The molecular chaperone Hsp90 is critical for the maintenance of cellular homeostasis and represents a promising drug target. Despite increasing knowledge on the structure of Hsp90, the molecular basis of substrate recognition and pro-folding by Hsp90/co-chaperone complexes remains unknown.

The most common mis-folding disease is Alzheimer’s disease (AD), which is hallmarked by the oligomeric aggregation and amyloid fibril formation of amyloid-β and the microtubule-associated protein Tau. Hsp90 promotes either degradation or toxic aggregation of its natively disordered client Tau, depending on the particular associated co-chaperone. In addition, expression levels of the Hsp90 pro-folding co-chaperone FKBP51 increase with age, are elevated in AD brain and correlate with Tau pathogenesis. Insights into the structural changes that depend on the Hsp90/FKBP51 pro-toxic complex and trigger Tau aggregation and toxicity are thus important to decipher the nature of Tau’s toxic pathway.

In this study, we used an integrative approach that combines functional assays with fluorescence-based and calorimetric binding studies, mutational analysis, NMR spectroscopy and chemical cross-linking.

Here, we report the solution structures of human full-length Hsp90 in complex with the PPIase FKBP51, as well as the 280 kDa Hsp90/FKBP51 complex bound to the Alzheimer’s disease-related protein Tau. We reveal that the FKBP51/Hsp90 complex, which synergizes to promote toxic Tau oligomers in vivo, is highly dynamic and stabilizes the extended conformation of the Hsp90 dimer resulting in decreased Hsp90 ATPase activity. Within the ternary Hsp90/FKBP51/Tau complex, Hsp90 serves as a scaffold that traps the PPIase and nucleates multiple conformations of Tau’s proline-rich region next to the PPIase catalytic pocket in a phosphorylation-dependent manner. A key finding of our study is that the interaction of the PPIase with Hsp90 is dynamic and involves Hsp90 surfaces distinct from other co-chaperones, thus enabling asymmetric chaperone/co-chaperone/client complexes.

Our study defines a conceptual model for dynamic Hsp90/co-chaperone/client recognition.