Transmembrane IV of the high-affinity sodium-glucose cotransporter participates in sugar binding

A systematic evaluation of sorting motifs in the sodium–iodide symporter (NIS)

Human sodium–iodide symporter (NIS) variants were created to suppress predicted binding motifs potentially implicated in trafficking of this protein. A leucine residue in an internal PDZ-binding motif was found to be essential for expression of the symporter at the plasma membrane.

The sodium/iodide symporter: state of the art of its molecular characterization

The sodium/iodide symporter (NIS or SLC5A5) is an intrinsic membrane protein implicated in iodide uptake into thyroid follicular cells. It plays a crucial role in iodine metabolism and thyroid regulation and its function is widely exploited in the diagnosis and treatment of benign and malignant thyroid diseases. A great effort is currently being made to develop a NIS-based gene therapy also allowing the radiotreatment of nonthyroidal tumors. NIS is also expressed in other tissues, such as salivary gland, stomach and mammary gland during lactation, where its physiological role remains unclear. The molecular identity of the thyroid iodide transporter was elucidated approximately fifteen years ago. It belongs to the superfamily of sodium/solute symporters, SSS (and to the human transporter family, SLC5), and is composed of 13 transmembrane helices and 643 amino acid residues in humans. Knowledge concerning NIS structure/function relationship has been obtained by taking advantage of the high resolution structure of one member of the SSS family, the Vibrio parahaemolyticus sodium/galactose symporter (vSGLT), and from studies of gene mutations leading to congenital iodine transport defects (ITD). This review will summarize current knowledge regarding the molecular characterization of NIS.

Bridging the gap between structure and kinetics of human SGLT1

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, K(0.5)); e.g., mutating sugar binding residues increases the glucose K(0.5) by 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 (K(i)) are proportional to the changes in sugar K(0.5), except in the case of F101C, where phlorizin K(i) increases by orders of magnitude without a change in glucose K(0.5). 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.

Role of transmembrane domain 4 in ligand permeation by Crithidia fasciculata equilibrative nucleoside transporter 2 (CfNT2)

Equilibrative nucleoside transporters play essential roles in nutrient uptake, cardiovascular and renal function, and purine analog drug chemotherapies. Limited structural information is available for this family of transporters; however, residues in transmembrane domains 1, 2, 4, and 5 appear to be important for ligand and inhibitor binding. In order to identify regions of the transporter that are important for ligand specificity, a genetic selection for mutants of the inosine-guanosine-specific Crithidia fasciculata nucleoside transporter 2 (CfNT2) that had gained the ability to transport adenosine was carried out in the yeast Saccharomyces cerevisiae. Nearly all positive clones from the genetic selection carried mutations at lysine 155 in transmembrane domain 4, highlighting lysine 155 as a pivotal residue governing the ligand specificity of CfNT2. Mutation of lysine 155 to asparagine conferred affinity for adenosine on the mutant transporter at the expense of inosine and guanosine affinity due to weakened contacts to the purine ring of the ligand. Following systematic cysteine-scanning mutagenesis, thiol-specific modification of several positions within transmembrane domain 4 was found to interfere with inosine transport capability, indicating that this helix lines the water-filled ligand translocation channel. Additionally, the pattern of modification of transmembrane domain 4 suggested that it may deviate from helicity in the vicinity of residue 155. Position 155 was also protected from modification in the presence of ligand, suggesting that lysine 155 is in or near the ligand binding site. Transmembrane domain 4 and particularly lysine 155 appear to play key roles in ligand discrimination and translocation by CfNT2.

Effects on conformational states of the rabbit sodium/glucose cotransporter through modulation of polarity and charge at glutamine 457

The high affinity sodium/glucose cotransporter (SGLT1) couples transport of Na(+) and glucose. Previous studies established that mutant Q457C human SGLT1 retains full activity, and sugar translocation is abolished in mutant Q457R or in mutant Q457C after reaction with methanethiosulfonate derivatives, but Na(+) and sugar binding remain intact. To explore the mechanism by which modulation of Q457 abolishes transport, Q457C and Q457R of rabbit SGLT1 were studied using chemical modification and the two-electrode voltage-clamp technique. Compared to wild-type SGLT1, Q457C exhibits approximately 20-fold reduction in phloridzin affinity and preferential occupancy of an inward-facing state. Alkylation of Q457C by [(2-trimethylammonium) ethyl] methanethiosulphonate bromide, (MTSET), reverses these changes while blocking transport. Analysis of pre-steady-state currents in the absence of sugar yields three decay constants for each of Q457C, Q457C-MTSET and Q457R. Comparison of Q457C-MTSET and Q457R with Q457C and wild-type, reveals that inhibition of transport is accompanied by a decrease in magnitude and voltage-independence of the slow decay constant at negative potentials. But fast and medium decays remain unchanged. Computer simulation of transient currents suggests that introduction of positive charge at position 457 leads to a predominant outward rather than inward-facing conformational state. Taken together, the 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.

Reanalysis of structure/function correlations in the region of transmembrane segments 4 and 5 of the rabbit sodium/glucose cotransporter

The predicted topology of the mammalian high-affinity sodium/glucose cotransporter (SGLT1), in the region surrounding transmembrane segments 4 and 5, disagrees with the recent published crystal structure of bacterial SGLT from Vibrio parahaemolyticus (vSGLT). To investigate this issue further, 38 residues from I143 to A180 in the N-terminal half of rabbit SGLT1 were each replaced with cysteine and then expressed in COS-7 cells or Xenopus laevis oocytes. The membrane orientations of the substituted cysteines were determined by treatment with the thiol-specific reagent N-Biotinoylaminoethyl methanethiosulfonate (biotin-MTSEA), combined with the membrane impermeant thiol-specific reagent sodium (2-sulfonatoethyl) methanethiosulfonate (MTSES). The present results combined with previous structure/function studies of SGLT1, suggest that transmembrane domain (TM) 4 of mammalian SGLT1 extends from residue 143-171 and support the topology observed in the crystal structure of vSGLT.