A “PRION” is an infectious agent comprised only of protein, called PrP (abbreviation for “Prion Protein”). Prions are remarkable in that they can fold in multiple, structurally distinct, means –– at least one of which is transmissible to other prion proteins, leading to diseases that are similar to viral infections. They are believed to be the cause of transmissible spongiform encephalopathies (TSEs), among other “”slow-virus” diseases.
Prions were initially identified as the causative agent in bovine spongiform encephopathy (BSE; commonly called “mad cow disease”) — and “scrapie” in sheep. Human prion diseases include Creutzfeldt-Jakob Disease (CJD) and a variant form of CJD (vCJD), Gerstmann-Sträussler-Scheinker syndrome, fatal familial insomnia, and kuru. A 2015 study [Prusiner et al., Proc Natl Acad Sci USA 112: E5308–5317] concluded that multiple system atrophy (MSA) –– a rare human neurodegenerative disease that was fatal (2o12) in my golfing buddy –– is caused by a misfolded version of a protein called alpha-synuclein, and is therefore also classifiable as a prion disease. Several yeast proteins have been identified as having prionogenic properties as well.
A protein –– as an INDEPENDENT infectious agent –– stands in contrast to all other known infectious agents such as viruses, bacteria, fungi, and parasites. All of these contain nucleic acids (DNA, RNA, or both). For this reason, a small number of researchers still consider the prion/TSE hypothesis “unproven.” The mechanisms by which prions are self-propagating protein aggregates –– that act as protein-based elements of inheritance –– will require further studies.
Although prevalent in eukaryotes (i.e. organisms having pairs of chromosomes), including yeast and fungi, prions have not been identified in bacteria. In the attached intriguing paper, authors found that a bacterial protein, transcription terminator Rho of Clostridium botulinum (Cb-Rho), could form a prion. Authors identified a candidate prion-forming domain (cPrD) in Cb-Rho and showed that it conferred amyloidogenicity (ability to generate amyloids) on Cb-Rho and could functionally replace the PrD of a yeast prion-forming protein..!!
Moreover, its cPrD enabled Cb-Rho, in Escherichia coli, to access alternative conformations — a soluble form that terminated transcription efficiently and an aggregated, self-propagating prion form that was functionally compromised. The prion form caused genome-wide changes in the transcriptome. Thus, Cb-Rho functions as a protein-based element of inheritance in bacteria, suggesting that the emergence of prions predates the evolutionary split between eukaryotes and bacteria (i.e. organisms having a single set of chromosomes).
Science 13 Jan 2o17; 355: 198–201