Proteomics of plant detergent-resistant membranes

Phylogeny, topology, structure and functions of membrane-bound class III peroxidases in vascular plants

Peroxidases are key player in the detoxification of reactive oxygen species during cellular metabolism and oxidative stress. Membrane-bound isoenzymes have been described for peroxidase superfamilies in plants and animals. Recent studies demonstrated a location of peroxidases of the secretory pathway (class III peroxidases) at the tonoplast and the plasma membrane. Proteomic approaches using highly enriched plasma membrane preparations suggest organisation of these peroxidases in microdomains, a developmentally regulation and an induction of isoenzymes by oxidative stress. Phylogenetic relations, topology, putative structures, and physiological function of membrane-bound class III peroxidases will be discussed.

Advances in qualitative and quantitative plant membrane proteomics

The membrane proteome consists of integral and membrane-associated proteins that are involved in various physiological and biochemical functions critical for cellular function. It is also dynamic in nature, where many proteins are only expressed during certain developmental stages or in response to environmental stress. These proteins can undergo post-translational modifications in response to these different conditions, allowing them to transiently associate with the membrane or other membrane proteins. Along with their increased size, hydrophobicity, and the additional organelle and cellular features of plant cells relative to mammalian systems, the characterization of the plant membrane proteome presents unique challenges for effective qualitative and quantitative analysis using mass spectrometry (MS) analysis. Here, we present the latest advancements developed for the isolation and fractionation of plant organelles and their membrane components amenable to MS analysis. Separations of membrane proteins from these enriched preparations that have proven effective are discussed for both gel- and liquid chromatography-based MS analysis. In this context, quantitative membrane proteomic analyses using both isotope-coded and label-free approaches are presented and reveal the potential to establish a wider-biological interpretation of the function of plant membrane proteins that will ultimately lead to a more comprehensive understanding of plant physiology and their response mechanisms.

The role of the plasma membrane H+-ATPase in plant-microbe interactions

Plasma membrane (PM) H+-ATPases are the primary pumps responsible for the establishment of cellular membrane potential in plants. In addition to regulating basic aspects of plant cell function, these enzymes contribute to signaling events in response to diverse environmental stimuli. Here, we focus on the roles of the PM H+-ATPase during plant-pathogen interactions. PM H+-ATPases are dynamically regulated during plant immune responses and recent quantitative proteomics studies suggest complex spatial and temporal modulation of PM H+-ATPase activity during early pathogen recognition events. Additional data indicate that PM H+-ATPases cooperate with the plant immune signaling protein RIN4 to regulate stomatal apertures during bacterial invasion of leaf tissue. Furthermore, pathogens have evolved mechanisms to manipulate PM H+-ATPase activity during infection. Thus, these ubiquitous plant enzymes contribute to plant immune responses and are targeted by pathogens to increase plant susceptibility.

Challenges and solutions for the identification of membrane proteins in non-model plants

The workhorse for proteomics in non-model plants is classical two-dimensional electrophoresis, a combination of iso-electric focusing and SDS-PAGE. However, membrane proteins with multiple membrane spanning domains are hardly detected on classical 2-DE gels because of their low abundance and poor solubility in aqueous media. In the current review, solutions that have been proposed to handle these two problems in non-model plants are discussed. An overview of alternative techniques developed for membrane proteomics is provided together with a comparison of their strong and weak points. Subsequently, strengths and weaknesses of the different techniques and methods to evaluate the identification of membrane proteins are discussed. Finally, an overview of recent plant membrane proteome studies is provided with the used separation technique and the number of identified membrane proteins listed.

Perspectives on remorin proteins, membrane rafts, and their role during plant-microbe interactions

Invasion of host cells by pathogenic or mutualistic microbes requires complex molecular dialogues that often determine host survival. Although several components of the underlying signaling cascades have recently been identified and characterized, our understanding of proteins that facilitate signal transduction or assemble signaling complexes is rather sparse. Our knowledge of plant-specific remorin proteins, annotated as proteins with unknown function, has recently advanced with respect to their involvement in host-microbe interactions. Current data demonstrating that a remorin protein restricts viral movement in tomato leaves and the importance of a symbiosis-specific remorin for bacterial infection of root nodules suggest that these proteins may serve such regulatory functions. Direct interactions of other remorins with a resistance protein in Arabidopsis thaliana, and differential phosphorylation upon perception of microbial-associated molecular patterns and during expression of bacterial effector proteins, strongly underline their roles in plant defense. Furthermore, the specific subcellular localization of remorins in plasma membrane microdomains now provides the opportunity to visualize membrane rafts in living plants cells. There, remorins may oligomerize and act as scaffold proteins during early signaling events. This review summarizes current knowledge of this protein family and the potential roles of remorins in membrane rafts.

Cross-talk of calcium-dependent protein kinase and MAP kinase signaling

Plants use different signalling pathways to acclimate to changing environmental conditions. Fast changes in the concentration of free Ca(2+) ions - so called Ca(2+) signals - are among the first responses to many stress situations. These signals are decoded by different types of calcium-dependent protein kinases, which - together with mitogen-activated protein kinases (MAPK) - present two major pathways that are widely used to adapt the cellular metabolism to a changing environment. Ca(2+)-dependent protein kinase (CDPK) and MAPK pathways are known to be involved in signalling of abiotic and biotic stress in animal, yeast and plant cells. In many cases both pathways are activated in response to the same stimuli leading to the question of a potential cross-talk between those pathways. Cross-talk between Ca(2+)-dependent and MAPK signalling pathways has been elaborately studied in animal cells, but it has hardly been investigated in plants. Early studies of CDPKs involved in the biotic stress response in tobacco indicated a cross-talk of CDPK and MAPK activities, whereas a recent study in Arabidopsis revealed that CDPKs and MAPKs act differentially in innate immune signalling and showed no direct cross-talk between CDPK and MAPK activities. Similar results were also reported for CDPK and MAPK activities in the salt stress response in Arabidopsis. Different modes of action are furthermore supported by the different subcellular localization of the involved kinases. In this review, we discuss recent findings on CDPK and MAPK signalling with respect to potential cross-talk and the subcellular localization of the involved components.

