6. Phosphorylation of Steroid Receptors GLUCOCORTICOID RECEPTOR PHOSPHORYLATION IN MOUSE L-CELLS

Summary-This paper summarizes our observations on the phosphorylation state of untransformed and transformed glucocorticoid receptors isolated from 32P-labeled L-cells. The 30&350-kDa 9S untrans-formed murine glucocorticoid receptor complex is composed of a lOO-kDa steroid-binding phosphopro-tein and one or possibly two units of the 90-kDa heat shock protein (hsp90), which is also a phosphoprotein. Transformation of this complex to the 4s DNA-binding state is accompanied by dissociation of hsp90. When receptors in cytosol are transformed by heating at 2S’C, there is no gross change in the degree of phosphorylation of the steroid-binding protein. Both receptors that are bound to DNA after transformation under cell-free conditions and receptors that are located in the nucleus of cells incubated at 37°C in the presence of glucocorticoid are labeled with 3ZP The results of experiments in which the 32P-labeled receptor was submitted to limited proteolysis suggest that the lh-kDa DNA-binding domain is phosphorylated and that the 28-kDa steroid-binding domain is not