Characterization of rat ovarian lutropin receptor

The LH/CG receptor activates canonical signaling pathway when expressed in Drosophila

G-protein coupled receptors (GPCRs) and their ligands provide precise tissue regulation and are therefore often restricted to specific animal phyla. For example, the gonadotropins and their receptors are crucial for vertebrate reproduction but absent from invertebrates. In mammals, LHR mainly couples to the PKA signaling pathway, and CREB is the major transcription factor of this pathway. Here we present the results of expressing elements of the human gonadotropin system in Drosophila. Specifically, we generated transgenic Drosophila expressing the human LH/CG receptor (denoted as LHR), a constitutively active form of LHR, and an hCG analog. We demonstrate activation-dependent signaling by LHR to direct Drosophila phenotypes including lethality and specific midline defects; these phenotypes were due to LHR activation of PKA/CREB pathway activity. That the LHR can act in an invertebrate demonstrates the conservation of factors required for GPCR function among phylogenetically distant organisms. This novel gonadotropin model may assist the identification of new modulators of mammalian fertility by exploiting the powerful genetic and pharmacological tools available in Drosophila.

Inefficient Maturation of the Rat Luteinizing Hormone Receptor

Increasing evidence suggests that the folding and maturation of monomeric proteins and assembly of multimeric protein complexes in the endoplasmic reticulum (ER) may be inefficient not only for mutants that carry changes in the primary structure but also for wild type proteins. In the present study, we demonstrate that the rat luteinizing hormone receptor, a G protein-coupled receptor, is one of these proteins that matures inefficiently and appears to be very prone to premature degradation. A substantial portion of the receptors in stably transfected human embryonic kidney 293 cells existed in immature form of M(r) 73,000, containing high mannose-type N-linked glycans. In metabolic pulse-chase studies, only approximately 20% of these receptor precursors were found to gain hormone binding ability and matured to a form of M(r) 90,000, containing bi- and multiantennary sialylated N-linked glycans. The rest had a propensity to form disulfide-bonded complexes with a M(r) 120,000 protein in the ER membrane and were eventually targeted for degradation in proteasomes. The number of membrane-bound receptor precursors increased when proteasomal degradation was inhibited, and no cytosolic receptor forms were detected, suggesting that retrotranslocation of the misfolded/incompletely folded receptors is tightly coupled to proteasomal function. Furthermore, a proteasomal blockade was found to increase the number of receptors that were capable of hormone binding. Thus, these results raise the interesting possibility that luteinizing hormone receptor expression at the cell surface may be controlled at the ER level by regulating the number of newly synthesized proteins that will mature and escape the ER quality control and premature degradation.

Constitutive and Agonist-dependent Self-association of the Cell Surface Human Lutropin Receptor

The human lutropin receptor (hLHR) is a G protein-coupled receptor (GPCR) that plays an essential role in reproductive physiology. The present studies were undertaken to determine whether the hLHR self-associates. We show that high molecular weight complexes of the hLHR can be co-immunoprecipitated from 293 cells transfected with differentially tagged hLHRs. These complexes are detected only in extracts from cells that have been co-transfected and not in extracts combined from cells expressing only one form of tagged hLHR, confirming the in vivo self-association of the receptor. In transiently transfected cells, in which a small percentage of cells overexpress hLHR and most of the hLHR is located intracellularly in the ER, the self-associated hLHR is composed predominantly of immature hLHR. When cells were transiently co-transfected with wild-type hLHR and a misfolded mutant of the hLHR, a physical association of the ER-localized misfolded mutant with the immature hLHR was observed, resulting in a decreased cell surface expression of the wild-type receptor. In contrast, in stably transfected cells, where the majority of cells express receptor and there is much less intracellular accumulation of hLHR, the self-associated forms of the hLHR are composed predominantly of cell surface receptor. The abundance of cell surface hLHR dimers and oligomers, as detected on SDS gels, is increased further upon human choriogonadotropin treatment of the stably transfected cells. In addition to documenting the self-association of cell surface hLHR, our results underscore the importance of the cellular distribution of recombinant GPCR as it relates to the nature of the GPCR dimerization and oligomerization.

