Unexpected Tolerance of α-Cleavage of the Prion Protein to Sequence Variations

The cellular form of the prion protein, PrP(C), undergoes extensive proteolysis at the α site (109K↓H110). Expression of non-cleavable PrP(C) mutants in transgenic mice correlates with neurotoxicity, suggesting that α-cleavage is important for PrP(C) physiology. To gain insights into the mechanisms of α-cleavage, we generated a library of PrP(C) mutants with mutations in the region neighbouring the α-cleavage site. The prevalence of C1, the carboxy adduct of α-cleavage, was determined for each mutant. In cell lines of disparate origin, C1 prevalence was unaffected by variations in charge and hydrophobicity of the region neighbouring the α-cleavage site, and by substitutions of the residues in the palindrome that flanks this site. Instead, α-cleavage was size-dependently impaired by deletions within the domain 106–119. Almost no cleavage was observed upon full deletion of this domain. These results suggest that α-cleavage is executed by an α-PrPase whose activity, despite surprisingly limited sequence specificity, is dependent on the size of the central region of PrP(C).