Functional expression of the human CHIP28 water channel in a yeast secretory mutant

Recombinant Microorganisms as Tools for High Throughput Screening for Nonantibiotic Compounds

Microorganisms were among the first tools used for the discovery of biologically active compounds. Their utility reached a zenith during the era of antibiotic development in the 1950s and 1960s, then declined. Subsequently, a substantial role for microorganisms in the pharmaceutical industry developed with the realization that microbial fermentations were intriguing sources of nonantibiotic natural products. From recombinant DNA technology emerged another important role for microorganisms in pharmaceutical research: the expression of heterologous proteins for therapeutic products or for in vitro high throughput screens (HTSs). Recent developments in cloning, genetics, and expression systems have opened up new applications for recombinant microorganisms in screening for nonantibiotic compounds in HTSs. These screens employ microorganisms that depend upon the function of a heterologous protein for survival under defined nutritional conditions. Compounds that specifically target the heterologous protein can be identified by measuring viability of the microorganism under different nutrient selection. Advantages of this approach include a built-in selection for target selectivity, an easily measured end point that can be used for a multitude of different targets, and compatibility with automation required for HTSs. Mechanism-based HTSs using recombinant microorganisms can also address drug targets that are not readily approachable in other HTS formats, including certain enzymes; ion channels and transporters; and protein::protein, protein::DNA, and protein::RNA interactions.

Saccharomyces cerivisiae as a model system for kidney disease: what can yeast tell us about renal function?

Ion channels, solute transporters, aquaporins, and factors required for signal transduction are vital for kidney function. Because mutations in these proteins or in associated regulatory factors can lead to disease, an investigation into their biogenesis, activities, and interplay with other proteins is essential. To this end, the yeast, Saccharomyces cerevisiae, represents a powerful experimental system. Proteins expressed in yeast include the following: 1) ion channels, including the epithelial sodium channel, members of the inward rectifying potassium channel family, and cystic fibrosis transmembrane conductance regulator; 2) plasma membrane transporters, such as the Na(+)-K(+)-ATPase, the Na(+)-phosphate cotransporter, and the Na(+)-H(+) ATPase; 3) aquaporins 1-4; and 4) proteins such as serum/glucocorticoid-induced kinase 1, phosphoinositide-dependent kinase 1, Rh glycoprotein kidney, and trehalase. The variety of proteins expressed and studied emphasizes the versatility of yeast, and, because of the many available tools in this organism, results can be obtained rapidly and economically. In most cases, data gathered using yeast have been substantiated in higher cell types. These attributes validate yeast as a model system to explore renal physiology and suggest that research initiated using this system may lead to novel therapeutics.

Yeast water channels: an overview of orthodox aquaporins

In yeast, the presence of orthodox aquaporins has been first recognized in Saccharomyces cerevisiae, in which two genes (AQY1 and AQY2) were shown to be related to mammal and plant water channels. The present review summarizes the putative orthodox aquaporin protein sequences found in available genomes of yeast and filamentous fungi. Among the 28 yeast genomes sequenced, most species present only one orthodox aquaporin, and no aquaporins were found in eight yeast species. Alignment of amino acid sequences reveals a very diverse group. Similarity values vary from 99% among species within the Saccharomyces genus to 34% between ScAqy1 and the aquaporin from Debaryomyces hansenii. All of the fungal aquaporins possess the known characteristic sequences, and residues involved in the water channel pore are highly conserved. Advances in the establishment of the structure are reviewed in relation to the mechanisms of selectivity, conductance and gating. In particular, the involvement of the protein cytosolic N-terminus as a channel blocker preventing water flow is addressed. Methodologies used in the evaluation of aquaporin activity frequently involve the measurement of fast volume changes. Particular attention is paid to data analysis to obtain accurate membrane water permeability parameters. Although the presence of aquaporins clearly enhances membrane water permeability, the relevance of these ubiquitous water channels in yeast performance remains obscure.

Fluconazole transport into Candida albicans secretory vesicles by the membrane proteins Cdr1p, Cdr2p, and Mdr1p

