Structural characteristics of bullfrog (Rana catesbeiana) transthyretin and its cDNA

Abstract

Transthyretin, an extracellular thyroid-hormone-binding protein (THBP) in higher vertebrates, is synthesized and secreted by the choroid plexus of all classes of vertebrates, except fish and amphibians, and synthesized in the liver of endothermic animals. Here, we report the nucleotide sequence of the cDNA for a THBP found in plasma of bullfrog (Rana catesbeiana) tadpoles before the climax of metamorphosis.The amino acid sequence clearly shows this protein to be an amphibian transthyretin. The three-dimensional structure of bullfrog transthyretin was derived using homology modeling. Compared with transthyretins from other vertebrate species, bullfrog transthyretin is highly conserved at the thyroid hormonebinding sites and other important structural regions of the subunits. Bullfrog transthyretin mRNA was found in tadpole liver, but not in tadpole choroid plexus. Thus, during evolution, synthesis of transthyretin in the liver of metamorphosing amphibians preceded that in the choroid plexus of reptiles, birds and mammals. It was previously observed that the protein most abundantly synthesized and secreted by the choroid plexus in adult amphibians is a lipocalin [Achen, M. G., Harms, P. J., Thomas, T., Richardson, S. J., Wettenhall, R. E. H. & Schreiber, G. (1992) J. Biol. Chem. 267, 23170Ϫ23 174], in contrast to transthyretin being the most abundantly synthesized and secreted protein in the choroid plexus of mammals, birds and reptiles. Lipocalin mRNA was found in large amounts in tadpole choroid plexus, but not livers. Keywords : transthyretin; lipocalin ; metamorphosis; bullfrog (Rana catesbeiana).Amphibian metamorphosis can be induced by both thyroxine and 3,3′,5-triiodothyronine [1], and is regulated via the hypothalamus-pituitary-thyroid gland axis, which is established just before the onset of metamorphosis [2]. The concentration of thyroid hormones in the blood increases gradually during prometamorphosis, reaches a maximum level at the beginning of metamorphic climax, and then decreases and ceases to be detectable at the end of metamorphosis [3,4]. Thyroid hormone receptors, the genes of which are thought to be among the earliest genes responding to the elevation of thyroid hormone concentration in blood plasma, are upregulated in several tissues during both spontaneous and induced metamorphosis [5,6], and in cell-culture systems [7,8]. Thus, amphibian metamorphosis has provided a good model for analyzing the control of gene expression by thyroid hormones [9Ϫ11]. However, it is still unclear how thyroid hormones are delivered to target cells. Previously, the presence of a thyroid hormone-binding protein (THBP) in blood plasma from metamorphosing Rana catesbeiana (bullfrog) tadpoles had been described [12]. This THBP had a higher affinity Correspondence to K. Yamauchi,

The evolutionary and integrative roles of transthyretin in thyroid hormone homeostasis

“…In amphibians, TTR mRNA was observed in the liver during metamorphosis, namely in tadpoles of the frog Rana catesbeiana (Yamauchi et al 1998), and of the African clawed toad Xenopus laevis (Prapunpoj et al 2000b). No TTR mRNA was observed in tadpole brain and in any of the studied tissues from adult amphibians.…”

“…No TTR mRNA was observed in tadpole brain and in any of the studied tissues from adult amphibians. Very high levels of the mRNA for a lipocalin-type protein (binding lipophilic compounds), instead of TTR mRNA, were found in the choroid plexus of the cane toad Bufo marinus (Achen et al 1992) and the bullfrog (Yamauchi et al 1998).…”

Developmentally regulated thyroid hormone distributor proteins in marsupials, a reptile, and fish

“…More recently, hepatic TTR synthesis during early development of fish (11,35,52), and during metamorphosis in amphibians (51,53) was demonstrated.…”

Characterization of little skate (Leucoraja erinacea) recombinant transthyretin: Zinc-dependent 3,3′,5-triiodo-l-thyronine binding

“…CV221819), trout (Yamauchi et al, 1999;accession.No.CX256523), seabream (Santos and Power, 1999), toad (Prapunpoj et al, 2000), bullfrog (Yamauchi et al, 1993(Yamauchi et al, , 1998, crocodile (Prapunpoj et al, 2002), chicken (Duan et al, 1991) and human (Kanda et al, 1974) were aligned. The N-terminal ends of the lamprey, skate and seabream TTRs are predicted, whereas those of the other TTRs are based on Edman degradation.…”