Janssen O E, Refetoff S
Department of Medicine, University of Chicago, Illinois 60637.
J Biol Chem. 1992 Jul 15;267(20):13998-4004.
Thyroxine-binding globulin (TBG) is a glycoprotein that transports thyroid hormones in blood. Of two naturally occurring variants in man that harbor single proline substitutions (TBG-CD5 and TBG-Montreal), only TBG-CD5 manifests as complete TBG deficiency. In order to determine the pathophysiology of these TBG disorders, we expressed TBG-CD5 and TBG-Montreal (TBG-M), as well as the common type TBG (TBG-C) in reticulocyte lysate and Xenopus oocytes. Vectors encoding the three TBG types were constructed, transcribed in vitro, and their products of cell-free translation and processing by canine microsomal membranes were analyzed. TBG-C and TBG-M had identical mobility on denaturing polyacrylamide gel electrophoresis but could be distinguished by differences in thyroxine (T4) binding. TBG-CD5 had altered electrophoretic mobility and did not bind T4. TBG-C and TBG-M expressed in microinjected Xenopus oocytes showed properties similar to their respective serum forms, whereas TBG-CD5 was found in small amounts only intracellularly. Our results confirm that the previously described alanine 113 to proline substitution is responsible for the altered properties of TBG-M. The substitution of leucine 227 by proline in TBG-CD5 appears to impair its cotranslational processing and secretion.
甲状腺素结合球蛋白（TBG）是一种在血液中运输甲状腺激素的糖蛋白。在人类中存在的两种携带单个脯氨酸替代的天然变体（TBG-CD5和TBG-蒙特利尔）中，只有TBG-CD5表现为完全性TBG缺乏。为了确定这些TBG疾病的病理生理学，我们在网织红细胞裂解物和非洲爪蟾卵母细胞中表达了TBG-CD5和TBG-蒙特利尔（TBG-M）以及常见类型的TBG（TBG-C）。构建了编码这三种TBG类型的载体，进行体外转录，并分析了它们的无细胞翻译产物以及犬微粒体膜对其的加工情况。TBG-C和TBG-M在变性聚丙烯酰胺凝胶电泳上具有相同的迁移率，但可以通过甲状腺素（T4）结合的差异来区分。TBG-CD5的电泳迁移率发生了改变，并且不结合T4。在显微注射的非洲爪蟾卵母细胞中表达的TBG-C和TBG-M表现出与其各自血清形式相似的特性，而TBG-CD5仅在细胞内少量存在。我们的结果证实，先前描述的丙氨酸113被脯氨酸替代是导致TBG-M特性改变的原因。TBG-CD5中亮氨酸227被脯氨酸替代似乎损害了其共翻译加工和分泌。
