Three J-proteins impact Hsp104-mediated variant-specific prion elimination: a new critical role for a low-complexity domain

Abstract

Prions are self-propagating protein isoforms that are typically amyloid. In Saccharomyces cerevisiae, amyloid prion aggregates are fragmented by a trio involving three classes of chaperone proteins: Hsp40s, also known as J-proteins, Hsp70s, and Hsp104. Hsp104, the sole Hsp100-class disaggregase in yeast, along with the Hsp70 Ssa and the J-protein Sis1, is required for the propagation of all known amyloid yeast prions. However, when Hsp104 is ectopically overexpressed, only the prion [PSI + ] is efficiently eliminated from cell populations via a highly debated mechanism that also requires Sis1. Recently, we reported roles for two additional J-proteins, Apj1 and Ydj1, in this process. Deletion of Apj1, a J-protein involved in the degradation of sumoylated proteins, partially blocks Hsp104-mediated [PSI + ] elimination. Apj1 and Sis1 were found to have overlapping functions, as overexpression of one compensates for loss of function of the other. In addition, overexpression of Ydj1, the most abundant J-protein in the yeast cytosol, completely blocks Hsp104-mediated curing. Yeast prions exhibit structural polymorphisms known as "variants"; most intriguingly, these J-protein effects were only observed for strong variants, suggesting variant-specific mechanisms. Here, we review these results and present new data resolving the domains of Apj1 responsible, specifically implicating the involvement of Apj1's Q/S-rich low-complexity domain.

Releasing the Lockdown: An Emerging Role for the Ubiquitin-Proteasome System in the Breakdown of Transient Protein Inclusions

“…A recent study from the late Stephan Jentsch lab [31] has demonstrated that the yeast Hsp70/Apj1 bi-chaperone pair, in cooperation with Hsp110, exclusively couples the dissolution of INQ to 26S proteasome-mediated degradation, independently of Hsp104 function [32]. The involvement of the Hsp40 co-chaperone in the proteasome-dependent disaggregation of transient-QC is in line with findings that established Hsp40 co-chaperones not only as Hsp70 activators but also as regulators of their binding specificity and functional diversity [32][33][34][35]. Now, the den Brave study, not only establishes the function of the nuclear Hsp40, Apj1, in INQ clearance, but it also indicates that Hsp104-dependent refolding and proteasome-mediated degradation of solubilized misfolded proteins are independent disaggregation triage reactions [32].…”

Yeast Models for Amyloids and Prions: Environmental Modulation and Drug Discovery

“…The Hsp104 is essential for all amyloid-based cytosolic yeast prions [18,23]. The machinery also includes members of the Hsp70 family Ssa [24,25,26,27] and cochaperones of the Hsp40 family, also known as J-proteins [28,29,30]. In the current model of [ PSI + ] prion propagation, the Hsp70/40 complex binds to amyloid fibrils and recruits Hsp104 [31].…”

How Do Yeast Cells Contend with Prions?

“…Among Hsp40s, Sis1p (the only essential Hsp40 [ 116 ]) is required by [PSI+], [URE3], [PIN+], and [SWI+] [ 94 , 117 , 118 ], while Swa2 is specifically required for [URE3] [ 97 ]. The specificity appears to reside in the TPR domain of Swa2.…”