Localization of residues that confer antibody binding specificity using human chorionic gonadotropin/luteinizing hormone beta subunit chimeras and mutants

Clinical significance and pitfalls of human chorionic gonadotropin-related tumor markers for intracranial germinomas

PURPOSE

Measuring serum and cerebrospinal fluid human chorionic gonadotropin (hCG) is essential for the diagnosis of intracranial germ cell tumors. There are three types of hCG-related markers in clinical use: hCGβ, intact hCG, and total hCG. The best marker for the diagnosis of intracranial germ cell tumors, especially germinoma, is currently unknown. This study aimed to evaluate the usefulness of these hCG-related markers.

METHODS

We investigated 19 serum samples obtained from 6 patients with histologically diagnosed germinoma treated in our institute. Serum hCGβ, intact hCG, and total hCG values were measured before, during, and after treatment. Samples with hCG values above the lower limits were considered positive.

RESULTS

The positivity rates of serum hCGβ, intact hCG, and total hCG were 6% (1/17), 47% (7/15), and 42% (8/19), respectively, with the latter two having significantly higher positivity rates than hCGβ (p = 0.041). Both intact and total hCGs showed similar values. The median values of hCGβ, intact hCG, and total hCG before treatment were 0.1 ng/mL, 4.6 mIU/mL, and 4.5 mIU/mL, respectively.

CONCLUSION

Serum intact and total hCGs have higher detection rates than hCGβ in patients with germinoma using available commercial measurement tools.

Structural biology of glycoprotein hormones and their receptors: insights to signaling

This article reviews the progress made in the field of glycoprotein hormones (GPH) and their receptors (GPHR) by several groups of structural biologists including ourselves aiming to gain insight into GPH signaling mechanisms. The GPH family consists of four members, with follicle-stimulating hormone (FSH) being the prototypic member. GPH members belong to the cystine-knot growth factor superfamily, and their receptors (GPHR), possessing unusually large N-terminal ectodomains, belong to the G-protein coupled receptor Family A. GPHR ectodomains can be divided into two subdomains: a high-affinity hormone binding subdomain primarily centered on the N-terminus, and a second subdomain that is located on the C-terminal region of the ectodomain that is involved in signal specificity. The two subdomains unexpectedly form an integral structure comprised of leucine-rich repeats (LRRs). Following the structure determination of hCG in 1994, the field of FSH structural biology has progressively advanced. Initially, the FSH structure was determined in partially glycosylated free form in 2001, followed by a structure of FSH bound to a truncated FSHR ectodomain in 2005, and the structure of FSH bound to the entire ectodomain in 2012. Comparisons of the structures in three forms led a proposal of a two-step monomeric receptor activation mechanism. First, binding of FSH to the FSHR high-affinity hormone-binding subdomain induces a conformational change in the hormone to form a binding pocket that is specific for a sulfated-tyrosine found as sTyr 335 in FSHR. Subsequently, the sTyr is drawn into the newly formed binding pocket, producing a lever effect on a helical pivot whereby the docking sTyr provides as the 'pull & lift' force. The pivot helix is flanked by rigid LRRs and locked by two disulfide bonds on both sides: the hormone-binding subdomain on one side and the last short loop before the first transmembrane helix on the other side. The lift of the sTyr loop frees the tethered extracellular loops of the 7TM domain, thereby releasing a putative inhibitory influence of the ectodomain, ultimately leading to the activating conformation of the 7TM domain. Moreover, the data lead us to propose that FSHR exists as a trimer and to present an FSHR activation mechanism consistent with the observed trimeric crystal form. A trimeric receptor provides resolution of the enigmatic, but important, biological roles played by GPH residues that are removed from the primary FSH-binding site, as well as several important GPCR phenomena, including negative cooperativity and asymmetric activation. Further reflection pursuant to this review process revealed additional novel structural characteristics such as the identification of a 'seat' sequence in GPH. Together with the 'seatbelt', the 'seat' enables a common heteodimeric mode of association of the common α subunit non-covalently and non-specifically with each of the three different β subunits. Moreover, it was possible to establish a dimensional order that can be used to estimate LRR curvatures. A potential binding pocket for small molecular allosteric modulators in the FSHR 7TM domain has also been identified.

Molecular dissection of an hCG-β epitope using single-step solid phase radioimmunoassay

BACKGROUND

Peptides and proteins have both sequence-specific (contiguous) and conformation-specific (discontiguous) epitopes. Sequence-specific epitopes are delineated by peptide approach and other robust methods like competition assays, gene expression assays, synthetic peptide library based assays etc. Available methods for delineation of conformation-specific epitopes are cumbersome (X-ray crystallography etc.), time-consuming and require costly sophisticated equipments. Hence, there is a need to develop a simple method for identification and mapping of conformation-specific epitopes.

METHOD

In the single-step solid phase radioimmunoassay (SS-SPRIA), an immunochemical bridge of 'mouse IgG-anti-mouse IgG' was prepared in the polypropylene wells followed by adsorption with hCG specific monoclonal antibody (MAb) G(1)G(10).1. The extent of competitive inhibition in binding ability of (125)IhCG-beta with chemically or enzymatically modified hCG-beta to immobilized MAb G(1)G(10).1 in comparison to hCG-beta standards was utilized to identify the epitopic amino acid involved in epitope-paratope interaction.

RESULTS

Data clearly suggest that the epitope under investigation consisted of Arg (94, 95) and Asp (99) at the core region with a Lys (104) and a His (106) in the proximity and absence of chymotrypsin susceptible Phe or Tyr in this region.

CONCLUSION

The data of SS-SPRIA revealed the 93-100 loop of amino acid sequence, as the core region of conformation-specific epitope of hCG-beta at or near the receptor-binding region. Hence, SS-SPRIA seems to be a simple method for identification and mapping of conformation-specific epitopes.

Prediction of orthologous relationship by functionally important sites

Making accurate functional predictions plays an important role in the era of proteomics. Reliable functional information can be extracted from orthologs in other species when annotating an unknown gene. Here a site-based approach called PORFIS is proposed to predict orthologous relationship. When applied to the bacterial transcription factor PurR/LacI family and the protein kinase AGC family, our method was able to identify, with few false positives, the important sites that agree with those verified by biological experiments. We also tested it on the alpha-proteasome family, the glycoprotein hormone family and the growth hormone family to demonstrate its ability to predict orthologous relationship. Compared with other prediction methods based on phylogenetic analysis or hidden Markov models, PORFIS not only has competitive prediction accuracy, but also provides valuable biological information of functionally important sites associated with orthologs which can be further studied in biological experiments.

