Copper induces increased beta-sheet content in the scrapie-susceptible ovine prion protein PrPVRQ compared with the resistant allelic variant PrPARR

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Abstract
Prion diseases are characterized by conformational change in the copper-binding protein PrP (prion protein). Polymorphisms in ovine PrP at amino acid residues 136, 154 and 171 are associated with variation in susceptibility to scrapie. PrPVRQ [PrP(Val136/Arg154/Gln171)] or PrPARQ [PrP(Ala136/Arg154/Gln171)] animals show susceptibility to scrapie, whereas those that express Ala136/Arg154/Arg171 (PrPARR) show resistance. Results are presented here that show PrPVRQ and PrPARR display different conformational responses to metal-ion interaction. At 37 degrees C copper induced different levels of b-sheet content in the allelic variants of ovine full-length prion protein (amino acid 25-232). PrPVRQ showed a significant increase in b-sheet content when exposed to copper at 37 degrees C, whereas PrPARR remained relatively unchanged. The conversion of a-helical PrPVRQ to b-sheet form was shown by CD spectroscopy and the decreased binding of C-terminal specific monoclonal anti-PrP antibodies. This conversion to an increased b-sheet form did not occur with truncated PrPVRQ (amino acids 89-233), which demonstrates that additional metal-binding sites outside of the N-terminus may not overtly influence the overall structure of ovine PrP. Despite the difference in b-sheet content, both the scrapie-susceptible and -resistant allelic forms of ovine PrP acquired resistance to proteinase K digestion following exposure to copper at 37 degrees C, suggesting the potential for disease-associated PrPARR to accumulate in vivo. Our present study demonstrates that allelic variants of ovine PrP differ in their structure and response to the interaction with copper. These observations will contribute to a better understanding of the mechanism of susceptibility and resistance to prion disease.