PRNP

Prion protein
250px PDB rendering based on 1ag2.
Available structures PDB Ortholog search: PDBe, RCSB List of PDB id codes 1E1G, 1E1J, 1E1P, 1E1S, 1E1U, 1E1W, 1FKC, 1FO7, 1H0L, 1HJM, 1HJN, 1I4M, 1OEH, 1OEI, 1QLX, 1QLZ, 1QM0, 1QM1, 1QM2, 1QM3, 2IV4, 2IV5, 2IV6, 2K1D, 2KUN, 2LBG, 2LEJ, 2LFT, 2LSB, 2LV1, 2M8T, 2OL9, 2W9E, 3HAF, 3HAK, 3HEQ, 3HER, 3HES, 3HJ5, 3HJX, 3MD4, 3MD5, 3NHC, 3NHD, 3NVF, 4DGI, 4E1H, 4E1I, 4KML, 4N9O
Identifiers
Symbols	PRNP ; ASCR; AltPrP; CD230; CJD; GSS; KURU; PRIP; PrP; PrP27-30; PrP33-35C; PrPc; p27-30
External IDs	OMIM: 176640 MGI: 97769 HomoloGene: 7904 ChEMBL: 4869 GeneCards: PRNP Gene
Gene ontology Molecular function • copper ion binding • protein binding • microtubule binding • tubulin binding • identical protein binding • ATP-dependent protein binding • ion channel binding • chaperone binding Cellular component • nucleus • cytoplasm • mitochondrial outer membrane • endoplasmic reticulum • Golgi apparatus • plasma membrane • cell surface • integral component of membrane • extrinsic component of membrane • anchored component of membrane • membrane raft • extracellular exosome Biological process • negative regulation of protein phosphorylation • cellular copper ion homeostasis • response to oxidative stress • cell cycle arrest • axon guidance • learning or memory • metabolic process • negative regulation of interferon-gamma production • negative regulation of interleukin-17 production • negative regulation of interleukin-2 production • regulation of protein localization • cellular response to drug • negative regulation of apoptotic process • negative regulation of sequence-specific DNA binding transcription factor activity • negative regulation of activated T cell proliferation • response to cadmium ion • negative regulation of T cell receptor signaling pathway • protein homooligomerization • negative regulation of calcineurin-NFAT signaling cascade • cellular response to copper ion • regulation of potassium ion transmembrane transport Sources: Amigo / QuickGO
RNA expression pattern
File:PBB GE PRNP 201300 s at tn.png
File:PBB GE PRNP 215707 s at tn.png
More reference expression data
Orthologs
Species	Human	Mouse
Entrez	5621	19122
Ensembl	ENSG00000171867	ENSMUSG00000079037
UniProt	F7VJQ1	P04925
RefSeq (mRNA)	NM_000311	NM_001278256
RefSeq (protein)	NP_000302	NP_001265185
Location (UCSC)	Chr 20: 4.69 – 4.7 Mb	Chr 2: 131.91 – 131.94 Mb
PubMed search	[2]	[3]

PRNP (PRioN Protein) is the human gene encoding for the major prion protein PrP (for prion protein or protease-resistant protein), also known as CD230 (cluster of differentiation 230). Expression of the protein is most predominant in the nervous system but occurs in many other tissues throughout the body.

The protein can exist in multiple isoforms, the normal PrP^C, the disease-causing PrP^Sc, and an isoform located in mitochondria. The misfolded version PrP^Sc is associated with a variety of cognitive disorders and neurodegenerative diseases such as Bovine Spongiform Encephalopathy, Chronic Wasting Disease, Creutzfeldt–Jakob disease, Fatal Familial Insomnia, Feline spongiform encephalopathy, Gerstmann–Sträussler–Scheinker syndrome, Kuru, Scrapie, Transmissible mink encephalopathy, Ungulate spongiform encephalopathy, and Variant Creutzfeldt-Jakob Disease.

Gene

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.

Structure

PrP is highly conserved through mammals, lending credence to application of conclusions from test animals such as mice. Comparison between primates is especially similar, ranging from 92.9-99.6% similarity in amino acid sequences. The human protein structure consists of a globular domain with three α-helices and a two-strand antiparallel β-sheet, an NH₂-terminal tail, and a short COOH-terminal tail. A glycophosphatidylinositol (GPI) membrane anchor at the COOH-terminal tethers PrP to cell membranes, and this proves to be integral to the transmission of conformational change; secreted PrP lacking the anchor component is unaffected by the infectious isoform.

The primary sequence of PrP is 253 amino acids long before post-translational modification. Signal sequences in the amino- and carboxy- terminal ends are removed posttranslationally, resulting in a mature length of 208 amino acids. For human and golden hamster PrP, two glycosylated sites exist on helices 2 and 3 at Asn181 and Asn197. Murine PrP has glycosylation sites as Asn180 and Asn196. A disulfide bond exists between Cys179 of the second helix and Cys214 of the third helix (human PrP^C numbering).