A major cause of azole resistance in Candida albicans is overexpression of CDR1, CDR2, and/or MDR1, which encode plasma membrane efflux pumps. To analyze the catalytic properties of these pumps, we used ACT1- and GAL1-regulated expression plasmids to overexpress CDR1, CDR2, or MDR1 in a C. albicans cdr1 cdr2 mdr1-null mutant. When the genes of interest were expressed, the resulting transformants were more resistant to multiple azole antifungals, and accumulated less [(3)H]fluconazole intracellularly, than empty-vector controls. Next, we used a GAL1-regulated dominant negative sec4 allele to cause cytoplasmic accumulation of post-Golgi secretory vesicles (PGVs), and we found that PGVs isolated from CDR1-, CDR2-, or MDR1-overexpressing cells accumulated much more [(3)H]fluconazole than did PGVs from empty-vector controls. The K(m)s (expressed in micromolar concentrations) and V(max)s (expressed in picomoles per milligram of protein per minute), respectively, for [(3)H]fluconazole transport were 0.8 and 0.91 for Cdr1p, 4.3 and 0.52 for Cdr2p, and 3.5 and 0.59 for Mdr1p. [(3)H]fluconazole transport by Cdr1p and Cdr2p required ATP and was unaffected by carbonyl cyanide 3-chlorophenylhydrazone (CCCP), whereas [(3)H]fluconazole transport by Mdr1p did not require ATP and was inhibited by CCCP. [(3)H]fluconazole uptake by all 3 pumps was inhibited by all other azoles tested, with 50% inhibitory concentrations (IC(50)s; expressed as proportions of the [(3)H]fluconazole concentration) of 0.2 to 5.6 for Cdr1p, 0.3 to 3.1 for Cdr2p, and 0.3 to 3.1 for Mdr1p. The methods used in this study may also be useful for studying other plasma membrane transporters in C. albicans and other medically important fungi.

STIM1 gates the store-operated calcium channel ORAI1 in vitro

Store-operated Ca(2+) entry through the plasma membrane Ca(2+) release-activated Ca(2+) (CRAC) channel in mammalian T cells and mast cells depends on the sensor protein stromal interaction molecule 1 (STIM1) and the channel subunit ORAI1. To study STIM1-ORAI1 signaling in vitro, we have expressed human ORAI1 in a sec6-4 strain of the yeast Saccharomyces cerevisiae and isolated sealed membrane vesicles carrying ORAI1 from the Golgi compartment to the plasma membrane. We show by in vitro Ca(2+) flux assays that bacterially expressed recombinant STIM1 opens wild-type ORAI1 channels but not channels assembled from the ORAI1 pore mutant E106Q or the ORAI1 severe combined immunodeficiency (SCID) mutant R91W. These experiments show that the STIM1-ORAI1 interaction is sufficient to gate recombinant human ORAI1 channels in the absence of other proteins of the human ORAI1 channel complex, and they set the stage for further biochemical and biophysical dissection of ORAI1 channel gating.

In vitro analysis and modification of aquaporin pore selectivity

Aquaporins enable the passage of a diverse set of solutes besides water. Many novel aquaporin permeants, such as antimonite and arsenite, silicon, ammonia, and hydrogen peroxide, have been described very recently. By the same token, the number of available aquaporin sequences has rapidly increased. Yet, sequence analyses and structure models cannot reliably predict permeability properties. Even the contribution to pore selectivity of individual residues in the channel layout is not fully understood. Here, we describe and discuss established in vitro assays for water and solute permeability. Measurements of volume change due to flux along osmotic or chemical gradients yield quantitative biophysical data, whereas phenotypic growth assays can hint at the relevance of aquaporins in the physiological setting of a certain cell. We also summarize data on the modification of pore selectivity of the prototypical water-specific mammalian aquaporin-1. We show that replacing residues in the pore constriction region allows ammonia, urea, glycerol, and even protons to pass the aquaporin pore.

Exceptional overproduction of a functional human membrane protein

Eukaryotic--especially human--membrane protein overproduction remains a major challenge in biochemistry. Heterologously overproduced and purified proteins provide a starting point for further biochemical, biophysical and structural studies, and the lack of sufficient quantities of functional membrane proteins is frequently a bottleneck hindering this. Here, we report exceptionally high production levels of a correctly folded and crystallisable recombinant human integral membrane protein in its active form; human aquaporin 1 (hAQP1) has been heterologously produced in the membranes of the methylotrophic yeast Pichia pastoris. After solubilisation and a two step purification procedure, at least 90 mg hAQP1 per liter of culture is obtained. Water channel activity of this purified hAQP1 was verified by reconstitution into proteoliposomes and performing stopped-flow vesicle shrinkage measurements. Mass spectrometry confirmed the identity of hAQP1 in crude membrane preparations, and also from purified protein reconstituted into proteoliposomes. Furthermore, crystallisation screens yielded diffraction quality crystals of untagged recombinant hAQP1. This study illustrates the power of the yeast P. pastoris as a host to produce exceptionally high yields of a functionally active, human integral membrane protein for subsequent functional and structural characterization.

