The alpha subunit of pituitary glycoprotein hormones. Formation of three-dimensional structure during cell-free biosynthesis

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.

Expression of biologically active beta subunit of bovine follicle-stimulating hormone in the methylotrophic yeast Pichia pastoris

Follicle-stimulating hormone (FSH), a pituitary gonadotropin, is a heterodimer composed of an alpha subunit, which is common to all the glycoprotein hormones, noncovalently associated with the hormone-specific beta subunit. The objective of the present study is to develop a recombinant DNA expression system for the beta subunit of FSH that can be applied to study structure-function relationships while producing large quantities of the hormone subunit for immuno-contraceptive, clinical, and veterinary purposes. We report here the expression of biologically active bovine FSH beta (bFSH beta) in the methylotrophic yeast Pichia pastoris. The Pichia-expressed FSH beta (pFSH beta) was secreted into the culture medium and was found to be immunologically very similar to pituitary-derived ovine FSH beta. Replacement of cognate signal peptide with the yeast alpha mating factor signal peptide increased the level of expression from 230 ng/ml (cognate signal peptide) to 4 micrograms/ml (alpha mating factor signal peptide) of the culture supernatant. pFSH beta His.tag (pFSH beta with six histidine residues at the C terminus) was purified to apparent homogeneity using one-step nickel affinity chromatography. The molecular weight of purified pFSH beta His.tag was approximately 22,000, which was slightly higher than that of the pituitary-derived ovine FSH beta. pFSH beta His.tag could assemble with the alpha subunit to yield a heterodimer capable of binding to the FSH receptors and also elicit biological response. These data show that pFSH beta His.tag is properly folded and biologically active.

Bacterial expression of human chorionic gonadotropin alpha subunit: studies on refolding, dimer assembly and interaction with two different beta subunits

Human chorionic gonadotropin (hCG) is a member of a family of heterodimeric glycoprotein hormones that have a common alpha subunit but differ in their hormone-specific beta subunit. The common alpha subunit contains two asparagine (N)-linked oligosaccharides. To study the function of carbohydrates on in vitro refolding of alpha subunit and dimer assembly, we generated recombinant non-glycosylated hCG alpha subunit (rNGl-hCGalpha) from E. coli. The expression vector was constructed by inserting hCGalpha cDNA coding for the mature form in-frame into a pQE-30 vector, which contains a 6 x His sequence immediately before the 5'-end of hCGalpha cDNA for subsequent purification of rNG-hCGalpha. The rNG-hCGalpha expressed in inclusion bodies was efficiently purified by immobilized metal chelate affinity chromatography on Ni-NTA resin. SDS-PAGE, solid-phase binding assay and immunoblotting demonstrated the expression of rNG-hCG. Its alpha molecular weight on SDS-PAGE was 14.7 kDa under reducing conditions and 15 kDa for a monomer accompanied with some higher molecular weight oligomer under non-reducing conditions. Reconstitution of rNG-hCGalpha with native hCGbeta and oFSHbeta occurred in very low yield under standard conditions. However, the oxidation-reduction system cystamine (1.34 mM) and cysteamine (7.3 mM) facilitated both the refolding of rNG-hCGalpha and reconstitution of rNG-hCGalpha with native hCGbeta to regain partially correct conformation. These were revealed by conformationally sensitive antibody and receptor binding assays. Cystamine and cysteamine were more effective in the recombination of rNG-hCGalpha with oFSHbeta as indicated by a 22-36-fold decrease in the amount required to cause a 50% competitive inhibition in radioreceptor assay. They have no effect on assembly of rNG-hCGalpha with oLHbeta. Our results suggest the carbohydrate moieties confer greater conformational flexibility to the backbone of the beta subunit and the relative rigidity of the beta subunit may serve as a conformational template of the alpha subunit. The present approach has made it possible to prepare the non-glycosylated gonadotropin alpha subunit in adequate amounts for further study on their biological and topographical features in complete absence of carbohydrate.

