Transformation of highly purified avian progesterone receptor

Transformation of the avian progesterone receptor to the nuclear form was studied using highly purified receptor preparations. The progesterone receptor was purified to near homogeneity in the presence of 10 mM sodium molybdate by affinity chromatography on deoxycorticosterone-Sepharose followed by DEAE-Sephadex chromatography. This latter step resolved the receptor into two 8S forms, I and II. Receptor transformation was measured by the binding of receptor to the polyanion resins (DNA-cellulose, phosphocellulose, or ATP-Sepharose) and to isolated nuclei and by the change in sedimentation coefficient from 8S to 4S. Molybdate was removed from the purified receptor preparations by agarose gel filtration. This step resulted in transformation of a major portion of receptor, as indicated by the above criteria. The extent of transformation was enhanced slightly by further incubation of the receptor in 0.2 M KCl. Control samples, which contained 10 mM molybdate, remained nontransformed, as tested by sedimentation or binding analysis. However, receptor transformation could not be reversed by adding molybdate back to transformed receptor. Although transformation of both receptor components I and II was observed, the extent of component I transformation was generally 2- to 5-fold greater than that of component II. About 50-90% of component I could be converted to a form that bound DNA-cellulose, ATP-Sepharose, and phosphocellulose. Since the progesterone receptor is a phosphoprotein, 32P-labeled receptor was tested for any loss of phosphate during transformation or receptor inactivation by incubation at 37 C. No observable loss of 32P occurred with either treatment. Our results show that transformation of the 8S receptor components I and II can be achieved in the absence of other cytosolic factors.