A Structural and Functional Role for Disulfide Bonds in a Class II Hydrophobin

Hydrophobins are multifunctional, highly surface active proteins produced in filamentous fungi and can be identified by eight conserved cysteine residues, which form four disulfide bridges. These proteins can be subdivided into two classes based on their hydropathy profiles, solubility, and structures formed upon interfacial assembly. Here, we probe the structural and functional roles of disulfide bonds for a class II hydrophobin in different interfacial contexts by reducing its disulfides with 1,4-dithiothreitol and blocking the free thiols with iodoacetamide and then examining the protein secondary structure, emulsification capability, hydrophobic surface wetting, and solution self-assembly. Changes in circular dichroism spectra upon reduction and blocking of disulfides are consistent with an increase in the level of random coil secondary structure. Emulsification of octane in water using reduced and unreduced forms of class II hydrophobin showed a substantial loss of emulsification ability without disulfides and stable emulsion formation for hydrophobin with disulfides. Additionally, water contact angle measurements performed on polytetrafluoroethylene treated with solutions of reduced and unreduced hydrophobin showed efficient wetting of the hydrophobic surface for unreduced samples only. Lastly, Förster resonance energy transfer (FRET) was used to assess the role of disulfides in self-assembly in solution, and near complete loss of the FRET signal is consistent with a model in which solution self-assembly does not occur after reduction and blocking of the disulfides. From this, we conclude that, in contrast to class I hydrophobins, the disulfides of this class II hydrophobin are required for protein structural stability, surface activity at both liquid-liquid and solid-liquid interfaces, and solution self-assembly.