identified the cryptococcal ortholog of neuronal calcium sensor 1 (Ncs1) as a member of the EF-hand superfamily. We demonstrated that Ncs1 has a role in Ca2+ homeostasis under stress and nonstress conditions, as the ncs1Δ mutant is sensitive to a high Ca2+ concentration and has an elevated basal Ca2+ level. Furthermore, NCS1 expression is induced by Ca2+, with the Ncs1 protein adopting a punctate subcellular distribution. We also demonstrate that, in contrast to the case with Saccharomyces cerevisiae, NCS1 expression in C. neoformans is regulated by the calcineurin pathway via the transcription factor Crz1, as NCS1 expression is reduced by FK506 treatment and CRZ1 deletion. Moreover, the ncs1Δ mutant shares a high temperature and high Ca2+ sensitivity phenotype with the calcineurin and calmodulin mutants (cna1Δ and cam1Δ), and the NCS1 promoter contains two calcineurin/Crz1-dependent response elements (CDRE1). Ncs1 deficiency coincided with reduced growth, characterized by delayed bud emergence and aberrant cell division, and hypovirulence in a mouse infection model. In summary, our data show that Ncs1 has a significant role as a Ca2+ sensor in C. neoformans, working with calcineurin to regulate Ca2+ homeostasis and, consequently, promote fungal growth and virulence.IMPORTANCECryptococcus neoformans is the major cause of fungal meningitis in HIV-infected patients. Several studies have highlighted the important contributions of Ca2+ signaling and homeostasis to the virulence of C. neoformans Here, we identify the cryptococcal ortholog of neuronal calcium sensor 1 (Ncs1) and demonstrate its role in Ca2+ homeostasis, bud emergence, cell cycle progression, and virulence. We also show that Ncs1 function is regulated by the calcineurin/Crz1 signaling cascade. Our work provides evidence of a link between Ca2+ homeostasis and cell cycle progression in C. neoformans.

Pseudohyphal Growth of the Emerging Pathogen Candida auris Is Triggered by Genotoxic Stress through the S Phase Checkpoint

Candida auris is a newly emerged fungal pathogen of humans. This species was first reported in 2009 when it was identified in an ear infection of a patient in Japan. However, despite intense interest in this organism as an often multidrug-resistant fungus, there is little knowledge about its cellular biology. During infection of human patients, fungi are able to change cell shape from ellipsoidal yeast cells to elongated filaments to adapt to various conditions within the host organism. There are different types of filaments, which are triggered by reactions to different cues. Candida auris fails to form filaments when exposed to triggers that stimulate yeast filament morphogenesis in other fungi. Here, we show that it does form filaments when its DNA is damaged. These conditions might arise when Candida auris cells interact with host immune cells or during growth in certain host tissues (kidney or bladder) or during treatment with antifungal drugs.

The morphogenetic switching between yeast cells and filaments (true hyphae and pseudohyphae) is a key cellular feature required for full virulence in many polymorphic fungal pathogens, such as Candida albicans. In the recently emerged yeast pathogen Candida auris, occasional elongation of cells has been reported. However, environmental conditions and genetic triggers for filament formation have remained elusive. Here, we report that induction of DNA damage and perturbation of replication forks by treatment with genotoxins, such as hydroxyurea, methyl methanesulfonate, and the clinically relevant fungistatic 5-fluorocytosine, cause filamentation in C. auris. The filaments formed were characteristic of pseudohyphae and not parallel-sided true hyphae. Pseudohyphal growth is apparently signaled through the S phase checkpoint and, interestingly, is Tup1 independent in C. auris. Intriguingly, the morphogenetic switching capability is strain specific in C. auris, highlighting the heterogenous nature of the species as a whole.

IMPORTANCECandida auris is a newly emerged fungal pathogen of humans. This species was first reported in 2009 when it was identified in an ear infection of a patient in Japan. However, despite intense interest in this organism as an often multidrug-resistant fungus, there is little knowledge about its cellular biology. During infection of human patients, fungi are able to change cell shape from ellipsoidal yeast cells to elongated filaments to adapt to various conditions within the host organism. There are different types of filaments, which are triggered by reactions to different cues. Candida auris fails to form filaments when exposed to triggers that stimulate yeast filament morphogenesis in other fungi. Here, we show that it does form filaments when its DNA is damaged. These conditions might arise when Candida auris cells interact with host immune cells or during growth in certain host tissues (kidney or bladder) or during treatment with antifungal drugs.

Regulation of the Candida albicans Hypha-Inducing Transcription Factor Ume6 by the CDK1 Cyclins Cln3 and Hgc1

The ability to switch between proliferation as yeast cells and development into hyphae is a hallmark of Candida albicans. The switch to hyphal morphogenesis depends on external inducing conditions, but its efficiency is augmented in stationary-phase cells. Ume6, a transcription factor that is itself transcriptionally induced under hypha-promoting conditions, is both necessary and sufficient for hyphal morphogenesis. We found that Ume6 is regulated posttranslationally by the cell cycle kinase Cdc28/Cdk1, which reduces Ume6 activity via different mechanisms using different cyclins. Together with the cyclin Hgc1, Cdk1 promotes degradation of Ume6 via the SCFCDC4 ubiquitin ligase. Since HGC1 is a key transcriptional target of Ume6, this results in a negative-feedback loop between Hgc1 and Ume6. In addition, we found that Cln3, a G1 cyclin that is essential for cell cycle progression and yeast proliferation, suppresses hyphal morphogenesis and that Cln3 suppresses Ume6 activity both in the heterologous Saccharomyces cerevisiae system and in C. albicans itself. This activity of Cln3 may provide the basis for the antagonistic relationship between yeast proliferation and hyphal development in C. albicans. IMPORTANCE The yeast to hypha (mold) morphogenetic switch of Candida albicans plays a role in its virulence and constitutes a diagnostic trait for this organism, the most prevalent systemic fungal pathogen in industrialized countries. It has long been known that hyphae are most efficiently induced from stationary cultures. Here, a molecular basis for this observation is provided. The G1 cyclin Cln3, an essential promoter of yeast proliferation, was found to suppress hyphal induction. Suppression of hyphal induction is achieved by inhibition of the activity of the central activator of hyphal morphogenesis, the transcription factor Ume6. Thus, levels of Cln3 control the switch between proliferation of C. albicans as individual yeast cells and development into extended hyphae, a switch that may preface the proliferation/differentiation switch in multicellular organisms.

