Multiple functions of DOA1 in Candida albicans

Degradation of α-Subunits, Doa1 and Doa4, are Critical for Growth, Development, Programmed Cell Death Events, Stress Responses, and Pathogenicity in the Watermelon Fusarium Wilt Fungus Fusarium oxysporum f. sp. niveum

The ubiquitin-proteasome system (UPS) regulates protein quality or control and plays essential roles in several biological and biochemical processes in fungi. Here, we present the characterization of two UPS components, FonDoa1 and FonDoa4, in watermelon Fusarium wilt fungus, Fusarium oxysporum f. sp. niveum (Fon), and their biological functions. FonDoa1 localizes in both the nucleus and cytoplasm, while FonDoa4 is predominantly present in the cytoplasm. Both genes show higher expression in germinating macroconidia at 12 h. Deletion of FonDoa1 or FonDoa4 affects vegetative growth, conidiation, conidial germination/morphology, apoptosis, and responses to environmental stressors. FonDoa1, but not FonDoa4, positively regulates autophagy. The targeted disruption mutants exhibit significantly attenuated pathogenicity on watermelon due to defects in the infection process and invasive fungal growth. Further results indicate that the WD40, PFU, and PUL domains are essential for the function of FonDoa1 in Fon pathogenicity and environmental stress responses. These findings demonstrate the previously uncharacterized biological functions of FonDoa1 and FonDoa4 in phytopathogenic fungi, providing potential targets for developing strategies to control watermelon Fusarium wilt.

The Antifungal and Antibiofilm Activities of Caffeine against Candida albicans on Polymethyl Methacrylate Denture Base Material

Background: In this study, the effect of pure caffeine was established against Candida albicans (C. albicans) using different microbiological techniques. Methods: Broth microdilution and colony forming units (CFUs) assays were used to detect the minimum inhibitory concentration (MIC) and minimum fungicidal concentration (MFC). The Live/Dead fluorescent dyes were implemented to determine the yeast viability. Polymethyl methacrylate acrylic resin (PMMA) discs were prepared to evaluate caffeine’s effects against adherent C. albicans using microplate reader, CFUs, and scanning electron microscope (SEM). Results: caffeine’s MIC was detected around 30 mg/mL, while the MFC was considered at 60 mg/mL. In an agar-well diffusion test, the inhibition zones were wider in caffeine groups. The Live/Dead viability test verified caffeine’s antifungal effects. The optical density of the adherent C. albicans on PMMA discs were lower at 620 nm or 410 nm in caffeine groups. CFU count was also reduced by caffeine treatments. SEM revealed the lower adherent C. albicans count in caffeine groups. The effect of caffeine was dose-dependent at which the 60 mg/mL dose demonstrated the most prominent effect. Conclusion: The study reinforced caffeine’s antifungal and antibiofilm properties and suggested it as an additive, or even an alternative, disinfectant solution for fungal biofilms on denture surfaces.

DNA damage checkpoint and repair: From the budding yeast Saccharomyces cerevisiae to the pathogenic fungus Candida albicans

Cells are constantly challenged by internal or external genotoxic assaults, which may induce a high frequency of DNA lesions, leading to genome instability. Accumulation of damaged DNA is severe or even lethal to cells and can result in abnormal proliferation that can cause cancer in multicellular organisms, aging or cell death. Eukaryotic cells have evolved a comprehensive defence system termed the DNA damage response (DDR) to monitor and remove lesions in their DNA. The DDR has been extensively studied in the budding yeast Saccharomyces cerevisiae. Emerging evidence indicates that DDR genes in the pathogenic fungus Candida albicans show functional consistency with their orthologs in S. cerevisiae, but may act through distinct mechanisms. In particular, the DDR in C. albicans appears critical for resisting DNA damage stress induced by reactive oxygen species (ROS) produced from immune cells, and this plays a vital role in pathogenicity. Therefore, DDR genes could be considered as potential targets for clinical therapies. This review summarizes the identified DNA damage checkpoint and repair genes in C. albicans based on their orthologs in S. cerevisiae, and discusses their contribution to pathogenicity in C. albicans.

Emerging roles of Wss1 in the survival of Candida albicans under genotoxic stresses

This perspective aims to discuss the potential physiological roles and regulation mechanisms of the recently identified Candida albicans Wss1 protease important in DNA-protein crosslink (DPC) tolerance and repair. DPC is a bulky DNA lesion that blocks essential DNA transactions; thus, it poses a significant threat to genome integrity if left unrepaired. Discoveries of Wss1 in Saccharomyces cerevisiae and SPRTN in human as DPC proteases have demonstrated the importance of protease function in DPC repair. Our recent study revealed that Wss1 in C. albicans, an opportunistic pathogen that can cause life-threatening infection in immunocompromised individuals, also promotes DPC tolerance similarly to both S. cerevisiae Wss1 and human SPRTN. However, its molecular mechanism and regulation are still poorly understood. Here, we briefly discuss the recent insights into C. albicans Wss1 based on the information from S. cerevisiae, as well as outline the aspect of this protein that could make it a potential target for antifungal drug development.

