Expression cloning and characterization of a novel sodium dicarboxylate cotransporter from winter flounder kidney

A cDNA coding for a Na+-dicarboxylate cotransporter, fNaDC-3, from winter f lounder (Pseudopleuronectes americanus) kidney was isolated by functional e xpression in Xenopus laevis oocytes, The fNaDC-3 cDNA is 2384 nucleotides l ong and encodes a protein of 601 amino acids with a calculated molecular ma ss of 66.4 kDa, Secondary structure analysis predicts at least eight membra ne-spanning domains. Transport of succinate by fNaDC-3 was sodium dependent , could be inhibited by lithium, and evoked an inward current. The apparent affinity constant (K-m) of fNaDC-3 for succinate of 30 mu M resembles that of Na+-dicarboxylate transport in the basolateral membrane of mammalian re nal proximal tubules. The substrates specific for the basolateral transport er, 2,3-dimethylsuccinate and cis-aconitate, not only inhibited succinate u ptake but also evoked inward currents, proving that they are transported by fNaDC-3. Succinate transport via fNaDC-3 decreased by lowering pH, as did citrate transport, although much more moderately. These characteristics sug gest that fNaDC-3 is a new type of Na+-dicarboxylate transporter that most likely corresponds to the Na+-dicarboxylate cotransporter in the basolatera l membrane of mammalian renal proximal tubules.