Expression cloning and cDNA sequencing of the Na⁺/glucose co-transporter

Abstract

Organic substrates (sugars, amino acids, carboxylic acids and neutrotransmitters) are actively transported into eukaryotic cells by Na⁺ co-transport. Some of the transport proteins have been identified—for example, intestinal brush border Na⁺/glucose and Na⁺/proline transporters^1,2 and the brain Na⁺/Cl^–/GABA transporter³—and progress has been made in locating their active sites and probing their conformational states^1,2,4–7. The archetypical Na⁺-driven transporter is the intestinal brush border Na⁺/glucose co-transporter (see ref. 8), and a defect in the co-transporter is the origin of the congenital glucose–galactose malabsorption syndrome⁹. Here we describe cloning of this co-transporter by a method new to membrane proteins. We have sequenced the cloned DNA and have found no homology between the Na⁺/glucose co-transporter and either the mammalian facilitated glucose carrier or the bacterial sugar transport proteins. This suggests that the mammalian Na⁺-driven transporter has no evolutionary relationship to the other sugar transporters.