The sequence of 967 amino acids at the carboxyl-end of rat thyroglobulin. Location and surroundings of two thyroxine-forming sites

Molecular advances in thyroglobulin disorders

Synthesis of tri-iodothyronine (T(3)) and thyroxine (T(4)) follows a metabolic pathway that depends on the integrity of the thyroglobulin structure. This large glycoprotein is a homodimer of 660 kDa synthesized and secreted by the thyroid cells into the lumen of thyroid follicle. In humans it is coded by a single copy gene, 270 kb long, that maps on chromosome 8q24 and contains an 8.5 kb coding sequence divided into 48 exons. The preprotein monomer is composed of a 19-amino acid signal peptide followed by a 2749-amino acid polypeptide. In the last decade, several mutations in the thyroglobulin gene were reported. In animals, four of them have been observed in Afrikander cattle (p.R697X), Dutch goats (p.Y296X), cog/cog mouse (p.L2263P) and rdw rats (p.G2300R). Mutations in the human thyroglobulin gene are associated with congenital goiter or endemic and nonendemic simple goiter. Thirty-five inactivating mutations have been identified and characterized in the human thyroglobulin gene: 20 missense mutations (p.C175G, p.Q310P, p.Q851H, p.S971I, p.R989C, p.P993L, p.C1058R, p.C1245R, p.S1447N, p.C1588F, p.C1878Y, p.I1912V, p.C1977S, p.C1987Y, p.C2135Y, p.R2223H, p.G2300D, p.R2317Q, p.G2355V, p.G2356R), 8 splice site mutations (g.IVS3-3C>G, g.IVS5+1G>A, g.IVS10-1G>A, g.IVS24+1G>C, g.IVS30+1G>T, g.IVS30+1G>A, g.IVS34-1G>C, g.IVS45+2T>A) 5 nonsense mutations (p.R277X, p.Q692X, p.W1418X, p.R1511X, p.Q2638X) and 2 single nucleotide deletions (p.G362fsX382, p.D1494fsX1547). The thyroglobulin gene has been also identified as the major susceptibility gene for familial autoimmune thyroid diseases (AITD) by linkage analysis using highly informative polymorphic markers. In conclusion the identification of mutations in the thyrogobulin gene has provided important insights into structure-function relationships.

[Enzymes involved in thyroid iodide organification]

Thyroid hormone biosynthesis depends on iodide uptake and its incorporation into the acceptor protein thyroglobulin (Tg), a high molecular weight protein secreted into the follicular lumen. The sodium-iodide symporter (NIS) is responsible for thyroid iodide uptake, the first step in thyroid hormonogenesis. Iodide is subsequently transported through the cellular membrane by pendrin (PDS) and then incorporated into Tg. Iodide oxidation and organification occur mainly in the thyrocyte apical surface and these reactions are catalyzed by thyroperoxidase (TPO) in the presence of hydrogen peroxide. Thus, thyroid iodide organification depends on TPO activity, which is modulated by the concentration of substrates (thyroglobulin and iodide) and cofactor (hydrogen peroxide). Hydrogen peroxide generation is catalyzed by the thyroid NADPH oxidase (ThOx), which is present in the apical pole of thyrocytes, is stimulated by thyrotropin and is inhibited by iodide. Hydrogen peroxide generation is the limiting step in thyroid hormone biosynthesis under iodine sufficiency conditions.

Delineation of five thyroglobulin T cell epitopes with pathogenic potential in experimental autoimmune thyroiditis

Experimental autoimmune thyroiditis (EAT) is a T cell-mediated disease that can be induced in mice after challenge with thyroglobulin (Tg) or Tg peptides. To date, five pathogenic Tg peptides have been identified, four of which are clustered toward the C-terminal end. Because susceptibility to EAT is under control of H-2A(k) genes, we have used an algorithm-based approach to identify A(k)-binding peptides with pathogenic potential within mouse Tg. Eight candidate synthetic peptides, varying in size from 9 to 15 aa, were tested and five of those (p306, p1579, p1826, p2102, and p2596) were found to induce EAT in CBA/J (H-2(k)) mice either after direct challenge with peptide in adjuvant or by adoptive transfer of peptide-sensitized lymph node cells (LNCs) into naive hosts. These pathogenic peptides were immunogenic at the T cell level, eliciting specific LNC proliferative responses and IL-2 and/or IFN-gamma secretion in recall assays in vitro, but contained nondominant epitopes. All immunogenic peptides were confirmed as A(k) binders because peptide-specific LNC proliferation was blocked by an A(k)-specific mAb, but not by a control mAb. Peptide-specific serum IgG was induced only by p2102 and p2596, but these Abs did not bind to intact mouse Tg. This study reaffirms the predictive value of A(k)-binding motifs in epitope mapping and doubles the number of known pathogenic T cell determinants in Tg that are now found scattered throughout the length of this large autoantigen. This knowledge may contribute toward our understanding of the pathogenesis of autoimmune thyroiditis.

Pathogenic thyroglobulin peptides as model antigens: insights on the induction and maintenance of autoimmune thyroiditis

In recent years, the discovery of pathogenic thyroglobulin (Tg) peptides has given a new impetus to study, at the basic level, mechanisms of induction and immunoregulation of autoimmune thyroiditis. The genetic control of the immune response against defined Tg epitopes and the diversity of the T-cell receptor repertoire recruited for their recognition were among the first issues examined. Some of these epitopes contained hormonogenic sites, i.e. thyroxine residues, and thus offered an excellent opportunity to study how post-translational modifications such as iodination, can influence induction of thyroiditogenic cells. The delineation of pathogenic Tg determinants also enabled the search for "molecular mimics" i.e. peptides of microbial origin that may be involved in the pathogenesis of the disease. In addition, factors promoting the generation of pathogenic epitopes during Tg processing in antigen presenting cells could now be systematically investigated. This review summarizes recent findings in these areas.

Megalin in thyroid physiology and pathology

Megalin, a member of the low density lipoprotein endocytic receptor family, is expressed on the apical surface of thyroid epithelial cells, directly facing the follicle lumen, where colloid is stored in high concentrations. Studies in vivo and with cultured thyroid cells have provided evidence that megalin expression on thyroid cells is TSH-dependent. Thyroglobulin (Tg), the major protein component of the colloid and the precursor of thyroid hormones, binds to megalin with high affinity and megalin mediates in part its uptake by thyrocytes. Tg internalized by megalin avoids the lysosomal pathway and is delivered by transepithelial transport (transcytosis) to the basolateral membrane of thyrocytes, from which it is released into the bloodstream. This process competes with pathways leading to thyroid hormone release from Tg molecules, which occurs following internalization of Tg molecules from the colloid by other means of uptake (fluid phase endocytosis or endocytosis mediated by low affinity receptors) that result in proteolytic cleavage in the lyosomes. During transcytosis of Tg, a portion of megalin (secretory component) remains complexed with Tg and enters the circulation, where its detection may serve as a tool to identify the origin of serum Tg in patients with thyroid diseases. Tg endocytosis via megalin is facilitated by the interaction of Tg with cell surface heparan sulfate proteoglycans, which occurs via a carboxyl terminal heparin binding site of Tg functionally related with a major megalin binding site. Although autoantibodies against megalin can be found in the serum of approximately 50% of patients with autoimmune thyroiditis, a role of megalin in this and other thyroid diseases remains to be established.

