The biological action of choriogonadotropin is not dependent on the complete native quaternary interactions between the subunits

Modulation of the steroidogenic related activity according to the design of single-chain bovine FSH analogs

Single-chain (SC) gonadotropins have been genetically engineered to increase the repertoire of analogs for potential use in humans and domestic animals. The major aim of the current study was to examine the steroidogenic related activity of SC FSH analogs carrying structural differences. To address this issue, we designed and expressed three SC bovine FSH analogs in CHO cells: (i) FSHβα in which the tethered subunit domains are linked in tandem; (ii) FSHβCTPα that contains the carboxy terminal peptide (CTP) of the human choriogonadotropin (hCG) β subunit as a spacer, and (iii) FSHβboCTPα in which the linker is derived from a CTP-like sequence (boCTP) decoded from the bovine LHβ DNA. The data suggested that the secretion efficiency of these variants from the transfected cells was unaffected by the presence or absence of the CTP linker, N-glycans were attached to the analogs and the hCGβ-CTP domain in the FSHβCTPα variant was O-glycosylated. In a rat immortalized granulosa cell bioassay the potency of the three variants towards progesterone secretion varied. In immature mice, the analogs increased the ovary weight and induced StAR, Cyp11a (P450scc), Cyp17 (P450c17) and Cyp19 (P450aromatase) transcripts. However, the dose dependence and amplitude of these transcript levels differed in response to FSHβα, FSHβboCTPα and FSHβCTPα. Collectively, these data suggest that the design of the FSH analog can modulate the bioactivity in vitro and in vivo. A systematic analysis of receptor activation with ligands carrying structural differences may identify new regulatory factor/s involved in the pleiotropic FSH activity.

The role of the 3' region of mammalian gonadotropin β subunit gene in the luteinizing hormone to chorionic gonadotropin evolution

CGβ subunits comprise a unique carboxyl-terminal peptide (CTP) that has multiple O-linked glycans and extends serum half-life of the protein. It has evolved by incorporating a previously untranslated region of the LHβ gene into the reading frame. Although CTP-like sequences are encrypted in the LHβ genes of several mammals, the CGβ subunit developed only in primates and equids. To study this restriction in evolution, we examined whether the cryptic CTP decoded from the bovine LHβ gene (boCTP) possesses key characteristics of the human (h) CGβ-CTP. The boCTP does not impede several crucial aspects of hormone biosynthesis, but compared to the hCGβ-CTP, the stretch lacks O-glycans and determinants for circulatory survival. O-glycan deficiency and the associated incapacity to extend serum half-life is a major drawback of the boCTP. This may explain why LH did not evolve into CG in ruminants and consequently alternative mechanisms evolved to delay luteolysis early in gestation.

A novel carboxyl-terminal heptapeptide initiates the regulated secretion of LH from unique sub-domains of the ER

The coordinated secretion of LH and FSH are critical for reproductive functions. After translocation into the endoplasmic reticulum (ER), their biosynthetic routes diverge at a determinative step prior to sorting in the regulated (LH) and constitutive (FSH) secretion pathways. Recently, we identified a C-terminal heptapeptide sequence, present only in the LHβ subunit, as a critical signal for entry of the LH dimer into the regulated pathway. We showed that an LHβ mutant lacking the heptapeptide (LHβΔT) assembled more efficiently with the α subunit than wild-type LHβ subunit, and this LHΔT dimer was secreted constitutively. Thus, an association exists between the presence of the C-terminal heptapeptide and sorting of the LH heterodimer to the regulated pathway. To study how this delayed LHβ subunit assembly is related to the trafficking of LH, we exploited the single subunit transfection model in rat somatotrope-derived GH₃ cells with the use of immunofluorescence confocal microscopy. The LHβ subunit showed a distinct immunofluorescent localization as compared to the FSHβ subunit and LHβ mutants. The wild-type LHβ subunit exhibited a perinuclear staining corresponding to the ER/nuclear envelope region. In contrast, the wild-type FSHβ subunit and the mutants LHβΔT and LHβL119A displayed no detectable perinuclear staining; only peripheral ER puncta were observed. Also, no perinuclear fluorescence was detected in cells expressing the LH heterodimer. We propose that the C-terminal heptapeptide is responsible for delayed heterodimer assembly within an ER sub-domain of the nuclear envelope, as an early partitioning event necessary for the entrance of LH into the regulated secretory pathway, whereas FSHβ does not traverse the nuclear envelope region. These data suggest that, at least for LH, the molecular decision to enter the regulated secretory pathway is a pre-Golgi event controlled by the novel C-terminal heptapeptide.