Characterization of plant aquaporins

Plants have been reported to contain a large set of aquaporins (38 for Arabidopsis), which has been divided into four subfamilies on the basis of similarities in their amino acid sequences. They belong to the large superfamily of major intrinsic proteins (MIP), which was the basis for the nomenclature PIP, TIP, and NIP, also indicating the subcellular localization plasma membrane, tonoplast, and nodule of the respective founding member. The fourth subfamily of small and basic intrinsic proteins is not well characterized so far. The increasing number of reports dealing with various aspects of plant aquaporins is starting to advance our understanding of aquaporin biology in plants. Fundamental questions include: what is the basic function of the different plant aquaporins, what is their primary substrate, and what is the consequence of function/malfunction of a particular aquaporin for the overall function of the plant? Biochemical and biophysical techniques can be employed to get information on the basic functional characteristics of plant aquaporins. An impressive set of techniques has been used to study aquaporin function on molecular, subcellular, and cellular levels in plants, as well as in heterologous expression systems. The physiological role of aquaporins in plants is much less well understood, but reports unraveling the physiological role of aquaporins, mainly employing genetic techniques and functional measurement on the whole plant level, are emerging. The goal of this chapter is to give an overview on the applied methods, together with some exemplary findings.

Expression of heterologous aquaporins for functional analysis in Saccharomyces cerevisiae

In this study the yeast Saccharomyces cerevisiae, which is a genetically tractable model for analysis of osmoregulation, has been used for analysis of heterologous aquaporins. Aquaporin water channels play important roles in the control of water homeostasis in individual cells and multicellular organisms. We have investigated the effects of functional expression of the mammalian aquaporins AQP1 and AQP5 and the aquaglyceroporins AQP3 and AQP9. Expression of aquaporins caused moderate growth inhibition under hyperosmotic stress, while expression of aquaglyceroporins mediated strong growth inhibition due to glycerol loss. Water transport was monitored in protoplasts, where the kinetics of bursting was influenced by presence of aquaporins but not aquaglyceroporins. We observed glycerol transport through aquaglyceroporins, but not aquaporins, in a yeast strain deficient in glycerol production, whose growth depends on glycerol inflow. In addition, a gene reporter assay allowed to indirectly monitor the effect of AQP9-mediated enhanced glycerol loss on osmoadaptation. Transport activity of certain aqua(glycero)porins was diminished by low pH or CuSO4, suggesting that yeast can potentially be used for screening of putative aquaporin inhibitors. We conclude that yeast is a versatile system for functional studies of aquaporins, and it can be developed to screen for compounds of potential pharmacological use.

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

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

Water channel activity of radish plasma membrane aquaporins heterologously expressed in yeast and their modification by site-directed mutagenesis

Plants contain a number of aquaporin isoforms. We developed a method for determining the water channel activity of individual isoforms of aquaporin. Six plasma membrane aquaporins (RsPIPs) and two vacuolar membrane aquaporins (RsTIPs) of radish (Raphanus sativus) were expressed heterologously in Saccharomyces cerevisiae BJ5458, which is deficient in endogenous functional aquaporin. Aquaporins were detected by immunoblot analysis with corresponding antibodies. Water permeability of membranes from yeast transformants was assayed by stopped-flow spectrophotometry. The water channel activity of members of the RsPIP2 and RsTIP subfamilies was about 10 times and 5 times greater, respectively, than that of the control; however, RsPIP1s had little (RsPIP1-2 and RsPIP1-3) or no activity (RsPIP1-1). Site-directed mutation of several residues conserved in RsPIP1s or RsPIP2s markedly altered the water transport activity. Exchange of Ile244 of RsPIP1-3 with valine increased the activity to 250% of the wild type RsPIP1-3. On the other hand, exchange of Val235 of RsPIP2-2, which corresponds to RsPIP1-3 Ile244, with isoleucine caused a marked inactivation to 45% of the original RsPIP2-2. Mutation at possible phosphorylation sites at the N- and C-terminal tails also altered the activity. These results suggest that these residues in the half-helix loop E and the tails are involved in the water transport and the functional regulation of RsPIP1 and RsPIP2.

N-glycosylation controls functional activity of Oatp1, an organic anion transporter

