A hyperosmotic stress-induced mRNA of carp cell encodes Na +- and Cl −-dependent high affinity taurine transporter

Abstract

A cDNA clone encoding a Na +- and Cl −-dependent high affinity taurine transporter was isolated from a common carp cell line, Epithelioma papulosum cyprini (EPC), as a hyperosmotic stress-inducible gene by RNA arbitrarily primed PCR. The clone contained a 2.5-kb cDNA fragment including an open reading frame of 1878 bp encoding a protein of 625 amino acids. The deduced amino acid sequence of carp taurine transporter shows 78–80% identity to those of cloned mammalian taurine transporters. The functional characteristics of the cloned transporter were analyzed by expression in COS-7 cells. Transfection with the cDNA induced Na +- and Cl −-dependent taurine transport activity with an apparent K m of 56 μM. The Na +/Cl −/taurine coupling ratio for cloned transporter was estimated as 2:1:1. Uptake of radiolabeled taurine was inhibited by excessive cold taurine, hypotaurine, β-alanine and γ-aminobutyric acid but not by α-alanine, glycine, leucine or glycinebetaine. Taurine transporter mRNA is ubiquitously distributed in carp tissues, and its level decreases in the order: kidney>spleen>heart>fin>eye≈intestine≈gill>skin>brain>muscle>hepatopancreas. Taurine transporter mRNA level increased up to 7.5-fold on raising the ambient osmolality from 300 to 450 mosmol/kgH 2O. These data suggest the significant role of taurine as an osmolyte in carp cells.