Function of Prion
The precise function of the prion protein is not known, but there is substantial evidence that it serves as a copper-dependent antioxidant.[
Tags: Proteins
Fungal prions have been investigated, leading to a deeper understanding of disease-forming mammalian prions. Prion-like proteins are found naturally in some plants and non-mammalian animals. Some of these are not associated with any disease state and may possibly even have a useful role[1]. Because of this, scientists reasoned that such proteins could give some sort of evolutionary advantage to their host. This was suggested to be the case in a species of fungus, Podospora anserina. Genetically compatible colonies of this fungus can merge together and share cellular contents such as nutrients and cytoplasm. A natural system of protective "incompatibility" proteins exists
Prion proteins were discovered in the yeast Saccharomyces cerevisiae by Reed Wickner in the early 1990s. Subsequently, a prion has also been found in the fungus Podospora anserina. These prions behave similarly to PrP, but are generally non-toxic to their hosts. Susan Lindquist's group at the Whitehead Institute has argued that some of the fungal prions are not associated with any disease state, but may have a useful role; however, researchers at the NIH have also provided strong arguments demonstrating that fungal prions should be considered a diseased state. Research into fungal prions has given strong support to the protein-only hypothesis
The infectious isoform of PrPC, known as PrPSc, is able to convert normal PrPC proteins into the infectious isoform by changing their conformation. Although the exact 3D structure of PrPSc is not known, there is increased ?-sheet content in the diseased form of the molecule, replacing normal areas of ?-helix.[16] Aggregations of these abnormal isoforms may form highly structured amyloid fibers. The end of a fiber acts as a template for the free protein molecules, causing the fiber to grow. Small differences in the amino acid sequence of prion-forming regions lead to distinct structural features on the surface of prion
Mark Purdey and Dr. David R. Brown have suggested that common prion is a beneficial molecule when bound to copper ions and that loss of this activity could cause disease. They have hypothesised that abnormal amounts of copper and manganese in the environment or animal feed could precipitate this. [49] Evidence favouring a pollutant cause: Manganese present increases the percentage of helical protein, while Copper decreases it.[50] Alzheimer's disease has similar symptoms, and has been attributed to excessive Aluminum at various times. Copper deficiency and Manganese proficiency have been found in the environment of affected cattle. Sporadic occurrences of diseased prion rule out genetics.
More than 20 mutations in the PRNP gene have been identified in people with inherited prion diseases, which include the following:[7][8] Creutzfeldt-Jakob disease - aspartic acid-178 is replaced by asparagine while valine is present at amino acid 129 Gerstmann-Sträussler-Scheinker syndrome - usually a change in codon 102 from proline to leucine[9] fatal insomnia - aspartic acid-178 is replaced by asparagine while methionine is present at amino acid 129[10] Some PRNP mutations lead to a change in single amino acids (the building blocks of proteins) in the prion protein. Others insert additional amino acids into the protein or cause an abnormally short protein to be
A gene for the normal protein has been isolated: the PRNP gene.[51] Some prion diseases can be inherited, and in all inherited cases there is a mutation in the PRNP gene. Many different PRNP mutations have been identified and it is thought that the mutations somehow make PrPC more likely to change spontaneously into the abnormal PrPSc form.[verification needed] While these mutations can occur throughout the gene encoding the prion protein the most notable code for the five octopeptide repeats found near the signal peptide of the protein, e.g. if the number of octopeptide repeats is increased to thirteen it
The protein that prions are made of is found throughout the body, even in healthy people and animals. However, the prion protein found in infectious material has a different folding pattern and is resistant to proteases, the enzymes in the body that can normally break down proteins. The normal form of the protein is called PrPC, while the infectious form is called PrPSc — the C refers to 'cellular' or 'common' PrP, while the Sc refers to 'scrapie', a prion disease occurring in sheep.[12] While PrPC is structurally well-defined, PrPSc is certainly polydisperse and defined at a relatively poor level.
