Functional characterization of a microbial aquaglyceroporin

Local Attraction of Substrates and Co-Substrates Enhances Weak Acid and Base Transmembrane Transport

The transmembrane transport of weak acid and base metabolites depends on the local pH conditions that affect the protonation status of the substrates and the availability of co-substrates, typically protons. Different protein designs ensure the attraction of substrates and co-substrates to the transporter entry sites. These include electrostatic surface charges on the transport proteins and complexation with seemingly transport-unrelated proteins that provide substrate and/or proton antenna, or enzymatically generate substrates in place. Such protein assemblies affect transport rates and directionality. The lipid membrane surface also collects and transfers protons. The complexity in the various systems enables adjustability and regulation in a given physiological or pathophysiological situation. This review describes experimentally shown principles in the attraction and facilitation of weak acid and base transport substrates, including monocarboxylates, ammonium, bicarbonate, and arsenite, plus protons as a co-substrate.

Aquaglyceroporin Modulators as Emergent Pharmacological Molecules for Human Diseases

Aquaglyceroporins, a sub-class of aquaporins that facilitate the diffusion of water, glycerol and other small uncharged solutes across cell membranes, have been recognized for their important role in human physiology and their involvement in multiple disorders, mostly related to disturbed energy homeostasis. Aquaglyceroporins dysfunction in a variety of pathological conditions highlighted their targeting as novel therapeutic strategies, boosting the search for potent and selective modulators with pharmacological properties. The identification of selective inhibitors with potential clinical applications has been challenging, relying on accurate assays to measure membrane glycerol permeability and validate effective functional blockers. Additionally, biologicals such as hormones and natural compounds have been revealed as alternative strategies to modulate aquaglyceroporins via their gene and protein expression. This review summarizes the current knowledge of aquaglyceroporins’ involvement in several pathologies and the experimental approaches used to evaluate glycerol permeability and aquaglyceroporin modulation. In addition, we provide an update on aquaglyceroporins modulators reported to impact disease, unveiling aquaglyceroporin pharmacological targeting as a promising approach for innovative therapeutics.

F0F1-ATPase Contributes to the Fluoride Tolerance and Cariogenicity of Streptococcus mutans

The phenotypic traits of Streptococcus mutans, such as fluoride tolerance, are usually associated with genotypic alterations. The aim of this study was to identify adaptive mutations of S. mutans to gradient fluoride concentrations and possible relationships between the mutations and fluoride tolerance. We identified a highly resistant S. mutans strain (FR1000) with a novel single nucleotide polymorphism (SNP, −36G→T) in the promoter region of F₀F₁-ATPase gene cluster (SMU_1527-SMU_1534) resistant to 1,000 ppm fluoride using the whole-genome Illumina PE250 sequencing. Thus, a −36G→T F₀F₁-ATPase promoter mutation from the parental strain S. mutans UA159 was constructed and named UA159-T. qRT-PCR showed that the F₀F₁-ATPase gene expression of both FR1000 and UA159-T was up-regulated, and fluoride tolerance of UA159-T was significantly improved. Complementation of Dicyclohexylcarbodiimide (DCCD), a specific inhibitor of F₀F₁-ATPase, increased fluoride susceptibility of FR1000 and UA159-T. Intracellular fluoride concentrations of fluoride tolerance strains were higher compared to UA159 strain as demonstrated by ¹⁸F analysis. Further validation with rat caries models showed that UA159-T caused more severe caries lesions under fluoride exposure compared with its parental UA159 strain. Overall, the identified −36G→T mutation in the promoter region of F₀F₁-ATPase gene drastically contributed to the fluoride tolerance and enhanced cariogenicity of S. mutans. These findings provided new insights into the mechanism of microbial fluoride tolerance, and suggested F₀F₁-ATPase as a potential target for suppressing fluoride resistant strains.

A generic high-throughput assay to detect aquaporin functional mutants: Potential application to discovery of aquaporin inhibitors

BACKGROUND

The discovery of stable, yet functional, protein mutants is a limiting factor in the development of biotechnological applications, structural studies or in drug discovery. Rapid detection of functional mutants is especially challenging for water channel aquaporins, as they do not have a directly measurable enzymatic or binding activity. Current methods available are time consuming and only applicable to specific aquaporins.

METHODS

Herein we describe an assay based on the protective effect of aquaporins on yeast S. cerevisiae in response to rapid freezing.

RESULTS

Yeast overexpressing a functional water-permeable aquaporin of choice are rescued after the challenge, while inactive or blocked aquaporins confer no protection and lead to cell death. The potential of this assay is shown by screening a small number of E. coli aquaporin Z (AQPZ) mutants. Additionally, a library of ~10,000 drug-like compounds was tested against human AQP1 (hAQP1).

CONCLUSIONS

Since rescue is only dependent on transmembrane water flux, the assay is applicable to water-permeable aquaporins of any origin.

