The scrapie agent causes a neurodegenerative disorder that is closely related to Creutzfeldt-Jakob disease and kuru in humans. The accumulation of amyloid proteins and plaques in scrapie infected brain makes scrapie the best current model for Alzheimer's disease. In purified, protease-treated fractions containing the scrapie agent, PrP 27-30 is the major protein identified and scrapie associated fibrils (SAF) are the major ultrastructural component. PrP 27-30 is produced from a larger precursor by proteolytic cleavage; the precursor protein is encoded by a normal host gene. We have identified a larger form of the scrapie agent protein (PrP 35-38) in homogenates of infected hamster and mouse brains. We developed a method for isolating PrP 35-38 which does not use protease digestion and have purified PrP 35-38 from the 263K strain of the hamster scrapie agent. PrP 35-38 has an N-terminal amino acid sequence distinct from PrP 27-30. Our data indicate that PrP 27-30 is not the major form of PrP in situ, but is produced by proteolytic cleavage of PrP 35-38 during purification. We theorize that the conversion of a normal protein to PrP 35-38 by proteolytic cleavage and abnormal processing leads to the accumulation of the infectious scrapie agent and scrapie amyloid fibrils. Also, we suggest that biologically distinct scrapie agent strains may result from structural differences in their proteins. This project will examine the mechanisms that convert a normal precursor protein to a pathological and disease-initiating form (PrP 35-38) by defining the primary structure of the purified, intact scrapie agent protein, PrP 35-38. We propose to purify the scrapie agent and its full length protein, PrP 35-38, from different scrapie agent strains propagated in animal hosts of two different genotypes. We will study the biological, biochemical, structural, and ultrastructural properties of the purified agent and PrP 35-38. Strain variation in purified preparations will be examined and we will determine if structural differences in the scrapie agent proteins correlate with biologically distinct strains. We propose to isolate monomeric PrP 35-38 after denaturation of the SAF structures and determine if it is biologically active. Also, we will attempt to renature isolated PrP 35-38 to a native state and measure its biological activity. Our studies will interface with other projects in progress at this Institute, including studies of protease-treated SAF and PrP 27-30 and characterization of the scrapie protein precursor, p54.
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