Structural characterization of two aquaporins isolated from native spinach leaf plasma membranes

Tissue-specific accumulation of PIP aquaporins of a particular heteromeric composition is part of the maize response to mycorrhiza and drought

The systemic coordination of accumulation of plasma membrane aquaporins (PIP) was investigated in this study in relation to mycorrhized maize response to a rapid development of severe drought followed by rewatering. In non-mycorrhizal roots, drought led to a drop in PIP abundance, followed by a transient increase under rewatering, whereas leaves showed an opposite pattern. In contrast, mycorrhiza contributed to maintenance of high and stable levels of PIPs in both plant organs after an initial increase, prolonged over the irrigation period. Isoelectric focusing electrophoresis resolved up to 13 aquaporin complexes with highly reproducible pl positions across leaf and root samples, symbiotic and non-symbiotic, stressed or not. Mass spectrometry recognized in leaves and roots a different ratio of PIP1 and PIP2 subunits within 2D spots that accumulated the most. Regardless of symbiotic status, drought regulation of aquaporins in roots was manifested as the prevalence of complexes that comprise almost exclusively PIP2 monomers. In contrast, the leaf response involved enrichment in PIP1s. PIP1s are thought to enhance water transport, facilitate CO2 diffusion but also affect stomatal movements. These features, together with elevated aquaporin levels, might explain a stress tolerance mechanism observed in mycorrhizal plants, resulting in faster recovery of stomatal water conductance and CO2 assimilation rate after drought.

Brassica oleracea L. var. italica Aquaporin Reconstituted Proteoliposomes as Nanosystems for Resveratrol Encapsulation

Aquaporins (AQPs), membrane proteins responsible for facilitating water transport, found in plant membrane vesicles (MV), have been related to the functionality and stability of MV. We focused on AQPs obtained from broccoli, as they show potential for biotechnological applications. To gain further insight into the role of AQPs in MV, we describe the heterologous overexpression of two broccoli AQPs (BoPIP1;2 and BoPIP2;2) in Pichia pastoris, resulting in their purification with high yield (0.14 and 0.99 mg per gram cells for BoPIP1;2 and BoPIP2;2). We reconstituted AQPs in liposomes to study their functionality, and the size of proteoliposomes did not change concerning liposomes. BoPIP2;2 facilitated water transport, which was preserved for seven days at 4 °C and at room temperature but not at 37 °C. BoPIP2;2 was incorporated into liposomes to encapsulate a resveratrol extract, resulting in increased entrapment efficiency (EE) compared to conventional liposomes. Molecular docking was utilized to identify binding sites in PIP2s for resveratrol, highlighting the role of aquaporins in the improved EE. Moreover, interactions between plant AQP and human integrin were shown, which may increase internalization by the human target cells. Our results suggest AQP-based alternative encapsulation systems can be used in specifically targeted biotechnological applications.

Aquaporin-mediated transport: Insights into metalloid trafficking

Metalloids in plants have diverse physiological effects. From being essential to beneficial to toxic, they have significant effects on many physiological processes, influencing crop yield and quality. Aquaporins are a group of membrane channels that have several physiological substrates along with water. Metalloids have emerged as one of their important substrates and they are found to have a substantial role in regulating plant metalloid homeostasis. The present review comprehensively details the multiple isoforms of aquaporins having specificity for metalloids and being responsible for their influx, distribution or efflux. In addition, it also highlights the usage of aquaporin-mediated transport as a selection marker in toxic screens and as tracer elements for closely related metalloids. Therefore, aquaporins, with their imperative contribution to the regulation of plant growth, development and physiological processes, need more research to unravel the metalloid trafficking mechanisms and their future applications.

Posttranslational regulation of transporters important for symbiotic interactions

Coordinated sharing of nutritional resources is a central feature of symbiotic interactions, and, despite the importance of this topic, many questions remain concerning the identification, activity, and regulation of transporter proteins involved. Recent progress in obtaining genome and transcriptome sequences for symbiotic organisms provides a wealth of information on plant, fungal, and bacterial transporters that can be applied to these questions. In this update, we focus on legume-rhizobia and mycorrhizal symbioses and how transporters at the symbiotic interfaces can be regulated at the protein level. We point out areas where more research is needed and ways that an understanding of transporter mechanism and energetics can focus hypotheses. Protein phosphorylation is a predominant mechanism of posttranslational regulation of transporters in general and at the symbiotic interface specifically. Other mechanisms of transporter regulation, such as protein-protein interaction, including transporter multimerization, polar localization, and regulation by pH and membrane potential are also important at the symbiotic interface. Most of the transporters that function in the symbiotic interface are members of transporter families; we bring in relevant information on posttranslational regulation within transporter families to help generate hypotheses for transporter regulation at the symbiotic interface.

Combined Transcriptomics and Metabolomics Analysis Reveals the Molecular Mechanism of Salt Tolerance of Huayouza 62, an Elite Cultivar in Rapeseed (Brassica napus L.)

Soil salinity is one of the most significant abiotic stresses affecting crop yield around the world. To explore the molecular mechanism of salt tolerance in rapeseed (Brassica napus L.), the transcriptome analysis and metabolomics analysis were used to dissect the differentially expressed genes and metabolites in two rapeseed varieties with significant differences in salt tolerance; one is an elite rapeseed cultivar, Huayouza 62. A total of 103 key differentially expressed metabolites (DEMs) and 53 key differentials expressed genes (DEGs) that might be related to salt stress were identified through metabolomics and transcriptomics analysis. GO and KEGG analysis revealed that the DEGs were mainly involved in ion transport, reactive oxygen scavenging, osmotic regulation substance synthesis, and macromolecular protein synthesis. The DEMs were involved in TCA cycle, proline metabolism, inositol metabolism, carbohydrate metabolic processes, and oxidation-reduction processes. In addition, overexpression of BnLTP3, which was one of the key DEGs, could increase tolerance to salt stress in Arabidopsis plants. This study reveals that the regulation mechanism of salt tolerance in rapeseed at the transcriptome and metabolism level and provides abundant data for further in-depth identification of essential salt tolerance genes.

