Sialylation of the prion protein glycans controls prion replication rate and glycoform ratio

Prion or PrP(Sc) is a proteinaceous infectious agent that consists of a misfolded and aggregated form of a sialoglycoprotein called prion protein or PrP(C). PrP(C) has two sialylated N-linked carbohydrates. In PrP(Sc), the glycans are directed outward, with the terminal sialic acid residues creating a negative charge on the surface of prion particles. The current study proposes a new hypothesis that electrostatic repulsion between sialic residues creates structural constraints that control prion replication and PrP(Sc) glycoform ratio. In support of this hypothesis, here we show that diglycosylated PrP(C) molecules that have more sialic groups per molecule than monoglycosylated PrP(C) were preferentially excluded from conversion. However, when partially desialylated PrP(C) was used as a substrate, recruitment of three glycoforms into PrP(Sc) was found to be proportional to their respective populations in the substrate. In addition, hypersialylated molecules were also excluded from conversion in the strains with the strongest structural constraints, a strategy that helped reduce electrostatic repulsion. Moreover, as predicted by the hypothesis, partial desialylation of PrP(C) significantly increased the replication rate. This study illustrates that sialylation of N-linked glycans creates a prion replication barrier that controls replication rate and glycoform ratios and has broad implications.