Characterization of the calf uterine progesterone receptor and its stabilization by nucleic acids.

The molecular and steroid hormone-binding properties of the calf uterine progesterone receptor and its interaction with nucleic acids were investigated. A positive cooperative binding interaction of [3H]progesterone with the receptor was evident from a nonlinear Scatchard plot and a Hill coefficient of 1.22 +/- 0.02. The range of progesterone receptor concentrations was 0.73-1.04 pmol/mg protein, approximately twice that of the estrogen receptor. Competitive binding assays revealed a high specificity for progesterone: R5020 greater than or equal to progesterone greater than deoxycorticosterone greater than 5 alpha-pregnane-3,20-dione much greater than 17 alpha-hydroxyprogesterone greater than or equal to 20 alpha-dihydroprogesterone greater than or equal to testosterone greater than or equal to estradiol greater than cortisol. Thus, a progesterone-specific receptor of high affinity and concentration is obtainable from calf uterus in large quantities without estrogen pretreatment. Thermal inactivation of the unoccupied progesterone receptor is inhibited by 10 mM sodium molybdate, whereas thermal inactivation of the ammonium sulfate-purified progesterone receptor is not. Thermal inactivation of the ammonium sulfate-purified receptor is inhibited by nucleic acids and polynucleotides; polyguanylate (poly G) is the most effective. DNA and poly G also effectively restore the progesterone-binding ability of the ammonium sulfate-purified receptor which had been lost due to heat inactivation. After incubation of the unoccupied receptor from 5-30 min at 25 C, the addition of poly G restored the receptor's [3H]progesterone-binding ability to control levels. These data suggest that the progesterone receptor's steroid-binding site is more readily inactivated by heat than is the DNA-binding site, and that nucleic acid binding induces a conformational change, which consequently restores the receptor's progesterone-binding site to functional activity.