hCG-receptor binding and transmembrane signaling

Restricted lateral diffusion of luteinizing hormone receptors in membrane microdomains

Single particle tracking was used to evaluate lateral motions of individual FLAG-tagged human luteinizing hormone (LH) receptors expressed on CHO cells and native LH receptors on both KGN human granulosa-derived tumor cells and M17 human neuroblastoma cells before and after exposure to human chorionic gonadotropin (hCG). Compared with LH receptors on untreated cells, LH receptors on cells treated with 100 nm hCG exhibit restricted lateral diffusion and are confined in small, nanometer-scale, membrane compartments. Similar to LH receptors labeled with Au-hCG, LH receptors labeled with gold-deglycosylated hCG, an hCG antagonist, also exhibit restricted lateral diffusion and are confined in nanoscale membrane compartments on KGN cells treated with 100 nm hCG. LH receptor point mutants lacking potential palmitoylation sites remain in large compartments despite treatment with 100 nm hCG as do LH receptors on cells treated with cytochalasin D. Finally, both polarization homotransfer fluorescence resonance energy transfer imaging and photon counting histogram analysis indicate that treatment with hCG induces aggregation of YFP-coupled LH receptors stably expressed on CHO cells. Taken together, our results demonstrate that binding of hCG induces aggregation of LH receptors within nanoscale, cell surface membrane compartments, that hCG binding also affects the lateral motions of antagonist binding LH receptors, and that receptor surface densities must be considered in evaluating the extent of hormone-dependent receptor aggregation.

Translational fusion of two beta-subunits of human chorionic gonadotropin results in production of a novel antagonist of the hormone

The strategy of translationally fusing the alpha- and beta-subunits of human chorionic gonadotropin (hCG) into a single-chain molecule has been used to produce novel analogs of hCG. Previously we reported expression of a biologically active single-chain analog hCGalphabeta expressed using Pichia expression system. Using the same expression system, another analog, in which the alpha-subunit was replaced with the second beta-subunit, was expressed (hCGbetabeta) and purified. hCGbetabeta could bind to LH receptor with an affinity three times lower than that of hCG but failed to elicit any response. However, it could inhibit response to the hormone in vitro in a dose-dependent manner. Furthermore, it inhibited response to hCG in vivo indicating the antagonistic nature of the analog. However, it was unable to inhibit human FSH binding or response to human FSH, indicating the specificity of the effect. Characterization of hCGalphabeta and hCGbetabeta using immunological tools showed alterations in the conformation of some of the epitopes, whereas others were unaltered. Unlike hCG, hCGbetabeta interacts with two LH receptor molecules. These studies demonstrate that the presence of the second beta-subunit in the single-chain molecule generated a structure that can be recognized by the receptor. However, due to the absence of alpha-subunit, the molecule is unable to elicit response. The strategy of fusing two beta-subunits of glycoprotein hormones can be used to produce antagonists of these hormones.

Trans-activation, cis-activation and signal selection of gonadotropin receptors

It has been thought that when a hormone binds to a receptor, the liganded receptor activates itself and generates hormone signals, such as the cAMP signal and the inositol phosphate signal (cis-activation). We describe that a liganded LH receptor or FSH receptor molecule is capable of intermolecularly activating nonliganded receptors (trans-activation). Particularly, intriguing is the possibility that a pair of compound heterozygous mutants, one defective in binding and the other defective in signaling, may cooperate and rescue signaling. Furthermore, trans-activation of the binding deficient receptors examined in our studies generates either the cAMP signal or the IP signal, but not both. Trans-activation and selective signal generation have broad implications on signal generation mechanisms, and suggest new therapeutic approaches.

Time-dependent folding of immunological epitopes of the human chorionic gonadotropin beta-subunit

We have explored the possibility to use 14 different monoclonal antibodies in order to follow the formation of the respective epitopes during the biosynthesis of hCG subunits and their association in JEG-3 choriocarcinoma cells using pulse (30s to 5 min)-chase (0-180 min) experiments. We found central cystine knot epitope structures (epitope beta1) to be formed immediately and simultaneously with epitopes on the protruding hCG-beta loops 1 and 3. We found also differences in the time-dependent folding of beta2 and beta4 epitopes, which are highly overlapping structures on the loops 1+3. These differences were reinforced by decreasing the temperature during the pulse-chase experiments to 25 degrees C. Moreover, we describe for the first time an intracellular intact hCG beta-subunit form that showed the transient expression of the hCG-beta-core fragment epitope beta11 in the course of the maturation of this subunit which casts new light on the presence of hCG-beta-core fragment in Down's syndrome, tumors and pregnancy.

