A novel glucose transporter, mouse GLUT9 (mGLUT9), was recently cloned from mouse 7d embryonic cDNA (1). Several splice variants of mGLUT9 were described, two of which were cloned (mGLUT9a and mGLUT9a _{( 209-316)}). This study describes the cloning and characterization of another splice variant, mGLUT9b. Cloned from adult liver, mGLUT9b is identical to mGLUT9a except at the amino terminus. Based on analysis of the genomic structure, the different amino termini result from alternative transcriptional/translational start sites. Expression and localization of these two mGLUT9 splice variants were examined in control and diabetic adult mouse tissues and in cell lines.

RT-PCR analysis demonstrated expression of mGLUT9a in several tissues while mGLUT9b was observed primarily in liver and kidney. Using a mGLUT9 specific antibody, western blot analysis of total membrane fractions from liver and kidney detected a single, wide band, migrating at 55 kDa. This band shifted to a lower molecular weight when deglycosylated with PGNase F. Both forms were present in liver and kidney. Immunohistochemical localization demonstrated basolateral distribution of mGLUT9 in liver hepatocytes and the expression of mGLUT9 in specific tubules in the outer cortex of the kidney.

To investigate the alternative amino termini, mGLUT9a and mGLUT9b were over expressed in kidney epithelium cell lines. Subcellular fractions localized both forms to the plasma membrane. Immunofluorescent staining of polarized MDCK cells over-expressing mGLUT9 depicted a basolateral distribution for both splice variants. Finally, mGLUT9 protein expression was significantly increased in the kidney and liver from streptozotocin-induced diabetic mice compared with non-diabetic animals.