Structure/Function Studies of the High Affinity Na+/Glucose Cotransporter (SGLT1)

Abstract

The high affinity sodium/glucose cotransporter (SGLT1) couples transport of Na+ and glucose. Investigation of the structure/function relationships of the sodium/glucose transporter (SGLT1) is crucial to understanding co-transporter mechanism. In the first project, we used cysteine-scanning mutagenesis and chemical modification by methanethiosulphonate (MTS) derivatives to test whether predicted TM IV participates in sugar binding. Charged and polar residues and glucose/galactose malabsorption (GGM) missense mutations in TM IV were replaced with cysteine. Mutants exhibited sufficient expression to be studied in detail using the two-electrode voltage-clamp method in Xenopus laevis oocytes and COS-7 cells. The results from mutants T156C and K157C suggest that TM IV participates in sugar interaction with SGLT1. This work has been published in Am J Physiol Cell Physiol 295 (1), C64-72, 2008. The crystal structure of Vibrio parahaemolyticus SGLT (vSGLT) was recently published (1) and showed discrepancy with the predicted topology of mammalian SGLT1 in the region surrounding transmembrane segments IV-V. Therefore, in the second project, we investigated the topology in this region, thirty-eight residues from I143 to A180 in the N-terminal half of rabbit SGLT1 were individually replaced with cysteine and then expressed in COS-7 cells or Xenopus laevis oocytes. Based on the results from biotinylation of mutants in intact COS-7 cells, MTSES accessibility of cysteine mutants expressed in COS-7 cells, effect of substrate on the accessibility of mutant T156C in TM IV expressed in COS-7 cells, and characterization of cysteine mutants in TM V expressed in Xenopus laevis oocytes, we suggest that the region including residues 143-180 forms part of the Na+- and sugar substrate-binding cavity. Our results also suggest that TM IV of mammalian SGLT1 extends from residue 143-171 and support the crystal structure of vSGLT. This work has been published in Biochem Biophys Res Commun 378 (1), 133-138, 2009 Previous studies established that mutant Q457C human SGLT1 retains full activity, and sugar translocation is abolished in mutant Q457R or in mutant Q457C following reaction with methanethiosulfonate derivatives, but Na+ and sugar binding remain intact. Therefore, in the third project, we explored the mechanism by which modulation of Q457 abolishes transport, Q457C and Q457R of rabbit SGLT1 expressed in Xenopus laevis oocytes were studied using chemical modification, the two-electrode voltage-clamp technique and computer model simulations. Our results suggest that glutamine 457, in addition to being involved in sugar binding, is a residue that is sensitive to conformational changes of the carrier. This work has been published in Biophysical Journal 96 (2), 748-760, 2009. Taken together our study along with previous biochemical characterization of SGLT1 and crystal structure of vSGLT, we propose a limited structural model that attempts to bring together the functions of substrate binding (Na+ and sugar), coupling, and translocation. We propose that both Na+ and sugar enter a hydrophilic cavity formed by multiple transmembrane helices from both N-terminal half of SGLT1 and C-terminal half of SGLT1, analogous to all of the known crystal structures of ion-coupled transporters (the Na+/leucine transporter, Na+/aspartate transporter and lactose permease). The functionally important residues in SGLT1 (T156 and K157 in TM 4, D454 and Q457 in TM 11) are close to sugar binding sites.