Residue 457 controls sugar binding and transport in the Na+/glucose cotransporter

The Na+/glucose cotransporter (SGLT1) is highly selective for its natural substrates, D-galactose and D-galactose. We have investigated the structural basis of this sugar selectivity on the human isoform of SGLT1, single site mutants of hSGLT1, and the pig SGLT3 isoform, expressed in Xenopus oocytes using electrophysiological methods and the effects of cysteine-specific reagents. Kinetics of transport of glucose analogues, each modified at one position of the pyranose ring, were determined for each transporter. Correlation of kinetics with amino acid sequences indicates that residue Gln-457 sequentially interacts with O1 of the pyranose in the binding site, and with O5 in the translocation pathway. Furthermore, correlation of the selectivity characteristics of the SGLT isoforms (SGLT1 transports both glucose and galactose, but SGLT2 and SGLT3 transport only glucose) with amino acid sequence differences, suggests that residue 460 (threonine in SGLT1, and serine in SGLT2 and SGLT3) are involved in hydrogen bonding to O4 of the pyranose. In addition, the results show that substrate specificity of binding is not correlated to substrate specificity of transport, suggesting there are at least two steps in the sugar translocation process.