GENERAL SIGNIFICANCE

Mapping of permissive mutations on the aquaporin structure can help delineate the minimal requirements for effective water transport. Alternatively, the assay can be potentially used to discover compounds that inhibit aquaporin water transport. When additionally screened for thermostability, functional aquaporin mutants can be useful in the development of biomimetic membranes for water purification, or to improve the likelihood of producing well-diffracting crystals, enabling rational design of much needed aquaporin inhibitors.

Identification and functional analysis of genome mutations in a fluoride-resistant Streptococcus mutans strain

It is known that fluoride-resistant microorganisms are different from fluoride-sensitive ones in growth, adherence and metabolic activity. It was hypothesized that these phenotypic differences were due to stable genotypic changes in the fluoride-resistant strains. However, until now, no studies have reported these genotypic changes. The aim of this study is to identify such changes in a fluoride-resistant Streptococcus mutans strain (C180-2FR) using whole-genome shotgun (WGS) sequencing and to examine the potential function of the identified mutations by comparing gene expression between the fluoride-sensitive (C180-2) and C180-2FR strains. We performed 50 bp paired-end Illumina shotgun sequencing for both strains. Through extensive bioinformatic analysis, we were able to identify 8 single nucleotide polymorphisms (SNPs) in the genome of C180-2FR, which were further confirmed by Sanger sequencing. Expression of the genes containing or in proximity to the SNPs in C180-2 and C180-2FR was then quantified by real-time PCR. A gene cluster containing genes coding for fluoride antiporters was up-regulated 10-fold in C180-2FR when compared to that in C180-2, independent of growth phase. Two SNPs are located in this gene cluster, one in its promoter region and the other in its protein-coding region. In addition, one gene, which codes for a putative glycerol uptake facilitator protein, was found to be down-regulated by 60% in C180-2FR at an early growth phase. The promoter region of this gene contained a SNP. No difference in expression was found for the other SNP-containing genes. In summary, using WGS sequencing, we were able to uncover genetic changes in the genome of a fluoride-resistant strain. These findings can provide new insights into the mechanism of microbial fluoride resistance.

Channel-mediated lactic acid transport: a novel function for aquaglyceroporins in bacteria

MIPs (major intrinsic proteins), also known as aquaporins, are membrane proteins that channel water and/or uncharged solutes across membranes in all kingdoms of life. Considering the enormous number of different bacteria on earth, functional information on bacterial MIPs is scarce. In the present study, six MIPs [glpF1 (glycerol facilitator 1)–glpF6] were identified in the genome of the Gram-positive lactic acid bacterium Lactobacillus plantarum. Heterologous expression in Xenopus laevis oocytes revealed that GlpF2, GlpF3 and GlpF4 each facilitated the transmembrane diffusion of water, dihydroxyacetone and glycerol. As several lactic acid bacteria have GlpFs in their lactate racemization operon (GlpF1/F4 phylogenetic group), their ability to transport this organic acid was tested. Both GlpF1 and GlpF4 facilitated the diffusion of D/L-lactic acid. Deletion of glpF1 and/or glpF4 in Lb. plantarum showed that both genes were involved in the racemization of lactic acid and, in addition, the double glpF1 glpF4 mutant showed a growth delay under conditions of mild lactic acid stress. This provides further evidence that GlpFs contribute to lactic acid metabolism in this species. This lactic acid transport capacity was shown to be conserved in the GlpF1/F4 group of Lactobacillales. In conclusion, we have functionally analysed the largest set of bacterial MIPs and demonstrated that the lactic acid membrane permeability of bacteria can be regulated by aquaglyceroporins.

Aquaporin AqpZ is involved in cell volume regulation and sensitivity to osmotic stress in Synechocystis sp. strain PCC 6803

The moderately halotolerant cyanobacterium Synechocystis sp. strain PCC 6803 contains a plasma membrane aquaporin, AqpZ. We previously reported that AqpZ plays a role in glucose metabolism under photomixotrophic growth conditions, suggesting involvement of AqpZ in cytosolic osmolarity homeostasis. To further elucidate the physiological role of AqpZ, we have studied its gene expression profile and its function in Synechocystis. The expression level of aqpZ was regulated by the circadian clock. AqpZ activity was insensitive to mercury in Xenopus oocytes and in Synechocystis, indicating that the AqpZ can be categorized as a mercury-insensitive aquaporin. Stopped-flow light-scattering spectrophotometry showed that addition of sorbitol and NaCl led to a slower decrease in cell volume of the Synechocystis ΔaqpZ strain than the wild type. The ΔaqpZ cells were more tolerant to hyperosmotic shock by sorbitol than the wild type. Consistent with this, recovery of oxygen evolution after a hyperosmotic shock by sorbitol was faster in the ΔaqpZ strain than in the wild type. In contrast, NaCl stress had only a small effect on oxygen evolution. The amount of AqpZ protein remained unchanged by the addition of sorbitol but decreased after addition of NaCl. This decrease is likely to be a mechanism to alleviate the effects of high salinity on the cells. Our results indicate that Synechocystis AqpZ functions as a water transport system that responds to daily oscillations of intracellular osmolarity.

