Dual polarisation interferometry analysis of copper binding to the prion protein: evidence for two folding states.

The prion protein is a copper binding glycoprotein expressed in neurones and other cells. Conversion of this protein to an abnormal isoform is central to the cause of prion diseases or transmissible spongiform encephalopathies. Detecting slight structural differences between different forms of the prion protein could be essential to understanding the role of the protein in health and disease. Dual polarisation interferometry (DPI) is a new method that allows detection of small structural differences. We used this technique to evaluate the effectiveness of DPI in the analysis of metal binding to recombinant mouse prion protein. DPI was able to measure mass change in the prion protein following addition of copper and could identify reproducible differences in the structure of prion protein dependent on how metal was added to the protein. These slight structural differences were confirmed by the use of circular dichroism spectroscopy and Fourier-transformed infra-red spectroscopy. These results suggest that DPI can provide important information on both transitory and stable structural difference that are induced in the prion protein. This technique could be important not only for the study of metal-protein interactions but also small structural differences that could define prion strains.