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Chapter 17 : The Cell Wall of Filamentous Fungi
Category: Microbial Genetics and Molecular Biology; Fungi and Fungal Pathogenesis
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This chapter describes the molecular composition and organization of the cell wall in the filamentous fungi. The major differences between the cell wall of the filamentous fungi and that of the better-studied yeast are stressed. The main fibrous component of the cell wall is glucan, a polymer of glucose. Chitin provides tensile strength to the cell wall and composes ~2% of the total cell wall dry weight in yeast, and 10 to 15% in filamentous fungi. Differences in the cell wall composition of yeast and filamentous fungi may have arisen because of differing evolutionary pressures; the filamentous fungal cell wall is adapted to extremely rapid deposition and growth at the hyphal tip and an ability to penetrate hard surfaces. The fungal cell wall in both yeasts and filamentous fungi is studied by various techniques that are addressed in this chapter. They include (i) the chemical analysis of carbohydrate composition; (ii) microscopic analysis (electron, light, and immunofluorescence); (iii) biochemical separation and identification of cell wall proteins (CWPs); (iv) use of cell wall biosynthesis inhibitors and cell wall mutants; and (v) genomics, transcriptomics, and proteomics. The chapter focuses on compounds affecting specific constituents of the fungal cell wall. The significance of the cell wall as a target for antifungals has long been appreciated, yet the number of successful drugs developed is very small. Important progress has also been made in the analysis of the cell wall.
Key Concept Ranking
- Mitogen-Activated Protein Kinase Pathway
Structure and schematic representation of the architecture of a prototypical fungal cell wall. The outer layer contains high levels of different types of mannoproteins. The inner layer is made mainly of polysaccharides (β-glucans and chitin) and small amounts of proteins. The plasma membrane contains enzymes involved in β-glucan and chitin biosynthesis.
Comparison between the cell wall carbohydrate composition of a yeast (S. cerevisiae) (A) and a mold (A. fumigatus) (B). Both contain a branched polysaccharide of β-1,3/1,6 glucan that is linked to chitin via a β-1,4 linkage. However, only S. cerevisiae contains β-1,6 glucan, whereas A. fumigatus contains α-glucans and galactomannans that are not found in yeast.
Schematic outlining the basic methods used to analyze the carbohydrate composition of the cell wall. Either the entire cell wall or alkali-digested soluble and insoluble fractions are completely digested by acid hydrolysis and analyzed by high-performance anion-exchange chromatography (HPIC) or chemical analysis and chromatography.
Outline of the fungal cell wall integrity pathway. Activation of the pathway involves perception of external signals (including stress- and nutrition-related cues) by membrane-associated sensors and signal transducers, intracellular transducers and messenger molecules, regulators, and structural components (including cytoskeletal proteins and cell wall biosynthetic enzymes). The pathway can also be activated via internal cues linked with other signaling pathways.