Differential glycosylation of the protein (PrP) forming scrapie-associated fibrils

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Abstract
PrP is a glycoprotein found in normal brain. In brain affected by scrapie it forms scrapie-associated fibrils (SAF). PrP from SAF shows considerable heterogeneity of size and charge on two-dimensional gels. It separates into six major regions, the three more acidic regions arising as a result of partial proteolytic degradation. The two more basic higher Mr forms (Mr 34,000 and 29,000) of PrP can be reduced in apparent Mr to a lower Mr form (Mr 25,000) with Peptide-N-glycosidase F. In addition, a series of lectins has been found to bind to PrP. Some bind preferentially to the higher Mr forms whereas others bind more strongly to the lower Mr form. Some of the heterogeneity of PrP is therefore due to differential N-glycosylation. We suggest that one or two N-linked carbohydrate chains are bound to the protein causing some of the differences in Mr. The major cause of heterogeneity of PrP is therefore proteolytic cleavage combined with differential glycosylation at the two potential N-glycosylation sites. The glycolipid moiety attached to PrP may be responsible for some lectin binding to all the three bands. Using lectins as a probe to study potential differences in N-glycosylation we have looked at their binding to PrP isolated from SAF, from different strains of scrapie and from different regions of the same brain. No major differences in the N-glycan moieties were found.