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 myo-inositol > D-glucose > D-galactose >/= alpha-methyl-D-glucoside. The current induced by myo-inositol increased with membrane hyperpolarization and depended on external myo-inositol 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.
D000595:Amino Acid Sequence; D000818:Animals; D001483:Base Sequence; D003001:Cloning, Molecular; D004553:Electric Conductivity; D015870:Gene Expression; D007294:Inositol / Q000494:pharmacology; D007421:Intestine, Small / Q000737:chemistry*; D007668:Kidney / Q000737:chemistry; D007700:Kinetics; D008562:Membrane Glycoproteins / Q000737:chemistry / Q000235:genetics* / Q000502:physiology; D008839:Microelectrodes; D008969:Molecular Sequence Data; D009004:Monosaccharide Transport Proteins / Q000737:chemistry / Q000235:genetics* / Q000502:physiology; D009865:Oocytes / Q000378:metabolism; D018408:Patch-Clamp Techniques; D018075:RNA, Complementary / Q000235:genetics; D012333:RNA, Messenger / Q000032:analysis; D012964:Sodium / Q000494:pharmacology; D051269:Sodium-Glucose Transporter 1; D014162:Transfection; D014982:Xenopus laevis / Q000235:genetics*