Only a Portion of the Small Seatbelt Loop in Human Choriogonadotropin Appears Capable of Contacting the Lutropin Receptor

Twenty residues of the human choriogonadotropin (hCG) beta-subunit that are wrapped around alpha-subunit loop 2 like a "seatbelt" stabilize the heterodimer and enable the hormone to distinguish lutropin (LHR), follitropin, and thyrotropin receptors. The N-terminal portion of the seatbelt contains a small disulfide-stabilized loop needed for heterodimer assembly and is thought to mediate hCG-LHR interactions. To test the latter notion, we compared the LHR binding and signal transduction activities of hCG analogs in which the alpha-subunit C terminus (alphaCT) was cross-linked to residues in the small seatbelt loop. Analogs having an intersubunit disulfide between a cysteine in place of alphaCT residue alphaSer-92 and cysteines substituted for loop residues betaArg-94, betaArg-95, or betaSer-96 had high activities in LHR binding and signaling assays despite the fact that both portions of the hormone are thought to be essential for hCG activity. Use of a larger probe blocked hormone activity when the alphaCT was cross-linked to cysteines in place of residues betaArg-95 and betaAsp-99, but not to cysteines in place of residues betaArg-94, betaSer-96, or betaThr-97. This suggested that the side chains of residues betaArg-95 and betaAsp-99, which face in the same outward direction from the heterodimer, are nearer than the others to the LHR interface. The finding that residue 95 can be cross-linked to small alphaCT probes without eliminating hormone activity indicates its side chain does not participate in essential LHR contacts. We suggest that contacts between the small seatbelt loop and the LHR, if any, involve its backbone atoms and possibly the side chain of residue betaAsp-99.

Model of Glycoprotein Hormone Receptor Ligand Binding and Signaling

Studies described here were initiated to develop a model of glycoprotein hormone receptor structure and function. We found that the region that links the lutropin receptor leucine-rich repeat domain (LRD) to its transmembrane domain (TMD) has substantial roles in ligand binding and signaling, hence we term it the signaling specificity domain (SSD). Theoretical considerations indicated the short SSDs in marmoset lutropin and salmon follitropin receptors have KH domain folds. We assembled models of lutropin, follitropin, and thyrotropin receptors by aligning models of their LRD, TMD, and shortened SSD in a manner that explains how substitutions in follitropin and thyrotropin receptors distant from their apparent ligand binding sites enable them to recognize lutropins. In these models, the SSD is parallel to the concave surface of the LRD and makes extensive contacts with TMD outer loops 1 and 2. The LRD appears to contact TMD outer loop 3 and a few residues in helices 1, 5, 6, and 7. We propose that signaling results from contacts of the ligands with the SSD and LRD that alter the LRD, which then moves TMD helices 6 and 7. The positions of the LRD and SSD support the notion that the receptor can be activated by hormones that dock with these domains in either of two different orientations. This would account for the abilities of some ligands and ligand chimeras to bind multiple receptors and for some receptors to bind multiple ligands. This property of the receptor may have contributed significantly to ligand-receptor co-evolution.

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.

Use of protein knobs to characterize the position of conserved alpha-subunit regions in lutropin receptor complexes

Efforts to identify the manner in which human choriogonadotropin (hCG) contacts lutropin receptors (LHR) have been stymied by the complex structure of the hormone and the likelihood that it contacts the receptor at multiple sites. During studies of hCG assembly in mammalian cells, we found that addition of a cysteine to the long disordered beta-subunit COOH terminus (betaCT) enabled it to become cross-linked by a disulfide to cysteines that are substituted for residues in loop alpha2 or in the alpha-subunit COOH terminus (alphaCT). This created a "knob" on the alpha-subunit at the location of the cysteine. Knobs of various sizes and charges were useful for probing surfaces of the alpha-subunit thought previously to contact the LHR. Attachment of the betaCT to residues in loop alpha2 facing loops beta1 and beta3 reduced hormone activity only a few fold revealing that this surface does not participate in essential high affinity receptor contacts, a finding inconsistent with our earlier view of the hCG-LHR complex. In contrast, this approach showed that the opposite surface of loop alpha2 appeared to be nearer the receptor interface. Although attachment of knobs to portions of the alphaCT reduced hormone activity substantially, this finding was difficult to interpret. As discussed, this procedure should be adapted readily to other proteins and may facilitate the introduction of fluorophores, enzymes, or other reagents at specific sites on protein surfaces. It may also permit one to cross-link proteins or to obscure specific protein surfaces during the development of "Trojan Horse" therapeutics.

Glycoprotein hormone assembly in the endoplasmic reticulum: II. Multiple roles of a redox sensitive beta-subunit disulfide switch

All three human glycoprotein hormone heterodimers are assembled in the endoplasmic reticulum by threading the glycosylated end of alpha-subunit loop two (alpha2) beneath a disulfide "latched" strand of the beta-subunit known as the "seatbelt." This remarkable event occurs efficiently even though the seatbelt effectively blocks the reverse process, thereby stabilizing each heterodimer. Studies described here show that assembly is facilitated by the formation, disruption, and reformation of a loop within the seatbelt that is stabilized by the most easily reduced disulfide in the free beta-subunit. We refer to this disulfide as the "tensor" because it shortens the seatbelt, thereby securing the heterodimer. Formation of the tensor disulfide appears to precede and facilitate seatbelt latching in most human choriogonadotropin beta-subunit molecules. Subsequent disruption of the tensor disulfide elongates the seatbelt, thereby increasing the space beneath the seatbelt and the beta-subunit core. This permits the formation of hydrogen bonds between backbone atoms of the beta-subunit cystine knot and the tensor loop with backbone atoms in loop alpha2, a process that causes the glycosylated end of loop alpha2 to be threaded between the seatbelt and the beta-subunit core. Contacts between the tensor loop and loop alpha2 promote reformation of the tensor disulfide, which explains why it is more stable in the heterodimer than in the uncombined beta-subunit. These findings unravel the puzzling nature of how a threading mechanism can be used in the endoplasmic reticulum to assemble glycoprotein hormones that have essential roles in vertebrate reproduction and thyroid function.

Tight attachment of chitin-binding-domain-tagged proteins to surfaces coated with acetylated chitosan

Several excellent procedures for trapping tagged proteins have been devised, but many of these are expensive, cannot be used outside a limited pH range, fail to work in the presence of chaotropic agents, or are difficult to use. The chitin binding domain (CBD) of Bacillus circulans chitinase, which binds to chitin matrices prepared from inexpensive reagents isolated from crab shells, is an alternative tag that can be used under a variety of pH and denaturing conditions. Kits based on the interaction between the CBD and the chitin beads are available commercially. Here, we show that simultaneous treatment of microtiter plates with chitosan, a deacetylated form of chitin, and acetic anhydride produces a surface-bound film of chitin that also interacts tightly with the CBD. Chitin-coated microtiter well plates captured a CBD-tagged heterodimeric human glycoprotein hormone analog directly from mammalian cell culture media, even when present in trace amounts. Binding to the surface was stable in sodium dodecylsulfate and reversed only partially at low pH or in 8M urea at 37 degrees C. This technique appears well suited to surface attachment and permits biochemical or other analyses of molecules that can be tagged with a CBD.

