Normal-Sized Thyroglobulin Messenger Ribonucleic Acid in Dutch Goats with a Thyroglobulin Synthesis Defect Is Translated into a 35,000 Molecular Weight N-Terminal Fragment*

Abstract

The translation product of the thyroglobulin (Tg) mRNA in Dutch goats with a Tg synthesis defect has been characterized. The Tg mRNA has a normal size of 8.4 kilobases. Translation of goitrous polysomal Tg mRNA resulted, after immunoprecipitation with polyclonal rabbit antigoat Tg antibodies, in a single 35,000 mol wt (Mr) Tg fragment. To characterize the Tg antigens produced in vivo, thyroid hormone release by the goiter was suppressed by injecting T4 sc in newborn goitrous goats. Immunohistochemical studies showed the presence of Tg antigens almost solely in the colloidal lumen. Electrophoresis of the reduced thyroid proteins demonstrated the presence of two Tg fragments with Mr of 40,000 and 32,000, respectively; the latter is probably a breakdown product of the 40,000 Mr fragment. The difference in Mr between the in vivo and in vitro translation products (40,000 and 35,000 Mr, respectively) can be explained by the carbohydrate content (10% wt/wt) of the in vivo product, as was shown by periodic acid-Schiff-positive staining. Using monoclonal antibodies against the hormonogenic sites at the first and last parts of the Tg molecule, we demonstrated that only the first part of the Tg molecule is present. Both in vivo and in vitro 10% of the goitrous Tg mRNA molecule is translated, resulting in an N-terminal Tg fragment that easily aggregates to large S-S complexes in the colloidal lumen of goiter by H2O2 oxidation.