Porphyromonas gingivalis fimbriae bind to cytokeratin of epithelial cells

The adherence of Porphyromonas gingivalis to host cells is likely a prerequisite step in the pathogenesis of P. gingivalis-induced periodontal disease. P. gingivalis binds to and invades epithelial cells, and fimbriae are shown to be involved in this process. Little is known regarding epithelial receptor(s) involved in binding of P. gingivalis fimbriae. Using an overlay assay with purified P. gingivalis fimbriae as a probe, two major epithelial cell proteins with masses of 50 and 40 kDa were identified by immunoblotting with fimbria-specific antibodies. Iodinated purified fimbriae also bound to the same two epithelial cell proteins. An affinity chromatography technique was utilized to isolate and purify the epithelial components to which P. gingivalis fimbriae bind. Purified fimbriae were coupled to CNBr-activated Sepharose-4B, and the solubilized epithelial cell extract proteins bound to the immobilized fimbriae were isolated from the column. A major 50-kDa component and a minor 40-kDa component were purified and could be digested with trypsin, suggesting that they were proteins. These affinity-eluted 50- and 40-kDa proteins were then subjected to amino-terminal sequencing, and no sequence could be determined, suggesting that these proteins have blocked amino-terminal residues. CNBr digestion of the 50-kDa component resulted in an internal sequence homologous to that of Keratin I molecules. Further evidence that P. gingivalis fimbriae bind to cytokeratin molecule(s) comes from studies showing that multicytokeratin rabbit polyclonal antibodies cross-react with the affinity-purified 50-kDa epithelial cell surface component. Also, binding of purified P. gingivalis fimbriae to epithelial components can be inhibited in an overlay assay by multicytokeratin rabbit polyclonal antibodies. Furthermore, we showed that biotinylated purified fimbriae bind to purified human epidermal keratin in an overlay assay. These studies suggest that the surface-accessible epithelial cytokeratins may act as receptor(s) for P. gingivalis fimbriae. We hypothesize that adherence of P. gingivalis fimbriae to cytokeratin may be important for colonization of oral mucous membranes and possibly also for activation of epithelial cells.

Molecular architecture and biorecognition processes of the cystine knot protein superfamily: part I. The glycoprotein hormones

In this review article, the reader is introduced to recent advances in our knowledge on a subset of the cystine knot superfamily of homo- and hetero-dimeric proteins, from the perspective of the endocrine glycoprotein hormone family of proteins: follitropin (FSH), Iutropin (LH), thyrotropin. (TSH) and chorionic gonadotropin (CG). Subsequent papers will address the structure-function behaviour of other members of this increasingly significant family of proteins, including various members of the transforming growth factor-beta (TGF-beta) family of proteins, the activins, inhibins, bone morphogenic growth factor, platelet derived growth factor-beta, nerve growth factor and more than 35 other proteins with similar topological features. In the present review article, specific emphasis has been placed on advances with the glycoprotein hormones (GPHs) that have facilitated greater insight into their physiological functions, molecular structures and most importantly the basis of the molecular recognition events that lead to the formation of hetero-dimeric structures as well as their specific and selective recognition by their corresponding receptors and antibodies. Thus, this review article focuses on the structural motifs involved in receptor recognition and the current techniques available to identify these regions, including the role of immunological methodology, peptide fragment design and synthesis and mutagenesis to delineate their structure-function relationships and molecular recognition behaviour.

Mutants of human choriogonadotropin lacking N-glycosyl chains in the alpha-subunit. 1. Mechanism for the differential action of the N-linked carbohydrates