Single chain human chorionic gonadotropin, hCGalphabeta: effects of mutations in the alpha subunit on structure and bioactivity

The strategy of translationally fusing the subunits of heterodimeric proteins into single chain molecules is often used to overcome the mutagenesis-induced defects in subunit interactions. The approach of fusing the alpha and beta subunits of human Chorionic Gonadotropin (hCG) to produce a single chain hormone (phCGalphabeta) was used to investigate roles of critical residues of the alpha subunit in hormone receptor interaction and biological activity. The alpha subunit was mutated using PCR-based site-directed mutagenesis, fused to the wild type beta subunit and the fusion protein was expressed using Pichia pastoris expression system. Following partial purification, the mutant proteins were extensively characterized using immunological probes, receptor assays, and in vitro bioassays. The mutation hCGalpha P38A, which disrupts subunit interaction in the heterodimeric molecule, produced a fusion molecule exhibiting altered subunit interactions as judged by the immunological criteria, but could bind to the receptor with lower affinity and elicit biological response. Mutation of hCGalpha T54A disrupting the glycosylation at Asparagine 52, believed to be important for bioactivity, also yielded a biologically active molecule suggesting that the glycosylation at this site is not as critical for bioactivity as it is in the case of the heterodimer. The fusion protein approach was also used to generate a superagonist of hormone action. Introduction of four lysine residues in the Loop 1 of the alpha subunit led to the generation of a mutant having higher affinity for the receptor and enhanced bioactivity. Immunological characterization of single chain molecules revealed that the interactions between the subunits were not identical to those seen in the heterodimeric hormone, and the subunits appeared to retain their isolated conformations, and also retained the ability to bind to the receptors and elicit response. These data suggest the plasticity of the hormone-receptor interactions.

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.

Epitope mapping from real time kinetic studies - role of crosslinked disulphides and incidental interacting regions in affinity measurements: study with human chorionic gonadotropin and monoclonal antibodies

Real time kinetic studies were used to map conformational epitopes in human chorionic gonadotropin (hCG) for two monoclonal antibodies (MAbs). The epitopes were identified in the regions (alpha 5--14 and alpha 55--62). The association rate constant (k+1) was found to be altered by chemical modification of hCG, and the ionic strength of the reaction medium. Based on these changes, we propose the presence of additional interactions away from the epitope- paratope region in the hCG-MAb reaction. We have identified such incidental interacting regions (IIRs) in hCG to be the loop region alpha 35--47 and alpha 60--84. The IIRs contribute significantly towards the KA of the interaction. Therefore, in a macromolecular interaction of hCG and its MAb, KA is determined not only by epitopeparatope interaction but also by the interaction of the nonepitopic-nonparatopic IIRs. However, the specificity of the interaction resides exclusively with the epitope-paratope pair.

A single-chain tetradomain glycoprotein hormone analog elicits multiple hormone activities in vivo

