Identification and characterization of human glucose transporter-like protein-9 (GLUT9): Alternative splicing alters trafficking

The recently cloned human GLUT9 gene, which maps to chromosome 4p15.3-p16, consists of 12 exons coding for a 540-amino acid protein. Based on a sequence entry (NCBI accession number BC018897) and screening of expressed sequence tags, we have cloned an alternative splice variant of GLUT9 from human kidney cDNA. The RNA of this splice variant consists of 13 exons and codes for a putative protein of 512 amino acids (GLUT9ΔN). The predicted proteins differ only in their N terminus, suggesting a different subcellular localization and possible physiological role. Screening human tissue RNA by reverse transcription-PCR showed that GLUT9 is expressed mainly in kidney, liver, placenta, and leukocytes, whereas GLUT9ΔN was detected only in kidney and placenta. The GLUT9 protein localized by immunohistochemistry to human kidney proximal tubules, and subcellular fractionation of human kidney revealed the GLUT9 protein in plasma membranes and high density microsomal membranes. Treatment of kidney membrane proteins with peptide N-glycosidase F showed that GLUT9 and GLUT9ΔN are expressed in vivo. Localization of GLUT9 and GLUT9ΔN in three kidney-derived cell lines revealed a plasma membrane distribution for GLUT9 in COS-7 and HEK293 cells, whereas GLUT9ΔN showed a perinuclear pattern and plasma membrane staining in COS-7 and HEK293 cells, respectively. In polarized Madin-Darby canine kidney cells, GLUT9 trafficked to the basolateral membrane, whereas GLUT9ΔN localized to the apical membrane. Using heterologous expression of GLUT9 in Xenopus oocytes, GLUT9 appears to be a functional isoform with low affinity for deoxyglucose. Deoxyglucose transport mediated by GLUT9 was not inhibited by cytochalasin B. GLUT9 did not bind cytochalasin B as shown by a cytochalasin B binding assay, indicating a similar behavior of GLUT9 compared with GLUT5.