The structures of membrane receptors mediating rapid, nongenomic actions of steroids have not been identified. We describe the cloning of a cDNA from spotted seatrout ovaries encoding a protein that satisfies the following seven criteria for its designation as a steroid membrane receptor: plausible structure, tissue specificity, cellular distribution, steroid binding, signal transduction, hormonal regulation, and biological relevance. For plausible structure, computer modeling predicts that the protein has seven transmembrane domains, typical of G protein-coupled receptors. The mRNA (4.0 kb) is only detected in the brain and reproductive tissues on Northern blots. Antisera only detect the protein (40 kDa) in plasma membranes of reproductive tissues. The recombinant protein produced in an Escherichia coli expression system has a high affinity (K d ‫؍‬ 30 nM), saturable, displaceable, single binding site specific for progestins. Progestins alter signal transduction pathways, activating mitogenactivated protein kinase and inhibiting adenylyl cyclase, in a transfected mammalian cell line. Inhibition of adenylyl cyclase is pertussis toxin sensitive, suggesting the receptor may be coupled to an inhibitory G protein. Progestins and gonadotropin up-regulate both mRNA and protein levels in seatrout ovaries. Changes in receptor abundance in response to hormones and at various stages of oocyte development, its probable coupling to an inhibitory G protein and inhibition of progestin induction of oocyte maturation upon microinjection of antisense oligonucleotides are consistent with the identity of the receptor as an intermediary in oocyte maturation. These characteristics suggest the fish protein is a membrane progestin receptor mediating a ''nonclassical'' action of progestins to induce oocyte maturation in fish. Many physiological effects of steroid hormones are too rapid to be mediated by the classical genomic mechanism of steroid action involving activation of nuclear steroid receptors (1-3). Rapid, nongenomic steroid actions initiated on the cell surface have been described in a wide variety of animal, tissue, and cell models (4-8). Steroid membrane receptors characterized in plasma membrane fractions of many target tissues are the likely intermediaries of these nonclassical steroid actions (6, 9-11). For example, extensive studies in our laboratory have demonstrated that a progestin membrane receptor characterized in spotted seatrout (Cynoscion nebulosus) ovaries mediates the induction of oocyte meiotic maturation by the maturationinducing steroid, 17,20␤,21-trihydroxy-4-pregnen-3-one (20␤-S), in this species (12-15). However, detailed knowledge of the molecular structures and mechanisms of action of steroid membrane receptors has eluded investigators, despite intensive research efforts in many laboratories over several decades to purify and sequence the receptor proteins (16)(17)(18)(19)(20). The minute quantities of receptor proteins present in target tissues and major losses of binding activity during solubilizatio...

Recently we discovered a previously uncharacterized gene with the characteristics of a membrane progestin receptor (mPR) in a fish model, spotted seatrout. Here, we report the identification, cloning, and characteristics of other members of this hitherto unknown family of putative mPRs from several vertebrate species, including human, mouse, pig, Xenopus, zebrafish, and Fugu, with highly conserved nucleotide and deduced amino acid sequences and similar structures to the spotted seatrout mPR. The 13 vertebrate genes identified seem to belong to an unknown gene family. Phylogenetic analysis indicates these cDNAs comprise three distinct groups (named ␣, ␤, and ␥) within this gene family. Structural analyses of the translated cDNAs suggest they encode membrane proteins with seven transmembrane domains. The transcript sizes of the human ␣, ␤, and ␥ putative mPR mRNAs varied from 2.8 to 5.8 kb and showed distinct distributions in reproductive, neural, kidney and intestinal tissues, respectively. Recombinant human ␣, ␥, and mouse ␤ proteins produced in an Escherichia coli expression system demonstrated high affinity (Kd ‫؍‬ 20 -30 nM) saturable binding for progesterone. Further analysis of binding to the ␥-subtype revealed binding was specific for progestins and was displaceable, with rapid rates of association and dissociation (t1/2 ‫؍‬ 2-8 min). These results suggest this is a new family of steroid receptors unrelated to nuclear steroid receptors, but instead having characteristics of G protein-coupled receptors.A lthough the existence of specific receptors on the surface of target cells mediating rapid nongenomic actions of steroids was recognized 20 years ago (1, 2), efforts to determine the structures of steroid membrane receptors have been unsuccessful until now (3-5). In the accompanying paper in this issue of PNAS (6), we described a gene discovered in a teleost species, spotted seatrout, whose protein fulfils the criteria for its designation as a steroid membrane receptor, including structural plausibility, specific tissue and plasma membrane localization, steroid binding characteristic of steroid and progestin receptors, coupling to second messenger pathways, regulation by steroid hormones, and biological relevance. Evidence was obtained that this progestin membrane receptor (mPR) is the intermediary in progestin induction of oocyte meiotic maturation in teleost fishes and activates an inhibitory G protein (G i/o ), which suggests it may be a G protein-coupled receptor (GPCR).The aims of this study were to search for related cDNA fragments of other vertebrates in the genomic databases, clone and sequence their full-length cDNAs, and partially characterize these genes and their recombinant proteins. This paper reports the identification of 13 additional vertebrate genes closely related to the spotted seatrout mPR. Structural and phylogenetic evidence is presented that these vertebrate genes encode for membrane proteins with seven or occasionally six transmembrane domains that can be classified into three subty...

