Selective endocytosis of thyroglobulin: A review of potential mechanisms for protecting newly synthesized molecules from premature degradation

In 1976 Cortese, Schneider and Salvatore (Eur. J. Biochem. 68 (1976) 121-12 9) showed that the thyroid gland protects newly synthesized, iodine and hor mone poor thyroglobulin from immediate degradation. Since then there has be en substantial progress in understanding the mechanism by which this select ivity of degradation occurs. Thyroglobulin in the follicular lumen is inter nalized mainly by receptor-specific endocytosis. Recycling of immature, poo rly iodinated thyroglobulin back to the follicular lumen is the pathway mos t likely responsible for selectivity. Since additional carbohydrate groups are added to the immature thyroglobulin, it appears that this recycling occ urs via the Golgi compartment. The molecular signal for recycling most like ly involves the complex carbohydrates and probably is exposed GlcNAc groups . A thyroid-specific GlcNAc receptor has been identified and cloned. Other Tg-binding sites have been identified in the thyroid, but their physiologic al role remains to be determined. (C) Societe francaise de biochimie et bio logie moleculaire / Elsevier, Paris.