Secondary structure and oligomerization of the E. coli glycerol facilitator

Transmembrane Helices 7 and 8 Confer Aggregation Sensitivity to the Cystic Fibrosis Transmembrane Conductance Regulator

The Cystic Fibrosis Transmembrane Conductance Regulator (CFTR) is a large multi-spanning membrane protein that is susceptible to misfolding and aggregation. We have identified here the region responsible for this instability. Temperature-induced aggregation of C-terminally truncated versions of CFTR demonstrated that all truncations up to the second transmembrane domain (TMD2), including the R region, largely resisted aggregation. Limited proteolysis identified a folded structure that was prone to aggregation and consisted of TMD2 and at least part of the Regulatory Region R. Only when both TM7 (TransMembrane helix 7) and TM8 were present, TMD2 fragments became as aggregation-sensitive as wild-type CFTR, in line with increased thermo-instability of late CFTR nascent chains and in silico prediction of aggregation propensity. In accord, isolated TMD2 was degraded faster in cells than isolated TMD1. We conclude that TMD2 extended at its N-terminus with part of the R region forms a protease-resistant structure that induces heat instability in CFTR and may be responsible for its limited intracellular stability.

Membrane Protein Production and Purification from Escherichia coli and Sf9 Insect Cells

A major obstacle to studying membrane proteins by biophysical techniques is the difficulty in producing sufficient amounts of materials for functional and structural studies. To overexpress the target membrane protein heterologously, especially an eukaryotic protein, a key step is to find the optimal host expression system and perform subsequent expression optimization. In this chapter, we describe protocols for screening membrane protein production using bacterial and insect cells, solubilization screening, large-scale production, and commonly used affinity chromatography purification methods. We discuss general optimization conditions, such as promoters and tags, and describe current techniques that can be used in any laboratory without specialized expensive equipment. Especially for insect cells, GFP fusions are particularly useful for localization and in-gel fluorescence detection of the proteins on SDS-PAGE. We give detailed protocols that can be used to screen the best expression and purification conditions for membrane protein study.

Th1-Dependent Cryptococcus-Associated Immune Reconstitution Inflammatory Syndrome Model With Brain Damage

Cryptococcus-associated immune reconstitution inflammatory syndrome (C-IRIS) is identified upon immune reconstitution in immunocompromised patients, who have previously contracted an infection of Cryptococcus neoformans (Cn). C-IRIS can be lethal but how the immune system triggers life-threatening outcomes in patients is still poorly understood. Here, we establish a mouse model for C-IRIS with Cn serotype A strain H99, which is highly virulent and the most intensively studied. C-IRIS in mice is induced by the adoptive transfer of CD4+ T cells in immunocompromised Rag1-deficient mice infected with a low inoculum of Cn. The mice with C-IRIS exhibit symptoms which mimic clinical presentations of C-IRIS. This C-IRIS model is Th1-dependent and shows host mortality. This model is characterized with minimal lung injury, but infiltration of Th1 cells in the brain. C-IRIS mice also exhibited brain swelling with resemblance to edema and upregulation of aquaporin-4, a critical protein that regulates water flux in the brain in a Th1-dependent fashion. Our C-IRIS model may be used to advance our understanding of the paradoxical inflammatory phenomenon of C-IRIS in the context of neuroinflammation.

