Structural analysis of cloned plasma membrane proteins by freeze-fracture electron microscopy

The Molecular Basis of Glucose Galactose Malabsorption in a Large Swedish Pedigree

Glucose-galactose malabsorption (GGM) is due to mutations in the gene coding for the intestinal sodium glucose cotransporter SGLT1 (SLC5A1). Here we identify the rare variant Gln457Arg (Q457R) in a large pedigree of patients in the Västerbotten County in Northern Sweden with the clinical phenotype of GGM. The functional effect of the Q457R mutation was determined in protein expressed in Xenopus laevis oocytes using biophysical and biochemical methods. The mutant failed to transport the specific SGLT1 sugar analog α-methyl-D-glucopyranoside (αMDG). Q457R SGLT1 was synthesized in amounts comparable to the wild-type (WT) transporter. SGLT1 charge measurements and freeze-fracture electron microscopy demonstrated that the mutant protein was inserted into the plasma membrane. Electrophysiological experiments, both steady-state and presteady-state, demonstrated that the mutant bound sugar with an affinity lower than the WT transporter. Together with our previous studies on Q457C and Q457E mutants, we established that the positive charge on Q457R prevented the translocation of sugar from the outward-facing to inward-facing conformation. This is contrary to other GGM cases where missense mutations caused defects in trafficking SGLT1 to the plasma membrane. Thirteen GGM patients are now added to the pedigree traced back to the late 17^th century. The frequency of the Q457R variant in Västerbotten County genomes, 0.0067, is higher than in the general Swedish population, 0.0015, and higher than the general European population, 0.000067. This explains the high number of GGM cases in this region of Sweden.

Nano-scale resolution of native retinal rod disk membranes reveals differences in lipid composition

Photoreceptors rely on distinct membrane compartments to support their specialized function. Unlike protein localization, identification of critical differences in membrane content has not yet been expanded to lipids, due to the difficulty of isolating domain-specific samples. We have overcome this by using SMA to coimmunopurify membrane proteins and their native lipids from two regions of photoreceptor ROS disks. Each sample's copurified lipids were subjected to untargeted lipidomic and fatty acid analysis. Extensive differences between center (rhodopsin) and rim (ABCA4 and PRPH2/ROM1) samples included a lower PC to PE ratio and increased LC- and VLC-PUFAs in the center relative to the rim region, which was enriched in shorter, saturated FAs. The comparatively few differences between the two rim samples likely reflect specific protein-lipid interactions. High-resolution profiling of the ROS disk lipid composition gives new insights into how intricate membrane structure and protein activity are balanced within the ROS, and provides a model for future studies of other complex cellular structures.

Endoplasmic reticulum phospholipid scramblase activity revealed after protein reconstitution into giant unilamellar vesicles containing a photostable lipid reporter

Transbilayer movement of phospholipids in biological membranes is mediated by a diverse set of lipid transporters. Among them are scramblases that facilitate a rapid bi-directional movement of lipids without metabolic energy input. Here, we established a new fluorescence microscopy-based assay for detecting phospholipid scramblase activity of membrane proteins upon their reconstitution into giant unilamellar vesicles formed from proteoliposomes by electroformation. The assay is based on chemical bleaching of fluorescence of a photostable ATTO-dye labeled phospholipid with the membrane-impermeant reductant sodium dithionite. We demonstrate that this new methodology is suitable for the study of the scramblase activity of the yeast endoplasmic reticulum at single vesicle level.

The mitochondrial ADP/ATP carrier exists and functions as a monomer

For more than 40 years, the oligomeric state of members of the mitochondrial carrier family (SLC25) has been the subject of debate. Initially, the consensus was that they were dimeric, based on the application of a large number of different techniques. However, the structures of the mitochondrial ADP/ATP carrier, a member of the family, clearly demonstrated that its structural fold is monomeric, lacking a conserved dimerisation interface. A re-evaluation of previously published data, with the advantage of hindsight, concluded that technical errors were at the basis of the earlier dimer claims. Here, we revisit this topic, as new claims for the existence of dimers of the bovine ADP/ATP carrier have emerged using native mass spectrometry of mitochondrial membrane vesicles. However, the measured mass does not agree with previously published values, and a large number of post-translational modifications are proposed to account for the difference. Contrarily, these modifications are not observed in electron density maps of the bovine carrier. If they were present, they would interfere with the structure and function of the carrier, including inhibitor and substrate binding. Furthermore, the reported mass does not account for three tightly bound cardiolipin molecules, which are consistently observed in other studies and are important stabilising factors for the transport mechanism. The monomeric carrier has all of the required properties for a functional transporter and undergoes large conformational changes that are incompatible with a stable dimerisation interface. Thus, our view that the native mitochondrial ADP/ATP carrier exists and functions as a monomer remains unaltered.

Nanoarchitecture and dynamics of the mouse enteric glycocalyx examined by freeze-etching electron tomography and intravital microscopy

The glycocalyx is a highly hydrated, glycoprotein-rich coat shrouding many eukaryotic and prokaryotic cells. The intestinal epithelial glycocalyx, comprising glycosylated transmembrane mucins, is part of the primary host-microbe interface and is essential for nutrient absorption. Its disruption has been implicated in numerous gastrointestinal diseases. Yet, due to challenges in preserving and visualizing its native organization, glycocalyx structure-function relationships remain unclear. Here, we characterize the nanoarchitecture of the murine enteric glycocalyx using freeze-etching and electron tomography. Micrometer-long mucin filaments emerge from microvillar-tips and, through zigzagged lateral interactions form a three-dimensional columnar network with a 30 nm mesh. Filament-termini converge into globular structures ~30 nm apart that are liquid-crystalline packed within a single plane. Finally, we assess glycocalyx deformability and porosity using intravital microscopy. We argue that the columnar network architecture and the liquid-crystalline packing of the filament termini allow the glycocalyx to function as a deformable size-exclusion filter of luminal contents.

Sun, Krystofiak et al. show the nanoarchitecture of the murine enteric glycocalyx, glycoprotein-rich coat covering cells and assess its porosity and deformability in mice, providing a comprehensive structural framework. This study suggests that the glycocalyx may function as a deformable size-exclusion filter of luminal contents.

Functional analysis of a triplet deletion in the gene encoding the sodium glucose transporter 3, a potential risk factor for ADHD

Sodium-glucose transporters (SGLT) belong to the solute carrier 5 family, which is characterized by sodium dependent transport of sugars and other solutes. In contrast, the human SGLT3 (hSGLT3) isoform, encoded by SLC5A4, acts as a glucose sensor that does not transport sugar but induces membrane depolarization by Na⁺ currents upon ligand binding. Whole-exome sequencing (WES) of several extended pedigrees with high density of attention-deficit/hyperactivity disorder (ADHD) identified a triplet ATG deletion in SLC5A4 leading to a single amino acid loss (ΔM500) in the hSGLT3 protein imperfectly co-segregating with the clinical phenotype of ADHD. Since mutations in homologous domains of hSGLT1 and hSGLT2 were found to affect intestinal and renal function, respectively, we analyzed the functional properties of hSGLT3[wt] and [ΔM500] by voltage clamp and current clamp recordings from cRNA-injected Xenopus laevis oocytes.

