Bridging the gap between structure and kinetics of human SGLT1
| Autor: | Bruce A. Hirayama, Jeff Abramson, Monica Sala-Rabanal, Donald D. F. Loo, Ernest M. Wright, Vincent Chaptal |
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| Rok vydání: | 2012 |
| Předmět: |
Patch-Clamp Techniques
Bridging (networking) Protein Conformation Physiology Stereochemistry Molecular Sequence Data Kinetics Electrochemistry Structure-Activity Relationship Xenopus laevis Sodium-Glucose Transporter 1 Protein structure Vascular Biology Animals Humans Amino Acid Sequence chemistry.chemical_classification Sequence Homology Amino Acid Chemistry Cell Biology Protein Structure Tertiary Amino acid Membrane protein Biophysics Cotransporter |
| Zdroj: | American Journal of Physiology-Cell Physiology. 302:C1293-C1305 |
| ISSN: | 1522-1563 0363-6143 |
| Popis: | The Na+-glucose cotransporter hSGLT1 is a member of a class of membrane proteins that harness Na+electrochemical gradients to drive uphill solute transport. Although hSGLT1 belongs to one gene family (SLC5), recent structural studies of bacterial Na+cotransporters have shown that Na+transporters in different gene families have the same structural fold. We have constructed homology models of hSGLT1 in two conformations, the inward-facing occluded (based on vSGLT) and the outward open conformations (based on Mhp1), mutated in turn each of the conserved gates and ligand binding residues, expressed the SGLT1 mutants in Xenopus oocytes, and determined the functional consequences using biophysical and biochemical assays. The results establish that mutating the ligand binding residues produces profound changes in the ligand affinity (the half-saturation concentration, K0.5); e.g., mutating sugar binding residues increases the glucose K0.5by up to three orders of magnitude. Mutation of the external gate residues increases the Na+to sugar transport stoichiometry, demonstrating that these residues are critical for efficient cotransport. The changes in phlorizin inhibition constant ( Ki) are proportional to the changes in sugar K0.5, except in the case of F101C, where phlorizin Kiincreases by orders of magnitude without a change in glucose K0.5. We conclude that glucose and phlorizin occupy the same binding site and that F101 is involved in binding to the phloretin group of the inhibitor. Substituted-cysteine accessibility methods show that the cysteine residues at the position of the gates and sugar binding site are largely accessible only to external hydrophilic methanethiosulfonate reagents in the presence of external Na+, demonstrating that the external sugar (and phlorizin) binding vestibule is opened by the presence of external Na+and closes after the binding of sugar and phlorizin. Overall, the present results provide a bridge between kinetics and structural studies of cotransporters. |
| Databáze: | OpenAIRE |
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