Biological Activities of Tethered Equine Chorionic Gonadotropin (eCG) and Its Deglycosylated Mutants

Equine chorionic gonadotropin (eCG), which consists of highly glycosylated alpha- and beta-subunits, is a unique member of the gonadotropin family because it elicits response characteristics of both follicle-stimulating hormone (FSH) and luteinizing hormone (LH) in species other than the horse. In this study, recombinant tethered-eCG as well as its deglycosylated mutants were produced to determine if alpha- and beta- subunits can be synthesized as a single polypeptide chain (tethered-eCG) and display biological activity. We found that tethered-eCG (T- betaalpha) had both LH- and FSH-like activities comparable to dimeric eCG. Luteinizing hormone-like activity of tethered-eCGs deglycosylated at Asn(56) (T-betaalpha56) was decreased. In contrast, LH-like activity of eCG without O-glycosylated carboxyl-terminal peptide (CTP) (T-betacalpha) was slightly decreased but still similar to T-betaalpha. Double mutation at Asn(56) and CTP (T-betacalpha56) caused marked decrease in the activity, indicating that both glycosylations at Asn(56) and CTP are involved in LH-like activity in the tethered form. Interestingly, FSH-like activity remained in all deglycosylated eCG mutants (T-betaalpha56, T-betacalpha and T-betacalpha56) as well as T-betaalpha. The biological roles of oligosaccharides at Asn(56) of eCG alpha-subunit and O-linked peptide of beta-subunit appear to be different in LH- and FSH-like activities in tethered-eCG.

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.

Immunochemical measurement of early pregnancy isoforms of HCG: potential applications to fertility research, prenatal diagnosis, and cancer

Human chorionic gonadotropin, the glycoprotein hormone of pregnancy, is found naturally in blood and urine in a variety of isoforms. These variants are related to both peptide bond cleavages (such as the nicked forms of hCG) and the beta core fragment urinary metabolite, as well as the larger variety of species resulting from carbohydrate heterogeneity. We have recently developed immunoassay systems that can measure nicked forms of hCG (antibody B151) as well as particular high carbohydrate variants (hyperglycosylated forms) of hCG (B152), which are associated with cancers producing hCG. Using the assay system for nicked hCG, we found that nicked hCG does not appear to be present as a significant hCG isoform during normal pregnancies if the urine specimens are well preserved. Applying the assay for hyperglycosylated hCG isoforms, we discovered that these forms are prevalent during very early pregnancy and decline rapidly to low concentration after the first 6 weeks of pregnancy. Persistence of these early pregnancy forms does not bode well for the pregnancy. Other investigators report that measurement of such hCG isoforms may aid in diagnosis of Down syndrome pregnancies. In summary, measurement of the hyperglycosylated hCG isoforms are useful for evaluation of healthy progress of normal pregnancy, as an additional detection marker for Down syndrome pregnancies, and as a potential new marker of trophoblastic malignancy. New reference preparations will soon be available for the calibration of assay systems for measurement of many of these hCG variants and metabolites.

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.

The surface of alpha-subunit loop 1 distant from the subunit interface is exposed in the hCG lutropin receptor complex

Interactions of the placental glycoprotein hormone human choriogonadotropin (hCG) with lutropin receptors (LHR) are required for maintenance of early pregnancy. Knowledge of how hCG interacts with LHR is useful for understanding the mechanism of receptor function, an issue of considerable debate. A large surface of hCG remains exposed after the hormone binds the LHR and can be readily detected with monoclonal antibodies. Here we show that the surface of hCG alpha-subunit loop 1 furthest from the beta-subunit interface can also be recognized by a monoclonal antibody when hCG is bound to the LHR. This extends the area of hCG known to be exposed in the hormone receptor complex, an observation that further restricts models of hCG-LHR interaction.

Deglycosylation of a bifunctional lutropin-follitropin agonist reduced its follitropin activity more than its lutropin activity

OBJECTIVE

To design a drug that blocks the gonadal actions of lutropins and follitropins.

DESIGN

Controlled in vitro study.

SETTING

Academic laboratory.

PATIENT(S)

None.

INTERVENTION(S)

We removed three glycosylation signals from an hCG-hFSH chimera known to have high affinity for LH and FSH receptors, expecting this would create a bifunctional antagonist (dgCFC). To offset the inhibition of subunit combination caused by deglycosylation of alpha-subunit loop 2, we prepared dgCFC as a single-chain fusion protein containing the alpha-subunit downstream of the chimeric beta-subunit.

MAIN OUTCOME MEASURE(S)

Receptor binding, cyclic adenosine monophosphate accumulation.

RESULT(S)

dgCFC bound LH or FSH receptors similar to hCG or hFSH. It was a partial agonist and had one tenth the efficacy of hFSH and two thirds the efficacy of hCG.

CONCLUSION(S)

The surprising high residual lutropin activity of dgCFC indicated that its FSH residues offset the effects of deglycosylation, suggesting this approach to preparing a bifunctional antagonist is unlikely to lead to a useful drug. The increased lutropin efficacy of dgCFC relative to deglycosylated hCG supports the idea that oligosaccharides modulate glycoprotein hormone efficacy through an influence on hormone conformation.

Synthesis of multi-subunit domain gonadotropin complexes: a model for alpha/beta heterodimer formation

The human glycoprotein hormones chorionic gonadotropin (CG), thyrotropin (TSH), lutropin (LH), and follitropin (FSH) are heterodimers, composed of a common alpha subunit assembled to a hormone-specific beta subunit. The subunits combine noncovalently early in the secretory pathway and exist as heterodimers, but not as multimers. Little information is available regarding the steps associated with the assembly reaction. It is unclear if the initial alpha beta engagement results either in the formation of only mature heterodimer or if the nascent complex is reversible and can undergo an exchange of subunits or combine transiently with an additional subunit. This is relevant for the case of LH and FSH, because both are synthesized in the same cell (i.e., pituitary gonadotrophs) and several of the alpha subunit sequences required for association with either the LH beta or FSH beta subunits are different. Such features could favor the generation of short-lived, multi-subunit forms prior to completion of assembly. Previously, we showed that the CG beta or FSH beta subunit genes can be genetically fused to the alpha gene to produce biologically active single chains, CG beta alpha and F beta alpha, respectively. Studies using monoclonal antibodies sensitive to the conformation of the hCG subunits suggested that in contrast to the highly compact heterodimer, the interactions between the beta and alpha domains in the single chain are in a more relaxed configuration. That the tethered domains do not interact tightly predicts that they could combine with an additional subunit to form triple domain complexes. We tested this point by cotransfecting CHO cells with the genes encoding F beta alpha and the CG beta subunit or the CG beta alpha and FSH beta monomer. The CG beta subunit combined noncovalently with F beta alpha to form a F beta alpha/CG beta complex. Ternary complex formation was not restricted to a specific set of single chain/monomeric subunit, because a CG beta alpha/FSH beta complex was also detected implying that triple domain intermediates could be transiently generated along the secretory pathway. Monoclonal antibodies specific for the CG heterodimer recognized the F beta alpha/CG beta complex, which suggests that the epitopes unique for dimeric CG were established. In addition, media containing F beta alpha/CG beta displayed high-affinity binding to both CG and FSH receptors. The presence of CG activity is presumptive for the existence of a functional F beta alpha/CG beta complex, because neither F beta alpha nor the uncombined CG beta subunit binds to CG receptor. These data show that the alpha subunit of the tether, although covalently linked to the FSH beta domain, can functionally interact with a different beta subunit implying that the contacts in the nascent alpha beta dimer are reversible. The formation of a functional single chain/subunit complex was not restricted to the FSH single chain/CG beta subunit since CG single chain interacts with the monomeric FSH beta subunit and exhibits FSH activity. The presence of the triple domain configuration does not abolish bioactivity, suggesting that although the gonadotropins are heterodimers, the cognate receptor is capable of recognizing a larger ligand composed of three subunit domains.

