Traffic Routes and Signals for the Tonoplast

NET4 and RabG3 link actin to the tonoplast and facilitate cytoskeletal remodelling during stomatal immunity

Members of the NETWORKED (NET) family are involved in actin-membrane interactions. Here we show that two members of the NET family, NET4A and NET4B, are essential for normal guard cell actin reorganization, which is a process critical for stomatal closure in plant immunity. NET4 proteins interact with F-actin and with members of the Rab7 GTPase RABG3 family through two distinct domains, allowing for simultaneous localization to actin filaments and the tonoplast. NET4 proteins interact with GTP-bound, active RABG3 members, suggesting their function being downstream effectors. We also show that RABG3b is critical for stomatal closure induced by microbial patterns. Taken together, we conclude that the actin cytoskeletal remodelling during stomatal closure involves a molecular link between actin filaments and the tonoplast, which is mediated by the NET4-RABG3b interaction. We propose that stomatal closure to microbial patterns involves the coordinated action of immune-triggered osmotic changes and actin cytoskeletal remodelling likely driving compact vacuolar morphologies.

Fluorescent Auxin Analogs Report Two Auxin Binding Sites with Different Subcellular Distribution and Affinities: A Cue for Non-Transcriptional Auxin Signaling

The complexity of auxin signaling is partially due to multiple auxin receptors that trigger differential signaling. To obtain insight into the subcellular localization of auxin-binding sites, we used fluorescent auxin analogs that can undergo transport but do not deploy auxin signaling. Using fluorescent probes for different subcellular compartments, we can show that the fluorescent analog of 1-naphthaleneacetic acid (NAA) associates with the endoplasmic reticulum (ER) and tonoplast, while the fluorescent analog of indole acetic acid (IAA) binds to the ER. The binding of the fluorescent NAA analog to the ER can be outcompeted by unlabeled NAA, which allows us to estimate the affinity of NAA for this binding site to be around 1 μM. The non-transportable auxin 2,4-dichlorophenoxyacetic acid (2,4-D) interferes with the binding site for the fluorescent NAA analog at the tonoplast but not with the binding site for the fluorescent IAA analog at the ER. We integrate these data into a working model, where the tonoplast hosts a binding site with a high affinity for 2,4-D, while the ER hosts a binding site with high affinity for NAA. Thus, the differential subcellular localization of binding sites reflects the differential signaling in response to these artificial auxins.

Sec24C mediates a Golgi-independent trafficking pathway that is required for tonoplast localisation of ABCC1 and ABCC2

Protein sorting is an essential biological process in all organisms. Trafficking membrane proteins generally relies on the sorting machinery of the Golgi apparatus. However, many proteins have been found to be delivered to target locations via Golgi-independent pathways, but the mechanisms underlying this delivery system remain unknown. Here, we report that Sec24C mediates the direct secretory trafficking of the phytochelatin transporters ABCC1 and ABCC2 from the endoplasmic reticulum (ER) to prevacuolar compartments (PVCs) in Arabidopsis thaliana. Genetic analysis showed that the sec24c mutants are hypersensitive to cadmium (Cd) and arsenic (As) treatments due to mislocalisation of ABCC1 and ABCC2, which results in defects in the vacuole compartmentalisation of the toxic metals. Furthermore, we found that Sec24C recognises ABCC1 and ABCC2 through direct interactions to mediate their exit from the ER to PVCs, which is independent of brefeldin A-sensitive post-Golgi trafficking pathway. These findings expand our understanding of Golgi-independent trafficking, which also provide key insights regarding the mechanism of tonoplast protein sorting and open a new perspective on the function of Sec24 proteins.

Current Methods to Unravel the Functional Properties of Lysosomal Ion Channels and Transporters

A distinct set of channels and transporters regulates the ion fluxes across the lysosomal membrane. Malfunctioning of these transport proteins and the resulting ionic imbalance is involved in various human diseases, such as lysosomal storage disorders, cancer, as well as metabolic and neurodegenerative diseases. As a consequence, these proteins have stimulated strong interest for their suitability as possible drug targets. A detailed functional characterization of many lysosomal channels and transporters is lacking, mainly due to technical difficulties in applying the standard patch-clamp technique to these small intracellular compartments. In this review, we focus on current methods used to unravel the functional properties of lysosomal ion channels and transporters, stressing their advantages and disadvantages and evaluating their fields of applicability.

