MOLECULAR BIOLOGY OF STEROID RECEPTORS IN XENOPUS OOCYTES

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This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. In full-grown ovarian follicles of amphibians gonadotropin induces the follicular cells to produce a maturation-inducing steroid, progesterone, that acts directly on the oocyte to initiate maturation. Maturation is the process by which oocytes arrested in prophase I of meiosis resume and complete their first meiotic division. Completion of oocyte maturation is required for normal fertilization. Early studies have shown that the action of progesterone in oocyte maturation involves a high-affinity receptor. However, little is known about the progesterone receptor. The objective of this study is to isolate and purify the receptor. Oocytes were collected from Xenopus laevis and processed to obtain plasma membrane. Progesterone receptor was solubilized from plasma membranes with digitonin and applied to native steady state polyacrylamide gel tubes to determine its binding activity. Following this procedure, a single peak of progesterone binding activity was observed in the tube gels. The migration distance of the peak varied in gels of different acrylamide concentrations. Proteins in the tube gels were then separated by placing the tube gels on sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE) slab gels. In silver-stained slabs, a single spot was identified that corresponded to, and exclusively co-migrated with, the peak of the progesterone binding activity in the tube gels. This protein was electro-eluted from the gels and shown to be homogenous by gradient SDS-PAGE. The molecular weight of the protein under reducing or nonreducing conditions was about 90 KDa. A specific immune serum directed against this 90 KDa protein significantly inhibited the progesterone binding activity of oocyte membrane preparations. In conclusion, the membrane progesterone receptor of Xenopus oocytes was solubilized and isolated and a monomeric protein at the molecular weight of approximately 90 KDa with electrophoretic and immunologic characteristics of the membrane-bound progesterone receptor was purified.