Identification of a hydrophobic residue as a key determinant of fructose transport by the facilitative hexose transporter SLC2A7 (GLUT7).
Until recently, the only facilitated hexose transporter GLUT proteins (SLC2A) known to transport fructose were GLUTs 2 and 5. However, the recently cloned GLUT7 can also transport fructose as well as glucose. Comparison of sequence alignments indicated that GLUTs 2, 5, and 7 all had an isoleucine residue at position "314" (GLUT7), whereas the non-fructose-transporting isoforms, GLUTs 1, 3, and 4, had a valine at this position. Mutation of Ile-314 to a valine in GLUT7 resulted in a loss of fructose transport, whereas glucose transport remained completely unaffected. Similar results were obtained with GLUTs 2 and 5. Energy minimization modeling of GLUT7 indicated that Ile-314 projects from transmembrane domain 7 (TM7) into the lumen of the aqueous pore, where it could form a hydrophobic interaction with tryptophan 89 from TM2. A valine residue at 314 appeared to produce a narrowing of the vestibule when compared with the isoleucine. It is proposed that this hydrophobic interaction across the pore forms a selectivity filter restricting the access of some hexoses to the substrate binding site(s) within the aqueous channel. The presence of a selectivity filter in the extracellular vestibule of GLUT proteins would allow for subtle changes in substrate specificity without changing the kinetic parameters of the protein.
Manolescu
Andrei
A
Department of Physiology, University of Alberta, Edmonton, Alberta, T6G 2H7, Canada.
Salas-Burgos
Alexis M
AM
Fischbarg
Jorge
J
Cheeseman
Chris I
CI
D000595:Amino Acid Sequence; D000818:Animals; D001665:Binding Sites; D001692:Biological Transport; D015153:Blotting, Western; D018076:DNA, Complementary / Q000378:metabolism; D004305:Dose-Response Relationship, Drug; D005632:Fructose / Q000737:chemistry* / Q000378:metabolism; D005947:Glucose / Q000737:chemistry; D051246:Glucose Transport Proteins, Facilitative / Q000737:chemistry* / Q000378:metabolism; D006601:Hexoses / Q000737:chemistry* / Q000378:metabolism; D006801:Humans; D007150:Immunohistochemistry; D007532:Isoleucine / Q000737:chemistry; D007700:Kinetics; D008958:Models, Molecular; D008969:Molecular Sequence Data; D016297:Mutagenesis, Site-Directed; D009154:Mutation; D009865:Oocytes / Q000378:metabolism; D011485:Protein Binding; D011487:Protein Conformation; D020033:Protein Isoforms; D017433:Protein Structure, Secondary; D021381:Protein Transport; D018075:RNA, Complementary / Q000378:metabolism; D017386:Sequence Homology, Amino Acid; D013379:Substrate Specificity; D014633:Valine / Q000737:chemistry; D014981:Xenopus; D014982:Xenopus laevis
D018076:DNA, Complementary; D051246:Glucose Transport Proteins, Facilitative; D006601:Hexoses; D020033:Protein Isoforms; D018075:RNA, Complementary; C487892:SLC2A7 protein, human; D007532:Isoleucine; D005632:Fructose; D014633:Valine; D005947:Glucose