Methionine 109 plays a key role in Cu(II) binding to His111 in the 92–115 fragment of the human prion protein

Abstract

The cellular prion protein (PrPC) is a monomeric glycoprotein expressed mainly in the central nervous system. Although its function is not understood, it has been proposed to be involved in Cu transport. Human PrP (hPrP) can bind up to six Cu(II) ions at its unstructured N-terminal region; two of them at the non-octarepeat region, encompassing His96 and His111. The Cu(II) coordination features of these two sites are practically identical, yet Cu ions have a preference for binding at the His111 site, which is unique in that it contains two Met residues in a MKHM motif. Here, the hPrP(92–115) peptide fragment, containing both His binding sites, and its variants with Met to Ile substitutions have been studied, to evaluate the role of Met109 and Met112 in the relative Cu(II) loading into the His111 and His96 sites. The fact that Cu(II) bound to His96 and His111 display distinct circular dichroism spectra allowed to discern Cu binding to each site under different experimental conditions. Through a combination of spectroscopic and calorimetric studies, we demonstrate that His111 has a slightly higher affinity for Cu(II) than His96, and that Met109 plays an important role in the binding preference of Cu(II) for the His111 site. Met109 profiles as a highly important residue that, not only assures a preferential loading of Cu(II) ions into the His111 site, but it also serves as a Cu(I) anchoring residue and it might be a key player for Cu transport by hPrP.