Spotlight on Class I Hydrophobins: Their Intriguing Biochemical Properties and Industrial Prospects

A particular class of amphiphilic proteins, the hydrophobins (HPBs), have been described as the most surface-active proteins known. They are small cysteine-rich proteins produced by filamentous fungi and are able to spontaneously self-assemble into amphipathic layers at air–water and hydrophobic/hydrophilic interfaces, playing a key role in different stages of fungal life cycle. Despite the conserved pattern of cysteines, the amino acid sequences of HPBs show low sequence similarity. They can usually be divided into two classes characterized by the different length of the inter-cysteine spaces. Class I HPBs are produced in both ascomycete and basidiomycete species and assemble into insoluble polymeric layers known as rodlets, sharing the cross-β structure with amyloid fibrils. These layers are extremely stable and can only be solubilized with harsh acid treatments, and the soluble forms can polymerize back into rodlets under appropriate conditions. The multiple potential applications of class I HPBs and interest in the industrial use of these proteins, both as emulsion stabilizers and for surface modification, are demonstrated by numerous research articles and patents which have been published and registered in recent years. The increasing demand of HPBs has led to challenges in their production and purification. HPB industrial-scale production and biotechnological applications can be advanced by the discovery and characterization of new members of this family, either naturally isolated or through protein genetic modification, thus also broadening opportunities for their uses.