Chaperone Titration as a Mechanism of Prion Appearance

Abstract

Prions modulate cellular phenotypes by adopting alternative folds called amyloid that change their function and act as templates to convert natively folded protein to the amyloid state. The initial appearance of an amyloid is rare but enhanced by the presence of other aggregated proteins. Previous work has suggested this enhancement arises from the ability of pre-existing aggregates to template amyloid assembly but fails to explain all experimental observations. Our studies in S. cerevisiae suggest an alternative role for pre-existing aggregates: the titration of chaperones, which normally act to disassemble nascent amyloid nuclei. Using the [PSI+]/[PIN+] dual prion system, we demonstrate that the presence of [PIN+] reduces the binding of the molecular chaperone Hsp104 to Sup35 amyloid, the determinant of the [PSI+] prion. In turn, this change reduces the fragmentation of Sup35 amyloid, promoting appearance of the [PSI+] prion. Notably, the combination of [PIN+] and reduced levels of Hsp104 can inhibit prion appearance and propagation for some conformational variants of Sup35 amyloid, providing further evidence of titration. Using conformational variants of [PIN+] known to promote different levels of [PSI+] formation, we demonstrate that differential chaperone titration explains differences in [PIN+] variant promotion of [PSI+] induction. Thus, [PIN+] acts as a decoy for chaperones and thereby promotes persistence of nascent aggregates.