Transcriptome using Illumina sequencing reveals the traits of spermatogenesis and developing testes in Eriocheir sinensis

Chinese mitten crab (Eriocheir sinensis) has the spermatozoa with typical aflagellate, decondensed chromatin, cup-shaped nuclei, and radial arms. However, the mechanism of spermatogenesis during which the specific spermatozoa are generated in this species is yet unclear. Here, the transcriptome of developing testis in E. sinensis was analyzed using the ways of RNA-seq and bioinformatics analysis to identify candidate genes potentially involved in development of testis and spermatogenesis. The Illumina HiSeq2500 sequencing of three replicons of samples produced a total of 145.19 M clean reads representing with a total of 21.34 Gb bases and 45.48% GC content. 56.30% clean reads were mapped to the draft genome of E. sinensis. The assembly of the transcriptome yielded contigs of 5691802 sequences and unigenes of 406527 sequences. Total 24246 and 40793 transcripts were annotated using Swissprot and Nr database, respectively. There were 48213 (70.31%) and 7858 (46.25%) transcripts with identity of more than 99 matching to mature testis unigenes in the databases of Nr and EST, respectively. The analytic results of KOG, GO and KEGG showed wide potential molecular functions of transcripts in the developing testes. KEGG analysis of unigenes yielded total 9422 predicted genes. Those predicted genes were involved in total 216 KEGG pathways related to the physiological activities of developing testis. 1975 predicted genes were involved in cellular and subcellular structural alteration of male germ cells. There were important roles of some pathways in the processes of morphological and structural biogenesis pertaining to testis development and spermatogenesis. Other 583 unigenes encoding the genetic and epigenetic factors also be found, which might contribute to the decondensation and stability of decondensed nuclei in the spermatozoa. These predicted events provide a view of the potential molecular mechanisms of development of testis and spermatogenesis in E. sinensis.

A Predictive Model for Yeast Cell Polarization in Pheromone Gradients

Budding yeast cells exist in two mating types, a and α, which use peptide pheromones to communicate with each other during mating. Mating depends on the ability of cells to polarize up pheromone gradients, but cells also respond to spatially uniform fields of pheromone by polarizing along a single axis. We used quantitative measurements of the response of a cells to α-factor to produce a predictive model of yeast polarization towards a pheromone gradient. We found that cells make a sharp transition between budding cycles and mating induced polarization and that they detect pheromone gradients accurately only over a narrow range of pheromone concentrations corresponding to this transition. We fit all the parameters of the mathematical model by using quantitative data on spontaneous polarization in uniform pheromone concentration. Once these parameters have been computed, and without any further fit, our model quantitatively predicts the yeast cell response to pheromone gradient providing an important step toward understanding how cells communicate with each other.

The dynamic structure of Spitzenkörpers of Trichosporon asahii examined by the fluorescent probe FM4-64

The Spitzenkörper is a dynamic and specialized multicomponent cell complex present in the tips of hyphal cells. The amphiphilic styryl dye FM4-64 was found to be ideal for imaging the dynamic changes of the apical vesicle cluster within growing hyphal tips. It is widely used as a marker of endocytosis and to visualize vacuolar membranes. Here we performed uptake experiments using FM4-64 to study the dynamic of the Spitzenkörper in Trichosporon asahii. We observed that Spitzenkörpers were present at the tip of the budding site of the spore, blastospore, and the germ tube of T. asahii. We also found that Spitzenkörpers were present at the tip of the hyphae as well as the subapical regions. Cytochalasin D, an inhibitor of actin polymerization, leads to abnormal Spitzenkörper formation and loss of cell polarity.

Cell aggregations in yeasts and their applications

Yeasts can display four types of cellular aggregation: sexual, flocculation, biofilm formation, and filamentous growth. These cell aggregations arise, in some yeast strains, as a response to environmental or physiological changes. Sexual aggregation is part of the yeast mating process, representing the first step of meiotic recombination. The flocculation phenomenon is a calcium-dependent asexual reversible cellular aggregation that allows the yeast to withstand adverse conditions. Biofilm formation consists of multicellular aggregates that adhere to solid surfaces and are embedded in a protein matrix; this gives the yeast strain either the ability to colonize new environments or to survive harsh environmental conditions. Finally, the filamentous growth is the ability of some yeast strains to grow in filament forms. Filamentous growth can be attained by two different means, with the formation of either hyphae or pseudohyphae. Both hyphae and pseudohyphae arise when the yeast strain is under nutrient starvation conditions and they represent a means for the microbial strain to spread over a wide area to survey for food sources, without increasing its biomass. Additionally, this filamentous growth is also responsible for the invasive growth of some yeast.

Choosing the right lifestyle: adhesion and development in Saccharomyces cerevisiae

The budding yeast Saccharomyces cerevisiae is a eukaryotic microorganism that is able to choose between different unicellular and multicellular lifestyles. The potential of individual yeast cells to switch between different growth modes is advantageous for optimal dissemination, protection and substrate colonization at the population level. A crucial step in lifestyle adaptation is the control of self- and foreign adhesion. For this purpose, S. cerevisiae contains a set of cell wall-associated proteins, which confer adhesion to diverse biotic and abiotic surfaces. Here, we provide an overview of different aspects of S. cerevisiae adhesion, including a detailed description of known lifestyles, recent insights into adhesin structure and function and an outline of the complex regulatory network for adhesin gene regulation. Our review shows that S. cerevisiae is a model system suitable for studying not only the mechanisms and regulation of cell adhesion, but also the role of this process in microbial development, ecology and evolution.

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

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

Dse1 may control cross talk between the pheromone and filamentation pathways in yeast

The filamentous/invasive growth pathway is activated by nutrient limitation in the haploid form of the yeast Saccharomyces cerevisiae, whereas exposure to mating-pheromone causes cells to differentiate into gametes. Although these two pathways respond to very different stimuli and generate very different responses, they utilize many of the same signaling components. This implies the need for robust mechanisms to maintain signal fidelity. Dse1 was identified in an allele-specific suppressor screen for proteins that interact with the pheromone-responsive Gbetagamma, and found to bind both to a Gbetagamma-affinity column, and to the shared MEKK, Ste11. Although overexpression of Dse1 stimulated invasive growth and transcription of both filamentation and mating-specific transcriptional reporters, deletion of DSE1 had no effect on these outputs. In contrast, pheromone hyper-induced transcription of the filamentation reporter in cells lacking Dse1 and in cells expressing a mutant form of Gbeta that exhibits diminished interaction with Dse1. Thus, the interaction of Dse1 with both Gbeta and Ste11 may be designed to control cross talk between the pheromone and filamentation pathways.

