Expression cloning of NaDC-2, an intestinal Na(+)- or Li(+)-dependent dicarboxylate transporter

Abstract

A cDNA coding for a Na(+)-dicarboxylate cotransporter from Xenopus laevis intestine, NaDC-2, was isolated by functional expression cloning in Xenopus oocytes. NaDC-2 encodes a 622-residue polypeptide with a predicted mass of 68.6 kDa. The sequence and secondary structure of NaDC-2 are related to the mammalian renal Na(+)-dicarboxylate and Na(+)-sulfate cotransporters. NaDC-2 mRNA is expressed only in the intestine. Oocytes injected with NaDC-2 cRNA exhibit increased transport of succinate, citrate, and glutarate. Transport of succinate by NaDC-2 is stimulated by Na+ or Li+, with Michaelis-Menten constant values for succinate of 0.3 mM (in Na+) and 0.7 mM (in Li+). Na+ and Li+ activation curves show sigmoid kinetics, with Hill coefficients of 1.4 (nNa) and 1.7 (nLi), indicating that multiple cations are involved in the transport of succinate. The transport of succinate by NaDC-2 is insensitive to pH, whereas the transport of citrate is inhibited at high pH. The differences in functional properties between NaDC-2 and the structurally related Na(+)-dicarboxylate cotransporters NaDC-1 and hNaDC-1 will form the basis of detailed structure-function studies.