Role of thyroglobulin endocytic pathways in the control of thyroid hormone release

Thyroglobulin (Tg), the thyroid hormone precursor, is synthesized by thyrocytes and secreted into the colloid. Hormone release requires uptake of Tg by thyrocytes and degradation in lysosomes. This process must be precisely regulated. Tg uptake occurs mainly by micropinocytosis, which can result from both fluid-phase pinocytosis and receptor-mediated endocytosis. Because Tg is highly concentrated in the colloid, fluid-phase pinocytosis or low-affinity receptors should provide sufficient Tg uptake for hormone release; high-affinity receptors may serve to target Tg away from lysosomes, through recycling into the colloid or by transcytosis into the bloodstream. Several apical receptors have been suggested to play roles in Tg uptake and intracellular trafficking. A thyroid asialoglycoprotein receptor may internalize and recycle immature forms of Tg back to the colloid, a function also attributed to an as yet unidentified N-acetylglucosamine receptor. Megalin mediates Tg uptake by thyrocytes, especially under intense thyroid-stimulating hormone stimulation, resulting in transcytosis of Tg from the colloid to the bloodstream, a function that prevents excessive hormone release.

Amalgam is a ligand for the transmembrane receptor neurotactin and is required for neurotactin-mediated cell adhesion and axon fasciculation in Drosophila

Neurotactin (NRT), a member of the cholinesterase-homologous protein family, is a heterophilic cell adhesion molecule that is required for proper axon guidance during Drosophila development. In this study, we identify amalgam (AMA), a member of the immunoglobulin superfamily, as a ligand for the NRT receptor. Using transfected Schneider 2 cells and embryonic primary cultures, we demonstrate that AMA is a secreted protein. Furthermore, AMA is necessary for NRT-expressing cells both to aggregate with themselves and to associate with embryonic primary culture cells. Aggregation assays performed with truncated NRT molecules reveal that the integrity of the cholinesterase-like extracellular domain was not required either for AMA binding or for adhesion, with only amino acids 347-482 of the extracellular domain being necessary for both activities. Moreover, the NRT cytoplasmic domain is required for NRT-mediated adhesion, although not for AMA binding. Using an ama-deficient stock, we find that ama function is not essential for viability. Pupae deficient for ama do exhibit defasciculation defects of the ocellar nerves similar to those found in nrt mutants.

Binding of rat thyroglobulin to heparan sulfate proteoglycans

We previously showed that rat thyroglobulin (Tg) is a heparin-binding protein and that heparin inhibits Tg binding to megalin (gp330), an endocytic Tg receptor found on the apical surface of thyrocytes. Cooperation between cell surface receptors and heparin-like molecules, namely heparan sulfate proteoglycans (HSPGs), can facilitate cell surface binding of some heparin-binding proteins. Based on our previous findings indicating that heparin and megalin-binding sites of rat Tg are functionally related, here we investigated whether rat Tg binds to HSPGs, which are expressed by thyroid cells. We showed in solid phase assays that unlabeled rat Tg binds to a heparan sulfate (HS) preparation in a dose-dependent, saturable manner, with moderately high affinity (Kd approximately 19 nM, Ki approximately 25 nM). Binding was inhibited by heparin and by HS itself. We then studied the role of HSPGs in Tg binding to FRTL-5 cells, a differentiated Fisher rat thyroid cell line. As previously reported, after incubation of FRTL-5 cells with unlabeled rat Tg at 4 degrees C, heparin released virtually all the cell-bound Tg. Co-incubation of Tg with HS or with a preparation of HSPGs resulted in a reduction of binding by 35%-40%. When FRTL-5 cells were preincubated with heparitinase or heparinase I, which released 20%-30% of cell surface HSPGs, Tg binding was reduced to a similar extent. An antibody against a Tg heparin-binding site functionally related to a major megalin-binding site virtually abolished Tg binding to HS and to FRTL-5 cells, supporting the hypothesis that combined interactions of Tg with HSPGs and with megalin are involved in Tg binding to rat thyroid cells.

Role of insulin and serum on thyrotropin regulation of thyroid transcription factor-1 and pax-8 genes expression in FRTL-5 thyroid cells

Thyrotropin (TSH), via its cyclic adenosine monophosphate (cAMP) signal, decreases thyrotropin receptor (TSHR) gene expression in FRTL-5 thyroid cells, whereas it increases expression of the thyroglobulin (Tg) gene. Despite the opposite effects of TSH on TSHR and Tg expression, both genes are positively controlled by thyroid transcription factor-1 (TTF-1) and evidence has accumulated that TSH can decrease TTF-1 mRNA levels. In this report, we further characterize the action of TSH on TTF-1 in order to understand its different activities on the TSHR and Tg genes better. The effect of TSH on the TSHR requires the presence of insulin and serum and we show here that also both factors are necessary for the TSH effect to decrease TTF-1 mRNA levels. The decrease is paralleled by a downregulation of TTF-1 protein levels as well as by a decrease in TTF-1/DNA complex when the TTF-1 site of the TSHR promoter was used as probe. Again, the decrease requires insulin and serum. The TSH downregulation of TTF-1 mRNA levels is due to a decrease in its transcription rate. Using a luciferase-linked chimera construct spanning 5.18 kb of the TTF-1 5'-flanking region, we show that TSH decreases TTF-1 promoter activity and that this effect depends on insulin and serum. These data contrast with the action of TSH on Tg and Pax-8 gene expression. TSH increases Pax-8 mRNA levels and the increase is evident whether insulin and serum are present or not. Moreover, this increase is paralleled by an increase in Pax-8 protein binding to an oligonucleotide derived from the C site of the Tg promoter, which can bind both TTF-1 and Pax-8. The present data thus show that TTF-1 gene expression is interdependently regulated by TSH and serum growth factors including insulin. They also show this interdependent-regulation is not duplicated in the case of Pax-8. We suggest that these differences may contribute to the distinct ability of TSH to regulate TSHR versus Tg gene expression in FRLT-5 thyroid cells.

