Identification of a protein from rat liver cytosol that enhances activation of the glucocorticoid receptor

Abstract

We have identified a factor from rat liver cytosol that enhances the DNA-cellulose-binding ability of the glucocorticoid receptor and lowers the sedimentation value from 9-10 S to 4-5 S. Cytosol is prepared in the presence of molybdate, and unactivated receptor is isolated by chromatography on DEAE-cellulose in the presence of molybdate. This receptor sediments at 9-10 S and has little affinity for DNA. If the molybdate is removed and the receptor is incubated at 25 degrees C with the low-salt wash of the DEAE-cellulose column, DNA binding is enhanced by 50-600% relative to controls incubated with buffer only. In addition, the factor present in the low-salt wash converts the 9-10 S receptor into a mixture of 5 S and 4 S forms. The factor must be present during the incubation in order to exert its maximal effect. Factor added after the incubation has only marginal effects on the DNA-binding ability of the receptor, indicating that the factor does not increase the DNA-binding ability of activated receptor. Moreover, the factor is significantly less effective on receptor that has been activated before incubation with the factor. These results suggest that the factor acts as an activation enhancer. Preliminary characterization indicates that the activation enhancer is a trypsin-sensitive protein of approx. 70,000 Da, whose activation-enhancing properties are inhibited by ATP. RNAase A, which has effects similar to those described above on the 7-8 S receptor, does not mimic the effects of the activation enhancer on the 9-10 S receptor.