Hunt J L, Licht P
Group in Endocrinology, University of California at Berkeley 94720, USA.
Comp Biochem Physiol B Biochem Mol Biol. 1998 Dec;121(4):397-406. doi: 10.1016/s0305-0491(98)10122-0.
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.
从彩龟（泽龟科）中分离出的一种血液蛋白，对两种不同的激素——甲状腺素（T4）和维生素D具有独特的双重结合亲和力，其与哺乳动物维生素D结合蛋白（DBP）具有明显的高度同源性，这促使我们进行进一步的种间比较，以更好地了解功能性结合位点的结构。我们使用聚合酶链反应（PCR），以从彩龟蛋白降解产物的推定核苷酸序列衍生而来的引物，克隆了cDNA。成熟的龟蛋白含有466个氨基酸，比哺乳动物DBP多约8个残基。编码区的核苷酸序列与哺乳动物DBP（人、大鼠、小鼠和兔）显示出63%的核苷酸同源性和73%的氨基酸同源性（约53%的一致性）。然而，与哺乳动物甲状腺素结合球蛋白（TBG）或转甲状腺素蛋白（TTR）没有显著同源性。与哺乳动物的比较有助于进一步确定维生素D和肌动蛋白结合位点的要求。用cDNA的5'部分探测从龟组织分离的RNA的Northern印迹显示，该转录本在肝脏、肾脏和大脑中表达（按丰度顺序），这与DBP表达主要局限于肝脏的哺乳动物序列形成对比。
