The steroid-binding subunit of the Na/K-ATPase as a progesterone receptor on the amphibian oocyte plasma membrane

Progesterone acts at a plasma membrane receptor on the Rana oocyte to initiate meiosis. A cascade of lipid messengers occurs within seconds, followed by sequential changes in membrane phospholipid composition. We now show that progesterone binding to the plasma membrane increases continuously over the first 4 h. Subsequently, about 60% of the total plasma membrane and >90% of membrane-bound progesterone, ouabain binding sites, and Na/K-ATPase activity are internalized. Until the completion of membrane internalization, oocytes must be continuously exposed to nanomolar concentrations of exogenous progesterone for meiosis to continue. The membrane-bound progesterone remains unchanged, whereas microinjected [3H]progesterone is rapidly metabolized. We find that progesterone and the plant steroid ouabain compete for one of two ouabain binding sites on the oocyte surface. Ouabain blocks progesterone action and inhibits subsequent meiosis if added at any time during the first 4-5 h. Western blots of SDS/PAGE extracts of isolated oocyte plasma membranes contain a −110 kDa band which binds an antibody to the steroid-binding c-terminal domain in rat and human PR. The number of binding sites and Kd for progesterone binding to the plasma membrane is comparable to those for low-affinity ouabain binding to the α-subunit of the Na/K-ATPase (112 kDa). Our results suggest that progesterone binding to the ouabain binding site on the N-terminal region of the α-subunit of Na/K-ATPase may modulate early plasma membrane events over the first 4-6 h. Progesterone may thus act in part through the plasma membrane Na/K-ATPase signaling system.