A protein required for prion generation: [URE3] induction requires the Ras-regulated Mks1 protein

Infectious proteins (prions) can arise de novo as well as by transmission from another individual. De novo prion generation is believed responsible for most cases of Creutzfeldt–Jakob disease and for initiating the mad cow disease epidemic. However, the cellular components needed for prion generation have not been identified in any system. The [URE3] prion of Saccharomyces cerevisiae is an infectious form of Ure2p, apparently a self-propagating amyloid. We now demonstrate a protein required for de novo prion generation. Mks1p negatively regulates Ure2p and is itself negatively regulated by the presence of ammonia and by the Ras–cAMP pathway. We find that in mks1Δ strains, de novo generation of the [URE3] prion is blocked, although [URE3] introduced from another strain is expressed and propagates stably. Ras2(Val19) increases cAMP production and also blocks [URE3] generation. These results emphasize the distinction between prion generation and propagation, and they show that cellular regulatory mechanisms can critically affect prion generation.