THE RBAT GENE IS RESPONSIBLE FOR L-CYSTINE UPTAKE VIA THE B(0,-LIKE AMINO-ACID-TRANSPORT SYSTEM IN A RENAL PROXIMAL TUBULAR CELL-LINE (OK CELLS)())

Several studies have shown that the cRNA of human, rabbit, or rat rBAT induces in Xenopus oocytes sodium-independent, high affinity uptake o f L-cystine via a system b(0,+)-like amino acid exchanger. We have sho wn that mutations in rBAT cause type I cystinuria (Calonge, M. J., Gas parini, P., Chillaron, J., Chillon, M., Gallucci, M., Rousaud, F., Zel ante, L., Testar, X., Dallapiccola, B., Di Silverio, F., Barcelo, P., Estivill, X., Zorzano, A., Nunes, V., and Palacin, M. (1994) Nat. Gene t. 6, 420-425; Calonge, M. J., Volipini, V., Bisceglia, L., Rousaud, F ., De Sanctis, L., Beccia, E., Zelante, L., Testar, X., Zorzano, A., E stivill, X., Gasparini, P., Nunes, V., and Palacin, M. (1995) Proc. Na tl. Acad. Sci. U. S. A. 92, 9667-9671). Apart from oocytes, no other e xpression system has been used for transfection of functional rBAT act ivity. Furthermore, the b(0,+)-like transport activity has not been cl early described in the kidney or intestine. Here, we report that a ''p roximal tubular-like'' cell line derived from opossum kidney (OR cells ) expresses an rBAT transcript. Poly(A)(+) RNA from OK cells induced s ystem b(0,+)-like transport activity in oocytes. This was hybrid-deple ted by human rBAT antisense oligonucleotides. A polymerase chain react ion amplified cDNA fragment (similar to 700 base pairs) from OK cell R NA corresponds to an rBAT protein fragment 65-69% identical to those f rom human, rabbit and rat kidneys. We have also examined transport of L-cystine in OK cells and found characteristics very similar to the am ino acid exchanger activity induced by rBAT cRNA in oocytes. Uptake of L-cystine was of high affinity, sodium-independent and shared with L- arginine and L-leucine. It was trans-stimulated by amino acids with th e same specificity as rBAT-induced transport activity in oocytes. Furt hermore, it was localized to the apical pole of confluent OK cells. To demonstrate that the rBAT protein is functionally related to this tra nsport activity, we have transfected OK cells with human rBAT antisens e and sense sequences. Transfection with rBAT antisense, but not with rBAT sense, resulted in the specific reduction of rBAT mRNA expression and b(0,+)-like transport activity. These results demonstrate that rB AT is functionally related to the L-cystine uptake via system b(0,+)-l ike in the apical pole of the renal OK cell line.