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