MEGALIN (GP330) - A PUTATIVE ENDOCYTIC RECEPTOR FOR THYROGLOBULIN (TG)

Megalin (gp330) is a large glycoprotein receptor found mainly on a gro up of absorptive epithelial cells, including renal proximal tubule, ep ididymal and thyroid cells. Megalin has been shown to bind multiple, u nrelated ligands, mainly in vitro, and to mediate endocytosis of ligan ds in cultured cells. However, physiologic ligands of megalin are larg ely unknown. In the present study we have demonstrated that purified r at megalin binds rat thyroglobulin (Tg) in solid phase assays, with an estimated K-d of 9.2+/-0.6 nM. Binding was calcium dependent and was almost completely inhibited by excess Tg, by three megalin ligands - l actoferrin, lipoprotein lipase and apolipoprotein J- and by the recept or associated protein (RAP), which inhibits binding of all megalin lig ands. Three anti-megalin antibodies partially inhibited Tg binding to megalin. I-125 labeled Tg bound to megalin was released by EDTA and he parin; the released product was shown by SDS-PAGE and autoradiography to be 660 kD (dimeric) Tg. However, an immunoblotting experiment showe d binding of megalin both to monomeric (330 kD) and dimeric Tg. We pro pose that megalin, which is known to mediate ligand endocytosis and is found on the apical surface of thyrocytes, may participate in the end ocytosis of Tg from the colloid, a process that is required for hormon e release from Tg.