This protein was first isolated by extracting proteins from cells stressed by heating, dehydrating or by other means, all of which caused the cell's proteins to begin to denature. Whereas cytoplasmic Hsp90 is essential for viability under all conditions in eukaryotes, the bacterial homologue HtpG is dispensable under non-heat stress conditions. Hsp90 is found in bacteria and all branches of eukarya, but it is apparently absent in archaea. A 90 kDa protein is considered fairly large for a non-fibrous protein. The "90" comes from the fact that it has a mass of roughly 90 kilodaltons. Heat shock protein 90 (Hsp90) is one of the most common of the heat-related proteins. However, when cells are heated, the fraction of heat shock proteins increases to 4–6% of cellular proteins. They account for 1–2% of total protein in unstressed cells. As their name implies, heat shock proteins protect cells when stressed by elevated temperatures. Heat shock proteins, as a class, are among the most highly expressed cellular proteins across all species. It also stabilizes a number of proteins required for tumor growth, which is why Hsp90 inhibitors are investigated as anti-cancer drugs. Hsp90 ( heat shock protein 90) is a chaperone protein that assists other proteins to fold properly, stabilizes proteins against heat stress, and aids in protein degradation. Also, X-X represents a mature properly folded protein dimer. Hsp40, Hsp70, and p23 are partner chaperones while Hop is a co-chaperone. X/Y represents an immature incompletely folded protein such a steroid receptor.
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