Solution structure and oligomeric state of the E. coliglycerol facilitator

Protein dynamics at atomic resolution can provide deep insights into the biological activities of proteins and enzymes but they can also make structure and dynamics studies challenging. Despite their well-known biological and pharmaceutical importance, integral membrane protein structure and dynamics studies lag behind those of water-soluble proteins mainly owing to solubility problems that result upon their removal from the membrane. Escherichia coli glycerol facilitator (GF) is a member of the aquaglyceroporin family that allows for the highly selective passive diffusion of its substrate glycerol across the inner membrane of the bacterium. Previous molecular dynamics simulations and hydrogen-deuterium exchange studies suggested that protein dynamics play an important role in the passage of glycerol through the protein pore. With the aim of studying GF dynamics by solution and solid-state nuclear magnetic resonance (NMR) spectroscopy we optimized the expression of isotope-labelled GF and explored various solubilizing agents including detergents, osmolytes, amphipols, random heteropolymers, lipid nanodiscs, bicelles and other buffer additives to optimize the solubility and polydispersity of the protein. The GF protein is most stable and soluble in lauryl maltose neopentyl glycol (LMNG), where it exists in a tetramer-octamer equilibrium. The solution structures of the GF tetramer and octamer were determined by negative-stain transmission electron microscopy (TEM), size-exclusion chromatography small-angle X-ray scattering (SEC-SAXS) and solid-state magic-angle spinning NMR spectroscopy. Although NMR sample preparation still needs optimization for full structure and dynamics studies, negative stain TEM and SEC-SAXS revealed low-resolution structures of the detergent-solubilized tetramer and octamer particles. The non-native octamer appears to form from the association of the cytoplasmic faces of two tetramers, the interaction apparently mediated by their disordered N- and C-termini. This information may be useful in future studies directed at reducing the heterogeneity and self-association of the protein.

The GlpF residue Trp219 is part of an amino-acid cluster crucial for aquaglyceroporin oligomerization and function

The vestibule loop regions of aquaglyceroporins are involved in accumulation of glycerol inside the channel pore. Even though most loop regions do not show high sequence similarity among aquaglyceroporins, loop E is highly conserved in aquaglyceroporins, but not in members of the homologous aquaporins. Specifically, a tryptophan residue is extremely conserved within this loop. We have investigated the role of this residue (Trp219) that deeply protrudes into the protein and potentially interacts with adjacent loops, using the E. coli aqualgyeroporin GlpF as a model. Replacement of Trp219 affects the activity of GlpF and impairs the stability of the tetrameric protein. Furthermore, we have identified an amino acid cluster involving Trp219 that stabilizes the GlpF tetramer. Based on our results we propose that Trp219 is key for formation of a defined vestibule structure, which is crucial for glycerol accumulation as well as for the stability of the active GlpF tetramer.

Folding and stability of the aquaglyceroporin GlpF: Implications for human aqua(glycero)porin diseases

Aquaporins are highly selective polytopic transmembrane channel proteins that facilitate the permeation of water across cellular membranes in a large diversity of organisms. Defects in aquaporin function are associated with common diseases, such as nephrogenic diabetes insipidus, congenital cataract and certain types of cancer. In general, aquaporins have a highly conserved structure; from prokaryotes to humans. The conserved structure, together with structural dynamics and the structural framework for substrate selectivity is discussed. The folding pathway of aquaporins has been a topic of several studies in recent years. These studies revealed that a conserved protein structure can be reached by following different folding pathways. Based on the available data, we suggest a complex folding pathway for aquaporins, starting from the insertion of individual helices up to the formation of the tetrameric aquaporin structure. The consequences of some known mutations in human aquaporin-encoding genes, which most likely affect the folding and stability of human aquaporins, are discussed.

The effects of a protein osmolyte on the stability of the integral membrane protein glycerol facilitator