PrP messenger RNA contains a pseudoknot structure (prion pseudoknot), which is thought to be involved in regulation of PrP protein translation.

Ligand-binding

The mechanism for conformational conversion to the scrapie isoform is speculated to be an elusive ligand-protein, but, so far, no such compound has been identified. However, a large body of research has developed on candidates and their interaction with the PrP^C.

Copper, zinc, manganese, and nickel are confirmed PrP ligands that bind to its octarepeat region. Ligand binding causes a conformational change with unknown effect. Heavy metal binding at PrP has been linked to resistance to oxidative stress arising from heavy metal toxicity.

PrP^C (normal cellular) isoform

Although the precise function of PrP is not yet known, it is possibly involved in the transport of ionic copper to cells from the surrounding environment. Researchers have also proposed roles for PrP in cell signaling or in the formation of synapses. PrP^C attaches to the outer surface of the cell membrane by a glycosylphosphatidylinositol anchor at its C-terminal Ser231.

Prion protein contains 5 amino-terminal octapeptide repeats with sequence PHGGGWGQ. This is thought to generate a copper-binding domain via nitrogen atoms in the histidine imidazole side-chains and deprotonated amide nitrogens from the 2nd and 3rd glycines in the repeat. The ability to bind copper is, therefore, pH-dependent. NMR shows copper binding results in a conformational change at the N-terminus.

PrP^Sc (scrapie) isoform

PrP^Sc is a conformational isoform of PrP^C, but this orientation tends to accumulate in compact, protease-resistant aggregates within neural tissue. The abnormal PrP^Sc isoform has a different secondary and tertiary structure from PrP^C, but identical primary sequence. Circular dichroism shows that normal PrP^C had 43% alpha helical and 3% beta sheet content, whereas PrP^Sc was only 30% alpha helix and 43% beta sheet. This refolding renders the PrP^Sc isoform extremely resistant to proteolysis.

The propagation of PrP^Sc is a topic of great interest, as its accumulation is a pathological cause of neurodegeneration. Based on the progressive nature of spongiform encephalopathies, the predominant hypothesis posits that the change from normal PrP^C is caused by the presence and interaction with PrP^Sc. Strong support for this is taken from studies in which PRNP-knockout mice are resistant to the introduction of PrP^Sc. Despite widespread acceptance of the conformation conversion hypothesis, some studies mitigate claims for a direct link between PrP^Sc and cytotoxicity.

Polymorphisms at sites 136, 154, and 171 are associated with varying susceptibility to scrapie. Polymorphisms of the PrP-VRQ form and PrP-ARQ form are associated with increased susceptibility, whereas PrP-ARR is associated with resistance.

The National Scrapie Plan aims to breed out these scrapie polymorphisms by increasing the frequency of the resistant allele. However, PrP-ARR polymorphisms are susceptible to atypical scrapie, so this may prove unfruitful.

Function

Nervous system

The strong association to neurodegenerative diseases raises many questions of the function of PrP in the brain. A common approach is using PrP-knockout and transgenic mice to investigate deficiencies and differences. Initial attempts produced two strains of PrP-null mice that shows no physiological or developmental differences when subjected to an array of tests. However, more recent strains have shown significant cognitive abnormalities.

As the null mice age, a marked loss of Purkinje cells in the cerebellum results in decreased motor coordination. However, this effect is not a direct result of PrP’s absence, and rather arises from increased Doppel gene expression. Other observed differences include reduced stress response and increased exploration of novel environments.

Circadian rhythm is altered in null mice. Fatal familial insomnia is thought to be the result of a point mutation in PRNP at codon 178, which corroborates PrP’s involvement in sleep-wake cycles. In addition, circadian regulation has been demonstrated in PrP mRNA, which cycles regularly with day-night.

Memory

While null mice exhibit normal learning ability and short-term memory, long-term memory consolidation deficits have been demonstrated. As with ataxia, however, this is attributable to Doppel gene expression. However, spatial learning, a predominantly hippocampal-function, is decreased in the null mice and can be recovered with the reinstatement of PrP in neurons; this indicates that loss of PrP function is the cause. The interaction of hippocampal PrP with laminin (LN) is pivotal in memory processing and is likely modulated by the kinases PKA and ERK1/2.

Further support for PrP’s role in memory formation is derived from several population studies. A test of healthy young humans showed increased long-term memory ability associated with an MM or MV genotype when compared to VV. Down Syndrome patients with a single valine substitution have been linked to earlier cognitive decline. Several polymorphisms in PRNP have been linked with cognitive impairment in the elderly as well as earlier cognitive decline. All of these studies investigated differences in codon 129, indicating its importance in the overall functionality of PrP, in particular with regard to memory.