The rat hepatic lectin-1 subunit of the asialoglycoprotein receptor is upregulated by thyrotropin and downregulated by neoplastic transformation of thyroid cells

We have recently shown that the rat hepatic lectin (RHL)-1 subunit of the asialoglycoprotein receptor (ASGPr) is expressed in the PC C13 differentiated thyroid cell line. To investigate in vivo the expression of RHL-1 and the ability of thyrotropin (TSH) to modulate its expression, reverse-transcriptase polymerase chain reaction (RT-PCR) and Western blot assays have been performed on thyroid extracts from rats treated with thyroxine (T4) or propylthiouracil (PTU), each of which modulates TSH levels. It is shown that RHL-1 expression is down-regulated by T4 (which decreases serum TSH) and upregulated by PTU (which increases serum TSH), at both mRNA and protein levels. The sensitivity of RHL-1 to neoplastic transformation of thyroid cells has been investigated. The RHL-1 expression pattern has been studied in PC C13 thyroid cells transformed by several oncogenes that induce different degrees of malignancy and dedifferentiation. RT-PCR and Western blot assays show that RHL-1 expression progressively decreases as PC C13 cells acquire a more transformed phenotype. Expression of glyceraldehyde-3-phosphate dehydrogenase (GAPDH) mRNA, a housekeeping gene used as internal control to normalize RHL-1 mRNA content, exhibits no variations in the different PC C13 cell lines used. In addition, we show that both native and asialo-thyroglobulin (Tg) bind RHL-1 in vitro, and native Tg binds RHL-1 on the surface of PC C13 cells. After thyroid cells transformation, the surface expression of RHL-1 is inhibited in a measure that correlates with the mRNA and protein levels. Therefore, the RHL-1 inhibition at the mRNA, protein and plasma membrane expression follows a gradient that parallels the progressive acquisition of the fully transformed phenotype in the PC C13 system. The results reported in the present article, together with our previous data, suggest that RHL-1 expression could be regulated, at least in part, by the same transcription factors involved in the expression of the other molecules characteristic of the thyroid differentiated state.

Identification of a heparin-binding region of rat thyroglobulin involved in megalin binding

We recently showed that thyroglobulin (Tg) is a heparin-binding protein and that heparin inhibits binding of Tg to its endocytic receptor megalin (gp330). Here we have identified a heparin-binding region in the carboxyl-terminal portion of rat Tg and have studied its involvement in megalin binding. Rat thyroid extracts, obtained by ammonium sulfate precipitation, were separated by column fractionation into four Tg polypeptides, with apparent masses of 660, 330, 210, and 50 kDa. As assessed by enzyme-linked immunoadsorbent assays and ligand blot binding assays, megalin bound to intact Tg (660 and 330 kDa) and, to a even greater extent, to the 210-kDa Tg polypeptide. Furthermore, the 210-kDa Tg polypeptide inhibited megalin binding to intact Tg by approximately 70%. Solid phase assays showed binding of biotin-labeled heparin to intact Tg and to the 210-kDa Tg polypeptide. We characterized the 210-kDa Tg polypeptide by matrix-assisted laser desorption/ionization mass spectrometry analysis and found that it corresponds to the carboxyl-terminal portion of rat Tg. We developed a synthetic peptide corresponding to a 15-amino acid sequence in the carboxyl-terminal portion of rat Tg (Arg(689)-Lys(703)), containing a heparin-binding consensus sequence (SRRLKRP) and demonstrated heparin binding to this peptide. A rabbit antibody raised against the peptide recognized intact Tg in its native conformation and under denaturing conditions. This antibody markedly reduced heparin-binding to intact Tg, indicating that the region of native Tg corresponding to the peptide is involved in heparin binding. Furthermore, the anti-Tg peptide antibody almost completely inhibited binding of megalin to Tg, suggesting that the Tg region containing the peptide sequence is required for megalin binding. Physiologically, Tg binding to megalin on thyroid cells may be facilitated by Tg interaction with heparin-like molecules (heparan sulfate proteoglycans) via adjacent binding sites.

A comparative review of the structure and biosynthesis of thyroglobulin

Thyroglobulin, the major iodoglycoprotein of the thyroid (Mr 669 kDa) has a sedimentation coefficient of 19 S and an isoelectric point (pI) of 4.4-4.7. The protein has been isolated and purified from saline extracts of the gland of several animal species, by methods such as ammonium sulfate fractionation, DEAE-cellulose chromatography and Sepharose 4B/6B gel-filtration. DEAE-cellulose chromatography of thyroglobulin from many species, by linear gradient, yielded a complex elution pattern, while camel thyroglobulin showed only a major and minor peak. As an iodoprotein, the protein has 0.1-2.0% iodine. The amino acid and iodoamino acid composition of thyroglobulins, in general, is similar. However, a high thyroxine content (15 mol/mol protein) has been noted for buffalo species. Asparagine or aspartic acid has been reported as the major N-terminal amino acid for thyroglobulins of several animal species whereas glutamic acid is the sole N-terminal amino acid for buffalo thyroglobulin. As a glycoprotein, thyroglobulin contains 8-10% total carbohydrate with galactose, mannose, fucose, N-acetyl glucosamine and sialic acid residues. The carbohydrate in the protein is distributed as two distinct units, A and B. In addition, human thyroglobulin has carbohydrate unit C. The occurrence of sulfate and phosphate as Gal-3-SO4 and Man-6-PO4, respectively, has been reported in few species. The quaternary structure of native thyroglobulin is comprised of two equal sized subunits of 330 kDa. However, the protein appears to contain 4-8 non-identical units in few species. The synthesis of thyroid hormones occurs in the matrix of the protein and is regulated by pituitary thyrotropin. The role of tyrosine residues 5 and 130 in thyroxine synthesis has been well documented.

Intrathymic expression of genes involved in organ specific autoimmune disease

Insulin, thyroglobulin and myelin basic protein (MBP) are implicated as autoantigens in the autoimmune diseases, insulin-dependent diabetes mellitus (IDDM), autoimmune thyroid-disease and multiple sclerosis. Self tolerance to these antigens, until recently only thought to be present extrathymically, is generally considered to be maintained by 'peripheral' mechanisms, such as clonal anergy or clonal ignorance. The techniques of reverse transcription and polymerase chain reaction (RT-PCR) were used to investigate the intrathymic expression of these genes. Expression was examined in mRNA isolated from complete adult rat thymus, various mouse thymic cell-types isolated from fetal thymic-organ cultures and from neonatal-mouse thymocyte subsets. mRNA for insulin, thyroglobulin and MBP were detected in unfractionated adult rat and embryonic mouse thymus. Rat thymus expressed both insulin I and II, while mouse thymus only expressed insulin II. Thyroglobulin and MBP, but not insulin mRNA were detected in mouse MHC class II+ thymic epthelial cells and class II+ dendritic cells and in certain thymocyte subsets. The presence of insulin, thyroglobubin and MBP mRNA in the thymus has important implications for the development of the T-cell repertoire, particularly for the mechanisms of tolerance that prevent autoreactivity to these antigens in healthy individuals.