Membrane rafts in plant cells

Over the past five years, the structure, composition and possible functions of membrane raft-like domains on plant plasma membranes (PM) have been described. Proteomic analyses have indicated that a high proportion of proteins associated with detergent-insoluble membranes (DIMs), supposed to contain raft-like domains isolated from the PM, might be involved in signalling pathways. Recently, the dynamic association of specific proteins with the DIM fraction upon environmental stress has been reported. Innovative imaging methods have shown that lateral segregation of lipids and proteins exists at the nanoscale level in the plant PM, correlating detergent insolubility and membrane-domain localization of presumptive raft proteins. These data suggest a role for plant rafts as signal transduction platforms, similar to those documented for mammalian cells.

Isobaric tags for relative and absolute quantification- based comparative proteomics reveals the features of plasma membrane-associated proteomes of pollen grains and pollen tubes from Lilium davidii

Mature pollen grains (PGs) from most plant species are metabolically quiescent. However, once pollinated onto stigma, they quickly hydrate and germinate. A PG can give rise to a vegetative cell-derived polarized pollen tube (PT), which represents a specialized polar cell. The polarized PT grows by the tip and requires interaction of different signaling molecules localized in the apical plasma membrane and active membrane trafficking. The mechanisms underlying the interaction and membrane trafficking are not well understood. In this work, we purified PG and PT plasma-membrane vesicles from Lilium davidii Duch. using the aqueous two-phase partition technique, then enriched plasma membrane proteins by using Brij58 and KCl to remove loosely bound contaminants. We identified 223 integral and membrane-associated proteins in the plasma membrane of PGs and PTs by using isobaric tags for relative and absolute quantification (iTRAQ) and 2-D high-performance liquid chromatography-tandem mass spectrometry. More than 68% of the proteins have putative transmembrane domains and/or lipid-modified motifs. Proteins involved in signal transduction, membrane trafficking and transport are predominant in the plasma-membrane proteome. We revealed most components of the clathrin-dependent endocytosis pathway. Statistical analysis revealed 14 proteins differentially expressed in the two development stages: in PTs, six upregulated and eight downregulated are mainly involved in signaling, transport and membrane trafficking. These results provide novel insights into polarized PT growth.

Endocytosis in plant-microbe interactions

Plants encounter throughout their life all kinds of microorganisms, such as bacteria, fungi, or oomycetes, with either friendly or unfriendly intentions. During evolution, plants have developed a wide range of defense mechanisms against attackers. In return, adapted microbes have developed strategies to overcome the plant lines of defense, some of these microbes engaging in mutualistic or parasitic endosymbioses. By sensing microbe presence and activating signaling cascades, the plasma membrane through its dynamics plays a crucial role in the ongoing molecular dialogue between plants and microbes. This review describes the contribution of endocytosis to different aspects of plant-microbe interactions, microbe recognition and development of a basal immune response, and colonization of plant cells by endosymbionts. The putative endocytic routes for the entry of microbe molecules or microbes themselves are explored with a special emphasis on clathrin-mediated endocytosis. Finally, we evaluate recent findings that suggest a link between the compartmentalization of plant plasma membrane into microdomains and endocytosis.

Plasma membrane sterol complexation, generated by filipin, triggers signaling responses in tobacco cells

The effects of changes in plasma membrane (PM) sterol lateral organization and availability on the control of signaling pathways have been reported in various animal systems, but rarely assessed in plant cells. In the present study, the pentaene macrolide antibiotic filipin III, commonly used in animal systems as a sterol sequestrating agent, was applied to tobacco cells. We show that filipin can be used at a non-lethal concentration that still allows an homogeneous labeling of the plasma membrane and the formation of filipin-sterol complexes at the ultrastructural level. This filipin concentration triggers a rapid and transient NADPH oxidase-dependent production of reactive oxygen species, together with an increase in both medium alkalinization and conductivity. Pharmacological inhibition studies suggest that these signaling events may be regulated by phosphorylations and free calcium. By conducting FRAP experiments using the di-4-ANEPPDHQ probe and spectrofluorimetry using the Laurdan probe, we provide evidence for a filipin-induced increase in PM viscosity that is also regulated by phosphorylations. We conclude that filipin triggers ligand-independent signaling responses in plant cells. The present findings strongly suggest that changes in PM sterol availability could act as a sensor of the modifications of cell environment in plants leading to adaptive cell responses through regulated signaling processes.

Structural sterols are involved in both the initiation and tip growth of root hairs in Arabidopsis thaliana

Structural sterols are abundant in the plasma membrane of root apex cells in Arabidopsis thaliana. They specifically accumulate in trichoblasts during the prebulging and bulge stages and show a polar accumulation in the tip during root hair elongation but are distributed evenly in mature root hairs. Thus, structural sterols may serve as a marker for root hair initiation and growth. In addition, they may predict branching events in mutants with branching root hairs. Structural sterols were detected using the sterol complexing fluorochrome filipin. Application of filipin caused a rapid, concentration-dependent decrease in tip growth. Filipin-complexed sterols accumulated in globular structures that fused to larger FM4-64-positive aggregates in the tip, so-called filipin-induced apical compartments, which were closely associated with the plasma membrane. The plasma membrane appeared malformed and the cytoarchitecture of the tip zone was affected. Trans-Golgi network/early endosomal compartments containing molecular markers, such as small Rab GTPase RabA1d and SNARE Wave line 13 (VTI12), locally accumulated in these filipin-induced apical compartments, while late endosomes, endoplasmic reticulum, mitochondria, plastids, and cytosol were excluded from them. These data suggest that the local distribution and apical accumulation of structural sterols may regulate vesicular trafficking and plasma membrane properties during both initiation and tip growth of root hairs in Arabidopsis.