Adaptable and Multifunctional Ion-Conducting Aquaporins

Aquaporins function as water and neutral solute channels, signaling hubs, disease virulence factors, and metabolon components. We consider plant aquaporins that transport ions compared to some animal counterparts. These are candidates for important, as yet unidentified, cation and anion channels in plasma, tonoplast, and symbiotic membranes. For those individual isoforms that transport ions, water, and gases, the permeability spans 12 orders of magnitude. This requires tight regulation of selectivity via protein interactions and posttranslational modifications. A phosphorylation-dependent switch between ion and water permeation in AtPIP2;1 might be explained by coupling between the gates of the four monomer water channels and the central pore of the tetramer. We consider the potential for coupling between ion and water fluxes that could form the basis of an electroosmotic transducer. A grand challenge in understanding the roles of ion transporting aquaporins is their multifunctional modes that are dependent on location, stress, time, and development.

Aquaporins - Expression, purification and characterization

Aquaporin water channels facilitate the bi-directional flow of water and small, neutral solutes down an osmotic gradient in all kingdoms of life. Over the last two decades, the availability of high-quality protein has underpinned progress in the structural and functional characterization of these water channels. In particular, recombinant protein technology has guaranteed the supply of aquaporin samples that were of sufficient quality and quantity for further study. Here we review the features of successful expression, purification and characterization strategies that have underpinned these successes and that will drive further breakthroughs in the field. Overall, Escherichia coli is a suitable host for prokaryotic isoforms, while Pichia pastoris is the most commonly-used recombinant host for eukaryotic variants. Generally, a two-step purification procedure is suitable after solubilization in glucopyranosides and most structures are determined by X-ray following crystallization.

Response Mechanism of Plants to Drought Stress

With the global climate anomalies and the destruction of ecological balance, the water shortage has become a serious ecological problem facing all mankind, and drought has become a key factor restricting the development of agricultural production. Therefore, it is essential to study the drought tolerance of crops. Based on previous studies, we reviewed the effects of drought stress on plant morphology and physiology, including the changes of external morphology and internal structure of root, stem, and leaf, the effects of drought stress on osmotic regulation substances, drought-induced proteins, and active oxygen metabolism of plants. In this paper, the main drought stress signals and signal transduction pathways in plants are described, and the functional genes and regulatory genes related to drought stress are listed, respectively. We summarize the above aspects to provide valuable background knowledge and theoretical basis for future agriculture, forestry breeding, and cultivation.

Parallelism and convergence in post-domestication adaptation in cereal grasses

The selection of desirable traits in crops during domestication has been well studied. Many crops share a suite of modified phenotypic characteristics collectively known as the domestication syndrome. In this sense, crops have convergently evolved. Previous work has demonstrated that, at least in some instances, convergence for domestication traits has been achieved through parallel molecular means. However, both demography and selection during domestication may have placed limits on evolutionary potential and reduced opportunities for convergent adaptation during post-domestication migration to new environments. Here we review current knowledge regarding trait convergence in the cereal grasses and consider whether the complexity and dynamism of cereal genomes (e.g., transposable elements, polyploidy, genome size) helped these species overcome potential limitations owing to domestication and achieve broad subsequent adaptation, in many cases through parallel means. This article is part of the theme issue 'Convergent evolution in the genomics era: new insights and directions'.

Plant Aquaporins in Infection by and Immunity Against Pathogens - A Critical Review

Plant aquaporins (AQPs) of the plasma membrane intrinsic protein (PIP) family face constant risk of hijack by pathogens aiming to infect plants. PIPs can also be involved in plant immunity against infection. This review will utilize two case studies to discuss biochemical and structural mechanisms that govern the functions of PIPs in the regulation of plant infection and immunity. The first example concerns the interaction between rice Oryza sativa and the bacterial blight pathogen Xanthomonas oryzae pv. oryzae (Xoo). To infect rice, Xoo uses the type III (T3) secretion system to secrete the proteic translocator Hpa1, and Hpa1 subsequently mediates the translocation of T3 effectors secreted by this system. Once shifted from bacteria into rice cells, effectors exert virulent or avirulent effects depending on the susceptibility of the rice varieties. The translocator function of Hpa1 requires cooperation with OsPIP1;3, the rice interactor of Hpa1. This role of OsPIP1;3 is related to regulatory models of effector translocation. The regulatory models have been proposed as, translocon-dependent delivery, translocon-independent pore formation, and effector endocytosis with membrane protein/lipid trafficking. The second case study includes the interaction of Hpa1 with the H2O2 transport channel AtPIP1;4, and the associated consequence for H2O2 signal transduction of immunity pathways in Arabidopsis thaliana, a non-host of Xoo. H2O2 is generated in the apoplast upon induction by a pathogen or microbial pattern. H2O2 from this source translocates quickly into Arabidopsis cells, where it interacts with pathways of intracellular immunity to confer plant resistance against diseases. To expedite H2O2 transport, AtPIP1;4 must adopt a specific conformation in a number of ways, including channel width extension through amino acid interactions and selectivity for H2O2 through amino acid protonation and tautomeric reactions. Both topics will reference relevant studies, conducted on other organisms and AQPs, to highlight possible mechanisms of T3 effector translocation currently under debate, and highlight the structural basis of AtPIP1;4 in H2O2 transport facilitated by gating and trafficking regulation.

PIP Water Transport and Its pH Dependence Are Regulated by Tetramer Stoichiometry

Many plasma membrane channels form oligomeric assemblies, and heterooligomerization has been described as a distinctive feature of some protein families. In the particular case of plant plasma membrane aquaporins (PIPs), PIP1 and PIP2 monomers interact to form heterotetramers. However, the biological properties of the different heterotetrameric configurations formed by PIP1 and PIP2 subunits have not been addressed yet. Upon coexpression of tandem PIP2-PIP1 dimers in Xenopus oocytes, we can address, for the first time to our knowledge, the functional properties of single heterotetrameric species having 2:2 stoichiometry. We have also coexpressed PIP2-PIP1 dimers with PIP1 and PIP2 monomers to experimentally investigate the localization and biological activity of each tetrameric assembly. Our results show that PIP2-PIP1 heterotetramers can assemble with 3:1, 1:3, or 2:2 stoichiometry, depending on PIP1 and PIP2 relative expression in the cell. All PIP2-PIP1 heterotetrameric species localize at the plasma membrane and present the same water transport capacity. Furthermore, the contribution of any heterotetrameric assembly to the total water transport through the plasma membrane doubles the contribution of PIP2 homotetramers. Our results also indicate that plasma membrane water transport can be modulated by the coexistence of different tetrameric species and by intracellular pH. Moreover, all the tetrameric species present similar cooperativity behavior for proton sensing. These findings throw light on the functional properties of PIP tetramers, showing that they have flexible stoichiometry dependent on the quantity of PIP1 and PIP2 molecules available. This represents, to our knowledge, a novel regulatory mechanism to adjust water transport across the plasma membrane.

