Heparin Binding by Murine Recombinant Prion Protein Leads to Transient Aggregation and Formation of RNA-Resistant Species

The conversion of cellular prion protein (PrP(C)) into the pathological conformer PrP(Sc) requires contact between both isoforms and probably also requires a cellular factor, such as a nucleic acid or a glycosaminoglycan (GAG). Little is known about the structural features implicit in the GAG-PrP interaction. In the present work, light scattering, fluorescence, circular dichroism, and nuclear magnetic resonance (NMR) spectroscopy were used to describe the chemical and physical properties of the murine recombinant PrP 23-231 interaction with low molecular weight heparin (LMWHep) at pH 7.4 and 5.5. LMWHep interacts with rPrP 23-231, thereby inducing transient aggregation. The interaction between murine rPrP and heparin at pH 5.5 had a stoichiometry of 2:1 (LMWHep:rPrP 23-231), in contrast to a 1:1 binding ratio at pH 7.4. At binding equilibrium, NMR spectra showed that rPrP complexed with LMWHep had the same general fold as that of the free protein, even though the binding can be indicated by significant changes in few residues of the C-terminal domain, especially at pH 5.5. Notably, the soluble LMWHep:rPrP complex prevented RNA-induced aggregation. We also investigated the interaction between LMWHep and the deletion mutants rPrP Δ51-90 and Δ32-121. Heparin did not bind these constructs at pH 7.4 but was able to interact at pH 5.5, indicating that this glycosaminoglycan binds the octapeptide repeat region at pH 7.4 but can also bind other regions of the protein at pH 5.5. The interaction at pH 5.5 was dependent on histidine residues of the murine rPrP 23-231. Depending on the cellular milieu, the PrP may expose different regions that can bind GAG. These results shed light on the role of GAGs in PrP conversion. The transient aggregation of PrP may explain why some GAGs have been reported to induce the conversion into the misfolded, scrapie conformation, whereas others are thought to protect against conversion. The acquired resistance of the complex against RNA-induced aggregation explains some of the unique properties of the PrP interaction with GAGs.