Structural elements in the flexible tail of the co-chaperone p23 coordinate client binding and progression of the Hsp90 chaperone cycle.

The co-chaperone p23 is a central part of the Hsp90 machinery. It stabilizes the closed conformation of Hsp90, inhibits its ATPase and is important for client maturation. Yet, how this is achieved has remained enigmatic. Here, we show that a tryptophan residue in the proximal region of the tail decelerates the ATPase by allosterically switching the conformation of the catalytic loop in Hsp90. We further show by NMR spectroscopy that the tail interacts with the Hsp90 client binding site via a conserved helix. This helical motif in the p23 tail also binds to the client protein glucocorticoid receptor (GR) in the free and Hsp90-bound form. In vivo experiments confirm the physiological importance of ATPase modulation and the role of the evolutionary conserved helical motif for GR activation in the cellular context. p23 is a co-chaperone of Hsp90 but its mode of action is mechanistically not well understood. Here, the authors combine in vitro and yeast in vivo assays, biochemical measurements and NMR experiments to characterize p23 and identify two conserved helical elements in the intrinsically disordered C-terminal tail of p23 that together with the folded domain of p23 regulate the Hsp90 ATPase activity and affect the binding and maturation of Hsp90 clients. [ABSTRACT FROM AUTHOR]