The Aquaporin Splice Variant NbXIP1;1α Is Permeable to Boric Acid and Is Phosphorylated in the N-terminal Domain

Aquaporins (AQPs) are membrane channel proteins that transport water and uncharged solutes across different membranes in organisms in all kingdoms of life. In plants, the AQPs can be divided into seven different subfamilies and five of these are present in higher plants. The most recently characterized of these subfamilies is the XIP subfamily, which is found in most dicots but not in monocots. In this article, we present data on two different splice variants (α and β) of NbXIP1;1 from Nicotiana benthamiana. We describe the heterologous expression of NbXIP1;1α and β in the yeast Pichia pastoris, the subcellular localization of the protein in this system and the purification of the NbXIP1;1α protein. Furthermore, we investigated the functionality and the substrate specificity of the protein by stopped-flow spectrometry in P. pastoris spheroplasts and with the protein reconstituted in proteoliposomes. The phosphorylation status of the protein and localization of the phosphorylated amino acids were verified by mass spectrometry. Our results show that NbXIP1;1α is located in the plasma membrane when expressed in P. pastoris, that it is not permeable to water but to boric acid and that the protein is phosphorylated at several amino acids in the N-terminal cytoplasmic domain of the protein. A growth assay showed that the yeast cells expressing the N-terminally His-tagged NbXIP1;1α were more sensitive to boric acid as compared to the cells expressing the C-terminally His-tagged isoform. This might suggest that the N-terminal His-tag functionally mimics the phosphorylation of the N-terminal domain and that the N-terminal domain is involved in gating of the channel.

Nanostructured self-assembly of inverted formin 2 (INF2) and F-actin-INF2 complexes revealed by atomic force microscopy

Self-organization of cytoskeletal proteins such as actin and tubulin into filaments and microtubules is frequently assisted by the proteins binding to them. Formins are regulatory proteins that nucleate the formation of new filaments and are essential for a wide range of cellular functions. The vertebrate inverted formin 2 (INF2) has both actin filament nucleating and severing/depolymerizing activities connected to its ability to encircle actin filaments. Using atomic force microscopy, we report that a formin homology 2 (FH2) domain-containing construct of INF2 (INF2-FH1-FH2-C or INF2-FFC) self-assembles into nanoscale ringlike oligomeric structures in the absence of actin filaments, demonstrating an inherent ability to reorganize from a dimeric to an oligomeric state. A construct lacking the C-terminal region (INF2-FH1-FH2 or INF2-FF) also oligomerizes, confirming the dominant role of FH2-mediated interactions. Moreover, INF2-FFC domains were observed to organize into ringlike structures around single actin filaments. This is the first demonstration that formin FH2 domains can self-assemble into oligomers in the absence of filaments and has important implications for observing unaveraged decoration and/or remodeling of filaments by actin binding proteins.

Aquaporins: highly regulated channels controlling plant water relations

Plant growth and development are dependent on tight regulation of water movement. Water diffusion across cell membranes is facilitated by aquaporins that provide plants with the means to rapidly and reversibly modify water permeability. This is done by changing aquaporin density and activity in the membrane, including posttranslational modifications and protein interaction that act on their trafficking and gating. At the whole organ level aquaporins modify water conductance and gradients at key "gatekeeper" cell layers that impact on whole plant water flow and plant water potential. In this way they may act in concert with stomatal regulation to determine the degree of isohydry/anisohydry. Molecular, physiological, and biophysical approaches have demonstrated that variations in root and leaf hydraulic conductivity can be accounted for by aquaporins but this must be integrated with anatomical considerations. This Update integrates these data and emphasizes the central role played by aquaporins in regulating plant water relations.

Two aquaporins of Jatropha are regulated differentially during drought stress and subsequent recovery

Jatropha has potential to be an important bio-fuel crop due to such advantages as high seed oil content and the ability to grow well on marginal lands less suited for food crops. Despite its ability to grow on marginal land, Jatropha is still susceptible to high salt and drought stresses, which can significantly reduce plant growth, stomatal conductance, sap-flow rate, and plant sap volume. This study was undertaken to collect basic knowledge of the physiological and molecular aspects of Jatropha response to salt and drought stresses, and to elucidate how Jatropha recovers from stress. From these studies we identified candidate genes that may be useful for the development of Jatropha cultivars that will grow efficiently in arid and barren lands. Of particular interest, two plasma membrane intrinsic proteins were identified: Jatropha plasma membrane intrinsic protein 1 (JcPIP1) and Jatropha plasma membrane intrinsic protein 2 (JcPIP2). The expression levels of JcPIP1 were dramatically increased in roots, stems, and leaves during the recovery from stress, whereas the JcPIP2 gene transcripts levels were induced in roots and stems during the water deficit stress. The protein levels of JcPIP1 and JcPIP2 were consistent with the gene expression patterns. Based on these results, we hypothesized that JcPIP1 plays a role in the recovery events from water stresses, while JcPIP2 is important in early responses to water stress. Virus induced gene silencing technology revealed that both JcPIP1 and JcPIP2 have positive roles in response to water deficit stresses, but have antagonistic functions at the recovery stage. We suggest that both JcPIP1 and JcPIP2 may play important roles in responses to water deficit conditions and both have potential as targets for genetic engineering.

