Folding of stringent clients requires transfer from Hsp70 to Hsp90. The co-chaperone Hop physically connects the chaperone machineries. Here we define its role from the remodeling of Hsp70/40-client complexes to the mechanism of client transfer and the conformational switching from stalled to active client-processing states of Hsp90. We show that Hsp70 together with Hsp40 completely unfolds a stringent client, the glucocorticoid receptor ligand binding domain (GR-LBD) in large assemblies. Hop remodels these for efficient transfer onto Hsp90. As p23 enters, Hsp70 leaves the complex via switching between binding sites in Hop. To proceed to client folding, current concepts assume that Hop dissociates and the co-chaperone p23 stabilizes the Hsp90 closed state. In contrast, we show that p23 directly interacts with Hop, relieves the stalling Hsp90-Hop interaction and closes Hsp90. This reaction allows folding of the client and is thus the key regulatory step for the progression of the chaperone cycle. To study the interaction between the different proteins, especially p23 and the DP2 domain of Hop, we performed crosslinking-MS.