Scrapie PrP 27-30 is a sialoglycoprotein

Abstract

The major scrapie prion protein, designated PrP 27-30, exhibited both charge and size heterogeneity after purification from infected hamster brains. Eight or more discrete charge isomers of PrP 27-30 with isoelectric points ranging from approximately pH 4.6 to 7.9 were found by using non-equilibrium pH gradient electrophoresis in the first dimension followed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis in the second dimension. The charge isomers were detected by silver staining as well as by radioiodination. The procedures used to disaggregate PrP 27-30 before electrophoresis in the first dimension do not appear to be responsible for the charge heterogeneity. However, heating PrP 27-30 to 100 degrees C for 15 min in 0.1 N NaOH or 0.1 N HCl resulted in modification of the protein and alteration of its electrophoretic pattern. A PrP 27-30 fragment (molecular weight, 17,100 to 21,900) obtained by cyanogen bromide cleavage also exhibited charge and size heterogeneity. Periodic acid-Schiff staining of PrP 27-30 electrophoresed into sodium dodecyl sulfate-polyacrylamide gels demonstrated that carbohydrate residues are attached to the protein. Digestion of PrP 27-30 with neuraminidase and endo-beta-N-acetylglucosaminidase H resulted in significant changes in the isoelectric pH of PrP 27-30 isomers, whereas digestion with alkaline phosphatase had no effect. Our results demonstrate that PrP 27-30 is a sialoglycoprotein; this is consistent with several properties of this protein and of the scrapie prion.