Correlative nanoscale imaging of actin filaments and their complexes

Actin remodeling is an area of interest in biology in which correlative microscopy can bring a new way to analyze protein complexes at the nanoscale. Advances in EM, X-ray diffraction, fluorescence, and single molecule techniques have provided a wealth of information about the modulation of the F-actin structure and its regulation by actin binding proteins (ABPs). Yet, there are technological limitations of these approaches to achieving quantitative molecular level information on the structural and biophysical changes resulting from ABPs interaction with F-actin. Fundamental questions about the actin structure and dynamics and how these determine the function of ABPs remain unanswered. Specifically, how local and long-range structural and conformational changes result in ABPs induced remodeling of F-actin needs to be addressed at the single filament level. Advanced, sensitive and accurate experimental tools for detailed understanding of ABP-actin interactions are much needed. This article discusses the current understanding of nanoscale structural and mechanical modulation of F-actin by ABPs at the single filament level using several correlative microscopic techniques, focusing mainly on results obtained by Atomic Force Microscopy (AFM) analysis of ABP-actin complexes.

Fusion of bolaamphiphile micelles: a method to prepare stable supported biomimetic membranes

Supported biomimetic membranes (SBMs) on solid substrates have been commonly prepared from vesicle-forming double-tail lipids, such as zwitterionic phospholipids, using the method of vesicle fusion. Here we report on the preparation of SBMs on silica surfaces via a similar process of "micelle fusion" from a cationic single-tail bolaamphiphile GLH-20 that forms spherical and elongated thread-like micelles in solution. We demonstrate that, in contrast to zwitterionic phospholipids, GLH-20 self-assembles into a stable contiguous SBM at both low and high ionic strengths. The cationic charge of GLH-20 promotes the formation of a stable SBM through enhanced double-layer interactions with the negatively charged silica surface. It is also shown that spinach aquaporin PM-28 was successfully incorporated within bolaamphiphile SBM in a manner similar to SBMs prepared by vesicle/proteoliposome fusion; thereby the inherent curvature of the micelle surface does not inhibit protein reconstitution. The results suggest that SBMs based on charged bolaamphiphiles might be an attractive platform for applications such as water purification and biosensors, where the stability and low defect rate of SBMs in diverse conditions are crucial for achieving desired performance.

Preparation of supported lipid membranes for aquaporin Z incorporation

There has been a recent surge of interest to mimic the performance of natural cellular membranes by incorporating water channel proteins-aquaporins (AQPs) into various ultrathin films for water filtration applications. To make biomimetic membranes one of the most crucial steps is preparing a defect-free platform for AQPs incorporation on a suitable substrate. In this study two methods were used to prepare supported lipid membranes on NF membrane surfaces under a benign pH condition of 7.8. One method was direct vesicle fusion on a hydrophilic membrane NF-270; the other was vesicle fusion facilitated by hydraulic pressure on a modified hydrophilic NF-270 membrane whose surface has been spin-coated with positively charged lipids. Experiments revealed that the supported lipid membrane without AQPs prepared by the spin coating plus vesicle fusion had a much lower defect density than that prepared by vesicle fusion alone. It appears that the surface roughness and charge are the main factors determining the quality of the supported lipid membrane. Aquaporin Z (AqpZ) proteins were successfully incorporated into 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC) liposomes and its permeability was measured by the stopped-flow experimental procedure. However, after the proteoliposomes have been fused onto the modified substrate, the AqpZ function in the resultant membrane was not observed and AFM images showed distinct aggregations of unfused proteoliposomes or AqpZ proteins on the substrate surface. It is speculated that the inhibition of AqpZ function may be caused by the low lipid mobility on the NF membrane surface. Further investigations to evaluate and optimize the structure-performance relationship are required.

Specific plasma membrane aquaporins of the PIP1 subfamily are expressed in sieve elements and guard cells

BACKGROUND INFORMATION

Transmembrane water flow is aided by water-specific channel proteins, aquaporins. Plant genomes code for approx. 35 expressed and functional aquaporin isoforms. Plant aquaporins fall into four different subfamilies of which the PIPs (plasma membrane intrinsic proteins) constitute the largest and evolutionarily most conserved subfamily with 13 members in Arabidopsis and maize. Furthermore, the PIPs can be divided into two phylogenetic groups, PIP1 and PIP2, of which the PIP1 isoforms are most tightly conserved, sharing >90% amino acid sequence identity. As the nomenclature implies, the majority of PIPs have been shown to be localized at the plasma membrane. Recently, two highly abundant plasma membrane aquaporins, SoPIP2;1 and SoPIP1;2, have been purified and structurally characterized.

RESULTS

We report the cloning of a cDNA encoding SoPIP1;2 and show that there are at least five additional sequences homologous with SoPIP2;1 and SoPIP1;2 in the spinach genome. To understand their role in planta, we have investigated the cellular localization of the aquaporin homologues SoPIP1;2 and SoPIP1;1. By Western- and Northern-blot analyses and by immunocytochemical detection at the light and electron microscopic levels, we show that SoPIP1;2 is highly expressed in phloem sieve elements of leaves, roots and petioles and that SoPIP1;1 is present in stomatal guard cells.

CONCLUSIONS

Localization of the two abundant aquaporin isoforms suggests roles for specific PIPs of the PIP1 subgroup in phloem loading, transport and unloading, and in stomatal movements.

Reconstitution of water channel function and 2D-crystallization of human aquaporin 8

Among the thirteen human aquaporins (AQP0-12), the primary structure of AQP8 is unique. By sequence alignment it is evident that mammalian AQP8s form a separate subfamily distinct from the other mammalian aquaporins. The constriction region of the pore determining the solute specificity deviates in AQP8 making it permeable to both ammonia and H(2)O(2) in addition to water. To better understand the selectivity and gating mechanism of aquaporins, high-resolution structures are necessary. So far, the structure of three human aquaporins (HsAQP1, HsAQP4, and HsAQP5) have been solved at atomic resolution. For mammalian aquaporins in general, high-resolution structures are only available for those belonging to the water-specific subfamily (including HsAQP1, HsAQP4 and HsAQP5). Thus, it is of interest to solve structures of other aquaporin subfamily members with different solute specificities. To achieve this the aquaporins need to be overexpressed heterologously and purified. Here we use the methylotrophic yeast Pichia pastoris as a host for the overexpression. A wide screen of different detergents and detergent-lipid combinations resulted in the solubilization of functional human AQP8 protein and in well-ordered 2D crystals. It also became evident that removal of amino acids constituting affinity tags was crucial to achieve highly ordered 2D crystals diffracting to 3Å.