Osmolytes are naturally occurring molecules used by a wide variety of organisms to stabilize proteins under extreme conditions of temperature, salinity, hydrostatic pressure, denaturant concentration, and desiccation. The effects of the osmolyte trimethylamine N-oxide (TMAO) as well as the influence of detergent head group and acyl chain length on the stability of the Escherichia coli integral membrane protein glycerol facilitator (GF) tetramer to thermal and chemical denaturation by sodium dodecyl sulphate (SDS) are reported. TMAO promotes the association of the normally tetrameric α-helical protein into higher order oligomers in dodecyl-maltoside (DDM), but not in tetradecyl-maltoside (TDM), lyso-lauroylphosphatidyl choline (LLPC), or lyso-myristoylphosphatidyl choline (LMPC), as determined by dynamic light scattering (DLS); an octameric complex is particularly stable as indicated by SDS polyacrylamide gel electrophoresis. TMAO increases the heat stability of the GF tetramer an average of 10 °C in the 4 detergents and also protects the protein from denaturation by SDS. However, it did not promote re-association to the tetramer when added to SDS-dissociated protein. TMAO also promotes the formation of rod-like detergent micelles, and DLS was found to be useful for monitoring the structure of the protein and the redistribution of detergent during thermal dissociation of the protein. The protein is more thermally stable in detergents with the phosphatidylcholine head group (LLPC and LMPC) than in the maltoside detergents. The implications of the results for osmolyte mechanism, membrane protein stability, and protein-protein interactions are discussed.

Using Haloarcula marismortui bacteriorhodopsin as a fusion tag for enhancing and visible expression of integral membrane proteins in Escherichia coli

Membrane proteins are key targets for pharmacological intervention because of their vital functions. Structural and functional studies of membrane proteins have been severely hampered because of the difficulties in producing sufficient quantities of properly folded and biologically active proteins. Here we generate a high-level expression system of integral membrane proteins in Escherichia coli by using a mutated bacteriorhodopsin (BR) from Haloarcula marismortui (HmBRI/D94N) as a fusion partner. A purification strategy was designed by incorporating a His-tag on the target membrane protein for affinity purification and an appropriate protease cleavage site to generate the final products. The fusion system can be used to detect the intended target membrane proteins during overexpression and purification either with the naked eye or by directly monitoring their characteristic optical absorption. In this study, we applied this approach to produce two functional integral membrane proteins, undecaprenyl pyrophosphate phosphatase and carnitine/butyrobetaine antiporter with significant yield enhancement. This technology could facilitate the development of a high-throughput strategy to screen for conditions that improve the yield of correctly folded target membrane proteins. Other robust BRs can also be incorporated in this system.

Oligomeric state of the oxalate transporter, OxlT

OxlT, the oxalate transporter of Oxalobacter formigenes, was studied to determine its oligomeric state in solution and in the membrane. Three independent approaches were used. First, we used triple-detector (SEC-LS) size exclusion chromatography to analyze purified OxlT in detergent/lipid micelles. These measurements evaluate protein mass in a manner independent of contributions from detergent and lipid; such work shows an average OxlT mass near 47 kDa for detergent-solubilized material, consistent with that expected for monomeric OxlT (46 kDa). A disulfide-linked OxlT mutant was used to verify that it was possible detect dimers under these conditions. A second approach used amino-reactive cross-linkers of varying spacer lengths to study OxlT in detergent/lipid micelles and in natural or artificial membranes, followed by analysis via sodium dodecyl sulfate-polyacrylamide gel electrophoresis. These tests, performed under conditions where the presence of dimers can be documented for either of two known dimeric transporters (AdiC or TetL), indicate that OxlT exists as a monomer in the membrane and retains this status upon detergent solubilization. In a final test, we showed that reconstitution of OxlT into lipid vesicles at variable protein/lipid ratios has no effect on the specific activity of subsequent oxalate transport, as the OxlT content varies between 0.027 and 5.4 OxlT monomers/proteoliposome. We conclude that OxlT is a functional monomer in the membrane and in detergent/lipid micelles.

Structure and stability of the spinach aquaporin SoPIP2;1 in detergent micelles and lipid membranes

Background

SoPIP2;1 constitutes one of the major integral proteins in spinach leaf plasma membranes and belongs to the aquaporin family. SoPIP2;1 is a highly permeable and selective water channel that has been successfully overexpressed and purified with high yields. In order to optimize reconstitution of the purified protein into biomimetic systems, we have here for the first time characterized the structural stability of SoPIP2;1.

