A new kind of prion: a modified protein necessary for its own modification

We recently described an infectious protein (prion) unrelated to amyloid formation, that is an enzyme whose precursor can only be activated by the active form of the enzyme. All previously described infectious proteins are self-propagating amyloid forms of chromosomally encoded proteins. The infectious enzyme, vacuolar protease B (PrB), can activate its own precursor in an indefinitely self-propagating process. Transfer from cell to cell of cytoplasm containing active protease B transmits this non-chromosomal gene. The importance of this system is that many protein-modifying enzymes may act on themselves, and if conditions are right, may become prions as well.