Hydrophobins produced by the split gill fungus protect surfaces by self-assembling into a highly water-repellent layer when exposed to an air-water interface.

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The split gill fungus synthesizes a class of small proteins called hydrophobins that self-assemble into barrier layers. This serves the fungus in two ways: first, the surfactant properties of the hydrophobins allow hyphae, the stringy filaments of the fungus, to breach the surface tension of water to form aerial protrusions used for reproduction; second, the hydrophobins spontaneously generate a robust water-proof barrier over the surface of the exposed hyphae to prevent water-logging of spores before they are released into the air. Hydrophobins self-assemble into barrier layers at the water-air interface by virtue of their specially adapted amino acid composition. They all contain 8 cysteine amino acids, which cross-link together via disulfide bridges when it is dissolved in water. This allows the protein to maintain a globular, water-soluble structure. When exposed to air, the disulfide bridges break and the proteins aggregate into amyloid rods. Amyloid rods are usually considered to be misfolded proteins and are the cause of numerous diseases, including Alzheimers. In fact, the amyloid nature of the hydrophobins in fungi is considered the first documented instance of amyloid proteins being useful in that state. The disulfide bridges keep the hydrophobins from prematurely self-assembling into a barrier layer.


Hydrophobins in formation of aerial structures. Artist: Han Wösten

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References

"Hydrophobins are a remarkable class of small cysteine-rich proteins found exclusively in fungi. They self-assemble to form robust polymeric monolayers that are highly amphipathic and play numerous roles in fungal biology." (Sunde et al. 2008: 773).

"[Hydrophobins] possessed the remarkable property of spontaneously polymerising at gas–water interfaces into SDS-insoluble amphipathic sheets with the highly hydrophobic surface facing the gas and the hydrophilic surface facing the water." (Sunde et al 2008: 774)

"Whereas the surface of the hypha wall growing in a moist substrate is hydrophilic, the cell wall surface of aerial hyphae and airborne spores is hydrophobic. This fundamental change in the surface properties of the cell wall is achieved through the secretion of hydrophobins. Hydrophobins act as natural surfactants and reduce the surface tension of the growth medium, thus allowing fungi to breach the water–air interface and to produce aerial structures such as hyphae. Similarly, the spores that develop on the end of the aerial structures are coated by an amphipathic hydrophobin layer that renders their surface hydrophobic and resists wetting, thus facilitating effective dispersal in air. While the hydrophobin layer is strongly hydrophobic, acting to prevent water penetration and consequent water logging, it is nevertheless highly permeable to gas exchange, behaving in a manner resembling GORE-TEXTM ." (Sunde et al. 2008:775)

"One side of the hydrophobin membrane is moderately to highly hydrophilic (water contact angles ranging between 22? and 63?), while the other side exposes a surface as hydrophobic as Teflon or paraffin (water contact angle 110?)...Monomeric class I and class II hydrophobins are rich in β-sheet structure. At the water-air interface, class I hydrophobins attain more β-sheet structure...The membrane of class I hydrophobins is characterized by a mosaic of bundles of 5–12 nm-wide parallel rodlets...The rodlets of the class I hydrophobins, SC3 and SC4, of S. [Schizophyllum] commune are very similar to the fibrils formed by amyloid proteins." (Wösten 2001:629)

"To our knowledge, hydrophobins are the first example of functional amyloids, with multiple functions in fungal development...the four disulfide bridges of the SC3 hydrophobin are essential to prevent the protein from forming the amyloid structures in the absence of a hydrophilic-hydrophobic interface...Apparently, the disulphide bridges of hydrophobins keep monomers soluble in water (e.g., within the cell or in the medium) and thus prevent precocious self-assembly." (Wösten 2001:630)

Journal article
Hyrophobins: Multipurpose Proteins

Journal article
Structural Analysis of Hydrophobins

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Living System/s

Organism
Common PorecrustSchizophyllum communeSpecies

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