Binding of the partially purified glucocorticoid receptor of rat liver to chromatin and DNA

Abstract

The binding of the glucocorticoid receptor of rat liver to chromatin and DNA has been studied with crude and partially purified preparations of cytosol receptor labelled with [ 3H]-triamcinolone acetonide in vitro. The use of crude preparations of receptor and increasing protein concentrations leads to an apparent saturation of chromatin and DNA, suggesting a limited number of high affinity nuclear acceptor sites for the receptor. Appropriate controls indicate that the observed saturability of chromatin acceptor sites is due to the presence in crude receptor preparations of heat-stable protein factors which interfere with the binding of the receptor to the genome; whereas the apparent saturation of DNA is due to contamination with deoxyribonucleases. If the activated complex of receptor and triamcinolone acetonide (R-TA) is partially purified to a step where it is free from nucleases and inhibitors, its binding to both chromatin and DNA is linearly dependent on the concentration of free (R-TA) in the incubation medium. There is no absolute specificity with respect to the source of DNA or chromatin, although liver chromatin has considerably higher receptor binding capacity than chromatin from avian erythrocytes. The rate kinetics of association and dissociation for the binding of (R-TA) to DNA and chromatin are very similar, but DNA exhibits a 10-fold higher receptor binding capacity than chromatin. These data, in conjunction with the effect of poly-(D)-lysine and NaCl on the binding of (R-TA) to chromatin and DNA, suggest that most of the receptor molecules bound to chromatin in vitro interact with the ‘accessible’ DNA stretches. Although a small population of receptor molecules may bind specifically to target tissue genome, the detection of these specific sites against the background of unspecific binding is not possible with unfractionated chromatin or DNA preparations.