Probable internal homology in thyroglobulin peptide chain

Epitope mapping of human thyroglobulin reveals a central immunodominant region

Thyroglobulin is the thyroid hormone precursor and the major antigen frequently involved in autoimmune diseases. The primary structure of human thyroglobulin is known but the spatial structure remains largely undetermined. By using fusion protein produced in prokaryotic system we have characterized seven short immunoreactive peptides carrying at least one epitope. None of them includes hormonogenic sites, but five are concentrated in the central part of the monomeric molecule, which thus emerges as the major immunogenic region of this protein.

Prediction of the secondary structure of the carboxy-terminal third of rat thyroglobulin

A secondary structure prediction has been made using the available primary sequence data of the proposed carboxy-terminal of rat thyroglobulin. The model predicts 22% alfa-helix, 28% beta-structure and 17% beta turns. Out of the 8 possible carbohydrate acceptor-sites (Asn-x-Ser/Thr), 3 (residues 136, 368, 782) are associated with peptide sequences which favour the formation of beta-turn or loop-structures and are located in high hydrophilic regions. The entire sequence is predicted to be made up of two domains: one of them is highly structured, contains the hormonogenic sites, a cluster of tyrosines and at least one carbohydrate acceptor site.

Precursor-product relationship between the 26-kDa and 18-kDa fragments formed by iodination of human thyroglobulin

At moderate iodination levels (20 iodine at atoms/molecule), human thyroglobulin (hTgb) produces after reduction a thyroxinyl-peptide of 26 kDa which represents the N-terminal part of the protein. At higher iodination levels, the 26-kDa peptide is accompanied by another T4-containing peptide of 18 kDa. A precursor-product relationship between the 26- and 18-kDa fragments was demonstrated by the study of the tryptic fragments of both hormonopeptides. In addition, comparison with the protein sequence deduced from the nucleotide sequence of the 5'-end of hTgb mRNA demonstrated that the N-terminal region of Htgb from which are issued the 26-kDa peptide and its 18-kDa derivative is especially sensitive to proteolysis. This character is possibly related with a facilitated release of thyroid hormones in vivo.

Molecular organization of 19 S calf thyroglobulin

Controlled freezing and thawing of 19 S calf thyroglobulin resulted in a specific and reproducible breakdown of the protein. Beside the elementary chain (300,000 Da), new, discrete bands are revealed by gel electrophoresis in sodium dodecyl sulfate under reducing conditions. These new species consist of a major peptide of 100,000 Da and several faster-migrating bands. Most of these polypeptides were purified by preparative gel electrophoresis and individually digested in formic acid with CNBr. A comparative gel electrophoresis under denaturating and reducing conditions of (i) the fragments obtained from the native protein, (ii) the electrophoretically purified elementary chain, (iii) the 100,000-Da peptide, and (iv) a smaller fragment (of about 50,000) was performed. It revealed a very close homology among the peptide maps of the intact 19 S, the elementary chain, and the 100,000-Da peptide. Furthermore, it was shown that the digestion products of the smaller fragment, present in the peptide map of the native protein, were absent in both the elementary chain and the 100,000-Da species. These results support the idea that calf thyroglobulin, even though it has an apparently complex molecular organization, contains structural motifs which are repeated in the elementary chain.

The structure of a naturally occuring 10K polypeptide derived from the amino terminus of bovine thyroglobulin

A combination of data derived from peptide sequencing and nucleic acid sequencing of cloned cDNA fragments has been used to define the complete amino acid sequence of a 10,000 M.W., thyroxine containing polypeptide derived from bovine thyroglobulin. This fragment, TG-F, which was obtained following reduction and alkylation, has been placed at the amino terminus of the parent protein with hormone located at residue 5 in the primary sequence of the thyroglobulin molecule. The carboxyl terminal sequence of this fragment -Cys-Gln-Leu-Gln is found on the N-terminal side of a lys residue, suggesting that the peptide bond cleavage which occurs to produce this 80 residue fragment from the parent (330K) thyroglobulin chain is a gln-lys. In addition, the amino acid sequence of this 10K fragment contains: No sequence which would be a substrate for glycosylation and no carbohydrate. Several repeated homologous amino acid sequences. A striking number of beta-bends predicted from Chou-Fasman analyses, particularly near its carboxyl terminus.

