The classical progesterone receptor mediates Xenopus oocyte maturation through a nongenomic mechanism

Xenopus laevis oocytes are physiologically arrested at G2 of meiosis I. Resumption of meiosis, or oocyte maturation, is triggered by progesterone. Progesterone-inducedXenopus oocyte maturation is mediated via an extranuclear receptor and is independent of gene transcription. The identity of this extranuclear oocyte progesterone receptor (PR), however, has remained a longstanding problem. We have isolated the amphibian homologue of human PR from aXenopus oocyte cDNA library. The clonedXenopus progesterone receptor (xPR) functioned in heterologous cells as a progesterone-regulated transcription activator. However, endogenous xPR was excluded from the oocyte nucleus and instead appeared to be a cytosolic protein not associated with any membrane structures. Injection of xPR mRNA intoXenopus oocytes accelerated the progesterone-induced oocyte maturation and reduced the required concentrations of progesterone. In enucleated oocytes, xPR accelerated the progesterone-induced mitogen-activated protein kinase activation. These data suggest that xPR is the long sought afterXenopus oocyte receptor responsible for progesterone-induced oocyte maturation.