Interaction of the middle domains stabilizes Hsp90 alpha dimer in a closed conformation with high affinity for p23

The human genome encodes two highly similar cytosolic Hsp90 proteins called isoforms Hsp90 alpha and Hsp90 beta. Of the 300 client proteins for Hsp90 identified so far only a handful interact specifically with one Hsp90 isoform. Here we report for the first time that Hsp90 cochaperone p23 binds preferentially to Hsp90 alpha and that this interaction is mediated by the middle domain of Hsp90 alpha. Based on the homology modeling, we infer that the middle domains in the Hsp90 alpha dimer bind stronger with each other than in the Hsp90 beta dimer. Therefore, compared to Hsp90 beta, Hsp90 alpha may adopt closed conformation more easily. Hsp90 interacts with p23 in the closed conformation. Hsp90 alpha binds human recombinant p23 about three times stronger than Hsp90 beta but with significantly smaller exothermic enthalpy as determined by isothermal titration calorimetry of direct binding between the purified proteins. As p23 binds to Hsp90 in a closed conformation, stabilization of the Hsp90 alpha dimer in the closed conformation by its middle domains explains preference of p23 to this Hsp90 isoform.