MIPDB: a relational database dedicated to MIP family proteins

BACKGROUND INFORMATION

The MIPs (major intrinsic proteins) constitute a large family of membrane proteins that facilitate the passive transport of water and small neutral solutes across cell membranes. Since water is the most abundant molecule in all living organisms, the discovery of selective water-transporting channels called AQPs (aquaporins) has led to new knowledge on both the physiological and molecular mechanisms of membrane permeability. The MIPs are identified in Archaea, Bacteria and Eukaryota, and the rapid accumulation of new sequences in the database provides an opportunity for large-scale analysis, to identify functional and/or structural signatures or to infer evolutionary relationships. To help perform such an analysis, we have developed MIPDB (database for MIP proteins), a relational database dedicated to members of the MIP family.

RESULTS

MIPDB is a motif-oriented database that integrates data on 785 MIP proteins from more than 200 organisms and contains 230 distinct sequence motifs. MIPDB proposes the classification of MIP proteins into three functional subgroups: AQPs, glycerol-uptake facilitators and aquaglyceroporins. Plant MIPs are classified into three specific subgroups according to their subcellular distribution in the plasma membrane, tonoplast or the symbiosome membrane. Some motifs of the database are highly selective and can be used to predict the transport function or subcellular localization of unknown MIP proteins.

CONCLUSIONS

MIPDB offers a user-friendly and intuitive interface for a rapid and easy access to MIP resources and to sequence analysis tools. MIPDB is a web application, publicly accessible at http://idefix.univ-rennes1.fr:8080/Prot/index.html.

Pore selectivity analysis of an aquaglyceroporin by stopped-flow spectrophotometry on bacterial cell suspensions

Background information. Transport of water and small neutral solutes across plasma membranes is facilitated by AQP (aquaporin) and aquaglyceroporin channels, which belong to the MIP (major intrinsic protein) family. So far, more than 800 MIP proteins have been identified on the basis of sequence homology, but only less than 10% of them have been functionally characterized. In most studies, the channel properties of MIP proteins have been determined by using Xenopus oocyte swelling assays or stopped-flow spectrophotometry on proteoliposomes. As both methods sometimes present disadvantages, we developed an alternative method for analysing MIP function.Results. The kinetics of plasmolysis or deplasmolysis of Escherichia coli cells in suspension, in response to osmotic challenges, was analysed by stopped-flow spectrophotometry. Cytoplasmic volume variations were monitored either by GFP (green fluorescent protein) fluorescence quenching or by 90 degrees scattered light. The single exponential response to up-shocks in the impermeant solute mannitol was strongly accelerated when the cells expressed the native E. coli AQP AqpZ (rate constant 37.24 versus 3.05 s(-1) for control cells). The responses to hyperosmotic shocks realized with glycerol were biphasic. First, a light-scattering increase corresponded to cell plasmolysis. Secondly, deplasmolysis occurred when glycerol entered into the cell. Both phases were accelerated when the aquaglyceroporin GlpF was present in cell membranes. We concluded that the behaviour of MIP-expressing bacteria in the stopped-flow system was qualitatively identical with that reported for MIP-expressing oocytes or MIP-containing proteoliposomes. We then used this system to analyse the effects of mutations in the pore constriction of Gla(Llac), the aquaglyceroporin from Lactococcus lactis. In the present study, we show that Gla(Llac) loses its ability to transport glycerol but retains its ability to transport water when Val(223) was replaced by a histidine, the residue at the equivalent position in strict AQPs.Conclusions. These results show that stopped-flow spectrophotometry performed on E. coli cell suspensions is a useful experimental system to analyse the selectivity of wild-type or mutant MIP proteins and that a bifunctional aquaglyceroporin switches to an AQP by a single amino acid mutation in the pore constriction.

Molecular dynamics study of the archaeal aquaporin AqpM

Background

Aquaporins are a large family of transmembrane channel proteins that are present throughout all domains of life and are implicated in human disorders. These channels, allow the passive but selective movement of water and other small neutral solutes across cell membranes. Aquaporins have been classified into two sub-families: i) strict aquaporins that only allow the passage of water and ii) the less selective aquaglyceroporins that transport water and other neutral solutes, such as glycerol, CO₂ or urea. Recently, the identification and characterization of a number of archaeal and bacterial aquaporins suggested the existence of a third sub-family; one that is neither a strict aquaporin nor an aquaglyceroporin. The function and phylogeny of this third family is still a matter of debate.

Results

Twenty nanosecond molecular dynamics (MD) simulation of a fully hydrated tetramer of AqpM embedded in a lipid bilayer permitted predictions to be made of key biophysical parameters including: single channel osmotic permeability constant (p_f), single channel diffusive permeability constant (p_d), channel radius, potential water occupancy of the channel and water orientation inside the pore. These properties were compared with those of well characterized representatives of the two main aquaporin sub-families. Results show that changes in the amino acid composition of the aromatic/arginine region affect the size and polarity of the selectivity filter (SF) and could help explain the difference in water permeability between aquaporins. In addition, MD simulation results suggest that AqpM combines characteristics of strict aquaporins, such as the narrow SF and channel radius, with those of aquaglyceroporins, such as a more hydrophobic and less polar SF.