Analogs of human choriogonadotropin (hCG) lacking N-glycosyl chains at alpha52Asn and alpha78Asn were purified from the culture media of insect cells by immunoaffinity chromatography using a monoclonal antibody column. As previously reported, while analogs lacking carbohydrate at alpha52Asn and alpha78Asn had similar receptor binding activities compared with the wild type recombinant hCG (hCGwt), they differed in their signal transduction properties. The mutant lacking carbohydrate at alpha78Asn had 20% less cAMP-stimulating activity than hCGwt, but the absence of glycosylation at alpha52Asn resulted in the reduction of cAMP accumulation by 90-95%. A similar effect of the mutations was observed on the stimulation of steroidogenesis. Circular dichroism spectra of the two mutants showed significant differences. The mutant lacking carbohydrate at alpha52Asn had a much higher negative mean residue ellipticity (MRE) at 200 nm and a lower negative MRE at 220 nm than that lacking carbohydrate at alpha78Asn and hCGwt. The dissociation rates of the alpha52Asn and alpha78Asn carbohydrate deficient mutants at pH 3 and room temperature, measured by using 1-anilino-8-naphthalenesulfonate, were 9.4 x 10(-5) and 3.8 x 10(-5) s-1, respectively, as compared with 1.5 x 10(-5) s-1 for hCGwt. The results of both CD measurements and dissociation studies strongly suggest that the absence of carbohydrate at alpha52Asn results in conformational changes in the mutant which might explain the loss in its signal transduction function. This is further supported by indirect evidence from two other lines of experimentation. Unlike the mutant lacking carbohydrate at alpha78Asn, the one lacking carbohydrate at alpha52Asn cross-reacted with the two subunit specific monoclonal antibodies, anti-hCGalpha and anti-hCGbeta, which normally did not cross-react with the native or the hCGwt. Also, polyclonal anti-hCGbeta but not anti-hCGalpha was able to restore the cAMP-producing activity of the alpha52Asn carbohydrate deficient mutant. From all the data taken together, it appears that the loss of second messenger-producing activity of hCG with the absence of the glycosyl chain at alpha52Asn was probably due to a conformational change in the heterodimer rather than due to the loss of the alpha52Asn-carbohydrate-receptor interaction.

Structure and functional significance of the carbohydrates of the LH/CG receptor

The LH/CG appears to contain 1-4 bi- or multi-antennary complex-type N-linked oligosaccharide side chains, which appear to locate apart from the hormone-binding regions. The exact sites to which the N-linked chains are attached remain to be delineated. The carbohydrates of the mature membrane-inserted receptor do not contribute to either specific high-affinity ligand-binding or signal transduction of the receptor. Thus, the polypeptide core of the receptor is responsible for both high affinity binding and dictating the hormone specificity. Moreover, the deglycosylated receptor, once inserted to the plasma membrane in a functionally mature form, retains its functional conformation or permits the conformational change that is required for coupling of the receptor to effector enzymes. Addition of oligosaccharides to the nascent LH/CG receptor but not their subsequent conversion to complex-type ones appears to be required for acquiring the hormone-binding conformation. On the other hand, neither addition of oligosaccharides to the nascent receptor, nor their further maturation are needed for the transport of the receptor to the plasma membrane. Thus, one function of the N-linked oligosaccharides in the LH/CG receptor appears to be to direct the proper folding of the receptor.

Effect of modification of all loop regions in the alpha- and beta-subunits of human choriogonadotropin on its signal transduction activity

