Subcellular colocalization of the cellular and scrapie prion proteins in caveolae-like membranous domains

Results of transgenetic studies argue that the scrapie isoform of the prion protein (PrPSc) interacts with the substrate cellular PrP (PrPC) during conversion into nascent PrPSc. While PrPSc appears to accumulate primarily in lysosomes, caveolae-like domains (CLDs) have been suggested to be the site where PrPC is converted into PrPSc. We report herein that CLDs isolated from scrapie-infected neuroblastoma (ScN2a) cells contain PrPC and PrPSc. After lysis of ScN2a cells in ice-cold Triton X-100, both PrP isoforms and an N-terminally truncated form of PrPC (PrPC-II) were found concentrated in detergent-insoluble complexes resembling CLDs that were isolated by flotation in sucrose gradients. Similar results were obtained when CLDs were purified from plasma membranes by sonication and gradient centrifugation; with this procedure no detergents are used, which minimizes artifacts that might arise from redistribution of proteins among subcellular fractions. The caveolar markers ganglioside GM1 and H-ras were found concentrated in the CLD fractions. When plasma membrane proteins were labeled with the impermeant reagent sulfo-N-hydroxysuccinimide-biotin, both PrPC and PrPSc were found biotinylated in CLD fractions. Similar results on the colocalization of PrPC and PrPSc were obtained when CLDs were isolated from Syrian hamster brains. Our findings demonstrate that both PrPC and PrPSc are present in CLDs and, thus, support the hypothesis that the PrPSc formation occurs within this subcellular compartment.

Identification of Discrete Classes of Endosome-derived Small Vesicles as a Major Cellular Pool for Recycling Membrane Proteins

Vesicles carrying recycling plasma membrane proteins from early endosomes have not yet been characterized. Using Chinese hamster ovary cells transfected with the facilitative glucose transporter, GLUT4, we identified two classes of discrete, yet similarly sized, small vesicles that are derived from early endosomes. We refer to these postendosomal vesicles as endocytic small vesicles or ESVs. One class of ESVs contains a sizable fraction of the pool of the transferrin receptor, and the other contains 40% of the total cellular pool of GLUT4 and is enriched in the insulin-responsive aminopeptidase (IRAP). The ESVs contain cellubrevin and Rab4 but are lacking other early endosomal markers, such as EEA1 or syntaxin13. The ATP-, temperature-, and cytosol-dependent formation of ESVs has been reconstituted in vitro from endosomal membranes. Guanosine 5′-[γ-thio]triphosphate and neomycin, but not brefeldin A, inhibit budding of the ESVs in vitro. A monoclonal antibody recognizing the GLUT4 cytoplasmic tail perturbs the in vitro targeting of GLUT4 to the ESVs without interfering with the incorporation of IRAP or TfR. We suggest that cytosolic proteins mediate the incorporation of recycling membrane proteins into discrete populations of ESVs that serve as carrier vesicles to store and then transport the cargo from early endosomes, either directly or indirectly, to the cell surface.

Prion protein (PrP) synthetic peptides induce cellular PrP to acquire properties of the scrapie isoform.

Conversion of the cellular isoform of prion protein (PrPC) into the scrapie isoform (PrPSc) involves an increase in the beta-sheet content, diminished solubility, and resistance to proteolytic digestion. Transgenetic studies argue that PrPC and PrPSc form a complex during PrPSc formation; thus, synthetic PrP peptides, which mimic the conformational pluralism of PrP, were mixed with PrPC to determine whether its properties were altered. Peptides encompassing two alpha-helical domains of PrP when mixed with PrPC produced a complex that displayed many properties of PrPSc. The PrPC-peptide complex formed fibrous aggregates and up to 65% of complexed PrPC sedimented at 100,000 x g for 1 h, whereas PrPC alone did not. These complexes were resistant to proteolytic digestion and displayed a high beta-sheet content. Unexpectedly, the peptide in a beta-sheet conformation did not form the complex, whereas the random coil did. Addition of 2% Sarkosyl disrupted the complex and rendered PrPC sensitive to protease digestion. While the pathogenic A117V mutation increased the efficacy of complex formation, anti-PrP monoclonal antibody prevented interaction between PrPC and peptides. Our findings in concert with transgenetic investigations argue that PrPC interacts with PrPSc through a domain that contains the first two putative alpha-helices. Whether PrPC-peptide complexes possess prion infectivity as determined by bioassays remains to be established.