The subunit structure of the follitropin receptor. Chemical cross-linking of the solubilized follitropin-receptor complex

Human FSH Glycoform α-Subunit Asparagine52 Glycans: Major Glycan Structural Consistency, Minor Glycan Variation in Abundance

Follicle-stimulating hormone (FSH), an α/β heterodimeric glycoprotein hormone, consists of functionally significant variants resulting from the presence or absence of either one of two FSHβ subunit N-glycans. The two most abundant variants are fully-glycosylated FSH24 (based on 24 kDa FSHβ band in Western blots) and hypo-glycosylated FSH21 (21 kDa band, lacks βAsn²⁴ glycans). Due to its ability to bind more rapidly to the FSH receptor and occupy more FSH binding sites than FSH24, hypo-glycosylated FSH21 exhibits greater biological activity. Endoglycosidase F1-deglycosylated FSH bound to the complete extracellular domain of the FSH receptor crystallized as a trimeric complex. It was noted that a single biantennary glycan attached to FSHα Asn⁵² might preemptively fill the central pocket in this complex and prevent the other two FSH ligands from binding the remaining ligand-binding sites. As the most active FSH21 preparations possessed more rapidly migrating α-subunit bands in Western blots, we hypothesized that Asn⁵² glycans in these preparations were small enough to enable greater FSH21 receptor occupancy in the putative FSHR trimer model. Highly purified hFSH oligosaccharides derived from each FSH subunit, were characterized by electrospray ionization-ion mobility-collision-induced dissociation (ESI-IM-CID) mass spectrometry. FSHβ glycans typically possessed core-linked fucose and were roughly one third bi-antennary, one third tri-antennary and one third tetra-antennary. FSHα oligosaccharides largely lacked core fucose and were bi- or tri-antennary. Those αAsn⁵² glycans exhibiting tetra-antennary glycan m/z values were found to be tri-antennary, with lactosamine repeats accounting for the additional mass. Selective αAsn⁵² deglycosylation of representative pituitary hFSH glycoform Superdex 75 gel filtration fractions followed by ESI-IM-CID mass spectrometry revealed tri-antennary glycans predominated even in the lowest molecular weight FSH glycoforms. Accordingly, the differences in binding capacity of the same receptor preparation to different FSH glycoforms are likely the organization of the FSH receptor in cell membranes, rather than the αAsn⁵² oligosaccharide.

Solubilization and characterization of high affinity follicle-stimulating hormone receptors from porcine granulosa cells

Follicle-stimulating hormone (FSH)-receptors were solubilized from immature porcine ovarian granulosa cells with retention of high affinity 125I-porcine FSH-binding activity. The optimal concentration of Triton X-100 for solubilization was 0.5% (w/v), and the optimal cellular protein concentration 25 mg/ml. Glycerol (30%) increased recovery of solubilized receptor. 125I-pFSH-binding affinity ranged from 4 x 10(10) M-1 to 8 x 10(10) M-1 in either the absence or presence of glycerol. 125I-pFSH-binding capacity was 5 fmol/mg protein in the absence of glycerol and 58 fmol/mg protein in the presence of glycerol as determined by equilibrium saturation binding analysis. By gel permeation chromatography, the apparent size of the 125I-pFSH-receptor complex was 462 kDa in the absence of glycerol and 762 kDa in the presence of glycerol. Ligand blotting of solubilized receptor yielded a single species with an apparent molecular weight of 200 kDa under nonreducing conditions and a single species with an apparent molecular weight of 60 kDa under reducing conditions. These studies indicated that high affinity FSH-binding activity can be solubilized from membranes of immature porcine granulosa.