Immunoreactive and bioactive luteinizing hormone in pubertal patients with chronic renal failure. Cooperative Study Group on Pubertal Development in Chronic Renal Failure

Disturbed pulsatile LH secretion has been suggested to play a role in the etiology of delayed puberty and disturbed reproductive function in chronic renal failure (CRF), but interpretation of gonadotropin secretion from plasma concentration measurements is confounded by alterations in hormone metabolic clearance. To simultaneously investigate LH secretion and clearance in children, we performed multiple-parameter deconvolution analysis of 11-hour over-night serum LH concentration-time series of bioactive (bio-LH) and immunoreactive (i-LH) hormone in 36 pubertal patients (18 boys) with various degrees of CRF and 10 healthy controls matched for sex and pubertal stage. Twelve patients received conservative treatment for advanced but compensated CRF, 12 were treated by dialysis, and 12 were studied after successful renal transplantation. We observed that: (1) the mean (+/- SE) plasma half-lives of bio-LH and i-LH were increased in the dialysis group (155 +/- 47 and 201 +/- 31 min) and in the patients on conservative treatment (148 +/- 45 and 135 +/- 70 min) compared to controls (59 +/- 28 and 63 +/- 21 min; all P < 0.05). The plasma half-life of bio-LH in patients on conservative treatment or after renal transplantation was inversely correlated with glomerular filtration rate (GFR) (r = -0.70; P < 0.0001). (2) Pulsatile bio-LH production rate was independently affected by pubertal stage (P = 0.018) and treatment status (P = 0.017), increasing across pubertal stages and being significantly lower in dialysis patients (20 +/- 4 IU/liter * 11 hr) and patients on conservative treatment (28 +/- 9) than in controls (43 +/- 9; all P < 0.05). In patients on conservative treatment or after transplantation, a significant positive correlation between pulsatile bio-LH production rate was observed (r = 0.53; P < 0.008). Pulsatile i-LH secretion rate was significantly reduced only in dialysis patients (15 +/- 34 vs. 46 +/- 18; P < 0.05). (3) The reduction of pulsatile i-LH and/or bio-LH production rates was attributable to a halving of the LH mass secreted per burst in patients on conservative (bio-LH: 4.9 +/- 1.9 IU/liter) and dialysis treatment (bio-LH: 3.2 +/- 0.7, i-LH: 2.4 +/- 0.6 IU/liter) versus controls (bio-LH: 6.9 +/- 1.3, i-LH: 5.4 +/- 2.1 IU/liter), whereas the LH pulse frequency was not different between controls and treatment groups.(ABSTRACT TRUNCATED AT 400 WORDS)

Helix formation in reduced, S-carboxymethylated human choriogonadotropin beta subunit and tryptic peptides

The beta subunit of human choriogonadotropin (hCG beta) and its asialoderivative were digested with trypsin and then reduced and S-carboxymethylated. A series of peptides were purified which corresponded to residues 1-43, 44-95, 96-114, and 123-145 of the 145 amino acid residue glycoprotein. The two N-linked oligosaccharides were present on the amino terminal peptide, and three of the four O-linked oligosaccharides were present on the carboxy terminal peptide. Circular dichroic spectra between 190-240 nm were obtained on reduced, S-carboxymethylated (RCM) hCG beta and the above peptides, both in aqueous solution and in the helicogenic solvent 80% (vol/vol) trifluoroethanol (TFE). In aqueous solution there was evidence of only limited helicity in the peptides and RCM-hCG beta; however, in the presence of TFE, peptides 1-43 and 44-95 exhibited significant helicity, as did the full-length linear chain. The helicity developed in TFE by RCM-hCG beta appears much greater than that which occurs in the native, disulfide-intact form, thus suggesting that the disulfides prevent expression of helicity in regions with alpha-helix potential. Application of the Chou-Fasman secondary structure predictive algorithm to hCG beta suggested that several regions of helix potential, in particular regions 14-21, 59-69, and perhaps 80-88, may account for much of the helicity observed in peptides 1-43 and 44-95, respectively, in TFE. The region from 96-145 has no significant potential for helicity, consistent with the measured circular dichroic spectra of peptides 96-114 and 123-145. These results demonstrate that helicity can occur in the linear form of hCG beta, and this secondary structure can best be attributed to the amino terminal and the middle portion of the molecular. Several potential regions of beta-structure and beta-turns were also suggested.

