Expression and engineering of fungal hydrophobins

Filamentous fungi secrete unique proteins called hydrophobins. Upon contact with a hydrophilic-hydrophobic interface these proteins self-assemble into an amphipathic membrane. Differences in the solubility of the assemblages divides hydrophobins into two groups. The class I hydrophobins form highly insoluble membranes that can only be dissolved in trifluoroacetic acid (TFA) and formic acid, while assemblies of class II hydrophobins can be readily dissolved in ethanol or SDS. Self-assembly allows hydrophobins to change the nature of a surface; hydrophobic surfaces turn hydrophilic and hydrophilic surfaces become hydrophobic. These properties make hydrophobins interesting candidates for use in technical and medical applications. Class I hydrophobins seem to be particularly interesting to coat solid surfaces, while class II seem to be the molecules of choice for use in liquid systems. Application of hydrophobins would benefit from the availability of a library of hydrophobin variants. Moreover, production should be increased. Nature provides a fast amount of hydrophobins with slightly different characteristics. In addition to these hydrophobins, new variants may be obtained via random mutagenesis or by rational design. Functional class I hydrophobins (i.e. capable of self-assembling into an amphipathic membrane) could only be produced by filamentous fungi that by nature secrete hydrophobins into the culture medium. However, yields are still relatively low. On the other hand, class II hydrophobins can already be produced at high levels using Trichoderma reesei as a host.