PRION PROTEIN SELECTIVELY BINDS COPPER(II) IONS

The infectious isoform of the prion protein (PrPSc) is derived from ce llular PrP (PrPC) in a conversion reaction involving a dramatic reorga nization of secondary and tertiary structure. While our understanding of the pathogenic role of PrPSc has grown, the normal physiologic func tion of PrPC still remains unclear. Using recombinant Syrian hamster p rion protein [SHaPrP(29-231)], we investigated metal ions as possible ligands of PrP. Near-UV circular dichroism spectroscopy (CD) indicates that the conformation of SHaPrP(29-231) resembles PrPC purified from hamster brain. Here we demonstrate by CD and tryptophan (Trp) fluoresc ence spectroscopy that copper induces changes to the tertiary structur e of SHaPrP(29-231). Binding of copper quenches the Trp fluorescence e mission significantly, shifts the emission spectrum to shorter wavelen gths, and also induces changes in the near-UV CD spectrum of SHaPrP(29 -231). The binding sites are highly specific for Cu2+, as indicated by the lack of a change in Trp fluorescence emission with Ca2+, Co2+, Mg 2+, Mn2+, Ni2+, and Zn2+. Binding of Cu2+ also promotes the conformati onal shift from a predominantly a-helical to a P-sheet structure. Equi librium dialysis experiments indicate a binding stoichiometry of simil ar to 2 copper molecules per PrP molecule at physiologically relevant concentrations, and pH titration of Cu2+ binding suggests a role for h istidine as a chelating ligand. NMR spectroscopy has recently demonstr ated that the octarepeats (PHGGGWGQ) in SHaPrP(29-231) lack secondary or tertiary structure in the absence of Cu2+. Our results suggest that each Cu2+ binds to a structure defined by two octarepeats (PHGGGWGQ) with one histidine and perhaps one glycine carbonyl chelating the ion. We propose that the binding of two copper ions to four octarepeats in duces a more defined structure to this region.