A large non-immunized human Fab fragment phage library that permits rapid isolation and kinetic analysis of high affinity antibodies

We report the design, construction, and use of the first very large non-immunized phage antibody library in Fab format, which allows the rapid isolation and affinity analysis of antigen-specific human antibody fragments. Individually cloned heavy and light chain variable region libraries were combined in an efficient two-step cloning procedure, permitting the cloning of a total of 3.7 x 10(10) independent Fab clones. The performance of the library was determined by the successful selection of on average 14 different Fabs against 6 antigens tested. These include tetanus toxoid, the hapten phenyl-oxazolone, the breast cancer-associated MUC1 antigen, and three highly related glycoprotein hormones: human chorionic gonadotropin, human luteinizing hormone, and human follicle-stimulating hormone. In the latter category, a panel of either homone-specific or cross-reactive antibodies were identified. The design of the library permits the monitoring of selections with polyclonal phage preparations and to carry out large scale screening of antibody off-rates with unpurified Fab fragments on BIAcore. Antibodies with off-rates in the order of 10(-2) to 10(-4) s-1 and affinities up to 2.7 nM were recovered. The kinetics of these phage antibodies are of the same order of magnitude as antibodies associated with a secondary immune response. This new phage antibody library is set to become a valuable source of antibodies to many different targets, and to play a vital role in target discovery and validation in the area of functional genomics.

Addition of an N-terminal dimerization domain promotes assembly of hCG analogs: implications for subunit combination and structure-function analysis

Human chorionic gonadotropin (hCG) is a heterodimeric placental glycoprotein hormone that acts through ovarian lutropin receptors (LHR) to maintain early pregnancy. Its ability to distinguish LHR and follitropin receptors (FSHR) is controlled by 20 beta-subunit 'seatbelt' residues that surround alpha-subunit loop 2. Positively charged amino acids between residues 93-100, a small loop within the seatbelt, have been postulated to make essential LH receptor contacts. Previous studies showed that analogs containing negatively charged amino acids in this small loop had 5-10% the activity of hCG and 1-10% the lutropin activities of hCG/hFSH chimeric analogs capable of binding LHR and FSHR. These effects might be due to the influence of these residues on receptor contacts or on hormone conformation. During efforts to distinguish these possibilities, we increased and decreased the number of residues in this loop, mutations we anticipated would distort its conformation. Consistent with this supposition, these changes inhibited dimer formation, precluding assessment of these mutations on hormone activity. Addition of Fos and Jun dimerization domains to the N-termini of hCGalpha- and hCG/hFSHbeta-subunit chimeras overcame the effects of the seatbelt mutations on subunit combination and enabled preparation of heterodimers containing six, seven, or nine residues in their seatbelt loops. These had 0.1-10% the lutropin and 3-60% the follitropin activities of bifunctional chimeras containing 8 residues derived from hCG in the seatbelt loop. The abilities of N-terminal dimerization domains to promote subunit combination may permit structure/function analysis of other residues that influence heterodimer formation.

Expression and characterization of recombinant beta-subunit hCG homodimer

We have linked two human chorionic gonadotropin (hCG) beta-subunit cDNAs in tandem such that the expressed fusion protein consists of two mature beta-subunits joined through the carboxy terminal peptide of the first beta-subunit. A single glycine residue is inserted between the two subunits in the fusion protein. Chinese hamster ovary (CHO) cells transformed with a clone that contains the fused cDNAs express and secrete a protein that is consistent with it being a beta-hCG homodimer protein. These beta-homodimer molecules can recombine with two free alpha-subunits indicating that both beta-subunits within the homodimer are likely folded in their native conformation. Our data also suggest that the two beta-subunits fold upon each other as a globular protein and do not appear to exist as a simple fusion of two linear beta-subunits. Furthermore, the two beta-monomer subunits in the fusion protein form a stable homodimer that can bind and activate the hLH/CG receptor specifically. Recombination of the fusion protein with alpha-subunits appears to favor an arrangement where two alpha-subunits combine with a single molecule of the fusion protein. The recombined molecule consists of four subunits and is comparable to two tethered hCG moieties, which constitutes a hCG dimer. This hormone dimer can bind and activate the hLH/CG receptor with an activity approximating that of native hCG.

Epitope-specific focusing of the immune response to a minimized human chorionic gonadotropin analog

Minimized proteins have long been used to elicit an immune response to particular regions of a protein antigen. Most efforts to derive minimized proteins have employed synthetic peptide fragments. This approach works well for linear epitopes but poorly for conformational epitopes. Here we describe a homodimeric human chorionic gonadotropin (hCG) analog that retains the conformation of related parts of hCG and elicits high affinity specific antibodies. This novel immunogen displays the tertiary structure of selected loops of the protein but lacks structures that could elicit potentially undesirable antibodies.

Cystine knot of the gonadotropin alpha subunit is critical for intracellular behavior but not for in vitro biological activity

The common alpha subunit of glycoprotein hormones contains five disulfide bonds. Based on the published crystal structure, the assignments are 7-31, 59-87, 10-60, 28-82, and 32-84; the last three comprise the cystine knot, a structure also seen in a variety of growth factors. Previously, we demonstrated that the efficiency of secretion and the ability to form heterodimers by alpha subunits bearing single cysteine residue mutants in the cystine knot were significantly reduced. These results suggested that the cystine knot is critical for the intracellular integrity of the subunit. To assess if the presence of the free thiol affected the secretion kinetics, we constructed paired cysteine mutants of each disulfide bond of the alpha subunit. The secretion rate for these monomers was comparable with wild type except for the alpha-10-60 mutant, which was 40% lower. The recovery of the alpha7-31 and alpha59-87 mutants was greater than 95%, whereas for the cystine knot mutants, it was 20-40%. Co-expression of the wild-type chorionic gonadotropin beta subunit with double cysteine mutants did not enhance the recovery of alpha mutants in the media. Moreover, compared with wild-type, the efficiency of heterodimer formation of the alpha10-60 or alpha32-84 mutants was less than 5%. Because subunit assembly is required for biological activity, studies on the role of these disulfide bonds in signal transduction were not possible. To bypass the assembly step, we exploited the single chain model, where the alpha and beta subunits are genetically fused. The recovery of secreted tethered gonadotropins bearing mutations in the cystine knot was increased significantly. Although dimer-specific monoclonal antibodies discriminated the conformation of single chain alpha10-60 and alpha32-84 mutants from the native heterodimer, these mutants were nevertheless biologically active. Thus, individual bonds of cystine knot are important for secretion and heterodimer formation but not for in vitro bioactivity. Moreover, the data suggest that the native heterodimer configuration is not a prerequisite for receptor binding or signal transduction.

