9 Chitin Synthesis and Fungal Cell Morphogenesis

Recognizing the β-glucan component of the Candida albicans cell wall is a necessary step involved in host immune system recognition. Compounds that result in exposed β-glucan recognizable to the immune system could be valuable antifungal drugs. Antifungal development is especially important because fungi are becoming increasingly drug resistant. This study demonstrates that lipopeptide, surfactin, unmasks β-glucan when the C. albicans cells lack ergosterol. This observation also holds when ergosterol is depleted by fluconazole. Surfactin does not enhance the effects of local chitin accumulation in the presence of fluconazole. Expression of the CHS3 gene, encoding a gene product resulting in 80% of cellular chitin, is downregulated. C. albicans exposure to fluconazole changes the composition and structure of the fungal plasma membrane. At the same time, the fungal cell wall is altered and remodeled in a way that makes the fungi susceptible to surfactin. In silico studies show that surfactin can form a complex with β-glucan. Surfactin forms a less stable complex with chitin, which in combination with lowering chitin synthesis, could be a second anti-fungal mechanism of action of this lipopeptide.

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The citron homology domain as a scaffold for Rho1 signaling

Proceedings of the National Academy of Sciences ◽

10.1073/pnas.2110298118 ◽

2021 ◽

Vol 118 (39) ◽

pp. e2110298118

Author(s):

Sergio G. Bartual ◽

Wenfan Wei ◽

Yao Zhou ◽

Veronica M. Pravata ◽

Wenxia Fang ◽

...

Keyword(s):

Cell Wall ◽

Opportunistic Pathogen ◽

Nucleotide Exchange ◽

Chitin Synthesis ◽

Fungal Cell ◽

Guanine Nucleotide Exchange ◽

Eukaryotic Genes ◽

Nucleotide Exchange Factor ◽

Domain Of Unknown Function ◽

Important Regulator

Aspergillus fumigatus is a human opportunistic pathogen showing emerging resistance against a limited repertoire of antifungal agents available. The GTPase Rho1 has been identified as an important regulator of the cell wall integrity signaling pathway that regulates the composition of the cell wall, a structure that is unique to fungi and serves as a target for antifungal compounds. Rom2, the guanine nucleotide exchange factor to Rho1, contains a C-terminal citron homology (CNH) domain of unknown function that is found in many other eukaryotic genes. Here, we show that the Rom2 CNH domain interacts directly with Rho1 to modulate β-glucan and chitin synthesis. We report the structure of the Rom2 CNH domain, revealing that it adopts a seven-bladed β-propeller fold containing three unusual loops. A model of the Rho1–Rom2 CNH complex suggests that the Rom2 CNH domain interacts with the Rho1 Switch II motif. This work uncovers the role of the Rom2 CNH domain as a scaffold for Rho1 signaling in fungal cell wall biosynthesis.

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Wall Structure, Wall Growth, and Fungal Cell Morphogenesis

Biochemistry of Cell Walls and Membranes in Fungi ◽

10.1007/978-3-642-74215-6_6 ◽

1990 ◽

pp. 81-95 ◽

Cited By ~ 27

Author(s):

J. G. H. Wessels ◽

P. C. Mol ◽

J. H. Sietsma ◽

C. A. Vermeulen

Keyword(s):

Wall Structure ◽

Cell Morphogenesis ◽

Fungal Cell ◽

Wall Growth

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Sbe2p and Sbe22p, Two Homologous Golgi Proteins Involved in Yeast Cell Wall Formation

Molecular Biology of the Cell ◽

10.1091/mbc.11.2.435 ◽

2000 ◽

Vol 11 (2) ◽

pp. 435-452 ◽

Cited By ~ 31

Author(s):

Beatriz Santos ◽

Michael Snyder

Keyword(s):

Cell Wall ◽

Cell Surface ◽

Yeast Cell ◽

Cell Wall Structure ◽

Wall Structure ◽

Cell Integrity ◽

Cell Morphogenesis ◽

Chitin Synthesis ◽

Cell Wall Components ◽

Wall Components

The cell wall of fungal cells is important for cell integrity and cell morphogenesis and protects against harmful environmental conditions. The yeast cell wall is a complex structure consisting mainly of mannoproteins, glucan, and chitin. The molecular mechanisms by which the cell wall components are synthesized and transported to the cell surface are poorly understood. We have identified and characterized two homologous yeast proteins, Sbe2p and Sbe22p, through their suppression of a chs5 spa2 mutant strain defective in chitin synthesis and cell morphogenesis. Althoughsbe2 and sbe22 null mutants are viable,sbe2 sbe22 cells display several phenotypes indicative of defects in cell integrity and cell wall structure. First,sbe2 sbe22 cells display a sorbitol-remediable lysis defect at 37°C and are hypersensitive to SDS and calcofluor. Second, electron microscopic analysis reveals that sbe2 sbe22cells have an aberrant cell wall structure with a reduced mannoprotein layer. Finally, immunofluorescence experiments reveal that in small-budded cells, sbe2 sbe22 mutants mislocalize Chs3p, a protein involved in chitin synthesis. In addition, sbe2 sbe22 diploids have a bud-site selection defect, displaying a random budding pattern. A Sbe2p–GFP fusion protein localizes to cytoplasmic patches, and Sbe2p cofractionates with Golgi proteins. Deletion of CHS5, which encodes a Golgi protein involved in the transport of Chs3p to the cell periphery, is lethal in combination with disruption of SBE2 andSBE22. Thus, we suggest a model in which Sbe2p and Sbe22p are involved in the transport of cell wall components from the Golgi apparatus to the cell surface periphery in a pathway independent of Chs5p.

