Changes in binding activity of luteinizing hormone receptors by site directed mutagenesis of potential glycosylation sites

N-linked glycosylation facilitates processing and cell surface expression of rat luteinizing hormone receptor

The extracellular domain of the luteinizing hormone (LH) receptor has six potential N-linked glycosylation sites. Although previous studies have shown that mutation of the first three sites results in decreased ligand binding at the cell surface, the role of glycosylation in LH receptor processing is not understood. In the present study, we examined whether mutation of the first three sites has any affect on receptor synthesis, processing, and degradation of the mutant receptors. The data show that mutation of N77, N152, or N173 did not affect receptor synthesis, but did significantly reduce processing of the receptor precursor to the mature, cell surface form. Furthermore, defective processing was due to increased degradation of the precursor rather than increased turnover of cell surface receptors. Thus, lack of glycosylation decreases LH receptor processing and targets the receptors for degradation thereby leading to decreased cell surface expression. These results show that glycosylation of the LH receptor plays an important role in receptor processing and cell surface expression.

Gonadotropin receptors: role of post-translational modifications and post-transcriptional regulation

This review focuses on the post-translational modifications of LH and FSH receptors and recent studies on the regulation of LH receptor expression mediated by an RNA binding protein. Both LH and FSH receptors undergo extensive post-translational modifications. N-linked glycosylation occurs co-translationally and plays a role in the maturation and processing of the receptor, while palmitoylation is involved in receptor endocytosis and post-endocytic trafficking. A third type of post-translational modification is phosphorylation and its function has been reviewed. Finally, the regulation of LH receptor at the mRNA level by an RNA binding protein is discussed in the context of ovarian function.

Glycosylation of the gastrin-releasing peptide receptor and its effect on expression, G protein coupling, and receptor modulatory processes

Many gastrointestinal G protein-coupled receptors are glycosylated; however, which potential glycosylation sites are actually glycosylated and their role in receptor transduction or receptor modulation (internalization, down-regulation, desensitization) is largely unknown. We used site-directed mutagenesis to address these issues with the gastrin-releasing peptide receptor (GRP-R). Each of the four potential glycosylation sites was mutated by converting the Asn (N) to Gln (Q). Transient expression in CHOP cells demonstrated that changing Asn(24) or Asn(191) inhibited GRP-R cell surface expression, whereas elimination of Asn(5) and Asn(20) had no effect. Using ligand cross-linking studies in stable mutants expressed in Balb 3T3 cells, all four potential extracellular sites were glycosylated with carbohydrate residues of approximately 13 kDa on Asn(5), 10 kDa on Asn(20), 5 kDa on Asn(24), and 9 kDa on Asn(191). Removal of three glycosylation sites (N5,20,24,Q mutant) did not alter receptor affinity or G protein coupling; therefore, it could be speculated that deglycosylation at Asn(191) might be responsible for the altered G protein coupling seen with complete enzymatic deglycosylation of the native receptor previously reported. Removal of any single glycosylation site did not interfere with GRP-R induced chronic desensitization or down-regulation. However, elimination of all three NH(2)-terminal sites (N5,20,24) markedly attenuated both processes, with no effect on acute homologous desensitization and with only a minimal alteration of GRP-R internalization, supporting the findings of other studies that suggest that chronic desensitization and down-regulation are functionally coupled, distinct from acute desensitization and distinct from internalization. These data show that separate and specific glycosylation sites are important for GRP-R trafficking to the cell surface, ligand binding, G protein coupling, chronic desensitization, and down-regulation.

