Histidines in the octapeptide repeat of PrPC react with PrPSc at an acidic pH.

Cellular PrP is actively cycled between the cell surface and the endosomal pathway. The exact site and mechanism of conversion from PrPC to PrPSc remain unknown. We have previously used recombinant antibodies containing grafts of PrP sequence to identify three regions of PrPC (aa23−27, 98−110, and 136−158) that react with PrPSc at neutral pH. To determine if any regions of PrPC react with PrPSc at an acidic pH similar to that of an endosomal compartment, we tested our panel of grafted antibodies for the ability to precipitate PrPSc in a range of pH conditions. At pH near or lower than 6, PrP-grafted antibodies representing the octapeptide repeat react strongly with PrPSc but not PrPC. Modified grafts in which the histidines of the octarepeat were replaced with alanines did not react with PrPSc. PrPSc precipitated by the octapeptide at pH 5.7 was able to seed conversion of normal PrP to PrPSc in vitro. However, modified PrP containing histidine to alanine substitutions within the octapeptide repeats was st...