Evidence for synthesis of scrapie prion proteins in the endocytic pathway

Infectious scrapie prions are composed largely, if not entirely, of an abnormal isoform of the prion protein (PrP) which is designated PrP^Sc. A chromosomal gene encodes both the cellular prion protein (PrP^C) as well as PrP^Sc. Pulse-chase experiments with scrapie-infected cultured cells indicate that PrP^Sc is formed by a post-translational process. PrP is translated in the endoplasmic reticulum, modified as it passes through the Golgi, and is transported to the cell surface. Release of nascent PrP from the cell surface by phosphatidylinositol-specific phospholipase C or hydrolysis with dispase prevented PrP^Sc synthesis. At 18°C, the synthesis of PrP^Sc was inhibited under conditions that other investigators report a blockage of endosomal fusion with lysosomes. Our results suggest that PrP^Sc synthesis occurs after PrP transits from the cell surface. Whether all of the PrP molecules have an equal likelihood to be converted into PrP^Sc or only a distinct subset is eligible for conversion remains to be established. Identifying the subcellular compartment(s) of PrP^Sc synthesis should be of considerable importance in defining the molecular changes that distinguish PrP^Sc from PrP^C.