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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 wall 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 (Zygomycotina), 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 matrix of the wall as 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 regulate movement of water, especially in 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 a variable polymer of aromatic components. The polymer is laid down in walls after the initiation of wall formation. 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 stabilised fraction of organic carbon in soil.

Table 1: Common wall constituents found in each division of fungi (adapted from Gooday in Gow & Gadd, 1995).
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.

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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 apex. The tip is highly plastic as the wall is laid down. Walls rigidify with maturity. 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.

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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 linking the cytoplasm of the two cells 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. 

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Hydrophobins in Walls

Hydrophobins are a large, diverse group of related proteins found widely and only 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 for the fungus and in biotechnology.

Monomers of the protein are excreted from the hyphal tip. If the hypha is in solution, the hydrophobins pass into solution. Monomers may polymerise in solution, and polymeric layers are formed at water/air interfaces. If the hypha emerges from the solution, then the polypeptide polymerises on the surface of the hypha resulting in an array of parallel rodlets covering the wall. 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 while still enabling signal molecules to pass to the environment. The presence of hydrophobins may also increase the strength of the wall.

The exposed hydrophobic domain enables attachment to hydrophobic surfaces. Hydrophobic 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. Further, hydrophobins are not detected by immunological repsonses in animals, and thus the cell or hyphae evades an important protective mechanism of the host. The attachment properties make hydrophobins extremely important in morphogenic development and interactions with other organisms.

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. The diversity of hydrophobins found in some lichen fungi indicates a high degree of functional specificity for each compound.

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 complex and multifacetted.

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 predict the presence of hydrophobins.

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Glomalin in Walls

Some members of the Glomeromycota produce a putative glycoprotein in the walls. The compound has been called glomalin. Glomalin is related to the Heat Shock Proteins in group 60. However, the structure, elucidation and characterisation of the function of glomalin are incomplete.

Glomalin has gained prominence because broad measures of its presence have been correlated with structural stability in soil. Correlative data indicates the molecule is long-lived. These data need to be viewed cautiously because of the methods used to develop the hypothesis. Experimental testing of hypotheses generated using correlative data remain incomplete.

Conclusion

Fungal walls consist of complex fibrillar material embedded in polysaccharide and other compounds, and functional complexity associated proteins and glycoproteins. 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.

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References

Cox PW & Hooley P. (2009) Hydrophobins: New prospects for biotechnology. Fungal Biology Reviews 23, 40-47.

Gooday GW in Gow NAR & Gadd GM 1995 The Growing Fungus. Chapman Hall

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