ANTIBIOTIC RADICICOL BINDS TO THE N-TERMINAL DOMAIN OF HSP90 AND SHARES IMPORTANT BIOLOGIC ACTIVITIES WITH GELDANAMYCIN

The molecular chaperone Hsp90 plays an essential role in the folding a nd function of important cellular proteins including steroid hormone r eceptors, protein kinases and proteins controlling the cell cycle and apoptosis, A 15 Angstrom deep pocket region in the N-terminal domain o f Hsp90 serves as an ATP/ADP-binding site and has also been shown to b ind geldanamycin, the only specific inhibitor of Hsp90 function descri bed to date. We now show that radicicol, a macrocyclic antifungal stru cturally unrelated to geldanamycin, also specifically binds to Hsp90. Moreover, radicicol competes with geldanamycin for binding to the N-te rminal domain of the chaperone, expressed either by in vitro translati on or as a purified protein, suggesting that radicicol shares the geld anamycin binding site. Radicicol, as does geldanamycin, also inhibits the binding of the accessory protein p23 to Hsp90, and interferes with assembly of the mature progesterone receptor complex. Radicicol does not deplete cells of Hsp90, but rather increases synthesis as well as the steady-state level of this protein, similar to a stress response. Finally, radicicol depletes SKBR3 cells of p185(erbB2), Raf-l and muta nt p53, similar to geldanamycin. Radicicol thus represents a structura lly unique antibiotic, and the first non-benzoquinone ansamycin, capab le of binding to Hsp90 and interfering with its function.