Subviral Agents and Prions

A number of virus-like agents have been discovered. These transmissible agents, including satellite viruses, viroids, and prions, are termed subviral agents, because they do not fit into the conventional definition of “virus” and they are smaller and simpler. A satellite virus possesses a particle structure indistinguishable from an ordinary virus, but it differs from a regular virus in that its propagation depends on another virus. Viroids, which are found only in plants, are constituted of “RNA only.” Conversely, prions, which are an etiologic agent of transmissible neurodegenerative diseases, are comprised of “protein only.” Unlike other infectious agents, a host-coded prion protein (PrP) solely constitutes the etiologic entity of prion diseases. In particular, PrP Sc , a misfolded aggregation-prone isoform of the cellular prion protein (PrP C ), is an infectious agent responsible for prion diseases.