SLA2 mutations cause SWE1-mediated cell cycle phenotypes in Candida albicans and Saccharomyces cerevisiae

The early endocytic patch protein Sla2 is important for morphogenesis and growth rates in Saccharomyces cerevisiae and Candida albicans, but the mechanism that connects these processes is not clear. Here we report that growth defects in cells lacking CaSLA2 or ScSLA2 are associated with a cell cycle delay that is influenced by Swe1, a morphogenesis checkpoint kinase. To establish how Swe1 monitors Sla2 function, we compared actin organization and cell cycle dynamics in strains lacking other components of early endocytic patches (Sla1 and Abp1) with those in strains lacking Sla2. Only sla2 strains had defects in actin cables, a known trigger of the morphogenesis checkpoint, yet all three strains exhibited Swe1-dependent phenotypes. Thus, Swe1 appears to monitor actin patch in addition to actin cable function. Furthermore, Swe1 contributed to virulence in a mouse model of disseminated candidiasis, implying a role for the morphogenesis checkpoint during the pathogenesis of C. albicans infections.

Vacuole inheritance regulates cell size and branching frequency of Candida albicans hyphae

Hyphal growth of Candida albicans is characterized by asymmetric cell divisions in which the subapical mother cell inherits most of the vacuolar space and becomes cell cycle arrested in G1, while the apical daughter cell acquires most of the cell cytoplasm and progresses through G1 into the next mitotic cell cycle. Consequently, branch formation in hyphal compartments is delayed until sufficient cytoplasm is synthesized to execute the G1 ‘START’ function. To test the hypothesis that this mode of vacuole inheritance determines cell cycle progression and therefore the branching of hyphae, eight tetracycline-regulated conditional mutants were constructed that were affected at different stages of the vacuole inheritance pathway. Under repressing conditions, vac7, vac8 and fab1 mutants generated mycelial compartments with more symmetrically distributed vacuoles and increased branching frequencies. Repression of VAC1, VAM2 and VAM3 resulted in sparsely branched hyphae, with large vacuoles and enlarged hyphal compartments. Therefore, during hyphal growth of C. albicans the cell cycle, growth and branch formation can be uncoupled, resulting in the investment of cytoplasm to support hyphal extension at the expense of hyphal branching.

Filament formation in Saccharomyces cerevisiae--a review

Many yeasts can produce filamentous elongated cells identifiable as hyphae, pseudohyphae or invasive filaments. Filament formation has been understood as a foraging response that occurs in nutrient-poor conditions. However, fusel alcohols were observed to induce filament formation in rich nutrient conditions in every yeast species examined. Fusel alcohols, e.g., 3-methyl-1-butanol (3Me-BuOH; 'isoamyl alcohol'), 2-methyl-1-propanol (isobutyl alcohol), (-)-2-methyl-1-butanol ('active amyl alcohol'), 2-phenylethanol and 3-(2-hydroxyethyl)indole (tryptophol) (the end products of leucine, valine, isoleucine, phenylalanine and tryptophan catabolism, respectively) are the end products of amino acid catabolism that accumulate when nutrients become limiting. Thus, yeast responds to its own metabolic by-products. Considerable effort was made to define the cell biological and biochemical changes that take place during 3Me-BuOH-induced filamentation. In Saccharomyces cerevisiae filaments contain significantly greater mitochondrial mass and increased chitin content in comparison with yeast-form cells. The global transcriptional response of S. cerevisiae during the early stages of 3Me-BuOH-induced filament formation has been described. Four ORFs displayed very significant (more than 10-fold) increases in their RNA species, and 12 ORFs displayed increases in transcription of more than 5-fold. The transcription of five genes (all of which encode transporters) decreased by similar amounts. Where examined, the activity of the proteins encoded reflected the transcriptional pattern of their respective mRNAs. To understand this regulation, studies were performed to see whether deletion or overexpression of key genes affects the ability to filament and invade solid YEPD medium. This has led to identification of those proteins that are essential for filament formation, repressors and those which are simply not required. It also leads to the conclusion that 3Me-BuOH-induced filament formation is not a foraging response but a response to reduced growth rate.

Polarized growth in fungi--interplay between the cytoskeleton, positional markers and membrane domains

One kind of the most extremely polarized cells in nature are the indefinitely growing hyphae of filamentous fungi. A continuous flow of secretion vesicles from the hyphal cell body to the growing hyphal tip is essential for cell wall and membrane extension. Because microtubules (MT) and actin, together with their corresponding motor proteins, are involved in the process, the arrangement of the cytoskeleton is a crucial step to establish and maintain polarity. In Saccharomyces cerevisiae and Schizosaccharomyces pombe, actin-mediated vesicle transportation is sufficient for polar cell extension, but in S. pombe, MTs are in addition required for the establishment of polarity. The MT cytoskeleton delivers the so-called cell-end marker proteins to the cell pole, which in turn polarize the actin cytoskeleton. Latest results suggest that this scenario may principally be conserved from S. pombe to filamentous fungi. In addition, in filamentous fungi, MTs could provide the tracks for long-distance vesicle movement. In this review, we will compare the interaction of the MT and the actin cytoskeleton and their relation to the cortex between yeasts and filamentous fungi. In addition, we will discuss the role of sterol-rich membrane domains in combination with cell-end marker proteins for polarity establishment.

Cell cycle and cell death are not necessary for appressorium formation and plant infection in the fungal plant pathogen Colletotrichum gloeosporioides

Background

In order to initiate plant infection, fungal spores must germinate and penetrate into the host plant. Many fungal species differentiate specialized infection structures called appressoria on the host surface, which are essential for successful pathogenic development. In the model plant pathogen Magnaporthe grisea completion of mitosis and autophagy cell death of the spore are necessary for appressoria-mediated plant infection; blocking of mitosis prevents appressoria formation, and prevention of autophagy cell death results in non-functional appressoria.

Results

We found that in the closely related plant pathogen Colletotrichum gloeosporioides, blocking of the cell cycle did not prevent spore germination and appressoria formation. The cell cycle always lagged behind the morphogenetic changes that follow spore germination, including germ tube and appressorium formation, differentiation of the penetrating hypha, and in planta formation of primary hyphae. Nuclear division was arrested following appressorium formation and was resumed in mature appressoria after plant penetration. Unlike in M. grisea, blocking of mitosis had only a marginal effect on appressoria formation; development in hydroxyurea-treated spores continued only for a limited number of cell divisions, but normal numbers of fully developed mature appressoria were formed under conditions that support appressoria formation. Similar results were also observed in other Colletotrichum species. Spores, germ tubes, and appressoria retained intact nuclei and remained viable for several days post plant infection.

