Grp78 destabilization of infectious prions is strain-specific and modified by multiple factors including accessory...

Lethal neurodegenerative prion diseases result from the continuous accumulation of infectious and variably protease-resistant prion protein aggregates (PrP^D) which are misfolded forms of the normally detergent soluble and protease-sensitive cellular prion protein. Molecular chaperones like Grp78 have been found to reduce the accumulation of PrP^D, but how different cellular environments and other chaperones influence the ability of Grp78 to modify PrP^D is poorly understood. In this work, we investigated how pH and protease-mediated structural changes in PrP^D from two mouse-adapted scrapie prion strains, 22L and 87V, influenced processing by Grp78 in the presence or absence of chaperones Hsp90, DnaJC1, and Stip1. We developed a cell-free in vitro system to monitor chaperone-mediated structural changes to, and disaggregation of, PrP^D. For both strains, Grp78 was most effective at structurally altering PrP^D at low pH, especially when additional chaperones were present. While Grp78, DnaJC1, Stip1, and Hsp90 were unable to disaggregate the majority of PrP^D from either strain, pretreatment of PrP^D with proteases increased disaggregation of 22L PrP^D compared to 87V, indicating strain-specific differences in aggregate structure were impacting chaperone activity. Hsp90 also induced structural changes in 87V PrP^D as indicated by an increase in the susceptibility of its n-terminus to proteases. Our data suggest that, while chaperones like Grp78, DnaJC1, Stip1, and Hsp90 disaggregate only a small fraction of PrP^D, they may still facilitate its clearance by altering aggregate structure and sensitizing PrP^D to proteases in a strain and pH-dependent manner.