Acts as a co-chaperone and mediates the association of the chaperones HSP70 and HSP90 probably facilitating substrate transfer from HSP70 to HSP90. Stimulates HSP70 ATPase activity and, in contrast, inhibits HSP90 ATPase activity. Monomer. Homodimer. Forms a complex composed of HOP and chaperones HSP70 and HSP90; the interaction is stronger in the absence of ATP. Interacts (via TPR 1, 2, 3, 7, 8 and 9 repeats) with HSP70 (via C-terminus); the interaction is direct and is stronger in the absence of ATP. Interacts (via TPR 4, 5 and 6 repeats) with HSP90 (via C-terminus); the interaction is direct. The TPR repeats form 3 domains; the TPR 1 domain is composed of TPR 1, 2 and 3 repeats, the TPR2A domain of TPR 4, 5 and 6 repeats and TPR2B domain of TPR 7, 8 and 9 repeats.