Methodology/Principal Finding

We have characterized the protein structural stability after purification and after reconstitution into detergent micelles and proteoliposomes using circular dichroism and fluorescence spectroscopy techniques. The structure of SoPIP2;1 was analyzed either with the protein solubilized with octyl-β-D-glucopyranoside (OG) or reconstituted into lipid membranes formed by E. coli lipids, diphytanoylphosphatidylcholine (DPhPC), or reconstituted into lipid membranes formed from mixtures of 1-palmitoyl-2-oleoyl-phosphatidylcholine (POPE), 1-palmitoyl-2oleoyl-phosphatidylethanolamine (POPE), 1-palmitoyl-2-oleoyl-phosphatidylserine (POPS), and ergosterol. Generally, SoPIP2;1 secondary structure was found to be predominantly α-helical in accordance with crystallographic data. The protein has a high thermal structural stability in detergent solutions, with an irreversible thermal unfolding occurring at a melting temperature of 58°C. Incorporation of the protein into lipid membranes increases the structural stability as evidenced by an increased melting temperature of up to 70°C.

Conclusion/Significance

The results of this study provide insights into SoPIP2;1 stability in various host membranes and suggest suitable choices of detergent and lipid composition for reconstitution of SoPIP2;1 into biomimetic membranes for biotechnological applications.

Refractive-index-based screening of membrane-protein-mediated transfer across biological membranes

Numerous membrane-transport proteins are major drug targets, and therefore a key ingredient in pharmaceutical development is the availability of reliable, efficient tools for membrane transport characterization and inhibition. Here, we present the use of evanescent-wave sensing for screening of membrane-protein-mediated transport across lipid bilayer membranes. This method is based on a direct recording of the temporal variations in the refractive index that occur upon a transfer-dependent change in the solute concentration inside liposomes associated to a surface plasmon resonance (SPR) active sensor surface. The applicability of the method is demonstrated by a functional study of the aquaglyceroporin PfAQP from the malaria parasite Plasmodium falciparum. Assays of the temperature dependence of facilitated diffusion of sugar alcohols on a single set of PfAQP-reconstituted liposomes reveal that the activation energies for facilitated diffusion of xylitol and sorbitol are the same as that previously measured for glycerol transport in the aquaglyceroporin of Escherichia coli (5 kcal/mole). These findings indicate that the aquaglyceroporin selectivity filter does not discriminate sugar alcohols based on their length, and that the extra energy cost of dehydration of larger sugar alcohols, upon entering the pore, is compensated for by additional hydrogen-bond interactions within the aquaglyceroporin pore.

Detergent binding explains anomalous SDS-PAGE migration of membrane proteins

Migration on sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) that does not correlate with formula molecular weights, termed "gel shifting," appears to be common for membrane proteins but has yet to be conclusively explained. In the present work, we investigate the anomalous gel mobility of helical membrane proteins using a library of wild-type and mutant helix-loop-helix ("hairpin") sequences derived from transmembrane segments 3 and 4 of the human cystic fibrosis transmembrane conductance regulator (CFTR), including disease-phenotypic residue substitutions. We find that these hairpins migrate at rates of -10% to +30% vs. their actual formula weights on SDS-PAGE and load detergent at ratios ranging from 3.4-10 g SDS/g protein. We additionally demonstrate that mutant gel shifts strongly correlate with changes in hairpin SDS loading capacity (R(2) = 0.8), and with hairpin helicity (R(2) = 0.9), indicating that gel shift behavior originates in altered detergent binding. In some cases, this differential solvation by SDS may result from replacing protein-detergent contacts with protein-protein contacts, implying that detergent binding and folding are intimately linked. The CF-phenotypic V232D mutant included in our library may thus disrupt CFTR function via altered protein-lipid interactions. The observed interdependence between hairpin migration, SDS aggregation number, and conformation additionally suggests that detergent binding may provide a rapid and economical screen for identifying membrane proteins with robust tertiary and/or quaternary structures.