Conclusion

We showed that in C. gloeosporioides the differentiation of infection structures including appressoria precedes mitosis and can occur without nuclear division. This phenomenon was also found to be common in other Colletotrichum species. Spore cell death did not occur during plant infection and the fungus primary infection structures remained viable throughout the infection cycle. Our results show that the control of basic cellular processes such as those coupling cell cycle and morphogenesis during fungal infection can be substantially different between fungal species with similar lifestyles and pathogenic strategies.

Special type of pheromone-induced invasive growth in Saccharomyces cerevisiae

The ability to invade a solid substrate is an important phenomenon due to its connection with pathogenic activity of fungi. We report here on invasion displayed by MATalpha cells of Saccharomyces cerevisiae lacking Isw2p, a subunit of the ISW2 chromatin remodelling complex. We found that on minimal medium, where the isw2Delta MATalpha mutant is not invasive, additional absence of another ISW2 complex subunit, Dls1p or Dpb4p, promoted invasion. Our microarray data showed that derepression of MAT a-specific genes caused by absence of Isw2p is very low. Their expression is increased only by the autocrine activation of the mating pathway. Invasion of isw2Delta MATalpha cells thus resembles the pheromone-induced invasion, including dependence on Fig2p. We show here that another pheromone-induced protein, mating agglutinin Aga1p, can play a role in the agar adhesion necessary for invasion. In contrast with MAT a-cells invading agar under low alpha-pheromone concentration, the invasive growth of isw2Delta cells specifically requires Fus3 kinase. Its function in the invasion of isw2Delta MATalpha cells cannot be completely substituted by Kss1 kinase, which plays a basic role in invasive growth signalling. We suggest that partial dependence of the isw2Delta MATalpha invasion on Fus3p and Aga1p corresponds to a weaker pheromone response of this mutant.

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.

Candida albicans hyphae have a Spitzenkörper that is distinct from the polarisome found in yeast and pseudohyphae

Fungi grow with a variety of morphologies: oval yeast cells, chains of elongated cells called pseudohyphae and long, narrow, tube-like filaments called hyphae. In filamentous fungi, hyphal growth is strongly polarised to the tip and is mediated by the Spitzenkörper, which acts as a supply centre to concentrate the delivery of secretory vesicles to the tip. In the budding yeast Saccharomyces cerevisiae, polarised growth is mediated by the polarisome, a surface cap of proteins that nucleates the formation of actin cables delivering secretory vesicles to the growing tip. The human fungal pathogen, Candida albicans, can grow in all three morphological forms. Here we show the presence of a Spitzenkörper at the tip of C. albicans hyphae as a ball-like localisation of secretory vesicles, together with the formin Bni1 and Mlc1, an ortholog of an S. cerevisiae myosin regulatory light chain. In contrast, in C. albicans yeast cells, pseudohyphae and hyphae Spa2 and Bud6, orthologs of S. cerevisiae polarisome components, as well as the master morphology regulator Cdc42, localise predominantly, but not exclusively, to a surface cap resembling the polarisome of S. cerevisiae yeast cells. A small amount of Cdc42 also localises to the Spitzenkörper. Furthermore, we show differences in the genetic and cytoskeletal requirements, and cell cycle dynamics of polarity determinants in yeast, pseudohyphae and hyphae. These results, together with the cytological differences between the cell types, suggest that the Spitzenkörper and polarisome are distinct structures, that the polarisome and Spitzenkörper coexist in hyphae, and that polarised growth in hyphae is driven by a fundamentally different mechanism to that in yeast and pseudohyphae.

The mitotic cyclins Clb2p and Clb4p affect morphogenesis in Candida albicans

The ability of Candida albicans to switch cellular morphologies is crucial for its ability to cause infection. Because the cell cycle machinery participates in Saccharomyces cerevisiae filamentous growth, we characterized in detail the two C. albicans B-type cyclins, CLB2 and CLB4, to better understand the molecular mechanisms that underlie the C. albicans morphogenic switch. Both Clb2p and Clb4p levels are cell cycle regulated, peaking at G2/M and declining before mitotic exit. On hyphal induction, the accumulation of the G1 cyclin Cln1p was prolonged, whereas the accumulation of both Clb proteins was delayed when compared with yeast form cells, indicating that CLB2 and CLB4 are differentially regulated in the two morphologies and that the dynamics of cyclin appearance differs between yeast and hyphal forms of growth. Clb2p-depleted cells were inviable and arrested with hyper-elongated projections containing two nuclei, suggesting that Clb2p is not required for entry into mitosis. Unlike Clb2p-depleted cells, Clb4p-depleted cells were viable and formed constitutive pseudohyphae. Clb proteins lacking destruction box domains blocked cell cycle progression resulting in the formation of long projections, indicating that both Clb2p and Clb4p must be degraded before mitotic exit. In addition, overexpression of either B-type cyclin reduced the extent of filamentous growth. Taken together, these data indicate that Clb2p and Clb4p regulate C. albicans morphogenesis by negatively regulating polarized growth.

Transcriptional profiling of the early stages of germination in by real-time RT-PCR

By using real-time RT-PCR, we profiled the expression of CGR1, CaMSI3, EFG1, NRG1, and TUP1 in Candida albicans strains JCM9061 and CAI4 under several conditions, including induction of morphological transition, heat shock, and treatment with calcium inhibitors. Expression of CaMSI3 changed under these growth conditions except during heat shock. CGR1 expression increased during the early stages of hyphal growth in JCM9061, while expression was strain-dependent during heat shock. Both EFG1 and NRG1 were similarly expressed under hypha-inducing conditions and heat shock. Expression of TUP1 was slightly different from the expression of EFG1 or NRG1.

