Figure 5.

Enzymes needed for their own activation can be prions. “Prion” means “infectious protein”, not necessarily amyloid based. If an enzyme is essential for activation of its own precursor, then cells without the active form produce the same as progeny, and those with the active form produce offspring of the same kind. Transmission of just the active form (the protein only) from one cell to another lacking it, transmits the self-propagating activity, and so is an infectious protein. Two such systems have been described, the vacuolar protease B of S. cerevisiae,⁶⁹ and a protein kinase of Podospora anserina.⁷²