Prion Proteins
المؤلف:
Baijayantimala Mishra
المصدر:
Textbook of Medical Virology
الجزء والصفحة:
2nd Edition , p285-286
2025-12-15
51
Prion proteins are of two types. One is normal or cellular form which is denoted as PrPc for prion protein cellular form. The other form is the pathogenic form denoted as PrPSc (Prion protein scrapie).
PrPc, which is the physiological form, is composed of 208 amino acids and is encoded by prion protein gene PRNP located at the short arm of chromosome 20 of human genome. It is also encoded in the gene of most mammals. The protein is located mainly on the cell membrane and expressed predominantly in CNS and lymphoreticular tissues and also in many other tissues such as kidney, skin, myocardium, endothelium. In the central nervous system, the protein is present in neurons (both pre- and post-synaptic), extraneural tissues and glial cells. Expression of PrPc is more in olfactory bulb, hippocampus, prefrontal cortex and striatum.
The possible physiological functions of PrPc are:
i. Cell receptor, cell adhesion and signal transduction
ii. Copper transporter
iii. Antioxidant by reducing oxidative stress
iv. Antiapoptotic activity
v. Zinc transporter which may associate it in neurodegeneration
vi. Synaptic formation and maintenance.
These proteins are protease sensitive in contrast to PrPSc which are protease resistant.
PrPSc: This is the pathological form of prion protein. It is devoid of any nucleic acid. Structurally, it is the abnormal isoform of PrPc. The amino acid sequence of PrPc and PrPSc is same in a single host. PrPc consists of predominantly alpha helices. Refolding of these alpha helices to b pleated sheets converts PrPc to PrPSc. The predominant b pleated sheet structure of PrPSc makes it more stable and confers it the ability to form aggregate.
Prion replication: The pathological form PrPSc acts as the template for prion replication. When it comes in contact with the cellular form PrPc, it catalyses the conversion of PrPc to PrPSc resulting in formation of two prions PrPSc via formation of a heterodimer of PrPSc and PrPc. The two PrPSc now turns two more PrPc to PrPSc which in turn, turns four more to PrPSc. This continues further leading to an exponential transformation and accumulation of prions. More and more accumulation of prions PrPSc leads to polymerization and formation of fibrillar masses that are visible as plaques (Fig. 1).
Fig1. Conversion of PrPc to PrPSc
Differences between PrPc and PrPSc are noted in Table 1.
Table1. Difference between normal (PrPc) and pathological prion proteins (PrPSc)
Properties of prion proteins: Prion protein (PrPSc) is highly resistant to protease, UV and g irradiation. These properties have major implications in sterilization of prion contaminated materials.
Prion proteins do not induce any immunological response or inflammation.
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