Functional asymmetry of the sodium-D-glucose cotransporter expressed in yeast secretory vesicles

The role of transporter ectodomains in drug recognition and binding: phlorizin and the sodium–glucose cotransporter

This article reviews the role of segments of SLCs located outside the plasma membrane bilayer (ectodomains) using the inhibition of SGLTs (SLC5 family) by the aromatic glucoside phlorizin as a model system.

SLC5 and SLC2 transporters in epithelia-cellular role and molecular mechanisms

Members of the SLC5 and SLC2 family are prominently involved in epithelial sugar transport. SGLT1 (sodium-glucose transporter) and SGLT2, as representatives of the former, mediate sodium-dependent uptake of sugars into intestinal and renal cells. GLUT2 (glucose transporter), as representative of the latter, facilitates the sodium-independent exit of sugars from cells. SGLT has played a major role in the formulation and experimental proof for the existence of sodium cotransport systems. Based on the sequence data and biochemical and biophysical analyses, the role of extramembranous loops in sugar and inhibitor binding can be delineated. Crystal structures and homology modeling of SGLT reveal that the sugar translocation involves operation of two hydrophobic gates and intermediate exofacial and endofacial occluded states of the carrier in an alternating access model. The same basic model is proposed for GLUT1. Studies on GLUT1 have pioneered the isolation of eukaryotic transporters by biochemical methods and the development of transport kinetics and transporter models. For GLUT1, results from extensive mutagenesis, cysteine substitution and accessibility studies can be incorporated into a homology model with a barrel-like structure in which accessibility to the extracellular and intracellular medium is altered by pinching movements of some of the helices. For SGLT1 and GLUT1, the extensive hydrophilic and hydrophobic interactions between sugars and binding sites of the various intramembrane helices occur and lead to different substrate specificities and inhibitor affinities of the two transporters. A complex network of regulatory steps adapts the transport activity to the needs of the body.

SGLT inhibitors as new therapeutic tools in the treatment of diabetes

Recently, the idea has been developed to lower blood glucose blood glucose levels in diabetes by inhibiting sugar reabsorption sugar reabsorption in the kidney kidney . The main target is thereby the early proximal tubule proximal tubule where secondary active transport secondary active transport of the sugar is mediated by the sodium-D: -glucose D-glucose cotransporter SGLT2 SGLT2 . A model substance for the inhibitors inhibitors is the O-glucoside O-glucoside phlorizin phlorizin which inhibits transport transport competitively. Its binding to the transporter involves at least two different domains: an aglucone binding aglucone binding site at the transporter surface, involving extramembranous loops extramembraneous loops , and the sugar binding sugar binding /translocation site buried in a hydrophilic pocket of the transporter. The properties of these binding sites differ between SGLT2 and SGLT1 SGLT1 , which mediates sugar absorption sugar absorption in the intestine intestine . Various O-, C-, N- and S-glucosides have been synthesized with high affinity affinity and high specificity specificity for SGLT2 SGLT2 . Some of these glucosides are in clinical trials clinical trials and have been proven to successfully increase urinary glucose excretion urinary glucose excretion and to decrease blood sugar blood sugar levels without the danger of hypoglycaemia hypoglycaemia during fasting fasting in type 2 diabetes type 2 diabetes .

A proteomic study of sodium/d-glucose cotransporter 1 (SGLT1): Topology of loop 13 and coverage of other functionally important domains

In order to obtain further information about the structure and function of human sodium/D-glucose cotransporter 1 (hSGLT1), the recombinant protein was subjected, either after reconstitution into liposomes or in its free form, to proteolysis followed by nanoscale microcapillary liquid chromatography electrospray ionization tandem mass spectrometry (LC-MS/MS). The peptides released from SGLT1 proteoliposomes by trypsin bead digestion represented the early N-terminal, loop 7, and loop 9, supporting topology models that place these domains on the extracellular side of the protein. Trypsin bead digestion generated, however, also a number of peptides derived from loop 13 whose topology with regard to the membrane is hitherto a point of debate. Sequence coverage was provided from amino acids 559 to 644, suggesting that loop 13 is almost completely accessible at the extravesicular face of the proteoliposomes. These results support the notion that major parts of loop 13, essential for the interaction with transport inhibitors in vivo, are located extracellularly in intact cells. In-gel trypsin, chymotrypsin, and in particular trypsin/chymotrypsin digestion of recombinant SGLT1 in combination with LC-MS/MS provide extensive sequence coverage of the protein, including domains involved in sugar and inhibitor binding and potential phosphorylation sites. These studies demonstrate that proteomic analysis combined with mass spectrometry is a useful tool to characterize regions of SGLT1 that are important for its function and regulation.

