prion: a standalone construct

Stanley B. Prusiner proposed the term prion in 1982 (Prusiner, 1982) to name the agent of scrapie, a fatal disease of the nervous system in sheep and goats, after finding that its infectivity survived treatments that destroy nucleic acids yet was abolished by treatments that damage protein. The word is a contraction of proteinaceous infectious particle, and it named a pathogen that carries no DNA or RNA and instead propagates by protein alone. In the refinement that followed, recognised by the 1997 Nobel Prize in Physiology or Medicine (Prusiner, 1998; Colby & Prusiner, 2011), a prion is understood as a misfolded form of a normal cellular protein, in mammals the prion protein PrP, that acts as a template and converts further copies of the normal protein into the misfolded form, so multiplying without a genome. This is what makes the construct a landmark: it is the recognised counterexample to the assumption that a transmissible agent must carry nucleic acid, and it names the mechanism behind a group of neurodegenerative conditions in animals and humans, among them scrapie, bovine spongiform encephalopathy, and Creutzfeldt-Jakob disease.[1][2][3]

Unit

A single self-propagating class of infectious agent: a misfolded protein that templates its own conformation without any nucleic acid

Recovery means

Not applicable

Measured by

Identified by transmission of disease that resists nucleic-acid-inactivating procedures but is sensitive to protein-denaturing ones, and by detecting the protease-resistant misfolded prion protein through immunoassays and in vitro conversion assays