The function of steroid receptors in the cell depends on the chaperone machinery of Hsp90, as Hsp90 primes steroid receptors for hormone binding and transcriptional activation. Several conserved proteins are known to additionally participate in receptor chaperone assemblies, but the regulation of the process is not understood in detail. Also, it is unknown to what extent the contribution of these cofactors is conserved in other eukaryotes. We here examine the reconstituted C. elegans and human chaperone assemblies. We find that the nematode phosphatase PPH-5 and the prolyl isomerase FKB-6 facilitate the formation of glucocorticoid receptor (GR) complexes with Hsp90. Within these complexes, Hsp90 can perform its closing reaction more efficiently. By combining chemical crosslinking and mass spectrometry, we define contact sites within these assemblies. Compared to the nematode Hsp90 system, the human system shows less cooperative client interaction and a stricter requirement for the co-chaperone p23 to complete the closing reaction of GR•Hsp90•Pp5/Fkbp51/ Fkbp52 complexes. In both systems, hormone binding to GR is accelerated by Hsp90 alone and in the presence of its cofactors. our results show that cooperative complex formation and hormone binding patterns are, in many aspects, conserved between the nematode and human systems. Hsp90 is an ATP-driven molecular machine, highly abundant in the cytosol. It is recruited to client proteins that require energy-intense rearrangements and supports these reactions by performing nucleotide-induced conformational changes 1,2. These changes lead to the compaction of the Hsp90 dimer and the transient formation of a ring-like structure, where the N-terminal domains are dimerized in addition to the chaperone's permanent C-terminal dimerization 3. Only in the bacterial Hsp90 system are these changes performed and controlled by Hsp90 itself. In the eukaryotic systems, client-specific cofactors are present, which tailor the Hsp90 machinery into a client-specific mode and regulate the conformational changes while providing further interaction sites for the clients 4. This has been investigated for the Hsp90-dependent maturation of protein kinases with the help of the cofactor Cdc37 and for the chaperoning of steroid receptors, where a larger set of cofactors participates 5-7. Partly, these reactions have been reconstituted with recombinant proteins to identify interaction mechanisms, but mostly addressing the cofactors' interaction with Hsp90 in the absence of clients. Hsp90-containing protein complexes were first identified in the 1980s 8,9. In these studies, steroid hormone receptors, such as the glucocorticoid, mineralocorticoid or progesterone receptors, were isolated from vertebrate cells and the associated proteins were identified and studied by western blot analyses. Hsp90 and several other proteins were detected in these complexes, leading to the identification of Fkbp51, Fkbp52, Cyp40, Hop, p23, Hip and Pp5 as components of steroid hormone receptor complexes 10-14. The gluco...