Steroid-free glucocorticoid receptor binds specifically to mouse mammary tumour virus DNA.

Steroid hormones are thought to modulate gene expression through their interaction with receptor proteins. The intracellular localization of unoccupied receptor proteins has been a subject of controversy: free glucocorticoid receptor appears to reside in the cytoplasm and moves to the cell nucleus only after binding the steroid. The purified hormone-bound glucocorticoid receptor has been shown to bind selectively to hormone regulatory elements (HRE) in the vicinity of hormonally-inducible promoters and, in particular, in the long terminal repeat (LTR) region of mouse mammary tumour virus (MMTV). We have tackled the question of whether the hormone itself is required for the interaction of the receptor protein with the HRE. Using monoclonal antibodies to the receptor we find that upon heat-activation the steroid-free glucocorticoid receptor present in rat liver cytosol binds specifically in vitro to the HRE of MMTV. No qualitative differences in the DNaseI-footprints were detected when hormone-free receptor was compared to the hormone-receptor complex or even receptor complexed with the hormone antagonist RU486. We conclude that the steroid ligand is not an absolute requirement for generating the conformation of the glucocorticoid receptor that allows its interaction with the HRE in vitro. An alternative function of the hormone in vivo could be to modulate nuclear partitioning of the receptor.