Cloning and characterization of a novel Na⁺-dependent glucose transporter (NaGLT1) in rat kidney

To identify novel transporters in the kidney, we have constructed an MRNA data base composed of 1000 overall clones by random sequencing of a male rat kidney cDNA library. After a BLAST search, ∼40% of the clones were unknown and/or unannotated and were screened by measuring the uptake of various compounds using Xenopus oocytes. One clone stimulated the uptake of α-methyl-D-glucopyranoside and therefore was termed rat Na⁺-dependent glucose transporter 1 (rNaGLT1). The rNaGLT1 cDNA (2173 bp) has an open reading frame encoding a 484-amino acid protein, showing <22% homology to known SGLT and GLUT glucose transporters. α-Methyl-D-glucopyranoside uptake by rNaGLT1 cRNA-injected oocytes showed saturability, with an apparent K_m of 3.7 mM and a coupling ratio of 1:1 with Na⁺. rNaGLT1 mRNA was expressed predominantly in the kidney upon Northern blot analysis and reverse transcription-PCR. Reverse transcription-PCR in microdissected nephron segments revealed that rNaGLT1 mRNA was primarily localized in the proximal tubules. A clear signal corresponding to rNaGLT1 protein was recognized in the brush-border (but not basolateral) membrane fraction by immunoblot analysis. The rNaGLT1 mRNA level in the kidney was significantly higher than rat SGLT1 and SGLT2 mRNA levels. These findings suggest that rNaGLT1 is a novel Na⁺-dependent glucose transporter with low substrate affinity that mediates tubular reabsorption of glucose.