Chimeric proteins can exceed the sum of their parts: implications for evolution and protein design

Chimeric analogs derived from pairs of homologous proteins routinely exhibit activities found in one or both parents. We describe chimeras of two glycoprotein hormones, human chorionic gonadotropin (hCG) and human follitropin (hFSH), that exhibit activity unique to a third family member, human thyrotropin (hTSH). The results show that biological activity can be separated from hormone-specific amino acid residues. This is consistent with a model for the evolution of homologous ligand-receptor pairs involving gene duplication and the creation of inhibitory determinants that restrict binding. Disruption of these determinants can unmask activities characteristic of other members of a protein family. Combining portions of two ligands to create analogs with properties of a third family member can facilitate identifying key determinants of protein-protein interaction and may be a useful strategy for creating novel therapeutics. In the case of the glycoprotein hormones, this showed that two different hormone regions (i.e., the seat-belt and the intersubunit groove) appear to limit inappropriate contacts with receptors for other members of this family. These observations also have important caveats for chimera-based protein design because an unexpected gain of function may limit the therapeutic usefulness of some chimeras.

Influence of subunit interactions on lutropin specificity. Implications for studies of glycoprotein hormone function

Bovine lutropin (bLH) and human chorionic gonadotropin (hCG) are heterodimeric glycoprotein hormones required for reproduction. Both bind rat LH receptors (rLHRs), but hCG binds human LH receptors (hLHRs) 1000-10,000 fold better than bLH. We tested the premise that this difference in affinity could be used to identify lutropin receptor contacts. Heterodimers containing hCG/bLH alpha- or beta-subunit chimeras that bound hLHR like hCG (or bLH) were expected to have hCG (or bLH) residues at the receptor contact sites. Analogs containing one subunit derived from hCG bound hLHR much more like hCG than bLH, indicating that each bLH subunit contains all the residues sufficient for high affinity hLHR binding. Indeed, the presence of bovine alpha-subunit residues increased the activities of some hCG analogs. The low hLHR activity of bLH was due primarily to an interaction between its alpha-subunit and beta-subunit residue Leu95. Leu95 does not appear to contact the hLHR since it did not influence the hLHR activity of heterodimers containing human alpha-subunit. These observations show that interactions within and between the subunits can significantly influence the activities of lutropins, thereby confounding efforts to identify ligand residues that contact these receptors.

Immunochemical mapping of gonadotropins

As a glycoprotein hormone, human chorionic gonadotropic (hCG) is not a single molecular entity but this term rather comprises an array of molecular variants such as hCG, hCG beta, hCGn, hCG beta n, hCG beta cf, -CTPhCG, hCG beta CTP, deglyhCG, asialohCG, hCGav and the closely related molecules hLH, hLH beta and hLH beta ef. The advent of monoclonal antibodies (MCA), the availability of ultrasensitive detection systems and the recent determination of the crystal structure of hCG, made it possible to design special purpose diagnostic and clinical research immunoassays for hCG-like molecules. For more than a decade we and others have tried to refine epitope maps for hCG and related molecules by means of a large panel of MCA, naturally occurring metabolic variants of hCG (hCGn, hCG beta, hCG alpha, hCG beta cf, hCG beta CTP), homologous hormones and subunits of various species (e.g. hLH, hLH beta, hFSH, hTSH, oLH, rLH beta), chemically modified molecules (deglyhCG, asialohCG, tryptic and chymotryptic hCG beta and hCG alpha fragments) and synthetic peptides (octapeptides and longer). It appeared that all epitopes on molecular hCG-variants recognized by our MCA are determined by the protein backbone. Except for the two major epitopes on hCG beta CTP and parts of two antigenic domains on hCG alpha, epitopes on hCG-derived molecules are determined by the tertiary and quarternary structure. Operationally useful descriptive epitope maps were designed including information on assay suitability of antigenic determinants. On this basis we established ultrasensitive time-resolved fluoroimmuno-assays for hCG, hCG and hCGn, hCG beta and hCG beta n and hCG beta cf, hCG alpha and additional assays recognizing different spectra of hCG-variants. Such assay have been applied by us and others to the detection of pregnancy, early pregnancy loss, choriocarcinoma, testicular cancer, other cancers and prenatal diagnosis. However, as the molecular structure of many epitopes utilized in immunoassays of different laboratories was not resolved, comparability of results was not satisfactory. Consequently, attempts were made to compare schematic epitope maps from different research institutions. The situation has been much improved by solving the three-dimensional (3D) structure of hCG. It has been shown that hCG is a member of the structural superfamily of cystine knot growth factors like NGF, PDGF-B and TGF-beta. Each of its subunits is stabilized in its topology by three disulfide bonds forming a cystine knot. Moreover, it turned out that the disulfide bridges in their majority have previously been wrongly assigned. Computer molecular modeling of crystallographic coordinates of hCG and subsequent selective combined--PCR-based and immunological--mutational analyses of hCG beta expressed via the transmembrane region of a MHC molecule made it possible to more precisely localize epitopes on hCG-derived molecules. Although the entire surface of hCG has to be regarded as potentially immunogenic there seems to be hot spots where epitopes are clustered in antigenic domains. These are located on the first and third loops protuding from the cystine knots of both subunits and are possibly centered around the knot itself. Ultimate answers on epitope localizations will be given by the crystal structure determination of hCG complexed with different Fabs.

hCG beta residues 94-96 alter LH activity without appearing to make key receptor contacts

The ability of human chorionic gonadotropin (hCG) to distinguish lutropin (LHR) and follitropin (FSHR) receptors is controlled principally by beta-subunit residues 94-117. To learn how residues 94-96 (Arg-Arg-Ser) influence LHR binding, we studied the effects of replacing them on the LH and FSH activities of a bifunctional hCG analog in which residues 101-109 were derived from FSH. Analogs containing 1-3 arginines and no aspartates at residues 94-96 bound LHR with 25-400% the potency of hCG. When residues 94-96 were neutral or contained 1-3 aspartates, LHR binding was reduced 6-100 fold but remained at least ten-fold greater than the negative control analog containing residues 94-117 derived from FSH. Residues 94-96 had little influence on FSHR binding. These observations support a model [Moyle et al. (1995) J. Biol. Chem. 270:20,020] in which residues 94-96 influence LHR binding specificity primarily through an effect on hormone conformation rather than by direct participation in essential high affinity receptor contacts.

Contribution of Specific Disulphide Bonds to Two Epitopes of Thyrotropin beta-Subunit Associated with Receptor Recognition

In previous studies, we have shown that two epitopes of bovine thyrotropin beta-subunit that are recognised by the monoclonal antibodies designated mAb 279 and mAb 299 are also associated with the receptor-binding site of bovine thyrotropin. The present investigation has examined the role of the six disulphide bonds of bovine thyrotropin beta-subunit in the conformational stabilisation of these two epitopes, and hence assessed the relative contribution that these disulphide bonds make to the stabilisation of the receptor-binding region of the beta-subunit. The experimental procedure involved the production of several bovine thyrotropin beta-subunit-related derivatives in which an increasing number of the disulphide bonds were selectively reduced with dithiothreitol and alkylated with iodoacetic acid. Antibody-binding properties of these derivatives were then evaluated in thyrotropin beta-subunit-specific immunoassays based on the use of the well characterised mAb 279 and mAb 299, to determine the effect of disulphide bond reduction and alkylation on each epitopic specificity. In separate experiments, the residual disulphide bonds that remained intact following these selective partial reductive alkylation procedures were then fully reduced and alkylated with the fluorescent reagent 5-N-[(iodoacetamidoethyl)amino]naphthalene 1-sulphonic acid. The relative contribution of individual disulphide bonds in the stabilisation of each epitope could then be assessed after application of reverse-phase HPLC peptide mapping methods. Epitope recognition by mAb 279 was not dependent on the preservation of the so-called determinant loop Cys88-Cys95 disulphide bond nor directly involved binding interactions via the Cys2-Cys52, Cys27-Cys83, and Cys31-Cys85 disulphide bonds. However, the experimental results indicated that the mAb 279 epitope was stabilised by the Cys19-Cys105 and Cys16-Cys67 disulphide bonds, which is consistent with other data on the role of the C-terminal region of the thyrotropin beta-subunit in this epitope. In contrast, the presence of an intact Cys88-Cys95 disulphide bond was required for the stabilisation of the mAb 299 epitope, although the location of this disulphide bond is distal to the hairpin loop structure that constitutes the mAb 299 epitope. These results on the relative contribution of these disulphide bonds are also discussed in terms of their relationship to the stabilisation of the predicted region of bovine thyrotropin beta-subunit involved in receptor binding.

