Heat shock protein 104 (HSP104) chaperones soluble Tau via a mechanism distinct from its disaggregase activity

Heat shock protein 104 (HSP104) is a conserved AAA+ protein disaggregase, can disassemble the toxic aggregates formed by different amyloid proteins, and is protective in various animal models associated with amyloid-related diseases. Extensive studies have attempted to elucidate how HSP104 disassembles the aggregated form of clients. Here, we found that HSP104 exhibits a potent holdase activity that does not require energy, prevents the soluble form of amyloid clients from aggregating, and differs from HSP104's disaggregase activity. Using cryo-EM, NMR, and additional biophysical approaches, we found that HSP104 utilizes its small subdomain of nucleotide-binding domain 2 (ssNBD2) to capture the soluble amyloid client (K19 of Tau) independent of its ATP hydrolysis activity. Our results indicate that HSP104 utilizes two fundamental distinct mechanisms to chaperone different forms of amyloid client and highlight the important yet previously unappreciated function of ssNBD2 in chaperoning amyloid client and thereby preventing pathological aggregation.

Ploidy controls [URE3] prion propagation in yeast

Previous genetic approaches have enabled the identification of key partners for prion propagation in yeast, such as HSP104. All the experiments performed thus far have been conducted in a haploid context. In this study, we used a diploid yeast strain to identify genes that interfere with [URE3] stability. Our screen, based on a multi-copy library, revealed an unsuspected role for centromeric sequences that appear to decrease the mitotic stability of this prion. Because an increase in centromeric sequences interferes with [URE3] transmission, we analyzed this property in tetraploid yeast cells. We found that in such strains, [URE3] is quite unstable, with the concentration of Hsp104p being a key factor for the stabilization of [URE3] in 4n yeast cells. We also showed that HSP104 stabilization can occur independently of its 'disaggregate' activity. These results may explain the discrepancy between wild strains bearing or not bearing prions because they differ in their ploidy. These results provide new insight into prion biology by linking the control of ploidy to protein misfolding and demonstrate that [URE3] is also a gain-of-function phenotype.