Relationship Between the Dimerization of Thyroglobulin and its Ability to Form Triiodothyronine

BALAJI VELUSWAMY; CINTIA E. CITTERIO; PETER ARVAN; YOSHIAKI MORISHITA; NADA DAKKA

JOURNAL OF BIOLOGICAL CHEMISTRY (ONLINE)

American Society of Biochemistry and Molecular Biology, Inc.

Año: 2018 vol. 293 p. 4860 - 4869

1083-351X

Thyroglobulin (TG) is the most abundant thyroid gland protein,a dimeric iodoglycoprotein (660 kDa). TG serves as theprotein precursor in the synthesis of thyroid hormones tetraiodothyronine(T4) and triiodothyronine (T3). The primary site forT3 synthesis inTGinvolves an iodotyrosine acceptor at the antepenultimateTyr residue (at the extreme carboxyl terminus ofthe protein). The carboxyl-terminal region of TG comprises acholinesterase-like (ChEL) domain followed by a short uniquetail sequence. Despite many studies, the monoiodotyrosinedonor residue needed for the coupling reaction to create T3 atthis evolutionarily conserved site remains unidentified. In thisreport, we have utilized a novel, convenient immunoblottingassay to detect T3 formation after protein iodination in vitro,enabling the study of T3 formation in recombinant TG secretedfrom thyrocytes or heterologous cells. With this assay, we confirmthe antepenultimate residue of TG as a major T3-formingsite, but also demonstrate that the side chain of this residueintimately interacts with the same residue in the apposedmonomer of the TG dimer. T3 formation in TG, or the isolatedcarboxyl-terminal region, is inhibited by mutation of this antepenultimateresidue, but we describe the first substitutionmutation that actually increases T3 hormonogenesis by engineeringa novel cysteine, 10 residues upstream of the antepenultimateresidue, allowing for covalent association of the uniquetail sequences, and that helps to bring residues Tyr2744 fromapposed monomers into closer proximity.