Cloning and characterization of murine thyroglobulin cDNA

Thyroglobulin is used to induce in mice experimental autoimmune thyroiditis (EAT), a model for Hashimoto thyroiditis. Because murine thyroglobulin is a more potent inducer of EAT than heterologous thyroglobulins, it has been hypothesized that it contains unique pathogenic epitopes. The validation of this hypothesis has been hampered by the lack of the murine thyroglobulin sequence. To identify murine-specific areas in thyroglobulin, we cloned, by reverse transcriptase PCR, and sequenced the complete murine thyroglobulin cDNA. This encodes a polypeptide of 2748 amino acids that is 73.5 and 71.8% identical to bovine and human thyroglobulin, respectively. Six regions are unique to each species. We also analyzed through EpiMer the sequences able to bind to the I-Ek major histocompatibility allele and, therefore, function as T cell epitopes. EpiMer analysis showed seven murine-specific T cell epitopes in thyroglobulin. The availability of the complete murine thyroglobulin sequence should promote the understanding of the pathogenesis and immunoregulation of EAT.

Identification of a new thyroglobulin variant: a guanine-to-adenine transition resulting in the substitution of arginine 2510 by glutamine

We analyzed thyroglobulin (Tg) reverse transcription polymerase chain reaction (RT-PCR) products from three congenital goiters and three normal thyroid tissues by Taq I digestion. Tg coding sequences were amplified from position 57 to 8448 in 12 amplification fragments. A Taq I restriction fragment length polymorphism was detected in the most 3' RT-PCR product (nt 7584 through 8448). Data from the sequence showed a G-->A transition (nt 7627) causing the disappearance of the Taq I site in position 7625. It produced the substitution of arginine for a glutamine at position 2510. Afterwards, we established that the glutamine allele is present in normal unrelated individuals, with an allelic frequency of 62%. This Tg variant is thus widely represented in the human population. The available sequence information from rat and bovine Tg showed the presence, in both, of glutamine at position 2510.

Searching for pathogenic epitopes in thyroglobulin: parameters and caveats

Last year marked the 40th anniversary of the discovery that thyroglobulin (Tg) is a major autoantigen in autoimmune thyroiditis. The Tg molecule presents unique challenges for epitope mapping owing to its large size and extensive iodination. Consequently, pathogenic determinants have only recently been identified. Here, George Carayanniotis and Varada Rao summarize the approaches used to determine pathogenic Tg T-cell epitopes and discuss caveats in this unusual quest.

Contrasting immunopathogenic properties of highly homologous peptides from rat and human thyroglobulin

The current lack of amino acid sequence data for mouse thyroglobulin (Tg) necessitates mapping of pathogenic T-cell epitopes on heterologous Tg in mouse experimental autoimmune thyroiditis (EAT). A prevailing assumption has been that epitopes sharing a high degree of amino acid homology among heterologous Tg are likely to exhibit the same immunopathogenic properties in the same host. In this report, we have examined this concept while working with the 18-mer rat(r)Tg(2695-13) peptide that was previously shown to elicit 'A'-restricted T cells and EAT in SJL mice. A major immunopathogenic T-cell epitope was localized within the 12-mer rTg(2695-06). It was found that the human 12-mer homologue that carries two Ser substitutions at Glu2703 and Thr2704 exhibited contrasting properties: it failed to activate Th1 cells in lymphokine and proliferation assays; it did not cross-react with rTg(2695-06) at the T-cell level; and it induced only focal thyroiditis following adoptive transfer of specific lymph node cells. These data highlight the caveat involved in extrapolating results of pathogenic T-cell epitope mapping across heterologous Tgs, even when such epitopes share a high degree of amino acid homology.

Structures, alternative splicing, and neurexin binding of multiple neuroligins

Neuroligin 1 is a neuronal cell surface protein that binds to a subset of neurexins, polymorphic cell surface proteins that are also localized on neurons (Ichtchenko, K., Hata, Y., Nguyen, T., Ullrich, B., Missler, M., Moomaw, C., and Südhof, T. C. (1995) Cell 81, 435-443). We now describe two novel neuroligins called neuroligins 2 and 3 that are similar in structure and sequence to neuroligin 1. All neuroligins contain an N-terminal hydrophobic sequence with the characteristics of a cleaved signal peptide followed by a large esterase homology domain, a highly conserved single transmembrane region, and a short cytoplasmic domain. The three neuroligins are alternatively spliced at the same position and are expressed at high levels only in brain. Binding studies demonstrate that all three neuroligins bind to beta-neurexins both as native brain proteins and as recombinant proteins. Tight binding of the three neuroligins to beta-neurexins is observed only for beta-neurexins lacking an insert in splice site 4. Thus, neuroligins constitute a multigene family of brain-specific proteins with distinct isoforms that may have overlapping functions in mediating recognition processes between neurons.

Mouse thyroglobulin: conservation of sequence homology in C-terminal immunogenic regions of thyroglobulin

cDNA encoding 287 amino acids of the C-terminus of mouse thyroglobulin was cloned and sequenced. The amino acid homology between mouse and rat thyroglobulin was 96%, and was 78% between mouse and human. It was found that mouse thyroglobulin completely shared homology with two thyroiditogenic peptides described by other investigators. These findings are consistent with our hypothesis that in murine experimental thyroiditis, the primary thyroiditogenic epitopes are encoded by mouse-specific regions of thyroglobulin.

Mutated human Kirsten ras, driven by a thyroglobulin promoter, induces a growth advantage and partially dedifferentiates rat thyroid epithelial cells in vitro

We have earlier shown that expression of the human activated Ki-ras, directed by the rat thyroglobulin (TG) promoter in the thyroid gland of transgenic mice, is able to induce thyroid benign tumors, albeit at low incidence. A likely explantation of our results is that the low levels of exogenous Ki-ras transcripts are not sufficient to induce multifocal tumors in the thyroid gland. We have performed experiments to analyze the effects of a similar construct in vitro upon thyroid-cell proliferation and differentiation. Transfection of FRTL-5 rat thyroid cells with the human Ki-rasval12 fused to the rat TG promoter is rapidly followed by reduced expression of the differentiation markers thyroglobulin, thyroperoxydase and thyrotropin receptor, but not by fully malignant cell transformation. The data reported support the hypothesis that Ki-ras mRNA levels are critical to the process of complete neoplastic transformation of thyroid epithelial differentiated cells in vitro.

