These results support the model of the ATPase cycle, according to which the thermal movements of the lid segment may open up the conformational space for the catalytic Arg residue from the Hsp90-MD to reach the nucleotide binding site and induce ATP hydrolysis. Although the lack of structural information about the full Hsp90 dimer interacting Sgt1 and Rar1 precludes direct modeling of this mechanism, our results point to the key role of Rar1-CHORD2 as an allosteric regulator of the lid dynamics. Rather interestingly, despite a considerable lid flexibility in the ternary complex, the lid movements in each of the Hsp90-NTD could be correlated with the functional displacements of the interacting Sgt1-CS and Rar1-CHORD2 domains. Hence, in contrast to the binary complex, functional dynamics of the Hsp90-Sgt1-Rar1 assembly may be characterized by correlated motions of the interacting molecules. These findings are consistent with the experiments suggesting that RAR1-CHORD2 facilitates cooperative assembly of the complex and can enhance the ATPase activity while destabilizing the closed lid conformation. According to previous studies structural plasticity and functional 9-methoxycamptothecine adaptation of Hsp90 to the vastly divergent families of interacting cochaperones and client proteins are enabled by modulating the proper balance of structural rigidity and flexibility in the Hsp90 interdomain interfaces. We have recently demonstrated that functionally important regulatory sites of Hsp90 may be strategically positioned at the interdomain regions separating structurally rigid and flexible regions. These Procyanidin-B1 residues often correspond to hinge sites around which large protein movements are organized. In this section, we analyzed how cochaperonemediated modulation of the Hsp90 dynamics could affect the distribution of structural rigid and flexible regions that are crucial for proper functioning of the chaperone. Using the force constant method within the framework of the discrete molecular dynamics formalism as implemented in we computed the fluctuation distance force constant for each residue in the Hsp90-Sgt1 and Hsp90-Sgt1-Rar1 complexes. The highest sharp peaks in force constant distributions are typically associated with the residues forming boundaries between structurally rigid and flexible regions, and could indicate the interdomain hinge sites. The residue-based force constant profiles of the Hsp90-Sgt1 and Hsp90-Sgt1-Rar1 complexes are characterized by several high value peaks separating structurally rigid and flexible residues. Interestingly, the most notable sharpest peaks that signify an abrupt transition from structurally stable to mobile regions were observed near the lid motif of the Hsp90-NTD. In the Hsp90-Sgt1 complex, the pronounced peak corresponds to the L95 residue that anchors one end of the lid motif and F126 that anchors the opposite end of the lid. Only a few very minor peaks could be spotted in the Sgt1-CS 
domain, corresponding to a stretch of structurally immobilized residues at the intermolecular Hsp90-Sgt1 interface. Consistent with the ENM-based analysis, the force constant profile of the binary Hsp90-Sgt1 complex similarly indicated the greater mobility of the Sgt1-CS domain. The residue-based dynamic profiles are based on the consecutive residue numbering for the Hsp90-NTD and Sgt1-CS domains in the complex. For clarity of the comparison with structural and functional experiments, we refer to the important functional residues according to their original numbering in the crystal structures. We observed three distinctive peaks in the Sgt1CS domain profile corresponding respectively to Y157, F168, and K221 residues.