Ligand binding promotes prion protein aggregation - Role of the octapeptide repeats

Abstract

Aggregation of the normal cellular prion protein, PrP, is important in the pathogenesis of prion disease. PrP binds glycosaminoglycan (GAG) and divalent cations, such as Cu2+ and Zn2+. Here, we report our findings that GAG and Cu2+ promote the aggregation of recombinant human PrP (rPrP). The normal cellular prion protein has five octapeptide repeats. In the presence of either GAG or Cu2+, mutant rPrPs with eight or ten octapeptide repeats are more aggregation prone, exhibit faster kinetics and form larger aggregates than wild-type PrP. When the GAG-binding motif, KKRPK, is deleted the effect of GAG but not that of Cu2+ is abolished. By contrast, when the Cu2+-binding motif, the octapeptide-repeat region, is deleted, neither GAG nor Cu2+ is able to promote aggregation. Therefore, the octapeptide-repeat region is critical in the aggregation of rPrP, irrespective of the promoting ligand. Furthermore, aggregation of rPrP in the presence of GAG is blocked with anti-PrP mAbs, whereas none of the tested anti-PrP mAbs block Cu2+-promoted aggregation. However, a mAb that is specific for an epitope at the N-terminus enhances aggregation in the presence of either GAG or Cu2+. Therefore, although binding of either GAG or Cu2+ promotes the aggregation of rPrP, their aggregation processes are different, suggesting multiple pathways of rPrP aggregation. © 2008 The Authors.