The cation conductance of hSGLT3[wt] was activated by application of glucose or the specific agonist 1-desoxynojirimycin (DNJ) as revealed by inward currents in the voltage clamp configuration and cell depolarization in the current clamp mode. Almost no currents and changes in membrane potential were observed when glucose or DNJ were applied to hSGLT3[ΔM500]-injected oocytes, demonstrating a loss of function by this amino acid deletion in hSGLT3. To monitor membrane targeting of wt and mutant hSGLT3, fusion constructs with YFP were generated, heterologously expressed in Xenopus laevis oocytes and analyzed for membrane fluorescence by confocal microscopy. In comparison to hSGLT3[wt] the fluorescent signal of mutant [ΔM500] was reduced by 43% indicating that the mutant phenotype might mainly result from inaccurate membrane targeting. As revealed by homology modeling, residue M500 is located in TM11 suggesting that in addition to the core structure (TM1-TM10) of the transporter, the surrounding TMs are equally crucial for transport/sensor function.

In conclusion, our findings indicate that the deletion [ΔM500] in hSGLT3 inhibits membrane targeting and thus largely disrupts glucose-induced sodium conductance, which may, in interaction with other ADHD risk-related gene variants, influence the risk for ADHD in deletion carriers.

Conformational transitions of the sodium-dependent sugar transporter, vSGLT

Sodium-dependent transporters couple the flow of Na+ ions down their electrochemical potential gradient to the uphill transport of various ligands. Many of these transporters share a common core structure composed of a five-helix inverted repeat and deliver their cargo utilizing an alternating-access mechanism. A detailed characterization of inward-facing conformations of the Na+-dependent sugar transporter from Vibrio parahaemolyticus (vSGLT) has previously been reported, but structural details on additional conformations and on how Na+ and ligand influence the equilibrium between other states remains unknown. Here, double electron-electron resonance spectroscopy, structural modeling, and molecular dynamics are utilized to deduce ligand-dependent equilibria shifts of vSGLT in micelles. In the absence and presence of saturating amounts of Na+, vSGLT favors an inward-facing conformation. Upon binding both Na+ and sugar, the equilibrium shifts toward either an outward-facing or occluded conformation. While Na+ alone does not stabilize the outward-facing state, gating charge calculations together with a kinetic model of transport suggest that the resting negative membrane potential of the cell, absent in detergent-solubilized samples, may stabilize vSGLT in an outward-open conformation where it is poised for binding external sugars. In total, these findings provide insights into ligand-induced conformational selection and delineate the transport cycle of vSGLT.

On the role of a specific insert in acetate permeases (ActP) for tellurite uptake in bacteria: Functional and structural studies

The oxyanion tellurite (TeO₃^2-) is extremely toxic to bacterial cells. In Rhodobacter capsulatus, tellurite enters the cytosol by means of the high uptake-rate acetate permease RcActP2, encoded by one of the three actP genes present in this species (actP1, actP2 and actP3). Conversely, in Escherichia coli a low rate influx of the oxyanion is measured, which depends mainly on the phosphate transporter EcPitA, even though E. coli contains its own EcActP acetate permease. Here we report that when the actP2 gene from R. capsulatus is expressed in wild-type E. coli HB101 and in E. coli JW3460 ΔpitA mutant, the cellular intake of tellurite increases up to four times, suggesting intrinsic structural differences between EcActP and RcActP2. Indeed, a sequence analysis indicated the presence in RcActP2 of an insert of 15-16 residues, located between trans-membrane (TM) helices 6 and 7, which is absent in both EcActP and RcActP1. Based on this observation, the molecular models of homodimeric RcActP1 and RcActP2 were calculated and analyzed. In the RcActP2 model, the insert induces a perturbation in the conformation of the loop between TM helices 6 and 7, located at the RcActP2 dimerization interface. This perturbation opens a cavity on the periplasmic side that is closed, instead, in the RcActP1 model. This cavity also features an increase of the positive electric potential on the protein surface, an effect ascribed to specific residues Lys261, Lys281 and Arg560. We propose that this positively charged patch in RcActP2 is involved in recognition and translocation of the TeO₃^2- anion, attributing to RcActP2 a greater ability as compared to RcActP1 to transport this inorganic poison inside the cells.

The Human Sodium-Glucose Cotransporter (hSGLT1) Is a Disulfide-Bridged Homodimer with a Re-Entrant C-Terminal Loop

Na-coupled cotransporters are proteins that use the trans-membrane electrochemical gradient of Na to activate the transport of a second solute. The sodium-glucose cotransporter 1 (SGLT1) constitutes a well-studied prototype of this transport mechanism but essential molecular characteristics, namely its quaternary structure and the exact arrangement of the C-terminal transmembrane segments, are still debated. After expression in Xenopus oocytes, human SGLT1 molecules (hSGLT1) were labelled on an externally accessible cysteine residue with a thiol-reactive fluorophore (tetramethylrhodamine-C5-maleimide, TMR). Addition of dipicrylamine (DPA, a negatively-charged amphiphatic fluorescence “quencher”) to the fluorescently-labelled oocytes is used to quench the fluorescence originating from hSGLT1 in a voltage-dependent manner. Using this arrangement with a cysteine residue introduced at position 624 in the loop between transmembrane segments 12 and 13, the voltage-dependent fluorescence signal clearly indicated that this portion of the 12–13 loop is located on the external side of the membrane. As the 12–13 loop begins on the intracellular side of the membrane, this suggests that the 12–13 loop is re-entrant. Using fluorescence resonance energy transfer (FRET), we observed that different hSGLT1 molecules are within molecular distances from each other suggesting a multimeric complex arrangement. In agreement with this conclusion, a western blot analysis showed that hSGLT1 migrates as either a monomer or a dimer in reducing and non-reducing conditions, respectively. A systematic mutational study of endogenous cysteine residues in hSGLT1 showed that a disulfide bridge is formed between the C355 residues of two neighbouring hSGLT1 molecules. It is concluded that, 1) hSGLT1 is expressed as a disulfide bridged homodimer via C355 and that 2) a portion of the intracellular 12–13 loop is re-entrant and readily accessible from the extracellular milieu.

KV1 channels identified in rodent myelinated axons, linked to Cx29 in innermost myelin: support for electrically active myelin in mammalian saltatory conduction

Saltatory conduction in mammalian myelinated axons was thought to be well understood before recent discoveries revealed unexpected subcellular distributions and molecular identities of the K(+)-conductance pathways that provide for rapid axonal repolarization. In this study, we visualize, identify, localize, quantify, and ultrastructurally characterize axonal KV1.1/KV1.2 channels in sciatic nerves of rodents. With the use of light microscopic immunocytochemistry and freeze-fracture replica immunogold labeling electron microscopy, KV1.1/KV1.2 channels are localized to three anatomically and compositionally distinct domains in the internodal axolemmas of large myelinated axons, where they form densely packed "rosettes" of 9-nm intramembrane particles. These axolemmal KV1.1/KV1.2 rosettes are precisely aligned with and ultrastructurally coupled to connexin29 (Cx29) channels, also in matching rosettes, in the surrounding juxtaparanodal myelin collars and along the inner mesaxon. As >98% of transmembrane proteins large enough to represent ion channels in these specialized domains, ∼500,000 KV1.1/KV1.2 channels define the paired juxtaparanodal regions as exclusive membrane domains for the voltage-gated K(+)conductance that underlies rapid axonal repolarization in mammals. The 1:1 molecular linkage of KV1 channels to Cx29 channels in the apposed juxtaparanodal collars, plus their linkage to an additional 250,000-400,000 Cx29 channels along each inner mesaxon in every large-diameter myelinated axon examined, supports previously proposed K(+)conductance directly from juxtaparanodal axoplasm into juxtaparanodal myeloplasm in mammalian axons. With neither Cx29 protein nor myelin rosettes detectable in frog myelinated axons, these data showing axon-to-myelin linkage by abundant KV1/Cx29 channels in rodent axons support renewed consideration of an electrically active role for myelin in increasing both saltatory conduction velocity and maximum propagation frequency in mammalian myelinated axons.