Human choriogonadotropin (hCG), according to its three dimensional structure as determined by X-ray diffraction, has three beta-hairpin loops each in the alpha and beta subunit designated as alpha 1, alpha 2 alpha 3 and beta 1 beta 2 and beta 3, respectively. Since similar beta-hairpin loops in NGF and TNF beta have been implicated in their direct interaction with the receptor, it prompted the present investigation to determine the role of such loops in receptor binding and post-receptor signaling events in hCG. Based on the three dimensional structure of hCG, radical mutations were introduced in the alpha loops by replacing hydrophobic alpha 18Phe and alpha 74Phe by hydrophilic Thr residues in the alpha 1 and alpha 3 loops, respectively, and positively charged alpha 45Lys by negatively charged Asp in the helical segment in the alpha 2 loop. The beta loops were mutated by replacement of the beta 1, beta 2 and beta 3 sequences with the corresponding hFSH sequences. These replacements included beta 22Gly, beta 24Pro and beta 25Val with Glu, Arg and Phe in beta 1, 45Leu Gln Gly Val Leu Pro Ala Leu Pro53 with Tyr Lys Asn Pro Ala Arg Pro Leu Ile in beta 2 and 73Pro Arg Gly with Ala His His in the beta 3 loop. Six mutants, hCG alpha 1 beta, hCG alpha 2 beta and hCG alpha 3 beta and hCG alpha beta 1, hCG alpha beta 2 and h CG alpha beta 3, were obtained by co-infection of the insect High-Five cells with baculovirus containing mutant alpha or beta cDNAs and that containing complimentary wild type beta or alpha cDNAs. The mutants were almost completely secreted in the culture medium and were over expressed at levels ranging between 4.5 to 29 micrograms/ml indicating that mutations had no effect on the secretion or subunit assembly of hCG. In order to remove any contaminating beta-subunit, the culture medium was passed through a column of an hCG beta-specific monoclonal antibody, B158. The receptor binding activity of the mutant hCG alpha 1 beta, in which alpha 18Phe was replaced with Thr, increased almost 200% relative to rehCG. Similarly, increase in the cAMP and progesterone stimulation by the mutant ranged between 150 to 200%. This increase is believed to be due to a short range conformational change in the mutant as a result of the mutation rather than direct involvement of alpha 18Phe in the receptor binding. The evidence in support of this was derived from the fact that the affinity or interaction between the two subunits was impaired as indicated by the first order rate constant of hCG alpha 1 beta (km = 4.1 x 10(-2) min-1) at pH 3.0 at 23 degrees C which is one order of magnitude greater relative to rehCG (kw = 4.6 x 10(-3) min-1). All other mutations had no effect on the receptor binding or signal transduction of hCG indicating that, unlike NGF or TNF beta, beta-hairpin loops in hCG were not directly involved in receptor binding or post-receptor signaling events. However, since the mutation in the alpha 1 loop affects the receptor binding site, its presence in the vicinity of the alpha 1 loop is highly likely.

Preparation of recombinant carbohydrate deficient active analogs of human chorionic gonadotropin from insect cells

Human chorionic gonadotropin (hCG) has four N-glycosyl chains, two in each subunit. Several analogs lacking one or more specific N-linked carbohydrate chains have been purified from insect cells by immunoaffinity chromatography on a monoclonal antibody, B17, column Traces of the hCG beta mutant present, if any, were removed by a second immunoaffinity chromatography on a column of hCG beta specific monoclonal antibody, B158. N-glycosylation was inhibited by the replacement of either Asn or Thr to Gln in the consensus sequence. -Asn x Ser/Thr-, for N-glycosylation. All analogs were overexpressed in High-Five insect cells with the expression levels ranging between 1.5 to 15 micrograms/ml and were found homogeneous by SDS-PAGE under nonreducing and reducing conditions. Their molecular sizes ranged between 34k to 44k. The receptor binding affinity of all the analogs was unaltered as determined by radio receptor assay using rat ovarian membranes. The availability of these analogs should facilitate studies on the effect of a specific carbohydrate chain on the conformation and in vivo properties of hCG.

Human choriogonadotropin-induced coupling of receptor and Gs protein and the effect of hormone deglycosylation

The detergent-soluble extract of rat ovary plasma membranes contained a Gs protein of about 100 kDa as shown by its elution behavior on a Bio Gel A-1.5m column. However, the cell membranes exposed to hCG (37 degrees C, 15 min) contained in addition a higher molecular weight Gs protein complex of 300 kDa comprised of human chorionic gonadotropin (hCG) receptor (hCGR) and Gs. The complex bound with an affinity column of GTP-Sepharose and could be released with Gpp(NH)p and GTP inhibited this binding. The presence of the hCGR in the complex was shown by its binding to 125I-hCG. Furthermore, GTP inhibited the binding of hCG to the complex. These results indicate the presence of hCGR and Gs protein complex in the hCG-treated membranes. hCGR and Gs protein were individually purified and reconstituted into phospholipid vesicles. The protein-phospholipid vesicles showed saturation kinetics of binding of 125I-hCG and 3H-Gpp(NH)p. Incubation of phospholipid vesicles with hCG resulted in a 2-3-fold increase in the binding of 3H-Gpp(NH)p and GTPase activity. Activation of Gs protein was dependent on the length of incubation and the hormone concentration. Deglycosylated hCG was about 10 times less potent than hCG suggesting a role of carbohydrates of hCG in inducing hCG-Gs protein interactions. The data with the in vitro reconstitution system rule out the involvement of a carbohydrate-binding lectin in the function of the hormone.

