Wall Composition and Construction
Introduction
Walls surround the cells of all fungi. Walls are composites of a variety of components. Typically, the walls contain fibrillar materials bound together by sugars, proteins, lipids and a variety of polysaccharides. While the fibrillar material of the wall is largely inert, the composition of the inserted materials changes with time. The functional components are important in nutrient transport, metabolism of non-permeable substrates, communication, and cell wall modifications.
Composition
Approximately 80% of the wall consists of polysaccharides. Most fungi have a fibrillar structure built on chitin, chitosan (Zygomycota only), and ß-glucans, and a variety of heteropolysaccharides. The fibres are contained in a complex gel-like matrix. Proteins constitute a small fraction of wall material, rarely more than 20%, and often as glycoprotein. Not all proteins have a structural role. Mating, recognition, wall modification and nutrition involve wall-bound proteins. Hydrophobins are expressed constitutively, and become bound in the wall when the hyphae emerge in air. Lipids are found in walls, usually in very small concentrations. Along with hydrophobins (see below), lipids and waxes appear to function in the control over movement of water, especially the prevention of desiccation of cells. Walls also contain a range of other minor components, including pigments and salts.
Of the pigments, melanin is particularly intriguing. Melanin is important for protecting the hyphae and spores from UV stress, is essential for pathogenisis, and attachment to surfaces of emerging hyphae from spores. Melanin also contributes to the humic fraction in soil because it is very resistant to degradation.
| Division | Fibrous | Gel-like Polymer |
|---|---|---|
| Basidiomycota | Chitin β -(1-3), β-(1-6) Glucan |
Xylomannoproteins α (1-3) Glucan |
| Ascomycota | chitin β -(1-3), β-(1-6) Glucan |
Galactomannoproteins α (1-3) Glucan |
| Zygomycota | Chitin Chitosan |
Polyglucuronic acid Glucuronomannoproteins Polyphosphate |
| Chytridiomycota | Chitin Glucan |
Glucan |
The constituents of cell walls are synthesised in the cytoplasm, linked in the walls at the hyphal tip, and polymerised and cross-linked in the wall matrix. Chitin (structure of the monomer shown right) and the glucans are synthesised at the plasma membrane by enzymes embedded in the membrane. Nucleotide sugar precursors are accepted from the cytoplasm, linked and passed to the wall. Wall glycoproteins are synthesised in the endoplasmic reticulum, carried through the Golgi to the plasma membrane, where vesicles release the glycoprotein to the wall. Enzymes cross-linking fibrils in the wall are released through the plasma membrane.
Construction
Wall construction takes place in the apical tip. Autoradiographic studies suggest that all synthesis of chitin and glucans takes place within 1 mm of the terminus. The tip is highly plastic as the wall is laid down. As walls mature, they become more rigid. The rigidity is provided by cross-linking of polymers, thickening of fibrils and the deposition of materials in the interfibrillar matrix. The process is highly polarised, and reliant on maintenance of a positive turgor pressure within the cytoplasm. LINK
The walls have a number of characteristics which can be attributed to components of the wall. For instance, hyphae are generally resistant to loss of water, and yet capable of detection of signals for mating and conjugation between compatible hyphae. The function of hydrophobins is typical of this functional complexity (see below) located in the wall.
Communication
Conjugation such as in anastomosis and mating reactions in fungi are similar. Different but compatible glycoproteins, called agglutinins, in the walls of each complementary hypha fuse to form a complex binding the cells together. In mating, release of hormone-like compounds precedes the binding. The hormone alters receptor glycoproteins in the wall leading to a cascade of changes to the fibrillar construction of the wall. Ultimately, the wall breaks down, a conjugation tube may connect the cells, membranes fuse and cytoplasm linkages are established. LINK
Adhesion is also mediated by fibrillar glycoproteins embedded in a gel-like matrix. Fibrils are commonly found where fungi attach to surfaces. The fibrils are highly specific in where they attach, and a complementary protein on the surface of the partner is assumed. This system of recognition and communication between fungi and partner is widespread in pathogenic and mutualistic interactions.
Hydrophobins in Walls
Hydrophobins are a large, diverse group of related proteins found widely in the fungi. Hydrophobins may constitute up to 10% of total wall protein. Each molecule consists of a hydrophobic domain and a hydrophilic domain. The amphipathic structure provides the molecules with an extraordinary potential array of functions.
The proteins are excreted from the hyphal tip. If the hypha is in solution, the hydrophobins pass into solution. If the hypha emerges from the solution, then the polypeptide polymerises on the surface resulting in an array of parallel rodlets. The protein is attached to the fungal wall by the hydrophilic end. The hydrophobic domain is exposed. This construction reduces movement of water through the wall of the hypha providing some protection from desiccation. It may also increase the strength of the wall.
The exposed hydrophobic domain enables attachment to hydrophobic surfaces. These surfaces may be other hyphae, leading to the formation of complex structures. The hyphae can also attach to hydrophobic (eg waxy) plant surfaces, enabling attachment of spores prior to formation of appressoria. The attachment properties make hydrophobins extremely important in morphogenic development.
The exposed hydrophobic domain also provides fungi in lichens with control over the movement of water within the thallus. The domains prevent waterlogging within airspaces, allowing the movement of water and nutrients through channels controlled by the fungus. This suggests that regulation of the photobiont output is essentially mechanistic.
Each fungus has genes for more than one, commonly more than 10 different hydrophobins. Different hydrophobin genes are expressed at different times. In Schizophyllum the protein found in the hyphal wall differs from that expressed in the tubes of the basidiocarp. Genes are expressed differently in even the monokaryons and the dikaryon of the same fungus. The regulation of expression appears to be part of complex regulatory processes that are not yet clearly understood.
Hydrophobins have been implicated in a wide range of processes. Essentially, wherever hyphae adhere to other surfaces, modify movement of solutes across the wall or require strength and rigidity, we might expect the presence of hydrophobins.
Glomalin in Walls
Some members of the Glomeromycota produce a putative glycoprotein in the walls. The compound has been called glomalin. Recent work suggests that the compound is related to the Heat Shock Proteins in group 60. However, the structure elucidation and characterisation of the function of glomalin are incomplete.
Glomalin is important because of its possible correlation with aggregation of soil particles and the purported resilience of the molecule. Correlative data indicates the molecule is long-lived, though these data need to be viewed cautiously because of the methods used to develop the hypothesis. It is clear that concentrations of glomalin in soil increase from cultivated soil to soil under undisturbed vegetation, and that these changes are correlated with soil micro-aggregation. It apears that glomlin will be studied much more rigorously now that sequence data is available to enable clearer biochemical, physiological and ecological characterisation.
Conclusion
Fungal walls consist of complex of fibrillar material embedded in polysaccharide that enable the proteins also present to make the wall a functional unit. The wall may be highly protected, or relatively susceptible to the environment, because of various constituents in the matrix. The wall also allows the cell to communicate with the environment, enabling reproduction, recognition and reception.
References
Gooday GW in Gow NAR & Gadd GM 1995 The Growing Fungus. Chapman Hall
Wessels JGH (1996) Fungal hydrophobins: proteins that function at an interface. Trends in Plant Science 1, 9 – 15.
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