NATIVE RAT-KIDNEY MINERALOCORTICOID RECEPTOR IS A PHOSPHOPROTEIN WHOSE TRANSFORMATION TO A DNA-BINDING FORM IS INDUCED BY PHOSPHATASES

Addition of alkaline phosphatase to rat kidney cytosol diminishes the ability of the mineralocorticoid receptor (MR) to bind aldosterone in a time-, temperature- and concentration-dependent form. A variety of p hosphatase inhibitors, including levamisole, are effective in preventi ng this inactivation. On the other hand, when the steroid-receptor com plex is incubated in the presence of alkaline phosphatase, an incremen t in the rate of receptor transformation is evidenced by a change in t he sedimentation coefficient from 8.8 S to 5.1 S, as well as increased DNA-binding capacity. The effects of alkaline phosphatase on activati on and transformation can also be observed when the MR is incubated at 20 degrees C in the cytosolic medium, indicating that the catalytic a ction of an endogenous phosphatase may be involved in the transformati on process. The ability of phosphatase inhibitors such as levamisole f or suppressing both alkaline phosphatase- and endogenous phosphatase-d irected transformation does not correspond well between them. Evidence is presented to affirm that the endogenous phosphatase activity is no t due to an alkaline phosphatase-type, but it may be due to a protein serine/threonine phosphatase, as evidenced by the inhibitory effects o f okadaic acid. The experimental results also show direct evidence tha t the MR undergoes phosphorylation in a physiological milieu.