Hydrophobins, the fungal coat unravelled

Hydrophobins are among the most surface active molecules and self-assemble at any hydrophilic–hydrophobic interface into an amphipathic film. These small secreted proteins of about 100 amino acids can be used to make hydrophilic surfaces hydrophobic and hydrophobic surfaces hydrophilic. Although differences in the biophysical properties of hydrophobins have not yet been related to differences in primary structure it has been established that the N-terminal part, at least partly, determines wettability of the hydrophilic side of the assemblage, while the eight conserved cysteine residues that form four disulphide bridges prevent self-assembly of the hydrophobin in the absence of a hydrophilic–hydrophobic interface. Three conformations of class I hydrophobins have been identified: the monomeric state, which is soluble in water, the α-helical state, which is the result of self-assembly at a hydrophobic solid, and the β-sheet state, which is formed during self-assembly at the water–air interface. Experimental evidence strongly indicates that the α-helical state is an intermediate and that the β-sheet state is the end form of assembly. The latter state has a typical ultrastructure of a mosaic of 10 nm wide rodlets, which have been shown to resemble the amyloid fibrils.