Investigation of the relationship between the rodlet formation and Cys3-Cys4 loop of the HGFI hydrophobin.

We used protein fusion technology to transplant the Cys3-Cys4 loop of HGFI (a class I hydrophobin from Grifola frondosa) into a nonamyloidogenic hydrophobin HFBI (a class II hydrophobin from Trichoderma reesei) and replace the corresponding amino acids between Cys3 and Cys4 in this protein to identify whether this loop renders it amyloidogenic. Water contact angle (WCA) and X-ray photoelectron spectroscopy (XPS) measurements demonstrated that the mutant protein HFBI-AR could form amphipathic membranes by self-assembling at the hydrophilic mica and hydrophobic polystyrene surfaces. This property enabled the mutant protein to alter the surface wettabilities of polystyrene and mica as well as to change the elemental composition of siliconized glass. Atomic force microscopy (AFM) measurements indicated that, unlike class I hydrophobins, no amyloid-like rodlets were observed on the mutant protein HFBI-AR coated mica surface. Moreover, the Cys3-Cys4 region could not catalyze the mutant protein HFBI-AR to drive intermolecular association and formation of a cross-β rodlet structure to resist depolymerization in organic solvents when it self-assembled at water-air interfaces. These results demonstrate that the Cys3-Cys4 loop is not the major determinant that initiates HGFI to form rodlets or account for the unique properties of the proteins.