Cloning and functional expression of an SGLT-1-like protein from the Xenopus laevis intestine

A cDNA encoding an Na⁺-glucose cotransporter type 1 (SGLT-1)-like protein was cloned from the Xenopus laevis intestine by the 5'- and 3'-rapid amplification of cDNA ends method. The deduced amino acid sequence was 673 residues long, with a predicted mass of 74.1 kDa and 52-53% identity to mammalian SGLT-1s. This gene was expressed in the small intestine and kidney, reflecting a tissue distribution similar to that of SGLT-1. The function of the protein was studied using the two-microelectrode voltage-clamp technique after injection of cRNA into Xenopus laevis oocytes. Perfusion with myo- inositol elicited about twofold larger inward currents than perfusion with D- glucose. The order of the substrate specificity was myoinositol > D-glucose > D-galactose ≥ α-methyl-D-glucoside. The current induced by myo-inositol increased with membrane hyperpolarization and depended on external myoinositol and Na⁺: the apparent Michaelis-Menten constant was 0.25 ± 0.07 (SD) mM with myo-inositol, whereas the apparent concentration for half- maximal activation was 12.5 ± 1.0 mM and the Hill coefficient was 1.6 ± 0.1 with Na⁺. In conclusion, the cloned protein shares features with both SGLT- 1 and the Na⁺-myo-inositol cotransporter.