Mutations of gonadotropins and gonadotropin receptors: elucidating the physiology and pathophysiology of pituitary-gonadal function

The recent unraveling of structures of genes for the gonadotropin subunits and gonadotropin receptors has provided reproductive endocrinologists with new tools to study normal and pathological functions of the hypothalamic-pituitary-gonadal axis. Rare inactivating mutations that produce distinctive phenotypes of isolated LH or FSH deficiency have been discovered in gonadotropin subunit genes. In addition, there is a common polymorphism in the LHbeta subunit gene with possible clinical significance as a contributing factor to pathologies of LH-dependent gonadal functions. Both activating and inactivating mutations have been detected in the gonadotropin receptor genes, a larger number in the LH receptor gene, but so far only a few in the gene for the FSH receptor. These mutations corroborate and extend our knowledge of clinical consequences of gonadotropin resistance and inappropriate gonadotropin action. The information obtained from human mutations has been complemented by animal models with disrupted or inappropriately activated gonadotropin ligand or receptor genes. These clinical and experimental genetic disease models form a powerful tool for exploring the physiology and pathophysiology of gonadotropin function and provide an excellent example of the power of molecular biological approaches in the study of pathogenesis of diseases.

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.

Structural features of heterotrimeric G-protein-coupled receptors and their modulatory proteins

Over the past 20 years, the general mechanism for signaling through 7-transmembrane helix receptors coupled to GTP hydrolysis has been worked out. Although similar in overall organization, subtype variability and subcellular localization of components have built in considerable signaling specificity. Atomic resolution structures for many of the components have delineated the domain organization of these complex proteins and have given physical form to the idea of subtype specificity. This review describes what is known about the physical structures of the 7-transmembrane helix receptors, the heterotrimeric GTP binding coupling proteins, the adenylate cyclase and phospholipase C effector proteins, and signaling modulatory proteins, such as arrestin, phosducin, recoverin-type myristoyl switch proteins, and the pleckstrin homology domain of G-protein receptor kinase-2. These images allow experimenters to contemplate the details of the supramolecular organization of the multiprotein complexes involved in the transmission of signals across the cellular lipid bilayer.

Characterization of the translated products of the alternatively spliced luteinizing hormone receptor in the ovine ovary throughout the oestrous cycle

The luteinizing hormone receptor (LHR) is alternatively spliced. It is not known if the alternatively spliced mRNAs are translated in vivo, or indeed if they have any vital role to play. The B splice form has been detected in every species examined, and it encodes a putative protein with a high affinity LH/CG binding domain but no trans-membrane or intra-cellular domains. We raised antisera that recognize the putative protein of the B form, and the closely related G form, and showed that the B form mRNA is translated in the ovine ovary, but not kidney or liver. It localized to the luteal cytosolic and microsomal fractions and the levels declined during regression induced by treatment with prostaglandin F2alpha. We examined alternative splicing by RNase protection analyses and RT-PCR analyses of healthy pre-ovulatory follicles, atretic or steroidogenically-inactive follicles, and of newly formed, mid-luteal and regressing corpora lutea. There was approximately 5-fold more B form mRNA than A form. Thus we have evidence that the LHR B form is translated in vivo, but no evidence that alternative splicing of the LHR mRNA is differentially regulated, throughout the oestrous cycle.

The ligand binding site of NPY at the rat Y1 receptor investigated by site-directed mutagenesis and molecular modeling

The ligand binding site of neuropeptide Y (NPY) at the rat Y1 (rY1,) receptor was investigated by construction of mutant receptors and [3H]NPY binding studies. Expression levels of mutant receptors that did not bind [3H]NPY were examined by an immunological method. The single mutations Asp85Asn, Asp85Ala, Asp85Glu and Asp103Ala completely abolished [3H]NPY binding without impairing the membrane expression. The single mutation Asp286Ala completely abolished [3H]NPY binding. Similarly, the double mutation Leu34Arg/Asp199Ala totally abrogated the binding of [3H]NPY, whereas the single mutations Leu34Arg and Asp199Ala decreased the binding of [3H]NPY 2.7- and 5.2-fold, respectively. The mutants Leu34Glu, Pro35His as well as Asp193Ala only slightly affected [3H]NPY binding. A receptor with a deletion of the segment Asn2-Glu20 or with simultaneous mutations of the three putative N-terminal glycosylation sites, displayed no detectable [3H]NPY binding, due to abolished expression of the receptor at the cell surface. Taken together, these results suggest that amino acids in the N-terminal part as well as in the first and second extracellular loops are important for binding of NPY, and that Asp85 in transmembrane helix 2 is pivotal to a proper functioning of the receptor. Moreover, these studies suggest that the putative glycosylation sites in the N-terminal part are crucial for correct expression of the rY1 receptor at the cell surface.

