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* Institut für Zellbiologie, Universität Bonn, Ulrich-Haberland-Str. 61a, D-53121 Bonn, Germany;
Washington University, School of Medicine, Department of Cell Biology, St. Louis, Missouri, 63110 USA; and
Northwestern University, Medical School, Department of Cell and Molecular Biology, Chicago, Illinois 60611, USA
1Correspondence: Institut für Zellbiologie, Universität Bonn, Ulrich-Haberland-Str. 61a, D-53121 Bonn, Germany. E-mail: herzog{at}uni-bonn.de
Extracellular storage of thyroglobulin (TG) is a prerequisite for
maintaining constant levels of thyroid hormones in vertebrates. Storage
of TG within the follicle lumen is achieved by compactation and by the
formation of covalent cross-links between TG molecules. In bovine
thyroids, ~75% of the cross-links are other than disulfide bonds
(J. Cell Biol. 180, 1071–1081). We have now shown that polymeric
TG contains a large number of N
(
-glutamyl)lysine
cross-links and that only traces of these can be found in the soluble
form of TG. Because such isopeptide bridges are generated usually by
the action of a transglutaminase, it is reasonable to propose that the
covalent polymerization of TG in the globules is under the control of
this enzyme. Soluble TG was shown to be a substrate for
transglutaminase in vitro; moreover, the presence of
transglutaminase was demonstrated by immunofluorescence and by
immunoblotting in freshly isolated bovine thyroid globules. With
immunoelectron microscopy, transglutaminase was detected in the
cytoplasm of thyrocytes, but not in compartments of the secretory
pathway. Only one messenger RNA for transglutaminase was found by
Northern blotting. Sequencing of the cloned gene failed to reveal a
secretory signal, which supports the notion that the thyroid
transglutaminase is the cytosolic type. Apparently, the enzyme reaches
the lumen of the follicle by an as yet unknown pathway to catalyze the
covalent cross-linking of thyroid globules in this extracellular
compartment.—Saber-Lichtenberg, Y., Brix, K.,
Schmitz, A., Heuser, J. E., Wilson,
J. H., Lorand, L., and Herzog, V. Covalent cross-linking of
secreted bovine thyroglobulin by transglutaminase
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