Neurons and synapses

PrP is present in both pre- and post-synaptic neuron cells, and the greatest concentration is in the pre-synaptic cells. Considering this and PrP’s suite of behavioral influences, the neural cell functions and interactions are of particular interest. Based on the copper ligand, one proposed function casts PrP as a copper buffer for the synaptic cleft. In this role, the protein could serve as either a copper homeostasis mechanism, a calcium modulator, or a sensor for copper or oxidative stress. Loss of PrP function has been linked to long-term potentiation (LTP). This effect can be positive or negative and is due to changes in neuronal excitability and synaptic transmission in the hippocampus.

Some research indicates PrP involvement in neuronal development, differentiation, and neurite outgrowth. The PrP-activated signal transduction pathway is associated with axon and dendritic outgrowth with a series of kinases.

Immune system

Though most attention is focused on PrP’s presence in the nervous system, it is also abundant in immune system tissue. PrP immune cells include haematopoietic stem cells, mature lymphoid and myeloid compartments, and certain lymphocytes; also, it has been detected in natural killer cells, platelets, and monocytes. T cell activation is accompanied by a strong up-regulation of PrP, though it is not requisite. The lack of immuno-response to transmissible spongiform encephalopathies (TSE), neurodegenerative diseases caused by prions, could stem from the tolerance for PrP^Sc.

Muscles, liver, and pituitary

PrP-null mice provide clues to a role in muscular physiology when subjected to a forced swimming test, which showed reduced locomotor activity. Ageing mice with an overexpression of PRNP showed significant degradation of muscle tissue.

Though present, very low levels of PrP exist in the liver and could be associated with liver fibrosis. Presence in the pituitary has been shown to affect neuroendrocrine function in amphibians, but little is known concerning mammalian pituitary PrP.

Cellular

Varying expression of PrP through the cell cycle has led to speculation on involvement in development. A wide range of studies has been conducted investigating the role in cell proliferation, differentiation, death, and survival. Engagement of PrP has been linked to activation of signal transduction.

Modulation of signal transduction pathways has been demonstrated in cross-linking with antibodies and ligand-binding (hop/STI1 or copper). Given the diversity of interactions, effects, and distribution, PrP has been proposed as dynamic surface protein functioning in signaling pathways. Specific sites along the protein bind other proteins, biomolecules, and metals. These interfaces allow specific sets of cells to communicate based on level of expression and the surrounding microenvironment. The anchoring on a GPI raft in the lipid bilayer supports claims of an extracellular scaffolding function.

Diseases caused by PrP misfolding

Main article: Transmissible spongiform encephalopathy

More than 20 mutations in the PRNP gene have been identified in people with inherited prion diseases, which include the following:

• Creutzfeldt-Jakob disease - glutamic acid-200 is replaced by lysine while valine is present at amino acid 129
• Gerstmann-Sträussler-Scheinker syndrome - usually a change in codon 102 from proline to leucine
• fatal familial insomnia - aspartic acid-178 is replaced by asparagine while methionine is present at amino acid 129

The conversion of PrP^C to PrP^Sc conformation is the mechanism of transmission of fatal, neurodegenerative transmissible spongiform encephalopathies (TSE). This can arise from genetic factors, infection from external source, or spontaneously for reasons unknown. Accumulation of PrP^Sc corresponds with progression of neurodegeneration and is the proposed cause. 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 made. These mutations cause the cell to make prion proteins with an abnormal structure. The abnormal protein PrP^Sc accumulates in the brain and destroys nerve cells, which leads to the mental and behavioral features of prion diseases.

Several other changes in the PRNP gene (called polymorphisms) do not cause prion diseases but may affect a person's risk of developing these diseases or alter the course of the disorders. An allele that codes for a PRNP variant, G127V, provides resistance to Kuru.

In addition, some prion diseases can be transmitted from external sources of PrP^Sc.

• Scrapie - fatal neurodegenerative disease in sheep, not transmissible to humans
• Bovine spongiform encephalopathy (mad-cow disease) - fatal neurodegenerative disease in cows, which can be transmitted to humans by ingestion of brain, spinal, or digestive tract tissue of an infected cow
• Kuru - TSE in humans, transmitted via funerary cannibalism. Generally, affected family members were given, by tradition, parts of the central nervous system according to ritual when consuming deceased family members.

Other clinical significance

Mutations in PRNP are associated with Alzheimer's disease .

Interactions

A strong interaction exists between PrP and cochaperone Hsp70/ Hsp90 organizing protein/Stress-induced protein 1 (hop (protein)/STI1).

External links

• = gene&part = prion GeneReviews/NIH/NCBI/UW entry on Genetic Prion Diseases,Transmissible Spongiform Encephalopathies (TSEs). Includes: Familial Creutzfeldt-Jakob Disease (fCJD), Fatal Familial Insomnia, Gerstmann-Sträussler-Scheinker Disease
• GeneCard
• PRNP protein, human at the US National Library of Medicine Medical Subject Headings (MeSH)
• Susan Lindquist's seminars: "The Surprising World of Prion Biology"