Characterization of the avian progesterone receptor through the use of inhibitors

Several chemical agents have been identified which block interaction of the avian progesterone receptor with isolated nuclei, ATP-Sepharose, DNA-cellulose or phosphocellulose. Four of these inhibitors, rifamycin AF/103, o-phenanthroline, aurintricarboxylic acid and pyridoxal 5-phosphate appear to block directly binding of the activated receptor complex to the above "acceptors." Another inhibitor, sodium molybdate, only blocks receptor interactions when added before receptor activation and therefore appears to interfere with the activation process. When nuclear receptor complexes were formed in vivo and labeled by nuclear exchange with [3H]progesterone in vitro, these complexes could not be disrupted by incubation of the nuclei with inhibitors. Therefore, the receptor complex bound in nuclei appears to be modified or masked in a way which resists the action of these chemical agents. These results indicate the value of inhibitors as chemical probes for the analysis of steroid receptors.