Evidence that the hormone-binding domain of the mouse glucocorticoid receptor directly represses DNA binding activity in a major portion of receptors that are "misfolded" after removal of hsp90.

After dissociation of cytosolic heteromeric glucocorticoid receptor complexes by steroid, salt, and other methods, only 35-60% of the dissociated receptors can bind to DNA-cellulose. The DNA-binding and non-DNA-binding forms of the dissociated receptors have the same Mr and are phosphorylated to the same extent (Tienrungroj, W., Sanchez, E. R., Housley, P. R., Harrison, R. W., and Pratt, W. B. (1987) J. Biol. Chem. 262, 17347-17349). The basis for the different DNA-binding activities is unknown, but the DNA-binding fraction of the receptor has a more basic pI than the non-DNA-binding fraction (Smith, A. C., Elsasser, M. S., and Harmon, J. M. (1986) J. Biol. Chem. 261, 13285-13292). We have separated the non-DNA-binding state of the receptor from the DNA-binding state and then cleaved it with trypsin and chymotrypsin. We find that the 15-kDa tryptic fragment derived from the non-DNA-binding state of the dissociated receptor is fully competent in binding DNA, whereas the 42-kDa chymotryptic fragment containing both the hormone-binding and DNA-binding domains does not bind DNA. Trypsin cleavage of the molybdate-stabilized untransformed receptor also yields a 15-kDa fragment that is fully competent in binding DNA. Reducing agents do not restore DNA-binding to the non-DNA-binding fraction of the receptor and the hormone-binding domain can be separated from the DNA-binding domain on nonreducing gel electrophoresis. These results argue that the two domains are not linked by disulfide bridges, and they are consistent with the proposal that there are two least energy states of folding after dissociation of hsp90. A significant portion of the receptors is "misfolded" in such a manner that the steroid binding domain is directly preventing DNA-binding activity.