The GRR1 gene of Candida albicans is involved in the negative control of pseudohyphal morphogenesis

Stress combined with loss of the Candida albicans SUMO protease Ulp2 triggers selection of aneuploidy via a two-step process

A delicate balance between genome stability and instability ensures genome integrity while generating genetic diversity, a critical step for evolution. Indeed, while excessive genome instability is harmful, moderated genome instability can drive adaptation to novel environments by maximising genetic variation. Candida albicans, a human fungal pathogen that colonises different parts of the human body, adapts rapidly and frequently to different hostile host microenvironments. In this organism, the ability to generate large-scale genomic variation is a key adaptative mechanism triggering dangerous infections even in the presence of antifungal drugs. Understanding how fitter novel karyotypes are selected is key to determining how C. albicans and other microbial pathogens establish infections. Here, we identified the SUMO protease Ulp2 as a regulator of C. albicans genome integrity through genetic screening. Deletion of ULP2 leads to increased genome instability, enhanced genome variation and reduced fitness in the absence of additional stress. The combined stress caused by the lack of ULP2 and antifungal drug treatment leads to the selection of adaptive segmental aneuploidies that partially rescue the fitness defects of ulp2Δ/Δ cells. Short and long-read genomic sequencing demonstrates that these novel genotypes are selected via a two-step process leading to the formation of novel chromosomal fragments with breakpoints at microhomology regions and DNA repeats.

Multiple F-Box Proteins Collectively Regulate Cell Development and Pathogenesis in the Human Pathogen Cryptococcus neoformans

The ubiquitin-proteasome system (UPS) mediates intracellular proteins degradation that influences various cellular functions in eukaryotic cells. The UPS is also involved in the development and virulence of pathogenic fungi. F-box proteins, which are part of the SCF (Skp1-Cullin-F-box protein) ligase, are a key component of UPS and are essential for the recognition of specific substrates. In this study, we identified 20 F-box proteins in C. neoformans and obtained deletion mutants for 19 of them. A comprehensive phenotypic analysis of these mutants revealed the diverse function of F-box proteins in stress response, cell size regulation, sexual reproduction, antifungal drug resistance, and fungal virulence in C. neoformans. The importance of three F-box proteins: Fbp4, Fbp8, and Fbp11, in these cellular functions were characterized in detail. This study provides an overall view of the F-box gene family in C. neoformans, which will lead to a better understanding of the function of fungal SCF E3 ligase-mediated UPS in fungal development and pathogenesis.

Role of F-box Protein Cdc4 in Fungal Virulence and Sexual Reproduction of Cryptococcus neoformans

Cryptococcus neoformans is an opportunistic yeast-like pathogen that mainly infects immunocompromised individuals and causes fatal meningitis. Sexual reproduction can promote the exchange of genetic material between different strains of C. neoformans, which is one of the reasons leading to the emergence of highly pathogenic and drug-resistant strains of C. neoformans. Although much research has been done on the regulation mechanism of Cryptococcus sexual reproduction, there are few studies on the sexual reproduction regulation of Cryptococcus by the ubiquitin-proteasome system. This study identified an F-box protein, Cdc4, which contains a putative F-box domain and eight WD40 domains. The expression pattern analysis showed that the CDC4 gene was expressed in various developmental stages of C. neoformans, and the Cdc4 protein was localized in the nucleus of cryptococcal cells. In vitro stress responses assays showed that the CDC4 overexpression strains are sensitive to SDS and MMS but not Congo red, implying that Cdc4 may regulate the cell membrane integrity and repair of DNA damage of C. neoformans. Fungal virulence assay showed that although the cdc4Δ mutant grows normally and can produce typical virulence factors such as capsule and melanin, the cdc4Δ mutant completely loses its pathogenicity in a mouse systemic-infection model. Fungal mating assays showed that Cdc4 is also essential for fungal sexual reproduction in C. neoformans. Although normal mating hyphae were observed during mating, the basidiospores' production was blocked in bilateral mating between cdc4Δ mutants. Fungal nuclei development assay showed that the nuclei failed to undergo meiosis after fusion inside the basidia during the bilateral mating of cdc4Δ mutants, indicating that Cdc4 is critical to regulating meiosis during cryptococcal mating. In summary, our study revealed that the F-box protein Cdc4 is critical for fungal virulence and sexual reproduction in C. neoformans.

Filament Negative Regulator CDC4 Suppresses Glycogen Phosphorylase Encoded GPH1 that Impacts the Cell Wall-Associated Features in Candida albicans

We have previously identified Candida albicans GPH1 (orf19.7021) whose protein product was associated with C. albicans Cdc4. The GPH1 gene is a putative glycogen phosphorylase because its Saccharomyces cerevisiae homolog participates in glycogen catabolism, which involves the synthesis of β-glucan of the fungal cell wall. We made a strain whose CaCDC4 expression is repressed, and GPH1 is constitutively expressed. We established a GPH1 null mutant strain and used it to conduct the in vitro virulence assays that detect cell wall function. The in vitro virulence assay is centered on biofilm formation in which analytic procedures are implemented to evaluate cell surface hydrophobicity; competence, either in stress resistance, germ tube formation, or fibronection association; and the XTT-based adhesion and biofilm formation. We showed that the constitutively expressed GPH1 partially suppresses filamentation when the CaCDC4 expression is repressed. The C. albicans Gph1 protein is reduced in the presence of CaCdc4 in comparison with the absence of CaCdc4. Compared with the wild-type strain, the gph1Δ/gph1Δ mutant displayed a reduction in the capability to form germ tubes and the cell surface hydrophobicity but an increase in binding with fibronectin. Compared with the wild-type strain, the gph1Δ/gph1Δ mutant showed a rise in adhesion, the initial stage of biofilm formation, but displayed a similar capacity to form a mature biofilm. There was no major impact on the gph1Δ/gph1Δ mutant regarding the conditions of cell wall damaging and TOR pathway-associated nutrient depletion. We conclude that GPH1, adversely regulated by the filament suppressor CDC4, contributes to cell wall function in C. albicans.

