In mammals, an adequate supply of thyroid hormones is essential for normal growth and neurological development. The biosynthesis of thyroid hormones involves an iodinated precursor protein, thyroglobulin, which may be considered an extreme example of a pro-hormone. Thyroglobulin is a dimeric glycoprotein of relative molecular mass (Mr) 660,000 (660K), which is secreted by the thyrocyte and stored in the lumen of the thyroid follicle. The hormonogenic reaction is extracellular, and involves iodination of tyrosyl residues of thyroglobulin and the intramolecular coupling of a subset of these into thyroxine (T4) and triiodothyronine (T3), which remain part of the polypeptide chain. Secretion of hormones results from the endocytosis of thyroglobulin followed by its complete hydrolysis in lysosomes. Considering that the maximum yield of hormones is approximately 6-8 per 660K protein, the whole process is apparently wasteful. However, the efficiency of thyroglobulin as a thyroid hormone precursor is extremely high when the supply of iodine is short; in such conditions, almost all the iodine incorporated is found in iodothyronine. Hence it is suggested that the thyroglobulin structure has evolved to allow for the preferential and efficient iodination and coupling of the hormonogenic tyrosines. Here we report the complete primary structure of bovine thyroglobulin, derived from the sequence of its 8,431-base-pair complementary DNA. The 2,769-amino-acid sequence is characterized by a pattern of imperfect repeats derived from three cysteine-rich motifs. Four hormonogenic tyrosines have been precisely localized near the amino and carboxyl ends of the protein.

The hereditary goitre of Afrikander cattle is an autosomal recessive disease characterized in homozygotes by the production of abnormal thyroglobulin (Tg) and the coexistence in the thyroid of normal-sized 8.4-kilobase (kb) Tg mRNA with a misspliced 7.3-kb message having lost exon 9. We have cloned and sequenced the cDNA segment corresponding to the abnormal exon 8-exon 10 junction and the relevant genomic DNA region. The mutation responsible for the disease is a cytosine to thymine transition creating a stop codon at position 697 in exon 9. The original reading frame is maintained in the 7.3-kb mRNA, which, as it lacks the mutated exon, is translatable into a potentially functional protein. This puzzling phenotype in which a mutated exon is apparently removed selectively from transcripts by alternative splicing leads us to suggest that the 7.3-kb transcript could be present in normal animals. Using a sensitive oligonucleotide hybridization assay, we have demonstrated that a 7.3-kb mRNA lacking exon 9 does exist in normal thyroids as a minor mRNA species. As it is fully translatable, the 7.3-kb mRNA is expected to be more stable than the normal-sized 8.4-kb message. This probably accounts for the higher proportion of 7.3-kb transcript found in the goitre.Thyroglobulin (Tg), a 660-kDa homodimer, is the biosynthetic precursor of the thyroid hormones (1). The Tg gene is -250,000 nucleotide pairs long, ofwhich 8431 nucleotides are represented in the mRNA (2). Congenital goitres with defective Tg production have been described in humans (3) and in animal models (4, 5). They are usually associated with hypothyroidism and may lead, in humans, to the development of the "cretin" phenotype (6). The hereditary goitre of the Afrikander cattle is inherited as an autosomal recessive disease (7). The goitre ofhomozygotes contains no normal Tg (8) and contains both a shorter and a normal-sized Tg mRNA (9); exon 9 is absent in the shorter Tg mRNA, suggesting that a splicing error is responsible for the disease (9). To identify precisely the mutation, we have cloned and sequenced the cDNA of the affected region of the smaller mRNA and the exon 9-intron 9 region of the genomic DNA from both a normal individual and an individual with goitre. We show that a cytosine to thymine transition in exon 9, changing codon 697 from CGA (arginine) to TGA (stop) is responsible for the disease. The nonsense mutation in exon 9 is apparently associated with and partially cured by removal of the defective exon from a portion ofTg transcripts. An explanation for this curious phenotype was obtained by the identification of a minor mRNA species lacking exon 9 in the thyroids of normal animals. Different stability of the shorter (fully translatable) and larger (containing an early stop codon) messages probably accounts for their relative amounts in goitre tissue.

The sequence of the first 2831 nucleotides of bovine thyroglobulin mRNA has been determined from the analysis of a cDNA clone. Following a 41‐nucleotide 5′ untranslated sequence, a single open‐reading frame encoding 930 amino acids was observed. This corresponds to the aminoterminal third of thyroglobulin, preceded by a putative signal peptide of 19 amino acids. The protein sequence was found to be essentially made of the sevenfold repetition of a 60‐amino‐acid‐long building unit, interrupted at fixed positions by unrelated segments of variable length. The presence of an internal homology within the repetitive unit itself suggests that the 5′ region of the thyroglobulin gene has evolved from the initial duplication of a relatively short sequence, followed by the serial duplication of the resulting unit. The tyrosine residue at position five has been assigned an important hormonogenic function [Mercken, L., Simons, M.‐J. and Vassart, G. (1982) FEBS Lett. 149, 285–287]. This residue is flanked by sequence elements related to the repeated unit, suggesting that the hormonogenic domain evolved also from the basic ancestor sequence.

No abstract

The sequence of 370 bases at the 5 '-end of bovine thyroglobulin mRNA has been determined. A41 base untranslated segment was found preceeding the ATG initiator codon. It is followed by an open reading frame providing the first data on thyroglobulin primary structure. Analysis of the amino acid sequence demonstrated the presence of an 18 residue hydrophobic segment representing a putative signal peptide. Comparison of the amino terminal sequence of thyroglobulin with that of peptides known to contain thyroid hormones [7,8] demonstrated that the first tyrosine in native thyroglobulin is mainly found as thyroxine in the mature iodinated protein [8]. Our results clearly identify the amino-terminal region of thyroglobulin as an important hormonogenic domain of the protein. Thyroid hormone Protein sequence cDNA cloning