The fungal hydrophobin Sc3p self-assembles at the surface of aerial hyphae as a protein membrane constituting the hydrophobic rodlet layer

Abstract

The Schizophyllum commune hydrophobin Sc3p is a small, hydrophobic, cysteine-rich protein involved in the formation of aerial hyphae. Using an antibody against purified Sc3p we found that the hydrophobin is secreted into the medium at the apices of growing submerged hyphae but in emerging aerial hyphae it accumulates at the hyphal surface. Here, the hydrophobin self-assembles at the wall/air interface into an SDS-insoluble protein membrane, at the aerial site very hydrophobic and with the appearance of a mosaic of 10 nm spaced parallel rodlets. Interfacial self-assembly of the hydrophobin also occurs in vitro. When solutions containing various concentrations of purified Sc3p were dried down onto a glass surface, the amount of assembled Sc3p depended on the area of the interface. Surplus of Sc3p remained in the monomeric form, apparently because formation of a monolayer of assembled Sc3p abolishes the hydrophilic/hydrophobic interface. The 10 nm thick layer of assembled Sc3p at the surface of aerial hyphae thus probably represents a monolayer of the protein.