Molecular basis of the interaction of Hsp90 with its co‐chaperone Hop

The heat shock protein (Hsp) Hsp90 is one of the most abundant proteins in the cell. It controls the functional turnover of proteins being involved in protein folding, refolding, transport as well as protein degradation. Co‐chaperones influence Hsp90's activity in different ways, among which the Hsp organizing protein (Hop) was found to inhibit its ATP hydrolysis upon binding. Despite the availability of a number of studies investigating the Hsp90:Hop complex, several aspects of the Hsp90:Hop interaction have remained unresolved. Here, we employed a combinatory approach comprising native polyacrylamide gel electrophoresis, isothermal titration calorimetry, multiangle light scattering, isothermal titration calorimetry, small‐angle X‐ray scattering, dynamic light scattering, and nuclear magnetic resonance, spectroscopy to obtain a comprehensive picture about the human Hsp90β:Hop association in solution. Our data show that only one Hop molecule binds the Hsp90β dimer, Hop can interact with the open and closed state of Hsp90β, and Hop's TPR2A‐2B domains determine the affinity for Hsp90's C‐terminal and middle domain, whereby the interaction with the C‐terminal domain of Hsp90β is sufficient to induce an allosteric conformational change between the two Hsp90β monomers in the Hsp90(2):Hop(1) complex. Together, this study highlights the important role of the co‐chaperone Hop in reorganizing Hsp90 for efficient client loading.