A prion (IPA: /?pri??n/[1]listen (help·info)) is thought to be an infectious agent that, according to current scientific consensus, is comprised entirely of a propagated, mis-folded protein.[2] The mis-folded form of the prion protein has been implicated in a number of diseases in a variety of mammals, including bovine spongiform encephalopathy (BSE, also known as "mad cow disease") in cattle and Creutzfeldt-Jakob disease (CJD) in humans. All hypothesized prion diseases affect the structure of the brain or other neural tissue, and all are currently untreatable and are always fatal.[3] In general usage, prion refers to the theoretical unit of infection. Scientifically
Prions cause neurodegenerative disease by aggregating extracellularly within the central nervous system to form plaques known as amyloids, which disrupt the normal tissue structure. This disruption is characterized by "holes" in the tissue with resultant spongy architecture due to the vacuole formation in the neurons.[21] Other histological changes include astrogliosis and the absence of an inflammatory reaction.[22] While the incubation period for prion diseases is generally quite long, once symptoms appear the disease progresses rapidly, leading to brain damage and death.[23] Neurodegenerative symptoms can include convulsions, dementia, ataxia (balance and coordination dysfunction), and behavioural or personality changes. All known prion diseases,
Prior to the discovery of prions, it was thought that all pathogens used nucleic acids to direct their replication. The "protein-only hypothesis" states that a protein structure can replicate without the use of nucleic acid. This was initially controversial as it contradicts the so-called "central dogma of molecular biology," which describes nucleic acid as the central form of replicative information. Evidence in favor of a protein-only hypothesis include:[42] No virus particles, bacteria, or fungi have been conclusively associated with prion diseases No nucleic acid has been conclusively associated with infectivity; agent is resistant to degradation by nucleases No immune response to infection PrPSc experimentally transmitted
PRNP (PRioN Protein (Creutzfeld-Jakob disease, Gerstmann-Sträussler-Scheinker syndrome, fatal familial insomnia)) is a gene[1][2][3][4] that provides instructions to make a protein called the prion protein (PrP), which is expressed in the brain and several other tissues.[5][6] The human PRNP gene is located on the short (p) arm of chromosome 20 between the end (terminus) of the arm and position 12, from base pair 4,615,068 to base pair 4,630,233. PRNP has also recently been designated CD230 (cluster of differentiation 230).
In 2007, biochemist Surachai Supattapone and his colleagues at Dartmouth College produced purified infectious prions de novo from defined components (PrPC, co-purified lipids, and a synthetic polyanionic molecule) [43]. These researchers also showed that the polyanionic molecule required for prion formation was selectively incorporated into high-affinity complexes with PrP molecules, leading them to hypothesize that infectious prions may be composed of multiple host components, including PrP, lipid, and polyanionic molecules, rather than PrPSc alone [44].
The main function of Hop is to link Hsp70 and Hsp90 together. But recent investigations indicate that it also modulates the chaperone activities of the linked proteins and possibly interacts with other chaperones and proteins. Apart from its role in the Hsp70/Hsp90 "chaperone machine" it seems to participate in other protein complexes too (for example in the signal transduction complex EcR/USP and in the Hepatitis B virus reverse transcriptase complex, which enables the viral replication). It acts as a receptor for prion proteins too.[2][3] Hop is located in diverse cellular regions and also moves between the cytoplasm and the nucleus.
There is evidence that PrP may have a normal function in maintenance of long term memory.[18] Maglio and colleagues have shown that mice without the genes for normal cellular PrP protein have altered hippocampal LTP.
Although the identity and general properties of prions are now well understood, the mechanism of prion infection and propagation remains mysterious. It is often assumed that the diseased form directly interacts with the normal form to make it rearrange its structure. One idea, the "Protein X" hypothesis, is that an as-yet unidentified cellular protein (Protein X) enables the conversion of PrPC to PrPSc by bringing a molecule of each of the two together into a complex.[35] Current research suggests that the primary method of infection in animals is through ingestion. It is thought that prions may be deposited in the environment