Involvement of the heterodimeric interface region of the nucleotide binding domain-2 (NBD2) in the CFTR quaternary structure and membrane stability

The cystic fibrosis transmembrane conductance regulator (CFTR) is the only member of the ATP-binding cassette (ABC) superfamily that functions as a chloride channel. The predicted structure of CFTR protein contains two membrane-spanning domains (MSDs), each followed by a nucleotide binding domain (NBD1 and NBD2). The opening of the Cl- channel is directly linked to ATP-driven tight dimerization of CFTR's NBD1 and NBD2 domains. The presence of a heterodimeric interfaces (HI) region in NBD1 and NBD2 generated a head to tail orientation necessary for channel activity. This process was also suggested to promote important conformational changes in the associated transmembrane domains of CFTR, which may impact the CFTR plasma membrane stability. To better understand the role of the individual HI region in this process, we generated recombinant CFTR protein with suppressed HI-NBD1 and HI-NBD2. Our results indicate that HI-NBD2 deletion leads to the loss of the dimerization profile of CFTR that affect its plasma membrane stability. We conclude that, in addition to its role in Cl- transport, HI-NBD2 domain confers membrane stability of CFTR by consolidating its quaternary structure through interactions with HI-NBD1 region.

A novel synaptic vesicle fusion path in the rat cerebral cortex: the "saddle" point hypothesis

We improved freeze-fracture electron microscopy to study synapses in the neuropil of the rat cerebral cortex at ∼2 nm resolution and in three-dimensions. In the pre-synaptic axon, we found that “rods” assembled from short filaments protruding from the vesicle and the plasma membrane connects synaptic vesicles to the membrane of the active zone. We equated these “connector rods” to protein complexes involved in “docking” and “priming” vesicles to the active zone. Depending on their orientation, the “rods” define two synaptic vesicle-fusion paths: When parallel to the plasma membrane, the vesicles hemi-fuse anywhere (“randomly”) in the active zone following the conventional path anticipated by the SNARE hypothesis. When perpendicular to the plasma membrane, the vesicles hemi-fuse at the base of sharp crooks, called “indentations,” that are spaced 75–85 nm center-to-center, arranged in files and contained within gutters. They result from primary and secondary membrane curvatures that intersect at stationary inflection (“saddle”) points. Computer simulations indicate that this novel vesicle-fusion path evokes neurotransmitter concentration domains on the post-synaptic spine that are wider, shallower, and that reach higher average concentrations than the more conventional vesicle fusion path. In the post-synaptic spine, large (∼9× ∼15 nm) rectangular particles at densities of 72±10/ µm² (170–240/spine) match the envelopes of the homotetrameric GluR2 AMPA-sensitive receptor. While these putative receptors join clusters, called the “post-synaptic domains,” the overwhelming majority of the rectangular particles formed bands in the “non-synaptic” plasma membrane of the spine. In conclusion, in the neuropil of the rat cerebral cortex, curvatures of the plasma membrane define a novel vesicle-fusion path that preconditions specific regions of the active zone for neurotransmitter release. We hypothesize that a change in the hybridization of the R-SNARE synaptobrevin from parallel to antiparallel swings the synapse into this novel vesicle-fusion path.

CFTR structure and cystic fibrosis

CFTR (cystic fibrosis transmembrane conductance regulator) is a member of the ATP-binding cassette family of membrane proteins. Although almost all members of this family are transporters, CFTR functions as a channel with specificity for anions, in particular chloride and bicarbonate. In this review we look at what is known about CFTR structure and function within the context of the ATP-binding cassette family. We also review current strategies aimed at obtaining the high resolution structure of the protein.

Structural and functional analysis of the native peripherin-ROM1 complex isolated from photoreceptor cells

Peripherin and its homologue ROM1 are retina-specific members of the tetraspanin family of integral membrane proteins required for morphogenesis and maintenance of photoreceptor outer segments, regions that collect light stimuli. Over 100 pathogenic mutations in peripherin cause inherited rod- and cone-related dystrophies in humans. Peripherin and ROM1 interact in vivo and are predicted to form a core heterotetrameric complex capable of creating higher order oligomers. However, structural analysis of tetraspanin proteins has been hampered by their resistance to crystallization. Here we present a simplified methodology for high yield purification of peripherin-ROM1 from bovine retinas that permitted its biochemical and biophysical characterization. Using size exclusion chromatography and blue native gel electrophoresis, we confirmed that the core native peripherin-ROM1 complex exists as a tetramer. Peripherin, but not ROM1, is glycosylated and we examined the glycosylation site and glycan composition of ROM1 by liquid chromatographic tandem mass spectrometry. Mass spectrometry was used to analyze the native complex in detergent micelles, demonstrating its tetrameric state. Our electron microscopy-generated structure solved to 18 Å displayed the tetramer as an elongated structure with an apparent 2-fold symmetry. Finally, we demonstrated that peripherin-ROM1 tetramers induce membrane curvature when reconstituted in lipid vesicles. These results provide critical insights into this key retinal component with a poorly defined function.

Exogenous expression of human SGLT1 exhibits aggregations in sodium dodecyl sulfate polyacrylamide gel electrophoresis

Sodium/glucose co-transporter 1 (SGLT1), which actively and energy-dependently uptakes glucose, plays critical roles in the development of various diseases including diabetes mellitus and cancer, and has been viewed as a promising therapeutic target for these diseases. Protein-protein interaction with EGFR has been shown to regulate the expression and activity of SGLT1. Exogenous expression of SGLT1 is one of the essential approaches to characterize its functions; however, exogenously expressed SGLT1 is not firmly detectable by Western blot at its calculated molecular weight, which creates a hurdle for further understanding the molecular events by which SGLT1 is regulated. In this study, we demonstrated that exogenous SGLT1 functions in glucose-uptake normally but is consistently detected near the interface between stacking gel and running gel rather than at the calculated molecular weight in Western blot analysis, suggesting that the overexpressed SGLT1 forms SDS-resistant aggregates, which cannot be denatured and effectively separated on SDS-PAGE. Co-expression of EGFR enhances both the glucose-uptake activity and protein level of the SGLT1. However, fusion with Flag or HA tag at its carboxy- but not its amino-terminus abolished the glucose-uptake activity of exogenous SGLT1 without affecting its protein level. Furthermore, the solubility of SGLT1 aggregates was not affected by other detergents but was partially improved by inhibition of o-link glycosylation. These findings suggested exogenous overexpression of SGLT1 can function normally but may not be consistently detectable at its formula weight due to its gel-shift behavior by forming the SDS-resistant aggregates.

Molecular organization and ATP-induced conformational changes of ABCA4, the photoreceptor-specific ABC transporter

ATP-binding cassette (ABC) transporters use ATP to translocate various substrates across cellular membranes. Several members of subfamily A of mammalian ABC transporters are associated with severe health disorders, but their unusual complexity and large size have so far precluded structural characterization. ABCA4 is localized to the discs of vertebrate photoreceptor outer segments. This protein transports N-retinylidene-phosphatidylethanolamine to the outer side of disc membranes to prevent formation of toxic compounds causing macular degeneration. An 18 Å-resolution structure of ABCA4 isolated from bovine rod outer segments was determined using electron microscopy and single-particle reconstruction. Significant conformational changes in the cytoplasmic and transmembrane regions were observed upon binding of a nonhydrolyzable ATP analog and accompanied by altered hydrogen/deuterium exchange in the Walker A motif of one of the nucleotide-binding domains. These findings provide an initial view of the molecular organization and functional rearrangements for any member of the ABCA subfamily of ABC transporters.