We previously demonstrated that genetically linking one or more of the glycoprotein hormone-specific beta subunit genes to the common alpha subunit resulted in single-chain analogues that were bioactive in vitro. The ability of such large structures to bind their cognate receptors with high affinity supported the hypothesis that extensive flexibility exists between the ligand and receptor to establish a functional complex. To further characterize the extent of this conformational flexibility, we engineered a single-chain analogue that consists of sequentially linked thyroid-stimulating hormone (TSH) beta, follicle-stimulating hormone (FSH) beta, and chorionic gonadotropin (CG) beta subunits to the alpha subunit and expressed this chimera in transfected CHO (Chinese hamster ovary) cells. Because the four subunits are genetically linked and expressed as a single-chain, this analogue presumably lacks significant native structural features of the individual heterodimers. However, it exhibited FSH, CG, and TSH activities in vitro. Here, we test whether this nonnative structure would be stable in vivo and thus biologically active. Using a variety of bioassay protocols, we demonstrate that the analogue elicits multihormone activities when injected in vivo. First, treatment with the analogue caused increases in ovarian and uterine weights and resulted in elevated serum estradiol. Second, the analogue-stimulated ovarian follicle growth and pharmacologically rescued in vivo FSH deficiency similar to recombinant human FSH or equine CG (eCG) as confirmed by induction of aromatase in the ovaries of FSHbeta knockout mice. Third, in a superovulation protocol, when primed with eCG, the analogue elicited a dose-dependent ovulatory response comparable with that by native heterodimeric human CG. Finally, the analogue-stimulated thyroxin production in hypothyroid mice similar to the pituitary-derived human TSH standard. Based on these data, we conclude that a single-chain tetradomain glycoprotein hormone analogue, despite its presumed altered conformation, is stable and biologically active in vivo. Our results establish the permissiveness and conformational plasticity with which the glycoprotein hormones are recognized in vivo by their target cell receptors.

A single-chain bifunctional gonadotropin analog is secreted from Chinese hamster ovary cells as two distinct bioactive species

One of the major developments in exploring structure activity relationships of the glycoprotein hormone family was the genetic engineering of single chains comprised of the common alpha subunit and one or more of the hormone-specific beta subunits tandemly arranged. These studies indicate that there is a structural permissiveness in the quaternary relationships between the subunits and biological activity. However, the conformational relationships between the ligand and the receptor are unclear. Bifunctional triple-domain analogs represent an ideal model to address this issue. Does a single molecule possess the ability to simultaneously interact with both specific receptors or are there two functionally distinct species in the chimeric population? Here we show, using a preadsorption protocol comprised of Chinese hamster ovary cells expressing either the luteinizing hormone (LH)/chorionic gonadotropin (CG) or follicle-stimulating hormone (FSH) receptor, that at least two distinct bioactive populations of the dually active triple-domain chimera FSHbeta-CGbeta-alpha are synthesized, each corresponding to a single activity (CG or FSH). Furthermore, we show that these bioactive populations form distinct stable heterodimer-like contacts. That there is not a single biologically active species formed during synthesis of the chimera implies that in vivo the heterodimer exists in multiple conformations and is not a static rigid molecule.

Orientation of follicle-stimulating hormone (FSH) subunits complexed with the FSH receptor. Beta subunit toward the N terminus of exodomain and alpha subunit to exoloop 3

Follicle-stimulating hormone (FSH) comprises an alpha subunit and a beta subunit, whereas the FSH receptor consists of two halves with distinct functions: the N-terminal extracellular exodomain and C-terminal membrane-associated endodomain. FSH initially binds to exodomain, and the resulting FSH/exodomain complex modulates the endodomain and generates signal. However, it has been difficult to determine which subunit of FSH contacts the exodomain or endodomain and in what orientation FSH interacts with them. To address these crucial issues, the receptor was Ala-scanned and the hormone subunits were probed with photoaffinity labeling with receptor peptides corresponding to the N-terminal region of the exodomain and exoloop 3 of the endodomain. Our results show that both regions of the receptors are important for hormone binding and signal generation. In addition, the FSH beta subunit is specifically labeled with the N-terminal peptide, whereas the alpha subunit is labeled with the exoloop 3 peptide. These contrasting results show that the FSH beta subunit is close to the N-terminal region and that the alpha subunit is projected toward exoloop 3 in the endodomain. The results raise the fundamental question whether the alpha subunit, common among the glycoprotein hormones, plays a major role in generating the hormone signal common to all glycoprotein hormones.