Plants and fungi in the era of heterogeneous plasma membranes

Examples from yeast and plant cells are described that show that their plasma membrane is laterally compartmented. Distinct lateral domains encompassing both specific lipids and integral proteins coexist within the plane of the plasma membrane. The compartments are either spatially stable and include distinct sets of proteins, or they are transiently formed to accomplish diverse functions. They are not related to lipid rafts or their clusters, as defined for mammalian cells. This review summarises only well-documented compartments of plasma membranes from plants and fungi, which have been recognised using microscopic approaches. In several cases, physiological functions of the membrane compartmentation are revealed.

The Ca(2+) -dependent protein kinase CPK3 is required for MAPK-independent salt-stress acclimation in Arabidopsis

Plants use different signalling pathways to respond to external stimuli. Intracellular signalling via calcium-dependent protein kinases (CDPKs) or mitogen-activated protein kinases (MAPKs) present two major pathways that are widely used to react to a changing environment. Both CDPK and MAPK pathways are known to be involved in the signalling of abiotic and biotic stresses in animal, yeast and plant cells. Here, we show the essential function of the CDPK CPK3 (At4g23650) for salt stress acclimation in Arabidopsis thaliana, and test crosstalk between CPK3 and the major salt-stress activated MAPKs MPK4 and MPK6 in the salt stress response. CPK3 kinase activity was induced by salt and other stresses after transient overexpression in Arabidopsis protoplasts, but endogenous CPK3 appeared to be constitutively active in roots and leaves in a strictly Ca(2+) -dependent manner. cpk3 mutants show a salt-sensitive phenotype comparable with mutants in MAPK pathways. In contrast to animal cells, where crosstalk between Ca(2+) and MAPK signalling is well established, CPK3 seems to act independently of those pathways. Salt-induced transcriptional induction of known salt stress-regulated and MAPK-dependent marker genes was not altered, whereas post-translational protein phosphorylation patterns from roots of wild type and cpk3 plants revealed clear differences. A significant portion of CPK3 was found to be associated with the plasma membrane and the vacuole, both depending on its N-terminal myristoylation. An initial proteomic study led to the identification of 28 potential CPK3 targets, predominantly membrane-associated proteins.

Comparative analysis of soybean plasma membrane proteins under osmotic stress using gel-based and LC MS/MS-based proteomics approaches

To study the soybean plasma membrane proteome under osmotic stress, two methods were used: a gel-based and a LC MS/MS-based proteomics method. Two-day-old seedlings were subjected to 10% PEG for 2 days. Plasma membranes were purified from seedlings using a two-phase partitioning method and their purity was verified by measuring ATPase activity. Using the gel-based proteomics, four and eight protein spots were identified as up- and downregulated, respectively, whereas in the nanoLC MS/MS approach, 11 and 75 proteins were identified as up- and downregulated, respectively, under PEG treatment. Out of osmotic stress responsive proteins, most of the transporter proteins and all proteins with high number of transmembrane helices as well as low-abundance proteins could be identified by the LC MS/MS-based method. Three homologues of plasma membrane H(+)-ATPase, which are transporter proteins involved in ion efflux, were upregulated under osmotic stress. Gene expression of this protein was increased after 12 h of stress exposure. Among the identified proteins, seven proteins were mutual in two proteomics techniques, in which calnexin was the highly upregulated protein. Accumulation of calnexin in plasma membrane was confirmed by immunoblot analysis. These results suggest that under hyperosmotic conditions, calnexin accumulates in the plasma membrane and ion efflux accelerates by upregulation of plasma membrane H(+)-ATPase protein.

Lipid asymmetry in plant plasma membranes: phosphate deficiency-induced phospholipid replacement is restricted to the cytosolic leaflet

As in other eukaryotes, plant plasma membranes contain sphingolipids, phospholipids, and free sterols. In addition, plant plasma membranes also contain sterol derivatives and usually <5 mol% of a galactolipid, digalactosyldiacylglycerol (DGDG). We earlier reported that compared to fully fertilized oats (Avena sativa), oats cultivated without phosphate replaced up to 70 mol% of the root plasma membrane phospholipids with DGDG. Here, we investigated the implications of a high DGDG content on membrane properties. The phospholipid-to-DGDG replacement almost exclusively occurred in the cytosolic leaflet, where DGDG constituted up to one-third of the lipids. In the apoplastic (exoplasmic) leaflet, as well as in rafts, phospholipids were not replaced by DGDG, but by acylated sterol glycosides. Liposome studies revealed that the chain ordering in free sterol/phospholipid mixtures clearly decreased when >5 mol% DGDG was included. As both the apoplastic plasma membrane leaflet (probably the major water permeability barrier) and rafts both contain only trace amounts of DGDG, we conclude that this lipid class is not compatible with membrane functions requiring a high degree of lipid order. By not replacing phospholipids site specifically with DGDG, negative functional effects of this lipid in the plasma membrane are avoided.-Tjellström, H., Hellgren, L. I., Wieslander, A., Sandelius, A. S. Lipid asymmetry in plant plasma membranes: phosphate deficiency-induced phospholipid replacement is restricted to the cytosolic leaflet.