Influence of cyclic nucleotides on the processing of the carbohydrate part of the alpha-subunit of human choriogonadotropin by first trimester human placenta tissue

In pulse-chase experiments ([35S]Met as radioactive label) 4 intracellular forms of the alpha-subunit (apparent molecular weights of 11, 16.5, 19.5, and 23.4 kDa) were observed whereas almost no label was incorporated into the beta-subunit. The 23.4 kDa form was secreted as free alpha-subunit, the others were precursors of the alpha-subunit contained in secreted human choriogonadotropin. The rate-limiting step seemed to be the processing of the 19.5 kDa precursor by alpha-mannosidase II. 8-bromo-cAMP increased the total amount of intracellular forms of the alpha-subunit and accelerated significantly the velocity of all glycosylation steps. It seemed to cause a higher efficacy of the alpha-mannosidase II reaction. In the presence of 8-bromo-cAMP intracellular as well as extracellular alpha-subunits showed a higher sialic acid content.

Production and secretion of the 21-23.5 kDa prolactin-like molecules

We recently described the presence of a series of prolactin (PRL)-like molecules (PLMs) in the rat pituitary gland and showed that their formation was not due to artifactual proteolysis of 24 kDa PRL during extraction or to degradation of PRL in lysosomes. In this study we have found (1) in vitro translation of pituitary cell RNA to result in the production of only 24 kDa monomer isoform 2 and no PLMs, (2) that secretion of newly synthesized PLMs is differently regulated than at least a proportion of newly synthesized monomers, (3) that secretion of newly synthesized PLMs occurs after at least a 6 h delay, (4) that cysteamine (a) inhibits the release of the PLMs, (b) causes an increase in their amount versus isoform 2, and (c) causes an intracellular accumulation of pleiomorphic, immature secretory granules, and (5) that cells grown under degranulating culture conditions do not contain PLMs. These results, using normal anterior pituitary cells in primary culture, demonstrate the potential for differential release of the PLMs versus monomer PRL in vivo and are consistent with the production of the PLMs from 24 kDa monomer isoform 2 during secretory granule condensation.

Production of monoclonal antibodies complementary to an antibody-antigen complex. Use in an immunoradiometric assay for follitropin

Monoclonal antibodies were prepared against human follitropin by fusion of the myeloma cell line P3-X63-Ag8-653 with spleen cells of mice immunized with either human follitropin or follitropin bound to an anti-beta hFSH monoclonal antibody. The latter immunization procedure permits shielding of the immunodominant specific site and allows the production of numerous specific monoclonal antibodies to human follitropin which are complementary to the monoclonal antibody used in the immunogen. In this investigation two specific monoclonal antibodies were used in a two site immunoradiometric assay which was highly specific, rapid, with one incubation step and demonstrated a sensitivity level of 0.1 mIU/ml. It was possible to differentiate between prepubertal and adult levels of follitropin and to recognize individuals with hyposecretory states.

The subcellular sites of sulfation of mouse thyrotropin and free alpha subunits: studies employing subcellular fractionation and inhibitors of the intracellular translocation of proteins

To determine the subcellular sites of sulfation of thyrotropin (TSH) and free alpha-subunits, mouse thyrotropic tumor minces were incubated simultaneously with [3H]Met and [35S]SO4 for 1 or 3h, homogenized, and fractionated by discontinuous sucrose gradient ultracentrifugation. Dual-labeled TSH or free alpha-subunits were immunoprecipitated, and analyzed by SDS-gel electrophoresis. Endoglycosidase F released all [35S], but little [3H], from the dual-labeled species, indicating that [35S]SO4 was incorporated into oligosaccharides of TSH and free alpha-subunits. Both [35S]TSH and [35S] free alpha-subunits were predominantly in Golgi fractions at 1 and 3 h, but small amounts were also detected in fractions enriched in rough endoplasmic reticulum (RER). Similar distributions of [35S]SO4-labeled species were noted in cell fractions prepared from mouse pituitaries. Pituitaries from hypothyroid mice were incubated with [3H]Met and [35S]SO4 for 2 h, then chased for 4 or 16 h in the absence or presence of 2 uM monensin (Mon) or 10 uM carboxyl cyanide m-chlorophenylhydrazone (CCCP). At 4h, release into the medium of [3H]TSH was inhibited 59% and 86% by Mon and CCCP, respectively; release of [35S]TSH was inhibited 28% and 46%. At 4h, release of [3H]free alpha-subunits was inhibited 58% and 81% by these drugs, respectively; release of [35S]free alpha-subunits was inhibited 6% and 50%. Thus, Mon and CCCP inhibited the release of each [3H] species more than the [35S] species, indicating that most sulfation occurred in Golgi.