The F-Box Protein Fbp1 Regulates Virulence of Cryptococcus neoformans Through the Putative Zinc-Binding Protein Zbp1

The ubiquitin-proteasome system (UPS) is the major protein turnover mechanism that plays an important role in regulating various cellular functions. F-box proteins are the key proteins of the UPS, responsible for the specific recognition and ubiquitination of downstream targets. Our previous studies showed that the F-box protein Fbp1 plays an essential role in the virulence of C. neoformans. However, the molecular mechanism of Fbp1 regulating the virulence of C. neoformans is still unclear. In this study, we analyzed the potential Fbp1 substrates using an iTRAQ-based proteomic approach and identified the zinc-binding protein Zbp1 as a substrate of Fbp1. Protein interaction and stability assays showed that Zbp1 interacts with Fbp1 and is a downstream target of Fbp1. Ubiquitination analysis in vivo showed that the ubiquitination of Zbp1 is dependent on Fbp1 in C. neoformans. Subcellular localization analysis revealed that the Zbp1 protein was localized in the nucleus of C. neoformans cells. In addition, both deletion and overexpression of the ZBP1 gene led to the reduced capsule size, while overexpression has a more significant impact on capsule size reduction. Fungal virulence assays showed that although the zbp1Δ mutants are virulent, virulence was significantly attenuated in the ZBP1 overexpression strains. Fungal load assay showed that the fungal burdens recovered from the mouse lungs decreased gradually after infection, while no yeast cells were recovered from the brains and spleens of the mice infected by ZBP1 overexpression strains. Thus, our results revealed a new determinant of fungal virulence involving the post-translational regulation of a zinc-binding protein.

Pathogen-Host Interaction Repertoire at Proteome and Posttranslational Modification Levels During Fungal Infections

Prevalence of fungal diseases has increased globally in recent years, which often associated with increased immunocompromised patients, aging populations, and the novel Coronavirus pandemic. Furthermore, due to the limitation of available antifungal agents mortality and morbidity rates of invasion fungal disease remain stubbornly high, and the emergence of multidrug-resistant fungi exacerbates the problem. Fungal pathogenicity and interactions between fungi and host have been the focus of many studies, as a result, lots of pathogenic mechanisms and fungal virulence factors have been identified. Mass spectrometry (MS)-based proteomics is a novel approach to better understand fungal pathogenicities and host–pathogen interactions at protein and protein posttranslational modification (PTM) levels. The approach has successfully elucidated interactions between pathogens and hosts by examining, for example, samples of fungal cells under different conditions, body fluids from infected patients, and exosomes. Many studies conclude that protein and PTM levels in both pathogens and hosts play important roles in progression of fungal diseases. This review summarizes mass spectrometry studies of protein and PTM levels from perspectives of both pathogens and hosts and provides an integrative conceptual outlook on fungal pathogenesis, antifungal agents development, and host–pathogen interactions.

A putative F-box-domain-encoding gene AOL_s00076g207 regulates the development and pathogenicity of Arthrobotrys oligospora

F-box protein is a key component of the Skp1-cullin-F-box-type ubiquitin ligase complex (SCF-ULC) that marks its target proteins with ubiquitin for proteasomal degradation. In this study, we explored the potential role of AOL_s00076g207 (Aog207) in Arthrobotrys oligospora, a model fungus for studying nematodes-fungi interactions. The Aog207 gene encodes a putative F-box protein of the SCF-ULC. Deletion of Aog207 could inhibit mycelial growth in TYGA and PDA media. More importantly, the conidial germination rate of ΔAog207 mutants was remarkably declined compared to that of wild-type (WT) strain, and the mutant strains were more sensitive toward chemical stressors than the WT strain. In addition, ΔAog207 mutants generated fewer traps and captured fewer nematodes than WT strain. In summary, Aog207 disruption significantly affected the pathogenicity, mycelial growth, conidial germination, environmental adaptation and trap formation of A. oligospora. These findings may facilitate a better understanding of the nematode predation mechanism of A. oligospora and provide an experimental basis for developing biological control agents against nematodes.

More Than Just Cleaning: Ubiquitin-Mediated Proteolysis in Fungal Pathogenesis

Ubiquitin-proteasome mediated protein turnover is an important regulatory mechanism of cellular function in eukaryotes. Extensive studies have linked the ubiquitin-proteasome system (UPS) to human diseases, and an array of proteasome inhibitors have been successfully developed for cancer therapy. Although still an emerging field, research on UPS regulation of fungal development and virulence has been rapidly advancing and has generated considerable excitement in its potential as a target for novel drugs. In this review, we summarize UPS composition and regulatory function in pathogenic fungi, especially in stress responses, host adaption, and fungal pathogenesis. Emphasis will be given to UPS regulation of pathogenic factors that are important for fungal pathogenesis. We also discuss future potential therapeutic strategies for fungal infections based on targeting UPS pathways.