Single-chain, triple-domain gonadotropin analogs with disulfide bond mutations in the alpha-subunit elicit dual follitropin and lutropin activities in vivo

The human glycoprotein hormones chorionic gonadotropin (CG), TSH, LH, and FSH are heterodimers composed of a common alpha-subunit and a hormone-specific beta-subunit. The subunits assemble noncovalently early in the secretory pathway. LH and FSH are synthesized in the same cell (pituitary gonadotrophs), and several of the alpha-subunit sequences required for association with either beta-subunit are different. Nevertheless, no ternary complexes are observed for LH and FSH in vivo, i.e. both beta-subunits assembled with a single alpha-subunit. To address whether the alpha-subunit can interact with more than one beta-subunit simultaneously, we genetically linked the FSHbeta- and CGbeta-subunit genes to the common alpha-subunit, resulting in a single-chain protein that exhibited both activities in vitro. These studies also indicated that the bifunctional triple-domain variant (FSHbeta-CGbeta-alpha), is secreted as two distinct bioactive populations each corresponding to a single activity, and each bearing the heterodimer-like contacts. Although the data are consistent with the known secretion events of gonadotropins from the pituitary, we could not exclude the possibility whether transient intermediates are generated in vivo in which the alpha-subunit shuttles between the two beta-subunits during early stages of accumulation in the endoplasmic reticulum. Therefore, constructs were engineered that would direct the synthesis of single-chain proteins completely devoid of heterodimer-like interactions but elicit both LH and FSH actions. These triple-domain, single-chain chimeras contain the FSHbeta- and CGbeta-subunits and an alpha-subunit with cystine bond mutations (cys10-60 or cys32-84), which are known to prevent heterodimer formation. Here we show that, despite disrupting the intersubunit interactions between the alpha- and both CGbeta- and FSHbeta-subunits, these mutated analogs exhibit both activities in vivo comparable to nonmutated triple-domain single chain. Such responses occurred despite the absence of quaternary contacts due to the disrupted bonds in the alpha-subunit. Thus, gonadotropin heterodimer assembly is critical for intracellular events, e.g. hormone-specific posttranslational modifications, but when heterodimers are present in the circulation, the alpha/beta-contacts are not a prerequisite for receptor recognition.

Conservation of the heterodimeric glycoprotein hormone subunit family proteins and the LGR signaling system from nematodes to humans

Glycoprotein hormones, follicle-stimulating hormones (FSHs), luteinizing hormones (LHs), thyroid-stimulating hormones (TSHs), and chorionic gonadotropin (CG) are key endocrine hormones secreted from the pituitary gonadotrophs and thyrotrophs and the placenta in primates. These hormones, consisting of a common alpha subunit and a specific beta subunit, act through the FSH receptor (FSHR), the LH receptor (LHR), and the TSH receptor (TSHR) that are highly specific for their cognate hormones. These glycoprotein hormones are structurally and functionally conserved in various vertebrates and have been identified in most lineages of actinopterygians (bony fish) and sarcopterygians (tetra-pods). Of interest, recent genomic studies showed that vertebrate glycoprotein hormone receptors belong to an ancient subfamily of G protein-coupled receptors (GPCRs) named as leucine-rich repeat-containing GPCRs (LGRs). These findings have prompted the hypothesis that there could be additional glycoprotein hormones in vertebrate genomes. Indeed, searches of vertebrate genomes have led to the identification of two novel glycoprotein hormone subunits, glycoprotein alpha 2 (GPA2) and glycoprotein beta 5 (GPB5), as well as their homologs in invertebrates. Subsequently, it was demonstrated that GPA2 and GPB5 form a heterodimeric hormone, thyrostimulin/OGH, capable of activating TSHR in vivoand the thyroid axis in transgenic mice. However, the exact role of this novel glycoprotein hormone and its homolog in invertebrates is not clear. To gain a better understanding of the physiological role of the novel glycoprotein hormone subunits and their evolution, it is imperative to carry out systematic studies of these genes in representative model species. In the present report, we summarize our findings based on studies of genomes of model organisms from sea anemones to humans. We found that GPA2 and GPB5 represent the ancient forms of glycoprotein hormone alpha and beta subunits, respectively, and that vertebrate and invertebrate glycoprotein hormone subunit proteins shared common ancestors that evolved during early metazoan evolution. It is important to note that glycoprotein hormone alpha and beta subunit proteins from invertebrates formed a heterodimer with structural functional characteristics similar to that of vertebrate glycoprotein hormones. Taken together, both glycoprotein hormone alpha and beta subunits evolved before the evolution of nematodes, arthropods, and vertebrates.

