The N-acetylglucosamine-specific receptor of the thyroid. Binding characteristics, partial characterization, and potential role.

The binding characteristics of the GlcNAc binding protein present in thyroid membranes (Consiglio, E., Shifrin, S., Yavin, Z., Ambesi-Impiombato, F.S., Rall, J.E., Salvatore, G., and Kohn, L.D. (1981) J. Biol. Chem. 256, 10592-10599) were reinvestigated using neoglycoproteins as probes. Plasma membrane preparations from porcine thyroid specifically bound 125I-GlcNAc35-bovine serum albumin. Binding was dependent on the presence of calcium. Binding of ligand to receptor was minimal at neutral pH and maximal at pH 5.0. Equilibrium binding studies indicated positive cooperativity of binding and a site capacity of about 60 pmol/mg of protein. Competition studies were compatible with a specificity hierarchy of GlcNAc much greater than Gal; no recognition of mannose, fucose, or glucose moieties was noted. The receptor was detergent-solubilized from plasma membrane preparations and on the basis of the defined binding properties, purified by chromatography on a GlcNAc-Sepharose affinity column. The purified GlcNAc thyroid receptor has a subunit molecular size of about 45 kDa and appears to be an oligomer composed of three subunits. The receptor was identified as a component of thyrocytes by in situ cytochemical localization with fluorescent neoglycoproteins. In certain cases it was mainly present on, or near, the apical cell surface. It is suggested that this GlcNAc receptor functions in thyroglobulin metabolism, possibly involved in recycling of internalized thyroglobulin molecules back into the follicular lumen.