Transmembrane topology and oligomeric structure of the high-affinity choline transporter

The high-affinity choline transporter CHT1 mediates choline uptake essential for acetylcholine synthesis in cholinergic nerve terminals. CHT1 belongs to the Na(+)/glucose cotransporter family (SLC5), which is postulated to have a common 13-transmembrane domain core; however, no direct experimental evidence for CHT1 transmembrane topology has yet been reported. We examined the transmembrane topology of human CHT1 using cysteine-scanning analysis. Single cysteine residues were introduced into the putative extra- and intracellular loops and probed for external accessibility for labeling with a membrane-impermeable, sulfhydryl-specific biotinylating reagent in intact cells expressing these mutants. The results provide experimental evidence for a topological model of a 13-transmembrane domain protein with an extracellular amino terminus and an intracellular carboxyl terminus. We also constructed a three-dimensional homology model of CHT1 based on the crystal structure of the bacterial Na(+)/galactose cotransporter, which supports our conclusion of CHT1 transmembrane topology. Furthermore, we examined whether CHT1 exists as a monomer or oligomer. Chemical cross-linking induces the formation of a higher molecular weight form of CHT1 on the cell surface in HEK293 cells. Two different epitope-tagged CHT1 proteins expressed in the same cells can be co-immunoprecipitated. Moreover, co-expression of an inactive mutant I89A with the wild type induces a dominant-negative effect on the overall choline uptake activity. These results indicate that CHT1 forms a homo-oligomer on the cell surface in cultured cells.

A lipid-dependent link between activity and oligomerization state of the M. tuberculosis SMR protein TBsmr

TBsmr is a secondary active multidrug transporter from Mycobacterium tuberculosis that transports a plethora of compounds including antibiotics and fluorescent dyes. It belongs to the small multidrug resistance (SMR) superfamily and is structurally and functionally related to E. coli EmrE. Of particular importance is the link between protein function, oligomeric state and lipid composition. By freeze fracture EM, we found three different size distributions in three different lipid environments for TBsmr indicating different oligomeric states. The link of these states with protein activity has been probed by fluorescence spectroscopy revealing significant differences. The drug binding site has been probed further by (19)F-MAS NMR through chemical labeling of native cysteine residues showing a water accessible environment in agreement with the alternating access model.

The ΔF508-CFTR mutation inhibits wild-type CFTR processing and function when co-expressed in human airway epithelia and in mouse nasal mucosa

BACKGROUND

Rescue or correction of CFTR function in native epithelia is the ultimate goal of CF therapeutics development. Wild-type (WT) CFTR introduction and replacement is also of particular interest. Such therapies may be complicated by possible CFTR self-assembly into an oligomer or multimer.

RESULTS

Surprisingly, functional CFTR assays in native airway epithelia showed that the most common CFTR mutant, ΔF508-CFTR (ΔF-CFTR), inhibits WT-CFTR when both forms are co-expressed. To examine more mechanistically, both forms of CFTR were transfected transiently in varying amounts into IB3-1 CF human airway epithelial cells and HEK-293 human embryonic kidney cells null for endogenous CFTR protein expression. Increasing amounts of ΔF-CFTR inhibited WT-CFTR protein processing and function in CF human airway epithelial cells but not in heterologous HEK-293 cells. Stably expressed ΔF-CFTR in clones of the non-CF human airway epithelial cell line, CALU-3, also showed reduction in cAMP-stimulated anion secretion and in WT-CFTR processing. An ultimate test of this dominant negative-like effect of ΔF-CFTR on WT-CFTR was the parallel study of two different CF mouse models: the ΔF-CFTR mouse and the bitransgenic CFTR mouse corrected in the gut but null in the lung and airways. WT/ΔF heterozygotes had an intermediate phenotype with regard to CFTR agonist responses in in vivo nasal potential difference (NPD) recordings and in Ussing chamber recordings of short-circuit current (ISC) in vitro on primary tracheal epithelial cells isolated from the same mice. In contrast, CFTR bitransgenic +/- heterozygotes had no difference in their responses versus +/+ wild-type mice.

CONCLUSIONS

Taken altogether, these data suggest that ΔF-CFTR and WT-CFTR co-assemble into an oligomeric macromolecular complex in native epithelia and share protein processing machinery and regulation at the level of the endoplasmic reticulum (ER). As a consequence, ΔF-CFTR slows WT-CFTR protein processing and limits its expression and function in the apical membrane of native airway epithelia. Implications of these data for the relative health of CF heterozygous carriers, for CFTR protein processing in native airway epithelia, and for the relative efficacy of different CF therapeutic approaches is significant and is discussed.

Functional consequences of sulfhydryl modification of the γ-aminobutyric acid transporter 1 at a single solvent-exposed cysteine residue

The aims of this study were to optimize the experimental conditions for labeling extracellularly oriented, solvent-exposed cysteine residues of γ-aminobutyric acid transporter 1 (GAT1) with the membrane-impermeant sulfhydryl reagent [2-(trimethylammonium)ethyl]methanethiosulfonate (MTSET) and to characterize the functional and pharmacological consequences of labeling on transporter steady-state and presteady-state kinetic properties. We expressed human GAT1 in Xenopus laevis oocytes and used radiotracer and electrophysiological methods to assay transporter function before and after sulfhydryl modification with MTSET. In the presence of NaCl, transporter exposure to MTSET (1–2.5 mM for 5–20 min) led to partial inhibition of GAT1-mediated transport, and this loss of function was completely reversed by the reducing reagent dithiothreitol. MTSET treatment had no functional effect on the mutant GAT1 C74A, whereas the membrane-permeant reagents N-ethylmaleimide and tetramethylrhodamine-6-maleimide inhibited GABA transport mediated by GAT1 C74A. Ion replacement experiments indicated that MTSET labeling of GAT1 could be driven to completion when valproate replaced chloride in the labeling buffer, suggesting that valproate induces a GAT1 conformation that significantly increases C74 accessibility to the extracellular fluid. Following partial inhibition by MTSET, there was a proportional reduction in both the presteady-state and steady-state macroscopic signals, and the functional and pharmacological properties of the remaining signals were indistinguishable from those of unlabeled GAT1. Therefore, covalent modification of GAT1 at C74 results in completely nonfunctional as well as electrically silent transporters.