Trans-activation of mutant follicle-stimulating hormone receptors selectively generates only one of two hormone signals

Previously, we reported that a liganded LH receptor (LHR) is capable of activating itself (cis-activation) and other nonliganded LHRs to induce cAMP (trans-activation). Trans-activation of the LHR raises two crucial questions. Is trans-activation unique to LHR or common to other G protein-coupled receptors? Does trans-activation stimulate phospholipase Cbeta as it does adenylyl cyclase? To address these questions, two types of novel FSH receptors (FSHRs) were constructed, one defective in hormone binding and the other defective in signal generation. The FSHR, a G protein-coupled receptor, comprises two major domains, the N-terminal extracellular exodomain that binds the hormone and the membrane-associated endodomain that generates the hormone signals. For signal defective receptors, the exodomain was attached to glycosyl phosphatidylinositol (ExoGPI) or the transmembrane domain of CD8 immune receptor (ExoCD). ExoGPI and ExoCD can trans-activate another nonliganded FSH. Surprisingly, the trans-activation generates a signal to activate either adenylyl cyclase or phospholipase Cbeta, but not both. These results indicate that trans-activation in these mutant receptors is selective and limited in signal generation, thus providing new approaches to investigating the generation of different hormone signals and a novel means to selectively generate a particular hormone signal. Our data also suggest that the FSHR's exodomain could not trans-activate LHR.

Functional, structural, and distribution analysis of the chorionic gonadotropin receptor using murine monoclonal antibodies

LH and human chorionic gonadotropin (hCG) control steroid production and gametogenesis. They also function as growth factors through interaction with a specific receptor that is a member of the seven-transmembrane receptor family coupled via G proteins to signal pathways involving cAMP and phospholipase C/inositol 3 phosphate. For this study, monoclonal antibodies (mAbs) were raised against the human LH receptor (LHR)/hCG receptor (hCGR), using Chinese hamster ovary LHR-transfected cells as the immunogen. Two reagents were then selected on the basis of their ability to recognize the full-length transmembrane receptor expressed both by Chinese hamster ovary LHR-transfected cells and by a limited number of tumor cell lines. One of these mAbs reacts with the LHR/hCGR in tissue sections of both frozen and paraffin-embedded specimens. This unique feature allowed us to map the cytological distribution of LHR/hCGR in human breast tissues at different stages of development in physiological and benign pathological conditions. The same mAb proved to be agonistic: receptor ligation elicits signals that modulate the growth of selected breast tumor cell lines. This observation suggests that the mAb recognizes an epitope that is included in the domain of the receptor involved in the interaction with the natural ligand.

Evolution of glycoprotein hormone subunit genes in bilateral metazoa: identification of two novel human glycoprotein hormone subunit family genes, GPA2 and GPB5

The canonical members of the human glycoprotein hormone subunit family of cystine knot-forming polypeptides include the common alpha-subunit, and four beta-subunit genes, FSHbeta, LHbeta, TSHbeta, and hCGbeta. Using pairwise sequence analysis of the complete human genome, we have identified two novel glycoprotein hormone subunit-related genes. Based on unique sequence similarity to the alpha- and beta-subunits of glycoprotein hormones, they were named glycoprotein-alpha2 (GPA2) and glycoprotein-beta5 (GPB5), respectively. PCR analysis using a panel of human cDNAs from 14 different tissues demonstrated that GPB5 is similar to other beta-subunits showing restricted tissue expression, mainly in pituitary and brain. In contrast, the GPA2 transcript is found in diverse tissues. Furthermore, immunoreactive GPA2 and GPB5 were detected in the anterior pituitary of mouse and frog, whereas the expression of GPA2 and GPB5 in transfected cells resulted in the secretion of recombinant polypeptides in conditioned medium. After GenBank searches in lower organisms, glycoprotein hormone beta-subunit-related genes were identified from the genome of nematode Caenorhabditis elegans, hookworm Ancylostoma caninum, and Drosophila melanogaster. The evolutionary conservation of these invertebrate homologs can be seen in several key sequence characteristics, and the data suggest that the glycoprotein hormone beta-subunit gene ancestor evolved before the emergence of bilateral metazoa, thus providing a better understanding of the evolution of this group of classic polypeptide hormones and their receptors. Studies of the complete inventory of genes homologous to glycoprotein hormone subunits in the human genome and lower organisms will allow future functional characterization and identification of their respective receptors.