Characterization of human follicle-stimulating hormone binding to human granulosa cells by an immunoenzymological method

An original, nonradiometric method has been developed for studying the binding parameters of native follicle-stimulating hormone (FSH) to its specific receptors in human ovarian granulosa cells. After binding and washing of the cells, hFSH was desorbed from its receptors and quantitatively measured by a specific enzyme immunoassay (EIA) in which nonspecific binding was estimated in the presence of an excess of equine chorionic gonadotropin (eCG/PMSG), which binds to human FSH receptors but does not interfere in the hFSH EIA. This method makes use of native nonmodified hFSH molecules (in contrast to radiometric methods) and permits direct estimation of the binding parameters (Kd and total number of sites). The Kd of hFSH for its human granulosa receptors measured by this technique (4.8 +/- 0.3 x 10(-10) M) is close to that determined by other methods. However, we found a total number of specific FSH receptors per granulosa cell (1 to 6 x 10(4) higher than that reported by others by Scatchard analysis of competition dose-response curves in radioreceptor assays. The method is also sensitive enough to measure the in vivo occupancy of receptors by endogenous hFSH, which was found to be less than 6% in women undergoing hormonal treatment for in vitro fertilization.

Guanine nucleotide binding in human placental syncytiotrophoblast membranes and comparative regulation of adenylate cyclase in syncytiotrophoblast, turkey erythrocyte and bovine calf testes membranes by guanosine-5'-triphosphate

Guanosine-5'-triphosphate (GTP) binds specifically to syncytiotrophoblast plasma membranes and increases the production of cyclic AMP in these membranes. 1. In syncytiotrophoblast membranes, GTP alone caused a significant increase in the basal levels of cyclic AMP in a dose dependent manner. 2. GTP alone did not significantly stimulate cyclic AMP production in turkey erythrocyte or bovine calf testes membranes. 3. GTP decreased Gpp(NH)p-mediated cyclic AMP production while increasing NaF-mediated cyclic AMP production in placental, erythrocyte and testes membranes. 4. Since cyclic AMP has been reported to regulate the levels of placental hormones, and it is shown in this study that GTP increases cyclic AMP production in the placenta, this study suggests: (A) placental GTP levels may indirectly regulate placental hormone production, (B) placental beta adrenergic (BA) mediated adenylate cyclase activity may not be regulated in the same manner as the BA system of avian erythrocytes.

Thermodynamics of follitropin binding to solubilized calf testis receptor

Thermodynamic parameters of follitropin binding to solubilized testicular receptors were measured in order to assess the forces involved in the binding reaction. Reversibility of follitropin binding to solubilized receptor decreased only 20% over the temperature range 4-24 degrees C, whereas earlier studies indicated reversibility of binding to membrane-bound receptor decreased by more than 40% over the same range [Anderson, T. T., Curatolo, L. M., & Reichert, L. E., Jr. (1983) Mol. Cell. Endocrinol. 33, 37-52]. Thermodynamic analysis of follitropin binding to solubilized receptors showed that the hydrophobic effect was important in the binding reaction. The mean values, at 25 degrees C, for delta H and delta S were -31.8 kcal/mol and -66.0 cal mol-1 K-1, respectively, and delta Cp was -3.0 kcal mol-1 K-1. This is an unusually large heat capacity for protein-protein association reactions, indicating an enhanced role for the hydrophobic effect with the solubilized (compared to membrane-bound) receptor. Since glycerol was necessary to stabilize the solubilized receptor, we determined whether glycerol affected the thermodynamic parameters measured for the binding reaction. Control experiments, performed with membrane-bound receptor in the presence or absence of glycerol, indicated that delta Cp actually decreased upon addition of glycerol (-0.8 kcal mol-1 K-1 in the presence of glycerol compared to -2.3 kcal mol-1 K-1 in the absence of glycerol). Thus, the large negative delta Cp observed for the soluble receptor was a result of its removal from the membrane and was not due to the presence of glycerol.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of different batches of 125iodine on properties of 125I-hFSH and characteristics of radioligand-receptor assays

