The polymorphic residue 129 of the prion protein determines the efficiency of prion propagation

Prion disorders result from the accumulation of a misfolded isoform (PrP Sc) of the normal host prion protein (PrPC). PrP Sc propagates by complexing with and transferring its conformation onto resident PrPC to produce new PrPSc. Although the nature of the PrPSc-PrPC complex has not been resolved, certain segments or single residues within PrP are proposed to be critical in its formation. We explored the role of a key site, residue 129 in the PrPSc-PrPC complex formation. Transmission of sporadic prion disease to susceptible transgenic mice expressing human PrP with either Met or Val at residue 129, was faster when residue 129 of the PrPSc donor matched that of the recipient mouse PrP C. Moreover, evidence suggests the polymorphism at codon 129 predicts each of two major conformational subtypes of PrPSc that are linked to different disease phenotypes. Because PrPSc-Type 1 (∼21 kDa) is more commonly associated with the 129MM genotype and PrP Sc-Type 2 (∼19 kDa) is more prevalent with 129VV, we also questioned whether residue 129 determines the "strain susceptibility" of PrPC during prion propagation. We compared the transmissible properties of Type 1 and Type 2 PrPSc from sCJD and fCJD(E200K) in the above mentioned transgenic. Overall, a greater degree of spongiform degeneration and the faithful propagation of PrPSc conformation was observed when residue 129 of the PrPSc inoculum matched the PrPC of the recipient mouse. However, independent of this effect, we found residue 129 to confer strain susceptibility to PrP C, although differentially; mice expressing PrPC(129M) were more susceptible to Type 1 PrPSc(129M) and PrP Sc(129V), and they propagated a "Type 1-like" PrPSc conformation when challenged with either sCJD(129MM1), sCJD(129VV2), or fCJD(E200K/129VV2); however, mice expressing PrPC(129V) were not more susceptible to Type 2 PrPSc, and they faithfully propagated Type 1 and Type 2 PrPSc from either PrPSc(129M) or PrP Sc(129V). These data suggest a complex relationship between residue 129 of host PrPC and prion strain propagation. Whereas sequence homology of PrPSc and PrPC at residue 129 provides the lowest barrier to efficient and accurate strain propagation, Met substitution at residue 129 results in a PrPSc that is more "restricted" in its conformational repertoire and displays a greater selectivity than PrP-129V for specific sCJD and fCJD(E200K) strains. Finally, using a previously described heterologous yeast expression system to which we applied live cell FRET to study the association of PrP pairs, an alteration in the physical association of homologous (128M/M or 128V/V) versus heterologous (128M/V) PrP pairs was detected. We further show that substitution of residue 128 with amino acids that favor a helical secondary structure (Ala, Leu) replicate the associative behavior of PrP(128M), while replacement with amino acids that favor beta sheet secondary structure (Ile, Tyr), replicate the associative behavior of PrP(128V). Finally substitution of the polar amino acid Lys dramatically impaired PrP association with either PrP(128M) or PrP(128V). These studies demonstrate a direct effect of residue 129 on the physical association of PrP molecules, and they reveal a fundamental feature that the predictive secondary structure of met and Val determine the associative nature of residue 129 in the formation of a productive PrPSc-PrPC complex.