Post-translational hydroxylation at the N-terminus of the prion protein reveals presence of PPII structure in vivo

The transmissible spongiform encephalopathies are characterized by conversi on of a host protein, PrPC (cellular prion protein), to a protease-resistan t isoform, PrPSc (prion protein scrapie isoform). The importance of the hig hly flexible, N-terminal region of PrP has recently become more widely appr eciated, particularly the biological activities associated with its metal i on-binding domain and its potential to form a poly(L-proline) II (PPII) hel ix. Circular dichroism spectroscopy of an N-terminal peptide, PrP37-53, sho wed that the PPII helix is formed in aqueous buffer; as it also contains an Xaa-Pro-Gly consensus sequence, it may act as a substrate for the collagen -modifying enzyme prolyl 4-hydroxylase. Direct evidence for this modificati on was obtained by mass spectrometry and Edman sequencing in recombinant mo use PrP secreted from stably transfected Chinese hamster ovary cells. Almos t complete conversion of proline to 4-hydroxyproline occurs specifically at residue Pro44 of this murine protein; the same hydroxylated residue was de tected, at lower levels, in PrPSc from the brains of scrapie-infected mice. Cation binding and/or post-translational hydroxylation of this region of P rP may regulate its role in the physiology and pathobiology of the cell.