Identifying Candida albicans Gene Networks Involved in Pathogenicity

Candida albicans is a normal member of the human microbiome. It is also an opportunistic pathogen, which can cause life-threatening systemic infections in severely immunocompromized individuals. Despite the availability of antifungal drugs, mortality rates of systemic infections are high and new drugs are needed to overcome therapeutic challenges including the emergence of drug resistance. Targeting known disease pathways has been suggested as a promising avenue for the development of new antifungals. However, <30% of C. albicans genes are verified with experimental evidence of a gene product, and the full complement of genes involved in important disease processes is currently unknown. Tools to predict the function of partially or uncharacterized genes and generate testable hypotheses will, therefore, help to identify potential targets for new antifungal development. Here, we employ a network-extracted ontology to leverage publicly available transcriptomics data and identify potential candidate genes involved in disease processes. A subset of these genes has been phenotypically screened using available deletion strains and we present preliminary data that one candidate, PEP8, is involved in hyphal development and immune evasion. This work demonstrates the utility of network-extracted ontologies in predicting gene function to generate testable hypotheses that can be applied to pathogenic systems. This could represent a novel first step to identifying targets for new antifungal therapies.

Candida albicans Ubiquitin and Heat Shock Factor-Type Transcriptional Factors Are Involved in 2-Dodecenoic Acid-Mediated Inhibition of Hyphal Growth

Cis-2-dodecenoic acid (i.e., Burkholderia cenocepacia Diffusible Signal Factor, BDSF), a signaling molecule produced by Burkholderia cenocepacia but not by Candida albicans, can prevent Candida albicans hyphal formation. The mechanism by which BDSF controls the morphological switch of C. albicans is still unknown. To address this issue, we used the cDNA microarray method to investigate the differential expression of genes in C. albicans in the presence and absence of BDSF. The microarray result indicated that 305 genes were significantly different in the expression level. This included the downregulation of 75 genes and the upregulation of 230 genes. Based on the microarray data, a mutant library was screened to search for genes, once mutated, conferred insensitivity to BDSF. The results showed that the repressors (Ubi4 and Sfl1 proteins) and the activator (Sfl2 protein) of filamentous growth are involved in the BDSF regulation of hyphal morphogenesis. Ubi4, an ubiquitin polypeptide that participates in ubiquitin-mediated protein turnover, is the protein required for the degradation of Sfl2. Sfl1 and Sfl2 proteins antagonistically control C. albicans morphogenesis. In the hyphal induction condition, the amount of Ubi4 and Sfl1 protein increased rapidly with the exogenous addition of BDSF. As a result, the protein level of the activator of filamentous growth, Sfl2, decreased correspondingly, thereby facilitating the C. albicans cells to remain in the yeast form.

Microbial volatile compounds-induced cytotoxicity in the yeast Saccharomyces cerevisiae: The role of MAPK signaling and proteasome regulatory pathway

Microbial volatile organic compounds (mVCs) are formed in the metabolism of microorganisms and widely distributed in nature and pose threats to human health. However, the air pollution by microorganisms is a situation which is poorly understood. In this study, the cytotoxicity of E. aerogenes VCs was evaluated in the model organism Saccharomyces cerevisiae. E. aerogenes VCs inhibited the survival of yeast and triggered the formation of intracellular reactive oxygen species (ROS). The hypersensitive of MAP kinase mpk1/slt2 and 19S regulatory assembly chaperone adc17 mutants to the E. aerogenes VCs indicated cell wall integrity (CWI) pathway together with stress-inducible proteasome assembly regulation are essentially involved in mVCs tolerance mechanism. Furthermore, exposure to the mVCs resulted in the transcriptional upregulation of the CWI pathway, the regulatory particle assembly chaperones, and genes involved in proteasome regulations. Our research suggested that the ROS/MAPK signaling and proteasome regulatory pathway play pivotal roles in the integration and fine-tuning of the mVCs stress response. This study provides a molecular framework for future study of the effects of mVCs on more complex organisms, such as humans.

THR1 mediates GCN4 and CDC4 to link morphogenesis with nutrient sensing and the stress response in Candida albicans

Candida albicans (C. albicans) CDC4 (CaCDC4), encoding the F-box protein for the substrate specificity of the Skp1-cullin-F-box E3 ubiquitin ligase complex, suppresses the yeast-to-filament transition in C. albicans. In our previous study, Thr1 was identified as a CaCdc4-associated protein using affinity purification. THR1 encodes a homoserine kinase, which is involved in the threonine biosynthesis pathway. The present study generated a strain with repressible CaCDC4 expression and continuous THR1 expression. Colony and cell morphology analyses, as well as immunoblotting, revealed that the Thr1 protein was detectable under conditions in which the expression of CaCDC4 was repressed and that the filaments resulting from the repressed expression of CaCDC4 were suppressed by the constitutive expression of THR1 in C. albicans. Additionally, by using the CaSAT1-flipper method, the present study produced null mutants of THR1, GCN4, and CaCDC4. The phenotypic consequences were evaluated by growth curves, spotting assays, microscopic analysis, reverse transcription-polymerase chain reaction and XTT-based biofilm formation ability. The results revealed that fewer cells lacking THR1 entered the stationary phase but had no apparent morphological alteration. It was observed that the expression of THR1 was upregulated concurrently with GCN4 during nutrient depletion and that cells lacking GCN4 rescued the lethality of cells in the absence of THR1 in conditions accumulating homoserine in the threonine biosynthesis pathway. Of note, it was found that cells with either CaCDC4 or THR1 loss were sensitive to oxidative stress and osmotic stress, with those with THR1 loss being more sensitive. In addition, it was observed that cells with loss of either CaCDC4 or THR1 exhibited the ability to increase biofilm formation, with those lacking CaCDC4 exhibiting a greater extent of enhancement. It was concluded that CaCDC4 is important in the coordination of morphogenesis, nutrient sensing, and the stress response through THR1 in C. albicans.

