MOLECULAR IDENTITY AND CALMODULIN-MEDIATED REGULATION OF THE TAURINE TRANSPORTER IN A HUMAN RETINAL-PIGMENT EPITHELIAL-CELL LINE

The molecular identity and calmodulin-mediated regulation of the tauri ne transporter were investigated in a human retinal pigment epithelial cell, line (HRPE). Reverse transcription-polymerase chain reaction am plification of HRPE cell mRNA using primers specific for a taurine tra nsporter cloned from human placenta yielded a product of expected size (similar to 0.9 kb) which hybridized to the placental cDNA probe unde r high stringency conditions. The nucleotide sequence of the product w as identical to the sequence of the portion of the placental taurine t ransporter cDNA flanked by the specific primers. The taurine transport er expressed in the HRPE cell line thus appears to be identical to the transporter cloned from the placenta. Treatment of the HRPE cells wit h a selective calmodulin antagonist CGS 9343 B (CGS) led to a marked d ecrease in taurine transport activity. This effect could be reproduced with W-7, another calmodulin antagonist. The inhibition caused by CGS occurred rapidly (t(1/2) approximate to 10 min). Treatment of the cel ls with CGS did not affect the transport of leucine, an amino acid not recognized by the taurine transporter as a substrate. The CGS-induced inhibition of taurine transport was accompanied by a decrease in the maximal velocity of the transporter with no detectable change in the s ubstrate affinity. The steady state levels of the transporter mRNA how ever remained unaffected by CGS treatment. It is concluded that the HR PE cell line expresses a taurine transporter identical to the transpor ter described in the human placenta and that the function of this tran sporter is regulated by calmodulin-dependent processes.