Structural analysis of yoked chorionic gonadotropin-luteinizing hormone receptor ectodomain complexes by circular dichroic spectroscopy

Binding of the heterodimeric glycoprotein hormone, chorionic gonadotropin (CG), occurs to the heptahelical LH receptor N-terminal ectodomain (ECD), a large portion of which has been modeled as a leucine-rich repeat protein. In this study, we expressed and purified three single chain N-CG-ECD-C complexes, one comprising the full-length ECD, 1-341 (encoded by exons 1-10 and a portion of 11), and two C-terminal ECD deletion fragments, 1-294 (encoded by exons 1-10) and 1-180 (encoded by exons 1-7). The fusion proteins, including yoked CG (N-beta-alpha-C), were characterized by Western blot analysis and circular dichroism (CD). Analysis of the CD spectra obtained on the CG-ECD fusion proteins, and of the difference spectrum of each after subtracting the CG contribution, yielded secondary structures consistent with a repeating beta-strand/alpha-helix fold as predicted in the homology model. A marked decrease in helicity was observed when the C-terminal 47 amino acid residues were removed from the ECD. Removal of an additional 114 residues, i.e. the region encoded by exons 8-10, results in the loss of fewer helical residues. These results suggest that the hinge region of the ECD, predicted to contain only limited secondary structure, interacts with and stabilizes the ligand-occupied N-terminal portion. Furthermore, the results support a repeating fold, consistent with the proposed model for the LHR ECD.

Consequences of single-chain translation on the structures of two chorionic gonadotropin yoked analogs in alpha-beta and beta-alpha configurations

Human chorionic gonadotropin (hCG) is a placental-derived heterodimeric glycoprotein hormone, which, through the binding and activation of the LH receptor, rescues the corpus luteum and maintains pregnancy. The three-dimensional structure of hCG is known; however, the relevance of its fold to bioactivity is unclear. Although both subunits (alpha and beta) are required for activity, recent data with single-chain analogs have suggested a diminished role for the cystine knot and an intact heterodimeric interface in binding and receptor activation in vitro. Herein, we report the purification and structural characterization of two yoked (Y) hCG analogs, YhCG1 (beta-alpha) and YhCG3 (alpha-beta). The fusion proteins yielded higher IC50s and EC50s than those of hCG; the maximal hCG-mediated cAMP production, however, was the same. Circular dichroic spectroscopy revealed that the three proteins exhibit distinct far UV circular dichroic spectra, with YhCG1 containing somewhat more secondary structure than YhCG3 and hCG. Limited proteolysis with proteinase K indicated that heterodimeric hCG was much more resistant to cleavage than the single-chain analogs. YhCG1 was more susceptible to proteolysis than YhCG3, and the fragmentation patterns were different in the two proteins. Taken together, the data presented herein provide direct structural evidence for altered three-dimensional conformations in the two single-chain hCG analogs. Thus, the cognate G protein-coupled receptor can recognize and functionally respond to multiple ligand conformations.

Yoked complexes of human choriogonadotropin and the lutropin receptor: evidence that monomeric individual subunits are inactive

Human choriogonadotropin (hCG) contains an alpha-subunit, common to other members of the glycoprotein hormone family, and a unique beta-subunit that determines hormone specificity. It is generally thought that heterodimer formation is obligatory for full hormonal activity, although other studies have indicated that individual subunits and homodimeric hCGbeta were capable of low affinity binding to the LH receptor (LHR) and subsequent activation. Previously, we constructed two yoked hormone (hCG)-LHR complexes, where the two hormone subunits and the heptahelical receptor were engineered to form single polypeptide chains, i.e. N-beta-alpha-LHR-C and N-alpha-beta-LHR-C. Expression of both complexes led to constitutive stimulation of cAMP production. In the present study, we investigated whether the human alpha-subunit and hCGbeta can act as functional agonists when covalently attached to or coexpressed with the LH receptor. Our initial results showed that hCGbeta, but not alpha, was able to activate LHR with an increase in intracellular cAMP in human embryonic kidney 293 cells but not in Chinese hamster ovary or COS-7 cells. Further examination of this apparent cell-specific agonist activity of hCGbeta revealed that low levels of endogenous alpha-subunit were expressed in human embryonic kidney 293 cells, thus enabling sufficient amounts of active heterodimer to form with the transfected hCGbeta to activate LHR. The studies in Chinese hamster ovary and COS-7 cells clearly demonstrate that, even under experimental conditions where hormone-receptor interactions are maximized, individual subunits of hCG can not act as functional agonists, at least in their monomeric form.

