Photoaffinity labeling of human chorionic gonadotropin-binding sites in rat ovarian plasma membranes

Partial characterization of chorionic gonadotropin-like binding sites from the bacteria Xanthomonas maltophilia

The gram-negative bacterium, Xanthomonas maltophilia, has low- and high-affinity luteinizing hormone/chorionic gonadotropin (LH/CG)-binding sites, similar to the LH/CG receptor found in mammals. Although the low-affinity site binds both LH and human CG (hCG), the high-affinity site is specific for hCG. In the current investigation, these two binding sites were independently isolated from X. maltophilia for further characterization. To isolate functional binding sites, we developed a solubilization method using the detergent zwittergent 3,14 and high glycerol concentrations that allowed for the maintenance of ligand-binding integrity. Gel filtration experiments established molecular weights of 170 and 11.5 kDa for the two binding sites, which were supported by data from photoaffinity labeling and ultracentrifugation experiments. Gel filtration data also suggested the presence of a third binding site of 5.4 kDa. The 170-kDa site had a binding affinity of Kd = 12 x 10(-6) and bound both LH and hCG. The small molecular weight site had an affinity of Kd = 9.4 x 10(-8) and was CG specific. Collectively, these data demonstrate the presence of multiple hormone binding sites in X. maltophilia that differ in molecular size, binding affinity, and ligand specificity.

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.

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.

Hydrodynamic properties of the gonadotropin receptor from a murine Leydig tumor cell line are altered by desensitization

The murine Leydig tumor cell line 1 (MLTC-1) contains gonadotropin receptors (GR) that are coupled to adenylate cyclase through the stimulatory guanine nucleotide binding protein (Gs). The binding of human choriogonadotropin (hCG) causes MLTC-1 cells to accumulate cAMP. With time, the ability of MLTC-1 cells to respond to hCG is attenuated by a process called desensitization. The hydrodynamic properties of GR from control and desensitized MLTC-1 cells were studied. Sucrose density gradient sedimentation in H2O and D2O and gel filtration chromatography were used to estimate the Stokes radius (a), partial specific volume (vc), sedimentation coefficient (S20,w), and molecular weight (Mr) of the detergent-solubilized hormone-receptor complex (hCG-GR). [125I]hCG was bound to MLTC-1 cells under conditions that allow (37 degrees C) or prevent (0 degree C) desensitization, and hCG-GR was solubilized in Triton X-100. In the absence of desensitization, control hCG-GR had a Mr of 213,000 (a = 6.2; vc = 0.76; S20,w = 7.3), whereas desensitized hCG-GR had a Mr of 158,000 (a = 6.1; Vc = 0.71; S20,w = 6.6). Deglycosylated hCG (DG-hCG) is an antagonist that binds to GR with high affinity but fails to stimulate adenylate cyclase or cause desensitization. [125I]DG-hCG was bound to MLTC-1 cells and DG-hCG-GR solubilized in Triton X-100. The hydrodynamic properties of DG-hCG-GR (Mr 213,000; a = 5.8; Vc = 0.77; S20;w = 7.6) were the same as that for control hCG-GR. There was no evidence for the association of adenylate cyclase or Gs with GR in Triton X-100 solubilized preparations.(ABSTRACT TRUNCATED AT 250 WORDS)

Covalent cross-linking of radiolabeled human chorionic gonadotropin to rat ovarian luteinizing hormone receptor with glutaraldehyde

Crude plasma membranes of pseudopregnant rat ovaries were incubated with 125I-labeled human chorionic gonadotropin (125I-hCG) and the formed luteinizing hormone (LH)/hCG receptor-125I-hCG complex was solubilized, partially purified by Sepharose 6B gel filtration, cross-linked with glutaraldehyde and subjected to sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) and fluorography. An apparent molecular weight (mol wt) of 130,000 was obtained for the non-reduced complex. A similar-sized complex was observed, when 3H-hCG (beta-subunit labeled) instead of 125I-hCG (alpha-subunit labeled) was used, indicating that the complex contains intact hCG. Upon reduction of the cross-linked receptor-125I-hCG complex, a 105,000 mol wt complex in addition to the 130,000 one was observed. No smaller complexes appeared, however, upon reduction of the receptor-3H-hCG complex, suggesting that the alpha-subunit of hCG predominantly interacts with the receptor. The cross-linking efficiency was dependent on protein concentration, glutaraldehyde concentration, pH, reaction time and temperature. Under optimal conditions (2 mM glutaraldehyde, pH 7.0-8.0, 60 min, 20 degrees C) no nonspecific complexes appeared and the efficiency of cross-linking of the partially purified detergent-solubilized receptor-125I-hCG complex was approximately 30%. Glutaraldehyde thus provides a rapid and efficient cross-linking reagent to covalently attach 125I-hCG to its receptor and is likely to be highly applicable to other receptor-ligand systems as well.

Visualization of the rat ovarian lutropin receptor by ligand blotting

A ligand blotting technique was developed to identify the lutropin receptor after size separation by sodium dodecyl sulfate-polyacrylamide gel electrophoresis under nonreducing conditions. The separated proteins were transferred to a nitrocellulose sheet, which was subsequently incubated with 125I-labeled human choriogonadotropin (125I-hCG), and subjected to autoradiography. An Mr 90,000 band was specifically and intensely labeled with 125I-hCG. The band was not observed, when the hormone incubation was performed in the presence of an excess of unlabeled hCG or human lutropin. The presence of rat follitropin or rat prolactin did not, however, abolish the labeling. No specific labeling was found when down-regulated ovarian tissue or rat liver was used as starting material. The Mr 90,000 band disappeared when the protein samples were treated with reducing agent, showing that integrity of receptor disulfide bonds is essential for the hormone-receptor interaction. In addition, a receptor-positive murine Leydig tumor cell line produced an Mr 90,000-92,000 band in ligand blotting, thus demonstrating the similarity between rat and murine lutropin receptors. These results provide strong evidence that the lutropin receptor is an Mr 90,000 protein.