# Major prion protein
**Major prion protein** (PrP; also called PrP^C^ in its normal cellular form) is a cell-surface glycoprotein encoded in humans by the *PRNP* gene on chromosome 20, best known because its misfolded isoform, PrP^Sc^, is the infectious agent of the transmissible spongiform encephalopathies, a group of invariably fatal neurodegenerative diseases that includes Creutzfeldt-Jakob disease (CJD), Gerstmann-Sträussler-Scheinker syndrome (GSS), fatal familial insomnia (FFI), scrapie in sheep, bovine spongiform encephalopathy (BSE) in cattle, and chronic wasting disease (CWD) in cervids[1](https://medlineplus.gov/genetics/gene/prnp/) • [2](https://pmc.ncbi.nlm.nih.gov/articles/PMC4213568/). [Stanley B. Prusiner](https://www.edgepedia.org/stanley-b-prusiner), a neurologist at the [University of California, San Francisco](https://www.edgepedia.org/university-of-california-san-francisco), named the proteinaceous infectious particle a "prion" and received the 1997 [Nobel Prize in Physiology or Medicine](https://www.edgepedia.org/nobel-prize-in-physiology-or-medicine) for its discovery[3](https://www.livescience.com/16342-nobel-prize-medicine-history-list.html).
| Key fact | Detail |
|---|---|
| Gene | *PRNP* on chromosome 20 (20p13), three exons[4](https://pmc.ncbi.nlm.nih.gov/articles/PMC10579786/) |
| Protein size | 253 amino acids before processing; mature membrane form of 209 residues[2](https://pmc.ncbi.nlm.nih.gov/articles/PMC4213568/) • [5](https://pmc.ncbi.nlm.nih.gov/articles/PMC6721516/) |
| Structure | N-terminal octapeptide repeat domain, three α-helices, a short β-sheet, one disulfide bond, two N-glycans (Asn181, Asn197), C-terminal GPI anchor[2](https://pmc.ncbi.nlm.nih.gov/articles/PMC4213568/) |
| Normal function | Unknown; proposed roles include copper transport, synapse formation, and cell signaling[6](https://ncbi.nlm.nih.gov/books/NBK1229/?report=reader) |
| Pathogenic conversion | PrP^C^ converts to β-sheet-rich PrP^Sc^, which templates further conversion[7](https://www.annualreviews.org/doi/10.1146/annurev.genet.31.1.139) |
| Key polymorphism | Met129Val modifies disease risk and age of onset; E219K appears protective against sporadic CJD[1](https://medlineplus.gov/genetics/gene/prnp/) • [2](https://pmc.ncbi.nlm.nih.gov/articles/PMC4213568/) |
| Disease frequency | Sporadic CJD incidence about 1.5 cases per year per million worldwide[8](https://pmc.ncbi.nlm.nih.gov/articles/PMC10113350/) |
## Gene and protein structure
The human *PRNP* gene sits on the short arm of chromosome 20 (20p13) and contains three exons[4](https://pmc.ncbi.nlm.nih.gov/articles/PMC10579786/). No prion disease-associated mutations have been reported in the noncoding exons or introns; the pathogenic variants cluster in the protein-coding region[2](https://pmc.ncbi.nlm.nih.gov/articles/PMC4213568/).
The unprocessed protein is 253 amino acids long. Its N-terminal signal peptide (residues 1–22) is cleaved during synthesis, and a C-terminal signal (residues 232–254) is replaced by a glycophosphatidylinositol (GPI) anchor that tethers the mature 209-residue protein to the outer leaflet of the plasma membrane[2](https://pmc.ncbi.nlm.nih.gov/articles/PMC4213568/) • [5](https://pmc.ncbi.nlm.nih.gov/articles/PMC6721516/). Residues 51–91 form an octapeptide repeat region: a nonapeptide (PQGGGGWGQ) followed by four copies of the octapeptide PHGGGWGQ. The histidine in each repeat binds copper ions (Cu^2+^) with high affinity, and the protein is thought to participate in delivering extracellular copper to endosomal compartments[2](https://pmc.ncbi.nlm.nih.gov/articles/PMC4213568/). The structured C-terminal domain contains three α-helices, two short β-strands, and a disulfide bond between cysteines 179 and 214[2](https://pmc.ncbi.nlm.nih.gov/articles/PMC4213568/) • [5](https://pmc.ncbi.nlm.nih.gov/articles/PMC6721516/). A palindromic sequence, AGAAAAGA (residues 113–120), lies within the hydrophobic region and is thought to be important in the conversion of PrP^C^ to PrP^Sc^[2](https://pmc.ncbi.nlm.nih.gov/articles/PMC4213568/).