Cellular differentiation in response to nutrient availability: The repressor of meiosis, Rme1p, positively regulates invasive growth in Saccharomyces cerevisiae

In the yeast Saccharomyces cerevisiae, the transition from a nutrient-rich to a nutrient-limited growth medium typically leads to the implementation of a cellular adaptation program that results in invasive growth and/or the formation of pseudohyphae. Complete depletion of essential nutrients, on the other hand, leads either to entry into a nonbudding, metabolically quiescent state referred to as G0 in haploid strains or to meiosis and sporulation in diploids. Entry into meiosis is repressed by the transcriptional regulator Rme1p, a zinc-finger-containing DNA-binding protein. In this article, we show that Rme1p positively regulates invasive growth and starch metabolism in both haploid and diploid strains by directly modifying the transcription of the FLO11 (also known as MUC1) and STA2 genes, which encode a cell wall-associated protein essential for invasive growth and a starch-degrading glucoamylase, respectively. Genetic evidence suggests that Rme1p functions independently of identified signaling modules that regulate invasive growth and of other transcription factors that regulate FLO11 and that the activation of FLO11 is dependent on the presence of a promoter sequence that shows significant homology to identified Rme1p response elements (RREs). The data suggest that Rme1p functions as a central switch between different cellular differentiation pathways.

Human enhancer of invasion-cluster, a coiled-coil protein required for passage through mitosis

In a cross-species overexpression approach, we used the pseudohyphal transition of Saccharomyces cerevisiae as a model screening system to identify human genes that regulate cell morphology and the cell cycle. Human enhancer of invasion-cluster (HEI-C), encoding a novel evolutionarily conserved coiled-coil protein, was isolated in a screen for human genes that induce agar invasion in S. cerevisiae. In human cells, HEI-C is primarily localized to the spindle during mitosis. Depletion of HEI-C in vivo with short interfering RNAs results in severe mitotic defects. Analysis by immunofluorescence, flow cytometry analysis, and videomicroscopy indicates that HEI-C-depleted cells form metaphase plates with normal timing after G(2)/M transition, although in many cases cells have disorganized mitotic spindles. Subsequently, severe defects occur at the metaphase-anaphase transition, characterized by a significant delay at this stage or, more commonly, cellular disintegration accompanied by the display of classic biochemical markers of apoptosis. These mitotic defects occur in spite of the fact that HEI-C-depleted cells retain functional cell cycle checkpoints, as these cells arrest normally following nocodazole or hydroxyurea treatment. These results place HEI-C as a novel regulator of spindle function and integrity during the metaphase-anaphase transition.

In Candida albicans, the Nim1 kinases Gin4 and Hsl1 negatively regulate pseudohypha formation and Gin4 also controls septin organization

In the development of hyphal germ tubes of Candida albicans, a band of septin forms at the base of the germ tube (basal septin band). Later, a septin ring forms, which organizes the first septum within the germ tube (septin ring). We have investigated the role of the Nim1 kinases, Gin4 and Hsl1, in the formation of these septin structures. We show that during germ tube formation, Gin4 is required for the organization of the septin ring but not the basal septin band. Hsl1 is not required for the formation of either septin rings or basal bands. Unexpectedly, we found that both gin4Δ and hsl1Δ mutants form pseudohyphae constitutively, in a fashion that in the case of gin4Δ, is partly independent of Swe1. Gin4-depleted pseudohyphae are unable to form hyphae when challenged with serum, but this can be overcome by ectopic expression of Gin4 from the MET3 promoter. Thus, Gin4 may regulate the developmental switch from pseudohyphae to hyphae.

Control of nutrient-sensitive transcription programs by the unconventional prefoldin URI

Prefoldins (PFDs) are members of a recently identified, small-molecular weight protein family able to assemble into molecular chaperone complexes. Here we describe an unusually large member of this family, termed URI, that forms complexes with other small-molecular weight PFDs and with RPB5, a shared subunit of all three RNA polymerases. Functional analysis of the yeast and human orthologs of URI revealed that both are targets of nutrient signaling and participate in gene expression controlled by the TOR kinase. Thus, URI is a component of a signaling pathway that coordinates nutrient availability with gene expression.

Asynchronous cell cycle and asymmetric vacuolar inheritance in true hyphae of Candida albicans

Candida albicans forms unconstricted hyphae in serum-containing medium that are divided into discrete compartments. Time-lapse photomicroscopy, flow cytometry, and a novel three-dimensional imaging system were used to demonstrate that the kinetics and cell cycle events accompanying hyphal development were correlated with dynamic changes in vacuole morphology and the pattern of vacuole inheritance. Apical cells of hyphae underwent continuous extension before and after the first cytokinesis event. However, the resulting mother cell and sub-apical compartments did not immediately reenter the cell cycle and instead underwent cell cycle arrest before reentering the cycle. Vacuole was inherited asymmetrically at cytokinesis so that the distal, arrested compartments inherited most vacuole and the growing apical cell inherited most cytoplasm. Hydroxyurea release experiments demonstrated that the arrested, vacuolated hyphal compartments were in the G(1) phase of the cycle. The period of cell cycle arrest was decreased by the provision of assimilatable forms of nitrogen, suggesting that the hyphal cell cycle is regulated by nitrogen limitation that results in sup-apical cell cycle arrest. This pattern of growth is distinct from that of the synchronous, symmetrical development of pseudohyphae of C. albicans and other yeast species. These observations suggest that the cellular vacuole space correlates with alterations in the cell cycles of different cell types and that the total organelle space may influence size-regulated functions and hence the timing of the eukaryotic cell cycle.

Depletion of a polo-like kinase in Candida albicans activates cyclase-dependent hyphal-like growth

Morphogenesis in the fungal pathogen Candida albicans is an important virulence-determining factor, as a dimorphic switch between yeast and hyphal growth forms can increase pathogenesis. We identified CaCDC5, a cell cycle regulatory polo-like kinase (PLK) in C. albicans and demonstrate that shutting off its expression induced cell cycle defects and dramatic changes in morphology. Cells lacking CaCdc5p were blocked early in nuclear division with very short spindles and unseparated chromatin. GFP-tagged CaCdc5p localized to unseparated spindle pole bodies, the spindle, and chromatin, consistent with a role in spindle elongation at an earlier point in the cell cycle than that described for the homologue Cdc5p in yeast. Strikingly, the cell cycle defects were accompanied by the formation of hyphal-like filaments under yeast growth conditions. Filament growth was determinate, as the filaments started to die after 24 h. The filaments resembled serum-induced hyphae with respect to morphology, organization of cytoplasmic microtubules, localization of nuclei, and expression of hyphal-specific components. Filament formation required CaCDC35, but not EFG1 or CPH1. Similar defects in spindle elongation and a corresponding induction of filaments occurred when yeast cells were exposed to hydroxyurea. Because CaCdc5p does not appear to act as a direct repressor of hyphal growth, the data suggest that a target of CaCdc5p function is associated with hyphal-like development. Thus, an internal, cell cycle-related cue can activate hyphal regulatory networks in Candida.

