PRION

also called Whalebird, any of several species of small Antarctic seabirds of the genus Pachyptila, in the family Procellariidae (order Procellariiformes). All are blue-gray above and whitish below. Among the broad-billed species, the bill, unique among petrels, is flattened, with the upper mandible fringed with strainers (lamellae) not unlike those in the mouths of ducks. The thin floor of the mouth is distensible, forming a small pouch like that of the pelican. In feeding, the bird "hydroplanes," skittering across the water with its wings out and propelling itself with its feet as it dips its bill into the water for small marine invertebrates, such as squid and crustaceans. The smallest of the four species is the fairy prion (P. turtur), about 20 cm (8 inches) long; the largest is the broad-billed prion (P. forsteri) at about 27 cm. Most of the prions breed in burrows on Antarctic and sub-Antarctic islands. The broad-billed prion is more northerly in distribution, breeding on islands located between 35 and 60 S. A related bird, the short-tailed shearwater (Puffinus tenuirostris), is known in Alaska as the whalebird (see shearwater). a disease-causing agent that is responsible for a variety of fatal neurodegenerative diseases of animals and humans called transmissible spongiform encephalopathies. A prion is an aberrant form of a normally harmless protein found in mammals and birds. The normal form of the protein, whose function is unknown, is located on the surface of cells in the brain. Only when it is in the aberrant configuration does the prion protein cause disease. The pathogenic protein can enter the brain through infection, or it can arise from a mutation in the gene that encodes the protein. Once present in the brain it multiplies by inducing benign proteins to refold into the aberrant shape. The mechanism by which the conformational charge is achieved is not fully understood, but an additional factor, possibly another protein normally found in the body, may be involved. The normal protein structure is believed to consist of a number of flexible coils called alpha helices . In the aberrant protein some of these helices are stretched out into flat structures called beta strands. The normal protein conformation can be degraded rather easily by cellular enzymes called proteases, but the irregular protein shape is more resistant to this enzymatic activity. Thus, as prion proteins multiply they are not broken down by proteases and instead accumulate within nerve cells, destroying them. Progressive nerve cell destruction eventually causes brain tissue to become riddled with holes in a spongelike, or spongiform, pattern. Diseases caused by prions include four disorders that affect humans: Creutzfeldt-Jakob disease, Gerstmann-Strussler-Scheinker disease, fatal familial insomnia, and kuru. Other prion diseases, such as scrapie, bovine spongiform encephalopathy (commonly called mad cow disease), and chronic wasting disease of mule deer and elk, are suffered by animals. For decades physicians believed that these diseases resulted from infection with slow-acting viruses, so-called because of the lengthy incubation times required for the illnesses to develop. These diseases were, and sometimes still are, referred to as slow infections. The pathogenic agent of these diseases does have certain viral attributes, such as extremely small size and strain variation, but other properties are atypical of viruses. In particular the agent is resistent to exposure to ionizing ultraviolet radiation, which normally inactivates viruses by destroying their nucleic acid. Prions are unlike all other known disease-causing organisms in that they appear to lack nucleic acid-i.e., DNA or RNA-which is the genetic material that all other life-forms contain. Another unusual characteristic of prions is that they not only cause disease through infection but can be responsible for hereditary and sporadic forms of disease as well-for example, Creutzfeldt-Jakob disease manifests in all three ways, with sporadic cases being the most common. The prion protein can act as an infectious agent, spreading disease when transmitted to another organism, or it can arise from an inherited mutation. Prion diseases also show a sporadic pattern of incidence, meaning that they seem to appear in the population at random. The underlying molecular process that causes the aberrant protein to form in these cases remains to be delineated. Researchers suspect that other neurodegenerative disorders such as Alzheimer's disease or Parkinson's disease may arise from molecular mechanisms similar to those that cause the prion diseases. The concept of an infectious particle that lacks nucleic acid is unprecedented in biology. Because of its unorthodoxy, the scientific community initially viewed the prion theory with skepticism. Attempts to purify the disease-causing agent proved difficult, but in the early 1980s the American biochemist Stanley B. Prusiner and colleagues identified the "proteinaceous infectious particle," a name that was shortened to "prion" (pronounced "pree-on"). Efforts to isolate an associated nucleic acid have proved fruitless, and the prion model has gained widespread acceptance. Nevertheless many questions remain to be answered about this unique particle.