Characterization of Oncorhynchus mykiss 5-hydroxyisourate hydrolase/transthyretin superfamily: evolutionary and functional analyses.

Teleosts have highly diverged genomes that resulted from whole genome duplication, which leads to an extensive diversity of paralogous genes. Transthyretin (TTR), an extracellular thyroid hormone (TH) binding protein, is thought to have evolved from an ancestral 5-hydroxyisourate hydrolase (HIUHase) by gene duplication at some stage of chordate evolution. To characterize the functions of proteins that arose from duplicated genes in teleosts, we investigated the phylogenetic relationship of teleost HIUHase and TTR aa sequences, the expression levels of Oncorhynchus mykiss HIUHase and TTR mRNA in various tissues and the biological activities of the O. mykiss re-HIUHase and re-TTR. Phylogenetic analysis of the teleost aa sequences revealed the presence of two HIUHase subfamilies, HIUHase 1 (which has an N-terminal peroxisomal targeting signal-2 [PTS2]) and HIUHase 2 (which does not have an N-terminal PTS2), and one TTR family. The tissue distributions of HIUHase 1 and TTR mRNA were similar in juvenile O. mykiss and the mRNA levels were highest in the liver. The O. mykiss re-HIUHase and re-TTR proteins were both 40-50 kDa homotetramers consisting of 14-15 kDa subunits, with 30% identity. HIUHase had 5-hydroxyisourate (5-HIU) hydrolysis activity with Zn(2+) sensitivity, whereas TTR had ligand binding activity with a preference for THs and several environmental chemicals, such as halogenated phenols. Our results suggest that O. mykiss HIUHase and TTR have diverged from a common ancestral HIHUase with no functional complementation.