The charge structure of helix 1 in the prion protein regulates conversion to pathogenic PrPSc

The prion diseases are transmissible neurodegenerative disorders linked to a pathogenic conformer (PrP(Sc)) of the normal prion protein (PrP(C)). Accumulation of PrP(Sc) occurs via a poorly defined process in which PrP(Sc) complexes with and converts endogenous PrP(C) to nascent PrP(Sc). Recent experiments have focused on the highly charged first alpha helix (H1) of PrP. It has been proposed that two putative asparagine-to-arginine intrahelical salt bridges stabilize H1 in PrP(C) yet form intermolecular ionic bonds with adjacent PrP molecules during conversion of PrP(C) to PrP(Sc) (M. P. Morrissey and E. I. Shakhnovich, Proc. Natl. Acad. Sci. USA 96:11293-11298, 1999). Subsequent work (J. O. Speare et al., J. Biol. Chem. 278:12522-12529, 2003 using a cell-free assay of PrP(Sc) conversion suggested that rather than promoting conversion, the salt bridges stabilize PrP(C) against it. However, the role of individual H1 charges in PrP(Sc) generation has not yet been investigated. To approach this question, we systematically reversed or neutralized each charged residue in H1 and tested the effect on conversion to PrP(Sc) in scrapie-infected murine neuroblastoma (ScN2a) cells. We find that replacements of charged H1 residues with like charges permit conversion, while charge reversals hinder it. Neutralization of charges in the N-terminal (amino acids 143 to 146) but not the C-terminal (amino acids 147 to 151) half of H1 permits conversion, while complete reversal of charge orientation of the putative salt bridges produces a nonconvertible PrP. Circular dichroism spectroscopy studies and confocal microscopy immunofluorescence localization studies indicated that charge substitutions did not alter the secondary structure or cell surface expression of PrP(C). These data support the necessity of specific charge orientations in H1 for a productive PrP(Sc)-PrP(C) complex.