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. Consequently, 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 (Bowman & Free, 2006; Klis, Boorsma & De Groot, 2006; Lesage & Bussey, 2006; Latgé, 2007).
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. As we will show below, because of the unique components in its structure, and the vital roles that the cell wall plays in the life of the fungus, the fungal wall is an excellent target for antifungal agents. Formation and remodelling of the cell wall involves several biosynthetic pathways and the combined actions of hundreds of gene products within the fungal cell. Although cell wall composition varies among fungal species, most of what we can describe 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, all of which are Ascomycota. We will emphasise information about other groups where 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.
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 December 17, 2016