Variant‐specific and reciprocal Hsp40 functions in Hsp104‐mediated prion elimination: A potential role for the ‘Anti‐prion J‐protein 1’ (Apj1)
Authors: Hines Justin K , Sporn Zachary A , Astor Michael T , Kamiya Erina 
Published: November 2023

The prions of Saccharomyces cerevisiae are heritable aggregates of misfolded yeast proteins. Yeast prions are incorporated into daughter cells through the fragmentation of these aggregates by molecular chaperone proteins, including J‐proteins (Hsp40s), Hsp70s, and Hsp104. Previous studies have shown that the overexpression of Hsp104 cures the prion [PSI+], but not other prions, a phenomenon which has promoted the exploration of Hsp104 as a potential therapeutic agent for neurodegenerative diseases, despite the fact that the mechanism of [PSI+] elimination by Hsp104 overabundance is currently the subject of significant debate. We recently reported that the human homolog of Sis1, Hdj1, was capable of substituting for Sis1 in the propagation of strong but not weak [PSI+] variants and was deficient in substituting for Sis1 in the elimination of [PSI+] by Hsp104 overexpression. The objective of this study was to extend that investigation to the Sis1‐domain requirements for Hsp104‐mediated [PSI+] curing, expanding upon another prior investigation by extending the analysis to multiple [PSI+] variants and genetic backgrounds. We also asked whether any of the other 12 J‐proteins located in the S. cerevisiae cytosol are also necessary for Hsp104‐induced [PSI+] curing and if prion amyloid structure (variant identity) affects experimental outcomes. The primary method used in this study was complementation by plasmid‐shuffling in the budding yeast S. cerevisiae.Summary of ResultsOur investigation revealed that at least two weak [PSI+] variants can be maintained in the absence of any of 12 non‐essential J‐proteins, ruling out essential roles for any of these J‐proteins in weak [PSI+] prion propagation. Likewise, elimination of these weak [PSI+] variants by Hsp104 overexpression was ubiquitous, demonstrating that no J‐protein other than Sis1 is necessary for the elimination of weak [PSI+] variants by overexpression of Hsp104. In sharp contrast however, we found that strong variants of [PSI+] exhibited exceptional resistance to Hsp104‐mediated elimination only in strains lacking the J‐protein Apj1 (originally named: Anti‐Yeast Prion J‐protein 1) and conversely Apj1 overexpression accelerates prion elimination. Apj1 has been implicated in prion biology several times; often acting similarly to Ydj1 in prion biology. Here we find a novel genetic interaction for Apj1 and prions that is distinct from Ydj1 function. Additional mutation and truncation experiments revealed domains required for Apj1 function in Hsp104‐mediated [PSI+] curing and may ultimately help to explain the prion‐specificity of this curing mechanism.Support or Funding InformationThis work was supported by the National Institute of General Medical Sciences of the National Institutes of Health under Award Number R15GM110606. The content is solely the responsibility of the authors and does not necessarily represent the official views of the National Institutes of Health. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.