TTR and RBP: Implications in fish physiology

Abstract

Transthyretin (TTR), a homotetrameric protein of 54 kDa, is one of the carriers of thyroid hormone and, as a macromolecular complex with retinol-binding protein 4(RBP4), is involved in the transport of vitamin A in blood, preventing kidney glomerular filtration of the small, 21 kDa RBP4. TTR cDNA has been cloned in many vertebrates (man, rat, chicken, etc.) and more recently also in several teleost fishes. Fish TTR shows a strong homology with the same protein of other vertebrates, with similar monomer-monomer and dimer-dimer interfaces and a conserved tetrameric structure. RBP4 was also isolated from several teleosts and characterized. The expression of the two proteins was investigated during embryonic development and in adult tissues. TTR is expressed in adult fish liver and is therefore present in their serum, even though RBP4 and TTR have never been detected in the blood as a complex. The lack of RBP4-TTR complex formation in fish has been explained, since the residues specifically involved in the interaction between the two proteins in mammals are different from the corresponding residues in fish. © 2009 Springer-Verlag Berlin Heidelberg.