Structure--function relationships of the glycoprotein hormones and their receptors

The primary structures of the glycoprotein hormones follitropin (FSH), lutropin (LH), human choriogonadotropin (hCG) and thyrotropin (TSH) have been determined, hCG has been crystallized and initial diffraction data obtained. Studies with synthetic peptides have provided information on regions involved in receptor interaction and signal transduction. The receptors for the glycoprotein hormones have been prepared by gene cloning methods and their primary structures deduced. As Leo Reichert and colleagues discuss here, although cAMP is involved in glycoprotein hormone signal transduction, recent evidence also implicates other second messengers, especially Ca2+ and may include both the phosphatidylinositol pathway and activation of Ca2+ channels.

Endocytosis of the lutropin receptor is mediated by a low affinity binding site

Porcine Leydig cells in primary culture were incubated to equilibrium with increasing doses of either porcine luteinizing hormone (pLH) or human chorionic gonadotropin (hCG). Then, free and total high affinity gonadotropin receptors on the cell surface were dosed with [125I]hCG as tracer. In isotonic high salt medium, the pLH concentration for half receptor occupancy (Kocc) and for half receptor endocytosis (Kendo) were nearly indistinguishable (1-3 x 10(-7) M). But, when the medium was changed to an isotonic low salt buffer, Kocc shifted to 1.2 x 10(-9) M and Kendo to 1.5 x 10(-8) M. However, with hCG, both values were largely independent of the ionic strength (Kocc = 10(-10) M and Kendo = 10(-8) M). The fact that Kendo is higher than Kocc suggests that the endocytosis of the high affinity gonadotropin receptor is controlled by the hormone binding to another lower affinity site.

Sulfhydryl reagents alter epidermal growth factor receptor affinity and association with the cytoskeleton

Sulfhydryl (SH) reagents are known to influence the characteristics of many ligand-receptor systems. The SH reagent N-ethylmaleimide has been demonstrated to interact with EGF receptors, and to inhibit EGF receptor kinase activity. The data presented in this paper concern the effect of SH reagents on two intriguing features of the EGF receptor system, namely the presence of low and high affinity EGF binding sites, and the interaction of EGF receptors with the cytoskeleton. SH reagents were observed to induce a disappearance of high, but not low, affinity EGF receptors from the cell surface, and an increase in receptor-cytoskeleton interaction. Comparison of the effects of membrane-permeant and membrane-impermeant SH reagents on wild type and structurally modified EGF receptors suggested that sulfhydryl groups on the cytoplasmic, rather than the extracellular, receptor domain are involved. This indicates that the cytoplasmic domain of the EGF receptor plays a role in the high affinity binding of EGF, and in the interaction of EGF receptors with the cytoskeleton. Experiments with an anti-EGF receptor antibody that specifically blocks the binding of EGF to low affinity receptors indicated that EGF induces a shift in the EGF receptor from low to high affinity. SH reagents probably affect EGF binding by inhibiting this EGF-induced receptor conversion.

Towards understanding the glycoprotein hormone receptors

Lutropin (LH), follitropin (FSH) and thyrotropin (TSH), as well as choriogonadotropin (CG, which binds to the LH receptor) constitute the glycoprotein hormone family. Their 3 receptors have been cloned during the last few months. They belong to the large group of G-protein coupled membrane proteins, with their specific N-terminal domain likely to bind the hormone and the characteristic 7 membrane-spanning segments in their C-terminal moiety. The present review discusses the main results of amino acid sequence analysis performed on the glycoprotein hormone receptors. The putative extracellular head exhibits less than 45% homology over the 3 receptors, while approximately 70% residue conservation is found in the transmembrane moiety. Here only, limited sequence homologies (approximately 20%) can be found with other G-protein coupled receptors. The secondary structure predictions performed on the 3 receptors revealed that the polypeptide sequence predicted as ordered (either alpha-helix or beta-strand) were repeated evenly throughout the extracellular head with a period of approximately 25 amino acids. This analysis helped to define the intervening loops between this ordered stretches as potential candidates for bearing at least part of the binding site of the hormones. Some of the perspectives opened by the cloning of the receptors are described, like the production of the extracellular head of the porcine LH receptor in baculovirus-infected insect cells, and the exploration of the LH receptor's mechanism of functioning as a dimer.

