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Sugar transporters in prokaryotes and eukaryotes belong to a large family of membrane proteins containing 12 transmembrane alpha-helices. They are divided into two classes: one facilitative (uniporters) and the other concentrative (cotransporters or symporters). The concentrative transporters are energised by either H⁺ or Na⁺ gradients, which are generated and maintained by ion pumps. The facilitative and H⁺-driven sugar transporters belong to a gene family with a distinctive secondary structure profile. The Na⁺-driven transporters belong to a separate, small gene family with no homology at either the primary or secondary structural levels. It is likely that the Na⁺-and H⁺-driven sugar cotransporters share common transport mechanisms. To explore these mechanisms, we have expressed cloned eukaryote Na⁺/sugar cotransporters (SGLT) in Xenopus laevis oocytes and measured the kinetics of sugar …