Single-molecule analysis of PIP2;1 dynamics and partitioning reveals multiple modes of Arabidopsis plasma membrane aquaporin regulation

PIP2;1 is an integral membrane protein that facilitates water transport across plasma membranes. To address the dynamics of Arabidopsis thaliana PIP2;1 at the single-molecule level as well as their role in PIP2;1 regulation, we tracked green fluorescent protein-PIP2;1 molecules by variable-angle evanescent wave microscopy and fluorescence correlation spectroscopy (FCS). Single-particle tracking analysis revealed that PIP2;1 presented four diffusion modes with large dispersion of diffusion coefficients, suggesting that partitioning and dynamics of PIP2;1 are heterogeneous and, more importantly, that PIP2;1 can move into or out of membrane microdomains. In response to salt stress, the diffusion coefficients and percentage of restricted diffusion increased, implying that PIP2;1 internalization was enhanced. This was further supported by the decrease in PIP2;1 density on plasma membranes by FCS. We additionally demonstrated that PIP2;1 internalization involves a combination of two pathways: a tyrphostin A23-sensitive clathrin-dependent pathway and a methyl-β-cyclodextrin-sensitive, membrane raft-associated pathway. The latter was efficiently stimulated under NaCl conditions. Taken together, our findings demonstrate that PIP2;1 molecules are heterogeneously distributed on the plasma membrane and that clathrin and membrane raft pathways cooperate to mediate the subcellular trafficking of PIP2;1, suggesting that the dynamic partitioning and recycling pathways might be involved in the multiple modes of regulating water permeability.

Increasing gene dosage greatly enhances recombinant expression of aquaporins in Pichia pastoris

Background

When performing functional and structural studies, large quantities of pure protein are desired. Most membrane proteins are however not abundantly expressed in their native tissues, which in general rules out purification from natural sources. Heterologous expression, especially of eukaryotic membrane proteins, has also proven to be challenging. The development of expression systems in insect cells and yeasts has resulted in an increase in successful overexpression of eukaryotic proteins. High yields of membrane protein from such hosts are however not guaranteed and several, to a large extent unexplored, factors may influence recombinant expression levels. In this report we have used four isoforms of aquaporins to systematically investigate parameters that may affect protein yield when overexpressing membrane proteins in the yeast Pichia pastoris.

Results

By comparing clones carrying a single gene copy, we show a remarkable variation in recombinant protein expression between isoforms and that the poor expression observed for one of the isoforms could only in part be explained by reduced transcript levels. Furthermore, we show that heterologous expression levels of all four aquaporin isoforms strongly respond to an increase in recombinant gene dosage, independent of the amount of protein expressed from a single gene copy. We also demonstrate that the increased expression does not appear to compromise the protein folding and the membrane localisation.

Conclusions

We report a convenient and robust method based on qPCR to determine recombinant gene dosage. The method is generic for all constructs based on the pPICZ vectors and offers an inexpensive, quick and reliable means of characterising recombinant P. pastoris clones. By using this method we show that: (1) heterologous expression of all aquaporins investigated respond strongly to an increase in recombinant gene dosage (2) expression from a single recombinant gene copy varies in an isoform dependent manner (3) the poor expression observed for AtSIP1;1 is mainly caused by posttranscriptional limitations. The protein folding and membrane localisation seems to be unaffected by increased expression levels. Thus a screen for elevated gene dosage can routinely be performed for identification of P. pastoris clones with high expression levels of aquaporins and other classes of membrane proteins.

Detection of membrane protein two-dimensional crystals in living cells

It is notoriously difficult to grow membrane protein crystals and solve membrane protein structures. Improved detection and screening of membrane protein crystals are needed. We have shown here that second-order nonlinear optical imaging of chiral crystals based on second harmonic generation can provide sensitive and selective detection of two-dimensional protein crystalline arrays with sufficiently low background to enable crystal detection within the membranes of live cells. The method was validated using bacteriorhodopsin crystals generated in live Halobacterium halobium bacteria and confirmed by electron microscopy from the isolated crystals. Additional studies of alphavirus glycoproteins indicated the presence of localized crystalline domains associated with virus budding from mammalian cells. These results suggest that in vivo crystallization may provide a means for expediting membrane protein structure determination for proteins exhibiting propensities for two-dimensional crystal formation.

Hormonal treatment of the bark of rubber trees (Hevea brasiliensis) increases latex yield through latex dilution in relation with the differential expression of two aquaporin genes

Natural rubber is synthesized in laticifers in the inner liber of the rubber tree (Hevea brasiliensis). Upon bark tapping, the latex is expelled due to liber turgor pressure. The mature laticifers are devoid of plasmodesmata; therefore a corresponding decrease in the total latex solid content is likely to occur due to water influx inside the laticifers. Auxins and ethylene used as efficient yield stimulants in mature untapped rubber trees, but, bark treatments with abscisic acid (ABA) and salicylic acid (SA) could also induce a transient increase latex yield. We recently reported that there are three aquaporin genes, HbPIP2;1, HbTIP1;1 and HbPIP1;1, that are regulated differentially after ethylene bark treatment. HbPIP2;1 was up-regulated in both the laticifers and the inner liber tissues, whereas HbTIP1;1 was up-regulated in the latex cells, but very markedly down-regulated in the inner liber tissues. Conversely, HbPIP1;1 was down-regulated in both tissues. In the present study, HbPIP2;1 and HbTIP1;1 showed a similar expression in response to auxin, ABA and SA, as seen in ethylene stimulation, while HbPIP1;1 was slightly regulated by auxin, but neither by ABA nor SA. The analysis of the HbPIP1;1 promoter region indicated the presence of only ethylene and auxin responsive elements. In addition, the poor efficiency of this HbPIP1;1 in increasing plasmalemma water conductance was confirmed in Xenopus oocytes. Thus, an increase in latex yield in response to all of these hormones was proposed to be the major function of aquaporins, HbPIP2;1 and HbTIP1;1. This study emphasized that the circulation of water between the laticifers and their surrounding tissues that result in latex dilution, as well as the probable maintenance of the liber tissues turgor pressure, favor the prolongation of latex flow.

