Different structural stability and toxicity of PrPARR and PrPARQ sheep prion protein variants

The polymorphisms at amino acid residues 136, 154, and 171 in ovine prion protein (PrP) have been associated with different susceptibility to scrapie: animals expressing PrPARQ [PrP(Ala136/Arg154/Gln171)] show vulnerability, whereas those that express PrPARR [PrP(Ala136/Arg154/Arg171)] are resistant to scrapie. The aim of this study was to evaluate the in vitro toxic effects of PrPARR and PrPARQ variants in relation with their structural characteristics. We show that both peptides cause cell death inducing apoptosis but, unexpectedly, the scrapie resistant PrPARR form was more toxic than the scrapie susceptible PrPARQ variant. Moreover, the alpha-helical conformation of PrPARR was less stable than that of PrPARQ and the structural determinants responsible of these different conformational stabilities were characterized by spectroscopic analysis. We observed that PrP toxicity was inversely related to protein structural stability, being the unfolded conformation more toxic than the native one. However, the PrPARQ variant displays a higher propensity to form large aggregates than PrPARR. Interestingly, in the presence of small amounts of PrPARR, PrPARQ aggregability was reduced to levels similar to that of PrPARR. Thus, in contrast to PrPARR toxicity, scrapie transmissibility seems to reside in the more stable conformation of PrPARQ that allows the formation of large amyloid fibrils