Mixed Electrical-Chemical Synapses in Adult Rat Hippocampus are Primarily Glutamatergic and Coupled by Connexin-36

Dendrodendritic electrical signaling via gap junctions is now an accepted feature of neuronal communication in mammalian brain, whereas axodendritic and axosomatic gap junctions have rarely been described. We present ultrastructural, immunocytochemical, and dye-coupling evidence for “mixed” (electrical/chemical) synapses on both principal cells and interneurons in adult rat hippocampus. Thin-section electron microscopic images of small gap junction-like appositions were found at mossy fiber (MF) terminals on thorny excrescences of CA3 pyramidal neurons (CA3pyr), apparently forming glutamatergic mixed synapses. Lucifer Yellow injected into weakly fixed CA3pyr was detected in MF axons that contacted four injected CA3pyr, supporting gap junction-mediated coupling between those two types of principal cells. Freeze-fracture replica immunogold labeling revealed diverse sizes and morphologies of connexin-36-containing gap junctions throughout hippocampus. Of 20 immunogold-labeled gap junctions, seven were large (328–1140 connexons), three of which were consistent with electrical synapses between interneurons; but nine were at axon terminal synapses, three of which were immediately adjacent to distinctive glutamate receptor-containing postsynaptic densities, forming mixed glutamatergic synapses. Four others were adjacent to small clusters of immunogold-labeled 10-nm E-face intramembrane particles, apparently representing extrasynaptic glutamate receptor particles. Gap junctions also were on spines in stratum lucidum, stratum oriens, dentate gyrus, and hilus, on both interneurons and unidentified neurons. In addition, one putative GABAergic mixed synapse was found in thin-section images of a CA3pyr, but none were found by immunogold labeling, suggesting the rarity of GABAergic mixed synapses. Cx36-containing gap junctions throughout hippocampus suggest the possibility of reciprocal modulation of electrical and chemical signals in diverse hippocampal neurons.

Reconstitution of glucosylceramide flip-flop across endoplasmic reticulum: implications for mechanism of glycosphingolipid biosynthesis

Most glycosphingolipids are synthesized by the sequential addition of monosaccharides to glucosylceramide (GlcCer) in the lumen of the Golgi apparatus. Because GlcCer is synthesized on the cytoplasmic face of Golgi membranes, it must be flipped to the non-cytoplasmic face by a lipid flippase in order to nucleate glycosphingolipid synthesis. Halter et al. (Halter, D., Neumann, S., van Dijk, S. M., Wolthoorn, J., de Mazière, A. M., Vieira, O. V., Mattjus, P., Klumperman, J., van Meer, G., and Sprong, H. (2007) Pre- and post-Golgi translocation of glucosylceramide in glycosphingolipid synthesis. J. Cell Biol. 179, 101-115) proposed that this essential flipping step is accomplished via a complex trafficking itinerary; GlcCer is moved from the cytoplasmic face of the Golgi to the endoplasmic reticulum (ER) by FAPP2, a cytoplasmic lipid transfer protein, flipped across the ER membrane, then delivered to the lumen of the Golgi complex by vesicular transport. We now report biochemical reconstitution studies to analyze GlcCer flipping at the ER. Using proteoliposomes reconstituted from Triton X-100-solubilized rat liver ER membrane proteins, we demonstrate rapid (t(½) < 20 s), ATP-independent flip-flop of N-(6-((7-nitro-2-1,3-benzoxadiazol-4-yl)amino)hexanoyl)-D-glucosyl-β1-1'-sphingosine, a fluorescent GlcCer analog. Further studies involving protein modification, biochemical fractionation, and analyses of flip-flop in proteoliposomes reconstituted with ER membrane proteins from yeast indicate that GlcCer translocation is facilitated by well characterized ER phospholipid flippases that remain to be identified at the molecular level. By reason of their abundance and membrane bending activity, we considered that the ER reticulons and the related Yop1 protein could function as phospholipid-GlcCer flippases. Direct tests showed that these proteins have no flippase activity.

Single-molecule recognition force spectroscopy of transmembrane transporters on living cells

Atomic force microscopy (AFM) has proven to be a powerful tool in biological sciences. Its particular advantage over other high-resolution methods commonly used is that biomolecules can be investigated not only under physiological conditions but also while they perform their biological functions. Single-molecule force spectroscopy with AFM tip-modification techniques can provide insight into intermolecular forces between individual ligand-receptor pairs of biological systems. Here we present protocols for force spectroscopy of living cells, including cell sample preparation, tip chemistry, step-by-step AFM imaging, force spectroscopy and data analysis. We also delineate critical steps and describe limitations that we have experienced. The entire protocol can be completed in 12 h. The model studies discussed here demonstrate the power of AFM for studying transmembrane transporters at the single-molecule level.

Biology of human sodium glucose transporters

There are two classes of glucose transporters involved in glucose homeostasis in the body, the facilitated transporters or uniporters (GLUTs) and the active transporters or symporters (SGLTs). The energy for active glucose transport is provided by the sodium gradient across the cell membrane, the Na(+) glucose cotransport hypothesis first proposed in 1960 by Crane. Since the cloning of SGLT1 in 1987, there have been advances in the genetics, molecular biology, biochemistry, biophysics, and structure of SGLTs. There are 12 members of the human SGLT (SLC5) gene family, including cotransporters for sugars, anions, vitamins, and short-chain fatty acids. Here we give a personal review of these advances. The SGLTs belong to a structural class of membrane proteins from unrelated gene families of antiporters and Na(+) and H(+) symporters. This class shares a common atomic architecture and a common transport mechanism. SGLTs also function as water and urea channels, glucose sensors, and coupled-water and urea transporters. We also discuss the physiology and pathophysiology of SGLTs, e.g., glucose galactose malabsorption and familial renal glycosuria, and briefly report on targeting of SGLTs for new therapies for diabetes.

Mitochondrial carriers function as monomers

Mitochondrial carriers link biochemical pathways in the mitochondrial matrix and cytosol by transporting metabolites, inorganic ions, nucleotides and cofactors across the mitochondrial inner membrane. Uncoupling proteins that dissipate the proton electrochemical gradient also belong to this protein family. For almost 35 years the general consensus has been that mitochondrial carriers are dimeric in structure and function. This view was based on data from inhibitor binding studies, small-angle neutron scattering, electron microscopy, differential tagging/affinity chromatography, size-exclusion chromatography, analytical ultracentrifugation, native gel electrophoresis, cross-linking experiments, tandem-fusions, negative dominance studies and mutagenesis. However, the structural folds of the ADP/ATP carriers were found to be monomeric, lacking obvious dimerisation interfaces. Subsequently, the yeast ADP/ATP carrier was demonstrated to function as a monomer. Here, we revisit the data that have been published in support of a dimeric state of mitochondrial carriers. Our analysis shows that when critical factors are taken into account, the monomer is the only plausible functional form of mitochondrial carriers. We propose a transport model based on the monomer, in which access to a single substrate binding site is controlled by two flanking salt bridge networks, explaining uniport and strict exchange of substrates.

The sodium/galactose symporter crystal structure is a dynamic, not so occluded state

The recent elucidation of the sodium/galactose symporter structure from the Vibrio parahaemolyticus bacterium, vSGLT, has revealed a similarity in the core architecture with transporters from different gene families, including the leucine transporter (LeuT). Even though several transporters sharing this core have been structurally determined over the past few years, vSGLT is the only one crystallized in the substrate-bound inward-facing conformation so far. In this study, we report the first insight into the dynamics and coordination of the galactose (Gal) and proposed Na+ ion in vSGLT using a series of molecular dynamics simulations with a total time of about 0.1 micros. Our study reveals new residues, not observed in the crystal structure, which closely interact with the Na(+) ion or the substrate for extended times, and shows that in the crystallized conformation, a Na+ ion placed at the site equivalent to Na2 in LeuT can escape into the intracellular (IC) space in the absence of external forces. We have identified the highly conserved Asp189 as a likely binding residue on the pathway of Na(+) into the IC cavity. The release of Gal, on the other hand, requires the rotation of the side chain of the inner hydrophobic gate, Tyr263, without a significant change in vSGLT backbone conformation. Our simulations further show that the crystal structure represents but one accessible binding pose of Gal and Na+ among an ensemble of microstates, and that the Gal undergoes versatile alternate interactions at the binding pocket.