Detergent binding explains anomalous SDS-PAGE migration of membrane proteins

Migration on sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) that does not correlate with formula molecular weights, termed "gel shifting," appears to be common for membrane proteins but has yet to be conclusively explained. In the present work, we investigate the anomalous gel mobility of helical membrane proteins using a library of wild-type and mutant helix-loop-helix ("hairpin") sequences derived from transmembrane segments 3 and 4 of the human cystic fibrosis transmembrane conductance regulator (CFTR), including disease-phenotypic residue substitutions. We find that these hairpins migrate at rates of -10% to +30% vs. their actual formula weights on SDS-PAGE and load detergent at ratios ranging from 3.4-10 g SDS/g protein. We additionally demonstrate that mutant gel shifts strongly correlate with changes in hairpin SDS loading capacity (R(2) = 0.8), and with hairpin helicity (R(2) = 0.9), indicating that gel shift behavior originates in altered detergent binding. In some cases, this differential solvation by SDS may result from replacing protein-detergent contacts with protein-protein contacts, implying that detergent binding and folding are intimately linked. The CF-phenotypic V232D mutant included in our library may thus disrupt CFTR function via altered protein-lipid interactions. The observed interdependence between hairpin migration, SDS aggregation number, and conformation additionally suggests that detergent binding may provide a rapid and economical screen for identifying membrane proteins with robust tertiary and/or quaternary structures.

Functional characterization of a human aquaporin 0 mutation that leads to a congenital dominant lens cataract

The aquaporin (AQP) transmembrane proteins facilitate the movement of water across the plasma membrane. In the lens, AQP0 is expressed in fiber cells and AQP1 in the epithelium. Recently, two individuals were identified with congenital polymorphic autosomal dominant cataract, due to a single nucleotide base deletion mutation in the lens AQP0. The deletion modified the reading frame resulting in the addition of a premature stop codon. In the present study, we examined the water permeability properties, trafficking and dominant negative effects as well as cytotoxicity due to the mutant AQP0 (Delta213-AQP0) protein. The membrane water permeability (P(w)) of Delta213-AQP0 expressing oocytes (14+/-1 microm/s) was significantly lower than those expressing WT-AQP0 (25+/-3 microm/s). P(w) of water injected control oocytes was 13+/-2 microm/s. Co-expression of WT-AQP0 with Delta213-AQP0 significantly lowered the P(w) (18+/-3 microm/s) compared to WT-AQP0. With or without the EGFP tag, WT-AQP0 protein localized in the plasma membranes of oocytes and cultured cells whereas Delta213-AQP0 was retained in the ER. Forster Resonance Energy Transfer (FRET) showed that WT-AQP0 partly localized with the co-expressed Delta213-AQP0. Co-localization studies suggest that the mutant AQP0 gained its dominant function by trapping the WT-AQP0 in the ER through hetero-oligomerization. Incubating the cells with chemical chaperones, namely, TMAO and DMSO, did not correct the folding/trafficking defects. Cell death in the Delta213-AQP0 expressing cells was due to necrosis caused by the accumulation of Delta213-AQP0 protein in the ER in cytotoxic proportions. The data show that replacement of the distal end of the 6th TM domain and the C-terminal domain of AQP0 due to the deletion mutation resulted in the impairment of cell membrane P(w), localization of the mutant protein in the ER without trafficking to the plasma membrane, and cytotoxicity due to the accumulation of the mutant protein. Cataracts in patients with this mutation might have resulted from the above mentioned consequences.