Thyrotropin, follitropin, and chorionic gonadotropin expressed as a single multifunctional unit reveal remarkable permissiveness in receptor-ligand interactions

The glycoprotein hormones [chorionic gonadotropin (CG), FSH, LH, and TSH] are composed of a common alpha-subunit and a hormone-specific beta-subunit. Subunit assembly is vital to the in vivo function of these hormones. However, recent in vitro studies using double domain (beta-alpha) and triple domain (beta-beta-alpha) single chains have shown that gonadotropin receptor recognition can accommodate conformationally modified ligands. To investigate the extent of flexibility of ligand-receptor interactions, we constructed a single chain tetramer containing three different beta-subunits (TSHbeta, FSHbeta, and CGbeta) and a single alpha-subunit. This analog was inefficiently secreted from transfected Chinese hamster ovary cells, but surprisingly, the protein exhibited all activities comparable to the corresponding heterodimers. Because the alpha-subunit presumably cannot form the entire array of heterodimeric contacts with all beta-subunits simultaneously in the tetra-domain analog, the data show that the complete quaternary subunit-subunit interactions are essential for the efficient intracellular trafficking of the glycoprotein hormones, but not for receptor recognition. From an evolutionary perspective, the organization of such a multifunctional analog is consistent with the hypothesis that glycoprotein hormone genes were originally linked in tandem and subsequently evolved as independent genes. Our results also indicate that both gonadal and thyroid stimulatory functions can be combined in a unique analog.

Alternatively folded choriogonadotropin analogs. Implications for hormone folding and biological activity

Most heterodimeric proteins are stabilized by intersubunit contacts or disulfide bonds. In contrast, human chorionic gonadotropin (hCG) and other glycoprotein hormones are secured by a strand of their beta-subunits that is wrapped around alpha-subunit loop 2 "like a seatbelt." During studies of hCG synthesis in COS-7 cells, we found that, when the seatbelt was prevented from forming the disulfide that normally "latches" it to the beta-subunit, its carboxyl-terminal end can "scan" the surface of the heterodimer and become latched by a disulfide to cysteines substituted for residues in the alpha-subunit. Analogs in which the seatbelt was latched to residues 35, 37, 41-43, and 56 of alpha-subunit loop 2 had similar lutropin activities to those of hCG; that in which it was latched to residue 92 at the carboxyl terminus had 10-20% the activity of hCG. Attachment of the seatbelt to alpha-subunit residues 45-51, 86, 88, 90, and 91 reduced lutropin activity substantially. These findings show that the heterodimer can form before the beta-subunit has folded completely and support the notions that the carboxyl-terminal end of the seatbelt, portions of alpha-subunit loop 2, and the end of the alpha-subunit carboxyl terminus do not participate in lutropin receptor interactions. They suggest also that several different architectures could have been sampled without disrupting hormone activity as the glycoprotein hormones diverged from other cysteine knot proteins.

The position of the alpha and beta subunits in a single chain variant of human chorionic gonadotropin affects the heterodimeric interaction of the subunits and receptor-binding epitopes

The glycoprotein hormone family represents a class of heterodimers, which include the placental hormone human chorionic gonadotropin (CG) and the anterior pituitary hormones follitropin, lutropin, and thyrotropin. They are composed of common alpha subunit and a hormone-specific beta subunit. Based on the CG crystal structure, it was suggested that the quaternary subunit interactions are crucial for biological activity. However, recent observations using single chain glycoprotein hormone analogs, where the beta and alpha subunits are linked (NH(2)-CGbeta-alpha; CGbetaalpha orientation), implied that the heterodimeric-like quaternary configuration is not a prerequisite for receptor binding/signal transduction. To study the heterodimeric alignment of the two subunit domains in a single chain and its role in the intracellular behavior and biological action of the hormone, a single chain CG variant was constructed in which the carboxyl terminus of alpha was fused to the CGbeta amino terminus (NH(2)-alpha-CGbeta; alphaCGbeta orientation). The secretion rate of alphaCGbeta from transfected Chinese hamster ovary cells was less than that seen for CGbetaalpha. The alphaCGbeta tether was not recognized by dimer-specific monoclonal antibodies and did not bind to lutropin/CG receptor. To define if one or both subunit domains were modified in alphaCGbeta, it was co-transfected with a monomeric alpha or CGbeta gene. In each case, alphaCGbeta/alpha and alphaCGbeta/CGbeta complexes were formed indicating that CG dimer-specific epitopes were established. The alphaCGbeta/alpha complex bound to receptor indicating that the beta domain in the alphaCGbeta tether was still functional. In contrast, no significant receptor binding of alphaCGbeta/CGbeta was observed indicating a major perturbation in the alpha domain. These results suggest that although dimeric-like determinants are present in both alphaCGbeta/alpha and alphaCGbeta/CGbeta complexes, the receptor binding determinants in the alpha domain of the tether are absent. These results show that generating heterodimeric determinants do not necessarily result in a bioactive molecule. Our data also indicate that the determinants for biological activity are distinct from those associated with intracellular behavior.