Aquaporins: a family of highly regulated multifunctional channels

Aquaporins (AQPs) were discovered as channels facilitatingwater movement across cellular membranes. Whereas much of the research has focused on characterizing AQPs with respect to cell water homeostasis, recent discoveries in terms of the transport selectivity of AQP homologs has shed new light on their physiological roles. In fact, whereas some AQPs behave as "strict" water channels, others can conduct a wide range ofnonpolar solutes, such as urea or glycerol and even more unconventional permeants, such as the nonpolar gases carbon dioxide and nitric oxide, the polar gas ammonia, the reactive oxygen species hydrogen peroxide and the metalloids antimonite, arsenite, boron and silicon. This suggests that AQPs are also key players in various physiological processes not related to water homeostasis. The function, regulation and biological importance of AQPs in the different kingdoms is reviewed in this chapter, with special emphasis on animal and plant AQPs.

Supported lipid bilayer membranes for water purification by reverse osmosis

Some biological plasma membranes pass water with a permeability and selectivity largely exceeding those of commercial membranes for water desalination using specialized trans-membrane proteins aquaporins. However, highly selective transport of water through aquaporins is usually driven by an osmotic rather mechanical pressure, which is not as attractive from the engineering point of view. The feasibility of adopting biomimetic membranes for water purification driven by a mechanical pressure, i.e., filtration is explored in this paper. Toward this goal, it is proposed to use a commercial nanofiltration (NF) membrane as a support for biomimetic lipid bilayer membranes to render them robust enough to withstand the required pressures. It is shown in this paper for the first time that by properly tuning molecular interactions supported phospholipid bilayers (SPB) can be prepared on a commercial NF membrane. The presence of SPB on the surface was verified and quantified by several spectroscopic and microscopic techniques, which showed morphology close to the desired one with very few defects. As an ultimate test it is shown that hydraulic permeability of the SPB supported on the NF membrane (NTR-7450) approaches the values deduced from the typical osmotic permeabilities of intact continuous bilayers. This permeability was unaffected by the trans-membrane flow of water and by repeatedly releasing and reapplying a 10 bar pressure. Along with a parallel demonstration that aquaporins could be incorporated in a similar bilayer on mica, this demonstrates the feasibility of the proposed approach. The prepared SPB structure may be used as a platform for preparing biomimetic filtration membranes with superior performance based on aquaporins. The concept of SPBs on permeable substrates of the present type may also be useful in the future for studying transport of various molecules through trans-membrane proteins.

The AQP structure and functional implications

Progress in the structure determination of AQPs has led to a deep understanding of water and solute permeation by these small integral membrane proteins. The atomic structures now available have allowed the water permeation and exclusion of protons to be monitored by molecular dynamics simulations, and have provided a framework for assessing the water and solute permeation in great detail by site-directed mutations. In spite of this, further structural and molecular dynamics analyses are required to elucidate the basis for regulation as well as for gas permeation, processes that are still to be deciphered.

Cryoelectron microscopy structure of purified gamma-secretase at 12 A resolution

Gamma-secretase, an integral membrane protein complex, catalyzes the intramembrane cleavage of the beta-amyloid precursor protein (APP) during the neuronal production of the amyloid beta-peptide. As such, the protease has emerged as a key target for developing agents to treat and prevent Alzheimer's disease. Existing biochemical studies conflict on the oligomeric assembly state of the protease complex, and its detailed structure is not known. Here, we report that purified active human gamma-secretase in digitonin has a total molecular mass of approximately 230 kDa when measured by scanning transmission electron microscopy. This result supports a complex that is monomeric for each of the four component proteins. We further report the three-dimensional structure of the gamma-secretase complex at 12 A resolution as obtained by cryoelectron microscopy and single-particle image reconstruction. The structure reveals several domains on the extracellular side, three solvent-accessible low-density cavities, and a potential substrate-binding surface groove in the transmembrane region of the complex.

Expanding roles of plant aquaporins in plasma membranes and cell organelles

Aquaporins facilitate water transport across biomembranes in a manner dependent on osmotic pressure and water-potential gradient. The discovery of aquaporins has facilitated research on intracellular and whole-plant water transport at the molecular level. Aquaporins belong to a ubiquitous family of membrane intrinsic proteins (MIP). Plants have four subfamilies: plasma-membrane intrinsic protein (PIP), tonoplast intrinsic protein (TIP), nodulin 26-like intrinsic protein (NIP), and small basic intrinsic protein (SIP). Recent research has revealed a diversity of plant aquaporins, especially their physiological functions and intracellular localisation. A few PIP members have been reported to be involved in carbon dioxide permeability of cells. Newly identified transport substrates for NIP members of rice and Arabidopsis thaliana have been demonstrated to transport silicon and boron, respectively. Ammonia, glycerol, and hydrogen peroxide have been identified as substrates for plant aquaporins. The intracellular localisation of plant aquaporins is diverse; for example, SIP members are localised on the ER membrane. There has been much progress in the research on the functional regulation of water channel activity of PIP members including phosphorylation, formation of hetero-oligomer, and protonation of histidine residues under acidic condition. This review provides a broad overview of the range of potential aquaporins, which are now believed to participate in the transport of several small molecules in various membrane systems in model plants, crops, flowers and fruits.

Plant aquaporins: membrane channels with multiple integrated functions

Aquaporins are channel proteins present in the plasma and intracellular membranes of plant cells, where they facilitate the transport of water and/or small neutral solutes (urea, boric acid, silicic acid) or gases (ammonia, carbon dioxide). Recent progress was made in understanding the molecular bases of aquaporin transport selectivity and gating. The present review examines how a wide range of selectivity profiles and regulation properties allows aquaporins to be integrated in numerous functions, throughout plant development, and during adaptations to variable living conditions. Although they play a central role in water relations of roots, leaves, seeds, and flowers, aquaporins have also been linked to plant mineral nutrition and carbon and nitrogen fixation.

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.

