Adrenocortical Nuclear Progesterone-Binding Protein: Identification by Photoaffinity Labeling and Evidence for Deoxyribonucleic Acid Binding and Stimulation by Adrenocorticotropin

Nuclei of the guinea pig adrenal cortex contain a protein that specifically binds progesterone and that, biochemically, is clearly distinct from the classical progesterone receptor. The adrenocortical nuclear progesterone-binding protein has now been purified more than 2000-fold by steroid-affinity chromatography with a 75% yield. The purified protein preparation demonstrated three major bands on sodium dodecyl sulfate-polyacrylamide gel of 79K, 74K, and 50K. To determine which of the three might represent the progesterone-binding protein, steroid photoaffinity labeling was performed which resulted in the specific and exclusive labeling of a 50K band. Thus, the adrenocortical nuclear progesterone-binding protein appears to be distinct from the classical progesterone receptor not only biochemically, but also on the basis of molecular size. To test whether the adrenocortical nuclear progesterone-binding protein can be hormonally stimulated, guinea pigs were treated with ACTH. The chronic administration of ACTH caused a 4- to 6-fold increase in the specific progesterone binding capacity without a change in the binding affinity. There appeared to be no significant difference in nuclear progesterone binding between the zona fasciculata and zona reticularis. This finding suggests a mediating role for the progesterone-binding protein in ACTH action. In addition, the nuclear progesterone-binding protein bound to nonspecific DNA sequences, further suggesting a possible transcriptional regulatory role.