Direct demonstration of intrinsic follicle-stimulating hormone receptor-binding activity in acid-treated equine luteinizing hormone

After dissociating equine gonadotropins as a function of time at pH 3, we examined them by radioligand assay and sodium dodecyl sulfate polyacrylamide gel electrophoresis under nondissociating conditions (low, 0.1% SDS). Equine follicle-stimulating hormone (FSH) rapidly lost its receptor-binding activity, and low SDS-polyacrylamide gels demonstrated dissociation into subunits. Maximum dissociation occurred after 20-30 min of pH 3 incubation. Equine luteinizing hormone (LH), however, retained most biologic activity and was largely intact after 72 h of pH 3 incubation. Dose-response curves of acid-treated equine LH and FSH and intact equine LH and FSH were compared in five types of radioligand receptor assays. LH and FSH receptor-binding activities of equine LH were unaffected by pH 3. Equine LH showed 19- and 32-times more activity in the rat testis FSH assay than it did in chicken or horse FSH assays, respectively, directly demonstrating the intrinsic FSH receptor-binding activity of equine LH and the relative lack of specificity for these hormone preparations of the rat FSH receptor. Acid-treated 95% of its biologic activity in FSH assays. In LH assays, the slight (0.2%) activity of equine FSH was relatively unaffected by acid treatment, suggesting that contamination by equine LH accounts for this activity.

Differential processing and regulation of thyroid-stimulating hormone subunit carbohydrate chains in thyrotropic tumors and in normal and hypothyroid pituitaries

Thyroid-stimulating hormone (TSH) alpha- and beta-subunit glycosylation was investigated in mouse thyrotropic tumor and in normal and hypothyroid pituitary cells for various periods of time in the presence of [3H]mannose or [3H]galactose. After sequential precipitation with anti-alpha and anti-beta sera, subunits were treated with Pronase followed by endo-beta-N-acetylglucosaminidase H (Endo H) and analyzed by paper chromatography. In primary cultures of thyrotropic tumor cells incubated for 60 min with [3H]mannose, primarily Man9GlcNAc and Man8GlcNAc were found on TSH + alpha subunits, whereas Glc1Man9GlcNAc and Man9GlcNAc were prominent on free beta subunits. After preincubation of cells for 16 h in the presence or absence of glucose followed by a 60-min pulse of [3H]mannose, there was an 8-fold increase in labeled TSH + alpha but only a minimal change in free beta or total proteins. In the absence of glucose, there was a selective accumulation of Man8GlcNAc on TSH + alpha but not on free beta or total proteins; however, there was no detectable accumulation of Endo H resistant forms during glucose starvation on TSH subunits or total proteins. Normal mouse and rat pituitary minces incubated for 60 min with either [3H]mannose or [3H]galactose showed no glucose-containing species on TSH subunits, but equal amounts of Man9GlcNAc and Man8GlcNAc on TSH + alpha, and mostly Man9GlcNAc on free beta subunits. In contrast, hypothyroid mouse and rat pituitaries exhibited an increase in Glc1Man9NAc and Glc1Man8GlcNAc on free beta but not on TSH + alpha or total proteins.(ABSTRACT TRUNCATED AT 250 WORDS)

Differential infection of receptor-modified host cells by receptor-specific influenza viruses