Neuroligin 1: a splice site-specific ligand for beta-neurexins

Neurexins are neuronal cell surface proteins with hundreds of isoforms generated by alternative splicing. Here we describe neuroligin 1, a neuronal cell surface protein that is enriched in synaptic plasma membranes and acts as a splice site-specific ligand for beta-neurexins. Neuroligin 1 binds to beta-neurexins only if they lack an insert in the alternatively spliced sequence of the G domain, but not if they contain an insert. The extracellular sequence of neuroligin 1 is composed of a catalytically inactive esterase domain homologous to acetylcholinesterase. In situ hybridization reveals that alternative splicing of neurexins at the site recognized by neuroligin 1 is highly regulated. These findings support a model whereby alternative splicing of neurexins creates a family of cell surface receptors that confers interactive specificity onto their resident neurons.

Detection of primary and mature transcripts of calcitonin-gene-related peptide genes in rat parafollicular cells by light, fluorescence and confocal microscopy

Alternative splicing of primary transcripts from the calcitonin/alpha calcitonin gene-related peptide (alpha CGRP) gene result in mature mRNAs encoding either calcitonin or alpha CGRP. We have produced sequence-specific, synthetic, biotinylated oligodeoxynucleotide probes that recognize calcitonin (exon 4), and alpha CGRP (exon 6) sequences as well as sequences common to both splice variants (exon 3) of this gene. Probes to exons 4 and 3 revealed strong cytoplasmic signals in rat parafollicular cells. In addition, a punctate nuclear signal was obtained with these probes. The alpha CGRP-specific (exon 6) probe resulted in weak cytoplasmic labelling of parafollicular cells, but produced a punctate nuclear labelling similar to that seen with the exon 4 and 3 probes. RNase digestion removed all the cytoplasmic and nuclear signals obtained with all probes. Hybridization with a thyroglobulin-specific probe failed to label parafollicular cells. A control (human enterovirus) probe yielded negative results, while a probe to rat somatostatin produced cytoplasmic labelling of a small subpopulation of parafollicular cells. Finally, a probe specific for beta CGRP mRNA labelled most, if not all, parafollicular cells. Fluorescent alkaline phosphatase development of in situ hybridizations could be combined with indirect immunofluorescence for CGRP. Analysis by fluorescence and confocal microscopy revealed that CGRP immunoreactive cells contained calcitonin, alpha CGRP and beta CGRP hybridization signals. Our results demonstrate that all three genes may be simultaneously expressed by thyroid parafollicular cells and show that synthetic biotinylated oligonucleotide probes can be used for highly precise localizations of primary transcripts in the nuclei of these cells.(ABSTRACT TRUNCATED AT 250 WORDS)

Distinct genetic pattern of mouse susceptibility to thyroiditis induced by a novel thyroglobulin peptide

Experimental autoimmune thyroiditis (EAT), induced by thyroglobulin (Tg) and adjuvant, is major histocompatibility complex-controlled and dependent on Tg-reactive T cells, but the immunopathogenic T-cell epitopes on Tg remain mostly undefined. We report here the thyroiditogenicity of a novel rat Tg peptide (TgP2; corresponding to human Tg amino acids 2695-2713), identified by algorithms as a site of putative T-cell epitope(s). TgP2 causes EAT in SJL (H-2s) but not in C3H or B10.BR (H-2k), BALB/c (H-2d), and B10 (H-2b) mice. This reveals a new genetic pattern of EAT susceptibility, since H-2k mice are known to be high responders (susceptible) after Tg challenge. Following in vivo priming with TgP2, T cells from only SJL mice proliferated significantly and consistently to TgP2 in vitro, whereas TgP2-specific IgG was observed in all strains tested. Adoptive transfer of TgP2-primed SJL lymph node cells to naive syngeneic recipients induced a pronounced mononuclear infiltration of the thyroid, which was more extensive than that observed after direct peptide challenge. TgP2 is non-immunodominant, since priming of SJL mice with rTg did not consistently elicit T-cell responses to TgP2 in vitro and a TgP2-specific T-cell hybridoma did not respond to antigen presenting cells pulsed with rTg. The data support the notion that Tg epitopes need not be either iodinated or immunodominant in order to cause severe thyroiditis and that the genetic pattern of the disease they induce can be distinct from that of Tg-mediated EAT.

Preferential sites of proteolytic cleavage of bovine, human and rat thyroglobulin. The use of limited proteolysis to detect solvent-exposed regions of the primary structure

The sites and the sequence of the proteolytic cleavages of bovine, human and rat thyroglobulin, during the limited proteolysis with thermolysin and trypsin, were determined by sequencing the NH2 termini of the peptides produced and comparing them to the cDNA-derived sequences of bovine, human and rat thyroglobulin. Major cleavage sites of bovine thyroglobulin included residues 240, 502, 993, 1218, 1784 with thermolysin, and 240, 520, 1142, 1783, 2515 with trypsin. Cleavage sites of human thyroglobulin included residues 503, 982, 990, 1405, 1831 with thermolysin, and 522, 1627, 2513 with trypsin. Those of rat thyroglobulin included residues 501, 1776, 1784 with thermolysin, and 522, 1771, 1825, 2515 with trypsin (numbered as in bovine thyroglobulin). Thus, thyroglobulin from various species presents well localized and conserved regions particularly sensitive to proteolysis. The most sensitive region extended for 30 residues after residue 500. Another major cluster of cleavages was centered around residue 1800; this region was only partially sensitive in human thyroglobulin. A conserved tryptic site lay at the COOH terminus of the molecule. Most cleavage sites occurred within the inserted sequences that disrupt the Cys-rich, tandem repeats of thyroglobulin and either contain or are located near exon-intron junctions. Several cleavage sites lay in proximity of early iodinated or hormonogenic tyrosyl residues or of putative N-linked glycosylation sites. While a predominantly beta-type secondary structure and a rigid three-dimensional structure were predicted for the Cys-rich repeats, stretches of predicted alpha-helices, beta-strands and irregular structure were interspersed in the regions surrounding the cleavage sites. These data demonstrate the existence of conserved regions of thyroglobulin inherently sensitive to proteolysis, which most likely represent solvent-exposed regions of the primary structure, possibly forming loops at the surface of thyroglobulin.

