Thyroxine hormones visualized by the cryo-EM structure of bovine thyroglobulin
In vertebrates, iodine has a unique role. Its circulation regulates metabolism and many other fundamental processes, including neuronal tissue growth (Caravalho & Dupuy, 2017; Di Jeso & Arvan, 2016). The cycle begins with the intake of iodine from the bloodstream into thyroid follicles by the Na+/I- symporter (Dai
In the cryo-EM structure of bovine TG from natural sources presented in this issue of Acta Cryst. D (Kim
These differences may arise from differences between the bovine and human thyroids and may also reflect the availability of iodine under
Although TG has been extensively studied, its three-dimensional structure could not be determined for decades. Isolation of purified TG (Heidelberger & Palmer, 1933) enabled the determination of its molecular weight (Heidelberger & Svedberg, 1934). The first complete sequence of TG was deduced from cDNA of bovine TG and contained 2769 amino-acid residues (Mercken et al., 1985). Yet, a successful determination of a 3D structure requires sufficient amounts of homogeneous material and technology that makes the determination possible. The material has to be stable enough to sustain treatment by crystallization conditions or during the preparation of grids. It is not customary to publish unsuccessful attempts, but we believe that determination of the bovine TG structure was at least as long-lasting as that of its human homologue. Indeed, we know from personal experience how difficult this project was to bring to fruition. The structure of human TG was the culmination of about two decades of long endeavour, the seeds of which were planted by the crystal structure of MHC class II-associated p41 invariant chain fragment bound to cathepsin L (Gunčar et al., 1999). The p41 fragment revealed the fold of thyroglobulin type-1 repeats, containing six cysteine residues and the CWCV signature (Molina et al., 1996).
For a long time, we struggled with attempts to crystallize TG from natural sources. Porcine material appeared to behave the best and was the easiest to isolate in large amounts. However, we were unable to obtain material that was stable over longer periods of time or reproducible crystals diffracting beyond 15 Å at a beamline such as the microfocus beamline at ESRF. During one visit (by DT) to MRC LMB, Jan Loewe suggested tackling the structure with electron cryo-microscopy, which was becoming capable of resolving the structures of macromolecules in atomic detail. Indeed, we progressed from porcine to purchased and later recombinantly expressed human thyroglobulin (Coscia et al., 2020). As Kim et al. stated, the bovine TG material at first also appeared unstable on the grids. They hypothesized that the instability was caused by the interactions at the solvent–air interface and that a small addition of detergent already successfully applied in other studies (Noble et al., 2018; Chen et al., 2019) would prevent sample denaturation. Indeed, they observed that the addition of CHAPS (CMC of ~0.49%; GoldBio) prior to vitrification of the samples on the grid significantly improved the micrographs and resulted in a data set of 2.6 Å resolution. In addition to visualizing the T4 structure on the backbone of its precursor, the structure of bovine TG by Kim et al. is a triumph of sample preparation and the maturation of electron cryo-microscopy. The structure demonstrates that structural details of biologically important chemical reactions can now be visualized by electron cryo-microscopy.
This article was originally published in Acta Cryst. (2021). D77, 1346-1347.
This is an open-access article distributed under the terms of the Creative Commons Attribution (CC-BY) Licence, which permits unrestricted use, distribution, and reproduction in any medium, provided the original authors and source are cited.