Depletion of divalent cations within the secretory pathway inhibits the terminal glycosylation of complex carbohydrates of thyroglobulin

Newly synthesized thyroglobulin transiting the secretory pathway is posttra nslationally modified by addition of oligosaccharides to asparagine N-linke d residues. The effect of divalent cation depletion on oligosaccharide proc essing of Tg was studied in FRTL-5 cells. Treatment with an ionophore, A231 87, or thapsigargin, an inhibitor of the sarcoplasmic/endoplasmic reticulum ATPases delayed Tg secretion. These effects were accompanied by a normal d istribution of the marker of the endoplasmic reticulum protein disulfide is omerase. Analysis of the thyroglobulin oligosaccharides by Bio-gel P4 chrom atography showed that in the presence of A23187 and thapsigargin the additi on of peripheral sialic acid and possibly galactose is inhibited. These fin dings were strengthened by experiments of exoglycosidase digestion and SDS- PAGE analysis of the resulting products. These results reveal a cellular me chanism of production of thyroglobulin with incompletely processed complex chains, i.e., the ligand of the recently described GlcNAc and asialoglycopr otein receptors of the thyroid. Since A23187 and thapsigargin inhibit biosy nthetically the addition of peripheral sugars on N-linked oligosaccharides chains, the thyroglobulin molecules secreted in the presence of A23187 and thapsigargin should greatly facilitate studies on the function of the GlcNA c and asialoglycoprotein receptors of the thyroid. (C) Societe francaise de biochimie et biologie moleculaire / Elsevier, Paris.