Functional characterization and chromosomal localization of a cloned taurine transporter from human placenta

Abstract

A cDNA clone highly related to the rat brain taurine transporter has been isolated from a human placental cDNA library. Transfection of this cDNA into HeLa cells results in a marked elevation of taurine transport activity. The activity of the cDNA-induced transporter is dependent on the presence of Na+ as well as Cl-. The Na+/Cl-/taurine stoichiometry for the cloned transporter is 2:1:1. The transporter is specific for taurine and other beta-amino acids, including beta-alanine, and exhibits high affinity for taurine (Michaelis-Menten constant approximately 6 microM). The clone consists of a coding region 1863 bp long (including the termination codon), flanked by a 376 bp-long 5′ non-coding region and a 625 bp-long 3′ non-coding region. The nucleotide sequence of the coding region predicts a 620-amino acid protein with a calculated M(r) of 69,853. Northern-blot analysis of poly(A)+ RNA from several human tissues indicates a complex expression pattern differing across tissues. The principal transcript, 6.9 kb in size, is expressed abundantly in placenta and skeletal muscle, at intermediate levels in heart, brain, lung, kidney and pancreas and at low levels in liver. Cultured human cell lines derived from placenta (JAR and BeWo), intestine (HT-29), cervix (HeLa) and retinal pigment epithelium (HRPE), which are known to possess Na(+)- and Cl(-)-coupled taurine transport activity, also contain the 6.9 kb transcript. Somatic cell hybrid and in situ hybridization studies indicate that the cloned taurine transporter is localized to human chromosome 3 p24-->p26.