TSH-induced galactose incorporation at the NH2 terminus of thyroglobulin secreted by FRTL-5 cells

FRTL-5 cells were cultured in media containing standard growth factors with or without TSH, plus labeled precursors of N-linked oligosaccharide chains. The thyroglobulin secreted in the medium was purified and fragmented with CNBr. Three peptides were identified by NH2-terminal sequencing, that were labeled mainly with D-[2-3H]mannose, independent of TSH. One of them, corresponding to the NH2-terminus of thyroglobulin, incorporated both more D-[2-3H]mannose and more D-[1-3H]galactose upon TSH addition. These data likely reflect a TSH-induced increment of N-linked glycosylation at the NH2-terminus of thyroglobulin, mostly with the maturation of high-mannose to complex chains.

Gene order and genetic distance of 13 loci spanning murine chromosome 15

Thirteen genetic loci spanning murine chromosome 15 from 15A2 (Mlvi-2) to 15F2-3 (Gdc-1) have been mapped. The genetic distance extends to 55.4 cM. Among 151 animals, only 1 animal with a double cross-over was found. The linear order is unambiguous, with the exception of the distal end on 15F1-3. Our analysis favors the order cen-Ela-1/Hox-3-Wnt-1-Gdc-1-ter. This ordering makes necessary the introduction of three tightly spaced double recombination events around and within the Hox-3 locus. Alternatively, Hox-3 may be most distal, and several double recombinations at the telomere lead to map expansion. Despite the unequal distribution along chromosome 15 of G-versus R-bands, a comparison of distances determined by physical and genetic mapping does not indicate an overt difference in distance between both mapping techniques.

An interspecific linkage map of mouse chromosome 15 positioned with respect to the centromere

We have used an interspecific backcross between C57BL/6J and Mus spretus to derive a molecular genetic linkage map of chromosome 15 that includes 25 molecular markers and spans 93% of the estimated length of chromosome 15. Using a second interspecific backcross that was analyzed with a centromere-specific marker, we were also able to position our map with respect to the chromosome 15 centromere. This map provides molecular access to many discrete regions on chromosome 15, thus providing a framework for establishing relationships between cloned DNA markers and known mouse mutations and for identifying homologous genes in mice and humans that may be involved in disease.

Molecular analysis of the aberrant replication banding pattern on chromosome 15 in murine T-cell lymphomas

Cytogenetic techniques revealed an altered early replication banding pattern on the distal part of chromosome 15 in some murine T-cell lymphomas. This pattern reverted back to normal replication in somatic cell hybrids that had become non-tumorigenic after fusion of leukemic cells with normal fibroblasts. The altered banding pattern was correlated with malignancy. To investigate the molecular basis of the aberrant pattern in more detail, centrifugal elutriation of cells containing bromodeoxyuridine labeled DNA was used to prepare newly replicated DNA from selected intervals of the S-phase from tumor cells, as well as from hybrid cells with the revertant phenotype. These different DNA fractions were probed for DNA sequences distributed over the distal half of chromosome 15. Only two out of ten chromosome 15 specific genes tested showed a clear change in replication timing between the two different cell lines tested. These two genes were the lymphocyte antigen-6, Ly-6, and the neighboring thyroglobulin gene, Tgn, which replicated at the beginning of S in the tumor cells and later in S in the non-tumorigenic hybrid cells.

Efficient thyroid hormone formation by in vitro iodination of a segment of rat thyroglobulin fused to Staphylococcal protein A

A polypeptide of 224 amino acids from the C terminus of rat thyroglobulin fused to Staphylococcal protein A (TgC 224), containing 3 tyrosines which have been shown to be hormonogenic in vivo (Tyr-2555, -2569 and -2748), forms thyroid hormones with relatively high efficiency upon in vitro enzymatic iodination using, most likely, the hormonogenic Tyr-2555 and Tyr-2569. Acetylcholinesterase, which has sequence and structural homology with the C terminus of the thyroglobulin molecule and bovine serum albumin, used as control proteins, formed thyroid hormones with lower efficiency. These results validate our experimental approach to define the structural requirements for thyroid hormone formation using thyroglobulin fragments.

Identification of a thyroxine-containing self-epitope of thyroglobulin which triggers thyroid autoreactive T cells

Although thyroglobulin (Tg), the thyroid prohormone, is well known as a T cell dependent autoantigen in human and experimental autoimmune thyroid disease, very little is known about the molecular basis of Tg recognition by T cells. In this paper, we have characterized the epitopes recognized by two clonotypically distinct, murine Tg autoreactive T cell hybridomas, CH9 and ADA2. In vitro iodination of a Tg preparation which was deficient in in vivo organified iodine was first used to confirm our previous observation that these T cells recognize iodination-related epitopes in the Tg molecule. Affinity chromatography of tryptic peptides derived from normally iodinated human Tg revealed that these epitopes were exclusively located in thyroxine (T4) containing peptides. Through the use of synthetic T4-containing peptides, representing the four major hormonogenic sites in Tg, we demonstrated that both CH9 and ADA2 recognize an epitope containing the T4 at position 2553 in human Tg. Sets of overlapping 5mer to 12mer peptides around this T4 showed that the most potent peptide was a 9mer beginning at Asp 2551. The T4 was shown to be a critical residue, since its replacement with any of the 20 naturally occurring amino acids produced only nonstimulatory peptides. Since the T cell hybridomas could also be stimulated by major histocompatibility complex class II positive (interferon-gamma-treated) thyroid epithelial cells in vitro, and their parent T cell lines can induce thyroiditis on adoptive transfer, the T4-containing Tg sequence described here is implicated as a pathogenic epitope in murine thyroid autoimmunity.

Cholinesterase-like domains in enzymes and structural proteins: functional and evolutionary relationships and identification of a catalytically essential aspartic acid

Primary sequences of cholinesterases and related proteins have been systematically compared. The cholinesterase-like domain of these proteins, about 500 amino acids, may fulfill a catalytic and a structural function. We identified an aspartic acid residue that is conserved among esterases and lipases (Asp-397 in Torpedo acetylcholinesterase) but that had not been considered to be involved in the catalytic mechanism. Site-directed mutagenesis demonstrated that this residue is necessary for activity. Analysis of evolutionary relationships shows that the noncatalytic members of the family do not constitute a separate subgroup, suggesting that loss of catalytic activity occurred independently on several occasions, probably from bifunctional molecules. Cholinesterases may thus be involved in cell-cell interactions in addition to the hydrolysis of acetylcholine. This would explain their specific expression in well-defined territories during embryogenesis before the formation of cholinergic synapses and their presence in noncholinergic tissues.

Evolutionary origin of thyroglobulin by duplication of esterase gene

Rat liver microsomal carboxyesterase E1 was found to have homology with five esterases and with the COOH-terminal parts of two thyroglobulins. A phylogenetic tree constructed for these proteins shows that this new superfamily has evolved from a common ancestral gene that encoded a carboxyesterase. The tree also shows that the ancestral gene already existed before the divergence of vertebrates and invertebrates and later its duplicated genes gained various kinds of esterase activity. According to the tree, one of the duplicated genes evolved into the COOH-terminal half of thyroglobulin by a gene fusion with a DNA sequence whose evolutionary origin is unknown.

