Molecular chaperone that promotes the maturation, structural maintenance and proper regulation of specific target proteins involved for instance in cell cycle control and signal transduction. Undergoes a functional cycle that is linked to its ATPase activity which is essential for its chaperone activity. This cycle probably induces conformational changes in the client proteins, thereby causing their activation. Interacts dynamically with various co-chaperones that modulate its substrate recognition, ATPase cycle and chaperone function. Engages with a range of client protein classes via its interaction with various co-chaperone proteins or complexes, that act as adapters, simultaneously able to interact with the specific client and the central chaperone itself. Recruitment of ATP and co-chaperone followed by client protein forms a functional chaperone. After the completion of the chaperoning process, properly folded client protein and co-chaperone leave HSP90 in an ADP-bound partially open conformation and finally, ADP is released from Hsp90 which acquires an open conformation for the next cycle. In the resting state, through the dimerization of its C-terminal domain, HSP90 forms a homodimer which is defined as the open conformation. Upon ATP-binding, the N-terminal domain undergoes significant conformational changes and comes in contact to form an active closed conformation. After HSP90 finishes its chaperoning tasks of assisting the proper folding, stabilization and activation of client proteins under the active state, ATP molecule is hydrolyzed to ADP which then dissociates from HSP90 and directs the protein back to the resting state. Homodimer. The TPR repeat-binding motif mediates interaction with TPR repeat-containing proteins. Belongs to the heat shock protein 90 family.