Fungal Fibrils: Application of the Amyloid Polymer Structure by Fungi - The Functional Fold: Amyloid Structures in Nature - page 50

offers resistance to killing by host cells. In contrast, the hydrophobin
RodB is thought to play a role in maintaining the structure of the
conidial cell wall.

55,56

, the rice blast fungus, expression
of the Class I hydrophobin MPG1 is necessary for formation of the
rodlet layer on aerial hyphae and conidia, and is also required for
normal appressoria formation and efficient infection and hence
pathogenicity.

In M. grisea

21

Interestingly, despite the functional distinctions
between different Class I hydrophobins, complementation experi-
ments showed that six of the seven Class I hydrophobin genes tested
(including SC1 and SC4 from S. commune , RodA and DewA from A.

nidulans ), could

at least partially fulfil the various roles played by MPG1. Therefore,
although being very diverse in amino acid sequence and expression
pattern, Class I hydrophobins constitute a closely related group of
functional amyloid-like proteins with similar physical properties
and underlying functions.

, EAS from N. crassa , and SsgA from M. anisopliae

57

3.7 Conclusions

Hydrophobins are ubiquitously found in filamentous fungi and are
required for the dissemination of spores. The fungal hydrophobin
rodlets represent a functional adaptation of the polymeric properties
of amyloid. Both Class I and II hydrophobins are secreted by fungi in
a soluble, monomeric form which then self-assembles into a highly
insoluble and ordered state. However, only the Class I hydrophobin
rodlets can be strictly classified as “functional amyloid” since their
assemblies have a fibrillar morphology and display typical amyloid
characteristics, such as binding to amyloid-specific dyes and giving
rise to a cross-

β

X-ray fibre diffraction pattern. Apart from sharing
significant structural and morphological similarities with amyloid
fibrils, two features are unique to the hydrophobin assemblies.
The first is that the polymerization process extends in both the
longitudinal and lateral directions, resulting in the formation of a
film rather than filaments. Secondly, hydrophobin monomers as
well as the assemblies that they form are highly amphipathic. In
fact, the amphipathicity of the hydrophobin molecules is thought
to be a major driver of the assembly process, and the amphipathic
character of the assemblies is thought to underlie most, if not
all aspects of hydrophobin function in fungal biology. A number