Sodium/D-glucose cotransporter charge movements involve polar residues

Na(+)-dependent glucose transporters (SGLT1) exhibit transient carrier currents with a time constant (tau) of 2-20 ms, and the charge transfer (Q) fits the Boltzmann equation. There is a 60-mV negative displacement in the tau/V and Q/V curves between the human and rabbit SGLT1 proteins, and the initial goal was to identify the charges responsible for these differences in kinetics. We have focused on residue 176 in putative transmembrane helix (M4) because this is an aspartic acid in rabbit and asparagine in human. Asp-176 in rabbit SGLT1 was replaced with asparagine and alanine residues, and the wild-type and mutant proteins were expressed in Xenopus laevis oocytes. A two-electrode voltage clamp was used to measure the kinetics of charge transfer. There was no difference between the wild-type and D176N, but there was a 60-mV negative shift in the tau/V and Q/V curves with D176A. This suggests that polar residues at position 176 play an important role in determining charge transfer, probably by electrostatic bonding to a neighboring polar residue in the membrane domain of the protein. The similarity between rabbit SGLT1 and the D176N mutant further indicates that other membrane residues account for the difference between rabbit and human SGLT1. There were only modest changes in the steady-state Na+/glucose cotransport kinetics between wild-type and D176A mutant transporters in the voltage range +50 to -50 mV. Model simulations show that the mutation alters the rate constants for conformational changes of the unloaded transporter. Phlorizin, a specific competitive inhibitor of sugar transport, has a lower affinity for the D176A mutant than for SGLT1. This indicates that polar residues at position 176 hydrogen bond with the -OH group on the B-phenyl ring of the inhibitor.