Different modes of Hsp90 action in signaling pathways. Different modes of Hsp90 action in signaling pathways. (A) Inactive steroid hormone receptors are folded by the Hsp90 machinery. Folded receptors either are stabilized and activated by hormone binding or remain unstable and are rebound by chaperones (Smith, 1993; Pratt and Toft, 1997). (B) Ecdysone receptor with its partner USP can bind hormone but requires folding by the Hsp90 machinery to become active for DNA binding (Arbeitman and Hogness, 2000). (C) Monomeric HSF1 is sequestered by Hsp90. Under stress conditions, misfolded proteins compete for Hsp90, and HSF1 is displaced from Hsp90 and can form the active trimer (Zou et al., 1998). Jason C. Young et al. J Cell Biol 2001;154:267-274 © 2001 Rockefeller University Press