Rat Oatp1 (Slc21a1) is an organic anion-transporting polypeptide believed to be an anion exchanger. To characterize its mechanism of transport, Oatp1 was expressed in Saccharomyces cerevisiae under control of the GAL1 promoter. Protein was present at high levels in isolated S. cerevisiae secretory vesicles but had minimal posttranslational modifications and failed to exhibit taurocholate transport activity. Apparent molecular mass (M) of Oatp1 in yeast was similar to that of unmodified protein, approximately 62 kDa, whereas in liver plasma membranes Oatp1 has an M of approximately 85 kDa. To assess whether underglycosylation of Oatp1 in yeast suppressed functional activity, Oatp1 was expressed in Xenopus laevis oocytes with and without tunicamycin, a glycosylation inhibitor. With tunicamycin, M of Oatp1 decreased from approximately 72 to approximately 62 kDa and transport activity was nearly abolished. Mutations to four predicted N-glycosylation sites on Oatp1 (Asn to Asp at positions 62, 124, 135, and 492) revealed a cumulative effect on function of Oatp1, leading to total loss of taurocholate transport activity when all glycosylation sites were removed. M of the quadruple mutant was approximately 62 kDa, confirming that these asparagine residues are sites of glycosylation in Oatp1. Relatively little of the quadruple mutant was able to reach the plasma membrane, and most remained in unidentified intracellular compartments. In contrast, two of the triple mutants tested (N62/124/135D and N124/135/492D) were present in the plasma membrane fraction yet exhibited minimal transport activity. These results demonstrate that both membrane targeting and functional activity of Oatp1 are controlled by the extent of N-glycosylation.

Aquaporin expression correlates with freeze tolerance in baker's yeast, and overexpression improves freeze tolerance in industrial strains

Little information is available about the precise mechanisms and determinants of freeze resistance in baker's yeast, Saccharomyces cerevisiae. Genomewide gene expression analysis and Northern analysis of different freeze-resistant and freeze-sensitive strains have now revealed a correlation between freeze resistance and the aquaporin genes AQY1 and AQY2. Deletion of these genes in a laboratory strain rendered yeast cells more sensitive to freezing, while overexpression of the respective genes, as well as heterologous expression of the human aquaporin gene hAQP1, improved freeze tolerance. These findings support a role for plasma membrane water transport activity in determination of freeze tolerance in yeast. This appears to be the first clear physiological function identified for microbial aquaporins. We suggest that a rapid, osmotically driven efflux of water during the freezing process reduces intracellular ice crystal formation and resulting cell damage. Aquaporin overexpression also improved maintenance of the viability of industrial yeast strains, both in cell suspensions and in small doughs stored frozen or submitted to freeze-thaw cycles. Furthermore, an aquaporin overexpression transformant could be selected based on its improved freeze-thaw resistance without the need for a selectable marker gene. Since aquaporin overexpression does not seem to affect the growth and fermentation characteristics of yeast, these results open new perspectives for the successful development of freeze-resistant baker's yeast strains for use in frozen dough applications.

AQP3 deficiency in humans and the molecular basis of a novel blood group system, GIL

AQP3 is a water and glycerol channel present on human erythrocytes and in various tissues. By protein and molecular biology analysis, two unrelated probands who developed alloantibodies to the high frequency antigen GIL were found to be AQP3-deficient. The defect is caused by homozygous mutation affecting the 5' donor splice site of intron 5 of the AQP3 gene. This mutation causes the skipping of exon 5 and generates a frameshift and premature stop codon. Functional studies by 90 degrees light scattering using a stopped-flow spectrometer revealed the absence of facilitated glycerol transport across red cell membranes from the probands, but the water and urea transports were normal. Expression studies into COS-7 cells followed by flow cytometry analysis showed that only cells transfected with AQP3 cDNA strongly reacted with anti-GIL antibodies. These findings represent the first reported cases of AQP3 deficiency in humans and provide the molecular basis of a new blood group system, GIL, encoded by the AQP3 protein.

Antigenic and functional properties of the human red blood cell urea transporter hUT-B1

The Kidd (JK) blood group locus encodes the urea transporter hUT-B1, which is expressed on human red blood cells and other tissues. The common JK*A/JK*B blood group polymorphism is caused by a single nucleotide transition G838A changing Asp-280 to Asn-280 on the polypeptide, and transfection of erythroleukemic K562 cells with hUT-B1 cDNAs carrying either the G838 or the A838 nucleotide substitutions resulted in the isolation of stable clones that expressed the Jk(a) or Jk(b) antigens, respectively, thus providing the first direct demonstration that the hUT-B1 gene encodes the Kidd blood group antigens. In addition, immunochemical analysis of red blood cells demonstrated that hUT-B1 also exhibits ABO determinants attached to the single N-linked sugar chain at Asn-211. Moreover, immunoadsorption studies, using inside-out and right-side-out red cell membrane vesicles as competing antigen, demonstrated that the C- and N-terminal ends of hUT-B1 are oriented intracellularly. Mutagenesis and functional studies by expression in Xenopus oocytes revealed that both cysteines Cys-25 and Cys-30 (but not alone) are essential for plasma membrane addressing. Conversely, the transport function was not affected by the JK*A/JK*B polymorphism, C-terminal deletion (residues 360-389), or mutation of the extracellular N-glycosylation consensus site and remains poorly para-chloromercuribenzene sulfonate (pCMBS)-sensitive. However, transport studies by stopped flow light scattering using Jk-K562 transfectants demonstrated that the hUT-B1-mediated urea transport is pCMBS-sensitive in an erythroid context, as reported previously for the transporter of human red blood cells. Mutagenesis analysis also indicated that Cys-151 and Cys-236, at least alone, are not involved in pCMBS inhibition. Altogether, these antigenic, topologic, and functional properties might have implications into the physiology of hUT-B1 and other members of the urea transporter family.

