Molecular Features of the Zn Binding Site in the Prion Protein Probed by Cd NMR.

The cellular prion protein (PrP) is a zinc-binding protein that contributes to the regulation of Zn and other divalent species of the central nervous system. Zn coordinates to the flexible, N-terminal repeat region of PrP and drives a tertiary contact between this repeat region and a well-defined cleft of the C-terminal domain. The tertiary structure promoted by Zn is thought to regulate inherent PrP toxicity. Despite the emerging consensus regarding the interaction between Zn and PrP, there is little direct spectroscopic confirmation of the metal ion's coordination details. Here, we address this conceptual gap by using Cd as a surrogate for Zn. NMR finds that Cd binds exclusively to the His imidazole side chains of the repeat segment, with a dissociation constant of ∼1.2 mM, and promotes an N-terminal-C-terminal cis interaction very similar to that observed with Zn. Analysis of Cd NMR spectra of PrP, along with relevant control proteins and peptides, suggests that coordination of Cd in the full-length protein is consistent with a three- or four-His geometry. Examination of the mutation E199K in mouse PrP (E200K in humans), responsible for inherited Creutzfeldt-Jakob disease, finds that the mutation lowers metal ion affinity and weakens the cis interaction. These findings not only provide deeper insight into PrP metal ion coordination but they also suggest new perspectives on the role of familial mutations in prion disease.