Self-assembled films of hydrophobin proteins HFBI and HFBII studied in situ at the air/water interface

Hydrophobins are a group of surface-active fungal proteins known to adsorb to the air/water interface and self-assemble into highly crystalline films. We characterized the self-assembled protein films of two hydrophobins, HFBI and HFBII from Trichoderma reesei, directly at the air/water interface using Brewster angle microscopy, grazing-incidence X-ray diffraction, and reflectivity. Already in zero surface pressure, HFBI and HFBII self-assembled into micrometer-sized rafts containing hexagonally ordered two-dimensional crystallites with lattice constants of 55 Å and 56 Å, respectively. Increasing the pressure did not change the ordering of the proteins in the crystallites. According to the reflectivity measurements, the thicknesses of the hydrophobin films were 28 Å (HFBI) and 24 Å (HFBII) at 20 mN/m. The stable films could also be transferred to a silicon substrate. Modeling of the diffraction data indicated that both hydrophobin films contained six molecules in the unit cell, but the ordering of the molecules was somewhat different for HFBI and HFBII, suggesting specific protein−protein interactions.