Inhibition of yeast-to-filamentous growth transitions in Candida albicans by a small molecule inducer of mammalian apoptosis

The opportunistic fungal pathogen of humans Candida albicans is able to grow in different morphological forms such as round or oval yeasts and filamentous hyphae and pseudohyphae. Morphogenesis, the ability to switch between the yeast and filamentous growth forms, is important for adapting to new microenvironments in the human host and for pathogenesis. The molecular pathways governing morphogenesis are complex and incompletely understood. Previously, we identified several small organic molecules that specifically inhibit the initiation of hyphal growth in C. albicans without affecting cell viability or budded growth. One molecule from that screen is known to induce apoptosis in mammalian cells. In this study, we have screened additional inducers of mammalian apoptosis and identified BH3I-1, as well as several structural derivatives of BH3I-1, that act as specific inhibitors of morphogenesis under a variety of environmental conditions. Chemical epistasis experiments suggest that BH3I-1 acts downstream of the hypha-specific gene regulators Rfg1, Nrg1 and Ume6.

The F-box protein Fbp1 functions in the invasive growth and cell wall integrity mitogen-activated protein kinase (MAPK) pathways in Fusarium oxysporum

F-box proteins determine substrate specificity of the ubiquitin-proteasome system. Previous work has demonstrated that the F-box protein Fbp1, a component of the SCF(Fbp1) E3 ligase complex, is essential for invasive growth and virulence of the fungal plant pathogen Fusarium oxysporum. Here, we show that, in addition to invasive growth, Fbp1 also contributes to vegetative hyphal fusion and fungal adhesion to tomato roots. All of these functions have been shown previously to require the mitogen-activated protein kinase (MAPK) Fmk1. We found that Fbp1 is required for full phosphorylation of Fmk1, indicating that Fbp1 regulates virulence and invasive growth via the Fmk1 pathway. Moreover, the Δfbp1 mutant is hypersensitive to sodium dodecylsulfate (SDS) and calcofluor white (CFW) and shows reduced phosphorylation levels of the cell wall integrity MAPK Mpk1 after SDS treatment. Collectively, these results suggest that Fbp1 contributes to both the invasive growth and cell wall integrity MAPK pathways of F. oxysporum.

Proteomic identification of potential target proteins regulated by the SCF(F) (bp1) -mediated proteolysis pathway in Fusarium oxysporum

F-box proteins function in the recruitment of proteins for SCF ubiquitination and proteasome degradation. Here, we studied the role of Fbp1, a nonessential F-box protein of the tomato pathogen Fusarium oxysporum f. sp. lycopersici. The Δfbp1 mutant showed a significant delay in the production of wilt symptoms on tomato plants and was impaired in invasive growth on cellophane membranes and on living plant tissue. To search for target proteins recruited by Fbp1, a combination of sodium dodecylsulphate-polyacrylamide gel electrophoresis (SDS-PAGE) and matrix-assisted laser desorption/ionization time-of-flight/time-of-flight (MALDI-TOF/TOF) was used to compare proteins in mycelia of the wild-type and Δfbp1 mutant. The proteomic approach identified 41 proteins differing significantly in abundance between the two strains, 17 of which were more abundant in the Δfbp1 mutant, suggesting a possible regulation by proteasome degradation. Interestingly, several of the identified proteins were related to vesicle trafficking. Microscopic analysis revealed an impairment of the Δfbp1 strain in directional growth and in the structure of the Spitzenkörper, suggesting a role of Fbp1 in hyphal orientation. Our results indicate that Fbp1 regulates protein turnover and pathogenicity in F. oxysporum.

Fbp1-mediated ubiquitin-proteasome pathway controls Cryptococcus neoformans virulence by regulating fungal intracellular growth in macrophages

Cryptococcus neoformans is a human fungal pathogen that often causes lung and brain infections in immunocompromised patients, with a high fatality rate. Our previous results showed that an F-box protein, Fbp1, is essential for Cryptococcus virulence independent of the classical virulence factors, suggesting a novel virulence control mechanism. In this study, we show that Fbp1 is part of the ubiquitin-proteasome system, and we further investigated the mechanism of Fbp1 function during infection. Time course studies revealed that the fbp1Δ mutant causes little damage in the infected lung and that the fungal burden in the lung remains at a low but persistent level throughout infection. The fbp1Δ mutant cannot disseminate to other organs following pulmonary infection in the murine inhalation model of cryptococcosis but still causes brain infection in a murine intravenous injection model, suggesting that the block of dissemination of the fbp1Δ mutant is due to its inability to leave the lung. The fbp1Δ mutant showed a defect in intracellular proliferation after phagocytosis in a Cryptococcus-macrophage interaction assay, which likely contributes to its virulence attenuation. To elucidate the molecular basis of the SCF(Fbp1) E3 ligase function, we analyzed potential Fbp1 substrates based on proteomic approaches combined with phenotypic analysis. One substrate, the inositol phosphosphingolipid-phospholipase C1 (Isc1), is required for fungal survival inside macrophage cells, which is consistent with the role of Fbp1 in regulating Cryptococcus-macrophage interaction and fungal virulence. Our results thus reveal a new determinant of fungal virulence that involves the posttranslational regulation of inositol sphingolipid biosynthesis.