Conclusions

MD simulations of AqpM extend previous evidence that this archaeal aquaporin exhibits hybrid features intermediate between the two known aquaporin sub-families, supporting the idea that it may constitute a member of a novel class of aquaporins.

Characterization of multiprotein complexes of the Borrelia burgdorferi outer membrane vesicles

Among bacterial cell envelopes, the Borrelia burgdorferi outer membrane (OM) is structurally unique in that the identities of many protein complexes remain unknown; however, their characterization is the first step toward our understanding of membrane protein interactions and potential functions. Here, we used two-dimensional blue native/SDS-PAGE/mass spectrometric analysis for a global characterization of protein-protein interactions as well as to identify protein complexes in OM vesicles isolated from multiple infectious sensu stricto isolates of B. burgdorferi. Although we uncovered the existence of at least 10 distinct OM complexes harboring several unique subunits, the complexome is dominated by the frequent occurrence of a limited diversity of membrane proteins, most notably P13, outer surface protein (Osp) A, -B, -C, and -D and Lp6.6. The occurrence of these complexes and specificity of subunit interaction were further supported by independent two-dimensional immunoblotting and coimmunoprecipitation assays as well as by mutagenesis studies, where targeted depletion of a subunit member (P66) selectively abolished a specific complex. Although a comparable profile of the OM complexome was detected in two major infectious isolates, such as B31 and 297, certain complexes are likely to occur in an isolate-specific manner. Further assessment of protein complexes in multiple Osp-deficient isolates showed loss of several protein complexes but revealed the existence of additional complex/subunits that are undetectable in wild-type cells. Together, these observations uncovered borrelial antigens involved in membrane protein interactions. The study also suggests that the assembly process of OM complexes is specific and that the core or stabilizing subunits vary between complexes. Further characterization of these protein complexes including elucidation of their biological significance may shed new light on the mechanism of pathogen persistence and the development of preventative measures against the infection.

Plasma membrane aquaporin AqpZ protein is essential for glucose metabolism during photomixotrophic growth of Synechocystis sp. PCC 6803

The genome of Synechocystis PCC 6803 contains a single gene encoding an aquaporin, aqpZ. The AqpZ protein functioned as a water-permeable channel in the plasma membrane. However, the physiological importance of AqpZ in Synechocystis remains unclear. We found that growth in glucose-containing medium inhibited proper division of ΔaqpZ cells and led to cell death. Deletion of a gene encoding a glucose transporter in the ΔaqpZ background alleviated the glucose-mediated growth inhibition of the ΔaqpZ cells. The ΔaqpZ cells swelled more than the wild type after the addition of glucose, suggesting an increase in cytosolic osmolarity. This was accompanied by a down-regulation of the pentose phosphate pathway and concurrent glycogen accumulation. Metabolite profiling by GC/TOF-MS of wild-type and ΔaqpZ cells revealed a relative decrease of intermediates of the tricarboxylic acid cycle and certain amino acids in the mutant. The changed levels of metabolites may have been the cause for the observed decrease in growth rate of the ΔaqpZ cells along with decreased PSII activity at pH values ranging from 7.5 to 8.5. A mutant in sll1961, encoding a putative transcription factor, and a Δhik31 mutant, lacking a putative glucose-sensing kinase, both exhibited higher glucose sensitivity than the ΔaqpZ cells. Examination of protein expression indicated that sll1961 functioned as a positive regulator of aqpZ gene expression but not as the only regulator. Overall, the ΔaqpZ cells showed defects in macronutrient metabolism, pH homeostasis, and cell division under photomixotrophic conditions, consistent with an essential role of AqpZ in glucose metabolism.

Algal MIPs, high diversity and conserved motifs

Background

Major intrinsic proteins (MIPs) also named aquaporins form channels facilitating the passive transport of water and other small polar molecules across membranes. MIPs are particularly abundant and diverse in terrestrial plants but little is known about their evolutionary history. In an attempt to investigate the origin of the plant MIP subfamilies, genomes of chlorophyte algae, the sister group of charophyte algae and land plants, were searched for MIP encoding genes.

Results

A total of 22 MIPs were identified in the nine analysed genomes and phylogenetic analyses classified them into seven subfamilies. Two of these, Plasma membrane Intrinsic Proteins (PIPs) and GlpF-like Intrinsic Proteins (GIPs), are also present in land plants and divergence dating support a common origin of these algal and land plant MIPs, predating the evolution of terrestrial plants. The subfamilies unique to algae were named MIPA to MIPE to facilitate the use of a common nomenclature for plant MIPs reflecting phylogenetically stable groups. All of the investigated genomes contained at least one MIP gene but only a few species encoded MIPs belonging to more than one subfamily.

