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.