Regulations of sugar transporters: insights from yeast

Transport across the plasma membrane is the first step at which nutrient supply is tightly regulated in response to intracellular needs and often also rapidly changing external environment. In this review, I describe primarily our current understanding of multiple interconnected glucose-sensing systems and signal-transduction pathways that ensure fast and optimum expression of genes encoding hexose transporters in three yeast species, Saccharomyces cerevisiae, Kluyveromyces lactis and Candida albicans. In addition, an overview of GAL- and MAL-specific regulatory networks, controlling galactose and maltose utilization, is provided. Finally, pathways generating signals inducing posttranslational degradation of sugar transporters will be highlighted.

The COP9 signalosome counteracts the accumulation of cullin SCF ubiquitin E3 RING ligases during fungal development

Defects in the COP9 signalosome (CSN) impair multicellular development, including embryonic plant or animal death or a block in sexual development of the fungus Aspergillus nidulans. CSN deneddylates cullin-RING ligases (CRLs), which are activated by covalent linkage to ubiquitin-like NEDD8. Deneddylation allows CRL disassembly for subsequent reassembly. An attractive hypothesis is a consecutive order of CRLs for development, which demands repeated cycles of neddylation and deneddylation for reassembling CRLs. Interruption of these cycles could explain developmental blocks caused by csn mutations. This predicts an accumulation of neddylated CRLs exhibiting developmental functions when CSN is dysfunctional. We tested this hypothesis in A. nidulans, which tolerates reduced levels of neddylation for growth. We show that only genes for CRL subunits or neddylation are essential, whereas CSN is primarily required for development. We used functional tagged NEDD8, recruiting all three fungal cullins. Cullins are associated with the CSN1/CsnA subunit when deneddylation is defective. Two CRLs were identified which are specifically involved in differentiation and accumulate during the developmental block. This suggests that an active CSN complex is required to counteract the accumulation of specific CRLs during development.

A MAP kinase pathway is implicated in the pseudohyphal induction by hydrogen peroxide in Candica albicans

Hydrogen peroxide (H(2)O(2)) functions as a ubiquitous intracellular messenger besides as an oxidative stress molecule. This dual role is based on the distinct cellular responses against different concentrations of H(2)O(2). Previously, we demonstrated that both low (> 1 mM) and high (4-10 mM) doses of exogenous H(2)O(2) induce filamentous growth with distinct cell morphology and growth rate in Candida albicans, suggesting the different transcription response. In this study, we revealed that the sub-toxic and toxic levels of H(2)O(2) indeed induced pseudohyphae, but not true hyphae. Supporting this, several hyphae-specific genes that are expressed in true hyphae induced by serum were not detected in either sub-toxic or toxic H(2)O(2) condition. A DNA microarray analysis was conducted to reveal the transcription profiles in cells treated with sub-toxic and toxic conditions of H(2)O(2). Under the sub-toxic condition, a small number of genes involved in cell proliferation and metabolism were up-regulated, whereas a large number of genes were up-regulated in the toxic condition where the genes required for growth and proliferation were selectively restricted. For pseudohyphal induction by sub-toxic H(2)O(2), Cek1 MAPK activating the transcription factor Cph1 was shown to be important. The absence of expression of several hyphae-specific genes known to be downstream targets of Cph1-signaling pathway for true hyphae formation suggests that the Cek1-mediated signaling pathway is not solely responsible for pseudohyphal formation by subtoxic H(2)O(2) and, but instead, complex networking pathway may exists by the activation of different regulators.

Molecular characterization of the Aspergillus nidulans fbxA encoding an F-box protein involved in xylanase induction

The filamentous fungus Aspergillus nidulans has been used as a fungal model system to study the regulation of xylanase production. These genes are activated at transcriptional level by the master regulator the transcriptional factor XlnR and repressed by carbon catabolite repression (CCR) mediated by the wide-domain repressor CreA. Here, we screened a collection of 42 A. nidulans F-box deletion mutants grown either in xylose or xylan as the single carbon source in the presence of the glucose analog 2-deoxy-D-glucose, aiming to identify mutants that have deregulated xylanase induction. We were able to recognize a null mutant in a gene (fbxA) that has decreased xylanase activity and reduced xlnA and xlnD mRNA accumulation. The ΔfbxA mutant interacts genetically with creAd-30, creB15, and creC27 mutants. FbxA is a novel protein containing a functional F-box domain that binds to Skp1 from the SCF-type ligase. Blastp analysis suggested that FbxA is a protein exclusive from fungi, without any apparent homologs in higher eukaryotes. Our work emphasizes the importance of the ubiquitination in the A. nidulans xylanase induction and CCR. The identification of FbxA provides another layer of complexity to xylanase induction and CCR phenomena in filamentous fungi.

