The invention relates to a method of producing useful materials from filament-forming α-helical proteins or filaments made of such proteins. The method comprises allowing filament-forming α-helical proteins to self-assemble into α-helix containing filaments and forming fibres, films or bulk materials from the filaments. The materials are stretched to strain the filaments so that the α-helices substantially irreversibly change to β-sheet forms. The filament-forming α-helical proteins can comprise intermediate filament proteins. In a specific embodiment, the filament-forming proteins comprise hagfish slime thread IF proteins.