The luteinizing hormone receptor

The luteinizing hormone receptor (LHR) is a member of the subfamily of glycoprotein hormone receptors within the superfamily of G protein-coupled receptor (GPCR)/seven-transmembrane domain receptors. Over the past eight years, major advances have been made in determining the structure and function of the LHR and its gene. The hormone-binding domain has been localized to exons 1-7 in the extracellular (EC) domain/region of the receptor, which contains several leucine-rich repeats. High-affinity binding of LH and human chorionic gonadotrophin (hCG) causes secondary hormone or receptor contacts to be established with regions of the EC loop/transmembrane module that initiate signal transduction. Models of hormone-receptor interaction have been derived from the crystal structures of hCG and of the ribonuclease inhibitor, which also contains leucine-rich repeats. Such models provide a framework for the interpretation of mutational studies and for further experiments. The extracellular domain of the receptor has been overexpressed in vitro, which will facilitate crystallographic resolution of the structure of the receptor-binding site. The transmembrane domain/loop/cytoplasmic module transduces the signal for coupling to G proteins. Several constitutive, activating mutations that cause human disease have been found in helix VI and adjacent structures. These mutations have provided valuable information about mechanisms of signal transfer and G protein coupling. The structure of the LHR gene has been elucidated, and the regulation of its transcription is beginning to be understood. Valuable insights into receptor evolution have been derived from analysis of sequence homologies, the gene structure of glycoprotein hormone receptors and other members of the GPCR family, and the glycoprotein hormone receptor-like precursors identified in several invertebrate species.

The six N-linked carbohydrates of the lutropin/choriogonadotropin receptor are not absolutely required for correct folding, cell surface expression, hormone binding, or signal transduction

Using two separate methods, we have determined that all six potential sites for N-linked glycosylation on the rat lutropin/choriogonadotropin receptor (rLHR) contain carbohydrates. The functional roles of the carbohydrates were analyzed initially through the use of two nonglycosylated receptor mutants rLHR(N(77,152,173,269,277,291)Q) and rLHR(N(77,152,269,277,291)Q;T(175)A). Although Western blot analyses demonstrated both mutant receptors to be stably expressed, little or no hCG binding activity could be detected in detergent solubilized extracts of 293 cells expressing either nonglycosylated LHR mutant. Although this loss of hCG binding was concluded to be due to misfolding, it was unknown whether this misfolding was due to the absence of carbohydrates or to the multiple amino acid substitutions that had been introduced into the polypeptide. To differentiate between these possibilities, hCG binding assays were performed with nonglycosylated receptors obtained after tunicamycin treatment of cells expressing the wild-type rLHR. Even though these wild-type receptors were confirmed to be devoid of all N-linked carbohydrates by Western blots, they were found to bind hCG with a normal high affinity. In addition, tunicamycin-derived, nonglycosylated LHRs were present at the cell surface and exhibited a phenotype consistent with mature receptors due to their capability to mediate hCG-stimulated cAMP production as well as bind oLH with high affinity. These results indicate that the loss of high affinity hormone binding by rLHR(N(77,152,173,269,277,291)Q) and rLHR(N(77,152,269,277,291)Q;T(175)A) is simply due to the collective amino acid substitutions rather than to the absence of carbohydrates. Therefore, N-linked carbohydrates are not absolutely required for the proper folding of the rLHR into a mature receptor capable of binding hormone and signaling. These results are in marked contrast to the follitropin receptor (FSHR), a very similar receptor which has been shown to strictly require N-linked carbohydrates for folding of the nascent protein.