Transport of glycerol by Pediococcus pentosaceus isolated from wine

Pediococcus pentosaceus N(5)p is a strain isolated from wine that uses glycerol as its sole carbon source, mainly via the glycerol kinase pathway. The transport of glycerol was investigated in resting cells of this strain. Glycerol uptake followed a Michaelis-Menten relationship with an observed apparent K(m) of 33 microM and a V(max) of 2.5 nmol/min/mg of cell protein. The transport system was specific for glycerol, which was present in the cells grown either on glycerol or glucose suggesting its constitutive nature. The presence of uptake when resting cells were treated with HgCl(2) and the absence of counterflow indicate that facilitated diffusion is not involved in glycerol transport. On the other hand, glycerol uptake was inhibited by the metabolic poisons that affect ATP availability by acting on either electron transport or ATPase activity, and by the proton-conducting uncouplers without any effect on glycerol kinase activity. The restoration of glycerol uptake in de-energized cells by the addition of glucose and low concentration of cyanide-m-chlorophenyl hydrazone was achieved. These results, the first in the genus Pediococcus, provide evidence for an energy-dependent uptake of glycerol that involves the proton motive force directly or coupled with ATP synthesis.

Eukaryotic integral membrane protein expression utilizing the Escherichia coli glycerol-conducting channel protein (GlpF)

A fusion protein expression system is described that allows for production of eukaryotic integral membrane proteins in Escherichia coli (E. coli). The eukaryotic membrane protein targets are fused to the C terminus of the highly expressed E. coli inner membrane protein, GlpF (the glycerol-conducting channel protein). The generic utility of this system for heterologous membrane-protein expression is demonstrated by the expression and insertion into the E. coli cell membrane of the human membrane proteins: occludin, claudin 4, duodenal ferric reductase and a J-type inwardly rectifying potassium channel. The proteins are produced with C-terminal hexahistidine tags (to permit purification of the expressed fusion proteins using immobilized metal affinity chromatography) and a peptidase cleavage site (to allow recovery of the unfused eukaryotic protein).

A novel tripartite motif involved in aquaporin topogenesis, monomer folding and tetramerization

Aquaporin (AQP) folding in the endoplasmic reticulum is characterized by two distinct pathways of membrane insertion that arise from divergent residues within the second transmembrane segment. We now show that in AQP1 these residues (Asn49 and Lys51) interact with Asp185 at the C terminus of TM5 to form a polar, quaternary structural motif that influences multiple stages of folding. Asn49 and Asp185 form an intramolecular hydrogen bond needed for proper helical packing, monomer formation and function. In contrast, Lys51 interacts with Asp185 on an adjacent monomer to stabilize the AQP1 tetramer. Although these residues are unique to AQP1, they share a highly conserved architecture whose functional properties can be transferred to other family members. These findings suggest a general mechanism by which evolutionary divergence of membrane proteins can confer new functional properties via alternative folding pathways that give rise to a common final structure.

Expression, purification, and biochemical characterization of the antiinflammatory tristetraprolin: a zinc-dependent mRNA binding protein affected by posttranslational modifications

Tristetraprolin (TTP) is a hyperphosphorylated protein that destabilizes mRNA by binding to an AU-rich element (ARE). Mice deficient in TTP develop a severe inflammatory syndrome. The biochemical properties of TTP have not been adequately characterized, due to the difficulties in protein purification and lack of a high-titer antiserum. Full-length human TTP was expressed in human HEK293 cells and purified to at least 70% homogeneity. The purified protein was free of endogenous ARE binding activity, and was used for investigating its size, zinc dependency, and binding kinetics for tumor necrosis factor alpha mRNA ARE. A high-titer rabbit antiserum was raised against the MBP-hTTP fusion protein expressed in Escherichia coli. Cellular localization studies of the transfected cells indicated that approximately 80% of the expressed TTP was in the cytosol, with 20% in the nuclei. TTP from both locations bound to the ARE and formed similar complexes. The purified TTP was shown to be intact by N-terminal His-tag purification, C-terminal peptide sequencing, and mass spectrometry analysis. Results from size exclusion chromatography are consistent with the predominant form of active TTP being a tetramer. TTP's ARE binding activity was increased by 10 microM Zn(2+). The half-maximal binding of TTP from HEK293 cells was approximately 30 nM in assays containing 10 nM ARE. This value was about twice that of TTP from E. coli. TTP from HEK293 cells was highly phosphorylated, and its electrophoretic mobility was increased by alkaline phosphatase treatment and somewhat by T271A mutation, but not by PNGase F or S186A mutation. The gel mobility of TTP from E. coli was decreased by in vitro phosphorylation with p42/ERK2 and p38 mitogen-activated protein kinases. These results suggest that TTP's zinc-dependent ARE binding affinity is reduced by half by posttranslational modifications, mainly by phosphorylation but not by glycosylation, in mammalian cells. The results support a model in which each subunit of the TTP tetramer binds to one of the five overlapping UUAUUUAUU sequences of the ARE, resulting in a stable TTP-ARE complex.