Conclusions

Our results suggest that at least two of the seven subfamilies found in land plants were present already in an algal ancestor. The total variation of MIPs and the number of different subfamilies in chlorophyte algae is likely to be even higher than that found in land plants. Our analyses indicate that genetic exchanges between several of the algal subfamilies have occurred. The PIP1 and PIP2 groups and the Ca²⁺ gating appear to be specific to land plants whereas the pH gating is a more ancient characteristic shared by all PIPs. Further studies are needed to discern the function of the algal specific subfamilies MIPA-E and to fully understand the evolutionary relationship of algal and terrestrial plant MIPs.

Functional characterization and hyperosmotic regulation of aquaporin in Synechocystis sp. PCC 6803

The genome of the cyanobacterium Synechocystis sp. PCC 6803 (hereafter, Synechocystis) contains an aqpZ gene (slr2057) which encodes an aquaporin (SsAqpZ), a membrane channel protein that might play a role in osmotic water transport and therefore the growth of Synechocystis. Structural characterization of SsAqpZ by protein sequence analysis and homology modelling revealed that it was more similar to bacterial aquaporin Z than the glycerol facilitator. To understand the functional role of SsAqpZ, the aqpZ knockout (KO) and myc-tagged aqpZ knockin (KI) Synechocystis were constructed. Water channel activity assays showed that SsAqpZ facilitated water transportation. SsAqpZ-mediated changes in cell volume were observed in wild-type (WT) and KI Synechocystis. Expression of SsAqpZ in KI Synechocystis was induced by extracellular hyperosmolarity. In the absence of hyperosmolarity, WT, KO and KI Synechocystis showed the same pattern of growth and no morphological or phenotypical perturbations. Under hyperosmotic condition, while the WT and also KI cells maintained a similar growth rate throughout the entire exponential phase, KO cells grew significantly slower. These results indicate that SsAqpZ has water channel activity and is involved in the adaptation and maintenance of growth of Synechocystis in a hyperosmotic environment.

Molecular and functional characterization of catfish (Heteropneustes fossilis) aquaporin-1b: changes in expression during ovarian development and hormone-induced follicular maturation

The oocytes of the freshwater catfish Heteropneustes fossilis hydrate during hormone-induced meiotic maturation. To investigate if this process may be mediated by aquaporins (AQPs), as it occurs in marine fish producing highly hydrated eggs, the cloning of ovarian AQPs in catfish was carried out. Using degenerate primers for conserved domains of the major intrinsic protein (MIP) family, and 5' and 3'end amplification procedures, a full-length cDNA encoding for an AQP1-like protein was isolated. The predicted protein showed the typical six transmembrane domains and two Asn-Pro-Ala (NPA) motifs conserved among the members of the AQP superfamily. Phylogenetic analysis indicated that the catfish AQP clustered with the teleost-specific aquaporin-1b subfamily, and accordingly it was termed HfAqp1b. Heterologous expression in Xenopus laevis oocytes indicated that HfAqp1b encoded for a functional AQP, water permeability being enhanced by cAMP. Site-directed mutagenesis revealed that cAMP induced the translocation of HfAqp1b into the oocyte plasma membrane most likely through the phosphorylation of HfAqp1b Ser(227). In adult catfish, hfaqp1b transcripts were detected exclusively in ovary and brain and showed significant seasonal variations; in the ovary, hfaqp1b was maximally expressed during the pre-spawning period, whereas in the brain the highest expression was detected during spawning. In vitro stimulation of isolated catfish ovarian follicles with vasotocin (VT) or human chorionic gonadotropin (hCG), which induce oocyte maturation and hydration, elevated the hfaqp1b transcript levels after 6 or 16 h of incubation, respectively. These results suggest that HfAqp1b may play a role during VT- and hCG-induced oocyte hydration in catfish, and that VT may regulate HfAqp1b at the transcriptional and post-translational level in a manner similar to the vasopressin-dependent mammalian AQP2.

Identification of the family of aquaporin genes and their expression in upland cotton (Gossypium hirsutum L.)

BACKGROUND

Cotton (Gossypium spp.) is produced in over 30 countries and represents the most important natural fiber in the world. One of the primary factors affecting both the quantity and quality of cotton production is water. A major facilitator of water movement through cell membranes of cotton and other plants are the aquaporin proteins. Aquaporin proteins are present as diverse forms in plants, where they function as transport systems for water and other small molecules. The plant aquaporins belong to the large major intrinsic protein (MIP) family. In higher plants, they consist of five subfamilies including plasma membrane intrinsic proteins (PIP), tonoplast intrinsic proteins (TIP), NOD26-like intrinsic proteins (NIP), small basic intrinsic proteins (SIP), and the recently discovered X intrinsic proteins (XIP). Although a great deal is known about aquaporins in plants, very little is known in cotton.