Structure and functional significance of the carbohydrates of the LH/CG receptor

The LH/CG appears to contain 1-4 bi- or multi-antennary complex-type N-linked oligosaccharide side chains, which appear to locate apart from the hormone-binding regions. The exact sites to which the N-linked chains are attached remain to be delineated. The carbohydrates of the mature membrane-inserted receptor do not contribute to either specific high-affinity ligand-binding or signal transduction of the receptor. Thus, the polypeptide core of the receptor is responsible for both high affinity binding and dictating the hormone specificity. Moreover, the deglycosylated receptor, once inserted to the plasma membrane in a functionally mature form, retains its functional conformation or permits the conformational change that is required for coupling of the receptor to effector enzymes. Addition of oligosaccharides to the nascent LH/CG receptor but not their subsequent conversion to complex-type ones appears to be required for acquiring the hormone-binding conformation. On the other hand, neither addition of oligosaccharides to the nascent receptor, nor their further maturation are needed for the transport of the receptor to the plasma membrane. Thus, one function of the N-linked oligosaccharides in the LH/CG receptor appears to be to direct the proper folding of the receptor.

Structural characterisation of human recombinant glycohormones follitropin, lutropin and choriogonadotropin expressed in Chinese hamster ovary cells

The alpha and beta chains from human recombinant gonadotropins follitropin, lutropin and choriogonadotropin expressed in CHO cells have been structurally characterised both at the protein and at the carbohydrate level by using advanced mass spectrometric procedures. The three alpha chains share the same amino acid sequence while they display different glycosylation patterns. The oligosaccharide structures detected belong to the complex-type glycans with different degree of sialylation. Partial proteolytic processing occurred at the N-terminus of the follitropin beta chain and at the C-terminus of the lutropin beta chain. The N-linked glycans from the three beta chains were found to contain fucosylated and sialylated diantennary and triantennary complex-type structures. The follitropin beta chain showed the presence of N-acetyllactosamine repeats on the antennae. The overall structure of the recombinant glycohormones corresponds to their natural counterparts with the exception that sulphated terminal glycosylation is missing.

Expression of extracellular domain peptides of the FSH receptor and their effect on receptor-ligand interactions in vitro

Follicle-stimulating hormone receptor (FSH-R) displays considerable homology to luteinizing hormone receptor (LH-R) in structure and amino acid sequence. Comparison of the sequences of the extracellular domains (ECD) of the receptors reveals two regions (amino acids 4-56 and 265-319 in FSH-R) that share relatively little amino acid sequence similarity. This suggests that these variable regions may be important in providing specificity of ligand binding. We have expressed overlapping ECD peptides containing one or both of these regions (RFI, amino acids 5-125; RF2, amino acids 201-319; and RF3, amino acids 5-319) as fusion proteins in E. coli using pRSET vector. The presence of polyhistidine at the N-terminal end allowed substantial purification of the expressed proteins by a single step of affinity chromatography. The purified peptides were characterized for direct binding of hormone and their ability to block the binding of FSH and LH to the receptors. None of the peptides bound labelled hormone, while all peptides inhibited the binding of FSH to its receptor in a dose-dependent manner. However, only RF2 peptide inhibited ligand binding in a hormone-specific manner. These data suggest there is a site between amino acids 201-319 of the FSH-R ECD that is involved in FSH binding.

Functional glycosylation sites of the rat luteinizing hormone receptor required for ligand binding

The contribution of N-linked glycosylation to the ligand binding activity of the rat luteinizing hormone receptor (LHR) was studied in wild-type and mutant LHR expressed in mammalian (COS1) cells and overexpressed in insect (Sf9) cells. The binding affinities of the holoreceptor and its truncated splice variant (form B) lacking the transmembrane domain were equivalent in both cell lines. Tunicamycin-treated transfected Sf9 cells expressed a carbohydrate-free LH receptor that lacked hormone binding activity. Functional carbohydrate chains essential for binding activity were localized to glycosylation sites at Asn-173 and Asn-152. Glycosidase treatment of the double mutant N173Q/N152Q revealed the presence of at least one additional carbohydrate chain at Asn-269, Asn-277, or Asn-291 that does not contribute to hormone binding. Asn-77 was not glycosylated, but its mutation to Gln reduced hormone binding. LHR expressed in insect cells contained only high mannose carbohydrate chains, and those located at Asn-173 and Asn-152 were sufficient for high-affinity hormone binding. Enzymatic cleavage of glycosyl chains indicated that only the proximal N-acetylglucosamine residue, which is common to high mannose and complex carbohydrate forms, is necessary for acquisition of the high affinity conformation of the receptor. The carbohydrate chains of the LHR appear to be involved in intramolecular folding of the nascent receptor rather than in its interaction with the hormone.

