Molecular Features of the Zn2+ Binding Site in the Prion Protein Probed by 113Cd NMR

The cellular prion protein (PrPc) is a zinc-binding protein that contributes to the regulation of Zn2+ and other divalent species of the central nervous system. Zn2+ coordinates to the flexible, N-terminal repeat region of PrPc and drives a tertiary contact between this repeat region and a well-defined cleft of the C-terminal domain. The tertiary structure promoted by Zn2+ is thought to regulate inherent PrPc toxicity. Despite the emerging consensus regarding the interaction between Zn2+ and PrPc, there is little direct spectroscopic confirmation of the metal ion's coordination details. Here, we address this conceptual gap by using Cd2+ as a surrogate for Zn2+. NMR finds that Cd2+ binds exclusively to the His imidazole side chains of the repeat segment, with a dissociation constant of similar to 1.2 mM, and promotes an N-terminal-C-terminal cis interaction very similar to that observed with Zn2+. Analysis of Cd-113 NMR spectra of PrPc, along with relevant control proteins and peptides, suggests that coordination of Cd2+ in the full-length protein is consistent with a three- or four-His geometry. Examination of the mutation E199K in mouse PrPc (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 PrPc metal ion coordination but they also suggest new perspectives on the role of familial mutations in prion disease.