Sodium-Independent Low-Affinity d-Glucose Transport by Human Sodium/d-Glucose Cotransporter 1: Critical Role of Tryptophan 561

Although there is no evidence of significant Na-independent glucose flux in tissues naturally expressing SGLT1, previous kinetic and biophysical studies suggest that sodium/d-glucose cotransporter 1 (hSGLT1) can facilitate sodium-independent d-glucose transport and may contain more than one sugar binding site. In this work, we analyze the kinetic properties and conformational states of isolated hSGLT1 reconstituted in liposomes by transport and fluorescence studies in the absence of sodium. In the transport studies with hSGLT1, significant sodium-independent phlorizin inhibitable alpha-methyl d-glucopyranoside (alpha-MDG) uptake was observed which amounted to approximately 20% of the uptake observed in the presence of a sodium gradient. The apparent affinity constant for alpha-MDG was thereby 3.4 +/- 0.5 mM, a value approximately 10-fold higher than that in the presence of sodium. In the absence of sodium, various sugars significantly decreased the intrinsic Trp fluorescence of hSGLT1 in proteoliposomes exhibiting the following sequence of affinities: alpha-MDG > d-glucose approximately d-galactose > 6-deoxy-d-glucose > 2-deoxy-d-glucose > d-allose. Furthermore, significant protection effects of d-glucose or phlorizin against potassium iodide, acrylamide, or trichloroethanol quenching were observed. To locate the Trps involved in this reaction, we generated mutants in which all Trps were sequentially substituted with Phe. None of the replacements significantly affected sodium-dependent uptake. Uptake in the absence of sodium and typical fluorescence changes depended, however, on the presence of Trp at position 561. This Trp residue is conserved in all known SGLT1 forms (except Vibrio parahaemolyticus SGLT) and all SGLT isoforms in humans (except hSGLT3). If all these data are taken into consideration, it seems that Trp-561 in hSGLT1 forms part of a low-affinity sodium-independent binding and/or translocation site for d-glucose. The rate of sodium-independent translocation via hSGLT1 seems, however, to be tightly regulated in the intact cell by yet unknown factors.

Kinetics of the reverse mode of the Na+/glucose cotransporter

This study investigates the reverse mode of the Na(+)/glucose cotransporter (SGLT1). In giant excised inside-out membrane patches from Xenopus laevis oocytes expressing rabbit SGLT1, application of alpha-methyl-D: -glucopyranoside (alphaMDG) to the cytoplasmic solution induced an outward current from cytosolic to external membrane surface. The outward current was Na(+)- and sugar-dependent, and was blocked by phlorizin, a specific inhibitor of SGLT1. The current-voltage relationship saturated at positive membrane voltages (30-50 mV), and approached zero at -150 mV. The half-maximal concentration for alphaMDG-evoked outward current (K(0.5) (alphaMDG)) was 35 mM (at 0 mV). In comparison, K(0.5) (alphaMDG) for forward sugar transport was 0.15 mM (at 0 mV). K(0.5) (Na) was similar for forward and reverse transport ( approximately 35 mM at 0 mV). Specificity of SGLT1 for reverse transport was: alphaMDG (1.0) > D: -galactose (0.84) > 3-O-methyl-glucose (0.55) > D: -glucose (0.38), whereas for forward transport, specificity was: alphaMDG approximately D: -glucose approximately D: -galactose > 3-O-methyl-glucose. Thus there is an asymmetry in sugar kinetics and specificity between forward and reverse modes. Computer simulations showed that a 6-state kinetic model for SGLT1 can account for Na(+)/sugar cotransport and its voltage dependence in both the forward and reverse modes at saturating sodium concentrations. Our data indicate that under physiological conditions, the transporter is poised to accumulate sugar efficiently in the enterocyte.