Wss1 homolog from Candida albicans and its role in DNA-protein crosslink tolerance

Candida albicans is an opportunistic yeast that can cause life-threatening systemic infection in immunocompromised individuals. During infections, C. albicans has to cope with genotoxic stresses generated by the host immune system. DNA-protein crosslink (DPC), the covalent linkage of proteins with DNA, is one type of DNA damages that can be caused by the host immune response. DPCs are bulky lesions that interfere with the progression of replication and transcription machineries, and hence threaten genomic integrity. Accordingly, either a DPC tolerance mechanism or a DPC repair pathway is essential for C. albicans to maintain genomic stability and survive in the host. Here, we identified Wss1 (weak suppressor of Smt3) in C. albicans (CaWss1) using bioinformatics, genetic complementation, and biochemical studies. We showed that CaWss1 promotes cell survival under genotoxic stress conditions that generate DPCs and that the catalytic metalloprotease domain of CaWss1 is essential for its cellular function. Interactions of CaWss1 with Cdc48 and small ubiquitin-like modifier, although not strictly required, contribute to the function of CaWss1 in the suppression of the growth defects under DPC-inducing conditions. This report is the first investigation of the role of CaWss1 in DPC tolerance in C. albicans.

Genome Wide Analysis of WD40 Proteins in Saccharomyces cerevisiae and Their Orthologs in Candida albicans

The WD40 domain containing proteins are present in the lower organisms (Monera) to higher complex metazoans with involvement in diverse cellular processes. The WD40 repeats fold into β propeller structure due to which the proteins harbouring WD40 domains function as scaffold by offering platform for interactions, bring together diverse cellular proteins to form a single complex for mediating downstream effects. Multiple functions of WD40 domain containing proteins in lower eukaryote as in Fungi have been reported with involvement in vegetative and reproductive growth, virulence etc. In this article insilico analysis of the WDR proteins in the budding yeast Saccharomyces cerevisiae was performed. By WDSP software 83 proteins in S. cerevisiae were identified with at least one WD40 motif. WD40 proteins with 6 or more WD40 motifs were considered for further studies. The WD40 proteins in yeast which are involved in various biological processes show distribution on all chromosomes (16 chromosomes in yeast) except chromosome 1. Besides the WD40 domain some of these proteins also contain other protein domains which might be responsible for the diversity in the functions of WD40 proteins in the budding yeast. These proteins in budding yeast were analysed by DAVID and Blast2Go software for functional and domains categorization. Candida albicans, an opportunistic fungal pathogen also have orthologs of these WD40 proteins with possible similar functions. This is the first time genome wide analysis of WD40 proteins in lower eukaryote i.e. budding yeast. This data may be useful in further study of the functional diversity of yeast proteomes.

Molecular determinants of the interaction between Doa1 and Hse1 involved in endosomal sorting

Yeast Doa1/Ufd3 is an adaptor protein for Cdc48 (p97 in mammal), an AAA type ATPase associated with endoplasmic reticulum-associated protein degradation pathway and endosomal sorting into multivesicular bodies. Doa1 functions in the endosomal sorting by its association with Hse1, a component of endosomal sorting complex required for transport (ESCRT) system. The association of Doa1 with Hse1 was previously reported to be mediated between PFU domain of Doa1 and SH3 of Hse1. However, it remains unclear which residues are specifically involved in the interaction. Here we report that Doa1/PFU interacts with Hse1/SH3 with a moderate affinity of 5 μM. Asn-438 of Doa1/PFU and Trp-254 of Hse1/SH3 are found to be critical in the interaction while Phe-434, implicated in ubiquitin binding via a hydrophobic interaction, is not. Small-angle X-ray scattering measurements combined with molecular docking and biochemical analysis yield the solution structure of the Doa1/PFU:Hse1/SH3 complex. Taken together, our results suggest that hydrogen bonding is a major determinant in the interaction of Doa1/PFU with Hse1/SH3.

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.

The MP65 gene is required for cell wall integrity, adherence to epithelial cells and biofilm formation in Candida albicans

Background

The MP65 gene of Candida albicans (orf19.1779) encodes a putative β-glucanase mannoprotein of 65 kDa, which plays a main role in a host-fungus relationship, morphogenesis and pathogenicity. In this study, we performed an extensive analysis of a mp65Δ mutant to assess the role of this protein in cell wall integrity, adherence to epithelial cells and biofilm formation.

Results

The mp65Δ mutant showed a high sensitivity to a range of cell wall-perturbing and degrading agents, especially Congo red, which induced morphological changes such as swelling, clumping and formation of hyphae. The mp65Δ mutant showed an activation of two MAPKs (Mkc1p and Cek1p), a high level of expression of two stress-related genes (DDR48 and SOD5), and a modulated expression of β-glucan epitopes, but no gross changes in cell wall polysaccharide composition. Interestingly, the mp65Δ mutant displayed a marked reduction in adhesion to BEC and Caco-2 cells and severe defects in biofilm formation when compared to the wild type. All of the mentioned properties were totally or partially recovered in a revertant strain, demonstrating the specificity of gene deletion.