## Normal function
The precise physiological function of PrP^C^ is unknown. Proposed roles include the transport of copper into cells, protection of neurons from injury, formation of synapses, and cell signaling[1](https://medlineplus.gov/genetics/gene/prnp/) • [6](https://ncbi.nlm.nih.gov/books/NBK1229/?report=reader). A major constraint on functional studies has been the striking lack of phenotype in the first *Prnp* knockout mouse: mice lacking the protein developed normally, while becoming completely resistant to infection with scrapie brain extracts[9](https://journals.physiology.org/doi/10.1152/physrev.00007.2007). One synthesis of the evidence proposes that PrP^C^ acts as a dynamic cell-surface platform for the assembly of signaling modules, with selective molecular interactions translating into wide-ranging consequences for physiology and behavior[9](https://journals.physiology.org/doi/10.1152/physrev.00007.2007).
## Conversion to PrPSc and disease
Prion diseases arise when the normal, predominantly α-helical PrP^C^ is converted into PrP^Sc^, an anomalous conformer in which much of the α-helical structure becomes β-sheet[7](https://www.annualreviews.org/doi/10.1146/annurev.genet.31.1.139). PrP^Sc^ is rich in β-sheet, aggregation-prone, often protease-resistant, and self-replicating: it templates the conversion of further PrP^C^ molecules and polymerizes into infectious assemblies[8](https://pmc.ncbi.nlm.nih.gov/articles/PMC10113350/). Unlike pathogens with a nucleic acid genome, prions encipher strain-specific properties in the tertiary structure of PrP^Sc^ rather than in genes[7](https://www.annualreviews.org/doi/10.1146/annurev.genet.31.1.139).
Disease presents in three ways. Sporadic disease arises spontaneously with an incidence of about 1 per 10^6^ population per year; inherited disease follows *PRNP* mutations, with an incidence of about 1 per 10^7^–10^8^ per year; and acquired disease follows medical procedures or contaminated food[2](https://pmc.ncbi.nlm.nih.gov/articles/PMC4213568/). Clinically, the diseases produce rapidly progressive dementia, ataxia, and changes in memory, personality, and behavior, ultimately leading to death[1](https://medlineplus.gov/genetics/gene/prnp/). How the accumulation of PrP^Sc^ kills cells remains unknown; notably, systematic examination of patient brains found no spatial correlation between cell death and PrP^Sc^ deposits, and PrP^Sc^ accumulating in grafts placed in *Prnp*-knockout brains does not damage the neighboring PrP-null tissue[9](https://journals.physiology.org/doi/10.1152/physrev.00007.2007).
## Mutations and polymorphisms
More than 30 mutations in *PRNP* have been linked to inherited prion disease, including CJD, GSS, and FFI[2](https://pmc.ncbi.nlm.nih.gov/articles/PMC4213568/). They fall into two classes: point mutations that change single amino acids, and insertions or deletions in the octapeptide repeat region. Insertions of one to nine additional octapeptide repeats are pathogenic, whereas a deletion of a single repeat is a non-pathogenic polymorphism[2](https://pmc.ncbi.nlm.nih.gov/articles/PMC4213568/). A pathogenic variant is presumed to destabilize the protein, enhancing its propensity to attain the PrP^Sc^ state, which then acts as a conformational template for PrP^C^[6](https://ncbi.nlm.nih.gov/books/NBK1229/?report=reader).
**Codon 129.** The best-studied polymorphism is Met129Val, at which a person carries methionine, valine, or one of each. It does not cause disease but modifies risk: most affected individuals are homozygous (Met/Met or Val/Val), and Met/Met homozygosity is associated with earlier onset and rapidly progressive dementia, while Val/Val is more often associated with a prolonged course with ataxic onset[1](https://medlineplus.gov/genetics/gene/prnp/) • [2](https://pmc.ncbi.nlm.nih.gov/articles/PMC4213568/). Allele frequencies differ across populations, from M(0.66):V(0.34) in Caucasians to M(0.96):V(0.04) in Japan[2](https://pmc.ncbi.nlm.nih.gov/articles/PMC4213568/). A glutamate-to-lysine substitution at codon 219 (E219K), reported at an allelic frequency of 6% in the Japanese population, appears to protect against sporadic CJD[2](https://pmc.ncbi.nlm.nih.gov/articles/PMC4213568/).