Functional contributions of noncysteine residues within the cystine knots of human chorionic gonadotropin subunits

Human chorionic gonadotropin (hCG) is a heterodimeric member of a family of cystine knot-containing proteins that contain the consensus sequences Cys-X(1)-Gly-X(2)-Cys and Cys-X(3)-Cys. Previously, we characterized the contributions that cystine residues of the hCG subunit cystine knots make in folding, assembly, and bioactivity. Here, we determined the contributions that noncysteine residues make in hCG folding, secretion, and assembly. When the X(1), X(2), and X(3) residues of hCG-alpha and -beta were substituted by swapping their respective cystine knot motifs, the resulting chimeras appeared to fold correctly and were efficiently secreted. However, assembly of the chimeras with their wild type partner was almost completely abrogated. No single amino acid substitution completely accounted for the assembly inhibition, although the X(2) residue made the greatest individual contribution. Analysis by tryptic mapping, high performance liquid chromatography, and SDS-polyacrylamide gel electrophoresis revealed that substitution of the central Gly in the Cys-X(1)-Gly-X(2)-Cys sequence of either the alpha- or beta-subunit cystine knot resulted in non-native disulfide bond formation and subunit misfolding. This occurred even when the most conservative change possible (Gly --> Ala) was made. From these studies we conclude that all three "X" residues within the hCG cystine knots are collectively, but not individually, required for the formation of assembly-competent hCG subunits and that the invariant Gly residue is required for efficient cystine knot formation and subunit folding.

Disulfide bond mutations in follicle-stimulating hormone result in uncoupling of biological activity from intracellular behavior

The crystal structure of human CG reveals that each subunit is a member of the superfamily of cystine-knot growth factors. Although the distribution of the cysteine residues in all the beta-subunits is conserved, the conformation of the human FSH dimer differs from that of the CG/LH dimers. This suggests that the function of the cystine bonded loops in the human FSHbeta-subunit may differ from that in the CGbeta-subunit. To address this issue, we deleted two disulfide bonds in the FSHbeta domain: cys 20-104 and cys 28-82, which correspond to the disulfide bonds 26-110 and 34-88, respectively, in the CGbeta-subunit. The cys 26-110 bond is associated with the "seat-belt" region and cys 34-88 is a bond in the cystine knot. Coexpression of the wild-type alpha-subunit with the FSHbeta cysteine mutants in CHO cells revealed no detectable heterodimer. The FSHbeta mutants were then incorporated into a single chain where the beta-subunit is genetically fused to the alpha-subunit. In such a model, the rate-limiting subunit assembly step is by-passed and mutations that otherwise block heterodimer formation can be evaluated in terms of biological activity. Compared with the nonmutated single chain, the single-chain 28-82 mutant is secreted more slowly and its recovery is substantially reduced, whereas secretion and recovery of the 20-104 mutant was not significantly affected. The receptor binding affinity of the cys 28-82 mutant did not differ from wild-type and binding of the cys 20-104 mutant was decreased only 2-fold. The signal transduction data parallel the binding affinities, although the maximal accumulation of cAMP is less for the cys 20-104 mutant than that seen for cys 28-82 and nonmutated single-chains variants. These data support the hypothesis that the determinants for intracellular behavior and bioactivity of the gonadotropins are not the same, and that the cystine knot is a critical determinant for the formation of a stable, assembly-competent subunit. In addition, the data imply that the "seat-belt" conformation does not play a prominent role in the bioactivity of FSH.