Structure of a repilian plasma thyroxine binding protein indicates homology to vitamin D-binding protein.
Biochemical analyses revealed that a thyroxine (T4)-binding protein (TBP) in the blood of some turtles exhibits a high degree of structural homology to mammalian vitamin D-binding protein (DBP), instead of mammalian T4-binding protein (TBG). Like DBP, TBP is rich in Cys (ca. 28/molecule). The 31 NH2-terminal amino acids of TBP show 68% identity to the corresponding region of mammalian (rat, mouse, and human) DBP and one peptide from Lys-C proteolysis shows similar homology to residues 35-49 of DBP (the predicted sterol binding domain). Another peptide shows a 75% identity to a sequence corresponding to residues 134-154 of DBP, but seven others showed no homology to any proteins in GenBank. Digestion with N-glycosidase F reduced the M(r) of TBP by only 4.5K compared with 14K for TBG; T4-binding activity of TBP was unaffected. TBP (and DBP) also behaves as a much smaller molecule (approximately 57 kDa) than TBG (> 70 kDa) by size-exclusion chromatography, despite similarities in their estimated sizes from sodium dodecyl sulfate electrophoresis (approximately 60 kDa). Binding studies confirm that the turtle T4-binding protein likely also represents the major blood DPB. Thus, in the turtle, a single binding protein resembling DBP performs two major transport functions which are normally served by proteins representing two different multigene families in mammals.
Licht
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Department of Integrative Biology, University of California, Berkeley 94720.
Moore
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D000595:Amino Acid Sequence; D000596:Amino Acids / Q000032:analysis; D000818:Animals; D002850:Chromatography, Gel; D002851:Chromatography, High Pressure Liquid; D004355:Drug Stability; D004591:Electrophoresis, Polyacrylamide Gel; D006358:Hot Temperature; D006801:Humans; D008969:Molecular Sequence Data; D008970:Molecular Weight; D017386:Sequence Homology, Amino Acid; D013975:Thyroxine-Binding Proteins / Q000737:chemistry*; D014426:Turtles / Q000097:blood*; D014809:Vitamin D-Binding Protein / Q000737:chemistry*
D000596:Amino Acids; D013975:Thyroxine-Binding Proteins; D014809:Vitamin D-Binding Protein