Relative location of epitopes involved in synergistic antibody binding using human chorionic gonadotropin as a model

We systematically screened a large panel of well-characterized monoclonal antibodies (mAb) directed towards various epitopes on human chorionic gonadotropin (hCG) for synergistic binding of 125I-hCG when they were adsorbed to a solid phase. The epitope locations involved in synergy were then related to the crystal structure of hCG and discussed in accordance with available data on the hCG epitopes. Enhanced binding of hCG was specific for certain pairs of mAb and was reflected in a 3-50-fold increased apparent functional affinity constant for hCG. Surface plasmon resonance revealed that when the mAb were captured by a polyclonal anti-IgG1 coupled to the Biacore chip, the off rates for hCG were significantly slower with synergistic mAb combinations than for the corresponding single mAb or nonsynergistic pairs of mAb, whereas the on rates did not differ appreciably. Each of the two antibodies involved in synergistic binding of hCG (more than 3-fold compared to additive binding of the two mAb) always belonged to a different epitope cluster in a separate antigenic domain on hCG. Synergistic epitope combinations on holo-hCG were located in similar structural planes. Combinations of mAb directed towards the epitope clusters alpha 2/beta 3/5, alpha 2/hCG beta CTP (C-terminal peptide) and beta 3/5/hCG beta CTP showed the strongest enhancement, with binding more than 10-fold greater than the sum of 125I-hCG bound to the individual mAb, followed by pairs of mAb directed towards the epitope groups beta 1/beta 3/5, c 1/2/beta 3/5, beta 1/alpha 2, and alpha 2/alpha 3/5 (3-9-fold). The greater frequency of synergy obtained with the linear epitopes of the hCG beta CTP can be ascribed to their greater molecular flexibility relative to the constrained discontinuous epitopes on hCG alpha and core-hCG beta (residues 1-112). In general, these studies provide a method for rapid screening of synergistic antibody pairs which also helps to identify non-overlapping epitopes that are accessible in similar structural planes. In turn, this facilitates the design of high-affinity bispecific antibodies targetted to a single antigen molecule.

Identification and selective destruction of shared epitopes in human chorionic gonadotropin beta subunit

The feasibility of producing epitope-specific antigens by mutation of the gene is demonstrated, the aim being to eliminate unwanted surface epitopes yet allowing the natural folding of the protein to maintain the desired epitope(s). The model protein is the beta subunit of human chorionic gonadotropin (hCG beta) which previously has been used as an immunological contraceptive vaccine but has extensive cross-reaction with human luteinizing hormone. Of a series of mutants made, the mutant with substitutions of Glu for Arg 68, Ser for Arg 74, His for Gly 75 and His for Val 79, lost the ability to react with a panel of cross-reacting monoclonal antibodies while retaining the discontinuous and linear epitopes specific to the holo-hormone. In addition, allocation of amino acid residues to established epitope clusters could be made: residues 24, 25, 68 and 71 probably contribute to the cluster termed beta 3, residues 20, 21, 22, 75 and 77 to cluster beta 6 and residue 68 to clusters beta 2, beta 4 and beta 5.

Model of human chorionic gonadotropin and lutropin receptor interaction that explains signal transduction of the glycoprotein hormones

The goal of these studies was to devise a model that explains how human chorionic gonadotropin (hCG) interacts with lutropin (LH) receptors to elicit a hormone signal. Here we show that alpha-subunit residues near the N terminus, the exposed surface of the cysteine knot, and portions of the first and third loops most distant from the beta-subunit interface were recognized by antibodies that bound to hCG-receptor complexes. These observations were combined with similar data obtained for the beta-subunit (Cosowsky, L., Rao, S.N.V., Macdonald, G.J., Papkoff, H., Campbell, R.K., and Moyle, W.R. (1995) J. Biol. Chem. 270, 20011-20019), information on residues of hCG that can be changed without disrupting hormone function, the crystal structure of deglycosylated hCG, and the crystal structure of a leucine-repeat protein to devise a model of hCG-receptor interaction. This model suggest that the extracellular domain of the LH receptor is "U-" or "J"-shaped and makes several contacts with the transmembrane domain. High affinity hormone binding results from interactions between residues in the curved portion of the extracellular domain of the receptor and the groove in the hormone formed by the apposition of the second alpha-subunit loop and the first and third beta-subunit loops. Most of the remainder of the hormone is found in the large space between the arms of the extracellular domain and makes few, if any, additional specific contacts with the receptor needed for high affinity binding. Signal transduction is caused by steric or other influences of the hormone on the distance between the arms of the extracellular domain, an effect augmented by the oligosaccharides. Because the extracellular domain is coupled at multiple sites to the transmembrane domain, the change in conformation of the extracellular domain is relayed to the transmembrane domain and subsequently to the cytoplasmic surface of the plasma membrane. While the model does not require the hormone to contact the transmembrane domain to initiate signal transduction, small portions of both subunits may be near the transmembrane domain and assist in initiating the hormonal signal. This is the first model that is consistent with all known information on the activity of the gonadotropins including the amounts of the hormone that are exposed in the hormone-receptor complex, the apparent lack of specific contacts between much of the hormone and the receptor, and the roles of the oligosaccharides in signal transduction.(ABSTRACT TRUNCATED AT 400 WORDS)

The groove between the alpha- and beta-subunits of hormones with lutropin (LH) activity appears to contact the LH receptor, and its conformation is changed during hormone binding

Gonadotropins are heterodimeric glycoprotein hormones that control vertebrate fertility through their actions on gonadal lutropin (luteinizing hormone, LH) and follitropin (follicle-stimulating hormone, FSH) receptors. The beta-subunits of these hormones control receptor binding specificity; however, the region of the beta-subunit that contacts the receptor has not been identified. By a process of elimination we show this contact to be the portions of beta-subunit loops one and three found in a hormone groove created by the juxtaposition of the alpha- and beta-subunits. Most other regions of the beta-subunit can be recognized by antibodies that bind to human chorionic hormone (hCG)-receptor complexes or replaced without disrupting hormone function. Using a series of bovine LH/hCG and human FSH/hCG beta-subunit chimeras we identified key hCG beta-subunit residues in the epitopes of two antibodies that bind to hCG-receptor complexes. These epitopes include the surfaces of beta-subunit loops one and three near residue 74 on the outside of the hormone groove and parts of the C-terminal end of the "seat belt" that holds the two subunits together. The antibody that recognized residue 74 bound to receptor complexes containing most mammalian lutropins better than to the free hormones, an indication that the outside surface of the beta-subunit groove is altered during hormone binding. This region of the beta-subunit is furthest from the alpha-subunit and is recognized equally well in the free beta-subunit and in the heterodimer. Thus, the receptor associated increase in antibody binding appears due to an interaction of this portion of the beta-subunit with the receptor and not to an effect of the receptor on the relative positions of the alpha- and beta-subunits. Unlike most previous studies designed to identify portions of the beta-subunit likely to contact the LH receptor, this indirect approach provides data that are more easily interpreted because it does not rely on the use of mutations that disrupt hormone function. The approach described here should be valuable for studying the receptor interactions of other complex ligands.