## Species barrier
Prions transmit between species only inefficiently, a phenomenon called the species barrier. It can manifest as a complete block of disease development or as a prolonged incubation period on first passage that shortens on subsequent passage within the new species[10](https://pmc.ncbi.nlm.nih.gov/articles/PMC3003427/). Mechanistically, barriers reflect interactions between prion strains and PrP sequence variants, which govern the rate of conformational self-replication in the recipient[10](https://pmc.ncbi.nlm.nih.gov/articles/PMC3003427/). The barrier's practical importance was demonstrated when BSE from cattle transmitted to humans as variant CJD[10](https://pmc.ncbi.nlm.nih.gov/articles/PMC3003427/). Expressing a PrP identical in sequence to the infecting prion generally abrogates the barrier in mice, rabbits, and flies, supporting the view that no non-PrP genes are essential to cross-species transmission[8](https://pmc.ncbi.nlm.nih.gov/articles/PMC10113350/).
## Therapeutics aimed at PrP
Because PrP^C^ is the substrate for conversion, lowering its production is the leading therapeutic strategy. Prion-lowering antisense oligonucleotides (ASOs) and divalent siRNAs both slow disease progression and extend survival in prion-infected mice. Two programs have reached human trials: Ionis's PrProfile (NCT06153966), a PrP-lowering ASO trial that enrolled 56 patients in 2024 and opened a third arm in 2026, and PRiSM (NCT07444580), a first-in-human trial of a PrP-lowering divalent siRNA launched in 2026 and recruiting symptomatic patients[11](https://www.prionalliance.org/2026/07/16/mid-year-update-2026/). No PrP-lowering therapy has human efficacy data yet[11](https://www.prionalliance.org/2026/07/16/mid-year-update-2026/).
## References
1. [PRNP gene, MedlinePlus Genetics, National Library of Medicine](https://medlineplus.gov/genetics/gene/prnp/)
2. [Acevedo-Morantes CY, Wille H (2014). The Structure of Human Prions: From Biology to Structural Models. Viruses 6(10):3875–3892, PMC](https://pmc.ncbi.nlm.nih.gov/articles/PMC4213568/)
3. [Nobel Prize in Medicine: 1901–Present, Live Science](https://www.livescience.com/16342-nobel-prize-medicine-history-list.html)
4. [Human prion diseases and the prion protein – what is the current state of knowledge? PMC](https://pmc.ncbi.nlm.nih.gov/articles/PMC10579786/)
5. [Structural Consequences of Copper Binding to the Prion Protein, PMC](https://pmc.ncbi.nlm.nih.gov/articles/PMC6721516/)
6. [Genetic Prion Disease, GeneReviews, NCBI Bookshelf](https://ncbi.nlm.nih.gov/books/NBK1229/?report=reader)
7. [Prusiner SB, Scott MR (1997). Genetics of Prions. Annual Review of Genetics 31:139–175](https://www.annualreviews.org/doi/10.1146/annurev.genet.31.1.139)
8. [Prion assemblies: structural heterogeneity, mechanisms of formation, and role in species barrier, PMC](https://pmc.ncbi.nlm.nih.gov/articles/PMC10113350/)
9. [Physiology of the Prion Protein, Physiological Reviews](https://journals.physiology.org/doi/10.1152/physrev.00007.2007)
10. [The prion hypothesis: from biological anomaly to basic regulatory mechanism, PMC](https://pmc.ncbi.nlm.nih.gov/articles/PMC3003427/)
11. [Mid-year update 2026, Prion Alliance](https://www.prionalliance.org/2026/07/16/mid-year-update-2026/)
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*Topic: Encyclopedia › Life and health › Microorganisms and fungi › Viruses and acellular agents › Viroids, satellites, and prions › Prions › Major prion protein (PRNP)*
*Initially written Sep 17, 2026 · Reviewed: Sep 30, 2026; Oct 21, 2026 · Edited: Sep 30, 2026; Oct 21, 2026 · Last review: Oct 21, 2026*
*Copyright 2026 EdgeChat AI, a subsidiary of Biostate AI.*
License: Edgepedia Community License 1.0, https://www.edgepedia.org/edgepedia/license