Rat liver chromatin non-histone proteins and glucocorticoid binding

We have previously characterized a specific corticosterone binding protein in chromosomal non histone proteins (NHP) from rat liver. In this paper, we present evidence that a relationship exists between this protein and the cytoplasmic glucocorticoid receptor. The binding capacity of NHP is reduced by 40 p. cent when this fraction is isolated from adrenalectomized animals. Incubation of isolated nuclei with the glucocorticoid hormone receptor complex results in a decrease in the specific radioactivity of the cytoplasmic proteins and simultaneously in a rapid uptake of the isotope by the nucleus; radioactive hormone was extracted along with the NHP. Evidence is presented that the NHP component binding the hormone is closely related or identical to the cytoplasmic receptor-proteins. Progesterone and corticosterone compete similarly for the binding of dexamethasone to nuclear and cytoplasmic forms of the receptor. However the nuclear form of the receptor has a higher affinity for corticosterone (K_a : 6 × 10⁹ M⁻¹) than for dexamethasone (K_A : 10⁸ M⁻¹) in vitro.A mixture of rat liver NHP and cytosol was shown to bind specifically more corticosterone than when the two proteins were incubated separately with the hormone. The Scatchard analysis shows that the enhancement of binding is due to an interaction of nuclear and cytoplasmic proteins leading to the appearance of a stable protein-protein complex which has a high affinity for the hormone (K_a : 2 × 10⁸ M⁻¹). KCl prevented this interaction. Complex formation does not require the presence of the hormone. The experiments presented here favor the hypothesis of the existence of a regulatory protein in the nucleus. This protein associated with the binding protein to reveal or enhance the active form of the receptor.