Expression and characterization of a Grifola frondosa hydrophobin in Pichia pastoris

Hydrophobins are small secreted proteins produced by filamentous fungi. Being amphipathic and self-assembling, hydrophobins have drawn great attention since their discovery. The increase of production can reduce the cost and open up several new applications of hydrophobins. We successfully expressed recombinant Class I hydrophobin HGFI (rHGFI) by using pPIC9 vector with an alcohol oxidase 1 promoter in Pichia pastoris. Tricine-SDS-PAGE and Western blotting demonstrated that rHGFI, an 8 kDa protein, was secreted into the culture medium. The culture conditions of the transformant strain were optimized by controlling the methanol concentration and induction time. Ultrafiltration and reverse-phase high performance liquid chromatography were used to perform a large-scale purification of rHGFI. A stable production of rHGFI around 86 mg/L was achieved after the two-step purification. X-ray photoelectron spectroscopy and water contact angle measurements indicated that the functional rHGFI could self-assemble on hydrophobic siliconized glass and Teflon as well as on hydrophilic mica surfaces. A methylthiazol tetrazolium assay showed that rHGFI film could facilitate human aortic smooth muscle cell proliferation due to its cytocompatibility.