Covalent labelling of the lutropin binding site. Evidence for a single Mr 90000 sialoglycopolypeptide

A novel membrane-anchored cytochrome c-550 of alkaliphilic Bacillus clarkii K24-1U: expression, molecular features and properties of redox potential

A membrane-anchored cytochrome c-550, which is highly expressed in obligately alkaliphilic Bacillus clarkii K24-1U, was purified and characterized. The protein contained a conspicuous sequence of Gly(22)-Asn(34), in comparison with the other Bacillus small cytochromes c. Analytical data indicated that the original and lipase-treated intermediate forms of cytochrome c-550 bind to fatty acids of C(15), C(16) and C(17) chain lengths and C(15) chain length, respectively, and it was considered that these fatty acids are bound to glycerol-Cys(18). Since there was a possibility that the presence of a diacylglycerol anchor contributed to the formation of dimeric states of this protein (20 and 17 kDa in SDS-PAGE), a C18M (Cys(18) --> Met)-cytochrome c-550 was constructed. The molecular mass of the C18M-cytochrome c-550 was determined as 15 and 10 kDa in SDS-PAGE and 23 kDa in blue native PAGE. The C18M-cytochrome c-550 bound with or without Triton X-100 formed a tetramer as the original cytochrome c-550 bound with Triton X-100, as determined by gel filtration. The midpoint redox potential of cytochrome c-550 as determined by redox titration was +83 mV, while that determined by cyclic voltammetric measurement was +7 mV. The above results indicate that cytochrome c-550 is a novel cytochrome c.

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.

Subunit interaction of human chorionic gonadotropin (hCG) with rat ovarian luteinizing hormone (LH)/CG receptor

The subunit interaction of hCG with its rat ovarian LH/CG receptor was studied by cross-linking the solubilized receptor-hormone complex with glutaraldehyde (GA), disuccinimidyl suberate (DSS) or dithiobis(succinimidyl propionate) (DSP) and analyzing the complexes by sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) and autoradiography. The hormone was labeled either in its alpha-subunit (125I-hCG) or in its beta-subunit (3H-hCG) or the label (3H) was introduced into the receptor molecule instead of the hormone. All of the labeling procedures led to the detection of only the receptor-(alpha,beta)hCG and receptor-(alpha)hCG complexes on the autoradiograms. The sizes of these complexes were 137,000 and 106,000, respectively, under reducing conditions. These results suggest that the receptor binds one hormone molecule, and that hCG interacts with the receptor mainly through its alpha-subunit. In addition, polyclonal antibodies directed against the LH/CG receptor and the alpha- and beta-subunits of hCG were used to detect the non-reduced receptor-(alpha,beta)hCG complex in immunoblotting. As antibodies directed against both the alpha-subunit and the beta-subunit were able to detect the Mr 130,000 complex, it is conceivable that both of the subunits are at least partially exposed on the receptor-hormone complex. 125I-hCG was also cross-linked to the membrane-bound receptor. The membrane-bound complex had an Mr of 144,000 under reducing conditions, i.e. approximately 7000 higher than that of the solubilized complex (Mr 137,000). This may indicate that the membrane-bound receptor is covalently modified or differs in conformation from the solubilized receptor.

Effect of deglycosylation on the structure and hormone-binding activity of the lutropin receptor

Affinity-purified rat ovarian lutropin (LH) receptor is a single 90 kDa polypeptide which binds to immobilized lectins, indicating that the receptor is a glycoprotein [Keinänen, Kellokumpu, Metsikkö & Rajaniemi (1987) J. Biol. Chem. 262, 7920-7926]. In the present study the glycoprotein nature of the rat ovarian LH receptor was investigated in order to determine the contribution of the glycan moiety to receptor's size and hormone-binding properties. Treatment of the 125I-labelled purified LH receptor with neuraminidase and peptide N-glycosidase F resulted in a decrease in size of LH receptor from 90 kDa to 79 kDa and 62 kDa respectively, as assessed by SDS/polyacrylamide-gel electrophoresis. Endo-alpha-N-acetylgalactosaminidase treatment did not affect the electrophoretic mobility of the intact or neuraminidase-treated LH receptor. Subjecting the membrane-bound LH receptor to similar enzymic treatments followed by ligand blotting showed that the 79 kDa and 62 kDa forms are capable of specific hormone binding. Furthermore, intact and peptide N-glycosidase F-treated membranes bound 125I-labelled human choriogonadotropin with similar affinities. These data suggest that molecular mass of the polypeptide backbone of the LH receptor is 62 kDa. The receptor contains N-glycosidically linked oligosaccharide chains with terminal sialic acid residues, with little or no O-linked oligosaccharide. N-Linked carbohydrate is not required for specific high-affinity hormone binding.

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.

Photoaffinity labeling of the gonadotropin receptor with native, asialo, and deglycosylated choriogonadotropin

Human choriogonadotropin (hCG) is a heterodimeric hormone composed of an alpha and a beta subunit. hCG and its asialo (ashCG) and deglycosylated (dghCG) forms vary in their ability to stimulate hormone responsive adenylate cyclase. ashCG is a partial agonist, and dghCG is an antagonist. Photoactivatable moieties were coupled to hCG, ashCG, and dghCG, and the derivatives were radioiodinated. Competitive binding studies indicate that all of the derivatives had a similar affinity for the gonadotropin receptor on porcine granulosa cell membranes. Radiolabeled derivatives were used to photoaffinity label the gonadotropin receptor. Radiolabeled complexes were separated by NaDodSO4/PAGE. All of the derivatives produced similar autoradiographic patterns, except that dghCG produced an additional 48-kDa complex. To investigate the structure of the complexes further, peptide mapping of proteolytic digests was used. All, except for the 48-kDa complex, generated similar peptide maps indicating a relationship between those complexes in which the smaller components are part of the larger. The 48-kDa complex contained both subunits of 40-kDa dghCG. Therefore, this complex is expected to contain an additional component of 8 kDa. The complex was generated whether the hormone-receptor complex was photoaffinity labeled on cells, on isolated membranes, or after solubilizing in detergent. Formation was blocked by excess hCG and did not occur in the absence of UV irradiation. We conclude that the hCG derivatives are able to photoaffinity label the hCG receptor but that the dghCG derivative can photoaffinity label an additional component that was not observed when derivatives of hCG or ashCG were used to label the receptor.

Hormone binding modified endogenous proteolysis of LH/hCG receptors in rat ovarian plasma membranes

Purified rat ovarian plasma membranes were subjected to incubation under conditions where luteinizing hormone receptors were either free or bound to hCG. When receptor proteolysis was followed by labeling the receptor with tritiated borohydride or [125I]hCG, occupied receptors were found to be more accessible to endogenous proteinases than unoccupied receptors. They exhibited greater rates of degradation and also produced an additional degradation product upon proteolysis. Degradation of other plasma membrane (glyco)polypeptides, however, was not affected by hormone binding. These results indicate that hCG binding induces a conformational or a structural change in its receptor, thereby increasing its susceptibility to endogenous plasma membrane proteinases.