Mol. Cell https://doi.org/10.1016/j.molcel.2022.01.005 (2022)
Heat-induced expression of chaperones, such as those of the molecular disaggregation system comprising Hsp104, Hsp70 and Hsp40, has been thought to counteract protein misfolding and aggregation. However, no specific mature endogenous eukaryotic protein has yet been shown to misfold after heat shock. Instead, several recent studies found that elevated temperatures cause reversible aggregation of certain yeast proteins into biomolecular condensates, with these condensates interacting with the disaggregation system, which may contribute to their regulation. Yet evidence for a direct role of the system in condensate dispersion is lacking. Writing in Molecular Cell, Drummond and colleagues now report the in vitro reconstitution of the yeast disaggregation machinery, together with heat-induced biomolecular condensates of Pab1, an abundant and broadly conserved eukaryotic RNA-binding protein. Using this system, the authors show that the system indeed mediates rapid dispersion of condensates. Only Hsp104, Hsp70 and Sis1 (a type II Hsp40) are strictly required for the activity, in contrast to the established requirements for dispersal of heat-misfolded model aggregates such as firefly luciferase. The condensate disaggregation process is much more rapid, and this seems to be due in part to activation of a previously known mode of action of Hsp104, which threads substrates through its central channel to actively extract them from aggregates. In contrast to misfolded luciferase, which is fully threaded and unfolded, the authors find that Pab1 is only partially threaded and unfolded by Hsp104, facilitating its rapid dispersal and refolding. Furthermore, assays with varying chaperone concentrations reveal that binding of multiple Hsp70 molecules cooperatively targets condensates for dispersal. The study establishes biomolecular condensates as substrates of the disaggregation system, which mediates their dispersal via a distinct mechanism.
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