ISOLATION OF A CDNA-ENCODING A TAURINE TRANSPORTER IN THE HUMAN RETINAL-PIGMENT EPITHELIUM

Reverse transcription-polymerase chain reaction (RT-PCR) was performed to amplify a cDNA encoding a taurine transporter in the human retinal pigment epithelium (HRPE). The coding region of a PCR product was fou nd to be 1863 bp long, predicting a 620-amino acid protein (69,826 Da) . This cDNA sequence is almost identical to those taurine transporters recently determined in the human thyroid and placenta: 12 and 1 base pair(s) different from the reported thyroid and placenta transporter c lones, respectively. The injection of mRNA in vitro transcribed from t he PCR product markedly increased taurine uptake in Xenopus laevis ooc ytes. Taurine uptake is Na+ and Cl- dependent. Unlabeled taurine, beta -alanine and gamma-amino-n-butyric acid at 100 mu M inhibited the upta ke of radiolabeled taurine whereas 100 mu M alpha-alanine and alpha-am inoisobutyric acid did not. A kinetic study showed that taurine uptake is mediated by a single carrier system with the apparent Michaelis-Me nten constant of approximately 2 mu M. These results suggest that the PCR product encodes a functional taurine transporter whose characteris tics are similar to those of taurine uptake observed in the original H RPE cells. A DNA encoding the reported placental transporter was made from the PCR product by site-directed mutagenesis but it was not funct ional in the oocyte expression. A similar RT-PCR was performed with po ly (A)(+) mRNA isolated from JAR human placenta choriocarcinoma cells. This PCR product was identical to that from the HRPE. In addition, th e clone of the human thyroid transporter was obtained and re-sequenced . Its translation coding region was also identical to that of the PCR product from the HRPE, showing that taurine transporters are identical in the human RPE, thyroid and placenta.