The action of luteinizing hormone on the testis

Luteinizing hormone (LH) and human chorionic gonadotrophin (hCG) receptors are coupled to intracellular effector systems, most notably adenylate cyclase, through guanyl nucleotide-binding proteins or G-proteins. The molecular mechanism involved in the dynamic coupling of the LH/hCG receptor however, are not known. It has been postulated that receptor aggregation at the molecular level plays a critical role in this process. There have been attempts to understand the receptor association and dissociation phenomena at the molecular level. One of them involves the participation of the major histocompatibility complex (MHC) class I antigen in the mechanism of receptor activation and/or expression. One molecular basis for these mechanisms consists of a physical interaction between MHC proteins and receptors to form "compound receptors" able to transfer a hormonal signal to the cell. Using a photo-reactive probe we demonstrated that the LH/hCG receptors and the class I antigens are closely associated in the membrane. Thus, it is possible to form covalent complexes of hCG and class I antigens through the binding of the hormone to specific receptors. These findings imply that LH/hCG receptors and the MHC class I antigens may interact at the level of the plasma membrane in the mechanism of LH action. We also performed experiments using a single cell and limiting stimulation to a patch of membrane. The results stimulating the cell in a localized area suggested that even if all components are entirely free to float there is a constraint in the localization of the receptor, G-protein, and/or the effector, supporting the constraint dissociation model. Within a limited area subunits could dissociate, but they would not be free to diffuse throughout the membrane. Moreover the concept of compartmentalization that has been utilized to explain some inconsistencies in second-messenger action now can be proved by experimental design.

Purification and characterization of the human ovarian LH/hCG receptor and comparison of the properties of mammalian LH/hCG receptors

Methods previously published by us [Wimalasena et al., J Biol Chem 260: 10689-10697, 1985; Wimalasena et al., J Biol Chem 261: 9416-9420, 1986] were utilized to solubilize the human corpus luteal leuteinizing hormone/human chorionic gonadotropin (LH/hCG) receptor with 3-[(3-cholamide-propyl)dimethylammonio]-1-propanesulfonate (CHAPS) and to purify the receptor by two steps of hCG-Sepharose affinity chromatography. The specific binding capacity (SBC) of the purified human receptor was 7510 pmol/mg protein, and KA = 2.2 x 10(9) M-1 when iodo hCG was competed by hCG; the yield was 4-7% of starting activity. When hLH was used in competition with hCG, specific binding capacity was 7900 pmol/mg protein and KA 1.0 x 10(9) M-1. Silver staining and autoradiography demonstrated a single protein of Mr 78,000 under reducing and Mr 58-62 x 10(3) under nonreducing conditions. Rat ovarian LH/hCG receptor was purified by similar methods and the KA of 3.5 x 10(10) M-1 for hCG was substantially different from the KA for hLH which was 2.1 x 10(9) M-1. Mr of the rat protein was 78-82 x 10(3) (reduced) and 58-62 x 10(3) (nonreduced) when analyzed by silver staining and autoradiography. For the first time, human LH/hCG receptor has been purified to apparent homogeneity, and its Mr of 78,000 was essentially identical to the Mr values of purified rat and porcine receptors.