A novel RING finger protein, human enhancer of invasion 10, alters mitotic progression through regulation of cyclin B levels

The process of cellular morphogenesis is highly conserved in eukaryotes and is dependent upon the function of proteins that are centrally involved in specification of the cell cycle. The human enhancer of invasion clone 10 (HEI10) protein was identified from a HeLa cell library based on its ability to promote yeast agar invasion and filamentation. Through two-hybrid screening, the mitotic cyclin B1 and an E2 ubiquitin-conjugating enzyme were isolated as HEI10-interacting proteins. Mutation of the HEI10 divergent RING finger motif (characteristic of E3 ubiquitin ligases) and Cdc2/cyclin binding and phosphorylation sites alter HEI10-dependent yeast phenotypes, including delay in G(2)/M transition. In vertebrates, the addition of HEI10 inhibits nuclear envelope breakdown and mitotic entry in Xenopus egg extracts. Mechanistically, HEI10 expression reduces cyclin B levels in cycling Xenopus eggs and reduces levels of the cyclin B ortholog Clb2p in yeast. HEI10 is itself a specific in vitro substrate of purified cyclin B/cdc2, with a TPVR motif as primary phosphorylation site. Finally, HEI10 is itself ubiquitinated in egg extracts and is also autoubiquitinated in vitro. These and other points lead to a model in which HEI10 defines a divergent class of E3 ubiquitin ligase, functioning in progression through G(2)/M.

A screen in Saccharomyces cerevisiae identified CaMCM1, an essential gene in Candida albicans crucial for morphogenesis

Morphogenesis in Saccharomyces cerevisiae and the pathogenic yeast Candida albicans is governed in part by the same molecular circuits. In S. cerevisiae, FLO11/MUC1 expression has been shown to be modulated by multiple signalling pathways required for pseudohyphal development. We have established a screen in S. cerevisiae to identify regulators of fungal development in C. albicans based on FLO11::lacZ expression as a reporter. This screen identified both known components of the mitogen-activated protein kinase (MAPK) cascade and the cAMP cascade that are important for hyphal development in C. albicans, as well as genes not yet known to be involved in morphogenesis. The Candida homologue of MCM1 is one of the novel factors identified in this screen as being important for morphogenesis. CaMcm1p levels do not vary significantly in different cell types and respond to an autoregulatory feedback mechanism, arguing that CaMcm1p activity is regulated by post-translational modifications. Both overexpression and repression of this essential gene led to the induction of hyphae. Moreover, we found that the expression of HWP1, a hyphae-specific gene, was induced by repression of CaMCM1. The changes in morphology and HWP1 expression were not the result of a change in expression levels of NRG1 or TUP1, known repressors of hyphal development. Thus, CaMcm1p is a component of a hitherto unknown regulatory mechanism of hyphal growth.

Ash1 protein, an asymmetrically localized transcriptional regulator, controls filamentous growth and virulence of Candida albicans

In response to a number of distinct environmental conditions, the fungal pathogen Candida albicans undergoes a morphological transition from a round, yeast form to a series of elongated, filamentous forms. This transition is believed to be critical for virulence in a mouse model of disseminated candidiasis. Here we describe the characterization of C. albicans ASH1, a gene that encodes an asymmetrically localized transcriptional regulatory protein involved in this response. We show that C. albicans ash1 mutants are defective in responding to some filament-inducing conditions. We also show that Ash1p is preferentially localized to daughter cell nuclei in the budding-yeast form of C. albicans cell growth and to the hyphal tip cells in growing filaments. Thus, Ash1p "marks" newly formed cells and presumably directs a specialized transcriptional program in these cells. Finally, we show that ASH1 is required for full virulence of C. albicans in a mouse model of disseminated candidiasis.

Hyphal tip-associated localization of Cdc42 is F-actin dependent in Candida albicans

The rho-type GTPase Cdc42 is important for the establishment and maintenance of eukaryotic cell polarity. To examine whether Cdc42 is regulated during the yeast-to-hypha transition in Candida albicans, we constructed a green fluorescence protein (GFP)-Cdc42 fusion under the ACT1 promoter and observed its localization in live C. albicans cells. As in Saccharomyces cerevisiae, GFP-Cdc42 was observed around the entire periphery of the cell. In yeast-form cells of C. albicans, it clustered to the tips and sides of small buds as well as to the mother-daughter neck region of large-budded cells. Upon hyphal induction, GFP-Cdc42 clustered to the site of hyphal evagination and remained at the tips of the hyphae. This temporal and spatial localization of Cdc42 suggests that its activity is regulated during the yeast-to-hypha transition. In addition to the accumulation at the hyphal tip, GFP-Cdc42 was also seen as a band within the hyphal tube in cells that had undergone nuclear separation. With the F-actin-assembly inhibitor latrunculin A, we found that GFP-Cdc42 accumulation at the bud site in yeast-form cells is F-actin independent, whereas GFP-Cdc42 accumulation at the hyphal tip requires F-actin. Furthermore, disruption of the F-actin cytoskeleton impaired the transcriptional induction of hypha-specific genes. Therefore, hypha formation resembles mating in Saccharomyces cerevisiae in that both require F-actin for GFP-Cdc42 localization and efficient signaling.

Depression of Saccharomyces cerevisiae invasive growth on non-glucose carbon sources requires the Snf1 kinase

Haploid Saccharomyces cerevisiae cells growing on media lacking glucose but containing high concentrations of carbon sources such as fructose, galactose, raffinose, and ethanol exhibit enhanced agar invasion. These carbon sources also promote diploid filamentous growth in response to nitrogen starvation. The enhanced invasive and filamentous growth phenotypes are suppressed by the addition of glucose to the media and require the Snf1 kinase. Mutations in the PGI1 and GND1 genes encoding carbon source utilization enzymes confer enhanced invasive growth that is unaffected by glucose but requires active Snf1. Carbon source does not modulate FLO11 flocculin expression, but enhanced polarized bud site selection is necessary for invasion on certain carbon sources. Interestingly, deletion of SNF1 blocks invasion without affecting bud site selection. Snf1 is also required for formation of spokes and hubs in multicellular mats. To examine glucose repression of invasive growth more broadly, we performed genome-wide microarray expression analysis in wild-type cells growing on glucose and galactose, and snf1 Delta cells on galactose. SNF1 probably mediates glucose repression of multiple genes potentially involved in invasive and filamentous growth. FLO11-independent cell-cell attachment, cell wall integrity, and/or polarized growth are affected by carbon source metabolism. In addition, derepression of cell cycle genes and signalling via the cAMP-PKA pathway appears to depend upon SNF1 activity during growth on galactose.

