MOLECULAR RECOGNITION OF THYROGLOBULIN BY SORTILIN

Thyroglobulin (TG) is a large glycoprotein essential for thyroid hormone synthesis and storage. Synthesised and secreted by thyrocytes, TG forms a dimeric structure within the thyroid follicle lumen, where it undergoes iodination and hormonogenesis before being retrieved through endocytosis for hormone release. Despite its central role in thyroid physiology, the mechanisms governing TG uptake remain poorly understood. Sortilin, a ubiquitous receptor involved in protein sorting and endocytosis, has been proposed as a key mediator of TG internalisation, particularly for its iodinated forms. However, the molecular basis of this interaction has remained elusive. Here, we elucidate the mechanisms of TG recognition by sortilin using an integrated approach combining cryo-electron microscopy, crosslinking mass spectrometry, biophysical characterisation, and endocytosis assays in human thyroid cells. We demonstrate that sortilin preferentially binds the monomeric form of TG over its native dimeric state, suggesting a conformational selectivity in cargo recognition. Using an in vitro iodination assay, we show that iodination has no detectable effect on sortilin binding, indicating that the interaction is independent from the hormone content of TG and may instead reflect recognition of partially unfolded or degraded TG. Structural analysis of sortilin in complex with TG and a TG-derived C-terminal peptide (TGpep) reveals a binding mode conserved across other sortilin ligands relying on the peptide fit in sortilin β-propeller domain. Henceforth, performing sequence alignments and AlphaPulldown predictions we computationally derived a shared recognition motif by sortilin that may explain its broad ligand repertoire. Overall, this work elucidates a molecular mechanism for TG endocytosis, identifies sortilin as a monomer-selective receptor, and provides a structural basis for their interaction. This knowledge opens new avenues in understanding thyroid physiology and its regulation, deepening our understanding of sortilin-mediated protein trafficking.