Plant aquaporins

Aquaporins are ubiquitous membrane channel proteins that facilitate and regulate the permeation of water across biological membranes. Aquaporins are members of the MIP family and some of them seem to be also able to transport other molecules such as urea or glycerol. In the plant kingdom, a single plant expresses a considerably large number of MIP homologues. These homologues can be subdivided into four groups (PIP, TIP, NIP, SIP) with highly conserved amino acid sequences and intron positions in each group. Since their discovery, advancing knowledge of their structure led to an understanding of the basic features of the water transport mechanism. An optimal water balance is essential to the homeostasis of most organisms, and aquaporins may be one of the mechanisms involved under changing environmental and developmental conditions. In fact, this may be one reason for the abundance and diversity of aquaporins, in particular in plants. In addition, exposure to different types of stress alters water relations and thus, aquaporins may be involved in stress responses as well. The transcriptional and/or post-translational regulation of aquaporins would determine changes in membrane water permeability. Both phosphorylation and translocation to/from vesicles have been reported as post-translational mechanisms. However, translocation in plants has not yet been shown. Although significant advances have been achieved, complete understanding of aquaporin function and regulation remains elusive.

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.

Existence of a tightly regulated water channel in Saccharomyces cerevisiae

The Saccharomyces cerevisiae strain Sigma1278b possesses two putative aquaporins, Aqy1-1p and Aqy2-1p. Previous work demonstrated that Aqy1-1p functions as a water channel in Xenopus oocyte. However, no function could be attributed to Aqy2-1p in this system. Specific antibodies were used to follow the expression of Aqy1-1p and Aqy2-1p in the yeast. Aqy1-1p was never detected whatever the growth phase and culture conditions tested. In contrast, Aqy2-1p was detected only during the exponential growth phase in rich medium containing glucose. Aqy2-1p expression was repressed by hyper-osmotic culture conditions. Both immunocytochemistry and biochemical subcellular fractionation demonstrated that Aqy2-1p is located on the endoplasmic reticulum (ER) as well as on the plasma membrane. In microsomal vesicles enriched in ER, a water channel activity due to Aqy2-1p was detected by stopped-flow analysis. Our results show that the expression of aquaporins is tightly controlled. The physiological relevance of aquaporin-mediated water transport in yeast is discussed.

Structure and function of aquaporin water channels

The aquaporins (AQPs) are a family of small membrane-spanning proteins (monomer size approximately 30 kDa) that are expressed at plasma membranes in many cells types involved in fluid transport. This review is focused on the molecular structure and function of mammalian aquaporins. Basic features of aquaporin structure have been defined using mutagenesis, epitope tagging, and spectroscopic and freeze-fracture electron microscopy methods. Aquaporins appear to assemble in membranes as homotetramers in which each monomer, consisting of six membrane-spanning alpha-helical domains with cytoplasmically oriented amino and carboxy termini, contains a distinct water pore. Medium-resolution structural analysis by electron cryocrystallography indicated that the six tilted helical segments form a barrel surrounding a central pore-like region that contains additional protein density. Several of the mammalian aquaporins (e.g., AQP1, AQP2, AQP4, and AQP5) appear to be highly selective for the passage of water, whereas others (recently termed aquaglyceroporins) also transport glycerol (e.g., AQP3 and AQP8) and even larger solutes (AQP9). Evidence for possible movement of ions and carbon dioxide through the aquaporins is reviewed here, as well as evidence for direct regulation of aquaporin function by posttranslational modification such as phosphorylation. Important unresolved issues include definition of the molecular pathway through which water and solutes move, the nature of monomer-monomer interactions, and the physiological significance of aquaporin-mediated solute movement. Recent results from knockout mice implicating multiple physiological roles of aquaporins suggest that the aquaporins may be suitable targets for drug discovery by structure-based and/or high-throughput screening strategies.

