Acid-induced Molten Globule State of a Prion Protein: Crucial Role of Strand 1-Helix 1-Strand 2 Segment [Protein Structure and Folding]

September 12th, 2014 by Honda, R. P., Yamaguchi, K.-i., Kuwata, K.

The conversion of a cellular prion protein (PrPC) to its pathogenic isoform (PrPSc) 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 PrPC 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 PrPC and discovered a unique acid-induced molten globule state at pH 2.0 termed ″ A-state″. We characterized the structure of the A-state using far/near-UV CD, 1-anilino-8-naphthalene sulfonate (ANS)-fluorescence, size exclusion chromatography, and NMR. Deuterium exchange experiments with NMR detection revealed its unique structure ever reported thus far, i.e., the Strand 1-Helix 1-Strand 2 segment at the N-terminal was preferentially unfolded, while the Helix 2-Helix 3 segment at the C-terminal remained marginally stable. This conformational change could be triggered by the protonation of Asp144, Asp147, and Glu196 followed by disrupture of key salt bridges in PrPC. Moreover, the initial population of the A-state at low 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 facilitate the design of novel medical chaperones for treating prion diseases.