Protostadienol synthase which cyclizes (3S)-oxidosqualene to (17Z)-protosta-17(20),24-dien-3-beta-ol (protostadienol), the biosynthetic precursor of helvolic acid, a secondary metabolite which promotes virulence. (S)-2,3-epoxysqualene = (17Z)-protosta-17(20),24-dien-3beta-ol Belongs to the terpene cyclase/mutase family.