Functional analysis of aquaporin-2 mutants associated with nephrogenic diabetes insipidus by yeast expression

Mutations of aquaporin-2 (AQP2) vasopressin water channel cause nephrogenic diabetes insipidus (NDI). It has been suggested that impaired routing of AQP2 mutants to the plasma membrane causes the disease; however, no determinations have been made of mutation-induced alterations of AQP2 channel water permeability. To address this issue, a series of AQP2 mutants were expressed in yeast, and the osmotic water permeability (P(f)) of the isolated vesicles was measured. Wild-type and mutant AQP2 were expressed equally well in vesicles. P(f) of the vesicles containing wild-type AQP2 was 22 times greater than that of the control, which was sensitive to mercury and weakly dependent on the temperature. P(f) measurements and mercury inhibition examinations suggested that mutants L22V and P262L are fully functional, whereas mutants N68S, R187C, and S216P are partially functional. In contrast, mutants N123D, T125M, T126M, A147T, and C181W had very low water permeability. Our results suggest that the structure between the third and fifth hydrophilic loops is critical for the functional integrity of the AQP2 water channel and that disruption of AQP2 water permeability by mutations may cause NDI.

Functional reconstitution and characterization of AqpZ, the E. coli water channel protein

Understanding the selectivity of aquaporin water channels will require structural and functional studies of wild-type and modified proteins; however, expression systems have not previously yielded aquaporins in the necessary milligram quantities. Here we report expression of a histidine-tagged form of Escherichia coli aquaporin-Z (AqpZ) in its homologous expression system. 10-His-AqpZ is solubilized and purified to near homogeneity in a single step with a final yield of approximately 2.5 mg/l of culture. The histidine tag is removed by trypsin, yielding the native protein with the addition of three N-terminal residues, as confirmed by microsequencing. Sucrose gradient sedimentation analysis showed that the native, solubilized AqpZ protein is a trypsin-resistant tetramer. Unlike other known aquaporins, AqpZ tetramers are not readily dissociated by 1% SDS at neutral pH. Hydrophilic reducing agents have a limited effect on the stability of the tetramer in 1% SDS, whereas incubations for more than 24 hours, pH values below 5.6, or exposure to the hydrophobic reducing agent ethanedithiol cause dissociation into monomers. Cys20, but not Cys9, is necessary for the stability of the AqpZ tetramer in SDS. Upon reconstitution into proteoliposomes, AqpZ displays very high osmotic water permeability (pf > or = 10 x 10(-14) cm3 s-1 subunit-1) and low Arrhenius activation energy (Ea = 3.7 kcal/mol), similar to mammalian aquaporin-1 (AQP1). No permeation by glycerol, urea or sorbitol was detected. Expression of native and modified AqpZ in milligram quantities has permitted biophysical characterization of this remarkably stable aquaporin tetramer, which is being utilized for high-resolution structural studies.

Water transport across yeast vacuolar and plasma membrane-targeted secretory vesicles occurs by passive diffusion

To determine whether solute transport across yeast membranes was facilitated, we measured the water and solute permeations of vacuole-derived and late secretory vesicles in Saccharomyces cerevisiae; all permeations were consistent with passive diffusive flow. We also overexpressed Fps1p, the putative glycerol facilitator in S. cerevisiae, in secretory vesicles but observed no effect on water, glycerol, formamide, or urea permeations. However, spheroplasts prepared from the strain overexpressing Fps1p showed enhanced glycerol uptake, suggesting that Fps1p becomes active only upon insertion in the plasma membrane.

Molecular and functional study of AQY1 from Saccharomyces cerevisiae: role of the C-terminal domain

The yeast YPR192w gene, which encodes a protein (Aqy1p) with strong homology to aquaporins (AQPs), was cloned from nine S. cerevisiae strains. The osmotic water permeability coefficient (Pf) of X. laevis oocytes expressing the gene cloned from the Sigma1278b strain (AQY1-1) was 5.7 times higher than the Pf of oocytes expressing the gene cloned from other strains (AQY1-2). Aqy1-1p, initially cloned without its C-terminus (Aqy1-1DeltaCp), mediated an approximately 3 times higher water permeability than the full-length protein. This corresponds to a 3-fold higher protein density in the oocyte plasma membrane, as shown by freeze-fracture electron microscopy. Pf measurements in yeast spheroplasts confirmed the presence of functional water channels in Sigma1278b and a pharmacological study indicated that this strain contains at least a second functional aquaporin.

Evidence for the presence of aquaporin-3 in human red blood cells

A facilitated diffusion for glycerol is present in human erythrocytes. Glycerol transporters identified to date belong to the Major Intrinsic Protein (MIP) family of integral membrane proteins, and one of them, aquaporin-3 (AQP3), has been characterized in mammals. Using an antibody raised against a peptide corresponding to the rat AQP3 carboxyl terminus, we examined the presence of AQP3 in normal and Colton-null (aquaporin-1 (AQP1)-deficient) human erythrocytes. Three immunoreactive bands were detected on immunoblots of both normal and Colton-null red cells, very similar to the bands revealed in rat kidney, a material in which AQP3 has been extensively studied. By immunofluorescence, anti-AQP3 antibodies stained the plasma membranes of both normal and Colton-null erythrocytes. Glycerol transport was measured on intact erythrocytes by stopped-flow light scattering and on one-step pink ghosts by a rapid filtration technique. Glycerol permeability values, similar in both cell types, suggest that AQP1 does not represent the major path for glycerol movement across red blood cell membranes. Furthermore, pharmacological studies showed that Colton-null red cells remain sensitive to water and glycerol flux inhibitors, supporting the idea that another proteinaceous path, probably AQP3, mediates most of the glycerol movements across red cell membranes and represents part of the residual water transport activity found in AQP1-deficient red cells.

