Class i Hydrophobin Vmh2 Adopts Atypical Mechanisms to Self-Assemble into Functional Amyloid Fibrils

Hydrophobins are fungal proteins whose functions are mainly based on their capability to self-assemble into amphiphilic films at hydrophobic-hydrophilic interfaces (HHI). It is widely accepted that Class I hydrophobins form amyloid-like structures, named rodlets, which are hundreds of nanometers long, packed into ordered lateral assemblies and do not exhibit an overall helical structure. We studied the self-assembly of the Class I hydrophobin Vmh2 from Pleurotus ostreatus in aqueous solutions by dynamic light scattering (DLS), thioflavin T (ThT) fluorescence assay, circular dichroism (CD), cryogenic trasmission electron microscopy (Cryo-TEM) and TEM. Vmh2 does not form fibrillar aggregates at HHI. It exhibits spherical and fibrillar assemblies whose ratio depends on the protein concentration, when freshly solubilized at pH ≥ 7. Moreover it spontaneously self-assembles into isolated, micrometer long, and twisted amyloid fibrils, observed for the first time in fungal hydrophobins. This process is promoted by acidic pH, temperature, and Ca2+ ions. A model of self-assembly into amyloid-like structures has been proposed.