The ability to convert the 4 S glucocorticoid receptor to the 7-8 S form is dependent on both RNA and protein factors.
The DEAE-cellulose-purified 4 S form of the rat liver glucocorticoid receptor can associate with cytosolic factors, as evidenced by an alteration of the sedimentation value of the 7-8 S form. On the basis of sedimentation profile, this form is indistinguishable from the activated, low-salt 7-8 S form isolated from rat liver cytosol. In addition, both the endogenous and reconstituted 7-8 S receptor can bind DNA as the 7-8 S form. In keeping with our reports that the endogenous form of the 7-8 S receptor is sensitive to RNAase digestion, treatment of the cytosol with RNAase prior to mixing with the 4 S receptor prevents the formation of the 7-8 S material. Moreover, warming the cytosol to 50 degrees C prior to mixing with the 4 S receptor also eliminates the ability to form the heavier material. Since RNA is heat-stable, this suggests that other factors may be involved. Treatment of the cytosol with N-ethylmaleimide, a reagent reported to be specific for sulfhydryl groups, also eliminates 7-8 S generating ability. These observations suggest that a protein may be a component of the 7-8 S generating material. This is substantiated by the observation that trypsin or chymotrypsin treatment of the cytosol mitigates the ability of the cytosol to form the 7-8 S material and results in the appearance of a form of the receptor that sediments at approximately 6 S. Protease treatment of partially purified material eliminates the 7-8 S generating activity entirely. We conclude that the 7-8 S form of the receptor can be reconstituted from the 4 S receptor via association with at least two other cytosolic factors, a protein and an RNA.
Tymoczko, JL; Anderson, EE; Lee, KA; Unger, AL
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