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A Mutation Causing Reduced Biological Activity and Stability of Thyroxine-Binding Globulin Probably as a Result of Abnormal Glycosylation of the Molecule

Yuichi Mori, Susumu Seino, Kyoko Takeda, Irwin L. Flink^*, Yoshiharu Murata, Graeme I. Bell and Samuel Refetoff

Department of Medicine, The University of Chicago Chicago, Illinois 60637
Department of Pediatrics, The University of Chicago Chicago, Illinois 60637
Department of Biochemistry and Molecular Biology and Howard Hughes Medical Institute, The University of Chicago Chicago, Illinois 60637

Address requests for reprints to: Dr. Samuel Refetoff, Box 138, University of Chicago, 5841 South Maryland Avenue, Chicago, Illinois 60637.

Abstract

T₄-binding globulin (TBG), a 54-kilodalton glycoprotein, is the major thyroid hormone transport protein in man. The exact nature of the mutations causing X chromosome-linked TBG deficiency, which affect about 1 in 2,500 newborn males, is unknown. Here we report the sequence of a unique variant TBG (TBG-Gary) encoding a protein with severely impaired T4 binding as well as decreased stability at 37 C, resulting in its rapid in vivo denaturation. A single nucleotide substitution in the codon for residue 96 of the mature protein replaces isoleucine with asparagine; this replacement creates an additional site for N-linked glycosylation. The anodal shift of TBG-Gary on isoelectric focusing gel electrophoresis suggests that this new site is likely glycosylated. Since glycosylation is required for TBG to assume its correct tertiary structure, but is not subsequently necessary for maintenance of the biological properties or stability of the molecule, we believe that the likely presence of additional carbohydrate probably affects a higher order structure of the molecule and is thus responsible for the reduced stability and hormone binding activity of TBG-Gary (TBG^ASN-96)

FOOTNOTES

This work was supported in part by USPHS Grant DK-15,070. Presented in part at the 62nd Annual Meeting of The American Thyroid Association, Washington, DC, September 17, 1987.

^* Present address: Departments of Medicine and Pharmacology, University of Arizona, Tucson, Arizona 85719.

Present address: The Research Institute of Environmental Medicine, Nagoya University, Nagoya, Japan.

Received for publication September 30, 1988. Revision received December 7, 1988. Accepted for publication December 9, 1988.

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