Misfolded prion proteins form extended amyloid strings on the surface of infected cells, Rouvinski et al. reveal.
The endogenous prion protein, PrPC, is attached to lipid rafts on the plasma membrane by a C-terminal glycosylphosphatidylinositol (GPI) anchor. Infectious prion particles convert PrPC to a misfolded conformation, PrPSc, that is subsequently trimmed down to a protease-resistant C-terminal fragment. PrPSc forms amyloid fibers when detergent-extracted from infected cells, but whether it aggregates on membranes in vivo and where it localizes in the cell are still uncertain, in part because antibodies to the protein’s C-terminal region fail to recognize PrPSc under physiological conditions.
Rouvinski et al. found, however, that antibodies against PrPSc’s N terminus could recognize the full-length prion in its native state in infected cells. Using these antibodies, the researchers discovered that PrPSc forms “strings,” up to 8 µm in length, on the surface of cells in culture and in the brains of infected mice. The strings fluoresced with thioflavin T, suggesting an amyloid structure, and contained both the full-length and cleaved forms of PrPSc, but they didn’t contain PrPC at a detectable level. Live imaging revealed that the strings were largely immobile and remained on the cell surface for several hours. The slow rate of internalization might limit PrPSc’s degradation by lysosomal proteases and increase the prion’s ability to convert PrPC to the misfolded conformation.
Senior author Albert Taraboulos now wants to investigate PrPSc string structure and determine how they affect the physiology of infected cells. The strings localize to enlarged lipid raft–like domains, which could have a major impact on cell signaling pathways.
Text By Ben Short