Properties of the Na+/H+ exchanger protein. Detergent-resistant aggregation and membrane microdistribution

The Na+/H+ exchanger is a ubiquitous membrane protein of bacteria, plants and mammals. The first isoform discovered (NHE1) is present on the mammalian plasma membrane and transports one H+ out of cells in exchange for one extracellular Na+. With solubilization in standard SDS/PAGE buffer, this protein had a high tendency to aggregate when subjected to elevated temperature. The aggregates were stable and did not dissociate in high concentrations of SDS or 2-mercaptoethanol. We examined the distribution of the Na+/H+ exchanger within membrane subfractions. The Na+/H+ exchanger was found both in caveolin-containing fractions and, in lesser amounts, in higher density membrane fractions where the bulk of proteins were contained. Treatment with cytochalasin D caused only a minor reduction of the amount of Na+/H+ exchanger present in caveolin-enriched fractions suggesting an intact cytoskeleton was not important for NHE1 localization to these microdomains. Treatment of cells with methyl beta-cyclodextrin had a small stimulatory effect on Na+/H+ exchanger activity and reduced the amount of Na+/H+ exchanger in low density membrane fractions. Our study demonstrates that SDS cannot maintain the protein in a monomeric state suggesting that strong hydrophobic interactions are responsible for this temperature dependent aggregation behavior. In addition a large proportion of the Na+/H+ exchanger protein is found to be enriched in low density caveolin-containing fractions.

Role of C-terminal domain and transmembrane helices 5 and 6 in function and quaternary structure of major intrinsic proteins: analysis of aquaporin/glycerol facilitator chimeric proteins

We previously observed that aquaporins and glycerol facilitators exhibit different oligomeric states when studied by sedimentation on density gradients following nondenaturing detergent solubilization. To determine the domains of major intrinsic protein (MIP) family proteins involved in oligomerization, we constructed protein chimeras corresponding to the aquaporin AQPcic substituted in the loop E (including the proximal part of transmembrane domain (TM) 5) and/or the C-terminal part (including the distal part of TM 6) by the equivalent domain of the glycerol channel aquaglyceroporin (GlpF) (chimeras called AGA, AAG, and AGG). The analogous chimeras of GlpF were also constructed (chimeras GAG, GGA, and GAA). cRNA corresponding to all constructs were injected into Xenopus oocytes. AQPcic, GlpF, AAG, AGG, and GAG were targeted to plasma membranes. Water or glycerol membrane permeability measurements demonstrated that only the AAG chimera exhibited a channel function corresponding to water transport. Analysis of all proteins expressed either in oocytes or in yeast by velocity sedimentation on sucrose gradients following solubilization by 2% n-octyl glucoside indicated that only AQPcic and AAG exist in tetrameric forms. GlpF, GAG, and GAA sediment in a monomeric form, whereas GGA and AGG were found mono/dimeric. These data bring new evidence that, within the MIP family, aquaporins and GlpFs behave differently toward nondenaturing detergents. We demonstrate that the C-terminal part of AQPcic, including the distal half of TM 6, can be substituted by the equivalent domain of GlpF (AAG chimera) without modifying the transport specificity. Our results also suggest that interactions of TM 5 of one monomer with TM 1 of the adjacent monomer are crucial for aquaporin tetramer stability.