Influenza viruses of contrasting receptor specificity have been examined for their ability to infect receptor-modified MDCK cells containing sialyloligosaccharide receptor determinants of defined sequence. Cells were treated with sialidase to remove sialic acid and render them resistant to infection and were then incubated with sialyltransferase and CMP-sialic acid to restore sialic acid in the SA alpha 2,6Gal or SA alpha 2,3Gal linkages. The viruses A/RI/5 + /57 and A/duck/Ukraine/1/63, previously shown to exhibit preferential binding of SA alpha 2,6Gal and SA alpha 2,3Gal linkages, respectively, were found to exhibit differential infection of the receptor-modified cells in accord with their receptor specificity. Coinfection of SA alpha 2,3Gal derivatized cells with a mixture of the two viruses resulted in selective propagation of the SA alpha 2,3Gal-specific A/duck/Ukraine/1/63 virus. The results demonstrate the potential for cell surface receptors to mediate selection of receptor-specific variants of influenza virus.

Influence of cyclic nucleotides on receptor binding, immunological activity, and microheterogeneity of human choriogonadotropin synthesized in placental tissue culture

The regulation of the biosynthesis of choriogonadotropin (hCG) in tissue culture by human first trimester placenta in the presence of the following cyclic nucleotides and 3-isobutyl-1-methylxanthine (IBMX) was studied (concentrations in parentheses): IBMX (0.1 mM), cAMP (1 mM) + IBMX (0.1 mM), cGMP (0.1 mM) + IBMX (0.1 mM), 8-bromo-cAMP (0.5 mM) and 8-bromo-cGMP (0.5 mM). The medium concentration of hCG follows an optimum curve at all conditions, showing highest values at day 3 of the culture. The efficacy of the substances to cause an increase in the hCG medium concentration was in the following order: IBMX less than control less than 8-bromo-cGMP = 8-bromo-cAMP less than cGMP + IBMX less than cAMP + IBMX. The synthesized hCG was examined with respect to its receptor binding activity (LH/hCG receptor of rat testes), the activities to stimulate adenylate cyclase as well as testosterone biosynthesis in purified mouse Leydig cells, the immunological activity, and the microheterogeneity in isoelectric focusing. Only in the presence of 8-bromo-cAMP, 8-bromo-cGMP, and cGMP + IBMX was hCG synthesized, which differs significantly in the investigated properties from hCG of the control cultures. Only in the presence of 8-bromo-cGMP is the ratio of receptor binding activity/immunological activity optimal (near 1). In the presence of both 8-bromo-cAMP and 8-bromo-cGMP, microheterogeneity of hCG in isoelectric focusing was diminished and the synthesis of more acidic hCG subpopulations was favoured.

The common alpha subunit of bovine glycoprotein hormones: limited formation of native structure by the totally nonglycosylated polypeptide chain

The folding of the bovine glycoprotein hormone alpha subunit, synthesized in bacteria following insertion of the nucleotide sequence coding for this polypeptide, has been studied to determine the effect that a complete lack of carbohydrate has on this process. The bacterially derived alpha polypeptide (bac-alpha), extracted from E. coli in the presence of reductant and denaturant, had an estimated 0.2% native structure as determined by a conformationally sensitive radioimmunoassay. Upon reduction of disulfide bonds and reoxidation in air, the amount of native structure increased about 18-fold. Approximately 2% of the refolded bac-alpha preparation combines with the beta subunit of human chorionic gonadotropin (hCG beta) to form a complex that binds to the gonadotropin receptor and elicits a biological response. Since the correct folding (by immunological criteria) of bac-alpha (ca 3%) is significantly greater than expected from a random formation of disulfide bonds (0.1%), it appears that correct folding of alpha subunit can occur in the complete absence of carbohydrate, though in very low yield. Native bovine lutropin alpha subunit (LH alpha) and chemically deglycosylated LH alpha (which retains two asparagine-linked N-acetyl glucosamine residues per alpha oligosaccharide) were subjected to the same reduction/reoxidation regimen as the bacterially produced alpha subunit. As has been reported previously [Giudice LC, Pierce, JG, J Biol Chem 251: 6392, 1976] intact LH alpha fully regained its native structure. The partially deglycosylated LH alpha also refolds to a native-like structure in high yield as assessed by immunological assays and by its ability to combine with HCG beta to form a biologically active complex. The data show that carbohydrate, while not obligatory for correct folding, greatly facilitates the formation of functional alpha subunit.