Giorgia Tanzi

Sortilin is a ubiquitous membrane receptor mediating trafficking of protein cargoes. In the thyroid, sortilin binds thyroglobulin (TG) during its endocytosis, a key process in thyroid homeostasis. Although sortilin has been proposed to recognise highly iodinated TG, the molecular details of this interaction remain unknown. In this work, using an integrative structural biology approach, we reveal that sortilin binds an unstructured TG C-terminal peptide and exhibits a strong preference for the monomeric TG over the commonly known dimeric form. We find that sortilin-TG interaction is independent of the iodination state of TG and instead relies on the conversion to its monomeric state, presumably promoted by extracellular degradation. Furthermore, using AlphaPulldown and sequence analysis, we show that recognition of other reported ligands by sortilin likely relies on similar unstructured peptide motifs, which are not constrained to a single binding orientation within the receptor cavity. Overall, this study reveals the TG-sortilin binding interface and provides insights into the recognition mechanism of other cargoes by sortilin.

Thyroid hormones (the common name for prohormone thyroxine and the bioactive form triiodothyronine) control major developmental and metabolic processes. Release of thyroid hormones from the thyroid gland into the bloodstream and their transport into target cells is facilitated by plasma membrane transporters, including monocarboxylate transporter (MCT)8 and the highly homologous MCT10. However, the molecular mechanism underlying thyroid hormone transport is unknown. The relevance of such transporters is illustrated in patients with MCT8 deficiency, a severe neurodevelopmental and metabolic disorder. Using cryogenic-sample electron microscopy (cryo-EM), we determined the ligand-free and thyroxine-bound human MCT8 structures in the outward-facing state and the thyroxine-bound human MCT10 in the inward-facing state. Our structural analysis revealed a network of conserved gate residues involved in conformational changes upon thyroxine binding, triggering ligand release in the opposite compartment. We then determined the structure of a folded but inactive patient-derived MCT8 mutant, indicating a subtle conformational change which explains its reduced transport activity. Finally, we report a structure of MCT8 bound to its inhibitor silychristin, locked in the outward-facing state, revealing the molecular basis of its action and specificity. Taken together, this study advances mechanistic understanding of normal and disordered thyroid hormone transport.

Thyroid hormones (the common name for prohormone thyroxine and the bioactive form triiodothyronine) control major developmental and metabolic processes. Release of thyroid hormones from the thyroid gland into the bloodstream and their transport into target cells is facilitated by plasma membrane transporters, of which monocarboxylate transporter (MCT)8 and the highly homologous MCT10 are most important. Patients with MCT8 mutations suffer from a severe neurodevelopmental and metabolic disorder, however, the molecular mechanism underlying thyroid hormone transport is unknown. Using cryogenic-sample electron microscopy (cryo-EM), we determined the ligand-free and thyroxine-bound human MCT8 structures in the outward-open state and the thyroxine-bound human MCT10 in the inward-facing state. Our structural analysis revealed a network of conserved gate residues involved in conformational changes upon thyroxine binding, triggering ligand release on the opposite compartment. We then determined the structure of a folded, but inactive patient-derived MCT8 mutant, indicating a subtle conformational change which explains its reduced transport activity. In addition, we determined the structure of MCT8 bound to its inhibitor silychristin, revealing an interaction with residues essential to drive transition to the inward-facing state, thereby locking the protein in the outward-facing state. This study provides molecular and structural insights into normal and disordered thyroid hormone transport.