Molecular chaperone that promotes the maturation, structural maintenance and proper regulation of specific target proteins involved in cell cycle control and signal transduction. Undergoes a functional cycle that is linked to its ATPase activity. The nucleotide-free form of the dimer is found in an open conformation in which the N-termini are not dimerized and the complex is ready for client protein binding. Binding of ATP induces large conformational changes, resulting in the formation of a ring-like closed structure in which the N-terminal domains associate intramolecularly with the middle domain and also dimerize with each other, stimulating their intrinsic ATPase activity and acting as a clamp on the substrate. Finally, ATP hydrolysis results in the release of the substrate. 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. Required for growth at high temperatures. Inhibited by geldanamycin, macbecin I and radicicol, which bind to the ATP-binding pocket. Co-chaperones CDC37, SBA1 and STI1 reduce ATPase activity. Co-chaperones AHA1 and HCH1 increase ATPase activity. Homodimer. Interacts with the co-chaperones AHA1, CDC37, CNS1, CPR6, CPR7, HCH1, SBA1, SSE1 and STI1. CNS1, CPR6, CPR7 and STI1. Interacts directly with the substrates GCN2, HAP1 and STE11. Expressed constitutively and induced by high temperatures dependent on transcription factor HSF1. According to PubMed:2674684, it is constitutively expressed at low levels, however, due to the specificity of the antibody, this result is unsure. The TPR repeat-binding motif mediates interaction with TPR repeat-containing proteins like the co-chaperones AHA1, CDC37, CNS1, CPR6, CPR7, HCH1, SBA1, SSE1 and STI1. CNS1, CPR6, CPR7 and STI1. Present with 444943 molecules/cell in log phase SD medium. Belongs to the heat shock protein 90 family.