Filamentous fungi colonise living or dead moist substrates by means of hyphae that extend at their apices, while branching subapically. After a submerged feeding mycelium has been established, aerial hyphae are formed that may develop into reproductive structures such as conidiophores or fruiting bodies (e.g. mushrooms or brackets). Spores that are formed by these structures are dispersed and may give rise to new colonising mycelia. In these and other stages in the life cycle of filamentous fungi small secreted proteins, called hydrophobins, fulfil a broad spectrum of functions (1,2).
Hydrophobins are proteins that occur uniquely in mycelial fungi. These proteins and their encoding genes have been isolated from ascomycetes and basidiomycetes (1). Some evidence indicates that hydrophobins occur in zygomycetes as well (3) but it is not yet clear whether they occur in the chytridiomycetes. Based on their hydropathy patterns and their solubility characteristics, class I and class II hydrophobins were distinguished (4). Class I hydrophobins have been identified in both ascomycetes and basidiomycetes but until now class II hydrophobins have been found in ascomycetes only.
Hydrophobins allow fungi to escape their aqueous environment (2), confer hydrophobicity to fungal surfaces in contact with air (5-12) and mediate attachment of hyphae to hydrophobic surfaces (13,14) resulting in morphogenetic signals (15). The latter is important in initial steps of fungal pathogenesis where the fungus must attach to the hydrophobic surface of the host before penetration and infection can occur. Moreover, hydrophobins seem to function in cases of symbioses between fungi and plants (ectomycorrhizae) (16) or algae and/or cyanobacteria (lichens) (17, 18). The mechanism underlying all these functions is based on the property of hydrophobins to self-assemble at a hydrophilic/hydrophobic interface into an amphipathic membrane (19-22). Upon self-assembly at the interface between the hydrophilic cell wall and a hydrophobic environment (the air or the hydrophobic surface of a host), the hydrophilic side of the amphipathic membrane will orient and attach itself to the cell wall, while the hydrophobic side becomes exposed to the hydrophobic environment. Aerial hyphae and spores thus become hydrophobic, while hyphae that grow over a hydrophobic substrate attach themselves (Fig. 1).
Recently, it was shown that hydrophobins not only function at hydrophilic-hydrophobic interfaces but also function within the matrix of the cell wall where they influence cell wall composition (23). The mechanism is not yet clear. Moreover, the class II hydrophobin cerato-ulmin is a toxin for elm that seems to function as a monomer by increasing the permeability of the plant plasma membrane (24).
By no means did I write this, or even understand it all. But with all the things that I've read over the net, Hydrophobins seem to be directly related to Rizo. mycl. growth.
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