The rat asialoglycoprotein receptor binds the amino-terminal domain of thyroglobulin

We have previously reported that the rat hepatic lectin-1 (RHL-1) subunit o f rat asialoglycoprotein receptor (ASGPr), the endocytic receptor found ore the basolateral surface of hepatocytes, was expressed in rat thyroid tissu e and localized on the apical surface of polarized rat thyroid FRT cells. H ere we show that PC Cl3 cells, a differentiated rat thyroid cell line, boun d thyroglobulin (Tg) via ASGPr. In fact, both the bacterial recombinant car bohydrate recognition domain of RHL-1 (rCRD(RHL-1)) and the anti-rCRD(RHL-1 ) antibody markedly inhibited I-125-Tg binding to the cell surface of PC Cl 3 cells. Ligand blot assays with deglycosylated Tg show that the rCRD(RHL-1 ), was able to interact with Tg even after remotion of sugars. The region o f Tg involved in the binding to RHL-1 was investigated by ligand blot assay s with biotinylated rCRD(RHL-1) on thermolysin-digested native and desialat ed rat thyroglobulin. It is shown that the rCRD(RHL-1), specifically recogn ized a thyroglobulin fragment with an apparent M-r of 68,000, corresponding to the amino-terminal part of the molecule. To our knowledge, this is the first report that attributes to the amino-terminal portion of Tg molecule, containing its earliest and major hormonogenic site, the function of bindin g to a cell surface receptor of the thyroid. Moreover, we show that oligosa ccharides are not the only molecular signals for binding to RHL-1, but amin o acidic determinants could also play a role, (C) 2000 Academic Press.