The lutropin/choriogonadotropin receptor, a 2002 perspective

Reproduction cannot take place without the proper functioning of the lutropin/choriogonadotropin receptor (LHR). When the LHR does not work properly, ovulation does not occur in females and Leydig cells do not develop normally in the male. Also, because the LHR is essential for sustaining the elevated levels of progesterone needed to maintain pregnancy during the first trimester, disruptions in the functions of the LHR during pregnancy have catastrophic consequences. As such, a full understanding of the biology of the LHR is essential to the survival of our species. In this review we summarize our current knowledge of the structure, functions, and regulation of this important receptor.

Hormone interactions to Leu-rich repeats in the gonadotropin receptors. III. Photoaffinity labeling of human chorionic gonadotropin with receptor Leu-rich repeat 4 peptide

Human chorionic gonadotropin (hCG) binds to the extracellular N-terminal domain, exodomain, of its receptor, and the resulting hCG-exodomain complex is thought to modulate the membrane associated domain, endodomain, of the receptor to generate hormone signal. The bulk of the exodomain is speculated to assume a crescent structure consisting of eight to nine Leu-rich repeats (LRRs), which may provide the hormone contact sites. Unfortunately, little experimental evidence is available for the precise hormone contact points in the exodomain and the endodomain. The two preceding articles (Song, Y., Ji, I., Beauchamp, J., Isaacs, N., and Ji, T. (2001) J. Biol. Chem. 276, 3426-3435; Song, Y., Ji, I., Beauchamp, J., Isaacs, N., and Ji, T. (2001) J. Biol. Chem. 276, 3436-3442) show that putative LRR2 and LRR4 are crucial for hormone binding. In particular, the N-terminal region of LRR4 assumes the hydrophobic core of the LRR4 loop, whereas the C-terminal region is crucial for signal generation. However, it is unclear whether LRR4 interacts hCG and the endodomain and how it might be involved in signal generation. In this article, our affinity labeling results present the first evidence that the N-terminal region of LRR4 interacts with hCG, preferentially the hCGalpha subunit and that the hCG/LRR4 complex interacts with exoloop 2 of the endodomain. This interaction offers a mechanism to generate hormone signal.

Purification and structural analysis of a soluble human chorionogonadotropin hormone-receptor complex

Receptors for the luteotropin/human chorionogonadotropin hormone belong to the G-protein-coupled receptor family by their membrane-anchoring domains. They also possess a large extracellular domain (ECD) responsible for most of the hormone-receptor interactions. Structure-function studies identified several contacts between hormone and receptor ECD, but the precise topology of the complex is still unknown because of the lack of suitable heterologous expression means. Receptor ECDs exhibit leucine repeats and have been modelized on the basis of the three-dimensional structure of the porcine ribonuclease inhibitor, the first structurally known leucine-rich repeats protein. Here we report overexpression (up to 20 mg per liter) and purification to homogeneity of a soluble human chorionogonadotropin-ECD receptor complex secreted by stably cotransfected Chinese hamster ovary cells. Biochemical analysis and surface plasmon resonance data were in favor of a unique dimer with a 1:1 ligand-receptor stoichiometry. Immunopurified complex was submitted to circular dichroism characterization; CD spectra deconvolution indicated more than 25% alpha helices contributed by the receptor, in agreement with the porcine ribonuclease inhibitor-based modelization.

Genetic engineering of single-chain gonadotropins and hormone-receptor fusion proteins

The gonadotropin hormone family is distinguished by its heterodimeric structure in which the members share a common alpha subunit and a hormone-specific beta subunit. Since assembly of the heterodimer is often the rate-limiting step in production of functional hormone, single-chain hormones have been engineered by genetically linking the two subunits. The single-chain hormone can in turn be fused to its receptor to produce a functional single-chain hormone-receptor complex. These fusion constructs offer a valuable new approach in structure-function studies and in the generation of hormone analogs. In this article we describe the experimental design for the generation of single-chain human chorionic gonadotropin and single-chain hormone-receptor fusion complex and strategies for the expression of these fusion proteins.

