The hormonogenic sites of turtle thyroglobulin and their homology with those of mammals

Tyrosine 130 is an important outer ring donor for thyroxine formation in thyroglobulin

The thyroid couples two iodotyrosine molecules to produce thyroid hormone at the acceptor site in thyroglobulin, leaving dehydroalanine or pyruvate at the donor position. Previous work has located the acceptors but not the principal iodotyrosine donors. We incorporated [14C]tyrosine into beef thyroid slices, isolated and iodinated the [14C]thyroglobulin (Tg I), separated its N-terminal approximately 22-kDa hormone-rich peptide, and digested the latter with trypsin and endoproteinase Glu-C (EC 3.4.21.19). Nonlabeled thyroglobulin (Tg II) was isolated from the same glands and processed similarly, without iodination in vitro. Tg I was used to initially recognize pyruvate in peptide fractions, and Tg II was used to then identify its location in the thyroglobulin polypeptide chain. Sequencing of a tryptic peptide by mass spectrometry and Edman degradation showed a cleavage after Val129. An endoproteinase Glu-C-generated peptide had the predicted molecular mass of a fragment containing residues 130-146 with Tyr130 replaced by pyruvate; the identification of this peptide was supported by obtaining the expected shortened fragment after tryptic digestion. 14C-labeled pyruvate was identified in the same fraction as this peptide. We conclude that Tyr130 is an important donor of the outer iodothyronine ring. Its likely acceptor is Tyr5, the most important hormonogenic site of thyroglobulin, because Tyr5 and Tyr130 are proximate, because they are the most prominent early iodination sites in this part of thyroglobulin, and because the N-terminal region was previously found capable of forming T4 by itself.

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 the membrane receptor binding domain of thyroglobulin. Insights into quality control of thyroglobulin biosynthesis

The last stages of thyroglobulin maturation occur in the thyroid follicular lumen and include thyroid hormone formation and glycan completion. In this compartment, newly secreted thyroglobulins interact with a thyrocyte membrane receptor that prevents their premature lysosomal transfer and degradation. Both GlcNAc moieties and thyroglobulin peptide determinants are involved in receptor interaction. Here we used monoclonal antibodies (mAbs) directed against human thyroglobulin either to inhibit (mAb78) or to enhance (mAb240) the thyroglobulin binding and to identify the region of the thyroglobulin involved in the receptor recognition. Peptides containing the mAb epitopes were obtained by immunoscreening cyanogen bromide-derived native human thyroglobulin peptides and a cDNA thyroglobulin expression library. Three peptides, localized in the thyroglobulin N-terminal domain, were obtained. Peptides N1 (Ala1148-Gln1295) and N2 (Ser789-Met1008) were recognized by mAb240 and mAb78, respectively. None of them bound the receptor. The third peptide, N3 (Ser789-Met1172), (i) overlapped all or part of the N1 and N2 peptide sequences and was recognized by both mAbs, (ii) carried two complex glycans at Asn797 and Asn928, of which a subset presented accessible GlcNAc residues, and (iii) inhibited the thyroglobulin binding to FRTL5 cell membrane preparations. The N3 peptide includes tyrosine residues that have been reported to be involved in hormone formation. These results suggest that structural modifications closely associated with hormone formation within this domain act as sensors for the receptor interaction and thus for the intrafollicular retention or lysosomal homing of the prohormone.

Antigenic mapping of human thyroglobulin--topographic relationship between antigenic regions and functional domains

We characterized 26 mAb to human thyroglobulin to obtain a topographic map of the thyroglobulin antigenic surface. Among these mAb, three bind thyroglobulin peptides that are located in the primary sequence of thyroglobulin at either the N terminus or in the middle part of the molecule, three bind thyroglobulin via epitopes comprising the thyroid-hormone moiety, and three bind thyroglobulin through epitopes involved in the recognition of the molecule by its receptor. The 18 remaining mAb bind thyroglobulin through undetermined epitopes; most of these epitopes are resistant to trypsinization. We used two methods to map the antigenic regions of thyroglobulin: all 26 mAb were grouped, by means of cross-inhibition experiments, in 11 clusters corresponding to 11 antigenic regions of the thyroglobulin surface; by means of thyroglobulin peptides of decreasing size, obtained by time-controlled tryptic digestion, we analyzed the relative distance between pairs of epitopes in sandwich immunoassays. By combining these two methods, we organized most of the 11 antigenic regions on a topographic representation of the thyroglobulin surface. This new topographic map of thyroglobulin led us to some unexpected features of the thyroglobulin structure. First, antigenic region 8 located far from the N-terminal region is in close contact with two remote N-terminal antigenic regions (1 and 4), both involved in hormone formation. This antigenic region is likely to play a role in the correct positioning of hormonogenic tyrosines so as to optimize iodination-coupling reactions. Secondly, the domain involved in the binding of thyroglobulin to its receptor, probed by three mAb, is shared by two distinct mid-molecule antigenic regions, one being the main autoantigenic region of thyroglobulin.

Identification of hormonogenic tyrosines in fragment 1218-1591 of bovine thyroglobulin by mass spectrometry. Hormonogenic acceptor TYR-12donor TYR-1375

A fragment of bovine thyroglobulin encompassing residues 1218-1591 was prepared by limited proteolysis with thermolysin and continuous-elution polyacrylamide gel electrophoresis in SDS. The reduced and carboxymethylated peptide was digested with endoproteinase Asp-N and fractionated by reverse-phase high performance liquid chromatography. The fractions were analyzed by electrospray and fast atom bombardment mass spectrometry in combination with Edman degradation. The post-translational modifications of all seven tyrosyl residues of the fragment were characterized at an unprecedented level of definition. The analysis revealed the formation of: 1) monoiodotyrosine from tyrosine 1234; 2) monoiodotyrosine, diiodotyrosine, triiodothyronine (T3), and tetraiodothyronine (thyroxine, T4) from tyrosine 1291; and 3) monoiodotyrosine, diiodotyrosine, and dehydroalanine from tyrosine 1375. Iodothyronine formation from tyrosine 1291 accounted for 10% of total T4 of thyroglobulin (0.30 mol of T4/mol of 660-kDa thyroglobulin), and 8% of total T3 (0.08 mol of T3/mol of thyroglobulin). This is the first documentation of the hormonogenic nature of tyrosine 1291 of bovine thyroglobulin, as thyroxine formation at a corresponding site was so far reported only in rabbit, guinea pig, and turtle thyroglobulin. This is also the first direct identification of tyrosine 1375 of bovine thyroglobulin as a donor residue. It is suggested that tyrosyl residues 1291 and 1375 may support together the function of an independent hormonogenic domain in the mid-portion of the polypeptide chain of thyroglobulin.

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.