Proteomics of plasma membranes from poplar trees reveals tissue distribution of transporters, receptors, and proteins in cell wall formation

By exploiting the abundant tissues available from Populus trees, 3-4 m high, we have been able to isolate plasma membranes of high purity from leaves, xylem, and cambium/phloem at a time (4 weeks after bud break) when photosynthesis in the leaves and wood formation in the xylem should have reached a steady state. More than 40% of the 956 proteins identified were found in the plasma membranes of all three tissues and may be classified as "housekeeping" proteins, a typical example being P-type H(+)-ATPases. Among the 213 proteins predicted to be integral membrane proteins, transporters constitute the largest class (41%) followed by receptors (14%) and proteins involved in cell wall and carbohydrate metabolism (8%) and membrane trafficking (8%). ATP-binding cassette transporters (all members of subfamilies B, C, and G) and receptor-like kinases (four subfamilies) were two of the largest protein families found, and the members of these two families showed pronounced tissue distribution. Leaf plasma membranes were characterized by a very high proportion of transporters, constituting almost half of the integral proteins. Proteins involved in cell wall synthesis (such as cellulose and sucrose synthases) and membrane trafficking were most abundant in xylem plasma membranes in agreement with the role of the xylem in wood formation. Twenty-five integral proteins and 83 soluble proteins were exclusively found in xylem plasma membranes, which identifies new candidates associated with cell wall synthesis and wood formation. Among the proteins uniquely found in xylem plasma membranes were most of the enzymes involved in lignin biosynthesis, which suggests that they may exist as a complex linked to the plasma membrane.

Quantitative proteomics reveals a dynamic association of proteins to detergent-resistant membranes upon elicitor signaling in tobacco

A large body of evidence from the past decade supports the existence, in membrane from animal and yeast cells, of functional microdomains playing important roles in protein sorting, signal transduction, or infection by pathogens. In plants, as previously observed for animal microdomains, detergent-resistant fractions, enriched in sphingolipids and sterols, were isolated from plasma membrane. A characterization of their proteic content revealed their enrichment in proteins involved in signaling and response to biotic and abiotic stress and cell trafficking suggesting that these domains were likely to be involved in such physiological processes. In the present study, we used (14)N/(15)N metabolic labeling to compare, using a global quantitative proteomics approach, the content of tobacco detergent-resistant membranes extracted from cells treated or not with cryptogein, an elicitor of defense reaction. To analyze the data, we developed a software allowing an automatic quantification of the proteins identified. The results obtained indicate that, although the association to detergent-resistant membranes of most proteins remained unchanged upon cryptogein treatment, five proteins had their relative abundance modified. Four proteins related to cell trafficking (four dynamins) were less abundant in the detergent-resistant membrane fraction after cryptogein treatment, whereas one signaling protein (a 14-3-3 protein) was enriched. This analysis indicates that plant microdomains could, like their animal counterpart, play a role in the early signaling process underlying the setup of defense reaction. Furthermore proteins identified as differentially associated to tobacco detergent-resistant membranes after cryptogein challenge are involved in signaling and vesicular trafficking as already observed in similar studies performed in animal cells upon biological stimuli. This suggests that the ways by which the dynamic association of proteins to microdomains could participate in the regulation of the signaling process may be conserved between plant and animals.

Proteome analysis of detergent-resistant membranes (DRMs) associated with OsRac1-mediated innate immunity in rice

OsRac1, a member of the Rac/Rop GTPase family, plays important roles as a molecular switch in rice innate immunity, and the active form of OsRac1 functions in the plasma membrane (PM). To study the precise localization of OsRac1 in the PM and its possible association with other signaling components, we performed proteomic analysis of DRMs (detergent-resistant membranes) isolated from rice suspension-cultured cells transformed with myc-tagged constitutively active (CA) OsRac1. DRMs are regions of the PM that are insoluble after Triton X-100 treatment under cold conditions and are thought to be involved in various signaling processes in animal, yeast and plant cells. We identified 192 proteins in DRMs that included receptor-like kinases (RLKs) such as Xa21, nucleotide-binding leucine-rich repeat (NB-LRR)-type disease resistance proteins, a glycosylphosphatidylinositol (GPI)-anchored protein, syntaxin, NADPH oxidase, a WD-40 repeat family protein and various GTP-binding proteins. Many of these proteins have been previously identified in the DRMs isolated from other plant species, and animal and yeast cells, validating the methods used in our study. To examine the possible association of DRMs and OsRac1-mediated innate immunity, we used rice suspension-cultured cells transformed with myc-tagged wild-type (WT) OsRac1 and found that OsRac1 and RACK1A, an effector of OsRac1, shifted to the DRMs after chitin elicitor treatment. These results suggest that OsRac1-mediated innate immunity is associated with DRMs in the PM.

Phospholipase D family interactions with the cytoskeleton: isoform delta promotes plasma membrane anchoring of cortical microtubules

Phospholipase D (PLD) is a key enzyme in signal transduction - mediating plant responses to various environmental stresses including drought and salinity. Isotype PLDδ interacts with the microtubule cytoskeleton, although it is unclear if, or how, each of the 12 PLD isotypes in Arabidopsis may be involved mechanistically. We employed RNA interference in epidermal cells of Allium porrum L. (leek) leaves, in which the developmental reorientation of cortical microtubule arrays to a longitudinal direction is highly sensitive to experimental manipulation. Using particle bombardment and transient transformation with synthetic siRNAs targeting AtPLDα, β, γ, δ, ॉ and ζ, we examined the effect of 'cross-target' silencing orthologous A. porrum genes on microtubule reorientation dynamics during cell elongation. Co-transformation of individual siRNAs together with a GFP-MBD microtubule-reporter gene revealed that siRNAs targeting AtPLDδ promoted, whereas siRNAs targeting AtPLDβ and γ reduced, longitudinal microtubule orientation in A. porrum. These PLD isotypes, therefore, interact, directly or indirectly, with the cytoskeleton and the microtubule-plasma membrane interface. The unique response of PLDδ to silencing, along with its exclusive localisation to the plasma membrane, indicates that this isotype is specifically involved in promoting microtubule-membrane anchorage.

