Prion Protein NMR Structure from Tammar Wallaby (Macropus eugenii) Shows that the β2–α2 Loop Is Modulated by Long-Range Sequence Effects

NMR structures are presented for the recombinant construct of residues 121–230 from the tammar wallaby (Macropus eugenii) prion protein (PrP) twPrP(121–230) and for the variant mouse PrPs mPrP[Y225A,Y226A](121–231) and mPrP[V166A](121–231) at 20 °C and pH 4.5. All three proteins exhibit the same global architecture as seen in other recombinant PrPCs (cellular isoforms of PrP) and shown to prevail in natural bovine PrPC. Special interest was focused on a loop that connects the β2-strand with helix α2 in the PrPC fold, since there are indications from in vivo experiments that this local structural feature affects the susceptibility of transgenic mice to transmissible spongiform encephalopathies. This β2–α2 loop and helix α3 form a solvent-accessible contiguous epitope, which has been proposed to be the recognition area for a hypothetical chaperone, the “protein X”. This hypothetical chaperone would affect the conversion of PrPC into the disease-related scrapie form (PrPSc) by moderating intermolecular interactions related to the transmission barrier of transmissible spongiform encephalopathies between different species. In contrast to mPrP(121–231) and most other mammalian PrPCs, the β2–α2 loop is well defined at 20 °C in tammar wallaby PrP and in the two aforementioned variants of mPrP, showing that long-range interactions with helix α3 can have an overriding influence on the structural definition of the β2–α2 loop. Further NMR studies with two variant mPrPs, mPrP[Y225A](121–231) and mPrP[Y226A](121–231), showed that these interactions are dominantly mediated by close contacts between residues 166 and 225. The results of the present study then lead to the intriguing indication that well-defined long-range intramolecular interactions could act as regulators of the functional specificity of PrPC.