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Candida albicans strains deficient in CHS7, a key regulator of chitin synthase III, exhibit morphogenetic alterations and attenuated virulence

Microbiology ◽

10.1099/mic.0.28093-0 ◽

2005 ◽

Vol 151 (8) ◽

pp. 2623-2636 ◽

Cited By ~ 23

Author(s):

María Sanz ◽

Lucia Carrano ◽

Cristina Jiménez ◽

Gianpaolo Candiani ◽

José A. Trilla ◽

...

Keyword(s):

Candida Albicans ◽

Sequence Similarity ◽

Yeast Cells ◽

Null Mutant ◽

Chitin Synthesis ◽

Fungal Cell ◽

Chitin Synthases ◽

Opportunistic Mycoses ◽

Null Mutants

Chitin is a structural polysaccharide present in most fungal cell walls, whose synthesis depends on a family of enzymic activities named chitin synthases (CSs). The specific role of each of them, as well as of their regulatory proteins, in cell morphogenesis and virulence is not well understood. Here, it is shown that most chitin synthesis in Candida albicans, one of the fungi most commonly isolated from opportunistic mycoses and infections, depends on CHS7. Thus, C. albicans chs7Δ null mutants showed reduced levels of chitin and CS activity, and were resistant to Calcofluor. Despite the sequence similarity and functional relationship with ScChs7p, CaChs7p was unable to restore CSIII activity in a Saccharomyces cerevisiae chs7Δ null mutant, because it was unable to direct ScChs3p export from the endoplasmic reticulum. C. albicans chs7Δ null mutants did not show any defect in growth rate, but yeast cells displayed minor morphogenetic defects affecting septum formation, and showed an increased tendency to form filaments. CaChs7p was not required for germ-tube emission, and null mutant strains underwent the dimorphic transition correctly. However, colony morphology appeared distinctively affected. chs7Δ hyphae were very curved and had irregular lateral walls, resulting in very compact colonies that seemed unable to spread out radially on the surface, unlike the wild-type. This growth pattern may be associated with the reduced virulence and high clearance rate observed when the chs7Δ strain was used in a murine model of infection. Therefore, CaChs7p is required for normal hyphal morphogenesis, suggesting that in C. albicans CSIII plays an important role in maintaining cell wall integrity, being essential when invading surrounding tissues.

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Elevated Chitin Content Reduces the Susceptibility of Candida Species to Caspofungin

Antimicrobial Agents and Chemotherapy ◽

10.1128/aac.01486-12 ◽

2012 ◽

Vol 57 (1) ◽

pp. 146-154 ◽

Cited By ~ 98

Author(s):

Louise A. Walker ◽

Neil A. R. Gow ◽

Carol A. Munro

Keyword(s):

Cell Wall ◽

Candida Albicans ◽

Point Mutations ◽

Antifungal Drugs ◽

Cell Wall Polysaccharide ◽

Chitin Synthesis ◽

Fungal Cell ◽

Content Type ◽

Chitin Content ◽

Compensatory Increase

ABSTRACTThe echinocandin antifungal drugs inhibit synthesis of the major fungal cell wall polysaccharide β(1,3)-glucan. Echinocandins have good efficacy againstCandida albicansbut reduced activity against otherCandidaspecies, in particularCandida parapsilosisandCandida guilliermondii. Treatment ofCandida albicanswith a sub-MIC level of caspofungin has been reported to cause a compensatory increase in chitin content and to select for sporadic echinocandin-resistantFKS1point mutants that also have elevated cell wall chitin. Here we show that elevated chitin in response to caspofungin is a common response in variousCandidaspecies. Activation of chitin synthesis was observed in isolates ofC. albicans,Candida tropicalis,C. parapsilosis, andC. guilliermondiiand in some isolates ofCandida kruseiin response to caspofungin treatment. However,Candida glabrataisolates demonstrated no exposure-induced change in chitin content. Furthermore, isolates ofC. albicans,C. krusei,C. parapsilosis, andC. guilliermondiiwhich were stimulated to have higher chitin levels via activation of the calcineurin and protein kinase C (PKC) signaling pathways had reduced susceptibility to caspofungin. Isolates containing point mutations in theFKS1gene generally had higher chitin levels and did not demonstrate a further compensatory increase in chitin content in response to caspofungin treatment. These results highlight the potential of increased chitin synthesis as a potential mechanism of tolerance to caspofungin for the major pathogenicCandidaspecies.

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