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Constraining the loop, releasing prion infectivity

Prions are the heretical protein-only agents that propagate biological information in the absence of nucleic acids. In mammals, prions are the infectious agents responsible for transmissible spongiform encephalopathies (TSEs), a group of fatal neurodegenerative diseases, including Creutzfeldt–Jakob disease in humans and bovine spongiform encephalopathy, scrapie, and chronic wasting disease in animals. Prions replicate by propagating the misfolding of the infectious prion protein (termed PrPSc) to the homologous, but natively-folded cellular version of the prion protein (termed PrPC) (1). Despite considerable recent advance in knowledge of the infectious agent's nature and its mechanism of replication, we are still far from understanding the structural requirements for the infectious folding of the prion protein. In this issue of PNAS, Sigurdsson et al. (2) provide an important step forward in elucidating the role of a specific domain of the prion protein in the generation of infectivity.