Amino acid sequence of the unique 3,5,3'-triiodothyronine-containing sequence from porcine thyroglobulin

A unique T3-containing peptide was isolated by chemical and enzyme treatments from pTgb of different iodine contents and microsequenced: Leu-Ala-Ser-Lys-Ser-T3. This site can also form T4 to a small extent. It contains all the T3 of the protein and presents a special susceptibility to proteolysis thus suggesting a likely mechanism to explain that the thyroid secretes T3 preferentially to T4 in vivo.

Evidence for homologous repeating segments within the elementary polypeptide chain of guinea pig thyroglobulin

In this paper we report on the peptide maps of the three polypeptides produced by reduction of denatured 19-S thyroglobulin from guinea pig. These maps were obtained by both chemical cleavage (CNBr) and by limited enzymatic proteolysis (Staphylococcus aureus SV 8 protease). Analysis of these peptides showed a strict correspondence among the electrophoretic bands produced by cleavage of the guinea pig thyroglobulin components. These results support the idea that thyroglobulin, even though it has an apparently complex molecular structure, contains polypeptide sequences which are repeated in the elementary chain. Since similar structures should correspond to similar functions, each of these regions may work as a functional domain for the biosynthesis of the thyroid hormones.

Characterization of the major polypeptide chains of reduced bovine thyroglobulin

The finding that reduced 19S bovine thyroglobulin showed two major, closely migrating polypeptide bands during electrophoresis in SDS-polyacrylamide gels was presented as evidence that thyroglobulin consists of two nonidentical subunits, referred to as S and F, of approximately the same size (J. Biol. Chem. 253, 1853-1858 (1978)). It was, however, not clear whether the difference in migration rates of the two subunits was due to differences in amino acid or carbohydrate composition or a small difference in size. In this study, several physical and chemical properties of purified S and F polypeptides were investigated. Three methods revealed important differences between them. It was found by cyanogen bromide degradation that S contained one fragment (Mr approximately equal to 20 000) more than F. S eluted ahead of F by column chromatography in SDS. The sedimentation coefficient of S was found to be greater than that of F. However, the two subunits were shown to have similar amino acid and carbohydrate compositions and to be indistinguishable in their interaction with SDS. The different migration rates of S and F can therefore be explained by a small difference in size which agrees with the finding (Endocrinology 108, 1285-1292 (1981)) that a proteolytic enzyme present in commercial horseradish peroxidase can convert S into F.

The isoelectric focusing of human thyroglobulin

The isoelectric focusing of human thyroglobulin has been studied on slab gels. Three bands, focusing between pH 4.4 and 4.7, are observed. Deglycosylation of thyroglobulin does not affect the distribution of focused bands, but increases the pH range of focusing slightly. Native thyroglobulin and its half-sized subunit show the same distribution of isoelectric bands. Refocusing of one band results in the appearance of the three original bands. It appears that soluble complexes of thyroglobulin with ampholyte account for the apparent heterogeneity observed on isoelectric focusing.

Restriction mapping of synthetic thyroglobulin structural gene as a means of investigating thyroglobulin structure

Bovine 33 S thyroglobulin mRNA was reverse transcribed into double-stranded DNA under conditions allowing the synthesis of a complete 8 kilobase pair copy. A physical map of the resulting synthetic thyroglobulin structural gene was constructed using six restriction endonucleases. The following conclusions could be drawn: (i) the polypeptide chains in thyroglobulin subunits are identical; (ii) thyroglobulin is composed of a major class of molecules sharing the same primary structure; (iii) there is no evidence for precise internal repetition in the structure of thyroglobulin subunits.

An approach to the structure of thyroglobulin. Hormone-forming sequences in porcine thyroglobulin