Expression of the extracellular domain of the thyrotropin receptor in the baculovirus system using a promoter active earlier than the polyhedrin promoter. Implications for the expression of functional highly glycosylated proteins

Conventional baculovirus vectors that utilize the very late polyhedrin promoter have not proved successful for expressing a thyrotropin (TSH) receptor capable of ligand and Graves' disease autoantibody binding comparable to the receptor produced in mammalian cells. Because of the clinical importance of high level expression of this protein, we reassessed the baculovirus system using a new transfer vector (pAcMP3) containing the late basic protein promoter, which functions earlier than the classical polyhedrin promoter. Maximal synthesis of the [35S]methionine-labeled TSH receptor extracellular domain, affinity-purified using a 6-histidine tag, occurred earlier (1 day after insect cell infection) than with a vector (pVL1393) containing the polyhedrin promoter. The pAcMP3-derived TSH receptor extracellular domain was larger (approximately 68 kDa) than the pVL1393-derived protein (approximately 63 kDa). Only the 68-kDa product was secreted, albeit in trace amounts detectable only by precursor labeling. Enzymatic deglycosylation reduced both 68- and 63-kDa cellular proteins to approximately 54 kDa, indicating that the pAcMP3 vector generated a protein with greater carbohydrate content. However, despite its greater degree of glycosylation, most of the 68-kDa protein remained within the cell, almost entirely in the particulate fraction. Remarkably, the trace amounts of 68-kDa receptor protein affinity-purified from the soluble cytosolic fraction of infected insect cells completely neutralized TSH receptor autoantibodies in patients' sera and partly inhibited TSH binding. In conclusion, a baculovirus vector with a promoter active earlier than the conventional polyhedrin promoter generates a more glycosylated and functional TSH receptor extracellular domain protein, albeit at low levels. These data carry important implications for the expression by baculovirus vectors of functional, highly glycosylated proteins.

Protein glycosylation. Structural and functional aspects

During the last decade, there have been enormous advances in our knowledge of glycoproteins and the stage has been set for the biotechnological production of many of them for therapeutic use. These advances are reviewed, with special emphasis on the structure and function of the glycoproteins (excluding the proteoglycans). Current methods for structural analysis of glycoproteins are surveyed, as are novel carbohydrate-peptide linking groups, and mono- and oligo-saccharide constituents found in these macromolecules. The possible roles of the carbohydrate units in modulating the physicochemical and biological properties of the parent proteins are discussed, and evidence is presented on their roles as recognition determinants between molecules and cells, or cell and cells. Finally, examples are given of changes that occur in the carbohydrates of soluble and cell-surface glycoproteins during differentiation, growth and malignancy, which further highlight the important role of these substances in health and disease.

Significance of the carbohydrate moiety of the rat ovarian luteinizing-hormone/chorionic-gonadotropin receptor for ligand-binding specificity and signal transduction