HvPIP1;6, a barley (Hordeum vulgare L.) plasma membrane water channel particularly expressed in growing compared with non-growing leaf tissues

The aim of the present study was to identify water channel(s) which are expressed specifically in the growth zone of grass leaves and may facilitate growth-associated water uptake into cells. Previously, a gene had been described (HvEmip) which encodes a membrane intrinsic protein (MIP) and which is particularly expressed in the base 1 cm of barley primary leaves. The functionality of the encoding protein was not known. In the present study on leaf 3 of barley (Hordeum vulgare L.), a clone was isolated, termed HvPIP1;6, which has 99% amino acid sequence identity to HvEmip and belongs to the family of plasma membrane intrinsic proteins (PIPs). Expression of HvPIP1;6 was highest in the elongation zone, where it accounted for >85% of expression of known barley PIP1s. Within the elongation zone, faster grower regions showed higher expression than slower growing regions. Expression of HvPIP1;6 was confined to the epidermis, with some expression in neighboring mesophyll cells. Expression of HvPIP1;6 in Xenopus laevis oocytes increased osmotic water permeability 4- to 6-fold. Water channel activity was inhibited by pre-incubation of oocytes with 50 microM HgCl(2) and increased following incubation with the phosphatase inhibitor okadaic acid or the plant hormone ABA. Plasma membrane preparations were analyzed by Western blots using an antibody that recognized PIP1s. Levels of PIP1s were highest in the elongation and adjacent non-elongation zone. The developmental expression profile of HvPIP2;1, the only known barley water channel belonging to the PIP2 subgroup, was opposite to that of HvPIP1;6.

Major intrinsic proteins (MIPs) in plants: a complex gene family with major impacts on plant phenotype

The ubiquitous cell membrane proteins called aquaporins are now firmly established as channel proteins that control the specific transport of water molecules across cell membranes in all living organisms. The aquaporins are thus likely to be of fundamental significance to all facets of plant growth and development affected by plant-water relations. A majority of plant aquaporins have been found to share essential structural features with the human aquaporin and exhibit water-transporting ability in various functional assays, and some have been shown experimentally to be of critical importance to plant survival. Furthermore, substantial evidence is now available from a number of plant species that shows differential gene expression of aquaporins in response to abiotic stresses such as salinity, drought, or cold and clearly establishes the aquaporins as major players in the response of plants to conditions that affect water availability. This review summarizes the function and regulation of these genes to develop a greater understanding of the response of plants to water insufficiency, and particularly, to identify tolerant genotypes of major crop species including wheat and rice and plants that are important in agroforestry.

Plant aquaporins: novel functions and regulation properties

Aquaporins are water channel proteins of intracellular and plasma membranes that play a crucial role in plant water relations. The present review focuses on the most recent findings concerning the molecular and cellular properties of plant aquaporins. The mechanisms of transport selectivity and gating (i.e. pore opening and closing) have recently been described, based on aquaporin structures at atomic resolution. Novel dynamic aspects of aquaporin subcellular localisation have been uncovered. Also, some aquaporin isoforms can transport, besides water, physiologically important molecules such as CO(2), H(2)O(2), boron or silicon. Thus, aquaporins are involved in many great functions of plants, including nutrient acquisition, carbon fixation, cell signalling and stress responses.

Proteomic knowledge of human aquaporins

Aquaporins (AQPs) are an ubiquitous family of proteins characterized by sequence similarity and the presence of two NPA (Asp-Pro-Ala) motifs. At present, 13 human AQPs are known and they are divided into two subgroups according to their ability to transport only water molecules (AQP0, AQP1, AQP2, AQP4, AQP5, AQP6, and AQP8), or also glycerol and other small solutes (AQP3, AQP7, AQP9, AQP10, AQP12). The genomic, structural, and functional aspects of this family are briefly described. In particular, proteomic approaches to identify and characterize the most studied AQPs, mainly through SDS-PAGE followed by MS analysis, are discussed. Moreover, the clinical importance of the best studied aquaporin (AQP1) in human diseases is also provided.

Expression and functional analysis of the rice plasma-membrane intrinsic protein gene family

Plasma membrane intrinsic proteins (PIPs) are a subfamily of aquaporins that enable fast and controlled translocation of water across the membrane. In this study, we systematically identified and cloned ten PIP genes from rice. Based on the similarity of the amino acid sequences they encoded, these rice PIP genes were classified into two groups and designated as OsPIP1-1 to OsPIP1-3 and OsPIP2-1 to OsPIP2-7 following the nomenclature of PIP genes in maize. Quantitative RT-PCR analysis identified three root-specific and one leaf-specific OsPIP genes. Furthermore, the expression profile of each OsPIP gene in response to salt, drought and ABA treatment was examined in detail. Analysis on transgenic plants over-expressing of either OsPIP1 (OsPIP1-1) or OsPIP2 (OsPIP2-2) in wild-type Arabidopsis, showed enhanced tolerance to salt (100 mM of NaCl) and drought (200 mM of mannitol), but not to salt treatment of higher concentration (150 mM of NaCl). Taken together, these data suggest a distinct role of each OsPIP gene in response to different stresses, and should add a new layer to the understanding of the physiological function of rice PIP genes.

Functional aquaporin diversity in plants

Due to the fact that most plants are immobile, a rapid response of physiological processes to changing environmental conditions is essential for their survival. Thus, in comparison to many other organisms, plants might need a more sophisticated tuning of water balance. Among others, this is reflected by the comparable large amount of aquaporin genes in plant genomes. So far, aquaporins were shown to be involved in many physiological processes like root water uptake, reproduction or photosynthesis. Their classification as simple water pores has changed according to their molecular function into channels permeable for water, small solutes and/or gases. An adjustment of the corresponding physiological process could be achieved by regulation mechanisms. Concerning aquaporins these range from posttranslational modification, molecular trafficking to heteromerization of aquaporin isoforms. The aim of this review is to underline the function of the four plant aquaporin family subclasses with regard to the substrate specificity, regulation and physiological relevance.

Modulating the expression of aquaporin genes in planta: A key to understand their physiological functions?

