B. Di Jeso et al., Depletion of divalent cations within the secretory pathway inhibits the terminal glycosylation of complex carbohydrates of thyroglobulin, BIOCHIMIE, 81(5), 1999, pp. 497-504
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.