Radioiodination of highly purified human follicle-stimulating hormone (hFSH) (4000 IU/mg) was performed every other week for 23 weeks using 2 mCI carrier free Na125I (Amersham Corp., 15 mCi/micrograms I2) in the presence of lactoperoxidase. Incorporation of 125I into hFSH was determined by the method of R. C. Greenwood, W. M. Hunter, and J. S. Grover (1963) Biochem. J. 89, 114). Hormone binding was studied in vitro under steady-state conditions (16 h, 20 degrees C) using different calf testis membrane preparations having similar receptor characteristics. Each 125I-hFSH preparation was characterized for maximum bindability, specific activity of bindable radioligand as determined by self-displacement analysis, and by determination of Ka and Rt. Incorporation of 125I into FSH was relatively constant over the large number of experiments (62.4 +/- 6.4 microCi/micrograms; n = 23). By comparison, however, specific radioactivity of the receptor bindable fraction of 125I-hFSH was related to the lot of 125I utilized, and was significantly (P less than or equal to 0.01) lower and more variable (28.7 +/- 10.5 microCi/micrograms). Maximum bindability of 125I-hFSH was not correlated to specific activity (r = 0.06) but was negatively correlated to hFSH 125I incorporation (r = -0.47; P less than or equal to 0.05). These observations demonstrate the need to assess the quality of each batch of radioligand before undertaking radioligand-receptor assays and suggest that differences in Na125I lots affect specific radioactivity of the radioligand and its receptor binding characteristics.

High-affinity receptors for human interferon in bovine lung and human placenta

Membrane fractions were prepared from fresh frozen bovine lung tissue and human placenta and used for receptor binding studies with recombinant DNA produced human-alpha-interferon (IFN-alpha). A single class of high-affinity binding sites was determined for bovine lung (Ka = 1.5 X 10(10) M) and human placenta (Ka = 3.7 X 10(9) M) membranes, respectively. These values for the affinity of IFN binding are comparable within the range of observed error to that observed for the binding of human IFN-alpha 2 to cultured Daudi cells (Ka = 2.5 X 10(10) M). The type I IFN receptor content of bovine lung and human placenta membranes was 1.3 and 2.5 fmoles/mg wet weight, respectively. In addition, alterations in specific and nonspecific binding were observed with the bovine lung membrane incubations in the presence of calcium. Increases in specific binding of three- to fourfold were observed in the presence of 1 mM calcium chloride.

Inhibition of iodine-125-labeled human follitropin binding to testicular receptor by epidermal growth factor and synthetic peptides

Two tetrapeptide sequence homologies between mouse epidermal growth factor precursor (mEGFP) and human follitropin (FSH) were revealed by a computer program that identifies identical residues among polypeptide sequences. The two tetrapeptides, Lys-Thr-Cys-Thr (KTCT) and Thr-Arg-Asp-Leu (TRDL), are present in the hormone-specific beta subunit of FSH from all species studied. These tetrapeptides are not present in the alpha subunit, which is common to all pituitary glycoprotein hormones. Both tetrapeptides are also found in mEGFP, and one tetrapeptide, TRDL, is located within the 53-residue form of mEGF purified from mouse submaxillary glands. Computer-generated hydropathy profiles predicted that both tetrapeptides are located in hydrophilic portions of the FSH beta subunit and that TRDL is in a hydrophilic portion of commercially available mEGF. Therefore, the tetrapeptides might be accessible to receptor binding sites for FSH. We report that mEGF inhibits binding of 125I-labeled human FSH to receptors in testis by 50% (I50) at a concentration of 1.8 X 10(-5) M. No binding inhibition was observed by GnRH or arginine-vasopressin at 10(-4) M, neither of which contain the tetrapeptide sequences. FSH beta subunit, which contains both tetrapeptides, also inhibited binding (I50 = 9 X 10(-8) M) of 125I-labeled human FSH to testis receptor. Thus, it appears that FSH beta subunit and mEGF are capable of inhibiting binding of FSH to testicular FSH receptors, presumably through interactions that include the homologous tetrapeptides. This presumption was supported by the observation that the synthetic tetrapeptides (KTCT or TRDL) were also active in inhibiting binding of 125I-labeled human FSH to testis receptor.