A positive feedback loop stabilizes the guanine-nucleotide exchange factor Cdc24 at sites of polarization

In Saccharomyces cerevisiae, activation of Cdc42 by its guanine-nucleotide exchange factor Cdc24 triggers polarization of the actin cytoskeleton at bud emergence and in response to mating pheromones. The adaptor protein Bem1 localizes to sites of polarized growth where it interacts with Cdc42, Cdc24 and the PAK-like kinase Cla4. We have isolated Bem1 mutants (Bem1-m), which are specifically defective for binding to Cdc24. The mutations map within the conserved PB1 domain, which is necessary and sufficient to interact with the octicos peptide repeat (OPR) motif of Cdc24. Although Bem1-m mutant proteins localize normally, bem1-m cells are unable to maintain Cdc24 at sites of polarized growth. As a consequence, they are defective for apical bud growth and the formation of mating projections. Localization of Bem1 to the incipient bud site requires activated Cdc42, and conversely, expression of Cdc42-GTP is sufficient to accumulate Bem1 at the plasma membrane. Thus, our results suggest that Bem1 functions in a positive feedback loop: local activation of Cdc24 produces Cdc42-GTP, which recruits Bem1. In turn, Bem1 stabilizes Cdc24 at the site of polarization, leading to apical growth.

Feasibility of copper uptake by the yeast Pichia guilliermondii isolated from sewage sludge

Copper is one of the most abundant toxic heavy metals in municipal wastewaters and, in consequence, in sewage sludge and compost. The ability of a strain of the yeast Pichia guilliermondii, which was isolated from sewage sludge, to eliminate copper has been evaluated, using both viable and nonviable biomass. It has been found that raising concentrations of copper affected both morphology and physiological parameters of the viable yeast, and it is thought that a process of bioaccumulation may be involved in its copper uptake. The growth rate of nonadapted cells decreased with increasing concentrations of copper, mainly due to a decrease in the biomass yield. The cells could be adapted by training with increasing copper concentrations up to 317.7 mg/l. This adaptation was an all-or-nothing process: once cells had adapted, the biomass yield, metabolic flux and consequent growth rate were constant and independent of the external copper concentration. Also, it was determined that up to 20 mg of copper per gram of viable adapted biomass could accumulate from the medium (i.e., double the amount when using nonadapted viable biomass). Finally, adsorption data on nonviable cells were found to be well modeled by the Langmuir isotherm (qmax = 9.09 mg/g of biomass).

MST12 regulates infectious growth but not appressorium formation in the rice blast fungus Magnaporthe grisea

In the rice blast fungus Magnaporthe grisea, a mitogen-activated protein kinase gene, PMK1, is known to regulate appressorium formation and infectious hyphae growth. Since PMK1 is homologous to the FUS3 and KSS1 genes that regulate the transcription factor STE12 in yeast, we functionally characterized the STE12 homologue in M. grisea (MST12). A polymerase chain reaction-based approach was used to isolate the MST12 gene that is homologous to yeast STE12. Four mst12 deletion mutants were isolated by gene replacement. No obvious defect in vegetative growth, conidiation, or conidia germination was observed in mst12 mutants. However, mst12 mutants were nonpathogenic on rice and barley leaves. In contrast to pmk1 mutants that did not form appressoria, mst12 mutants produced typical dome-shaped and melanized appressoria. However, the appressoria formed by mst12 mutants failed to penetrate onion epidermal cells. When inoculated through wound sites, mst12 mutants failed to cause spreading lesions and appeared to be defective in infectious growth. These data indicate that MST12 may function downstream of PMK1 to regulate genes involved in infectious hyphae growth. A transcription factor or factors other than MST12 must exist in M. grisea and function downstream from PMK1 for appressorium formation.

Metabolic engineering of the morphology of Aspergillus

The morphology of filamentous organisms in submerged cultivation is a subject of considerable interest, notably due to the influence of morphology on process productivity. The relationship between process parameters and morphology is complex: the interactions between process variables, productivity, rheology, and macro- and micro-morphology create difficulties in defining and separating cause and effect. Additionally, organism physiology contributes a further level of complexity which means that the desired morphology (for optimum process performance and productivity) is likely to be process specific. However, a number of studies with increasingly powerful image analysis systems have yielded valuable information on what these desirable morphologies are likely to be. In parallel, studies on a variety of morphological mutants means that information on the genes involved in morphology is beginning to emerge. Indeed, we are now beginning to understand how morphology may be controlled at the molecular level. Coupling this knowledge with the tools of molecular biology means that it is now possible to design and engineer the morphology of organisms for specific bioprocesses. Tailor making strains with defined morphologies represents a clear advantage in optimization of submerged bioprocesses with filamentous organisms.

Hyphal elongation is regulated independently of cell cycle in Candida albicans

The mechanism for apical growth during hyphal morphogenesis in Candida albicans is unknown. Studies from Saccharomyces cerevisiae indicate that cell morphogenesis may involve cell cycle regulation by cyclin-dependent kinase. To examine whether this is the mechanism for hyphal morphogenesis, the temporal appearance of different spindle pole body and spindle structures, the cell cycle-regulated rearrangements of the actin cytoskeleton, and the phosphorylation state of the conserved Tyr19 of Cdc28 during the cell cycle were compared and found to be similar between yeast and serum-induced hyphal apical cells. These data suggest that hyphal elongation is not mediated by altering cell cycle progression or through phosphorylation of Tyr19 of Cdc28. We have also shown that germ tubes can evaginate before spindle pole body duplication, chitin ring formation, and DNA replication. Similarly, tip-associated actin polarization in each hypha occurs before the events of the G(1)/S transition and persists throughout the cell cycle, whereas cell cycle-regulated actin assemblies come and go. We have also shown that cells in phases other than G(1) can be induced to form hyphae. Hyphae induced from G(1) cells have no constrictions, and the first chitin ring is positioned in the germ tube at various distances from the base. Hyphae induced from budded cells have a constriction and a chitin ring at the bud neck, beyond which the hyphae continue to elongate with no further constrictions. Our data suggest that hyphal elongation and cell cycle morphogenesis programs are uncoupled, and each contributes to different aspects of cell morphogenesis.