Erythroid expression and oligomeric state of the AQP3 protein

Biochemical and biophysical studies have shown that the strictly water-permeable aquaporins have a tetrameric structure, whereas results concerning the oligomeric state of GlpF, the glycerol facilitator of Escherichia coli, are dependent upon the analytical technique used. Here, we analyzed the oligomerization of the AQP3 aquaglyceroporin, which presents a mixed selectivity for water, glycerol, and urea. At first, based on transcript detection by reverse transcription-PCR from human erythroid tissues and membrane expression detected by flow cytometry analysis, we demonstrated that AQP3 is expressed on human and rat but not on mouse red blood cells. Then, the quaternary structure of AQP3 was determined using as models human red blood cell membranes, which carry both AQP1 and AQP3, and two heterologous expression systems: Xenopus laevis oocyte, for density and size estimation of aquaporins, and Saccharomyces cerevisiae yeast, which expressed a non-glycosylated form of AQP3. By velocity sedimentation in sucrose gradient after non-denaturing detergent solubilization, AQP3 was essentially found as mono- and dimeric species in conditions under which AQP1 preserved its tetrameric structure. Freeze-fracture studies on oocyte plasma membranes gave a size of AQP3 particles in favor of a dimeric or trimeric structure. Finally, by cross-linking experiments with red blood cell membranes, AQP3 is visible as different oligomeric structures, including a tetrameric one.

Oligomerization of water and solute channels of the major intrinsic protein (MIP) family

Water and small solute fluxes through cell membranes are ensured in many tissues by selective pores that belong to the major intrinsic protein family (MIP). This family includes the water channels or aquaporins (AQP) and the neutral solute facilitators such as the glycerol facilitator (GlpF). We have compared the characteristics of representatives of each subfamily. Following solubilization in the nondenaturing detergents n-octyl-glucoside (OG) and Triton X-100 (T-X100), AQPs remain in their native homotetrameric state, while GlpF always behaves as a monomer. Solute facilitators are fully solubilized by the detergent N-lauroyl sarcosine (NLS), while AQPs are not. Analyses of mutants and chimeras demonstrate a close correlation between the water transport function and the resistance to NLS solubilization. Thus, AQPs and solute facilitators exhibit different behaviors in mild detergents; this could reflect differences in quaternary organization within the membranes. We propose that the oligomerization state or the strength of self-association is part of the mechanisms used by MIP proteins to ensure solute selectivity.

Reconstitution and functional comparison of purified GlpF and AqpZ, the glycerol and water channels from Escherichia coli

A large family of membrane channel proteins selective for transport of water (aquaporins) or water plus glycerol (aquaglyceroporins) has been found in diverse life forms. Escherichia coli has two members of this family-a water channel, AqpZ, and a glycerol facilitator, GlpF. Despite having similar primary amino acid sequences and predicted structures, the oligomeric state and solute selectivity of AqpZ and GlpF are disputed. Here we report biochemical and functional characterizations of affinity-purified GlpF and compare it to AqpZ. Histidine-tagged (His-GlpF) and hemagglutinin-tagged (HA-GlpF) polypeptides encoded by a bicistronic construct were expressed in bacteria. HA-GlpF and His-GlpF appear to form oligomers during Ni-nitrilotriacetate affinity purification. Sucrose gradient sedimentation analyses showed that the oligomeric state of octyl glucoside-solubilized GlpF varies: low ionic strength favors subunit dissociation, whereas Mg(2+) stabilizes tetrameric assembly. Reconstitution of affinity-purified GlpF into proteoliposomes increases glycerol permeability more than 100-fold and water permeability up to 10-fold compared with control liposomes. Glycerol and water permeability of GlpF both occur with low Arrhenius activation energies and are reversibly inhibited by HgCl(2). Our studies demonstrate that, unlike AqpZ, a water-selective stable tetramer, purified GlpF exists in multiple oligomeric forms under nondenaturing conditions and is highly permeable to glycerol but less well permeated by water.