Sugar binding residue affects apparent Na+ affinity and transport stoichiometry in mouse sodium/glucose cotransporter type 3B.

SGLT1 is a sodium/glucose cotransporter that moves two Na(+) ions with each glucose molecule per cycle. SGLT3 proteins belong to the same family and are described as glucose sensors rather than glucose transporters. Thus, human SGLT3 (hSGLT3) does not transport sugar, but extracellular glucose depolarizes the cell in which it is expressed. Mouse SGLT3b (mSGLT3b), although it transports sugar, has low apparent sugar affinity and partially uncoupled stoichiometry compared with SGLT1, suggesting that mSGLT3b is also a sugar sensor. The crystal structure of the Vibrio parahaemolyticus SGLT showed that residue Gln(428) interacts directly with the sugar. The corresponding amino acid in mammalian proteins, 457, is conserved in all SGLT1 proteins as glutamine. In SGLT3 proteins, glutamate is the most common residue at this position, although it is a glycine in mSGLT3b and a serine in rat SGLT3b. To test the contribution of this residue to the function of SGLT3 proteins, we constructed SGLT3b mutants that recapitulate residue 457 in SGLT1 and hSGLT3, glutamine and glutamate, respectively. The presence of glutamine at residue 457 increased the apparent Na(+) and sugar affinities, whereas glutamate decreased the apparent Na(+) affinity. Moreover, glutamate transported more cations per sugar molecule than the wild type protein. We propose a model where cations are released intracellularly without the release of sugar from an intermediate state. This model explains the uncoupled charge:sugar transport phenotype observed in wild type and G457E-mSGLT3b compared with SGLT1 and the sugar-activated cation transport without sugar transport that occurs in hSGLT3.