A novel progestin receptor (mPR) with seven-transmembrane domains was recently discovered in spotted seatrout and homologous genes were identified in other vertebrates. We show that cDNAs for the mPR ␣ subtypes from spotted seatrout A LTHOUGH THE IMPORTANCE of rapid (i.e. nonclassical) steroid actions initiated at the cell surface through binding to steroid membrane receptors has become more widely accepted within the past few years, details of the initial steroid-mediated events, including the identities of the steroid membrane receptors and their mechanisms of action, remain unclear and are surrounded by controversy (1-3). There is clear evidence that a variety of receptor proteins are involved in initiating these nonclassical steroid actions in different cell models, including nuclear steroid receptors or nuclear steroid receptor-like forms (1, 2, 4), receptors for other ligands that also bind steroids (2, 5), and unidentified receptors with different characteristics from those of any known receptors (2, 6). Recently, a novel cDNA was discovered in spotted seatrout ovaries that has several characteristics of the progestin membrane receptor (mPR) mediating progestin induction of oocyte maturation in this species by a nongenomic mechanism (7). The seatrout cDNA (st-mPR␣) encodes a 40 kDa protein, which has seven transmembrane domains, and receptor activation alters pertussis toxin-sensitive adenylyl cyclase activity, both of which suggest stmPR␣ is a G protein-coupled receptor (GPCR) or GPCR-like protein (7). More than 20 closely related genes have been cloned from other vertebrate species, including three mPR subtypes in humans, named ␣, ␤, and ␥, which show high levels of expression in human reproductive, brain, and kidney tissues, respectively (8). The identification of a new class of putative steroid receptors, unrelated to nuclear steroid receptors, but instead related to GPCRs, provides a plausible explanation of how steroids can initiate rapid hormonal responses in target cells by activating receptors on the cell surface. There has been broad recognition of the potential significance of these findings (1, 9, 10) and also an extensive research effort to determine the distribution, hormonal regulation, and biological roles of the mPRs in various vertebrate models (11-16). However, critical information is still lacking on several key features of mPRs essential for clearly establishing this proposed alternative model of steroid action and for understanding its likely evolutionary origins.The st-mPR␣ protein has been localized to the plasma membrane of seatrout oocytes (7), but progestin binding and activation of signal transduction pathways in the plasma membranes of cells transfected with the st-mPR␣ and human mPRs remain to be demonstrated. To date, progestin binding has only First Published Online November 9, 2006Abbreviations: GPCR, G protein-coupled receptor; HLY3, hemolysin 3; hu-mPR␣, human membrane progestin receptor ␣; MMD, monocyte to macrophage differentiation protein; mPR, membrane progestin rece...

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