Reconstitution of water channel function of aquaporins 1 and 2 by expression in yeast secretory vesicles

Aquaporins 1 (AQP1) and 2 (AQP2) were expressed in the yeast secretory mutant sec6-4. The mutant accumulates post-Golgi, plasma membrane-targeted vesicles and may be used to produce large quantities of membrane proteins. AQP1 or AQP2 were inducibly expressed in yeast and were localized within isolated sec6-4 vesicles by immunoblot analysis. Secretory vesicles containing AQP1 and AQP2 exhibited high water permeabilities and low activation energies for water flow, indicating expression of functional AQP1 and AQP2. AQP1 solubilized from secretory vesicles was successfully reconstituted into proteoliposomes, demonstrating the ability to use the yeast system to express aquaporins for reconstitution studies. The AQP2-containing secretory vesicles showed no increased permeability toward formamide, urea, glycerol, or protons compared with control vesicles, demonstrating that AQP2 is highly selective for water over these other substances. We conclude that the expression of aquaporins in yeast sec6 vesicles is a valid system to further study mammalian water channel function.

Purification and functional reconstitution of the human CHIP28 water channel expressed in Saccharomyces cerevisiae

The yeast Saccharomyces cerevisiae was used for heterologous expression of the human CHIP28 water Aquaporin-1 channel (Aquaporin-1). A nine-amino-acid epitope of the influenza hemagglutinin protein (HA epitope), recognized by the monoclonal antibody 12CA5, was chosen to tag CHIP28 at its N-terminus. Epitope-tagged CHIP28 was purified from yeast extracts by immunochromatography on protein A/ 12CA5-coupled beads, after KI extraction and detergent solubilization, then concentrated by anion exchange chromatography. Purified protein was reconstituted in proteoliposomes and was shown to function as a water channel by stopped-flow spectrophotometry. This study demonstrates that the yeast has the capacity to produce functional aquaporins at levels sufficient for biochemical and biophysical analyses.

Purified vesicles of tobacco cell vacuolar and plasma membranes exhibit dramatically different water permeability and water channel activity

The vacuolar membrane or tonoplast (TP) and the plasma membrane (PM) of tobacco suspension cells were purified by free-flow electrophoresis (FFE) and aqueous two-phase partitioning, with enrichment factors from a crude microsomal fraction of >/=4- to 5-fold and reduced contamination by other cellular membranes. For each purified fraction, the mean apparent diameter of membrane vesicles was determined by freeze-fracture electron microscopy, and the osmotic shrinking kinetics of the vesicles were characterized by stopped-flow light scattering. Osmotic water permeability coefficients (Pf) of 6.1 +/- 0.2 and 7.6 +/- 0.9 microm . s(-1) were deduced for PM-enriched vesicles purified by FFE and phase partitioning, respectively. The associated activation energies (Ea; 13.7 +/- 1.0 and 13.4 +/- 1.4 kcal . mol(-1), respectively) suggest that water transport in the purified PM occurs mostly by diffusion across the lipid matrix. In contrast, water transport in TP vesicles purified by FFE was characterized by (i) a 100-fold higher Pf of 690 +/- 35 microm . s(-1), (ii) a reduced Ea of 2.5 +/- 1.3 kcal . mol(-1), and (iii) a reversible inhibition by mercuric chloride, up to 83% at 1 mM. These results provide functional evidence for channel-mediated water transport in the TP, and more generally in a higher plant membrane. A high TP Pf suggests a role for the vacuole in buffering osmotic fluctuations occurring in the cytoplasm. Thus, the differential water permeabilities and water channel activities observed in the tobacco TP and PM point to an original osmoregulatory function for water channels in relation to the typical compartmentation of plant cells.

