UCSF

Hsp90 is a ubiquitous ATP dependent molecular chaperone with numerous "client" proteins that depend on Hsp90 for activation. In eukaryotes, Hsp90's ability to hydrolyze ATP is essential, yet it is entirely unclear how hydrolysis is promoting client activation. One of Hsp90's clients, the Glucocorticoid Receptor (GR), is a ligand activated transcription factor that regulates many biologically important genes. Although Hsp90 is required for GR ligand binding in vivo, purified apoGR is fully capable of binding ligand with no enhancement from Hsp90. We reveal that Hsp70, known to facilitate client delivery to Hsp90, inactivates GR through partial unfolding of GR in a region important for ligand binding. Hsp90, with the cochaperones HOP and p23, reverses this inactivation, promoting ligand binding. Recovery of ligand binding requires ATP hydrolysis on Hsp90. This indicates that energy from ATP hydrolysis on Hsp90 regulates client transfer from Hsp70, likely through the coupling of the two chaperone's ATP cycles. Such coupling is embodied in novel contacts between Hsp90 and Hsp70 in the GR:Hsp70:Hsp90:HOP complex by cryoEM. While GR released from Hsp70 is aggregation prone, release from Hsp90 protects GR from aggregation and enhances its affinity for ligand. Together, this illustrates how the coordinated chaperone interactions can enhance stability, function and regulation.