Reversible activation of non-steroid binding oestrogen receptor

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Abstract
Two high-affinity oestrogen receptors have been identified in the chick oviduct with equilibrium dissociation constants (Kd) of 0.1 and 1 nM, differing in their binding kinetics, role in ovalbumin synthesis and independent regulation in vivo. The higher-affinity receptor (X) increases RNA polymerase II activity directly1, whereas the low-affinity receptor (Y) seems to be necessary to confer specificity to transcription of oestrogen-dependent genes2. Acute administration of progesterone to oestrogen-stimulated chicks results in preferential destruction of the nuclear Y receptor3 accompanied by interruption of ovalbumin gene transcription4. Here we demonstrate that receptor Y exists in a non-oestradiol binding form (Ynb) which can be activated to the binding form in vitro by treatment with either ATP or ADP. Furthermore, dialysis of oviduct cytosol, which has no effect on the high-affinity receptor X, converts receptor Y to Ynb; receptor Y can then be recovered by treatment with ATP in the presence of Mg2+ and independently of Ca2+. This is the first report of the controlled interconversion between a non-steroid binding form of oestrogen receptor and active receptor in a tissue that contains two independently regulated oestrogen receptor types.