Skewes, B. | bioRxiv Science

bioRxiv2026

Single-molecule visualisation of human Hsp70-driven conformational remodelling during stress

The Hsp70 chaperone system plays a central role in the folding of nascent polypeptides and in preventing protein misfolding and aggregation during cellular stress. However, the precise mechanism by which the human Hsp70, HspA8, remodels the conformations of individual chemically misfolded clients remains unclear. Moreover, it is unknown whether this mechanism differs from that used by Hsp70 to engage clients during heat stress to preserve native function. To address these questions, we report here the use of single-molecule fluorescence resonance energy transfer (smFRET) to temporally interrogate how the human Hsp70 system regulates the conformation of a heat-sensitive client protein, firefly luciferase (Fluc), following chemical denaturation and during heat stress. We find that Hsp70 recognises both chemically denatured and heat-induced misfolded states of Fluc and resolves them by conformational expansion. Release from a Hsp70-bound state, a process driven by the nucleotide exchange factor, Hsp110, guides Fluc toward productive folding trajectories that would otherwise be unlikely to occur spontaneously following collapse from a conformationally unfolded state. Moreover, we demonstrate that both temperature and the conformational state of misfolded Fluc dictate the ability of HspA8 to meaningfully resolve non-native structure within the protein. Collectively, this work provides direct visualisation of the mechanisms by which Hsp70 modulates client conformations under diverse stress conditions to preserve proteome integrity.

biophysics↗

bioRxiv2025

Direct single-molecule visualization of Hsp90-mediated relief of a Hsp70-folding block

Hsp70 and Hsp90 are ubiquitous molecular chaperones that cooperate to promote the correct folding and maturation of client proteins. Despite their central role in proteostasis, the molecular mechanisms by which Hsp90 coordinates with Hsp70 to remodel clients remain poorly understood. In particular, how ATP hydrolysis by Hsp90 is coupled to client engagement and conformational change has been a long-standing question. This gap in understanding is largely due to the challenge of visualizing client conformations during the highly dynamic and heterogeneous interactions with the Hsp70/Hsp90 chaperone machinery. To address this, we used a combination of single-molecule fluorescence resonance energy transfer (smFRET) and total internal reflection fluorescence microscopy to observe individual firefly luciferase client proteins as they are sequentially engaged by Hsp70 and Hsp90. Here, we show that Hsp90 reduces rebinding of Hsp70 to folding intermediates while still allowing engagement to misfolded clients, thereby enabling productive refolding in the presence of typically inhibitory concentrations of Hsp70. Furthermore, Hsp90 uses ATP binding and hydrolysis to actively remodel the conformational landscape of the client, promoting controlled folding through localized compaction across multiple regions. These controlled folding events reduce misfolding and are essential for establishing native interdomain contacts. Using smFRET and kinetic simulations, we further demonstrate that heterogeneous Hsp70 binding generates region-specific folding kinetics and conformational dynamics, which are likely driven by variations in the number of available Hsp70-binding sites. This is then exploited during Hsp70/Hsp90-mediated folding to support localized folding of client subdomains, thereby reducing non-native interactions from distal regions to facilitate proper folding of multi-domain proteins.

biophysics↗