21st Century Guidebook to Fungi, SECOND EDITION, by  David Moore, Geoffrey D. Robson and Anthony P. J. Trinci

Table of Contents

1. Chapter 5: Fungal cell biology
2. Chapter 6: Structure and synthesis of fungal cell walls
   1. The fungal wall as a working organelle
   2. Fundamentals of wall structure and function
   3. Fundamentals of wall architecture
   4. The chitin component
   5. The glucan component
   6. The glycoprotein component
   7. Wall synthesis and remodelling
   8. On the far side
   9. The fungal wall as a clinical target
   10. Chapter 6 References and further reading
   11. Buy a PDF of Chapter 6
3. Chapter 7: From the haploid to the functional diploid; homokaryons, heterokaryons, dikaryons and compatibility

6.1 The fungal wall as a working organelle

The fungal cell wall is an important phylogenetic and taxonomic character but it is a dynamic organelle, which, generally speaking, has four major functions:

• maintenance of cell shape. By so doing, the wall determines the morphology of the hypha or other fungal cell, like yeast cells and spores. The more spectacular multicellular fruit bodies of higher fungi are a striking example of how the cell wall contributes to morphogenesis;
• stabilisation of the internal osmotic conditions is an important outcome of maintaining cell shape. The wall is sturdy and elastic and creates a counteracting pressure, which stops excessive water influx;
• the wall is also a protection against physical stress, and functions as a protective coat. The combination of mechanical strength and high elasticity allows the wall to transmit and redistribute physical stresses, offering effective protection against mechanical damage and allowing the hypha to penetrate aggressively the substrata into which it grows (Money, 2004; 2008);
• the cell wall is a scaffold for proteins. Most of the mechanical strength of the wall is provided by its stress-bearing polysaccharides, but these also function as a scaffold to which an external layer of glycoproteins is attached. These glycoproteins function to limit the permeability of the wall (in both directions), create a microenvironment in the inner region of the wall adjacent to the plasma membrane which is under the control of the fungus. Negatively charged phosphate groups in their carbohydrate side-chains probably contribute to water retention, and proteins of this external layer allow recognition of mating partners, substrata, substrates and hosts, and subsequent adhesion and capture of any or all of these.

Cell wall construction is tightly controlled. The wall is the first part of the fungal cell to challenge the environment or potential host, so it is also the first to experience whatever demands the environment imposes or defensive inhibitors the host can produce. The wall is the target for environmental stress, natural plant and animal defences and for clinical pharmaceutical agents. Polysaccharide composition, structure and thickness of the wall all vary with environmental conditions and are coordinated with the cell cycle and developmental progress of the organism (Latgé, & Beauvais, 2014).

Formation and remodelling of the cell wall involve several biosynthetic pathways and the combined, and coordinated, actions of hundreds of gene products within the fungal cell. An indication of the significance of the cell wall in the cell biology of the fungal cell is that about 20% of the genome of Saccharomyces cerevisiae (i.e. approximately 1,200 genes) is devoted to the construction of the cell wall. Some provide substrates for the components of the wall; others are concerned with the delivery and assembly of wall materials; and many regulate the assembly and maintenance processes, as well as wall modifications in response to environmental challenges (Latgé, & Beauvais, 2014). The gene products include enzymes that create, modify or split glycosidic bonds, including multigene families of chitin and glucan synthases as well as enzymes such as glucanases and chitinases (glycohydrolases) and transglycosidases, which are concerned with remodelling the structure of the wall (Gow et al., 2017). Any disruption to the structure of the cell wall will have a profound effect on the growth and morphology of the fungal cell; ultimately making the cell vulnerable to lysis and death.

Although many of the building blocks of the cell wall are conserved in different fungal species, other aspects of wall composition vary between species (Coronado et al., 2007; Xie & Lipke, 2010). Many of the enzymes involved in wall construction and for cross-linking walls were strongly conserved; including glycosyl hydrolases and transferases, proteases, lipases, enzymes of the GPI-anchor synthesis pathway, and the chaperone proteins that assist polypeptide folding or unfolding and the assembly or disassembly of macromolecular structures. Indeed, many of these proteins are also conserved in other eukaryotes and are also associated with wall synthesis in plants. On the other hand the wall-resident proteins have diversified rapidly; these are the structural glycoproteins, adhesins and sexual agglutinins (in Saccharomyces). Coronado et al. (2007) suggest that fungal cell walls are assembled by the products of a conserved set of genes that is ancestral, like the basic architecture of the wall itself, to the evolutionary divergence of the ascomycete and basidiomycete clades. Xie & Lipke (2010) point out that the relatively fast sequence divergence of wall-resident proteins, together with their unusually high content of tandem and nontandem repeats, may have served as a driver for yeast speciation; enabling the yeast clade to adapt and exploit diverse environments.

Most of what we can describe here about the fungal wall has come from detailed molecular, genomic and proteomic analyses of model fungal systems, especially Saccharomyces cerevisiae, Schizosaccharomyces pombe, Candida albicans, Aspergillus fumigatus and Neurospora crassa, and, in recent years, a growing number of pathogenic fungi, including human pathogens (Free, 2013). We will summarise information dealing with as wide a range of species as we can, but you should be aware that the story we can tell is still incomplete. 

Structurally, the fungal wall is a three-dimensional network of polysaccharides, glycoproteins and proteins, many of which are unique to fungi. Typically, the walls contain fibrillar polysaccharides of high tensile strength embedded in a more gel-like matrix comprising a variety of polysaccharides, glycoproteins, and proteins with a range of minor components, including lipids, pigments, inorganic ions and salts.

The structural fibrillar material of the wall is largely inert, forming the main structural component, but the composition of the other materials changes with time and position as these components can serve as:

• nutrient reserves,
• in transport and translocation,
• for the metabolism of non-permeable substrates,
• for communication and interaction with the exterior,
• for protection from outside attack. 

Updated July, 2019