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Am J Physiol Regul Integr Comp Physiol 283: R885-R896, 2002. First published July 11, 2002; doi:10.1152/ajpregu.00042.2002
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Vol. 283, Issue 4, R885-R896, October 2002

Crocodile transthyretin: structure, function, and evolution

Porntip Prapunpoj1, Samantha J. Richardson2, and Gerhard Schreiber2

1 Department of Biochemistry, Faculty of Science, Prince of Songkla University, Hat-Yai, Songkhla 90112, Thailand; and 2 Department of Biochemistry and Molecular Biology, The University of Melbourne, Parkville 3010, Victoria, Australia

Structure and function were studied for Crocodylus porosus transthyretin (crocTTR), an important intermediate in TTR evolution. The cDNA for crocTTR mRNA was cloned and sequenced and the amino acid sequence of crocTTR was deduced. In contrast to mammalian TTRs, but similar to avian and lizard TTRs, the subunit of crocTTR had a long and hydrophobic NH2-terminal region. Different from the situation in mammals, triiodothyronine (T3) was bound by crocTTR with higher affinity than thyroxine (T4). Recombinant crocTTR and a chimeric construct, with the NH2-terminal region of crocTTR being replaced by that of Xenopus laevis TTR, were synthesized in the yeast Pichia pastoris. Analysis of the affinity of the chimeric TTRs showed that the NH2-terminal region modulates T4 and T3 binding characteristics of TTR. The structural differences of the NH2-terminal regions of reptilian and amphibian TTRs were caused by a shift in splice sites at the 5' end of exon 2. The comparison of crocodile and other vertebrate TTRs shows that TTR evolution is an example for positive Darwinian evolution and identifies its molecular mechanism.

thyroid hormone-binding plasma proteins; choroid plexus; Crocodylus porosus; thyroid hormone homeostasis; eye; lizard; Tiliqua rugosa


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