Characterization of a Na+/glucose cotransporter cloned from rabbit small intestine.

The Na+/glucose cotransporter from rabbit intestinal brush border membranes has been cloned, sequenced, and expressed in Xenopus oocytes. Injection of cloned RNA into oocytes increased Na+/sugar cotransport by three orders of magnitude. In this study, we have compared and contrasted the transport properties of this cloned protein expressed in Xenopus oocytes with the native transporter present in rabbit intestinal brush borders. Initial rates of 14C-alpha-methyl-D-glucopyranoside uptake into brush border membrane vesicles and Xenopus oocytes were measured as a function of the external sodium, sugar, and phlorizin concentrations. Sugar uptake into oocytes and brush borders was Na+-dependent (Hill coefficient 1.5 and 1.7), phlorizin inhibitable (Ki 6 and 9 microM), and saturable (alpha-methyl-D-glucopyranoside Km 110 and 570 microM). The sugar specificity was examined by competition experiments, and in both cases the selectivity was D-glucose greater than alpha-methyl-D-glucopyranoside greater than D-galactose greater than 3-O-methyl-D-glucoside. In view of the close similarity between the properties of the cloned protein expressed in oocytes and the native brush border transporter, we conclude that we have cloned the classical Na+/glucose cotransporter.