Cloning and functional characterization of the human GLUT7 isoform SLC2A7 from the small intestine

Abstract

Facilitated glucose transporters (GLUTs) mediate transport of sugars across cell membranes by using the chemical gradient of sugars as the driving force. Improved cloning techniques and database analyses have expanded this family of proteins to a total of 14 putative members. In this work a novel hexose transporter isoform, GLUT7, has been cloned from a human intestinal cDNA library by using a PCR-based strategy (GenBank accession no. AY571960). The encoded protein is comprised of 524 amino acid residues and shares 68% similarity and 53% identity with GLUT5, its most closely related isoform. When GLUT7 was expressed in Xenopus oocytes, it showed high-affinity transport for glucose (K(m) = 0.3 mM) and fructose (IC(50) = 0.060 mM). Galactose, 2-deoxy-d-glucose, and xylose were not transported. Uptake of 100 microM d-glucose was not inhibited by 200 microM phloretin or 100 microM cytochalasin B. Northern blotting indicated that the mRNA for GLUT7 is present in the human small intestine, colon, testis, and prostate. Western blotting and immunohistochemistry of rat tissues with an antibody raised against the predicted COOH-terminal sequence confirmed expression of the protein in the small intestine and indicated that the transporter is predominantly expressed in the enterocytes' brush-border membrane. The unusual substrate specificity and close sequence identity with GLUT5 suggest that GLUT7 represents an intermediate between class II GLUTs and the class I member GLUT2. Comparison between these proteins may provide key information as to the structural determinants for the recognition of fructose as a substrate.

Structure of, and functional insight into the GLUT family of membrane transporters

“…22,26,27,30 A major difference between Class I and II is that Class II lacks the tryptophan residue after the GPXXXP motif. 58 Another striking difference is that all Class II GLUTs have a single hydrophobic residue, isoleucine, in TM7 associated with fructose/glucose selectivity.…”

“…25 In 2004, human GLUT7 was the last member of the GLUTs to be cloned using an intestinal cDNA and a polymerase chain reaction-based strategy. 26 Having 528 amino acids, GLUT7 shares 68% similarity and 53% identity with GLUT5 and is predominantly expressed at the apical membrane of the enterocytes in the small intestine. This GLUT can transport both glucose and fructose when expressed in Xenopus oocytes with high affinity, but with very low transport capacity, and it is not sensitive to cytochalasin B.…”

“…This GLUT can transport both glucose and fructose when expressed in Xenopus oocytes with high affinity, but with very low transport capacity, and it is not sensitive to cytochalasin B. 26,28 It is likely that its correct physiological substrate has yet to be identified (see GLUT9 below).…”

Overexpression of GLUT5 in Diabetic Muscle Is Reversed by Pioglitazone

“…GLUT5, GLUT7, GLUT9, and GLUT11 are all members of class II of the GLUT family, grouped together because of their high degree of amino acid identity (40 -60%) and deduced to have fructose transport activity because of their similarity to GLUT5 (16). GLUT7 (17,18) and GLUT9 (19) subsequently have been confirmed to have fructose-inhibitable glucose uptake. In addition, GLUT8 and GLUT12 have been shown to transport both glucose and fructose (20).…”

Expressional Comparison of Glucose Cotransporter Isoforms in the Rat Epididymis During Postnatal Development

“…In the testis of the male reproductive tract, predominant expression of Slc2a8 has been demonstrated by Chen et al (2003) and Zhao et al (2004). Others have also shown that Slc2a1, Slc2a5, and Slc2a7 are localized in the testis Davidson et al, 1992;Li et al, 2004;Ulisse et al, 1992). Even though Schürmann et al (2002) have shown the presence of GLUT8 at the acrosomal region of mature spermatozoa within the epididymis, the expression of other Slc2a isoforms in the epididymis has not been determined in detail.…”