Enhanced cell polarity in mutants of the budding yeast cyclin-dependent kinase Cdc28p

The yeast cyclin-dependent kinase Cdc28p regulates bud morphogenesis and cell cycle progression via the antagonistic activities of Cln and Clb cyclins. Cln G1 cyclins direct polarized growth and bud emergence, whereas Clb G2 cyclins promote isotropic growth of the bud and chromosome segregation. Using colony morphology as a screen to dissect regulation of polarity by Cdc28p, we identified nine point mutations that block the apical-isotropic switch while maintaining other functions. Like a clb2 Delta mutation, each confers tubular bud shape, apically polarized actin distribution, unipolar budding, and delayed anaphase. The mutations are all suppressed by CLB2 overexpression and are synthetically lethal with a CLB2 deletion. However, defects in multiple independent pathways may underlie their common phenotype, because the mutations are scattered throughout the CDC28 sequence, complement each other, and confer diverse biochemical properties. Glu12Gly, a mutation that alters a residue involved in Swe1p inhibition of Cdc28p, was unique in being suppressed by deficiency of SWE1 or CLN1. With wild-type CDC28, filament formation induced by CLN1 overexpression was markedly decreased in a SWE1 deletion. These results suggest that Swe1p, via inhibition of Clb2p/Cdc28p, may mediate much of the effect of Cln1p on filamentous morphogenesis.

Xbp1-mediated repression of CLB gene expression contributes to the modifications of yeast cell morphology and cell cycle seen during nitrogen-limited growth

Yeast cells undergo morphological transformations in response to diverse environmental signals. One such event, called pseudohyphal differentiation, occurs when diploid yeast cells are partially starved for nitrogen on a solid agar medium. The nitrogen-starved cells elongate, and a small fraction form filaments that penetrate the agar surface. The molecular basis for the changes in cell morphology and cell cycle in response to nitrogen limitation are poorly defined, in part because the heterogeneous growth states of partially starved cells on agar media are not amenable to biochemical analysis. In this work, we used chemostat cultures to study the role of cell cycle regulators with respect to yeast differentiation in response to nitrogen limitation under controlled, homogeneous culture conditions. We found that Clb1, Clb2, and Clb5 cyclin levels are reduced in nitrogen-limited chemostat cultures compared to levels in rich-medium cultures, whereas the Xbp1 transcriptional repressor is highly induced under these conditions. Furthermore, the deletion of XBP1 prevents the drop in Clb2 levels and inhibits cellular elongation in nitrogen-limited chemostat cultures as well as inhibiting pseudohyphal growth on nitrogen-limited agar media. Deletion of the CLB2 gene restores an elongated morphology and filamentation to the xbp1Delta mutant in response to nitrogen limitation. Transcriptional activation of the XBP1 gene and the subsequent repression of CLB gene expression are thus key responses of yeast cells to nitrogen limitation.

Cell polarity and hyphal morphogenesis are controlled by multiple rho-protein modules in the filamentous ascomycete Ashbya gossypii

Polarized cell growth requires a polarized organization of the actin cytoskeleton. Small GTP-binding proteins of the Rho-family have been shown to be involved in the regulation of actin polarization as well as other processes. Hyphal growth in filamentous fungi represents an ideal model to investigate mechanisms involved in generating cell polarity and establishing polarized cell growth. Since a potential role of Rho-proteins has not been studied so far in filamentous fungi we isolated and characterized the Ashbya gossypii homologs of the Saccharomyces cerevisiae CDC42, CDC24, RHO1, and RHO3 genes. The AgCDC42 and AgCDC24 genes can both complement conditional mutations in the S. cerevisiae CDC42 and CDC24 genes and both proteins are required for the establishment of actin polarization in A. gossypii germ cells. Agrho1 mutants show a cell lysis phenotype. Null mutant strains of Agrho3 show periodic swelling of hyphal tips that is overcome by repolarization and polar hyphal growth in a manner resembling the germination pattern of spores. Thus different Rho-protein modules are required for distinct steps during polarized hyphal growth of A. gossypii.

Glucose depletion causes haploid invasive growth in yeast

Haploid yeast invades solid agar in response to nutrient limitation. To decipher the cues that underlie invasion, we have developed a single cell invasive growth assay. Using this assay, as well as the traditional plate-washing assay, we show that invasive growth occurs in response to glucose depletion. In the absence of glucose (or other fermentable sugar), individual cells adopted a nonaxial budding pattern and elongated morphology within the first cell divisions, and invasion into the agar was observed in microcolonies containing as few as 10 cells. In support of this observation, we found that glucose suppressed the hyperinvasive growth morphology of STE11-4, pbs2, hsl7, and RAS2V19 mutations. In addition, removal of glucose from YPD medium caused constitutive invasion in wild-type cells. We tested glucose control proteins for a role in invasion and found that Snf1, a protein required for derepression of glucose-repressed genes, was required for invasive growth. The transcription factor Sip4, which interacts with Snf1 and is induced during the diauxic shift, had an inhibitory role on invasive growth, suggesting that multiple mechanisms are required for glucose depletion-dependent invasion.

Genetic analysis reveals that FLO11 upregulation and cell polarization independently regulate invasive growth in Saccharomyces cerevisiae

Under inducing conditions, haploid Saccharomyces cerevisiae perform a dimorphic transition from yeast-form growth on the agar surface to invasive growth, where chains of cells dig into the solid growth medium. Previous work on signaling cascades that promote agar invasion has demonstrated upregulation of FLO11, a cell-surface flocculin involved in cell-cell adhesion. We find that increasing FLO11 transcription is sufficient to induce both invasive and filamentous growth. A genetic screen for repressors of FLO11 isolated mutant strains that dig into agar (dia) and identified mutations in 35 different genes: ELM1, HSL1, HSL7, BUD3, BUD4, BUD10, AXL1, SIR2, SIR4, BEM2, PGI1, GND1, YDJ1, ARO7, GRR1, CDC53, HSC82, ZUO1, ADH1, CSE2, GCR1, IRA1, MSN5, SRB8, SSN3, SSN8, BPL1, GTR1, MED1, SKN7, TAF25, DIA1, DIA2, DIA3, and DIA4. Indeed, agar invasion in 20 dia mutants requires upregulation of the endogenous FLO11 promoter. However, 13 mutants promote agar invasion even with FLO11 clamped at a constitutive low-expression level. These FLO11 promoter-independent dia mutants establish distinct invasive growth pathways due to polarized bud site selection and/or cell elongation. Epistasis with the STE MAP kinase cascade and cytokinesis/budding checkpoint shows these pathways are targets of DIA genes that repress agar invasion by FLO11 promoter-dependent and -independent mechanisms, respectively.