Prion glycoprotein: structure, dynamics, and roles for the sugars.

The prion protein contains two N-linked glycosylation sites and a glycosylphosphatidylinositol (GPI) anchor. The large size of the N-linked sugars, together with their dynamic properties, enables them to shield two orthogonal faces of the protein almost completely. Thus, the sugars can protect large regions of the protein surface from proteases and from nonspecific protein-protein interactions. Immunoprecipitation of prion protein with calnexin suggests that in the ER the oligosaccharides may provide a route for protein folding via the calnexin pathway. Major questions relate to the relevance of the glycoform distribution (as defined by glycan site occupancy) to strain type and disease transmission. Glycan analysis has shown that prion protein contains at least 52 different sugars, that these consist of a subset of brain sugars, and that there is site specific glycan processing. PrP(Sc) from the brains of Syrian hamsters contains the same set of glycans as PrP(C), but a higher proportion of tri- and tetra-antennary sugars. This may be attributed to a decrease in the activity of GnTIII. The GPI anchor, which is modified with sialic acid, may allow the prion protein to be mobile in the lipid bilayer. Potentially, this provides a possible means for translocating the prions from one cell to another.