Abstract Maintaining a healthy proteome is fundamental for organism survival 1,2 . Integral to this are Hsp90 and Hsp70 molecular chaperones that together facilitate the folding, remodeling and maturation of Hsp90’s many “client” proteins 3–7 . The glucocorticoid receptor (GR) is a model client strictly dependent upon Hsp90/Hsp70 for activity 8–13 . Chaperoning GR involves a cycle of inactivation by Hsp70, formation of an inactive GR:Hsp90:Hsp70:Hop “loading” complex, conversion to an active GR:Hsp90:p23 “maturation” complex, and subsequent GR release 14 . Unfortunately, a molecular understanding of this intricate chaperone cycle is lacking for any client. Here, we report the cryo-EM structure of the GR loading complex, in which Hsp70 loads GR onto Hsp90, revealing the molecular basis of direct Hsp90/Hsp70 coordination. The structure reveals two Hsp70s—one delivering GR and the other scaffolding Hop. Unexpectedly, the Hop cochaperone interacts with all components of the complex including GR, poising Hsp90 for subsequent ATP hydrolysis. GR is partially unfolded and recognized via an extended binding pocket composed of Hsp90, Hsp70 and Hop, revealing the mechanism of GR loading and inactivation. Together with the GR maturation complex (Noddings et al., 2020), we present the first complete molecular mechanism of chaperone-dependent client remodeling, establishing general principles of client recognition, inhibition, transfer and activation.

GR chaperone cycle mechanism revealed by cryo-EM: inactivation of GR by GR:Hsp90:Hsp70:Hop client-loading complex