Raman optical activity demonstrates poly(l-proline) II helix in the N-terminal region of the ovine prion protein: Implications for function and misfunction

The aqueous solution structure of the full-length recombinant ovine prion protein PrP^25-233, together with that of the N-terminal truncated version PrP^94-233, have been studied using vibrational Raman optical activity (ROA) and ultraviolet circular dichroism (UVCD). A sharp positive band at ∼1315 cm^-1 characteristic of poly(l-proline) II (PPII) helix that is present in the ROA spectrum of the full-length protein is absent from that of the truncated protein, together with bands characteristic of β-turns. Although it is not possible similarly to identify PPII helix in the full-length protein directly from its UVCD spectrum, subtraction of the UVCD spectrum of PrP^94-233 from that of PrP^25-233 yields a difference UVCD spectrum also characteristic of PPII structure and very similar to the UVCD spectrum of murine PrP^25-113. These results provide confirmation that a major conformational element in the N-terminal region is PPII helix, but in addition show that the PPII structure is interspersed with β-turns and that little PPII structure is present in PrP^94-233. A principal component analysis of the ROA data indicates that the α-helix and β-sheet content, located in the structured C-terminal domain, of the full-length and truncated proteins are similar. The flexibility imparted by the high PPII content of the N-terminal domain region may be an essential factor in the function and possibly also the misfunction of prion proteins.