Chapter Two - Fungal Cell Wall Organization and Biosynthesis

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Abstract

The composition and organization of the cell walls from Saccharomyces cerevisiae, Candida albicans, Aspergillus fumigatus, Schizosaccharomyces pombe, Neurospora crassa, and Cryptococcus neoformans are compared and contrasted. These cell walls contain chitin, chitosan, β-1,3-glucan, β-1,6-glucan, mixed β-1,3-/β-1,4-glucan, α-1,3-glucan, melanin, and glycoproteins as major constituents. A comparison of these cell walls shows that there is a great deal of variability in fungal cell wall composition and organization. However, in all cases, the cell wall components are cross-linked together to generate a cell wall matrix. The biosynthesis and properties of each of the major cell wall components are discussed. The chitin and glucans are synthesized and extruded into the cell wall space by plasma membrane-associated chitin synthases and glucan synthases. The glycoproteins are synthesized by ER-associated ribosomes and pass through the canonical secretory pathway. Over half of the major cell wall proteins are modified by the addition of a glycosylphosphatidylinositol anchor. The cell wall glycoproteins are also modified by the addition of O-linked oligosaccharides, and their N-linked oligosaccharides are extensively modified during their passage through the secretory pathway. These cell wall glycoprotein posttranslational modifications are essential for cross-linking the proteins into the cell wall matrix. Cross-linking the cell wall components together is essential for cell wall integrity. The activities of four groups of cross-linking enzymes are discussed. Cell wall proteins function as cross-linking enzymes, structural elements, adhesins, and environmental stress sensors and protect the cell from environmental changes.

Section snippets

Introduction and Overview: General Organization of the Fungal Cell Wall

The cell wall is vital to the growth, survival, and morphogenesis of fungi. Mutational analysis has proved that it provides a protective barrier against a wide range of environmental conditions such as heat, cold, desiccation, and osmotic stress. It also provides protection against other microbes. Cell wall sensor proteins allow the fungus to assess and respond to changes in the environment. Cell wall adhesion and mucins mediate the adhesive properties of the fungal cell and play critical roles

Methods Used in Studying the Cell Wall

Because the cell wall components are all cross-linked together and the proteins are heavily glycosylated, it is challenging to study the organization and components of the fungal cell wall. However, genetic, microscopic, 2D-gel electrophoresis, proteomic, and immunological approaches have allowed for the identification of cell wall proteins, and the major polymers present in cell walls have been identified by carbohydrate analyses (Klis et al., 2006, Latgé, 2007, Pitarch et al., 2002). EM

Chitin and chitosan

Chitin makes up between 1% and 15% of the cell wall mass, with yeast species generally having 1–2% chitin and filamentous fungi having up to 15% chitin (Table 2.2). Chitin is synthesized by plasma membrane-associated chitin synthases. These enzymes utilize UDP-N-acetylglucosamine from the cytoplasmic side of the membrane as a substrate to synthesize linear chitin molecules (polymers of β-1,4-N-acetylglucosamine). The chitin polymer is elongated by the addition of N-acetylglucosamine to the

Well-Characterized Fungal Cell Walls

A great deal of work and a large literature exists for the S. cerevisiae cell wall and for the cell walls of the pathogens A. fumigatus and C. albicans. Somewhat smaller, but still helpful, literatures exist for the pathogen C. neoformans, and for S. pombe and N. crassa. Aspects of each of these cell walls were compared and contrasted when the individual cell wall components were discussed. A short overview of each of these cell walls will be given here. These short descriptions are helpful in

Conclusions: Variability in Cell Wall Components and Redundancy in Cell Wall Biosynthesis

Although only a handful of fungal cell walls have been characterized, it is clear that a great deal of variability can exist in the components used in constructing the cell wall. The presence and importance of the various cell wall components (β-1,3-glucan, mixed β-1,3/1,4-glucan, β-1,6-glucan, chitin, α-1,3-glucan, melanin, mannans, and galactomannans) have been examined, and the information from the different species is summarized in Table 2.2. As is apparent from Table 2.2 and the narratives

Acknowledgments

I would like to thank Dr. Abhiram Maddi for critical reading of the chapter and James Stamos for help in chapter preparation. Funding for the S. J. F. laboratory has been provided by a grant from the UB Foundation and by grants R01GM0785879 and R03Al103897 from the National Institutes of Health.

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