Aquaporins (AQPs) are believed to act as "cellular plumbers", allowing plants to rapidly alter their membrane water permeability in response to environmental cues. This study of AQP regulation at both the RNA and protein levels has revealed a large number of possible mechanisms. Currently, modulation of AQP expression in planta is considered the strategy of choice for elucidating the role of AQPs in plant physiology. This review highlights the fact that this strategy is complicated by many factors, such as the incomplete characterization of transport selectivity of the targeted AQP, the fact that AQPs might act as multifunctional channels with multiple physiological roles, and the number of post-translational regulation mechanisms. The classification of AQPs as constitutive or stress-responsive isoforms is also proposed.

Regulation of plant aquaporin activity

Accumulating evidence indicates that aquaporins play a key role in plant water relations. Plant aquaporins are part of a large and highly divergent protein family that can be divided into four subfamilies according to amino acid sequence similarity. As in other organisms, plant aquaporins facilitate the transcellular movement of water, but, in some cases, also the flux of small neutral solutes across a cellular membrane. Plant cell membranes are characterized by a large range of osmotic water permeabilities, and recent data indicate that plant aquaporin activity might be regulated by gating mechanisms. The factors affecting the gating behaviour possibly involve phosphorylation, heteromerization, pH, Ca2+, pressure, solute gradients and temperature. Regulation of aquaporin trafficking may also represent a way to modulate membrane water permeability. The aim of this review is to integrate recent molecular and biophysical data on the mechanisms regulating aquaporin activity in plant membranes and to relate them to putative changes in protein structure.

A Regulatory Domain in the C-terminal Extension of the Yeast Glycerol Channel Fps1p

The Saccharomyces cerevisiae gene FPS1 encodes an aquaglyceroporin of the major intrinsic protein (MIP) family. The main function of Fps1p seems to be the efflux of glycerol in the adaptation of the yeast cell to lower external osmolarity. Fps1p is an atypical member of the family, because the protein is much larger (669 amino acids) than most MIPs due to long hydrophilic extensions in both termini. We have shown previously that a short domain in the N-terminal extension of the protein is required for restricting glycerol transport through the channel (Tamás, M. J., Karlgren, S., Bill, R. M., Hedfalk, K., Allegri, L., Ferreira, M., Thevelein, J. M., Rydström, J., Mullins, J. G. L., and Hohmann, S. (2003) J. Biol. Chem. 278, 6337-6345). Deletion of the N-terminal domain results in an unregulated channel, loss of glycerol, and osmosensitivity. In this work we have investigated the role of the Fps1p C terminus (139 amino acids). A set of eight truncations has been constructed and tested in vivo in a yeast fps1Delta strain. We have performed growth tests, membrane localization following cell fractionation, and glycerol accumulation measurements as well as an investigation of the osmotic stress response. Our results show that the C-terminal extension is also involved in restricting transport through Fps1p. We have identified a sequence of 12 amino acids, residues 535-546, close to the sixth transmembrane domain. This element seems to be important for controlling Fps1p function. Similar to the N-terminal domain, the C-terminal domain is amphiphilic and has a potential to dip into the membrane.

A proteomic study reveals novel insights into the diversity of aquaporin forms expressed in the plasma membrane of plant roots

Aquaporins are channel proteins that facilitate the diffusion of water across cell membranes. The genome of Arabidopsis thaliana encodes 35 full-length aquaporin homologues. Thirteen of them belong to the plasma membrane intrinsic protein (PIP) subfamily and predominantly sit at the plasma membrane (PM). In the present work we combine separations of membrane proteins (by one- and two-dimensional gel electrophoresis) with identification by MS (matrix-assisted laser-desorption ionization-time-of-flight and electrospray-ionization tandem MS) to take an inventory of aquaporin isoforms expressed in the PM of Arabidopsis thaliana roots. Our analysis provides direct evidence for the expression of five PIPs (PIP1;1, PIP1;5, PIP2;1, PIP2;2 and PIP2;7) in the root PM and suggests the presence of at least three other PIP isoforms. In addition, we show that the same PIP isoform can be present under several forms with distinct isoelectric points. More specifically, we identify phosphorylated aquaporins in the PIP1 and PIP2 subgroups and suggest the existence of other post-translational modifications. Their identification should provide clues to reveal novel molecular mechanisms for aquaporin regulation.

Reconstitution of water channel function of an aquaporin overexpressed and purified from Pichia pastoris

The aquaporin PM28A is one of the major integral proteins in spinach leaf plasma membranes. Phosphorylation/dephosphorylation of Ser274 at the C-terminus and of Ser115 in the first cytoplasmic loop has been shown to regulate the water channel activity of PM28A when expressed in Xenopus oocytes. To understand the mechanisms of the phosphorylation-mediated gating of the channel the structure of PM28A is required. In a first step we have used the methylotrophic yeast Pichia pastoris for expression of the pm28a gene. The expressed protein has a molecular mass of 32462 Da as determined by matrix-assisted laser desorption ionization-mass spectrometry, forms tetramers as revealed by electron microscopy and is functionally active when reconstituted in proteoliposomes. PM28A was efficiently solubilized from urea- and alkali-stripped Pichia membranes by octyl-beta-D-thioglucopyranoside resulting in a final yield of 25 mg of purified protein per liter of cell culture.

Molecular physiology of aquaporins in plants

In plants, membrane channels of the major intrinsic protein (MIP) super-family exhibit a high diversity with, for instance, 35 homologues in the model species Arabidopsis thaliana. As has been found in other organisms, plant MIPs function as membrane channels permeable to water (aquaporins) and in some cases to small nonelectrolytes. The aim of the present article is to integrate into plant physiology what has been recently learned about the molecular and functional properties of aquaporins in plants. Exhaustive compilation of data in the literature shows that the numerous aquaporin isoforms of plants have specific expression patterns throughout plant development and in response to environmental stimuli. The diversity of aquaporin homologues in plants can also be explained in part by their presence in multiple subcellular compartments. In recent years, there have been numerous reports that describe the activity of water channels in purified membrane vesicles, in isolated organelles or protoplasts, and in intact plant cells or even tissues. Altogether, these data suggest that the transport of water and solutes across plant membranes concerns many facets of plant physiology. Because of the high degree of compartmentation of plant cells, aquaporins may play a critical role in cell osmoregulation. Water uptake in roots represents a typical process in which to investigate the role of aquaporins in transcellular water transport, and the mechanisms and regulations involved are discussed.