Fungal Fibrils: Application of the Amyloid Polymer Structure by Fungi - The Functional Fold: Amyloid Structures in Nature - page 48

bonds lie, must be co-planar with the long axis of the rodlets. Other
studies of SC3 oligomerization, which have been monitored by both
EM and PM-IRRAS,

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suggest that the hydrophobin must localize to
the surface and reach a critical concentration, in order to initiate
the structural conversion to the mainly

β

-sheet form and for rodlets

to form.

3.6 Specialization and Functional Significance of

These Functional Amyloids

Hydrophobins have been identified in three of the major fungi:
Ascomycota, Basidiomycota, and Zygomycota. These include plant
and human pathogens, edible mushrooms, and fungi that form
symbiotic relationships with cyanobacteria (lichens) and plant
roots (mycorrhiza).

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In lichens, hydrophobins line the air pockets
preventing waterlogging and facilitating gas exchange. Hydro-
phobins are highly expressed during the formation of mycorrhizae,
but a role for these proteins has not yet been determined.

Indeed
the potential role of hydrophobins in soil ecology has only recently
been appreciated.

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This wide distribution and the many different
functions of hydrophobins are indicative of the utility of these
proteins and shows how important bridging the interfaces between
the air, solid surfaces, and liquids is to the fungal life cycle.
Some fungi appear to have evolved a different family of proteins
that carry out some of the functions of hydrophobins. A recently
discovered example occurs in , the fungus that

causes maize smut. Although the genome of this fungus encodes two
hydrophobins, it uses a repellent protein to generate a hydrophobic
surface on aerial hyphae and to mediate attachment to hydrophobic
surfaces.

Ustilago maydis

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In contrast, no hydrophobins or repellents have been
identified in yeasts, even though several genomes have been
sequenced, so hydrophobins may be dispensable for growth in
generally aqueous habitats.
Multiple hydrophobins are expressed within individual fungi, and
the expression of these genes is temporally and spatially regulated,
suggesting specialization for different roles in development. More
than twice the number of Class I hydrophobins have been discovered,
compared to Class II; members and the latter are found only in asco-
mycetes, suggesting that they may have evolved convergently.

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