The specificity of polygalacturonase-inhibiting protein (PGIP): a single amino acid substitution in the solvent-exposed beta-strand/beta-turn region of the leucine-rich repeats (LRRs) confers a new recognition capability

Two members of the pgip gene family (pgip-1 and pgip-2) of Phaseolus vulgaris L. were expressed separately in Nicotiana benthamiana and the ligand specificity of their products was analysed by surface plasmon resonance (SPR). Polygalacturonase-inhibiting protein-1 (PGIP-1) was unable to interact with PG from Fusarium moniliforme and interacted with PG from Aspergillus niger; PGIP-2 interacted with both PGs. Only eight amino acid variations distinguish the two proteins: five of them are confined within the beta-sheet/beta-turn structure and two of them are contiguous to this region. By site-directed mutagenesis, each of the variant amino acids of PGIP-2 was replaced with the corresponding amino acid of PGIP-1, in a loss-of-function approach. The mutated PGIP-2s were expressed individually in N.benthamiana, purified and subjected to SPR analysis. Each single mutation caused a decrease in affinity for PG from F.moniliforme; residue Q253 made a major contribution, and its replacement with a lysine led to a dramatic reduction in the binding energy of the complex. Conversely, in a gain-of-function approach, amino acid K253 of PGIP-1 was mutated into the corresponding amino acid of PGIP-2, a glutamine. With this single mutation, PGIP-1 acquired the ability to interact with F.moniliforme PG.

Characterization of two LGR genes homologous to gonadotropin and thyrotropin receptors with extracellular leucine-rich repeats and a G protein-coupled, seven-transmembrane region

The receptors for LH, FSH, and TSH belong to the large G protein-coupled, seven-transmembrane (TM) protein family and are unique in having a large N-terminal extracellular (ecto-) domain containing leucine-rich repeats important for interaction with the glycoprotein ligands. We have identified two new leucine-rich repeat-containing, G protein-coupled receptors and named them as LGR4 and LGR5, respectively. The ectodomains of both receptors contain 17 leucine-rich repeats together with N- and C-terminal flanking cysteine-rich sequences, compared with 9 repeats found in known glycoprotein hormone receptors. The leucine-rich repeats in LGR4 and LGR5 are arrays of 24 amino acids showing similarity to repeats found in the acid labile subunit of the insulin-like growth factor (IGF)/IGF binding protein complexes as well as slit, decorin, and Toll proteins. The TM region and the junction between ectodomain and TM 1 are highly conserved in LGR4, LGR5, and seven other LGRs from sea anemone, fly, nematode, mollusk, and mammal, suggesting their common evolutionary origin. In contrast to the restricted tissue expression of gonadotropin and TSH receptors in gonads and thyroid, respectively, LGR4 is expressed in diverse tissues including ovary, testis, adrenal, placenta, thymus, spinal cord, and thyroid, whereas LGR5 is found in muscle, placenta, spinal cord, and brain. Hybridization analysis of genomic DNA indicated that LGR4 and LGR5 genes are conserved in mammals. Comparison of overall amino acid sequences indicated that LGR4 and LGR5 are closely related to each other but diverge, during evolution, from the homologous receptor found in snail and the mammalian glycoprotein hormone receptors. The identification and characterization of new members of the LGR subfamily of receptor genes not only allow future isolation of their ligands and understanding of their physiological roles but also reveal the evolutionary relationship of G protein-coupled receptors with leucine-rich repeats.

The amino-terminal region of the luteinizing hormone/choriogonadotropin receptor contacts both subunits of human choriogonadotropin. II. Photoaffinity labeling