The lutropin beta-subunit N-terminus facilitates subunit combination by offsetting the inhibitory effects of residues needed for LH activity

Human chorionic gonadotropin (hCG) contains a beta-subunit N-terminal amino acid extension that contacts the alpha-subunit and is needed for efficient alpha and hCG beta-subunit combination. Here we report that an hCG beta-subunit analog, lacking residues 2-8, combined with the alpha-subunit more efficiently when positively charged residues between beta-subunit cysteines 10 and 11 were replaced with negatively charged residues found in the corresponding portion of follitropin. Residues 2-8 had no influence on binding of hCG to lutropin receptors. Positive charges between cysteines 10 and 11 are essential for high affinity binding of lutropins to their receptors. Therefore, the N-terminal extension found in all lutropin beta-subunits appears to have evolved to offset the inhibition of subunit combination by beta-subunit residues that are essential for lutropin activity. This beta-subunit extension is not found in follitropins or thyrotropins, hormones that have negatively charged residues between cysteines 10 and 11.

Secondary structure prediction of beta-subunits of the gonadotropin-thyrotropin family from its aligned sequences using environment-dependent amino-acid substitution tables and conformational propensities

The secondary structures of beta-subunits of the glycoprotein hormone family, LH (luteinizing hormone), CG (chorionic gonadotropin), FSH (follicle stimulating hormone), TSH (thyroid stimulating hormone), and GTH I/GTH II (two types of fish gonadotropins), are predicted by comparing an amino-acid substitution pattern at equivalent sites in their aligned sequences with environment-dependent amino-acid substitution tables and conformational propensities calculated from other protein families whose three-dimensional structures are known. According to the prediction results, together with other structural information obtained from experiments, the following points come up as important structural features of the beta-subunits of this family; The regions assigned to regular secondary structures (one alpha-helix and three beta-strands) are considered to constitute a core of the beta-subunits. They involve interaction sites with carbohydrate and alpha-subunit. Out of the six disulfide bonds formed in the beta-subunit, four are located together on one side of the core, and the other two on the opposite side. The two regions assumed to be a receptor binding region from experiments (therefore, species-specific regions) are predicted as loops located on the same side of the beta-subunit in this study. Some of the predicted loops are rich in proline residues. While the positions of proline residues are conserved in the family generally, there are hormone- or species-specific ones in the loop that is assumed to take part in receptor binding. The possible importance of proline residues in hormone or species specificity is discussed. (After submitting the manuscript the X-ray crystal structure of human CG was published. In order to evaluate the prediction, the original manuscript is kept intact and a comparison has been made between the prediction results and the crystal structure in an appendix).

Immunochemical mapping of human lutropin: I. Delineation of a conformational antigenic determinant

Lutropin (LH), follitropin (FSH), thyrotropin (TSH) and choriogonadotropin (CG) are assembled of two non-covalently alpha (alpha) and beta (beta) subunits. We studied the discontinuous antigenic regions recognized by a monoclonal anti-hLH antibody designated as LH05 which binds to hLH, hCG and hTSH and does not cross-react with either the free subunits or hFSH. LH05 and an antibody designated HT13, recognizing an epitope partly comprizing the alpha64-76 region, did not bind simultaneously to hCG. Furthermore, LH05 was unable to combine with an anti-peptide antibody (LHP03) directed to residues 43-52 of hLHbeta. Thus, LH05 recognizes an epitope partly overlapping with those recognized by HT13 and LHP03. Using various hybrid molecules, we showed that the human alpha-subunit plays a critical role in the assembly of the epitope that, in contrast, contains amino acid residues conserved in the various beta-subunit of several species. Together, our results suggest that the amino acids Leu49-Pro50, Tyr59-Arg60 and Leu86-Ser87 in the hLHbeta and the alpha64-76 region are probably included in the epitope recognized by LH05 which appeared to be not accessible on the CG/LH receptor.

Co-evolution of ligand-receptor pairs

Specific receptors for lutropin (luteinizing hormone; LH) and follitropin (follicle-stimulating hormone; FSH) mediate the actions of human chorionic gonadotropin (hCG) and FSH5 on the gonads. Here we report that short independent sequences of the beta-subunit enable hCG to distinguish between the receptors for FSH and LH. Residues between the 11th and 12th cysteines restrict FSH receptor binding; residues between the 10th and 11th cysteines and, to a much lesser extent, residues carboxy-terminal to the 12th cysteine also affect LH receptor binding. CF101-109, an hCG analogue containing hFSH beta residues between the 11th and 12th cysteines, had high affinity for both LH and FSH receptors. Modifications to CF101-109 that reduce binding to either LH or FSH receptors yield gonadotropin analogues having differing ratios of LH:FSH activity. Ligand-binding specificity of the LH receptor is determined by residues encoded by parts of exons 2-4 and 7-9 which prevent hFSH binding but have little effect on hCG binding. FSH receptor specificity is controlled primarily by residues encoded by exons 5 and 6 that prevent hCG binding but have little effect on hFSH binding. These determinants can be interchanged to create receptor analogues that bind hCG and hFSH. Our observations support a model in which distinct negative determinants restrict ligand-receptor interaction. This explains coevolution of binding specificity in families of homologous ligands and their receptors. Natural or designed manipulation of these determinants leads to the 'evolution' of new, specific protein-protein interactions.

Immunochemical study of equine chorionic gonadotropin (eCG/PMSG): antigenic determinants on alpha- and beta-subunits

In the present study we have established an immunochemical mapping of equine Chorionic Gonadotropin (eCG/PMSG) using three monoclonal antibodies (mAbs), namely the antibodies ECG01, E10 and D7, raised against the native hormone. These antibodies do not bind to reduced, alkylated hormone, suggesting that they recognize discontinuous rather than continuous epitopes. We have also assessed the reactivity of mAbs towards human CG, and ovine, porcine, equine and bovine LH and FSH. The antigenic determinant recognized by ECG01 is localized on the alpha-subunit of equine gonadotropins and of human CG and LH. The epitopes recognized by E10 and D7 mAbs appear to be very similar and are present on the beta-subunit of eCG and of LHs from all species tested, except hLH, as well as on porcine and equine FSHs. Attempts to specify the amino-acid residues involved in these epitopes suggest that ECG01 mAb might preferentially bind to residues around position 70 whereas the region around disulfide bridges Cys-88-Cys-90 might be involved in the epitopes recognized by D7 and E10 mAbs. Topographical relationships of epitopes show that ECG01 mAb never binds to eCG simultaneously with either D7 or E10 mAbs. Furthermore, simultaneous binding of D7 and E10 mAbs on eCG could not be achieved. Thus, these three epitopes appear to be closely located on the surface of eCG. Finally, ECG01 mAb inhibits eCG binding to LH and FSH receptors, suggesting that its antigenic site is closely related to hormone-receptor interaction site(s).