Oligomerization of human ATP-binding cassette transporters and its potential significance in human disease

Human ATP-binding cassette transporters (ABC transporter) belong to an extremely important superfamily of membrane transporters. They use energy from ATP hydrolysis to transport a wide variety of substrates across the cellular membrane. Due to the physiological and pharmacological importance of their diverse substrates, ABC transporters have been shown to have close relationship with various human diseases such as cystic fibrosis and multi-drug resistance in cancer chemotherapy. While it has been thought traditionally that functional ABC transporters exist as monomeric full or dimeric half transporters, emerging evidence indicates that some ABC transporters oligomerize on cellular membranes and this oligomerization seems to have functional relevance. Therefore, this oligomerization process might be a promising drug target for ABC transporter-related human diseases, especially in overcoming multi-drug resistance in cancer chemotherapy. In this review, we perform a critical analysis of the past studies on the oligomerization of ABC transporters.

The ATP-binding cassette family: a structural perspective

The ATP-binding cassette family is one of the largest groupings of membrane proteins, moving allocrites across lipid membranes, using energy from ATP. In bacteria, they reside in the inner membrane and are involved in both uptake and export. In eukaryotes, these transporters reside in the cell's internal membranes as well as in the plasma membrane and are unidirectional-out of the cytoplasm. The range of substances that these proteins can transport is huge, which makes them interesting for structure-function studies. Moreover, their abundance in nature has made them targets for structural proteomics consortia. There are eight independent structures for ATP-binding cassette transporters, making this one of the best characterised membrane protein families. Our understanding of the mechanism of transport across membranes and membrane protein structure in general has been enhanced by recent developments for this family.

Architecture of the cystic fibrosis transmembrane conductance regulator protein and structural changes associated with phosphorylation and nucleotide binding

We describe biochemical and structural studies of the isolated cystic fibrosis transmembrane conductance regulator (CFTR) protein. Using electron cryomicroscopy, low resolution three-dimensional structures have been obtained for the non-phosphorylated protein in the absence of nucleotide and for the phosphorylated protein with ATP. In the latter state, the cytosolic nucleotide-binding domains move closer together, forming a more compact packing arrangement. Associated with this is a reorganization within the cylindrical transmembrane domains, consistent with a shift from an inward-facing to outward-facing configuration. A region of density in the non-phosphorylated protein that extends from the bottom of the cytosolic regions up to the transmembrane domains is hypothesised to represent the unique regulatory region of CFTR. These data offer insights into the architecture of this ATP-binding cassette protein, and shed light on the global motions associated with nucleotide binding and priming of the chloride channel via phosphorylation of the regulatory region.

Monomeric CFTR in plasma membranes in live cells revealed by single molecule fluorescence imaging

The cystic fibrosis transmembrane conductance regulator (CFTR) is a cAMP-regulated chloride channel. There is indirect and conflicting evidence about whether CFTR exists in cell membranes as monomers, dimers, or higher order oligomers. We measured fluorescence intensities and photobleaching dynamics of distinct fluorescent spots in cells expressing functional CFTR-green fluorescent protein (GFP) chimeras. Intensity analysis of GFP-labeled CFTR in live cells showed single-component distributions with mean intensity equal to that of purified monomeric GFP, indicating monomeric CFTR in cell membranes. Fluorescent spots showed single-step photobleaching, independently verifying that CFTR is monomeric. Results did not depend on whether GFP was added to the CFTR N terminus or fourth extracellular loop or on whether CFTR chloride conductance was stimulated by cAMP agonists. Control measurements with a CFTR chimera containing two GFPs showed two-step photobleaching and a single-component intensity distribution with mean intensity twice that of monomeric GFP. These results provide direct evidence for monomeric CFTR in live cells.

Three-dimensional reconstruction of human cystic fibrosis transmembrane conductance regulator chloride channel revealed an ellipsoidal structure with orifices beneath the putative transmembrane domain

The cystic fibrosis transmembrane conductance regulator (CFTR) chloride channel is a membrane-integral protein that belongs to an ATP-binding cassette superfamily. Mutations in the CFTR gene cause cystic fibrosis in which salt, water, and protein transports are defective in various tissues. Here we expressed wild-type human CFTR as a FLAG-fused protein in HEK293 cells heterologously and purified it in three steps: anti-FLAG and wheat germ agglutinin affinity chromatographies and size exclusion chromatography. The stoichiometry of the protein was analyzed using various biochemical approaches, including chemical cross-linking, blue-native PAGE, size exclusion chromatography, and electron microscopy (EM) observation of antibody-decorated CFTR. All these data support a dimeric assembly of CFTR. Using 5,039 automatically selected particles from negatively stained EM images, the three-dimensional structure of CFTR was reconstructed at 2-nm resolution assuming a 2-fold symmetry. CFTR, presumably in a closed state, was shown to be an ellipsoidal particle with dimensions of 120 x 106 x 162 A. It comprises a small dome-shaped extracellular and membrane-spanning domain and a large cytoplasmic domain with orifices beneath the putative transmembrane domain. EM observation of CFTR.anti-regulatory domain antibody complex confirmed that two regulatory domains are located around the bottom end of the larger oval cytoplasmic domain.

The cytoplasmic domain of the cellulose-synthesizing complex in vascular plants

The cytoplasmic domain of the rosette terminal complex has been imaged in situ in patches of plasma membrane isolated from tobacco BY-2 protoplasts. By partially extracting the plasma membrane lipids, cellulose microfibrils were observed through the plasma membrane. Rosette terminal complexes were identified on the basis of their association with the ends of these cellulose microfibrils. The cytoplasmic domain of the rosette terminal complex has been shown to be hexagonal in shape and has been measured to be 45-50 nm in diameter and 30-35 nm tall. These findings demonstrate that the terminal complex does indeed have a substantial cytoplasmic component, and that the hexagonal array observed in the lipid bilayer by freeze fracture is actually only a small part of the overall complex. These findings will allow better modeling of the terminal complex and may facilitate predictions of how many proteins are associated with the rosette terminal complex in vivo.

The crystal structure of a sodium galactose transporter reveals mechanistic insights into Na+/sugar symport

Membrane transporters that use energy stored in sodium gradients to drive nutrients into cells constitute a major class of proteins. We report the crystal structure of a member of the solute sodium symporters (SSS), the Vibrio parahaemolyticus sodium/galactose symporter (vSGLT). The approximately 3.0 angstrom structure contains 14 transmembrane (TM) helices in an inward-facing conformation with a core structure of inverted repeats of 5 TM helices (TM2 to TM6 and TM7 to TM11). Galactose is bound in the center of the core, occluded from the outside solutions by hydrophobic residues. Surprisingly, the architecture of the core is similar to that of the leucine transporter (LeuT) from a different gene family. Modeling the outward-facing conformation based on the LeuT structure, in conjunction with biophysical data, provides insight into structural rearrangements for active transport.

