The interaction of the prion protein with lipid membranes and implications for prion conversion

Prion diseases are characterised by the conversion of the normal a-helical prion protein (PrPc), to the abnormal P-sheet isoform (PrPSc). Biophysical studies using recombinant prion proteins have helped our understanding of the structural events associated with the transition of PrPc to PrPSc. These studies were performed in an aqueous environment, however PrP in living cells is anchored to cholesterol/sphingomyelin rich raft-domains. Evidence suggests that the lipid environment plays’ a role in the conversion of PrPc to PrPSc. Therefore in this thesis the biophysical properties of PrP in lipid membranes were evaluated. The binding of Syrian hamster prion protein, SHaPrP (90-231) to model lipid membranes was investigated by tryptophan fluorescence spectroscopy. Membranes composed of negatively charged lipids, zwitterionic lipids and raft-like membranes containing DPPC, cholesterol and sphingomyelin, were investigated. The results show that PrP has an affinity for negatively charged and gel-phase zwitterionic membranes. Binding of PrP to raft-like membranes was also observed. The interaction of PrP to negatively charged membranes involved both electrostatic and hydrophobic lipid- protein interactions, whereas the binding of PrP to raft-like membranes was predominantly driven by hydrophobic lipid-protein interactions. CD and ATR FTIR were used to investigate the secondary structure of PrP in lipid membranes. The interaction of PrP to negatively charged membranes was accompanied by an increase in P-sheet structure and EM showed extensive aggregation of the protein. In contrast, the binding of PrP to gel-phase zwitterionic membranes and raft-like membranes induced the formation of stable a-helical structure. No evidence of aggregation was detected in the EM images. The ability of PrP to perturb lipid membranes was examined. Evidence for the partial insertion of PrP into negatively charged membranes was obtained and the resulting p-sheet state was capable of disrupting lipid membranes, leading to total release of vesicle contents. The a-helical membrane bound state of PrP did not affect the integrity of the raft-like membranes. These finding support the view that the interaction of PrP with lipid membranes may play a role in PrP conversion