Hydrophobin Rodlets on the Fungal Cell Wall

Abstract

The conidia of airborne fungi are protected by a hydrophobic protein layer that coats the cell wall polysaccharides and renders the spores resistant to wetting and desiccation. A similar layer is presented on the outer surface of the aerial hyphae of some fungi. This layer serves multiple purposes, including facilitating spore dispersal, mediating the growth of hyphae into the air from moist environments, aiding host interactions in symbiotic relationships and increasing infectivity in pathogenic fungi. The layer consists of tightly packed, fibrillar structures termed "rodlets", which are approximately 10 nm in diameter, hundreds of nanometres long and grouped in fascicles. Rodlets are an extremely stable protein structure, being resistant to detergents, denaturants and alcohols and requiring strong acids for depolymerisation. They are produced through the self-assembly of small, surface-active proteins that belong to the hydrophobin protein family. These small proteins are expressed by all filamentous fungi and are characterised by a high proportion of hydrophobic residues and the presence of eight cysteine residues. Rodlets are a form of the functional amyloid fibril, where the hydrophobin monomers are held together in the rodlets by intermolecular hydrogen bonds that contribute to a stable β-sheet core.