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I.
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I. Introduction
Heat Shock Protein 90 (HSP 90) is a 90 kilo Dalton molecular chaperone in eukaryotes that will activate and facilitate many regulatory “client” signaling proteins. Client proteins range from proteins involved in cell signaling, proliferation, and cell survival. This protein is involved in the late stages of protein folding by binding to exposed hydrophobic surfaces of the client proteins. If HSP90 does not bind to the client proteins, then the protein’s signaling dependent activities, such as ligand binding, will be disrupted.
Due to the role that HSP90 plays in client protein signaling pathways, HSP90 has been a new target for chemotherapy treatment of multiple cancers. HSP90 can block cell signaling process that will activate oncogene protein kinase. A treatment that targets HSP90 will block the pathways that turn on oncogene protein kinases, which would limit the growth of cancerous cells.
HSP90 undergoes important conformational changes from activated ATP bound closed state to inactive open state. The open state allows for client proteins to bind and as ATP binding causes HSP90 to close, the client protein is released. The open state was isolated in Escherichia coli and the closed conformation was isolated in Yeast. For more information about the HSP90 chaperone cycle, please see the Chaperone Cycle popup in E. coli.
II. Open Structure in Escherichia coli
The inactive open structure consists of the N terminal domain, middle segment, and C domain.
The N domain is composed of eight β sheets and eight α helices. The Middle segment is formed by two α-β-α segments and three helices. Finally, The C domain has an alpha β fold.
Helix 21 in the C domain and a loop in the middle segment are amphipathic, and are expected to be a client protein binding site.
III. Closed Structure in Yeast
The closed structure of HSP90 consists of the N-terminal domain, C-terminal domain, small middle segment, and a large middle segment. ATP molecules are bound in the N-terminal domain.
The N domain is formed by a twisted β sheet and α helices. An ATP binding pocket is observed. The N domain connects to the middle segments via an antiparallel beta strand.
The middle segment is formed by a large α-β-α domain with a smaller α-β-α domain, which are connected via a helical coil.
The C domain is connected to the middle segment via a curved α helix sandwiched between a 3-stranded β sheet and another α helix. An α helix involved in dimeric interactions extends from the core of the C domain facing the N terminal.
An Amphipathic loop, which is a loop with both hydrophilic and hydrophobic components, projects from the larger middle segment towards the other side of the protein. The middle segment and the beginning of the C domain are connected via an extended loop. Note: This structure is missing part of the amphipathic loop on chain A.
IV. Conformational Changes in Yeast
HSP90 undergoes a major conformational change from inactive open (described in E. coli) to an active closed form (described in Yeast). ATP binding will trigger the molecule to change from open to close. When ATP binds, the small middle segment is brought 10 Angstroms closer together and the large middle segment is brought 20 Angstroms closer together. This conformational change is essential to the function of HSP90. This structure represents the C domain and Middle Segment found in Yeast.
During the conformational change, the N-terminal β strand will first hydrogen bond to the main β sheet in the other N terminal monomer.
Secondly, the lid segment will open by 180 degrees hinged at Glycene 94 and 121, which folds over the pocket containing nucleotide binding. Once the lid moves, the hydrophobic patches on Leucine 15 and 18 are exposed. Threonine 22 was found to activate the ATPase of HSP90; however, current research is unclear as to how.
A section of the lid in the N domain will interact with a section of the middle segment catalytic loop in the closed conformation. Arginine 380 is an essential catalytic residue that allows for the N domain and the middle segment to interact in the ATP bound state. However, Arg380 is not present in E. coli HSP90.
γ phosphate is in a glycine rich loop near the lid segment, which hydrogen bonds with Glutamine 119, Glycine 121, Valine 122, and Glycine 123.
V. Co-Chaperone p23
Co-chaperones in yeast, such as p23, are involved in stabilizing the ATP conformation and increasing the activated state of HSP90 to activate the client protein.
HSP90 sections will interact with the β hairpins in p23 in the N domain. The N terminal strain of p23 will interact with a loop in the N domain which is next to the lid segment.
VI. References
1. Goodsell, David. “PDB101: Molecule of the Month: Hsp90.” RCSB, 2008, pdb101.rcsb.org/motm/108.
2. Ali, M.M.U., Roe, S.N., Vaughan, C.K., Meyer, P., Panaretou, B., Piper, PW., Prodromou, C., and Pearl, LH. 2006. Crystal structure of an HSP90-nucleotide-p23/Sba1 closed chaperone complex. Nature. 440: 1013 – 1017.
3. Shiau, AK., Harris, SF., Southworth, DR. and Agard, DA. 2006. Structural Analysis of E. coli hsp90 Reveals Dramatic Nucleotide-Dependent Conformational Rearrangements. Cell. 127:329-340.