Structure of a Reptilian 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 Mr 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 (∼57 kDa) than TBG (>70 kDa) by size-exclusion chromatography, despite similarities in their estimated sizes from sodium dodecyl sulfate electrophoresis (∼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.