Characterization of the ion transport activity of the budding yeast Na+/H+ antiporter, Nha1p, using isolated secretory vesicles

The Saccharomyces cerevisiae Nha1p, a plasma membrane protein belonging to the monovalent cation/proton antiporter family, plays a key role in the salt tolerance and pH regulation of cells. We examined the molecular function of Nha1p by using secretory vesicles isolated from a temperature sensitive secretory mutant, sec4-2, in vitro. The isolated secretory vesicles contained newly synthesized Nha1p en route to the plasma membrane and showed antiporter activity exchanging H+ for monovalent alkali metal cations. An amino acid substitution in Nha1p (D266N, Asp-266 to Asn) almost completely abolished the Na+/H+ but not K+/H+ antiport activity, confirming the validity of this assay system as well as the functional importance of Asp-266, especially for selectivity of substrate cations. Nha1p catalyzes transport of Na+ and K+ with similar affinity (12.7 mM and 12.4 mM), and with lower affinity for Rb+ and Li+. Nha1p activity is associated with a net charge movement across the membrane, transporting more protons per single sodium ion (i.e., electrogenic). This feature is similar to the bacterial Na+/H+ antiporters, whereas other known eukaryotic Na+/H+ antiporters are electroneutral. The ion selectivity and the stoichiometry suggest a unique physiological role of Nha1p which is distinct from that of other known Na+/H+ antiporters.

Functional expression of heterologous proteins in yeast: insights into Ca2+signaling and Ca2+-transporting ATPases

The baker's yeast Saccharomyces cerevisiae is a well-developed, versatile, and widely used model organism. It offers a compact and fully sequenced genome, tractable genetics, simple and inexpensive culturing conditions, and, importantly, a conservation of basic cellular machinery and signal transducing pathways with higher eukaryotes. In this review, we describe recent technical advances in the heterologous expression of proteins in yeast and illustrate their application to the study of the Ca2+homeostasis machinery, with particular emphasis on Ca2+-transporting ATPases. Putative Ca2+-ATPases in the newly sequenced genomes of organisms such as parasites, plants, and vertebrates have been investigated by functional complementation of an engineered yeast strain lacking endogenous Ca2+pumps. High-throughput screens of mutant phenotypes to identify side chains critical for ion transport and selectivity have facilitated structure-function analysis, and genomewide approaches may be used to dissect cellular pathways involved in Ca2+transport and trafficking. The utility of the yeast system is demonstrated by rapid advances in the study of the emerging family of Golgi/secretory pathway Ca2+,Mn2+-ATPases (SPCA). Functional expression of human SPCA1 in yeast has provided insight into the physiology, novel biochemical characteristics, and subcellular localization of this pump. Haploinsufficiency of SPCA1 leads to Hailey-Hailey disease (HDD), a debilitating blistering disorder of the skin. Missense mutations, identified in patients with HHD, may be conveniently assessed in yeast for loss-of-function phenotypes associated with the disease.

Functional asymmetry of the human Na+/glucose transporter (hSGLT1) in bacterial membrane vesicles

The functional characteristics of the forward and reverse transport modes of the human Na(+)/glucose transporter (hSGLT1) were investigated using plasma membrane vesicles of E. coli expressing the recombinant transporter. Correctly and inverse-oriented vesicles were employed to measure the initial rates of methyl-alpha-D-glucose uptake, under zero-trans conditions, as a function of Na(+), sugar, and phlorizin concentrations and membrane potential. This approach enabled the analysis of the two faces of hSGLT1 in parallel, revealing the reversibility of Na(+)/sugar cotransport. While the key characteristics of secondary active sugar transport were maintained in both modes, namely, Na(+) and voltage dependence, the kinetic properties of the two sides indicated a functional asymmetry of the transporter. That is, the apparent affinity for sugar and driver cation Na(+) exhibited a difference of more than 1 order of magnitude between the two modes. Furthermore, the selectivity pattern of ligands and the interaction of the transporter with the competitive inhibitor phlorizin were different. Whereas the high-affinity substrates, D-glucose and D-galactose, inhibited uptake of radioactive sugar tracer at their physiological concentrations (10 mM) in the forward reaction, they were poor inhibitors even at high concentrations in the reverse transport mode. Taken together, these results confirm the successful employment of E. coli to express and characterize a human membrane protein (hSGLT1), elucidating the functional asymmetry of this cotransporter.