A mutant of hydrophobin HGFI tuning the self-assembly behaviour and biosurfactant activity

Hydrophobins are a series of low molecular weight proteins produced by filamentous fungi that play an important role in fungal growth. They have a globular structure and possess a unique hydrophobic patch on their surface that makes them amphiphilic, making them among the most surface-active proteins. Herein, the surface charge properties of HGFI, a class I hydrophobin from Grifola frondosa, were altered by replacing the negatively charged Glu24 with a positively charged Lys to generate the ME24 mutant. Pichia pastoris GS115 was used for recombinant expression of the ME24 mutant, which was purified by a two-step procedure. The function of the mutated residue in HGFI self-assembly was investigated. Reverse-phase high-performance liquid chromatography analysis revealed that the polarity of ME24 was enhanced compared with HGFI. Circular dichroism, thioflavin T assay, water contact angle and atomic force microscopy indicated that Glu24 participates in rodlet formation. Water solubility detection and dynamic light scattering showed that Glu24 affects the assembled state of HGFI in aqueous solution. The behaviour of the mutant in an emulsion, in the dispersion of insoluble materials and in large-scaled protein production suggests the functions of hydrophobins can be tuned for new applications.