Membrane Interactions and Conformational Preferences of Human and Avian Prion N-Terminal Tandem Repeats: The Role of Copper(II) Ions, pH, and Membrane Mimicking Environments

The flexible N-terminal domain of the prion protein (PrPc) is believed to play a pivotal role in both trafficking of the protein through the cell membrane and its pathogenic conversion into the beta sheet-rich scrapie isoform (PrPsc). Unlike mammalian PrPc, avian prion proteins are not known to undergo any pathogenic conformational conversions. Consequently, some critical advances in our understanding of the molecular mechanisms underlying prion pathogenesis are expected from comparative studies of the biophysical properties of the N-terminal domains of the two proteins. The present study addresses the role played by different environmental factors, i.e., copper(II), pH, and membrane-mimicking environments, in assisting the conformational preferences of huPrP60-91 and chPrP53-76, two soluble peptides encompassing the N-terminal copper(II) binding domains of the human and chicken prion proteins, respectively. Moreover, the membrane interactions of huPrP60-91, chPrP53-76, and their copper(II) complexes were evaluated by Trp fluorescence in conjunction with measurements of the variation in thermotropic properties of 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) unilamellar vesicles. Circular dichroism experiments revealed that huPrP60-91 adopts a predominant polyproline II conformation in aqueous solution that is destabilized at basic pH or in the presence of trifluoroethanol (TFE). Unlike anionic sodium dodecyl sulfate (SDS), which seems to stabilize the polyproline II conformation further, zwitterionic dodecylphosphocholine (DPC) micelles do not affect the peptide structure. On the contrary, copper(II) promptly promotes an increase in beta-turn-rich structures. Differential scanning calorimetry (DSC) and Trp fluorescence assays carried out on DPPC model membranes after incubation with huPrP60-91 showed a marked tendency of the peptide to slowly penetrate the lipid bilayer with a concomitant conformational transition toward an extended beta-sheet-like structure. Such an event, which was ascribed to the hyrophobic Trp side chain residues, was shown to also depend on the level of copper(II) occupancy along the peptide. Conversely, the CD spectra of chPrP53-76 aqueous solutions indicated the presence of a mixture of random-coil/beta-turn-like structures whose resulting equilibrium was influenced by SDS and copper(II) addition. Furthermore, chPrP53-76 did not exhibit any tendency to interact with model membranes in either the presence or absence of copper(II). The results reported here provide evidence of the different roles played by environmental factors in affecting the conformation and membrane activity of human and avian prion N-terminal domains.