Tonoplast and Peroxisome Targeting of γ-tocopherol N-methyltransferase Homologs Involved in the Synthesis of Monoterpene Indole Alkaloids

Many plant species from the Apocynaceae, Loganiaceae and Rubiaceae families evolved a specialized metabolism leading to the synthesis of a broad palette of monoterpene indole alkaloids (MIAs). These compounds are believed to constitute a cornerstone of the plant chemical arsenal but above all several MIAs display pharmacological properties that have been exploited for decades by humans to treat various diseases. It is established that MIAs are produced in planta due to complex biosynthetic pathways engaging a multitude of specialized enzymes but also a complex tissue and subcellular organization. In this context, N-methyltransferases (NMTs) represent an important family of enzymes indispensable for MIA biosynthesis but their characterization has always remained challenging. In particular, little is known about the subcellular localization of NMTs in MIA-producing plants. Here, we performed an extensive analysis on the subcellular localization of NMTs from four distinct medicinal plants but also experimentally validated that two putative NMTs from Catharanthus roseus exhibit NMT activity. Apart from providing unprecedented data regarding the targeting of these enzymes in planta, our results point out an additional layer of complexity to the subcellular organization of the MIA biosynthetic pathway by introducing tonoplast and peroxisome as new actors of the final steps of MIA biosynthesis.

Detailed characterization of the UMAMIT proteins provides insight into their evolution, amino acid transport properties, and role in the plant

Amino acid transporters play a critical role in distributing amino acids within the cell compartments and between plant organs. Despite this importance, relatively few amino acid transporter genes have been characterized and their role elucidated with certainty. Two main families of proteins encode amino acid transporters in plants: the amino acid-polyamine-organocation superfamily, containing mostly importers, and the UMAMIT (usually multiple acids move in and out transporter) family, apparently encoding exporters, totaling 63 and 44 genes in Arabidopsis, respectively. Knowledge of UMAMITs is scarce, based on six Arabidopsis genes and a handful of genes from other species. To gain insight into the role of the members of this family and provide data to be used for future characterization, we studied the evolution of the UMAMITs in plants, and determined the functional properties, the structure, and localization of the 47 Arabidopsis UMAMITs. Our analysis showed that the AtUMAMITs are essentially localized at the tonoplast or the plasma membrane, and that most of them are able to export amino acids from the cytosol, confirming a role in intra- and intercellular amino acid transport. As an example, this set of data was used to hypothesize the role of a few AtUMAMITs in the plant and the cell.

An integrative view on vacuolar pH homeostasis in Arabidopsis thaliana: Combining mathematical modeling and experimentation

The acidification of plant vacuoles is of great importance for various physiological processes, as a multitude of secondary active transporters utilize the proton gradient established across the vacuolar membrane. Vacuolar-type H⁺ -translocating ATPases and a pyrophosphatase are thought to enable vacuoles to accumulate protons against their electrochemical potential. However, recent studies pointed to the ATPase located at the trans-Golgi network/early endosome (TGN/EE) to contribute to vacuolar acidification in a manner not understood as of now. Here, we combined experimental data and computational modeling to test different hypotheses for vacuolar acidification mechanisms. For this, we analyzed different models with respect to their ability to describe existing experimental data. To better differentiate between alternative acidification mechanisms, new experimental data have been generated. By fitting the models to the experimental data, we were able to prioritize the hypothesis in which vesicular trafficking of Ca²⁺ /H⁺ -antiporters from the TGN/EE to the vacuolar membrane and the activity of ATP-dependent Ca²⁺ -pumps at the tonoplast might explain the residual acidification observed in Arabidopsis mutants defective in vacuolar proton pump activity. The presented modeling approach provides an integrative perspective on vacuolar pH regulation in Arabidopsis and holds potential to guide further experimental work.

The trafficking machinery of lytic and protein storage vacuoles: how much is shared and how much is distinct?

Plant cells contain two types of vacuoles, the lytic vacuole (LV) and protein storage vacuole (PSV). LVs are present in vegetative cells, whereas PSVs are found in seed cells. The physiological functions of the two types of vacuole differ. Newly synthesized proteins must be transported to these vacuoles via protein trafficking through the endomembrane system for them to function. Recently, significant advances have been made in elucidating the molecular mechanisms of protein trafficking to these organelles. Despite these advances, the relationship between the trafficking mechanisms to the LV and PSV remains unclear. Some aspects of the trafficking mechanisms are common to both types of vacuole, but certain aspects are specific to trafficking to either the LV or PSV. In this review, we summarize recent findings on the components involved in protein trafficking to both the LV and PSV and compare them to examine the extent of overlap in the trafficking mechanisms. In addition, we discuss the interconnection between the LV and PSV provided by the protein trafficking machinery and the implications for the identity of these organelles.