Conclusions

We demonstrate that the MP65 gene of Candida albicans plays a significant role in maintaining cell wall integrity, as well as in adherence to epithelia and biofilm formation, which are major virulence attributes of this fungus.

Molecular and proteomic analyses highlight the importance of ubiquitination for the stress resistance, metabolic adaptation, morphogenetic regulation and virulence of Candida albicans

Post-translational modifications of proteins play key roles in eukaryotic growth, differentiation and environmental adaptation. In model systems the ubiquitination of specific proteins contributes to the control of cell cycle progression, stress adaptation and metabolic reprogramming. We have combined molecular, cellular and proteomic approaches to examine the roles of ubiquitination in Candida albicans, because little is known about ubiquitination in this major fungal pathogen of humans. Independent null (ubi4/ubi4) and conditional (MET3p-UBI4/ubi4) mutations were constructed at the C. albicans polyubiquitin-encoding locus. These mutants displayed morphological and cell cycle defects, as well as sensitivity to thermal, oxidative and cell wall stresses. Furthermore, ubi4/ubi4 cells rapidly lost viability under starvation conditions. Consistent with these phenotypes, proteins with roles in stress responses (Gnd1, Pst2, Ssb1), metabolism (Acs2, Eno1, Fba1, Gpd2, Pdx3, Pgk1, Tkl1) and ubiquitination (Ubi4, Ubi3, Pre1, Pre3, Rpt5) were among the ubiquitination targets we identified, further indicating that ubiquitination plays key roles in growth, stress responses and metabolic adaptation in C. albicans. Clearly ubiquitination plays key roles in the regulation of fundamental cellular processes that underpin the pathogenicity of this medically important fungus. This was confirmed by the observation that the virulence of C. albicans ubi4/ubi4 cells is significantly attenuated.

Structure and function of the PLAA/Ufd3-p97/Cdc48 complex

PLAA (ortholog of yeast Doa1/Ufd3, also know as human PLAP or phospholipase A2-activating protein) has been implicated in a variety of disparate biological processes that involve the ubiquitin system. It is linked to the maintenance of ubiquitin levels, but the mechanism by which it accomplishes this is unclear. The C-terminal PUL (PLAP, Ufd3p, and Lub1p) domain of PLAA binds p97, an AAA ATPase, which among other functions helps transfer ubiquitinated proteins to the proteasome for degradation. In yeast, loss of Doa1 is suppressed by altering p97/Cdc48 function indicating that physical interaction between PLAA and p97 is functionally important. Although the overall regions of interaction between these proteins are known, the structural basis has been unavailable. We solved the high resolution crystal structure of the p97-PLAA complex showing that the PUL domain forms a 6-mer Armadillo-containing domain. Its N-terminal extension folds back onto the inner curvature forming a deep ridge that is positively charged with residues that are phylogenetically conserved. The C terminus of p97 binds in this ridge, where the side chain of p97-Tyr⁸⁰⁵, implicated in phosphorylation-dependent regulation, is buried. Expressed in doa1Δ null cells, point mutants of the yeast ortholog Doa1 that disrupt this interaction display slightly reduced ubiquitin levels, but unlike doa1Δ null cells, showed only some of the growth phenotypes. These data suggest that the p97-PLAA interaction is important for a subset of PLAA-dependent biological processes and provides a framework to better understand the role of these complex molecules in the ubiquitin system.

DOA1/UFD3 plays a role in sorting ubiquitinated membrane proteins into multivesicular bodies

Ubiquitin (Ub) is a sorting signal that targets integral membrane proteins to the interior of the vacuole/lysosome by directing them into lumenal vesicles of multivesicular bodies (MVBs). The Vps27-Hse1 complex, which is homologous to the Hrs-STAM complex in mammalian cells, serves as a Ub-sorting receptor at the surface of early endosomes. We have found that Hse1 interacts with Doa1/Ufd3. Doa1 is known to interact with Cdc48/p97 and Ub and is required for maintaining Ub levels. We find that the Hse1 Src homology 3 domain binds directly to the central PFU domain of Doa1. Mutations in Doa1 that block Hse1 binding but not Ub binding do not alter Ub levels but do result in the missorting of the MVB cargo GFP-Cps1. Loss of Doa1 also causes a synthetic growth defect when combined with loss of Vps27. Unlike the loss of Doa1 alone, the doa1Delta vps27Delta double mutant phenotype is not suppressed by Ub overexpression, demonstrating that the effect is not due to indirect consequence of lowered Ub levels. Loss of Doa1 results in a defect in the accumulation of GFP-Ub within yeast vacuoles, implying that there is a reduction in the flux of ubiquitinated membrane proteins through the MVB pathway. This defect was also reflected by an inability to properly sort Vph1-GFP-Ub, a modified subunit of the multiprotein vacuolar ATPase complex, which carries an in-frame fusion of Ub as an MVB sorting signal. These results reveal novel roles for Doa1 in helping to process ubiquitinated membrane proteins for sorting into MVBs.