The motor protein prestin is a bullet-shaped molecule with inner cavities

Prestin is a transmembrane motor protein localized at the outer hair cells (OHCs) of the mammalian inner ear. Voltage-dependent conformational changes in prestin generate changes in the length of OHCs. A loss of prestin function is reported to induce severe auditory deficiencies, suggesting prestin-dependent changes of OHC length may be at least a part of cochlear amplification. Here we expressed the recombinant FLAG-fused prestin proteins in Sf9 cells and purified to particles of a uniform size in EM. The square-shaped top view of purified prestin, the binding of multiple anti-FLAG antibodies to each prestin particle, the native-PAGE analysis, and the much larger molecular weight obtained from size exclusion chromatography than the estimation for the monomer all support that prestin is a tetramer (Zheng, J., Du, G. G., Anderson, C. T., Keller, J. P., Orem, A., Dallos, P., and Cheatham, M. (2006) J. Biol. Chem. 281, 19916-19924). From negatively stained prestin particles, the three-dimensional structure was reconstructed at 2 nm resolution assuming 4-fold symmetry. Prestin is shown to be a bullet-shaped particle with a large cytoplasmic domain. The surface representation demonstrates indentations on the molecule, and the slice images indicate the inner cavities of sparse densities. The dimensions, 77 x 77 x 115 A, are consistent with the previously reported sizes of motor proteins on the surface of OHCs.

Turnover rate of the gamma-aminobutyric acid transporter GAT1

We combined electrophysiological and freeze-fracture methods to estimate the unitary turnover rate of the gamma-aminobutyric acid (GABA) transporter GAT1. Human GAT1 was expressed in Xenopus laevis oocytes, and individual cells were used to measure and correlate the macroscopic rate of GABA transport and the total number of transporters in the plasma membrane. The two-electrode voltage-clamp method was used to measure the transporter-mediated macroscopic current evoked by GABA (I(NaCl)(GABA)), macroscopic charge movements (Q (NaCl)) evoked by voltage pulses and whole-cell capacitance. The same cells were then examined by freeze-fracture and electron microscopy in order to estimate the total number of GAT1 copies in the plasma membrane. GAT1 expression in the plasma membrane led to the appearance of a distinct population of 9-nm freeze-fracture particles which represented GAT1 dimers. There was a direct correlation between Q (NaCl) and the total number of transporters in the plasma membrane. This relationship yielded an apparent valence of 8 +/- 1 elementary charges per GAT1 particle. Assuming that the monomer is the functional unit, we obtained 4 +/- 1 elementary charges per GAT1 monomer. This information and the relationship between I(NaCl)(GABA) and Q (NaCl) were used to estimate a GAT1 unitary turnover rate of 15 +/- 2 s(-1) (21 degrees C, -50 mV). The temperature and voltage dependence of GAT1 were used to estimate the physiological turnover rate to be 79-93 s(-1) (37 degrees C, -50 to -90 mV).

Probing the substrate specificity of the ergothioneine transporter with methimazole, hercynine, and organic cations

Recently, we have identified the ergothioneine (ET) transporter ETT (gene symbol SLC22A4). Much interest in human ETT has been generated by case-control studies that suggest an association of polymorphisms in the SLC22A4 gene with susceptibility to chronic inflammatory diseases. ETT was originally designated a multispecific novel organic cation transporter (OCTN1). Here we reinvestigated, based on stably transfected 293 cells and with ET as reference substrate, uptake of quinidine, verapamil, and pyrilamine. ETT from human robustly catalyzed transport of ET (68micfrol/(minmgprotein)), but no transport of organic cations was discernible. With ET as substrate, ETT was relatively resistant to inhibition by selected drugs; the most potent inhibitor was verapamil (K(i)=11micromol/l). The natural compound hercynine and antithyroid drug methimazole are related in structure to ET. However, efficiency of ETT-mediated transport of methimazole (K(i)=7.5mmol/l) was 130-fold lower, and transport of hercynine (K(i)=1.4mmol/l) was 25-fold lower than transport of ET. ETT from mouse, upon expression in 293 cells, catalyzed high affinity, sodium-driven uptake of ET very similar to ETT from human. Additional real-time PCR experiments based on 16 human tissues revealed ETT mRNA levels considerably lower than in bone marrow. Our experiments establish that ETT is highly specific for its physiological substrate ergothioneine. ETT is not a cationic drug transporter, and it does not have high affinity for organic cation inhibitors. Detection of ETT mRNA or protein can therefore be utilized as a specific molecular marker of intracellular ET activity.

Regulation of function by dimerization through the amino-terminal membrane-spanning domain of human ABCC1/MRP1

Overexpression of some ATP-binding cassette (ABC) membrane transporters such as ABCB1/P-glycoprotein/MDR1 and ABCC1/MRP1 causes multidrug resistance in cancer chemotherapy. It has been thought that half-ABC transporters with one nucleotide-binding domain and one membrane-spanning domain (MSD) likely work as dimers, whereas full-length transporters with two nucleotide-binding domains and two or three MSDs function as monomers. In this study, we examined the oligomeric status of the human full-length ABC transporter ABCC1/MRP1 using several biochemical approaches. We found 1) that it is a homodimer, 2) that the dimerization domain is located in the amino-terminal MSD0L0 (where L0 is loop 0) region, and 3) that MSD0L0 has a dominant-negative function when coexpressed with wild-type ABCC1/MRP1. These findings suggest that ABCC1/MRP1 may exist and function as a dimer and that MSD0L0 likely plays some structural and regulatory functions. It is also tempting to propose that the MSD0L0-mediated dimerization may be targeted for therapeutic development to sensitize ABCC1/MRP1-mediated drug resistance in cancer chemotherapy.

NHE-RF1 protein rescues DeltaF508-CFTR function

In cystic fibrosis (CF), the DeltaF508-CFTR anterograde trafficking from the endoplasmic reticulum to the plasma membrane is inefficient. New strategies for increasing the delivery of DeltaF508-CFTR to the apical membranes are thus pathophysiologically relevant targets to study for CF treatment. Recent studies have demonstrated that PDZ-containing proteins play an essential role in determining polarized plasma membrane expression of ionic transporters. In the present study we have hypothesized that the PDZ-containing protein NHE-RF1, which binds to the carboxy terminus of CFTR, rescues DeltaF508-CFTR expression in the apical membrane of epithelial cells. The plasmids encoding DeltaF508-CFTR and NHE-RF1 were intranuclearly injected in A549 or Madin-Darby canine kidney (MDCK) cells, and DeltaF508-CFTR channel activity was functionally assayed using SPQ fluorescent probe. Cells injected with DeltaF508-CFTR alone presented a low chloride channel activity, whereas its coexpression with NHE-RF1 significantly increased both the basal and forskolin-activated chloride conductances. This last effect was lost with DeltaF508-CFTR deleted of its 13 last amino acids or by injection of a specific NHE-RF1 antisense oligonucleotide, but not by NHE-RF1 sense oligonucleotide. Immunocytochemical analysis performed in MDCK cells transiently transfected with DeltaF508-CFTR further revealed that NHE-RF1 specifically determined the apical plasma membrane expression of DeltaF508-CFTR but not that of a trafficking defective mutant potassium channel (KCNQ1). These data demonstrate that the modulation of the expression level of CFTR protein partners, like NHE-RF1, can rescue DeltaF508-CFTR activity.

Purification and 3D structural analysis of oligomeric human multidrug transporter ABCG2

ABCG2 is a multidrug efflux pump associated with resistance of cancer cells to a plethora of unrelated drugs. ABCG2 is a "half-transporter," and previous studies have indicated that it forms homodimers and higher oligomeric species. In this manuscript, electron microscopic structural analysis directly addressed this issue. An N-terminal hexahistidine-tagged ABCG2(R482G) isoform was expressed to high levels in insect cells. An extensive detergent screen was employed to effect extraction of ABCG2(R482G) from membranes and identified only the fos-choline detergents as efficient. Soluble protein was purified to >95% homogeneity by a three-step procedure while retaining the ability to bind substrates. Cryonegative stain electron microscopy of purified ABCG2(R482G) provided 3D structural data at a resolution of approximately 18 A. Single-particle analysis revealed that the complex forms a tetrameric complex ( approximately 180 A in diameter x approximately 140 A high) with an aqueous central region. We interpret the tetrameric structure as comprising four homodimeric ABCG2(R482G) complexes.