The mRNA levels of thyrotropin receptor, thyroglobulin and thyroid peroxidase in neoplastic human thyroid tissues

Expression levels of thyrotropin receptor (TSH-R), thyroglobulin (Tg) and thyroid peroxidase (TPO) mRNA in normal and neoplastic human thyroid tissues (6 adenomas and 7 carcinomas) were investigated by Northern-blot and slot-blot analyses. We found that TSH-R mRNA levels were significantly lower in carcinoma tissues than in normal tissues. The levels of Tg mRNA were also significantly lowered in adenoma and carcinoma tissues as compared to normal tissues. In contrast, no significant difference was observed in the expression levels of TPO mRNA between these tissues. Furthermore, TSH-R mRNA levels were well-correlated with Tg mRNA levels in neoplastic tissues. These results suggest that mRNAs of TSH-R and Tg are expressed in relation to their degree of differentiation.

Characterization of a murine cDNA encoding a member of the carboxylesterase multigene family

We have characterized a mRNA sequence containing the entire coding region of a mouse carboxylesterase (EC 3.1.1.1). pEs-N, an 1840-bp composite of five overlapping cDNA clones, contains an open reading frame of 554 amino acids that display a high degree of similarity with rat and rabbit carboxylesterases. Genetic mapping studies place this carboxylesterase in cluster 1 of the esterase region on chromosome 8. Results of blot hybridization analysis of genomic DNA probed with a pEs-N cDNA under both low and high stringency conditions suggest membership in a carboxylesterase multigene family, as would be expected for a nonspecific carboxylesterase. A message size of 1850-1900 nucleotides was revealed by RNA blot hybridization analysis. S1 nuclease protection analyses with a probe representing a segment of pEs-N detected message in liver, kidney, and lung, but not in spleen, brain, testes, and submandibular gland, with higher levels in female than in male kidney. Additional S1 nuclease-protected mRNA species were found, suggesting the expression of distinct members of a multigene family. In vitro translation of a full-size transcript of pEs-N resulted in a product of 51.5 kDa. Upon the addition of microsomes, this product was processed into a protein of 60.4 kDa, which is within the size range of monomeric units of mouse carboxylesterases.

The site of the molecular defect in the thyroid gland of the hyt/hyt mouse: abnormalities in the TSH receptor-G protein complex

The hyt/hyt mouse has a severe and pervasive primary inherited hypothyroidism with significantly depressed serum T4, elevated serum and pituitary TSH, and reduced thyroid gland iodide uptake. Previous ultrastructural and histologic analysis of the hyt/hyt thyroid gland along with these biochemical abnormalities support an inherited defect in TSH responsiveness of the hyt/hyt thyroid gland. In order to evaluate the potential site of the defect in the hyt/hyt mouse, we have studied the hyt/hyt gland and hyt/hyt TSH from a biochemical and molecular standpoint. Based on demonstrated bioactivity of hyt/hyt serum in the McKenzie bioassay, this reduced responsiveness to TSH in the hyt/hyt mouse is not due to reduced bioactivity of hyt/hyt TSH or a major structural abnormality in the hyt/hyt TSH molecule. In comparison to hyt/ + euthyroid littermates and +/+ BALB/cBY progenitor strain mice, the hyt/hyt mouse demonstrates a twofold reduction in thyroid gland basal cAMP and a markedly diminished response of adenylyl cyclase to exogenous TSH. However, hyt/hyt cAMP production is equivalent to the euthyroid mice after stimulation of thyroid glands by forskolin, cholera toxin, PGE1, and isoproterenol. These results support a defect in the TSH-G protein-adenylyl cyclase system in the hyt/hyt thyroid gland. Specifically, these findings suggest that the hyt/hyt mouse has a defect in TSH responsivity due to an inherited defect in the thyroid gland TSH receptor molecule. Since the hyt/hyt gland makes T3 and T4 but at diminished levels, the proposed defect in the TSH receptor would still impart partial function. Both hyt/hyt and euthyroid hyt/ + littermates make TSH receptor mRNAs of 5500 and 2400 base pairs. This suggests that the receptor defect does not represent a major structural abnormality of the gene. The receptor defect could represent a reduction in receptor number, receptor-TSH affinity, or TSH receptor-G protein coupling. The specificity of this effect on adenylyl cyclase-cAMP is shown by the reduction of TSH-cAMP regulated thyroid peroxidase (TPO) and thyroglobulin mRNAs in the hyt/hyt thyroid gland. Given the importance of TPO and thyroglobulin in normal thyroid hormone synthesis, the reductions in TPO and thyroglobulin mRNAs in the hyt/hyt thyroid gland may underlie the significant decrease in thyroid hormone production by the hyt/hyt mouse.

cDNA cloning of human-milk bile-salt-stimulated lipase and evidence for its identity to pancreatic carboxylic ester hydrolase

We have isolated and sequenced cDNA clones covering the entire coding sequence of human-milk bile-salt-stimulated lipase, as well as 996 nucleotides of the 3' end of the pancreatic enzyme carboxylic ester hydrolase. The deduced amino acid sequence of the lipase starts with a 23-residue leader peptide. The open reading frame continues with 722 amino acid residues. The sequence contains in the C-terminal part a proline-rich repeat, 16 repeats of 11 amino acid residues each. The mRNA was estimated to be approximately 2500 nucleotides from Northern blot and of similar size in mammary and pancreatic tissues. Data obtained indicate that the lipase and the carboxylesterase are identical and coded for by the same gene. The cDNA is 2428 bases long, which indicates that a near full-length copy of the transcript has been isolated. Comparisons with other enzymes show that the lipase is a new member of the supergene family of serine hydrolases. It is not only closely related (and in its N-terminal half virtually identical) to lysophospholipase from rat pancreas and cholesterol esterase from bovine pancreas, but also shows a high degree of similarity to several esterases, e.g. acetylcholine esterase. In contrast, no such similarity could be found to typical lipases.