The luteinizing hormone/choriogonadotropin receptor, a seven-transmembrane receptor, is composed of two equal halves, the N-terminal extracellular exodomain and the C-terminal membrane-associated endodomain. Unlike most seven-transmembrane receptors, the exodomain alone is responsible for high affinity hormone binding, whereas signal is generated in the endodomain. These physical separations of hormone-binding and receptor activation sites are attributed to unique mechanisms for hormone binding and receptor activation of this receptor and its subfamily members. However, the precise hormone contact sites in the exodomain are unclear. In the preceding article (Hong, S., Phang, T., Ji, I., and Ji, T. H. (1998) J. Biol. Chem. 273, 13835-13840), a region immediately downstream of the N terminus of the exodomain was shown to be crucial for hormone binding. To test if the region interacts with the hormone, human choriogonadotropin (hCG) was photoaffinity-labeled with a peptide mimic corresponding to Gly18-Tyr36 of the receptor. This peptide mimic specifically photoaffinity-labeled both the alpha- and beta-subunits of hCG. Interestingly, hCGalpha was preferentially labeled. On the other hand, denatured hCG was not labeled, and a mutant analog of the peptide failed to label hCG. Furthermore, the affinity labeling was UV-dependent and saturable, indicating the specificity of the photoaffinity labeling. Our results indicate that the region of the exodomain interacts with hCG and that the contact points are near both subunits of hCG. Particularly, the alternate residues (Leu20, Cys22, and Gly24) are crucial for hCG binding. In addition, the results underscore the fact that there is a crucial hormone contact site outside of the popularly believed primary hormone-binding site that is composed of Leu-rich repeats and is located in the middle of the exodomain. Our observations are crucial for understanding the molecular mechanism through which the initial high affinity hormone binding leads to receptor activation in the endodomain.

The luteinizing hormone receptor

The luteinizing hormone receptor (LHR) is a member of the subfamily of glycoprotein hormone receptors within the superfamily of G protein-coupled receptor (GPCR)/seven-transmembrane domain receptors. Over the past eight years, major advances have been made in determining the structure and function of the LHR and its gene. The hormone-binding domain has been localized to exons 1-7 in the extracellular (EC) domain/region of the receptor, which contains several leucine-rich repeats. High-affinity binding of LH and human chorionic gonadotrophin (hCG) causes secondary hormone or receptor contacts to be established with regions of the EC loop/transmembrane module that initiate signal transduction. Models of hormone-receptor interaction have been derived from the crystal structures of hCG and of the ribonuclease inhibitor, which also contains leucine-rich repeats. Such models provide a framework for the interpretation of mutational studies and for further experiments. The extracellular domain of the receptor has been overexpressed in vitro, which will facilitate crystallographic resolution of the structure of the receptor-binding site. The transmembrane domain/loop/cytoplasmic module transduces the signal for coupling to G proteins. Several constitutive, activating mutations that cause human disease have been found in helix VI and adjacent structures. These mutations have provided valuable information about mechanisms of signal transfer and G protein coupling. The structure of the LHR gene has been elucidated, and the regulation of its transcription is beginning to be understood. Valuable insights into receptor evolution have been derived from analysis of sequence homologies, the gene structure of glycoprotein hormone receptors and other members of the GPCR family, and the glycoprotein hormone receptor-like precursors identified in several invertebrate species.

Co-purification of a ribonuclease and human chorionic gonadotrophin beta-core protein from human urine and displacement of 125I-human luteinizing hormone from Candida albicans binding sites by ribonucleases

An 18 kDa pregnancy urine protein preparation, purified to apparent electrophoretic homogeneity as judged by silver-staining of polyacrylamide gels, inhibited binding of 125I-hLH (human luteinizing hormone) to Candida albicans microsomes, reacted with monoclonal and polyclonal antibodies raised against human chorionic gonadotrophin (hCG) beta-core protein and exhibited ribonuclease (RNase) activity. Eleven of the 12 amino acids at the N-terminus of a protein in this preparation were identical to those of the N-terminus of human non-secretory ribonuclease. These results indicate co-purification of hCG beta-core with a RNase. An 18 kDa RNase was also purified from a commercial hCG preparation (Chorulon). However, no RNase activity was detected in a highly purified commercial preparation (Profasi). Three commercial RNase preparations displaced 125I-hLH from C. albicans binders at extremely low concentrations (< 0.001 microg/ml RNase) whereas only slight displacement of 125I-hLH from sheep luteal binding sites was observed with very high concentrations of the RNases (100 microg/ml RNase). The co-purification of hCG beta-core and RNase from pregnancy urine and the displacement of 125I-hLH from C. albicans binding sites by RNases may be related to the close relationship that has been identified between mammalian RNase inhibitors and the extracellular domain of gonadotrophin receptors. The presence of RNase in commercial preparations of gonadotrophins should be borne in mind during any investigations that involve impure preparations of these hormones.