Neddylation and CAND1 independently stimulate SCF ubiquitin ligase activity in Candida albicans

SCF (Skp1-cullin/Cdc53-F-box protein) ubiquitin ligases bind substrates via the variable F-box protein and, in conjunction with the RING domain protein Rbx1 and the ubiquitin-conjugating enzyme Ubc3/Cdc34, catalyze substrate ubiquitination. The cullin subunit can be modified covalently by conjugation of the ubiquitin-like protein Rub1/NEDD8 (neddylation) or bound noncovalently by the protein CAND1 (cullin-associated, neddylation-dissociated). Expression of the Candida albicans CAND1 gene homolog CaTIP120 in Saccharomyces cerevisiae is toxic only in the presence of CaCdc53, consistent with a specific interaction between CaTip120 and CaCdc53. To genetically analyze this system in C. albicans, we deleted the homologs of RUB1/NEDD8, TIP120/CAND1, and the deneddylase gene JAB1, and we also generated a temperature-sensitive allele of the essential CaCDC53 gene by knock-in site-directed mutagenesis. Deletion of CaRUB1 and CaTIP120 caused morphological, growth, and protein degradation phenotypes consistent with a reduction in SCF ubiquitin ligase activity. Furthermore, the double Carub1(-/-) Catip120(-/-) mutant was more defective in SCF activity than either individual deletion mutant. These results indicate that CAND1 stimulates SCF ubiquitin ligase activity and that it does so independently of neddylation. Our data do not support a role for CAND1 in the protection of either the F-box protein or cullin from degradation but are consistent with the suggested role of CAND1 in SCF complex remodeling.

The F-Box protein Fbp1 regulates sexual reproduction and virulence in Cryptococcus neoformans

Cryptococcus neoformans is the leading cause of fungal meningitis in immunocomprised populations. Although extensive studies have been conducted on signal transduction pathways important for fungal sexual reproduction and virulence, how fungal virulence is regulated during infection is still not understood. In this study, we identified the F-box protein Fbp1, which contains a putative F-box domain and 12 leucine-rich repeats (LRR). Although fbp1 mutants showed normal growth and produced normal major virulence factors, such as melanin and capsule, Fbp1 was found to be essential for fungal virulence, as fbp1 mutants were avirulent in a murine systemic-infection model. Fbp1 is also important for fungal sexual reproduction. Basidiospore production was blocked in bilateral mating between fbp1 mutants, even though normal dikaryotic hyphae were observed during mating. In vitro assays of stress responses revealed that fbp1 mutants are hypersensitive to SDS, but not calcofluor white (CFW) or Congo red, indicating that Fbp1 may regulate cell membrane integrity. Fbp1 physically interacts with Skp1 homologues in both Saccharomyces cerevisiae and C. neoformans via its F-box domain, suggesting it may function as part of an SCF (Skp1, Cullins, F-box proteins) E3 ligase. Overall, our study revealed that the F-box protein Fbp1 is essential for fungal sporulation and virulence in C. neoformans, which likely represents a conserved novel virulence control mechanism that involves the SCF E3 ubiquitin ligase-mediated proteolysis pathway.

Orthologues of the anaphase-promoting complex/cyclosome coactivators Cdc20p and Cdh1p are important for mitotic progression and morphogenesis in Candida albicans

The conserved anaphase-promoting complex/cyclosome (APC/C) system mediates protein degradation during mitotic progression. Conserved coactivators Cdc20p and Cdh1p regulate the APC/C during early to late mitosis and G(1) phase. Candida albicans is an important fungal pathogen of humans, and it forms highly polarized cells when mitosis is blocked through depletion of the polo-like kinase Cdc5p or other treatments. However, the mechanisms governing mitotic progression and associated polarized growth in the pathogen are poorly understood. In order to gain insights into these processes, we characterized C. albicans orthologues of Cdc20p and Cdh1p. Cdc20p-depleted cells were blocked in early or late mitosis with elevated levels of Cdc5p and the mitotic cyclin Clb2p, suggesting that Cdc20p is essential and has some conserved functions during mitosis. However, the yeast cells formed highly polarized buds in contrast to the large doublets of S. cerevisiae cdc20 mutants, implying a distinct role in morphogenesis. In comparison, cdh1Δ/cdh1Δ cells were viable but showed enrichment of Clb2p and Cdc5p, suggesting that Cdh1p may influence mitotic exit. The cdh1Δ/cdh1Δ phenotype was pleiotropic, consisting of normal or enlarged yeast, pseudohyphae, and some elongated buds, whereas S. cerevisiae cdh1Δ yeast cells were reduced in size. Thus, C. albicans Cdh1p may have some distinct functions. Finally, absence of Cdh1p or Cdc20p had a minor or no effect on hyphal development, respectively. Overall, the results suggest that Cdc20p and Cdh1p may be APC/C activators that are important for mitosis but also morphogenesis in C. albicans. Their novel features imply additional variations in function and underscore rewiring in the emerging mitotic regulatory networks of the pathogen.

G1/S transcription factor orthologues Swi4p and Swi6p are important but not essential for cell proliferation and influence hyphal development in the fungal pathogen Candida albicans

