Progesterone-binding components of chick oviduct. VIII. Receptor activation and hormone-dependent binding to purified nuclei.

A cell-free system prepared from the estrogen-primed chick oviduct was developed and used to study the uptake of cytoplasmic progesterone-receptor complex by isolated nuclei. The receptor and purified nuclei were shown to be stable at 25 degrees, but not at 37 degrees. Thus, nuclear incubations were routinely performed at 25 degrees. Such incubations revealed greater nuclear uptake of the cytoplasmic hormone-receptor complex as compared to control incubations performed at 0 degrees. The uptake process showed a quantitative preference for oviduct nuclei. No net uptake occurred during 0 degrees incubations when the nuclei were preincubated in the absence of cytoplasmic components at 25 degrees. In contrast, the temperature requirement was partially removed by preincubation of the hormone-receptor complex at 25 degrees prior to incubation with nuclei at 0 degrees. Nuclear uptake was not accompanied by measurable alterations in the sedimentation properties of the progesterone receptor. The activation and nuclear uptake of receptor was clearly dependent upon prior binding of steroid hormone to the receptor indicating that the active nuclear form of the receptor could not be generated in the absence of the hormone. Receptor precipitation with ammonium sulfate also partially removed the temperature requirement for nuclear binding. In contrast to temperature activation, ammonium sulfate precipitation activated the receptor in the absence of hormone. It thus seemed likely that temperature and salt activation of receptor occurred via different mechanisms. Although we were able to destroy up to 60% of the nuclear DNA content by treatment with DNase prior to nuclear incubation, some 80 to 85% of the receptor-binding capacity was still present in the treated nuclei. Thus, chick progesterone receptors apparently bind to a relatively DNase-resistant portion of the oviduct genome. The properties of this system indicate its value for further investigation into the initial events of progesterone action in the chick oviduct.