Hsp90 (heat shock protein 90) is a molecular chaperone and is one of the most abundant proteins expressed in cells. It is a member of the heat shock protein family, which is upregulated in response to stress. Hsp90 is found in bacteria and all branches of eukarya, but it is apparently absent in archaea. Whereas cytoplasmic Hsp90 is essential for viability under all conditions in eukaryotes, the bacterial homologue HtpG is dispensable under non-heat stress conditions.
Heat shock proteins, as a class, are among the most highly expressed cellular proteins across all species. As their name implies, heat shock proteins protect cells when stressed by elevated temperatures. They account for 1–2% of total protein in unstressed cells. However, when cells are heated, the fraction of heat shock proteins increases to 4–6% of cellular proteins. Heat shock protein 90 (Hsp90) is one of the most common of the heat-related proteins. The "90" comes from the fact that it weighs roughly 90 kiloDaltons. A 90 kDa protein is considered fairly large for a non-fibrous protein.
The functions of Hsp90 include assisting in protein folding, stabilizing various proteins such as steroid receptors, and aiding in protein degradation. It also stabilizes a number of proteins involved in tumor growth, which is why Hsp90 inhibitors are investigated as anti-cancer drugs.
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. As discussed in more detail below, researchers later realized that Hsp90 has other essential functions in unstressed cells.
Read more about Hsp90: Isoforms, Mechanism, Clinical Significance, Hsp90 Phylogeny, Gene Duplications and Evolutionary Implications