Mechanisms of protein adhesion on surface films of hydrophobin

Hydrophobins are adhesive proteins produced by filamentous fungi. They are in many cases secreted into the medium and adsorb readily to a number of different surfaces. They fulfill many different tasks such as the formation of various coatings and mediating adhesion of fungi to surfaces. The mechanism of how hydrophobins adhere and how they mediate fungal adhesion is of interest both from the point of view of fungal biology and for various biotechnical immobilization applications. It has been shown that hydrophobins typically form a monomolecular layer on solid substrates. We are especially interested in how a surface layer of hydrophobin can mediate the adhesion of a second layer of another protein. In this work we systematically studied how proteins adsorb onto hydrophobins that are bound as monomolecular layers on nonpolar surfaces. We found that several types of proteins readily adsorb onto hydrophobins, but only under defined conditions of pH and ionic strength. The binding conditions were also highly dependent on the adhering protein. By studying solution conditions such as pH and ionic strength, we conclude that the surface adhesion is due to selective Coulombic charge interactions. We conclude that hydrophobins can transform a nonpolar surface into one that efficiently recruits other proteins by charge interactions.