NADPH oxidase-mediated reactive oxygen species production: subcellular localization and reassessment of its role in plant defense

Chemiluminescence detection of reactive oxygen species (ROS) triggered in tobacco BY-2 cells by the fungal elicitor cryptogein was previously demonstrated to be abolished in cells transformed with an antisense construct of the plasma membrane NADPH oxidase, NtrbohD. Here, using electron microscopy, it has been confirmed that the first hydrogen peroxide production occurring a few minutes after challenge of tobacco cells with cryptogein is plasma membrane located and NtrbohD mediated. Furthermore, the presence of NtrbohD in detergent-resistant membrane fractions could be associated with the presence of NtrbohD-mediated hydrogen peroxide patches along the plasma membrane. Comparison of the subcellular localization of ROS in wild-type tobacco and in plants transformed with antisense constructs of NtrbohD revealed that this enzyme is also responsible for the hydrogen peroxide production occurring at the plasma membrane after infiltration of tobacco leaves with cryptogein. Finally, the reactivity of wild-type and transformed plants to the elicitor and their resistance against the pathogenic oomycete Phytophthora parasitica were examined. NtrbohD-mediated hydrogen peroxide production does not seem determinant for either hypersensitive response development or the establishment of acquired resistance but it is most likely involved in the signaling pathways associated with the protection of the plant cell.

Membrane-associated, boron-interacting proteins isolated by boronate affinity chromatography

Boron deficiency symptoms point to a role for boron in plant membranes, but the molecular partners interacting with boron have not yet been identified. The objective of the present study was to isolate and identify membrane-associated proteins with an ability to interact with boron. Boron-interacting proteins were isolated from root microsomal preparations of arabidopsis (Arabidopsis thaliana) and maize (Zea mays) using phenylboronate affinity chromatography, subsequently separated by two-dimensional gel electrophoresis and identified using MALDI-TOF (matrix-assisted laser desorption ionization-time of flight) peptide mass fingerprinting. Twenty-six boron-binding membrane-associated proteins were identified in A. thaliana, and nine in Z. mays roots. Additional unidentified proteins were also present. Common to both species were the beta-subunit of mitochondrial ATP synthase, several beta-glucosidases, a luminal-binding protein and fructose bisphosphate aldolase. In A. thaliana, binding of these proteins to boron was significantly reduced after 4 d of boron deprivation. The relatively high number of diverse proteins identified as boron interacting, many of which are usually enriched in membrane microdomains, supports the hypothesis that boron plays a role in plant membranes by cross-linking glycoproteins, and may be involved in their recruitment to membrane microdomains.

Remorin, a solanaceae protein resident in membrane rafts and plasmodesmata, impairs potato virus X movement

Remorins (REMs) are proteins of unknown function specific to vascular plants. We have used imaging and biochemical approaches and in situ labeling to demonstrate that REM clusters at plasmodesmata and in approximately 70-nm membrane domains, similar to lipid rafts, in the cytosolic leaflet of the plasma membrane. From a manipulation of REM levels in transgenic tomato (Solanum lycopersicum) plants, we show that Potato virus X (PVX) movement is inversely related to REM accumulation. We show that REM can interact physically with the movement protein TRIPLE GENE BLOCK PROTEIN1 from PVX. Based on the localization of REM and its impact on virus macromolecular trafficking, we discuss the potential for lipid rafts to act as functional components in plasmodesmata and the plasma membrane.

The cell wall and secretory proteome of a tobacco cell line synthesising secondary wall

The utility of plant secondary cell wall biomass for industrial and biofuel purposes depends upon improving cellulose amount, availability and extractability. The possibility of engineering such biomass requires much more knowledge of the genes and proteins involved in the synthesis, modification and assembly of cellulose, lignin and xylans. Proteomic data are essential to aid gene annotation and understanding of polymer biosynthesis. Comparative proteomes were determined for secondary walls of stem xylem and transgenic xylogenic cells of tobacco and detected peroxidase, cellulase, chitinase, pectinesterase and a number of defence/cell death related proteins, but not marker proteins of primary walls such as xyloglucan endotransglycosidase and expansins. Only the corresponding detergent soluble proteome of secretory microsomes from the xylogenic cultured cells, subjected to ion-exchange chromatography, could be determined accurately since, xylem-specific membrane yields were of poor quality from stem tissue. Among the 109 proteins analysed, many of the protein markers of the ER such as BiP, HSP70, calreticulin and calnexin were identified, together with some of the biosynthetic enzymes and associated polypeptides involved in polymer synthesis. However 53% of these endomembrane proteins failed identification despite the use of two different MS methods, leaving considerable possibilities for future identification of novel proteins involved in secondary wall polymer synthesis once full genomic data are available.

Plasma membrane microdomains from hybrid aspen cells are involved in cell wall polysaccharide biosynthesis

Detergent-resistant plasma membrane microdomains [DRMs (detergent-resistant membranes)] were isolated recently from several plant species. As for animal cells, a large range of cellular functions, such as signal transduction, endocytosis and protein trafficking, have been attributed to plant lipid rafts and DRMs. The data available are essentially based on proteomics and more approaches need to be undertaken to elucidate the precise function of individual populations of DRMs in plants. We report here the first isolation of DRMs from purified plasma membranes of a tree species, the hybrid aspen Populus tremula x tremuloides, and their biochemical characterization. Plasma membranes were solubilized with Triton X-100 and the resulting DRMs were isolated by flotation in sucrose density gradients. The DRMs were enriched in sterols, sphingolipids and glycosylphosphatidylinositol-anchored proteins and thus exhibited similar properties to DRMs from other species. However, they contained key carbohydrate synthases involved in cell wall polysaccharide biosynthesis, namely callose [(1-->3)-beta-D-glucan] and cellulose synthases. The association of these enzymes with DRMs was demonstrated using specific glucan synthase assays and antibodies, as well as biochemical and chemical approaches for the characterization of the polysaccharides synthesized in vitro by the isolated DRMs. More than 70% of the total glucan synthase activities present in the original plasma membranes was associated with the DRM fraction. In addition to shedding light on the lipid environment of callose and cellulose synthases, our results demonstrate the involvement of DRMs in the biosynthesis of important cell wall polysaccharides. This novel concept suggests a function of plant membrane microdomains in cell growth and morphogenesis.

