The Hsp90 molecular chaperone is responsible for the conformational activation and assembly of proteins that are essential for cell signaling and regulation. Many of these client proteins control functions involved in malignant transformation and the chaperone has attracted considerable interest as anticancer target. Maturation of client proteins is an ATP-dependent phenomenon where the chaperone conformational changes are coupled to the binding and hydrolysis of the nucleotide. This Hsp90 ATPase/chaperone cycle is regulated by a set of cochaperone proteins. The human R2TP cochaperone complex is composed of RUVBL1 and RUVBL2 AAA+ ATPases, PIH1D1 interacting platform and the RPAP3 adaptator. The R2TP-HSP90 chaperone machinery is essential for the biogenesis of macromolecular complexes such as small nucleolar ribonucleoparticles, phosphatidyl-inositol 3-kinase-related kinases, telomerase and RNA polymerases. RPAP3 cochaperone contains tetratricopeptide repeats (TPR) domains anchoring HSP70 and HSP90 to R2TP1. We have solved the crystal structure of human RPAP3 in complex with PIH1D1 cochaperone and we have shown that Hsp90 activity is regulated by these cochaperones. This regulation is correlated with induced conformational changes of the HSP90 dimer as evaluated by FRET and we propose a mechanism for the action of the cochaperone. 1: Benbahouche et al. (2014) J Biol Chem 289:6236–6247. 2: Henri et al. (2018) Structure, 26, 1–14.