The G(1)/S transition is a critical control point for cell proliferation and involves essential transcription complexes termed SBF and MBF in Saccharomyces cerevisiae or MBF in Schizosaccharomyces pombe. In the fungal pathogen Candida albicans, G(1)/S regulation is not clear. To gain more insight into the G(1)/S circuitry, we characterized Swi6p, Swi4p and Mbp1p, the closest orthologues of SBF (Swi6p and Swi4p) and MBF (Swi6p and Mbp1p) components in S. cerevisiae. The mbp1Δ/Δ cells showed minor growth defects, whereas swi4Δ/Δ and swi6Δ/Δ yeast cells dramatically increased in size, suggesting a G(1) phase delay. Gene set enrichment analysis (GSEA) of transcription profiles revealed that genes associated with G(1)/S phase were significantly enriched in cells lacking Swi4p and Swi6p. These expression patterns suggested that Swi4p and Swi6p have repressing as well as activating activity. Intriguingly, swi4Δ/Δ swi6Δ/Δ and swi4Δ/Δ mbp1Δ/Δ strains were viable, in contrast to the situation in S. cerevisiae, and showed pleiotropic phenotypes that included multibudded yeast, pseudohyphae, and intriguingly, true hyphae. Consistently, GSEA identified strong enrichment of genes that are normally modulated during C. albicans-host cell interactions. Since Swi4p and Swi6p influence G(1) phase progression and SBF binding sites are lacking in the C. albicans genome, these factors may contribute to MBF activity. Overall, the data suggest that the putative G(1)/S regulatory machinery of C. albicans contains novel features and underscore the existence of a relationship between G(1) phase and morphogenetic switching, including hyphal development, in the pathogen.

Affinity purification of Candida albicans CaCdc4-associated proteins reveals the presence of novel proteins involved in morphogenesis

Candida albicans CDC4 is nonessential and plays a role in suppressing filamentous growth, in contrast to its evolutionary counterparts involved in the G1-S transition of the cell cycle. Genetic epistasis analysis has indicated that proteins besides Sol1 are targets of C. albicans Cdc4. Moreover, no formal evidence suggests that C. albicans Cdc4 functions through the ubiquitin E3 ligase of the Skp1-Cul1/Cdc53-F-box complex. To elucidate the role of C. albicans CDC4, C. albicans Cdc4-associated proteins were sought by affinity purification. A 6xHis epitope-tagged C. albicans Cdc4 expressed from Escherichia coli was used in affinity purifications with the cell lysate of C. albicans cdc4 homozygous null mutant. Candida albicans Cdc4 and its associated proteins were resolved by SDS-PAGE and visualized by silver staining. The candidate proteins were recovered and trypsin-digested to generate MALDI-TOF spectra profiles, which were used to search against those of known proteins in the database to reveal their identities. Two out of four proteins encoded by GPH1 and THR1 genes were further verified to interact with C. albicans Cdc4 using a yeast two-hybrid assay. We conclude that in vitro affinity purification using C. albicans Cdc4 generated from E. coli as the bait and proteins from cell lysate of C. albicans cdc4 homozygous null mutant as a source of prey permit the identification of novel proteins that physically interact and functionally associate with C. albicans Cdc4.

Impaired colonization and infection of tomato roots by the Deltafrp1 mutant of Fusarium oxysporum correlates with reduced CWDE gene expression

The vascular wilt pathogen Fusarium oxysporum f. sp. lycopersici efficiently invades roots and colonizes vascular tissues of its host tomato. For these processes, the F-box protein Frp1 is required. The Fusarium oxysporum Deltafrp1 mutant was characterized in detail to uncover the cause of its colonization defect. Using growth assays, we could attribute poor root colonization to reduced assimilation of organic acids, amino acids (except proline), or polysaccharides, singly or in combination. External root colonization by the Deltafrp1 mutant is restored by the addition of 0.1% glucose or proline but infection still does not occur. This is due to the inability of the Deltafrp1 mutant to penetrate the roots, as demonstrated by the lack of expression of SIX1 in the Deltafrp1 strain, which is a gene exclusively expressed inside roots, and loss of cell wall-degrading enzyme (CWDE) gene expression. Many of the metabolic defects of the Deltafrp1 strain can be attributed to reduced expression of the ICL1 (isocitrate lyase) gene. Strikingly, an Deltaicl1 mutant is still fully pathogenic and capable of external root colonization. We conclude that the inability of the Deltafrp1 strain to colonize and invade roots is not primarily due to metabolic defects but can be attributed to reduced expression of several CWDE genes.

Multiple functions of DOA1 in Candida albicans

While searching for regulators of virulence attributes of the human-pathogenic fungus Candida albicans, a gene was identified similar to the genes encoding the mammalian phospholipase A2-activating protein (PLAP) and the Saccharomyces cerevisiae protein Doa1, which is known to play a key role during ubiquitin (Ub)-dependent protein degradation. All three proteins contain WD-repeats. Both PLAP and CaDoa1 contain a mellitin-like sequence with a central 'KVL'. This mellitin-like sequence was shown to be necessary for full function of CaDoa1. CaDOA1 was expressed under all conditions investigated. Gene disruption of CaDOA1 caused phenotypes including modified colony morphologies, temperature sensitivity, reduced secretion of hydrolytic enzymes and hypersensitivity to various compounds such as propranolol, butanol, caffeine, chelators, azoles, nocodazole and cadmium. Strikingly, mutants lacking DOA1 were filamentous and grew as pseudohyphae and true hyphae under conditions that normally support yeast growth. Transcriptional profiling of Deltadoa1 indicated that several genes associated with Ub-mediated proteolysis, including CDC48 and UBI4, are upregulated. These data suggest that DOA1 of C. albicans, like its orthologue in S. cerevisiae, is associated with Ub-mediated proteolysis and has multiple functions. However, some functions of CaDoa1 seem to be unique for C. albicans. These results support the hypothesis that Ub-mediated proteolysis plays an important role in the regulation of morphology in C. albicans.

