NMR Structure of a Variant Human Prion Protein with Two Disulfide Bridges

The nuclear magnetic resonance structure of the globular domain with residues 121–230 of a variant human prion protein with two disulfide bonds, hPrP(M166C/E221C), shows the same global fold as wild-type hPrP(121–230). It contains three α-helices of residues 144–154, 173–194 and 200–228, an anti-parallel β-sheet of residues 128–131 and 161–164, and the disulfides Cys166-Cys221 and Cys179-Cys214. The engineered extra disulfide bond in the presumed “protein X”-binding site is accommodated with slight, strictly localized conformational changes. High compatibility of hPrP with insertion of a second disulfide bridge in the protein X epitope was further substantiated by model calculations with additional variant structures. The ease with which the hPrP structure can accommodate a variety of locations for a second disulfide bond within the presumed protein X-binding epitope suggests a functional role for the extensive perturbation by a natural second disulfide bond of the corresponding region in the human doppel protein.