The relationship between glucocorticoid receptor binding to Hsp90 and receptor function.

In this minireview we summarize evidence that the association of the glucocorticoid receptor (GR) with hsp90 may determine three functional states of the receptor. First, there is a direct correlation between hsp90 binding to the receptor and repression of DNA binding activity. Temperature-dependent dissociation of hsp90 from the cytosolic GR-hsp90 complex is promoted by hormone with simultaneous conversion of the receptor to the DNA binding state. GR that is translated in rabbit reticulocyte lysate binds to hsp90 at or near the termination of receptor translation and is in the non-DNA-binding form. Second, there is a direct correlation between binding of the immunopurified GR to hsp90 and the presence of a high affinity steroid binding conformation of the receptor. GR translated in reticulocyte lysate binds steroid with high affinity, but GR translated in wheat germ extract is not bound to hsp90, does not bind steroid with high affinity, and is in the DNA-binding form. When immunopurified, hsp90-free GR is incubated with rabbit reticulocyte lysate, hsp90 associates with the receptor and high affinity steroid binding capacity is completely reactivated. Third, there is a correlation between binding of hsp90 to steroid receptors and their retention in an inactive "docking" state until the binding of hormone in the intact cell triggers a progression to high affinity nuclear binding sites where the primary events involved in transcriptional activation occur. In contrast to the receptors that are retained in the docking state, the unliganded thyroid hormone receptor proceeds directly to high affinity nuclear binding sites. Consistent with this difference in behavior, the thyroid hormone receptor is translated in reticulocyte lysate in its DNA binding form and is not associated with hsp90.