Purification of low-abundance Arabidopsis plasma-membrane protein complexes and identification of candidate components

Purification of low-abundance plasma-membrane (PM) protein complexes is a challenging task. We devised a tandem affinity purification tag termed the HPB tag, which contains the biotin carboxyl carrier protein domain (BCCD) of Arabidopsis 3-methylcrotonal CoA carboxylase. The BCCD is biotinylated in vivo, and the tagged protein can be captured by streptavidin beads. All five C-terminally tagged Arabidopsis proteins tested, including four PM proteins, were functional and biotinylated with high efficiency in Arabidopsis. Transgenic Arabidopsis plants expressing an HPB-tagged protein, RPS2::HPB, were used to develop a method to purify protein complexes containing the HPB-tagged protein. RPS2 is a membrane-associated disease resistance protein of low abundance. The purification method involves microsomal fractionation, chemical cross-linking, solubilization, and one-step affinity purification using magnetic streptavidin beads, followed by protein identification using LC-MS/MS. We identified RIN4, a known RPS2 interactor, as well as other potential components of the RPS2 complex(es). Thus, the HPB tag method is suitable for the purification of low-abundance PM protein complexes.

Cell wall polysaccharide synthases are located in detergent-resistant membrane microdomains in oomycetes

The pathways responsible for cell wall polysaccharide biosynthesis are vital in eukaryotic microorganisms. The corresponding synthases are potential targets of inhibitors such as fungicides. Despite their fundamental and economical importance, most polysaccharide synthases are not well characterized, and their molecular mechanisms are poorly understood. With the example of Saprolegnia monoica as a model organism, we show that chitin and (1-->3)-beta-d-glucan synthases are located in detergent-resistant membrane microdomains (DRMs) in oomycetes, a phylum that comprises some of the most devastating microorganisms in the agriculture and aquaculture industries. Interestingly, no cellulose synthase activity was detected in the DRMs. The purified DRMs exhibited similar biochemical features as lipid rafts from animal, plant, and yeast cells, although they contained some species-specific lipids. This report sheds light on the lipid environment of the (1-->3)-beta-d-glucan and chitin synthases, as well as on the sterol biosynthetic pathways in oomycetes. The results presented here are consistent with a function of lipid rafts in cell polarization and as platforms for sorting specific sets of proteins targeted to the plasma membrane, such as carbohydrate synthases. The involvement of DRMs in the biosynthesis of major cell wall polysaccharides in eukaryotic microorganisms suggests a function of lipid rafts in hyphal morphogenesis and tip growth.

Very long chain fatty acid and lipid signaling in the response of plants to pathogens

Recent findings indicate that lipid signaling is essential for plant resistance to pathogens. Besides oxylipins and unsaturated fatty acids known to play important signaling functions during plant-pathogen interactions, the very long chain fatty acid (VLCFA) biosynthesis pathway has been recently associated to plant defense through different aspects. VLCFAs are indeed required for the biosynthesis of the plant cuticle and the generation of sphingolipids. Elucidation of the roles of these lipids in biotic stress responses is the result of the use of genetic approaches together with the identification of the genes/proteins involved in their biosynthesis. This review focuses on recent observations which revealed the complex function of the cuticle and cuticle-derived signals, and the key role of sphingolipids as bioactive molecules involved in signal transduction and cell death regulation during plant-pathogen interactions.

Alterations in detergent-resistant plasma membrane microdomains in Arabidopsis thaliana during cold acclimation

Microdomains in the plasma membrane (PM) have been proposed to be involved in many important cellular events in plant cells. To understand the role of PM microdomains in plant cold acclimation, we isolated the microdomains as detergent-resistant plasma membrane fractions (DRMs) from Arabidopsis seedlings and compared lipid and protein compositions before and after cold acclimation. The DRM was enriched in sterols and glucocerebrosides, and the proportion of free sterols in the DRM increased after cold acclimation. The protein-to-lipid ratio in the DRM was greater than that in the total PM fraction. The protein amount recovered in DRMs decreased gradually during cold acclimation. Cold acclimation further resulted in quantitative changes in DRM protein profiles. Subsequent mass spectrometry and Western blot analyses revealed that P-type H(+)-ATPases, aquaporins and endocytosis-related proteins increased and, conversely, tubulins, actins and V-type H(+)-ATPase subunits decreased in DRMs during cold acclimation. Functional categorization of cold-responsive proteins in DRMs suggests that plant PM microdomains function as platforms of membrane transport, membrane trafficking and cytoskeleton interaction. These comprehensive changes in microdomains may be associated with cold acclimation of Arabidopsis.

Exocytosis and cell polarity in plants - exocyst and recycling domains

In plants, exocytosis is a central mechanism of cell morphogenesis. We still know surprisingly little about some aspects of this process, starting with exocytotic vesicle formation, which may take place at the trans-Golgi network even without coat assistance, facilitated by the local regulation of membrane lipid organization. The RabA4b guanosine triphosphatase (GTPase), recruiting phosphatidylinositol-4-kinase to the trans-Golgi network, is a candidate vesicle formation organizer. However, in plant cells, there are obviously additional endosomal source compartments for secretory vesicles. The Rho/Rop GTPase regulatory module is central for the initiation of exocytotically active domains in plant cell cortex (activated cortical domains). Most plant cells exhibit several distinct plasma membrane domains, established and maintained by endocytosis-driven membrane recycling. We propose the concept of a 'recycling domain', uniting the activated cortical domain and the connected endosomal compartments, as a dynamic spatiotemporal entity. We have recently described the exocyst tethering complex in plant cells. As a result of the multiplicity of its putative Exo70 subunits, this complex may belong to core regulators of recycling domain organization, including the generation of multiple recycling domains within a single cell. The conventional textbook concept that the plant secretory pathway is largely constitutive is misleading.

