Identification and structural characterization of a novel member of the vitamin D binding protein family

The apparent high degree of homology of a blood protein with a unique dual binding affinity for two distinct hormones, thyroxin (T4) and vitamin D, isolated from a turtle, Trachemys scripta (Family Emydidae) and mammalian vitamin D binding protein (DBP) prompted further interspecific comparison to better understand the structure of functional binding sites. Using polymerase-chain reaction (PCR) with primers derived from the putative nucleotide sequences encoding peptides from the degradation of the T. scripta protein, we cloned the cDNA. The mature turtle protein contains 466 amino acids, about eight residues more than in mammalian DBP. The nucleotide sequence of the coding region showed 63% nucleotide and 73% amino acid homology (approximately 53% identity) to mammalian DBP (human, rat, mouse, and rabbit). However, there was no significant homology to mammalian T4-binding globulin (TBG) or transthyretin (TTR). Comparisons with mammals help define further the requirements for the vitamin D and actin binding sites. Northern blots of RNA isolated from turtle tissue probed with the 5' portion of cDNA established expression of the transcript in liver, kidney, and brain (in order of abundance), in contrast to mammal sequences in which expression of DBP is largely confined to the liver.