Abstract
Hsp70s use energy from ATP hydrolysis to unfold protein structures and solubilize stable aggregates and accumulate native species, even under adverse non-native conditions. To carry out its catalytic polypeptide-unfolding activity, Hsp70 needs to reversibly interact with a J-domain (JDP) catalyst, a misfolded or alternatively-folded polypeptide substrate and a nucleotide exchange factor (NEF), which binds to the nucleotide-binding domain (NBD), accelerates ADP-release, and controls the dissociation of the unfolded polypeptide product of the unfolding reaction. In bacteria, GrpE is the ubiquitous NEF, and yet, during the process of eukaryotization, it was lost from the cytosol, to be replaced by novel NEF proteins, among which the Hsp110 family stands out. Curiously, Hsp110s belong to the Hsp70 superfamily, but the evolutionary steps that led from an ancestral Hsp70 unfoldase to a Hsp110 NEF catalyzing other Hsp70's activity remain unsolved. Combining experiments using wild-type Sse1 (yeast Hsp110) and rationally designed mutants, we show that Hsp110 is likely built upon some distinctive features already present in Hsp70 by repurposing them, rather than by fashioning novel molecular properties. Taking all results together, we suggest a novel mechanism of action of Hsp110, whereby it is a NEF that also enhances the unfolding/disaggregating entropic pulling forces generated by Hsp70, by transiently increasing the chaperone's effective volume.