Evidence for a novel thioredoxin-like catalytic property of gonadotropic hormones

It has been proposed that dithiol-disulfide interchange and oxidation-reduction reactions may play a role in hormone-induced receptor activation. Inspection of the sequences of the gonadotropic hormones revealed a homologous tetrapeptide (Cys-Gly-Pro-Cys) between the beta subunit of lutropin (LH) and the active site of thioredoxin (TD). The beta subunit of follitropin (FSH) has a similar sequence (Cys-Gly-Lys-Cys). Thioredoxin is a ubiquitous protein serving as an electron donor for ribonucleotide reductase, but it also exhibits disulfide isomerase activity. The catalytic activity of TD was assayed by its ability to reactivate reduced and denatured ribonuclease. In this assay, the purified ovine FSH and bovine LH preparations tested were approximately 60 and approximately 300 times, respectively, as active as TD on a molar basis. This heretofore unsuspected catalytic property of FSH and LH may be important in understanding their mechanism of receptor activation and signal transduction.

Structure of the lutropin/choriogonadotropin receptor

In summary, the LH/CG receptor is a single polypeptide which contains a large hydrophilic domain that is situated extracellularly, attached to a region that spans the plasma membrane seven times, the carboxy-terminal region being intracellular. This topology was predicted by the amino acid sequence and has been confirmed by our immunofluorescence studies. The extracellular domain, which is related to a family of leucine-rich glycoproteins, is presumably involved in binding the large glycoprotein hormones hCG and LH. The carboxy-terminal half of the receptor, which is related to the family of rhodopsinlike receptors, is (by analogy with these receptors) presumably involved in the coupling of the receptor to the G protein. Our transfection studies confirm that this single polypeptide is capable of binding hormone and activating adenylyl cyclase. Therefore, not only is the structure of the LH/CG receptor unique compared to other cell surface receptors characterized to date, but also its structure suggests that the mechanism of the translation of hormone binding to G protein coupling in this receptor is different from other G protein-coupled receptors whose ligands are much smaller and intercalcate among the transmembrane helices. We predict that, due to the homology among the glycoprotein hormones, the structures of the FSH and TSH receptors share extensive amino acid and structural homology with the LH/CG receptor. Last, our newly acquired knowledge about the structure of the LH/CG receptor, and the development of a cDNA and antibodies for this receptor, should enable more detailed studies on the function and regulation of the LH/CG receptor, not previously possible.

Lutropin-choriogonadotropin receptor: an unusual member of the G protein-coupled receptor family

A complementary DNA (cDNA) for the rat luteal lutropin-choriogonadotropin receptor (LH-CG-R) was isolated with the use of a DNA probe generated in a polymerase chain reaction with oligonucleotide primers based on peptide sequences of purified receptor protein. As would be predicted from the cDNA sequence, the LH-CG-R consists of a 26-residue signal peptide, a 341-residue extracellular domain displaying an internal repeat structure characteristic of members of the leucine-rich glycoprotein (LRG) family, and a 333-residue region containing seven transmembrane segments. This membrane-spanning region displays sequence similarity with all members of the G protein-coupled receptor family. Hence, the LH-CG-R gene may have evolved by recombination of LRG and G protein-coupled receptor genes. Cells engineered to express LH-CG-R cDNA bind human choriogonadotropin with high affinity and show an increase in cyclic adenosine monophosphate when exposed to hormone. As revealed by RNA blot analysis and in situ hybridization, the 4.4-kilobase cognate messenger RNA is prominently localized in the rat ovary.

Cloning and sequencing of porcine LH-hCG receptor cDNA: variants lacking transmembrane domain

Complementary DNA clones, encoding the LH-hCG (luteinizing hormone-human choriogonadotropic hormone) receptor were isolated by screening a lambda gt11 library with monoclonal antibodies. The primary structure of the protein was deduced from the DNA sequence analysis; the protein contains 696 amino acids with a putative signal peptide of 27 amino acids. Hydropathy analysis suggests the existence of seven transmembrane domains that show homology with the corresponding regions of other G protein-coupled receptors. Three other types of clones corresponding to shorter proteins were observed, in which the putative transmembrane domain was absent. These probably arose through alternative splicing. RNA blot analysis showed similar patterns in testis and ovary with a major RNA of 4700 nucleotides and several minor species. The messenger RNA was expressed in COS-7 cells, yielding a protein that bound hCG with the same affinity as the testicular receptor.