title: Membrane topology of the human Na+/glucose cotransporter SGLT1.
abstract: The membrane topology of the human Na+/glucose cotransporter SGLT1 has been probed using N-glycosylation scanning mutants and nested truncations. Functional analysis proved essential for establishment of signal-anchor topology. The resultant model diverges significantly from previously held suppositions of structure based primarily on hydropathy analysis. SGLT1 incorporates 14 membrane spans. The N terminus resides extracellularly, and two hydrophobic regions form newly recognized membrane spans 4 and 12; the large charged domain near the C terminus is cytoplasmic. This model was evaluated further using two advanced empirically-based algorithms predictive of transmembrane helices. Helix ends were predicted using thermo-dynamically-based algorithms known to predict x-ray crystallographically determined transmembrane helix ends. Several considerations suggest the hydrophobic C terminus forms a 14th transmembrane helix, differentiating the eukaryotic members of the SGLT1 family from bacterial homologues. Our data inferentially indicate that these bacterial homologues incorporate 13 spans, with an extracellular N terminus. The model of SGLT1 secondary structure and the predicted helix ends signify information prerequisite for the rational design of further experiments on structure/function relationships.
authors.affiliation: Department of Physiology, UCLA School of Medicine 90095-1751, USA.
authors.forename: E
authors.initials: E
authors.lastname: Turk
authors.forename: C J
authors.initials: CJ
authors.lastname: Kerner
authors.forename: M P
authors.initials: MP
authors.lastname: Lostao
authors.forename: E M
authors.initials: EM
authors.lastname: Wright
mesh_terms: D000595:Amino Acid Sequence; D000818:Animals; D001692:Biological Transport; D006031:Glycosylation; D006801:Humans; D008562:Membrane Glycoproteins / Q000737:chemistry*; D008969:Molecular Sequence Data; D009004:Monosaccharide Transport Proteins / Q000737:chemistry*; D016297:Mutagenesis, Site-Directed; D009865:Oocytes; D017433:Protein Structure, Secondary; D051269:Sodium-Glucose Transporter 1; D013329:Structure-Activity Relationship; D014982:Xenopus laevis
chemical_list: D008562:Membrane Glycoproteins; D009004:Monosaccharide Transport Proteins; C495674:SLC5A1 protein, human; D051269:Sodium-Glucose Transporter 1