Steroid and G Protein Binding Characteristics of the Seatrout and Human Progestin Membrane Receptor α Subtypes and Their Evolutionary Origins

Abstract

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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