Fungal Fibrils: Application of the Amyloid Polymer Structure by Fungi - The Functional Fold: Amyloid Structures in Nature - page 36

specific biological purposes, in contrast to the amyloid deposits that
are commonly associated with disease states and protein misfolding.
This chapter will focus on what is currently known about the
sequence, structure, assembly mechanism, and physical properties
of hydrophobin rodlets and discuss the diverse functions played by
these fibrils in fungal biology and development.

3.1 Introduction

Fungi are heterotrophic organisms that play a key role in nutrient
recycling in the ecosystem. As such, fungi generally grow in moist
environments such as leaf litter, wood, and living plants, and
less commonly in animal tissue through the apical growth of
hyphae.

1,2

Once nutrients have been depleted from the immediate
environment, and in order to colonize new territory, fungi produce
aerial hyphae and reproductive structures such as conidiophores
and fruiting bodies (e.g., mushrooms or brackets) to enable spore
dispersal. The transition from the moist hydrophilic environment to
the hydrophobic conditions of the air is essential for the effective
dispersal of spores. Filamentous fungi are able to transverse the
boundary between dry and moist environments successfully due
to the presence of a unique family of proteins, called hydrophobins.
Many of these fungal hydrophobins self-assemble into a functional
amyloid structure that forms an amphipathic layer at critical
hydrophilic/hydrophobic interfaces, allowing fungi to successfully
colonise a range of environments.

3.2 Early Observations of Rodlets

A regular, fibrillar structure was first observed 40 years ago on
the outer surface of freeze-etched spores from mould fungi such
as

3,4 5,6 7

Penicillium , Aspergillus , Oidiodendron truncatum , and

7

Geotrichum candidum These fibrillar structures were given the

name “rodlets”. Micrographs showed that rodlets ranged from
approximately 100 to 18,000 Å in length and were usually grouped
in bundles of approximately 300 to 2500 Å in width (Fig. 3.1a,b).

.

5

Beever and colleagues showed that the rodlets from Neurospora

crassa formed a monolayer on the spore surface and were not

interwoven or overlaid.

8-10

The first clue as to their function came