Deconvoluting the Cu2+ binding modes of full-length prion protein

The prion protein (PrP) is a cell-surface Cu2+-binding glycoprotein that when misfolded is responsible for a number of transmissible spongiform encephalopathies. Full-length PrP(23-231) and constructs in which the octarepeat region has been removed, or His(95) and His(110) is replaced by alanine residues, have been used to elucidate the order and mode of Cu2+ coordination to PrP-(23-231). We have built on our understanding of the appearance of visible CD spectra and EPR for various PrP fragments to characterize Cu2+ coordination to full-length PrP. At physiological pH, Cu2+ initially binds to full-length PrP in the amyloidogenic region between the octarepeats and the structured domain at His(95) and His(110). Only subsequent Cu2+ ions bind to single histidine residues within the octarepeat region. Ni2+ ions are used to further probe metal binding and, like Cu2+, Ni2+ will bind individually to His(95) and His(110), involving preceding main chain amides. Competitive chelators are used to determine the affinity of the first mole equivalent of Cu2+ bound to full-length PrP; this approach places the affinity in the nanomolar range. The affinity and number of Cu2+ binding sites support the suggestion that PrP could act as a sacrificial quencher of free radicals generated by copper redox cycling.