Influence of hydrophobic interactions on the structure and steroid-binding properties of glucocorticoid receptors

Glucocorticoid-receptor complexes in rat thymus cytosol were characterized by gel-filtration and ion-exchange chromatography and by other procedures. Two forms of non-transformed complex were identified at low ionic strength in the presence of molybdate, with Stokes radii of approx. 8 and 6 nm. The 8 nm molybdate-stabilized form could be converted to the 6 nm form by chromatography on Sephacryl S-300 or Lipidex 1000 or by incubation with charcoal or phospholipase C, but not by chromatography on Sephadex G-25. The dissociation rate of the complex was reduced by treatment with charcoal or Lipidex 1000, but none of the treatments caused transformation to a DNA-binding form. Transformation of the complex, by exposure to elevated temperature or ionic strength in the absence of molybdate, resulted in the appearance of a different 6 nm form, distinguished by an increased affinity for DNA-cellulose and a reduced affinity for DEAE-cellulose. These results suggest that receptor transformation is preceded by structural changes associated with the loss of a lipid factor from the complex. Non-polar steroid antagonists, and lipophilic compounds such as phenothiazines, were found to bind to secondary, hydrophobic sites on the receptor and to exert allosteric effects on the primary steroid-binding site; these and other observations emphasize the importance of hydrophobic interactions as determinants of the structure and properties of glucocorticoid receptors.