Studies of a plasma membrane steroid receptor in Xenopus oocytes using the synthetic progestin RU 486.
A steroid binding protein (Mr = 110,000) has previously been identified in the plasma membrane of Xenopus laevis oocytes by photoaffinity labeling with [3H]R5020. In order to further characterize this steroid receptor, the photoaffinity labeled receptor protein was solubilized with 0.1% Brij 35. The solubilized labeled receptor yielded an approximate mol. wt of 102,000 +/- 2,000 by sucrose density gradient centrifugation, suggesting that the solubilized receptor exists as a monomer. RU 486, a synthetic progestin antagonist for mammalian cytosolic receptor systems, inhibited up to 70% of [3H] R5020 photoaffinity binding to the 110,000-Dalton receptor with an IC50 of 5 microM and induced germinal vesicle breakdown (GVBD) with an EC50 of 9.0 +/- 0.6 microM. GVBD induced by RU 486 was slower than with progesterone, and RU 486 was less powerful than progesterone. Micromolar concentrations of RU 486 also potentiated GVBD induced by sub-optimal concentrations of progesterone or R5020. Furthermore, RU 486 inhibited oocyte plasma membrane adenylate cyclase with an apparent IC50 of 7.5 +/- 2.5 microM. The close correlation of the EC50 value for RU 486 induction of GVBD with the IC50 values for inhibition of [3H]R5020 photoaffinity labeling of the 110,000-Dalton receptor and inhibition of adenylate cyclase activity further supports the physiological significance of the oocyte plasma membrane steroid receptor.
D000345:Affinity Labels / Q000378:metabolism; D000818:Animals; D002462:Cell Membrane / Q000378:metabolism; D004963:Estrenes / Q000378:metabolism*; D005260:Female; D015735:Mifepristone; D008970:Molecular Weight; D009865:Oocytes / Q000378:metabolism*; D010777:Photochemistry; D011397:Promegestone / Q000378:metabolism; D011987:Receptors, Steroid / Q000378:metabolism*; D014982:Xenopus laevis
D000345:Affinity Labels; D004963:Estrenes; D011987:Receptors, Steroid; D015735:Mifepristone; D011397:Promegestone