Immunologically distinct binding molecules for progesterone and RU38486 in the chick oviduct cytosol

3H]Progesterone and 3H]RU38486 binding in the chick oviduct cytosol is associated with macromolecules which sediment as 8 S and 4 S moieties, respectively, in molybdate-containing 5-20% sucrose gradients. The 3H]progesterone binding could be displaced by excess progesterone, but not by RU38486. Conversely, the 3H]RU38486 binding was able to compete with RU38486 but not by excess progesterone. A preparation containing antibodies against chick oviduct progesterone receptor recognized only the 3H]progesterone-receptor complex but not the 4 S, 3H]RU38486 binding component of the chick cytosol. In the calf uterus cytosol, 3H]R5020 (a synthetic progestin) and 3H]RU38486 were associated with 8 S molecules and the peaks of radioactivity were displaceable upon preincubation with radionert steroids. In addition, the complexes were recognized by antibodies to chick oviduct progesterone receptor. Our data suggest that in the chick oviduct cytosol, RU38486 does not bind to progesterone receptor, but interacts with an immunologically distinct macromolecule.