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 (T-4) and vitamin D, i solated from a turtle, Trachemys scripta (Family Emydidae) and mammalian vi tamin D binding protein (DBP) prompted further interspecific comparison to better understand the structure of functional binding sites. Using polymera se-chain reaction (PCR) with primers derived from the putative nucleotide s equences encoding peptides from the degradation of the T. scripta protein, we cloned the cDNA. The mature turtle protein contains 466 amino acids, abo ut eight residues more than in mammalian DBP. The nucleotide sequence of th e coding region showed 63% nucleotide and 73% amino acid homology (approxim ate to 53% identity) to mammalian DBP (human, rat, mouse, and rabbit). Howe ver, there was no significant homology to mammalian T-4-binding globulin (T BG) or transthyretin (TTR). Comparisons with mammals help define further th e requirements for the vitamin D and actin binding sites. Northern blots of RNA isolated from turtle tissue probed with the 5' portion of cDNA establi shed expression of the transcript in liver, kidney, and brain (in order of abundance), in contrast to mammal sequences in which expression of DBP is l argely confined to the liver. (C) 1998 Published by Elsevier Science Inc. A ll rights reserved.