HSP90 is an abundantly and ubiquitously expressed heat shock protein. It is understood to exist in two principal
forms α and β, which share 85% sequence amino acid homology. The two isoforms of Hsp90 are expressed in
the cytosolic compartment (1). Despite the similarities, HSP90α exists predominantly as a homodimer while
HSP90β exists mainly as a monomer.(2) From a functional perspective, hsp90 participates in the folding, assembly, maturation, and stabilization of specific proteins as an integral component of a chaperone complex. (3-6) Furthermore, Hsp90 is highly conserved between species; having 60% and 78% amino acid similarity between mammalian and the corresponding yeast and Drosophila proteins, respectively.
Hsp90 is a highly conserved and essential stress protein that is expressed in all eukaryotic cells. Despite its label of being a heat-shock protein, hsp90 is one of the most highly expressed proteins in unstressed cells (1–2% of cytosolic protein). It carries out a number of housekeeping functions – including controlling the activity, turnover, and trafficking of a variety of proteins. Most of the hsp90- regulated proteins that have been discovered to date are involved in cell signaling (7-8). The number of proteins now know to interact with Hsp90 is about 100. Target proteins include the kinases v-Src, Wee1, and c-Raf, transcriptional regulators such as p53 and steroid
receptors, and the polymerases of the hepatitis B virus and telomerase.5 When bound to ATP, Hsp90 interacts with cochaperones Cdc37, p23, and an assortment of immunophilin-like proteins, forming a complex that
stabilizes and protects target proteins from proteasomal degradation.
In most cases, hsp90-interacting proteins have been shown to co-precipitate with hsp90 when carrying out immune adsorption studies, and to exist in cytosolic heterocomplexes with it. In a number of cases, variations
in hsp90 expression or hsp90 mutation has been shown to degrade signaling function via the protein or to impair a specific function of the protein (such as steroid binding, kinase activity) in vivo. Ansamycin antibiotics, such as geldanamycin and radicicol, inhibit hsp90 function (9).
Recently, Prof. Tatu’s laboratory has shown the importance of Hsp90 in parasite growth. They have shown
that inhibition of P.falciparum Hsp90 (PfHsp90), blocks the erythrocytic cycle by inhibiting stage transformation, leading to inhibition of parasite growth (10, 11)
1. Nemoto, T. et al. (1997) J.Biol Chem. 272, 26179-26187.
2. Minami, Y, et al. (1991), J.Biol Chem. 266, 10099-10103.
3. Arlander SJH, et al. (2003) J Biol Chem 278, 52572-52577.
4. Pearl H, et al. (2001) Adv Protein Chem 59,157-186.
5. Neckers L, et al. (2002) Trends Mol Med 8:S55-S61.
6. Pratt W, Toft D. (2003) Exp Biol Med 228:111-133.
7. Pratt W, Toft D. (1997) Endocr Rev 18,306–360.
8. Pratt WB. (1998) Proc Soc Exptl Biol Med 217, 420–434.
9. Whitesell L, et al. (1994) Proc Natl Acad Sci USA 91, 8324–8328.
10. Banumathy G., Singh V., Pavithra S.R., and Tatu U.(2003) J Biol Chem. 278(20):18336-45.
11. Pavithra S.R, Banumathy G., Joy O., Singh V., and Tatu U. (2004) J Biol Chem. 279(45):46692-9.
