Prion

A prion ˈpriːɒn is a misfolded protein that can transmit its misfoldedness to normal variants of the same protein and trigger cellular death. Prions cause prion diseases known as transmissible spongiform encephalopathies (TSEs) that are transmissible, fatal neurodegenerative diseases in humans and animals. The proteins may misfold sporadically, due to genetic mutations, or by exposure to an already misfolded protein. The consequent abnormal three-dimensional structure confers on them the ability to cause misfolding of other proteins. The word prion is derived from the term "proteinaceous infectious particle". The hypothesized role of a protein as an infectious agent stands in contrast to all other known infectious agents such as viroids, viruses, bacteria, fungi, and parasites, all of which contain nucleic acids (DNA, RNA, or both). Prion isoforms of the major prion protein (PrP), whose specific function is uncertain, are hypothesized as the cause of transmissible spongiform encephalopathies (TSEs). These include scrapie in sheep, chronic wasting disease (CWD) in deer, bovine spongiform encephalopathy (BSE) in cattle (commonly known as "mad cow disease") and Creutzfeldt–Jakob disease (CJD) in humans. All known prion diseases in mammals affect the structure of the brain or other neural tissue; all are progressive, have no known effective treatment, and are always fatal. All known mammalian prion diseases were caused by the prion protein (PrP) until 2015, when a prion form of alpha-synuclein was hypothesized to cause multiple system atrophy (MSA). Prions are a type of intrinsically disordered protein, which change their conformation unless they are bound to a specific partner such as another protein. With a prion, two protein chains are stabilized if one binds to another in the same conformation. The probability of this happening is low, but once it does, the combination of the two is very stable. Then more units can get added, making a sort of "fibril".