Trafficking of prion proteins through a caveolae-mediated endosomal pathway

To understand the posttranslational conversion of the cellular prion protein (PrP^C) to its pathologic conformation, it is important to define the intracellular trafficking pathway of PrP^C within the endomembrane system. We studied the localization and internalization of PrP^C in CHO cells using cryoimmunogold electron microscopy. At steady state, PrP^C was enriched in caveolae both at the TGN and plasma membrane and in interconnecting chains of endocytic caveolae. Protein A–gold particles bound specifically to PrP^C on live cells. These complexes were delivered via caveolae to the pericentriolar region and via nonclassical, caveolae-containing early endocytic structures to late endosomes/lysosomes, thereby bypassing the internalization pathway mediated by clathrin-coated vesicles. Endocytosed PrP^C-containing caveolae were not directed to the ER and Golgi complex. Uptake of caveolae and degradation of PrP^C was slow and sensitive to filipin. This caveolae-dependent endocytic pathway was not observed for several other glycosylphosphatidyl inositol (GPI)-anchored proteins. We propose that this nonclassical endocytic pathway is likely to determine the subcellular location of PrP^C conversion.