Acid-induced Molten Globule State of a Prion Protein: CRUCIAL ROLE OF STRAND 1-HELIX 1-STRAND 2 SEGMENT*

The conversion of a cellular prion protein (PrP^C) to its pathogenic isoform (PrP^Sc) is a critical event in the pathogenesis of prion diseases. Pathogenic conversion is usually associated with the oligomerization process; therefore, the conformational characteristics of the pre-oligomer state may provide insights into the conversion process. Previous studies indicate that PrP^C is prone to oligomer formation at low pH, but the conformation of the pre-oligomer state remains unknown. In this study, we systematically analyzed the acid-induced conformational changes of PrP^C and discovered a unique acid-induced molten globule state at pH 2.0 termed the “A-state.” We characterized the structure of the A-state using far/near-UV CD, 1-anilino-8-naphthalene sulfonate fluorescence, size exclusion chromatography, and NMR. Deuterium exchange experiments with NMR detection revealed its first unique structure ever reported thus far; i.e. the Strand 1-Helix 1-Strand 2 segment at the N terminus was preferentially unfolded, whereas the Helix 2-Helix 3 segment at the C terminus remained marginally stable. This conformational change could be triggered by the protonation of Asp¹⁴⁴, Asp¹⁴⁷, and Glu¹⁹⁶, followed by disruption of key salt bridges in PrP^C. Moreover, the initial population of the A-state at low pH (pH 2.0–5.0) was well correlated with the rate of the β-rich oligomer formation, suggesting that the A-state is the pre-oligomer state. Thus, the specific conformation of the A-state would provide crucial insights into the mechanisms of oligomerization and further pathogenic conversion as well as facilitating the design of novel medical chaperones for treating prion diseases.