Topographic analysis of human follicle-stimulating hormone-beta using anti-peptide antisera

The purpose of this study was to identify peptide sequences of human follicle-stimulating hormone-beta (hFSH beta) which are accessible subsequent to association with hFSH alpha in heterodimeric hFSH. Antisera were raised against synthetic peptides (Abpep) corresponding to hFSH beta sequences 1-20, 16-36, 33-53, 49-67, 66-85, 81-100 and 98-111. The topography of hFSH beta was studied by testing the binding of these antisera to hFSH beta and hFSH captured by monoclonal antibodies (MAb) in an enzyme-linked immunosorbent assay (ELISA). When hFSH and hFSH beta were captured by the same MAb, binding of Ab16-36, Ab33-53, Ab81-100 and Ab98-111 to hFSH was significantly lower compared to hFSH beta. However, compared to other Abpep, binding of Ab35-53 to hFSH was strong. Similar results were obtained when hFSH was captured by an alpha-specific MAb (10.3A6). Using 10.3A6, it was also possible to demonstrate significant binding of Ab49-67 to hFSH. The data suggests that residues in regions 33-53 and 49-67 of hFSH beta appear to be accessible in the heterodimeric hFSH in addition to the glycosylated region of 1-15. Regions 16-36, 33-53, 81-100 and 98-111 of hFSH beta appear to contain subunit contact-associated sequences which are either masked or structurally altered subsequent to association with hFSH alpha in the heterodimeric hFSH.

Identification of assembled epitopes on the alpha-subunit of human follicle stimulating hormone

In order to characterize further the antigenic structure of human follitropin (hFSH), BALB/c mice were immunized with hFSH and anti-hFSH monoclonal antibodies (mAbs) were generated. The hFSH subunit specificity of the mAbs was assessed by a solid-phase enzyme-linked immunosorbent assay (ELISA) and a solution-phase radioimmunoassay (RIA), each using hFSH, hFSH alpha, and hFSH beta. Five mAbs bound hFSH and hFSH alpha in the ELISA and the RIA. In addition, some mAbs recognized hFSH beta, albeit to a much lower degree, as demonstrated by displacement of [125I]hFSH binding to the mAbs by hFSH beta, in the solution-phase RIAs. Next, synthetic peptides corresponding to the hFSH alpha-subunit sequence were used to identify sequences specific to the epitopes of each of the five mAbs. Using this epitope mapping strategy, two assembled epitopes were identified. mAbs 3A and 4B distinguish one discontinuous epitope comprised minimally of sequences alpha-16-21 and alpha-66-92, whereas mAbs 5F and 2E distinguish a second discontinuous epitope comprised minimally of sequences alpha-40-50 and alpha-66-72.

Assembly and expression of a synthetic gene encoding the bovine glycoprotein hormone alpha-subunit

The glycoprotein hormones are a family of alpha beta heterodimeric proteins which are responsible for gonadal and thyroid function. In previous studies we employed chimeric glycoprotein hormone beta-subunits to identify amino acid residues critical for binding to receptors and antibodies. To facilitate similar studies of the alpha-subunit of these hormones, we assembled a 406 bp synthetic gene which encodes the human alpha-subunit leader sequence and the secreted portion of the bovine alpha-subunit. It contains unique restriction sites that can be used for cassette mutagenesis or for making human/bovine alpha-subunit chimeras. The gene was assembled from eight long oligodeoxynucleotides in a single ligation and its structure verified by DNA sequencing. Co-transfection of COS-7 cells with the synthetic gene and the cDNA for human chorionic gonadotropin (hCG) beta-subunit resulted in the secretion of a functional alpha beta heterodimer which bound to luteinizing hormone receptors. The protein was recognized by several monoclonal antibodies including B109, an antibody to a conformational epitope which binds hCG but not the free bovine alpha-, human alpha-, or hCG beta-subunits. This suggests that the binding site for B109 may be formed by residues located primarily within the hCG beta-subunit and that formation of this epitope requires a change in conformation of the beta-subunit when it combines with the alpha-subunit.

Progress and approaches in mapping the surfaces of human follicle-stimulating hormone: comparison with the other human pituitary glycoprotein hormones

The pituitary glycoprotein hormones, including human follicle-stimulating hormone (hFSH), are involved in the physiologic functions of receptor binding, in subunit association during assembly and processing, and in untoward effects such as antibody formation during clinical intervention. These activities derive from specific macromolecular associations; a major research trend has been to map the surfaces of these glycoprotein hormones (hFSH, hLH, hCG, and hTSH) into discrete areas responsible for each activity by using a variety of biochemical approaches. Each surface domain or determinant of the hormone is likely to comprise discontinuous amino acid sequences, from one or both subunits, assembled into a precise, unique, macromolecular surface. The paradigm of antigen-antibody interaction may help to explain how the surfaces are assembled, how the common alpha-subunit combines with the unique beta-subunit of each, and how the receptor interacts with heterodimeric hormone.

Contributions of arginines-43 and -94 of human choriogonadotropin beta to receptor binding and activation as determined by oligonucleotide-based mutagenesis

Members of the glycoprotein hormone family contain a common alpha subunit and a hormone-specific beta subunit. Human choriogonadotropin (hCG) beta is a 145 amino acid residue protein glycosylated at 6 positions (2 N-linked and 4 O-linked oligosaccharides). In an effort to elucidate receptor determinants on hCG beta, we have used site-directed mutagenesis to prepare and express several mutant cDNAs with replacements at arginines-43 and -94. Arg-43 is invariant in all known mammalian CG/lutropin beta amino acid sequences, and Arg-94 is conserved in 10 of the 12 sequences. Moreover, various studies involving synthetic peptides and enzymatic digestions of intact beta chains suggest that these residues may be important in hCG receptor binding. Point mutants were made in which these two arginines were replaced with the corresponding residues in human follitropin beta, Leu-43 and Asp-94. The wild-type and mutant beta chains were expressed in CHO cells containing a stably integrated gene for bovine alpha, and heterodimer formation occurred. These heterologous gonadotropins were active in assays using transformed Leydig cells, competitive binding with standard 125I-hCG, and cAMP and progesterone production, but the potency was considerably less than that associated with the hCG beta wild-type-containing gonadotropin. The double-mutant protein Arg-43 to Leu/Arg-94 to Asp also associated with bovine alpha, but the resultant heterodimer exhibited only low activity. Replacement of each arginine with lysine yielded heterodimers that were at least as potent as bovine alpha-hCG beta wild type, but the Lys-43-containing beta chain appeared to exhibit a low degree of subunit association or reduced stability relative to the expressed hCG beta wild type. These results demonstrate that arginines-43 and -94 contribute to receptor binding through a positive charge.