Extinction and activation of the thyroglobulin promoter in hybrids of differentiated and transformed thyroid cells

Thyroglobulin gene expression was repressed in a rat thyroid cell line transformed with Kirsten murine sarcoma virus. Expression of a dominant selectable marker driven by the thyroglobulin promoter was also inhibited. Somatic cell hybridization of transformed and differentiated thyroid cells resulted in extinction of thyroglobulin gene expression. When transformed cells carrying a dominant selectable marker driven by the thyroglobulin promoter were fused to differentiated cells and expression of this marker was selected, we obtained stable hybrid cell lines expressing both the endogenous and the exogenous thyroglobulin promoters. Although the expression of v-ras remained unchanged compared with expression in the parental transformed cells, transformation was suppressed in the hybrid cell lines. The other thyroid differentiation markers, iodide uptake and thyroid-stimulating hormone-dependent growth, were inhibited in all the hybrids tested. We show that activity of the thyroglobulin promoter correlates with the presence of a thyroid nuclear factor that binds the promoter at position -60 from the transcription start site. Loss of this factor accompanies the extinction of thyroglobulin gene expression in hybrids selected for expression of a non-thyroid-specific promoter.

Extinction and activation of the thyroglobulin promoter in hybrids of differentiated and transformed thyroid cells

Thyroglobulin gene expression was repressed in a rat thyroid cell line transformed with Kirsten murine sarcoma virus. Expression of a dominant selectable marker driven by the thyroglobulin promoter was also inhibited. Somatic cell hybridization of transformed and differentiated thyroid cells resulted in extinction of thyroglobulin gene expression. When transformed cells carrying a dominant selectable marker driven by the thyroglobulin promoter were fused to differentiated cells and expression of this marker was selected, we obtained stable hybrid cell lines expressing both the endogenous and the exogenous thyroglobulin promoters. Although the expression of v-ras remained unchanged compared with expression in the parental transformed cells, transformation was suppressed in the hybrid cell lines. The other thyroid differentiation markers, iodide uptake and thyroid-stimulating hormone-dependent growth, were inhibited in all the hybrids tested. We show that activity of the thyroglobulin promoter correlates with the presence of a thyroid nuclear factor that binds the promoter at position -60 from the transcription start site. Loss of this factor accompanies the extinction of thyroglobulin gene expression in hybrids selected for expression of a non-thyroid-specific promoter.

Isolation and characterization of hagfish thyroid iodoprotein by its non-thyroglobulin nature, very high iodine and carbohydrate contents and low hormone/iodine ratio

We have characterized the thyroid iodoprotein of a hagfish, Eptatretus burgeri, one of the lowest marine vertebrates. The iodoprotein was not very homogeneous in its apparent molecular mass which decreased with the increase in hormone/iodotyrosine ratio. Four subfractions with an apparent molecular mass of about 400 kDa were purified from one major fraction by size-exclusion and Mono Q ion-exchange HPLC. The subfractions appeared to have the same peptide backbone, since they showed a single band with the same mobility as a 160-kDa protein in SDS/PAGE and the same amino acid composition. However they differed from each other in having increasing iodine contents (1.9% to 5.9% by mass of total amino acids) associated with the increase in hormonal iodine proportion (8.4% to 16.7% of total iodine) and carbohydrate content (35.6% to 53.5% by mass). These values are strikingly different from those of thyroglobulin with an iodine content of less than 1%, hormonal iodine of 20-40% and carbohydrate content of less than 10%. The amino acid composition of the hagfish iodoprotein, especially the cysteine content of less than 1%, was also entirely different from that of thyroglobulin. These results suggest that most, if not all, tyrosine residues of the hagfish thyroid glycoprotein with a less rigid structure are susceptible to an iodinating system, but hormone residues are formed by a much less efficient mechanism than those in thyroglobulin, when poorly iodinated.

Consensus residues at the acetylcholine binding site of cholinergic proteins

The nicotinic (nAcChR) and muscarinic (mAcCh) acetylcholine receptors and acetylcholinesterase (AcChEase) are structurally unrelated but share a common functional property: interaction with acetylcholine (AcCh). Alignment of the probable AcCh binding site regions of the nAcChR and mAcChR protein sequences revealed the presence of ten nearly identically spaced consensus residues, six of which contain potentially ligand-interactive side chains. Important elements of the consensus residues also were found in one unique sequence region of the AcChEases. Alignments among the two receptors and AcChEase outside the apparent binding region were rare, and the consensus AcCh binding residues were largely substituted in the homologous proteins, which do not bind AcCh. The consensus residues include two possible anionic subsite Asp residues and a Ser that may hydrogen bond to the AcCh carbonyl in the receptors. These residues correspond to positions Asp-166, Ser-173, and Asp-200 in the neuromuscular nAcChR; Asp-71, Ser-78, and Asp-105 in the M1 mAcChR; and Asp-93 and Asp-128 in Torpedo AcChEase. No corresponding consensus Ser is found in the AcChEase sequence; this is expected because of a downstream esterase active-site Ser-200 (Torpedo). A receptor-conserved and disulfide-linked Cys corresponding to neuromuscular nAcChR residue 193 and M1 mAcChR residue 97 may be important in energy transduction associated with agonist-mediated events. The presence of additional binding-site aromatic residues that may form a hydrophobic environment near the anionic subsite are aligned within, but not between, the three cholinergic protein groups. These observations target specific regions and residues within these proteins for structure-function studies of the cholinergic binding domain.

Studies on the structures of the normal and abnormal goat thyroglobulin genes

We have cloned the thyroglobulin (Tg) gene of normal goats and goitrous goats which have a Tg synthesis defect. At the 5'-end of the gene, we studied cosmid clones covering a region from 20 kilobases (kb) upstream from the Tg gene to 42 kb into it. Electron microscopy and restriction mapping show that this part of the gene contains 20 exons of 90-1190 bp, in total 4.9 kb of exonic information (56% of the mRNA) split by 19 introns of 150-9100 bp. The exons comprise 12% of the 5' sequences cloned. At the 3'-end, 55 kb were cloned, containing 10 kb of the gene which comprises only 3 exons of 550 bp in total. Sequence analysis of the 3'-end of the normal and abnormal Tg genes has revealed one transition mutation 3' to the reading frame in a stem-loop structure region of the last exon near the poly(A) addition site. Analysis of the promoter site and the first 5 exons has revealed only one difference between the normal and goitrous Tg genes: a Ser----Leu transition in exon 5. We also found an insertion in the fifth intron of the abnormal gene.

Alternative splicing may be responsible for heterogeneity of thyroglobulin structure

During the cloning of the bovine thyroglobulin cDNA, the restriction map of one of the recombinant plasmids was in disagreement with that of the full-length double-stranded thyroglobulin cDNA. When compared to the bovine Tg mRNA sequence, this cDNA clone exhibits a 333-nucleotide deletion which corresponds precisely to 2 exons of the Tg gene. It is thus likely that alternative processing of the premessenger RNA is at the origin of the deletion. The presence of giant introns in the vicinity of the dispensable exons may also reflect some error level in the splicing mechanism. Together with previous results the alternative splicing described in this study indicates that alternative processing of the Tg transcripts may be at the origin of thyroglobulin isoforms.