RESULTS

From a molecular cloning effort, together with a bioinformatic homology search, 71 upland cotton (G. hirsutum) aquaporin genes were identified. The cotton aquaporins consist of 28 PIP and 23 TIP members with high sequence similarity. We also identified 12 NIP and 7 SIP members that showed more divergence. In addition, one XIP member was identified that formed a distinct 5th subfamily. To explore the physiological roles of these aquaporin genes in cotton, expression analyses were performed for a select set of aquaporin genes from each subfamily using semi-quantitative reverse transcription (RT)-PCR. Our results suggest that many cotton aquaporin genes have high sequence similarity and diverse roles as evidenced by analysis of sequences and their expression.

CONCLUSION

This study presents a comprehensive identification of 71 cotton aquaporin genes. Phylogenetic analysis of amino acid sequences divided the large and highly similar multi-gene family into the known 5 aquaporin subfamilies. Together with expression and bioinformatic analyses, our results support the idea that the genes identified in this study represent an important genetic resource providing potential targets to modify the water use properties of cotton.

Aquaporin biology and nervous system

Our understanding of the movement of water through cell membranes has been greatly advanced by the discovery of a family of water-specific, membrane-channel proteins: the Aquaporins (AQPs). These proteins are present in organisms at all levels of life, and their unique permeability characteristics and distribution in numerous tissues indicate diverse roles in the regulation of water homeostasis.Phenotype analysis of AQP knock-out mice has confirmed the predicted role of AQPs in osmotically driven transepithelial fluid transport, as occurs in the urinary concentrating mechanism and glandular fluid secretion. Regarding their expression in nervous system, there are evidences suggesting that AQPs are differentially expressed in the peripheral versus central nervous system and that channel-mediated water transport mechanisms may be involved in cerebrospinal fluid formation, neuronal signal transduction and information processing.Moreover, a number of recent studies have revealed the importance of mammalian AQPs in both physiological and pathophysiological mechanisms and have suggested that pharmacological modulation of AQP expression and activity may provide new tools for the treatment of variety of human disorders in which water and small solute transport may be involved. For all the AQPs, new contributions to physiological functions are likely to be discovered with ongoing work in this rapidly expanding field of research.

Expression and functional characterization of four aquaporin water channels from the European eel (Anguilla anguilla)

The European eel is a euryhaline teleost which has been shown to differentially up- and downregulate aquaporin (AQP) water channels in response to changes in environmental salinity. We have characterized the transport properties of four aquaporins localized to osmoregulatory organs - gill, esophagus, intestine and kidney. By sequence comparison these four AQP orthologs resemble human AQP1 (eel AQP1), AQP3 (eel AQP3) and AQP10 (AQPe). The fourth member is a duplicate form of AQP1 (AQP1dup) thought to arise from a duplication of the teleost genome. Using heterologous expression in Xenopus oocytes we demonstrate that all four eel orthologs transport water and are mercury inhibitable. Eel AQP3 and AQPe also transport urea and glycerol, making them aquaglyceroporins. Eel AQP3 is dramatically inhibited by extracellular acidity (91% and 69% inhibition of water and glycerol transport respectively at pH 6.5) consistent with channel gating by protons. Maximal water flux of eel AQP3 occurred around pH 8.2 - close to the physiological pH of plasma in the eel. Exposure of AQP-expressing oocytes to heavy metals revealed that eel AQP3 is highly sensitive to extracellular nickel and zinc (88.3% and 86.3% inhibition, respectively) but less sensitive to copper (56.4% inhibition). Surprisingly, copper had a stimulatory effect on eel AQP1 (153.7% activity of control). Copper, nickel and zinc did not affect AQP1dup or AQPe. We establish that all four eel AQP orthologs have similar transport profiles to their human counterparts, with eel AQP3 exhibiting some differences in its sensitivity to metals. This is the first investigation of the transport properties and inhibitor sensitivity of salinity-regulated aquaporins from a euryhaline species. Our results indicate a need to further investigate the deleterious effects of metal pollutants on AQP-containing epithelial cells of the gill and gastrointestinal tract at environmentally appropriate concentrations.

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.

The PIP and TIP aquaporins in wheat form a large and diverse family with unique gene structures and functionally important features

Aquaporins, members of major intrinsic proteins (MIPs), transport water across cellular membranes and play vital roles in all organisms. Adversities such as drought, salinity, or chilling affect water uptake and transport, and numerous plant MIPs are reported to be differentially regulated under such stresses. However, MIP genes have been not yet been characterized in wheat, the largest cereal crop. We have identified 24 PIP and 11 TIP aquaporin genes from wheat by gene isolation and database searches. They vary extensively in lengths, numbers, and sequences of exons and introns, and sequences and cellular locations of predicted proteins, but the intron positions (if present) are characteristic. The putative PIP proteins show a high degree of conservation of signature sequences or residues for membrane integration, water transport, and regulation. The TIPs are more diverse, some with potential for water transport and others with various selectivity filters including a new combination. Most genes appear to be expressed as expressed sequence tags, while two are likely pseudogenes. Many of the genes are highly identical to rice but some are unique, and many correspond to genes that show differential expression under salinity and/or drought. The results provide extensive information for functional studies and developing markers for stress tolerance.

