102 CHAPTER 5
Roles of hydrophobins in fungal
differentiation
Hydrophobinsare a class of small secreted proteins
that were discovered relatively recently and that
seem to be unique to fungi. These proteins were dis-
covered by chance, when Wessels and his colleagues
were investigating the transcriptome (messenger RNA
profiles) of monokaryotic and dikaryotic strains of
a small bracket-producing fungus, Schizophyllum com-
mune. Among the most abundantly expressed genes
were a group that encoded hydrophobic proteins.
Because of their strongly hydrophobic nature, these
proteins cannot be detected by conventional protein
purification. Subsequent studies have revealed similar
proteins in a wide range of fungi and have shown that
they have important roles in fungal differentiation.All the hydrophobins studied to date have similar
structure and properties. They consist of about 100
amino acids, including eight cysteine residues that
occur in a specific pattern so that the protein can
fold to produce a highly hydrophobic domain. The
hydrophobins are soluble in water, but at an inter-
face with air they self-assemble into a film with a
hydrophobic face and a hydrophilic face. As shown in
Fig. 5.21, this is thought to happen when a hypha forms
an aerial branch so that the aerial portion is coated
by a hydrophobic film. The potential significance for
differentiation is that hyphae with hydrophobic sur-
faces might interact with one another in specific ways,
as discussed later. For conidial fungi the most obvious
significance is that hydrophobins could affect the
surface properties and thus the functions of the
spores (Chapter 10). The conidia of Emericella nidulans,Fig. 5.20Botrytis cinerea(sexual stage: Botryotinia fuckeliana), which commonly causes “grey mould” of soft fruits. (a)
Low-power view of the characteristicbranching conidiophores which bear clusters of grey conidia at their tips. (b) Close-
up view showing the bulbous tips of the conidiophore branches, bearing immature conidia.
(a) (b)Fig. 5.21Release of hydrophobin proteins
from hyphae of Schizophyllum commune
submerged in a culture medium (left), and
spontaneous polymerization of hydro-
phobins into water-repellent films (right)
when the hyphae emerge into an aerial
environment. (Based on a diagram in
Wessels 1996.)