Prion proteins are of two types. One is normal or cellular form which is denoted as PrP^c for prion protein cellular form. The other form is the pathogenic form denoted as PrP^Sc (Prion protein scrapie).

PrP^c, 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 PrP^c is more in olfactory bulb, hippocampus, prefrontal cortex and striatum.

The possible physiological functions of PrP^c 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 PrP^Sc which are protease resistant.

PrP^Sc: This is the pathological form of prion protein. It is devoid of any nucleic acid. Structurally, it is the abnormal isoform of PrP^c. The amino acid sequence of PrP^c and PrP^Sc is same in a single host. PrP^c consists of predominantly alpha helices. Refolding of these alpha helices to b pleated sheets converts PrP^c to PrP^Sc. The predominant b pleated sheet structure of PrP^Sc makes it more stable and confers it the ability to form aggregate.

Prion replication: The pathological form PrP^Sc acts as the template for prion replication. When it comes in contact with the cellular form PrP^c, it catalyses the conversion of PrP^c to PrP^Sc resulting in formation of two prions PrP^Sc via formation of a heterodimer of PrP^Sc and PrP^c. The two PrP^Sc now turns two more PrP^c to PrP^Sc which in turn, turns four more to PrP^Sc. This continues further leading to an exponential transformation and accumulation of prions. More and more accumulation of prions PrP^Sc leads to polymerization and formation of fibrillar masses that are visible as plaques (Fig. 1).

Fig1. Conversion of PrP^c to PrP^Sc

Differences between PrP^c and PrP^Sc are noted in Table 1.

Table1. Difference between normal (PrP^c) and pathological prion proteins (PrP^Sc)

Properties of prion proteins: Prion protein (PrP^Sc) 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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عادة بسيطة تدعم الصحة وتحافظ على الوزن المثالي

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مواضيع ذات صلة

HIV Infections in Humans: Laboratory Diagnosis

HIV Infections in Humans :Clinical Findings

HIV Infections in Humans : Pathogenesis and Pathology

Animal Lentivirus Systems

Classification of Lentiviruses

Viral DNA Is Integrated into the Host-Cell Genome in Some Nonlytic Viral Growth Cycles

Lytic Viral Growth Cycles Lead to Death of Host Cells

Viral Capsids Are Regular Arrays of One or a Few Types of Protein

Structure and Composition of Lentiviruses

HERPESVIRUSES

POLYOMAVIRUSES

Human T-Lymphotropic Viruses