How phosphotransferase system-related protein phosphorylation regulates carbohydrate metabolism in bacteria

The phosphoenolpyruvate(PEP):carbohydrate phosphotransferase system (PTS) is found only in bacteria, where it catalyzes the transport and phosphorylation of numerous monosaccharides, disaccharides, amino sugars, polyols, and other sugar derivatives. To carry out its catalytic function in sugar transport and phosphorylation, the PTS uses PEP as an energy source and phosphoryl donor. The phosphoryl group of PEP is usually transferred via four distinct proteins (domains) to the transported sugar bound to the respective membrane component(s) (EIIC and EIID) of the PTS. The organization of the PTS as a four-step phosphoryl transfer system, in which all P derivatives exhibit similar energy (phosphorylation occurs at histidyl or cysteyl residues), is surprising, as a single protein (or domain) coupling energy transfer and sugar phosphorylation would be sufficient for PTS function. A possible explanation for the complexity of the PTS was provided by the discovery that the PTS also carries out numerous regulatory functions. Depending on their phosphorylation state, the four proteins (domains) forming the PTS phosphorylation cascade (EI, HPr, EIIA, and EIIB) can phosphorylate or interact with numerous non-PTS proteins and thereby regulate their activity. In addition, in certain bacteria, one of the PTS components (HPr) is phosphorylated by ATP at a seryl residue, which increases the complexity of PTS-mediated regulation. In this review, we try to summarize the known protein phosphorylation-related regulatory functions of the PTS. As we shall see, the PTS regulation network not only controls carbohydrate uptake and metabolism but also interferes with the utilization of nitrogen and phosphorus and the virulence of certain pathogens.

Why do microorganisms have aquaporins?

Aquaporins are channel proteins that enhance the permeability of cell membranes for water. They have been found in Bacteria, Archaea and Eukaryotes. However, their absence in many microorganisms suggests that aquaporins do not fulfill a broad role such as turgor regulation or osmoadaptation but, instead, fulfill a role that enables microorganisms to have specific lifestyles. The recent discovery that aquaporins enhance cellular tolerance against rapid freezing suggests that they have ecological relevance. We have identified several examples of large-scale freeze-thawing of microbes in nature and we also draw attention to alternative lifestyle-related functions for aquaporins, which will be a focus of future research.

Heterologous Aquaporin (AQY2–1) Expression Strongly Enhances Freeze Tolerance of Schizosaccharomyces pombe

Aquaporin membrane proteins enable the transport of water across membranes in various organisms. In yeast their expression has been shown to correlate strongly with freeze tolerance. When we analyzed the freeze tolerance of Schizosaccharomyces pombe, an organism whose genome sequence has revealed no genes encoding a bona fide water channel, we found very low intrinsic freeze tolerance compared to other yeast species with aquaporin-encoding genes. Deletion of Spac977.17, which encodes a putative glycerol facilitator, resulted in no significant differences in freeze tolerance with its corresponding wild-type strain in all growth conditions tested. However, when we expressed the Saccharomyces cerevisiae aquaporin-encoding gene AQY2-1 in S. pombe cells, we found that the relatively low freeze tolerance of S. pombe could be significantly enhanced. Therefore, (i) the absence of a bona fide water channel in S. pombe might provide in part an explanation for its overall low freeze tolerance compared to other yeast species, and (ii) aquaporin overexpression might be a tool to improve cryopreservation of many other cell types as well, as has recently been shown for mouse oocytes and fish embryos.

A novel plant major intrinsic protein in Physcomitrella patens most similar to bacterial glycerol channels

A gene encoding a novel fifth type of major intrinsic protein (MIP) in plants has been identified in the moss Physcomitrella patens. Phylogenetic analyses show that this protein, GlpF-like intrinsic protein (GIP1;1), is closely related to a subclass of glycerol transporters in bacteria that in addition to glycerol are highly permeable to water. A likely explanation of the occurrence of this bacterial-like MIP in P. patens is horizontal gene transfer. The expressed P. patens GIP1;1 gene contains five introns and encodes a unique C-loop extension of approximately 110 amino acid residues that has no obvious similarity with any other known protein. Based on alignments and structural comparisons with other MIPs, GIP1;1 is suggested to have retained the permeability for glycerol but not for water. Studies on heterologously expressed GIP1;1 in Xenopus laevis oocytes confirm the predicted substrate specificity. Interestingly, proteins of one of the plant-specific subgroups of MIPs, the NOD26-like intrinsic proteins, are also facilitating the transport of glycerol and have previously been suggested to have evolved from a horizontally transferred bacterial gene. Further studies on localization and searches for GIP1;1 homologs in other plants will clarify the function and significance of this new plant MIP.

