Membrane Topology Motifs in the SGLT Cotransporter Family
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Homologues of the Na+/glucose cotransporter, the SGLT family, include sequences of mammalian, eubacterial, yeast, insect and nematode origin. The cotransported substrates are sugars, inositol, proline, pantothenate, iodide, urea and undetermined solutes. It is reasonable to expect that the SGLT family members share a similar or identical topology of membrane spanning elements, by virtue of their common ancestry and similar coupling of solute transport to downhill sodium flux. Here we examine their membrane topologies as deduced from diverse analyses of their primary sequences, and from their sequence correlations with the experimentally determined topology of the human Na+/glucose cotransporter SGLT1. Our analyses indicate that all family members share a common core of 13 transmembrane helices, but that some, like SGLT1 itself, have one additional span appended to the C-terminus, and still others, two. One bacterial member incorporates an additional span at the N-terminus. Sequence comparisons indicative of common ancestry of the SGLT and the [Na++ Cl−] transporter families are introduced, and evaluated in light of their topologies. New evidence concerning the previously asserted common ancestry of SGLT1 and an N-acetylglucosamine permease of the bacterial phosphotransferase system is considered. Finally, we analyze observations which lead us to conjecture that the experimental strategy most commonly employed to reveal the topology of bacterial transporters (i.e., the fusion of reporter enzymes such as phoA alkaline phosphatase, beta-lactamase or beta-galactosidase, to progressively C-truncated fragments of the transporter) has often instead so perturbed local topology as to have entirely missed pairs of adjacent membrane spans.
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