Post-transcriptional regulation of auxin transport proteins: cellular trafficking, protein phosphorylation, protein maturation, ubiquitination, and membrane composition

Auxin concentration gradients, established by polar transport of auxin, are essential for the establishment and maintenance of polar growth and morphological patterning. Three families of cellular transport proteins, PIN-formed (PIN), P-glycoprotein (ABCB/PGP), and AUXIN RESISTANT 1/LIKE AUX1 (AUX1/LAX), can independently and co-ordinately transport auxin in plants. Regulation of these proteins involves intricate and co-ordinated cellular processes, including protein-protein interactions, vesicular trafficking, protein phosphorylation, ubiquitination, and stabilization of the transporter complexes on the plasma membrane.

ABCB19/PGP19 stabilises PIN1 in membrane microdomains in Arabidopsis

Auxin transport is mediated at the cellular level by three independent mechanisms that are characterised by the PIN-formed (PIN), P-glycoprotein (ABCB/PGP) and AUX/LAX transport proteins. The PIN and ABCB transport proteins, best represented by PIN1 and ABCB19 (PGP19), have been shown to coordinately regulate auxin efflux. When PIN1 and ABCB19 coincide on the plasma membrane, their interaction enhances the rate and specificity of auxin efflux and the dynamic cycling of PIN1 is reduced. However, ABCB19 function is not regulated by the dynamic cellular trafficking mechanisms that regulate PIN1 in apical tissues, as localisation of ABCB19 on the plasma membrane was not inhibited by short-term treatments with latrunculin B, oryzalin, brefeldin A (BFA) or wortmannin--all of which have been shown to alter PIN1 and/or PIN2 plasma membrane localisation. When taken up by endocytosis, the styryl dye FM4-64 labels diffuse rather than punctuate intracellular bodies in abcb19 (pgp19), and some aggregations of PIN1 induced by short-term BFA treatment did not disperse after BFA washout in abcb19. Although the subcellular localisations of ABCB19 and PIN1 in the reciprocal mutant backgrounds were like those in wild type, PIN1 plasma membrane localisation in abcb19 roots was more easily perturbed by the detergent Triton X-100, but not other non-ionic detergents. ABCB19 is stably associated with sterol/sphingolipid-enriched membrane fractions containing BIG/TIR3 and partitions into Triton X-100 detergent-resistant membrane (DRM) fractions. In the wild type, PIN1 was also present in DRMs, but was less abundant in abcb19 DRMs. These observations suggested a rationale for the observed lack of auxin transport activity when PIN1 is expressed in a non-plant heterologous system. PIN1 was therefore expressed in Schizosaccharomyces pombe, which has plant-like sterol-enriched microdomains, and catalysed auxin transport in these cells. These data suggest that ABCB19 stabilises PIN1 localisation at the plasma membrane in discrete cellular subdomains where PIN1 and ABCB19 expression overlaps.

SNAREs--molecular governors in signalling and development

SNARE (soluble N-ethylmaleimide-sensitive factor protein attachment protein receptor) proteins drive membrane fusion and contribute to membrane and protein targeting and delivery in all eukaryotic cells. SNAREs are essential to the mechanics of cell growth and development, and they facilitate a number of homeostatic and evoked responses in plants, from hormone signalling to pathogen defence. Additionally, there is now unambiguous evidence that SNAREs play roles in anchoring other membrane proteins and in facilitating ion channel gating through direct, physical interaction with channel proteins. What is the physiological significance of these additional features of plant SNAREs? We explore possible interpretations and suggest functions as scaffolds for effective signal transmission between proteins and, by analogy with a mechanical device invented by James Watt, as molecular governors to coordinate solute transport with cell expansion and growth.

Heterogeneity and lateral compartmentalization of plant plasma membranes

Membrane specialization through lateral compartmentalization is pivotal to the development of organisms and their response to environmental signals. The membrane raft hypothesis is lively discussed as a concept for domain formation. In recent years plant scientists have begun to critically assess the membrane raft hypothesis, and this provided the first insights into the mechanisms underlying microdomain formation in plant plasma membranes. Several groups have now shown that phytosterols can induce phase separation, a prerequisite for the formation of membrane rafts. Furthermore, the protein repertoire of detergent-resistant membranes (DRMs) has been extensively characterized and the degree of fatty acid desaturation has been identified as an important factor in DRM formation. Recent studies comprising sterol-deficient mutants demonstrated the importance of correct sterol composition and endocytosis for proper membrane compartmentalization.

Structure-function analysis of plant aquaporin AtPIP2;1 gating by divalent cations and protons

Water channel proteins, AQPs (aquaporins), of the PIP (plasma membrane intrinsic protein) subfamily, provide a means for fine and quick adjustments of the plant water status. A molecular model for gating of PIPs by cytosolic protons (H(+)) and divalent cations was derived from the atomic structure of spinach SoPIP2;1 (Spinacia oleracea PIP2;1) in an open- and a closed-pore conformation. In the present study, we produced the Arabidopsis AtPIP2;1 (Arabidopsis thaliana PIP2;1) homologue in Pichia pastoris, either WT (wild-type) or mutations at residues supposedly involved in gating. Stopped-flow spectrophotometric measurements showed that, upon reconstitution in proteoliposomes, all forms function as water channels. The first functional evidence for a direct gating of PIPs by divalent (bivalent) cations was obtained. In particular, cadmium and manganese were identified, in addition to calcium (Ca(2+)) and H(+) as potent inhibitors of WT AtPIP2;1. Our results further show that His(199), the previously identified site for H(+) sensing, but also N-terminal located Glu(31), and to a lesser extent Asp(28), are involved in both divalent-cation- and H(+)-mediated gating. In contrast, mutation of Arg(124) rendered AtPIP2;1 largely insensitive to Ca(2+) while remaining fully sensitive to H(+). The role of these residues in binding divalent cations and/or stabilizing the open or closed pore conformations is discussed.