Very high single channel water permeability of aquaporin-4 in baculovirus-infected insect cells and liposomes reconstituted with purified aquaporin-4

The insect cell/baculovirus system was used to express the mercurial-insensitive water channel aquaporin-4 (AQP4) for purification and reconstitution. Immunoblot analysis of Sf9 cells infected with recombinant baculovirus showed greatest AQP4 expression at 72 h after infection at a multiplicity-of-infection of 5. Immunostaining and cell membrane fractionation indicated AQP4 plasma membrane expression. Quantitative immunoblot analysis showed approximately 60 microg of AQP4 per milligram of plasma membrane protein (approximately 2 mg of AQP4 protein per liter of Sf9 cell culture). Functional analysis by stopped-flow light scattering indicated that AQP4 functioned as a mercurial-insensitive water-selective transporter. Osmotic water permeability (Pf) in plasma membrane vesicles from AQP4-expressing Sf9 cells was very high (0.053 cm/s at 10 degrees C), weakly temperature dependent (activation energy, 4.5 kcal/mol), and not inhibited by HgCl2. The AQP4 single channel water permeability (p(f)), estimated from Pf and protein amount, was 19 x 10(-14) cm3/s. Purification of AQP4 to a single Coomassie blue-stained protein on SDS-PAGE (1300-fold over homogenate) was achieved by membrane fractionation, carbonate stripping of nonintegral proteins, solubilization in octyl-beta-glucoside, and anion exchange chromatography. AQP4 protein identity was confirmed by mass spectrometry. Reconstitution of purified AQP4 into proteoliposomes increased osmotic water permeability by >40-fold, giving a p(f) of 15 x 10(-14) cm3/s, remarkably greater than that of 4.9 x 10(-14) cm3/s measured in parallel for AQP1. These results establish the first purification of an aquaporin from a heterologous expression system. The high AQP4 p(f) suggests (a) significant functional differences among the aquaporins, (b) inadequacy of existing pore models to account for high water flow and water permselectivity, and (c) possible enhancement of water flow by AQP4 assembly in orthogonal arrays.

Expression of an Arabidopsis plasma membrane aquaporin in Dictyostelium results in hypoosmotic sensitivity and developmental abnormalities

The rd28 gene of Arabidopsis thaliana encodes a water channel protein, or aquaporin, of the plasma membrane. A construct in which transcription of the rd28 cDNA is controlled by the Dictyostelium actin15 promoter was transformed into Dictyostelium discoideum cells. Transformants contained RD28 protein in their plasma membranes. When shifted to a low-osmotic-strength buffer, cells expressing rd28 swelled rapidly and burst, indicating that the plant aquaporin allowed rapid water entry in the amoebae. The rate of osmotic lysis was a function of the osmotic pressure of the buffer. We also selected transformants in which the expression of the rd28 cDNA is driven by the promoter of the prespore cotB gene. These transformants accumulated rd28 mRNA uniquely in prespore cells. In low-osmotic-strength buffer, the cotB::rd28 cells aggregated and formed normally proportioned slugs but failed to form normal fruiting bodies. The number of spores was reduced 20-fold, and the stalks of the fruiting bodies were abnormally short. The consequences of expressing RD28 in prespore cells could be partially overcome by increasing the osmolarity of the medium. Under these conditions, the cotB::rd28 cells formed fruiting bodies of more normal appearance, and the number of viable spores increased slightly. Because prespore cells have to shrink and dehydrate to form spores, it was not unexpected that expression of an aquaporin would disrupt this process, but it was surprising to find that stalk differentiation was also affected by expression of rd28 in prespore cells. It appears that osmotic stress on prespore cells alters their ability to signal terminal differentiation in prestalk cells. The results provide independent confirmation that plant aquaporins can function in the cells of other organisms, and that D. discoideum can be used to study the properties of these water channels.

Functional cell surface expression of the anion transport domain of human red cell band 3 (AE1) in the yeast Saccharomyces cerevisiae

We expressed the 52-kDa integral membrane domain (B3mem) of the human erythrocyte anion transporter (band 3; AE1) in a protease-deficient strain of the yeast Saccharomyces cerevisiae under the control of the inducible GAL10-CYC1 promoter. Immunoblots of total protein from transformed yeast cells confirmed that the B3mem polypeptide was overexpressed shortly after induction with galactose. Cell surface expression of the functional anion transporter was detected by using a simple transport assay to measure stilbene disulfonate-inhibitable chloride influx into intact yeast cells. The B3mem polypeptide was recycled and degraded by the cells with a half-life of approximately 1-3 hr, which led to a steady-state level of expression in exponentially growing cultures. Our data suggest that 5-10% of total B3mem is functionally active at the cell surface at any one time and that overexpression of this anion transport protein does not interfere with cell growth or survival. This is one of only a few reports of the functional expression of a plasma membrane transport protein in the plasma membrane of yeast cells and to our knowledge is the first report of red cell band 3-mediated anion transport at the plasma membrane of cDNA-transformed cells. The cell surface expression system we describe will provide a simple means for future study of the functional properties of band 3 by using site-directed mutagenesis.