The mixture of CNBr peptides obtained by treatment of porcine thyroglobulin with cyanogen bromide was separated into three fractions by Sephadex G-200 gel filtration in 1 M propionic acid. When one of these fractions was reduced and S-alkylated, a hormone-containing CNBr peptide was purified by filtration on Biogel A-1.5 m and ion-exchange chromatography on DEAE-Sephadex. Similarly, two other hormone-containing CNBr peptides were separated from another reduced and S-alkylated fraction by Sephacryl S-200 gel filtration and DEAE-Sephadex chromatography. The three purified CNBr peptides have the same Mr (15000), differ in amino acid composition and contain 60-70% of the hormones present in the initial thyroglobulin. Fragmentation of these peptides into smaller hormone peptides was carried out by trypsin digestion followed by gel filtration. This resulted in the purification of eight discrete hormone-containing tryptic peptides whose homogeneity was established by the study of their N-terminal and C-terminal sequences. At least four sites for the synthesis of thyroxine and two sites for the synthesis of triiodothyronine have been identified in the three CNBr peptides. A possible additional site was recovered in the form of the peptides seryl-triiodothyronine and probably seryl-thyroxine. To gain information on the number of hormone-forming sites in thyroglobulin iodinated in vivo and on the distribution of these sites in the different hormone-containing peptides isolated, estimation of thyroxine and triiodothyronine content of batches of thyroglobulin of iodine content comprised between 0.73% and 1.6% was carried out, together with the estimation of the hormone content of all the CNBr peptides separated from two preparations of thyroglobulin of different iodine contents (0.73% and 1.23%). It was shown that the number of thyroxine and triiodothyronine residues increased linearly with increasing thyroglobulin iodine content in the range 0.73-1.6% and reached six residues of thyroxine and two residues of triiodothyronine/mol protein for a thyroglobulin iodine content of 1.6% with no indication of saturation. This result is at variance with previous findings on porcine thyroglobulin iodinated in vivo which suggested that the thyroxine content reached a plateau of three to four residues/molecule for an iodine content comprised between 0.7% and 1.1%.

Molecular cloning of bovine thyroglobulin complementary DNA. Characterization of 2500-base-pair and 1900-base-pair fragments

Double-stranded thyroglobulin complementary DNA (cDNA) was synthesized from purified 33-S bovine thyroglobulin mRNA. This synthetic structural gene has previously been shown to contain three sites for the restriction endonuclease HindIII, yielding two internal fragments of 1900 and 2500 base pairs respectively. Recombinant molecules were prepared by ligating the HindIII-restricted cDNA to the plasmid pBR322 which had been linearized by the same enzyme. When Escherichia coli was transformed with this mixture, it yielded two kinds of colonies each harboring recombinant plasmids containing one of the two cDNA fragments. Both recombinant molecules hybridized specifically to translatable thyroglobulin mRNA. Sequence homology between the two cloned DNAs could not be detected by cross-hybridization experiments; this argues against the existence of internal structural repetition in thyroglobulin subunits. Together, the two cloned DNA fragments represent 55% of the 8000-base-pair double-stranded thyroglobulin DNA.

Subcytoplasmic distribution of thyroglobulin mRNA in normal sheep thyroid

The thyroglobulin 33-S mRNA was isolated from sheep thyroid total polysomes. The 33-S RNA, twice purified on a 1% sodium dodecylsulfate/sucrose gradient, was 30-fold enriched in thyroglobulin messenger activity and was estimated as 50% pure by its messenger activity and 80% pure by the electrophoretic profile. It was used as template for complementary DNA synthesis and hybridized up to 85% of the DNA copy with pseudo-first-order kinetics. Back-hybridization kinetics showed that the purified mRNA corresponds to a major kinetic component with a base sequence complexity of 10000 nucleotides as determined by comparison to globin mRNA. Cross-reactivity of [3H]cDNA with liver RNA is less than 10%. Restriction endonuclease digestion of [3H]cDNA yielded a discrete band pattern. The distribution of thyroglobulin mRNA among free polysomes, membrane-bound polysomes and extrapolysomal pools was analyzed using hybridization to the specific [3H]cDNA probe. Free particles were recovered in the supernatant and membrane-bound particles in the pellet after a brief centrifugation of detergent-free homogenate (5 min at 27000 x g: procedure A; 12 min at 130000 x g: procedure B) with precautions taken to avoid cross-contamination. Using procedure A, 80% of thyroglobulin mRNA sequences were found in the membrane-bound fraction. Using procedure B, where contamination of free particles by membrane-bound particles was avoided by high-speed initial centrifugation and further isolation through a discontinuous sucrose gradient, 95-98% of thyroglobulin mRNA sequences were recovered in membrane-bound polysomes. In total polysomes, 89% of thyroglobulin mRNA sequences were in the polysomal area and shifted to ribosomal subunits after EDTA treatment.