Active sugar transport in health and disease

Secondary active glucose transport occurs by at least four members of the SLC5 gene family. This review considers the structure and function of two premier members, SGLT1 and SGLT2, and their role in intestinal glucose absorption and renal glucose reabsorption. Genetics disorders of SGLTs include Glucose-Galactose Malabsorption, and Familial Renal Glucosuria. SGLT1 plays a central role in Oral Rehydration Therapy used so effectively to treat secretory diarrhoea such as cholera. Increasing attention is being focused on SGLTs as drug targets for the therapy of diabetes.

Proximal tubular handling of phosphate: A molecular perspective

Members of the SLC34 gene family of solute carriers encode for three Na+-dependent phosphate (P i) cotransporter proteins, two of which (NaPi-IIa/SLC34A1 and NaPi-IIc/SLC34A3) control renal reabsorption of P i in the proximal tubule of mammals, whereas NaPi-IIb/SCLC34A2 mediates P i transport in organs other than the kidney. The P i transport mechanism has been extensively studied in heterologous expression systems and structure-function studies have begun to reveal the intricacies of the transport cycle at the molecular level using techniques such as cysteine scanning mutagenesis, and voltage clamp fluorometry. Moreover, sequence differences between the three types of cotransporters have been exploited to obtain information about the molecular determinants of hormonal sensitivity and electrogenicity. Renal handling of P i is regulated by hormonal and non-hormonal factors. Changes in urinary excretion of P i are almost invariably mirrored by changes in the apical expression of NaPi-IIa and NaPi-IIc in proximal tubules. Therefore, understanding the mechanisms that control the apical expression of NaPi-IIa and NaPi-IIc as well as their functional properties is critical to understanding how an organism achieves P i homeostasis.

Transmembrane transport of endo- and xenobiotics by mammalian ATP-binding cassette multidrug resistance proteins

Multidrug Resistance Proteins (MRPs), together with the cystic fibrosis conductance regulator (CFTR/ABCC7) and the sulfonylurea receptors (SUR1/ABCC8 and SUR2/ABCC9) comprise the 13 members of the human "C" branch of the ATP binding cassette (ABC) superfamily. All C branch proteins share conserved structural features in their nucleotide binding domains (NBDs) that distinguish them from other ABC proteins. The MRPs can be further divided into two subfamilies "long" (MRP1, -2, -3, -6, and -7) and "short" (MRP4, -5, -8, -9, and -10). The short MRPs have a typical ABC transporter structure with two polytropic membrane spanning domains (MSDs) and two NBDs, while the long MRPs have an additional NH2-terminal MSD. In vitro, the MRPs can collectively confer resistance to natural product drugs and their conjugated metabolites, platinum compounds, folate antimetabolites, nucleoside and nucleotide analogs, arsenical and antimonial oxyanions, peptide-based agents, and, under certain circumstances, alkylating agents. The MRPs are also primary active transporters of other structurally diverse compounds, including glutathione, glucuronide, and sulfate conjugates of a large number of xeno- and endobiotics. In vivo, several MRPs are major contributors to the distribution and elimination of a wide range of both anticancer and non-anticancer drugs and metabolites. In this review, we describe what is known of the structure of the MRPs and the mechanisms by which they recognize and transport their diverse substrates. We also summarize knowledge of their possible physiological functions and evidence that they may be involved in the clinical drug resistance of various forms of cancer.

New insights into cystic fibrosis: molecular switches that regulate CFTR

Cystic fibrosis transmembrane conductance regulator (CFTR), a Cl(-)-selective ion channel, is a prototypic member of the ATP-binding cassette transporter superfamily that is expressed in several organs. In these organs, CFTR assembles into large, dynamic macromolecular complexes that contain signalling molecules, kinases, transport proteins, PDZ-domain-containing proteins, myosin motors, Rab GTPases, and SNAREs. Understanding how these complexes regulate the intracellular trafficking and activity of CFTR provides a unique insight into the aetiology of cystic fibrosis and other diseases.

Membrane lateral diffusion and capture of CFTR within transient confinement zones

The cystic fibrosis transmembrane conductance regulator (CFTR) channel interacts with scaffolding and other proteins that are expected to restrict its lateral movement, yet previous studies have reported predominantly free diffusion. We examined the lateral mobility of CFTR channels on live baby hamster kidney cells using three complementary methods. Channels bearing an extracellular biotinylation target sequence were labeled with streptavidin conjugated with fluorescent dyes (Alexa Fluor 488 or 568) or quantum dots (qDot605). Fluorescence recovery after photobleaching and image correlation spectroscopy of the dye-labeled channels revealed a significant immobile population ( approximately 50%), which was confirmed by direct single particle tracking (SPT) of qDot605-labeled CFTR. Adding 10 histidine residues at the C-terminus of CFTR to mask the postsynaptic density 95, Discs large, ZO-1 (PDZ) binding motif abolished its association with EBP50/NHERF1, reduced the immobile fraction, and increased mobility. Other interactions that are not normally detected on this timescale became apparent when binding of PDZ domain proteins was disrupted. SPT revealed that CFTR(His-10) channels diffuse randomly, become immobilized for periods lasting up to 1 min, and in some instances are recaptured at the same location. The impact of transient confinement on the measured diffusion using the three fluorescence techniques were assessed using computer simulations of the biological experiments. Finally, the impact of endosomal CFTR on mobility measurements was assessed by fluorescence correlation spectroscopy. These results reveal unexpected features of CFTR dynamics which may influence its ion channel activity.

Evidence that the rabbit proton-peptide co-transporter PepT1 is a multimer when expressed in Xenopus laevis oocytes

To test whether the rabbit proton-coupled peptide transporter PepT1 is a multimer, we have employed a combination of transport assays, luminometry and site-directed mutagenesis. A functional epitope-tagged PepT1 construct (PepT1-FLAG) was co-expressed in Xenopus laevis oocytes with a non-functional but normally trafficked mutant form of the same transporter (W294F-PepT1). The amount of PepT1-FLAG cRNA injected into the oocytes was kept constant, while the amount of W294F-PepT1 cRNA was increased over the mole fraction range of 0 to 1. The uptake of [(3)H]-D: -Phe-L: -Gln into the oocytes was measured at pH(out) 5.5, and the surface expression of PepT1-FLAG was quantified by luminometry. As the mole fraction of injected W294F-PepT1 increased, the uptake of D: -Phe-L: -Gln decreased. This occurred despite the surface expression of PepT1-FLAG remaining constant, and so we can conclude that PepT1 must be a multimer. Assuming that PepT1 acts as a homomultimer, the best fit for the modelling suggests that PepT1 could be a tetramer, with a minimum requirement of two functional subunits in each protein complex. Western blotting also showed the presence of higher-order complexes of PepT1-FLAG in oocyte membranes. It should be noted that we cannot formally exclude the possibility that PepT1 interacts with unidentified Xenopus protein(s). The finding that PepT1 is a multimer has important implications for the molecular modelling of this protein.