Hsp90 is one of the most abundant proteins of eukaryotic cells, comprising 1-2% of total proteins under nonstress conditions. It is evolutionarily conserved among species, and is essential for cell survival. HSP90 binds to various substrate proteins and regulates their function. Hsp90 forms complexes with heat shock factor-1 (HSF-1) and maintains HSF-1 in an inactive state as the monomer to downregulate a heat shock reaction. A substance that binds and inhibits the activity of Hsp90 can induce the expression of Hsp70 and is expected to be used as a cell protecting agent. Thus Hsp90 regualtes its own expression by sequestering the transcription factor, HSF1, under non-stress conditions. Upon heat shock, HSF1 is released from Hsp90 leading to transcription and increased synthesis of Hsp90, thereby controlling the cellular stress response.
Isoforms: There are 5 protein isoforms of Hsp90 (1) Hsp90-alpha1 (2) Hsp90-alpha2 (3) Hsp90-beta (4) endoplasmin/GRP-94 and (5) TNF Receptor Associated Protein 1.
1. Endoplasmin/GRP94:
Structure: HSP90 is composed of 3 main domains. The C terminal domain contains a dimerization site as well as docking sites for various cochaperones. The central domain contains a large hydrophobic surface that is involved in the binding of HSP90 client protiens, and the N-terminal region contains the molecule’s ATPase domain, where ATP is bound and subsequently hydroyzed. Thus Hsp90 is an ATPase. It is believed that unfolded, or partially folded substrate proteins, also called Hsp90 client proteins, are stably bound to Hsp90 in its ATP bound state, and released upon ATP hydrolysis.
Hsp90 client proteins: Unlike other chaperones, most known client proteins of HSP90 are involved in the regulation of survival and growth. Geldanamycin, for example, binds the ATP binding pocket of HSP90, and this leads to allosteric changes in HSP90 that result in release of HSF1 and subsequenct expresion of HSP25 and HSP70. However, although some reports indicate that geldanamycin confers protection to cells from stress, most studies show that geldanamycin is cytotoxic which is due to the fact that i not only releases bound HSF1, but, but occupying the ATP binding pocket, blocks HSP90 chaperone activity and reduces the stability and activity of HSP90 client proteins such as Akt and Raf.
Hsp90 interacts with and stablizes a growing list of various kinases including several key members of malignant transformation, such as the ErbB2, Src, Abl or Met tyrosine kinases, or the Raf, Akt and cyclin-dependent serine kinases. Chaperone inhibitors can inhibit the ability of a chaperone to maintain its activation competent conformation. As a result, the client proteins become degraded by proteasome.
Inhibitors of Hsp90: The most important Hsp90 inhibitors are geldanamycin and its less toxic analog 17AAG. Geldanamycin, a natural product, binds to the ATP binding site of Hsp90 inhibitting ATP hydrolysis but not substrate protein bidning. Substrate proteins that reside longer on Hsp90 when ATP hydrolysis is inhibited are ubiquinated and subsequently degraded. Disrupting the function of the Hsp90 complex has been shown to deplete oncogenic kinases (via ubiquitin-mediated proteasomal degradation) and decrease tumor growth. The Hsp90 complex present in tumor cells exhibits much higher affinity for geldanamycin than Hsp90 in non-tumor cells. Thus inhibitors of the Hsp90 complex have the ability to convert this protein from a chaperone that insures correct protein folding of oncogenic proteins to a selective protein degradation tool.