Regulation of sugar transport and metabolism by the Candida albicans Rgt1 transcriptional repressor

The ability of the fungal pathogen Candida albicans to cause systemic infections depends in part on the function of Hgt4, a cell surface sugar sensor. The orthologues of Hgt4 in Saccharomyces cerevisiae, Snf3 and Rgt2, initiate a signalling cascade that inactivates Rgt1, a transcriptional repressor of genes encoding hexose transporters. To determine whether Hgt4 functions similarly through the C. albicans orthologue of Rgt1, we analysed Cargt1 deletion mutants. We found that Cargt1 mutants are sensitive to the glucose analogue 2-deoxyglucose, a phenotype probably due to uncontrolled expression of genes encoding glucose transporters. Indeed, transcriptional profiling revealed that expression of about two dozen genes, including multiple HGT genes encoding hexose transporters, is increased in the Cargt1 mutant in the absence of sugars, suggesting that CaRgt1 represses expression of several HGT genes under this condition. Some of the HGT genes (probably encoding high-affinity transporters) are also repressed by high levels of glucose, and we show that this repression is mediated by CaMig1, the orthologue of the major glucose-activated repressor in S. cerevisiae, but not by its paralogue CaMig2. Therefore, CaRgt1 and CaMig1 collaborate to control expression of C. albicans hexose transporters in response to different levels of sugars. We were surprised to find that CaRgt1 also regulates expression of GAL1, suggesting that regulation of galactose metabolism in C. albicans is unconventional. Finally, Cargt1 mutations cause cells to hyperfilament, and suppress the hypofilamented phenotype of an hgt4 mutant, indicating that the Hgt4 glucose sensor may affect filamentation by modulating sugar import and metabolism via CaRgt1.

The F-box protein Grr1 regulates the stability of Ccn1, Cln3 and Hof1 and cell morphogenesis in Candida albicans

Both G1 and mitotic cyclins have been implicated in regulating Candida albicans filamentous growth. We have investigated the functions of Grr1 whose orthologue in Saccharomyces cerevisiae is known to mediate ubiquitin-dependent degradation of the G1 cyclins Cln1 and Cln2. Here, we report that deleting C. albicans GRR1 causes significant stabilization of two G1 cyclins Ccn1 and Cln3 and pseudohyphal growth. grr1Delta cells are highly heterogeneous in length and many of them fail to separate after cytokinesis. Interestingly, some isolated rod-like G1 cells of similar sizes are present in the grr1Delta culture. Time-lapse microscopy revealed that the rod-shaped G1 cells first grew exclusively in width before budding and then the bud grew exclusively by apical extension until after cytokinesis, yielding rod-like daughter cells. Consistently, actin patches persistently localize to the bud tip until around the time of cytokinesis. Despite the pseudohyphal phenotype, grr1Delta cells respond normally to hyphal induction. Hyperphosphorylated Cln3 isoforms accumulate in grr1Delta cells, indicating that Grr1 selectively mediates their degradation in wild-type cells. grr1Delta pseudohyphal growth requires neither Hgc1 nor Swel, two important regulators of cell morphogenesis. Furthermore, the cellular level of Hof1, a protein having a role in cytokinesis, is also significantly increased in grr1Delta cells.

Temporal and spatial control of HGC1 expression results in Hgc1 localization to the apical cells of hyphae in Candida albicans

The human fungal pathogen Candida albicans can undergo a morphological transition from a unicellular yeast growth form to a multicellular hyphal growth form. During hyphal growth, cell division is asymmetric. Only the apical cell divides, whereas subapical cells remain in G(1), and cell surface growth is highly restricted to the tip of the apical cell. Hgc1, a hypha-specific, G(1) cyclin-like protein, is essential for hyphal development. Here, we report, using indirect immunofluorescence, that Hgc1 is preferentially localized to the dividing apical cells of hyphae. Hgc1 protein is rapidly degraded in a cell cycle-independent manner, and the protein turnover likely occurs in both the apical and the subapical cells of hyphae. In addition to rapid protein turnover, the HGC1 transcript is also dynamically regulated during cell cycle progression in hyphal growth. It is induced upon germ tube formation in early G(1); the transcript level is reduced during the G(1)/S transition and peaks again around the G(2)/M phase in the subsequent cell cycles. Transcription from the HGC1 promoter is essential for its apical cell localization, as Hgc1 no longer exhibits preferential apical localization when expressed under the MAL2 promoter. Using fluorescence in situ hybridization, the HGC1 transcript is detected only in the apical cells of hyphae, suggesting that HGC1 is transcribed in the apical cell. Therefore, the preferential localization of Hgc1 to the apical cells of hyphae results from the dynamic temporal and spatial control of HGC1 expression.

Morphogenesis and cell cycle progression in Candida albicans

Candida albicans, an opportunistic human pathogen, displays three modes of growth: yeast, pseudohyphae and true hyphae, all of which differ both in morphology and in aspects of cell cycle progression. In particular, in hyphal cells, polarized growth becomes uncoupled from other cell cycle events. Yeast or pseudohyphae that undergo a cell cycle delay also exhibit polarized growth, independent of cell cycle progression. The Spitzenkörper, an organelle composed of vesicles associated with hyphal tips, directs continuous hyphal elongation in filamentous fungal species and also in C. albicans hyphae. A polarisome mediates cell cycle dependent growth in yeast and pseudohyphae. Regulation of morphogenesis and cell cycle progression is dependent upon specific cyclins, all of which affect morphogenesis and some of which function specifically in yeast or hyphal cells. Future work will probably focus on the cell cycle checkpoints involved in connecting morphogenesis to cell cycle progression.