The contribution of the carbohydrate moiety of the rat ovarian luteinizing-hormone (LH)/chorionic-gonadotropin (CG) receptor to ligand-binding specificity and signal transduction was investigated by using glycosidases. Purified membranes from pseudo-pregnant rat ovaries were treated with neuraminidase or peptide N-glycosidase F, to remove terminal sialic acids and N-linked oligosaccharides of the receptor, respectively. Ligand blotting and densitometric scanning of the autoradiograms showed that 90-95% of the receptors were deglycosylated, and that desialylation was virtually complete. Neither the desialylated nor the deglycosylated receptors were able to bind human follicle-stimulating hormone or bovine thyroid-stimulating hormone, as revealed by competition binding experiments. The 50% effective dose of hCG for adenylate cyclase activation, as determined by measuring the formation of cyclic [32P]AMP from [alpha-32P]ATP for 15 min at 30 degrees C, was similar in the control and deglycosylated membranes: 10.2 +/- 3.3 nM and 12.2 +/- 3.8 nM respectively. The same was true for the time course of the basal, hCG- and forskolin-stimulated enzyme activity. In addition, removal of oligosaccharides from the receptor did not restore the ability of desialylated hCG, nor of the deglycosylated hormone, to stimulate adenylate cyclase. In conclusion, the carbohydrate moiety of the native membrane-inserted rat ovarian LH/CG receptor does not contribute to the ligand-binding specificity, and it is not required for the functional coupling of the occupied receptor and the adenylate cyclase system. These functions are associated with the polypeptide portion of the receptor.

High expression of the hormone binding active extracellular domain (1-294) of rat lutropin receptor in Escherichia coli

Using the polymerase chain reaction (PCR) technique, the cDNA fragment corresponding to the receptor coding region for residues 1-294 was prepared from rat lutropin receptor (LHR) cDNA and subsequently subcloned into Escherichia coli expression vector pT7-7. This truncated receptor was efficiently expressed in E. coli as indicated by sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) and silver staining. The recombinant protein present in the inclusion bodies was solubilized in 6 M guanidine-HCl and purified in two successive steps of fast performance liquid chromatography (FPLC) using Superose-12 and Mono Q columns. Refolding of the purified recombinant protein was achieved in 1.5 M guanidine-HCl in the presence of an equimolar proportion of cysteine and cystine. The refolded soluble truncated LHR(1-294) had apparent molecular weights of 33 kDa and 140 kDa under reducing and nonreducing conditions, respectively. The multimeric nature of the extracellular domain of the receptor is believed to be due to its self-association by intermolecular disulfide bond formation since the 1-294 amino acid segment has nine cysteine residues and thus has one or possibly more free sulfhydryl groups. The purified truncated receptor had high binding affinity for human choriogonadotropin (hCG) as indicated by ligand blotting on SDS-PAGE and radioligand receptor assays. The amount of hCG required for 50% inhibition of binding of 125I-hCG to the soluble truncated receptor was 2.7 x 10(-10) M (or 12.25 ng). Similarly, the amount of soluble truncated receptor required for 50% inhibition of 125I-hCG binding to the rat ovarian receptor in the radioligand receptor assay was 58 ng.(ABSTRACT TRUNCATED AT 250 WORDS)

Variant forms of the pig lutropin/choriogonadotropin receptor

The cloning and sequencing of porcine lutropin/choriogonadotropin (LH/hCG) receptor messenger RNAs have shown the presence of a full-length receptor (pLHR-A) and of shorter variants lacking either the transmembrane and the intracellular domains (pLHR-B and pLHR-C) or only the transmembrane domain (pLHR-D). Moreover, immunoblotting of testicular membrane extracts has detected 85-, 68-, and 45-48-kDa proteins reacting with antireceptor antibodies. Transfection experiments were performed to assign the protein species to the various messenger RNAs and to study the function of the various receptor species. COS-7 and L-cells transfected with an expression vector encoding full-length receptor pLHR-A yielded a protein of apparent molecular mass of 105 kDa. This corresponded to the complete receptor which had undergone a different glycosylation pattern to that found in testis, since after digestion with peptide N-glycosidase F both the 105-kDa COS-7 protein and the 85-kDa testicular glycoprotein yielded a holoprotein of approximately 63 kDa. Transfection with pLHR-A also yielded a high proportion of the 68-kDa glycoprotein which was shown by digestion with endoglycosidase H to be a high-mannose precursor of the full-length receptor. The existence of a large pool of precursor species in both transfected cells and Leydig cells evokes possible physiological regulations at the level of receptor maturation.(ABSTRACT TRUNCATED AT 250 WORDS)