The structure of natively iodinated bovine thyroglobulin

Abstract

Thyroglobulin is a homodimeric glycoprotein that is essential for the generation of thyroid hormones in vertebrates. Upon secretion into the lumen of follicles in the thyroid gland, tyrosine residues within the protein become iodinated to produce monoiodotyrosine (MIT) and diiodotyrosine (DIT). A subset of evolutionarily conserved pairs of DIT (and MIT) residues can then engage in oxidative coupling reactions that yield either thyroxine (T4; produced from coupling of a DIT `acceptor' with a DIT `donor') or triiodothyronine (T3; produced from coupling of a DIT acceptor with an MIT donor). Although multiple iodotyrosine residues have been identified as potential donors and acceptors, the specificity and structural context of the pairings (i.e. which donor is paired with which acceptor) have remained unclear. Here, single-particle cryogenic electron microscopy (cryoEM) was used to generate a high-resolution reconstruction of bovine thyroglobulin (2.3 Å resolution in the core region and 2.6 Å overall), allowing the structural characterization of two post-reaction acceptor–donor pairs as well as tyrosine residues modified as MIT and DIT. A substantial spatial separation between donor Tyr149 and acceptor Tyr24 was observed, suggesting that for thyroxine synthesis significant peptide motion is required for coupling at the evolutionarily conserved thyroglobulin amino-terminus.

Cryo-EM structure of native human thyroglobulin

“…While this work was in revision, Kim et al reported the structure of natively iodinated bovine thyroglobulin 35 . Interestingly, they observed three modified tyrosines with additional densities in their structure, indicative of acceptor sites: Y24 (T 4 ), Y2041 (DIT), and Y2575 (T 4 ) (Supplementary Table 4 ).…”

Maintaining the thyroid gland in mutant thyroglobulin–induced hypothyroidism requires thyroid cell proliferation that must continue in adulthood

“…When the same samples were analyzed under denaturing conditions by nonreducing SDS-PAGE—although the molecular masses of cog-Tg-L2263P, rdw-Tg-G2298R, and WT-Tg are identical—the mobility of WT-Tg still appeared slightly faster ( Fig. 1 C ), suggesting a more tightly disulfide-linked structure ( 29 , 30 ). Moreover, in WT thyroid glands, nearly all Tg proteins had migrated out of the ER so that most glycans on those Tg molecules were processed to complex sugars resistant to digestion with endoglycosidase H ( Fig.…”

Formation of thyroid hormone revealed by a cryo-EM structure of native bovine thyroglobulin

“…While this work was undergoing peer review, a paper was published describing a cryo-EM structure of native bovine TG with nascent thyroid hormone present at hormonogenic sites A and B 9 . The structural organization of the region encompassing site B is essentially identical to that reported here.…”