Definition of Arabidopsis sterol-rich membrane microdomains by differential treatment with methyl-beta-cyclodextrin and quantitative proteomics

Plasma membranes are dynamic compartments with key functions in solute transport, cell shape, and communication between cells and the environment. In mammalian cells and yeast, the plasma membrane has been shown to be compartmented into so-called lipid rafts, which are defined by their resistance to treatment with non-ionic detergents. In plants, the existence of lipid rafts has been postulated, but the precise composition of this membrane compartment is still under debate. Here we were able to experimentally clearly distinguish (i) true sterol-dependent “raft proteins” and (ii) sterol-independent “non-raft” proteins and co-purifying “contaminants” in plant detergent-resistant membranes. We used quantitative proteomics techniques involving ¹⁵N metabolic labeling and specific disruption of sterol-rich membrane domains by methyl-β-cyclodextrin. Among the sterol-dependent proteins we found an over-representation of glycosylphosphatidylinositol-anchored proteins. A large fraction of these proteins has functions in cell wall anchoring. We were able to distinguish constant and variable components of plant sterol-rich membrane microdomains based on their responsiveness to the drug methyl-β-cyclodextrin. Predominantly proteins with signaling functions, such as receptor kinases, G-proteins, and calcium signaling proteins, were identified as variable members in plant lipid rafts, whereas cell wall-related proteins and specific proteins with unknown functions make up a core set of sterol-dependent plant plasma membrane proteins. This allows the plant to maintain a balance between static anchoring of cell shape forming elements and variable adjustment to changing external conditions.

RING1 E3 ligase localizes to plasma membrane lipid rafts to trigger FB1-induced programmed cell death in Arabidopsis

Ubiquitination plays important roles in plant development, including programmed cell death. Here, we characterize a novel membrane-bound RING motif protein, encoded by RING1, that is expressed at a low level in all Arabidopsis tissues but can be upregulated by fumonisin B1 (FB1) treatment and pathogen infection. RING1 displays E3 ubiquitin ligase activity in vitro, which is dependent on the integrity of the RING motif. GFP fusion protein localization and cell fractionation experiments show that this E3 ligase is associated with the lipid rafts of plasma membranes. Knock-down of RING1 transcripts using artificial microRNA (amiR-R1(159)) leads to FB1 hyposensitivity, but overexpression of RING1 confers hypersensitivity. Additionally, expression of the pathogenesis-related 1 (PR-1) gene is lower and delayed in amiR-R1(159) plants compared with wild-type and RING1-overexpressing plants. The FB1 hyposensitivity of amiR-R1(159) plants can be rescued by expression of cleavage-resistant RING1mut transcripts. Our results suggest that RING1 acts as a signal from the plasma membrane lipid rafts to trigger the FB1-induced plant programmed cell death pathway.

Depletion of phytosterols from the plant plasma membrane provides evidence for disruption of lipid rafts

Involvement of sterols in membrane structural properties has been extensively studied in model systems but rarely assessed in natural membranes and never investigated for the plant plasma membrane (PM). Here, we address the question of the role of phytosterols in the organization of the plant PM. The sterol composition of tobacco BY-2 cell PM was determined by gas chromatography. The cyclic oligosaccharide methyl-beta-cyclodextrin, commonly used in animal cells to decrease cholesterol levels, caused a drastic reduction (50%) in the PM total free sterol content of the plant material, without modification in amounts of steryl-conjugates. Fluorescence spectroscopy experiments using DPH, TMA-DPH, Laurdan, and di-4-ANEPPDHQ indicated that such a depletion in sterol content increased lipid acyl chain disorder and reduced the overall liquid-phase heterogeneity in correlation with the disruption of phytosterol-rich domains. Methyl-beta-cyclodextrin also prevented isolation of a PM fraction resistant to solubilization by nonionic detergents, previously characterized in tobacco, and induced redistribution of the proteic marker of this fraction, NtrbohD, within the membrane. Altogether, our results support the role of phytosterols in the lateral structuring of the PM of higher plant cells and suggest that they are key compounds for the formation of plant PM microdomains.

The secretory system of Arabidopsis

Over the past few years, a vast amount of research has illuminated the workings of the secretory system of eukaryotic cells. The bulk of this work has been focused on the yeast Saccharomyces cerevisiae, or on mammalian cells. At a superficial level, plants are typical eukaryotes with respect to the operation of the secretory system; however, important differences emerge in the function and appearance of endomembrane organelles. In particular, the plant secretory system has specialized in several ways to support the synthesis of many components of the complex cell wall, and specialized kinds of vacuole have taken on a protein storage role-a role that is intended to support the growing seedling, but has been co-opted to support human life in the seeds of many crop plants. In the past, most research on the plant secretory system has been guided by results in mammalian or fungal systems but recently plants have begun to stand on their own as models for understanding complex trafficking events within the eukaryotic endomembrane system.

Enemy at the gates: traffic at the plant cell pathogen interface

The plant apoplast constitutes a space for early recognition of potentially harmful non-self. Basal pathogen recognition operates via dynamic sensing of conserved microbial patterns by pattern recognition receptors or of elicitor-active molecules released from plant cell walls during infection. Recognition elicits defence reactions depending on cellular export via SNARE (soluble N-ethylmaleimide-sensitive factor attachment protein receptor) complex-mediated vesicle fusion or plasma membrane transporter activity. Lipid rafts appear also involved in focusing immunity-associated proteins to the site of pathogen contact. Simultaneously, pathogen effectors target recognition, apoplastic host proteins and transport for cell wall-associated defence. This microreview highlights most recent reports on the arms race for plant disease and immunity at the cell surface.