Hijacking client instability to selectively disrupt HSP90-driven glucocorticoid receptor activation.

The molecular chaperone HSP90 drives the folding and activation of a broad spectrum of client proteins through dynamic, transient multiprotein assemblies. Although HSP90 has been widely pursued as an anticancer target, ATP-competitive inhibitors can trigger indiscriminate client depletion and protective heat shock responses, limiting clinical utility. Here, we report a client-directed strategy to perturb HSP90-dependent maturation by targeting unstable client regions that act as transient recognition sites for chaperone networks. Using energy-decomposition analysis of the ligand-binding domain of the stringent HSP90 client glucocorticoid receptor (GR), we identified surface-exposed, weakly coupled substructures predicted to sample locally unfolded conformations. We translated these motifs into a set of short GR-derived peptide mimics designed to compete with chaperone engagement events required for productive GR folding. The peptides selectively bind purified HSP90, are cell permeable, and induce GR degradation in triple-negative breast cancer cells. Functionally, the most active mimic potentiates paclitaxel response under hormone-supplemented conditions and suppresses glucocorticoid-induced tumor cell quiescence. These results establish a framework that integrates computation-guided identification of client unfolding regions to the rational design of chemical probes that modulate chaperone-dependent signaling pathways, with relevance for cancer progression. More broadly, our study illustrates how targeting weak, client-specific interactions within proteostasis assemblies can yield new entry points for therapeutic development.