Molecular mechanisms and drug development in aquaporin water channel diseases: aquaporin superfamily (superaquaporins): expansion of aquaporins restricted to multicellular organisms

Eleven aquaporins are identified in mammals. They all have highly conserved two asparagine-proline-alanine (NPA) boxes that are important for the formation of the water permeating pore. Recently we identified two new proteins in mammals distantly related to aquaporins (AQPs) with poorly conserved NPA boxes. Similarly poorly conserved AQP-like proteins were found in several genome projects of multicellular organisms. They may be subgrouped as the AQP superfamily or superaquaporins. Their function is still unknown. AQP11 knockout mice suffer from polycystic kidneys and neonatal fatality. The identification of superaquaporins will provide new insights into the role of AQPs in organogenesis.

Automated selection of positions determining functional specificity of proteins by comparative analysis of orthologous groups in protein families

The increasing volume of genomic data opens new possibilities for analysis of protein function. We introduce a method for automated selection of residues that determine the functional specificity of proteins with a common general function (the specificity-determining positions [SDP] prediction method). Such residues are assumed to be conserved within groups of orthologs (that may be assumed to have the same specificity) and to vary between paralogs. Thus, considering a multiple sequence alignment of a protein family divided into orthologous groups, one can select positions where the distribution of amino acids correlates with this division. Unlike previously published techniques, the introduced method directly takes into account nonuniformity of amino acid substitution frequencies. In addition, it does not require setting arbitrary thresholds. Instead, a formal procedure for threshold selection using the Bernoulli estimator is implemented. We tested the SDP prediction method on the LacI family of bacterial transcription factors and a sample of bacterial water and glycerol transporters belonging to the major intrinsic protein (MIP) family. In both cases, the comparison with available experimental and structural data strongly supported our predictions.

The aquaporin gene aqpX of Brucella abortus is induced in hyperosmotic conditions

An aquaporin gene (aqpX) was previously detected in the pathogenic bacterium Brucella abortus. Earlier studies showed that AqpX mediated rapid and large water fluxes in both directions in response to sudden osmotic up- or downshifts. Here, to study the role and the expression of the aqpX gene in B. abortus, an aqpX null mutant was constructed using an aqpX : : lacZ gene fusion. This mutant showed no significant difference in growth rate compared to the wild-type strain when grown in rich and minimal media, demonstrating that disruption of the aqpX gene was not lethal for B. abortus. The role of the B. abortus AqpX water channel was investigated by exposing the cells to hypo- and hyperosmolar conditions. While in hyperosmolar environments the growth rate of the knockout mutant was not affected, in hypo-osmolar conditions this mutant showed reduced viability after 50 h of growth. beta-Galactosidase assays and RT-PCR revealed that aqpX gene expression and the amount of aqpX mRNA were markedly increased in hyperosmolar conditions. Moreover, B. abortus aqpX expression levels were enhanced during the mid-exponential phase of growth. These results indicated that the expression of aqpX was regulated during the growth curve and induced in hyperosmolar conditions. This report is believed to be the first example of the induction of a bacterial aquaporin in hypertonic conditions.

Functional expression and characterization of an archaeal aquaporin. AqpM from methanothermobacter marburgensis

Researchers have described aquaporin water channels from diverse eubacterial and eukaryotic species but not from the third division of life, Archaea. Methanothermobacter marburgensis is a methanogenic archaeon that thrives under anaerobic conditions at 65 degrees C. After transfer to hypertonic media, M. marburgensis sustained cytoplasmic shrinkage that could be prevented with HgCl(2). We amplified aqpM by PCR from M. marburgensis DNA. Like known aquaporins, the open reading frame of aqpM encodes two tandem repeats each containing three membrane-spanning domains and a pore-forming loop with the signature motif Asn-Pro-Ala (NPA). Unlike other known homologs, the putative Hg(2+)-sensitive cysteine was found proximal to the first NPA motif in AqpM, rather than the second. Moreover, amino acids distinguishing water-selective homologs from glycerol-transporting homologs were not conserved in AqpM. A fusion protein, 10-His-AqpM, was expressed and purified from Escherichia coli. AqpM reconstituted into proteoliposomes was shown by stopped-flow light scattering assays to have elevated osmotic water permeability (P(f) = 57 microm x s(-1) versus 12 microm x s(-1) of control liposomes) that was reversibly inhibited with HgCl(2). Transient, initial glycerol permeability was also detected. AqpM remained functional after incubations at temperatures above 80 degrees C and formed SDS-stable tetramers. Our studies of archaeal AqpM demonstrate the ubiquity of aquaporins in nature and provide new insight into protein structure and transport selectivity.

Aquaglyceroporins, one channel for two molecules

In the light of the recently published structure of GlpF and AQP1, we have analysed the nature of the residues which could be involved in the formation of the selectivity filter of aquaporins, glycerol facilitators and aquaglyceroporins. We demonstrate that the functional specificity for major intrinsic protein (MIP) channels can be explained on one side by analysing the polar environment of the residues that form the selective filter. On the other side, we show that the channel selectivity could be associated with the oligomeric state of the membrane protein. We conclude that a non-polar environment in the vicinity of the top of helix 5 could allow aquaglyceroporins and GlpF to exist as monomers within the hydrophobic environment of the membrane.