Go your own way: membrane-targeting sequences

Membrane-targeting sequences, connected targeting mechanisms, and co-factors orchestrate primary targeting of proteins to membranes.

Tonoplast-localized Ca2+ pumps regulate Ca2+ signals during pattern-triggered immunity in Arabidopsis thaliana

One of the major events of early plant immune responses is a rapid influx of Ca2+ into the cytosol following pathogen recognition. Indeed, changes in cytosolic Ca2+ are recognized as ubiquitous elements of cellular signaling networks and are thought to encode stimulus-specific information in their duration, amplitude, and frequency. Despite the wealth of observations showing that the bacterial elicitor peptide flg22 triggers Ca2+ transients, there remain limited data defining the molecular identities of Ca2+ transporters involved in shaping the cellular Ca2+ dynamics during the triggering of the defense response network. However, the autoinhibited Ca2+-ATPase (ACA) pumps that act to expel Ca2+ from the cytosol have been linked to these events, with knockouts in the vacuolar members of this family showing hypersensitive lesion-mimic phenotypes. We have therefore explored how the two tonoplast-localized pumps, ACA4 and ACA11, impact flg22-dependent Ca2+ signaling and related defense responses. The double-knockout aca4/11 exhibited increased basal Ca2+ levels and Ca2+ signals of higher amplitude than wild-type plants. Both the aberrant Ca2+ dynamics and associated defense-related phenotypes could be suppressed by growing the aca4/11 seedlings at elevated temperatures. Relocalization of ACA8 from its normal cellular locale of the plasma membrane to the tonoplast also suppressed the aca4/11 phenotypes but not when a catalytically inactive mutant was used. These observations indicate that regulation of vacuolar Ca2+ sequestration is an integral component of plant immune signaling, but also that the action of tonoplast-localized Ca2+ pumps does not require specific regulatory elements not found in plasma membrane-localized pumps.

A Review of Plant Vacuoles: Formation, Located Proteins, and Functions

Vacuoles, cellular membrane-bound organelles, are the largest compartments of cells, occupying up to 90% of the volume of plant cells. Vacuoles are formed by the biosynthetic and endocytotic pathways. In plants, the vacuole is crucial for growth and development and has a variety of functions, including storage and transport, intracellular environmental stability, and response to injury. Depending on the cell type and growth conditions, the size of vacuoles is highly dynamic. Different types of cell vacuoles store different substances, such as alkaloids, protein enzymes, inorganic salts, sugars, etc., and play important roles in multiple signaling pathways. Here, we summarize vacuole formation, types, vacuole-located proteins, and functions.

Arabidopsis Class I Formin FH1 Relocates between Membrane Compartments during Root Cell Ontogeny and Associates with Plasmodesmata

Formins are evolutionarily conserved eukaryotic proteins engaged in actin nucleation and other aspects of cytoskeletal organization. Angiosperms have two formin clades with multiple paralogs; typical plant Class I formins are integral membrane proteins that can anchor cytoskeletal structures to membranes. For the main Arabidopsis housekeeping Class I formin, FH1 (At3g25500), plasmalemma localization was documented in heterologous expression and overexpression studies. We previously showed that loss of FH1 function increases cotyledon epidermal pavement cell shape complexity via modification of actin and microtubule organization and dynamics. Here, we employ transgenic Arabidopsis expressing green fluorescent protein-tagged FH1 (FH1-GFP) from its native promoter to investigate in vivo behavior of this formin using advanced microscopy techniques. The fusion protein is functional, since its expression complements the fh1 loss-of-function mutant phenotype. Accidental overexpression of FH1-GFP results in a decrease in trichome branch number, while fh1 mutation has the opposite effect, indicating a general role of this formin in controlling cell shape complexity. Consistent with previous reports, FH1-GFP associates with membranes. However, the protein exhibits surprising actin- and secretory pathway-dependent dynamic localization and relocates between cellular endomembranes and the plasmalemma during cell division and differentiation in root tissues, with transient tonoplast localization at the transition/elongation zones border. FH1-GFP also accumulates in actin-rich regions of cortical cytoplasm and associates with plasmodesmata in both the cotyledon epidermis and root tissues. Together with previous reports from metazoan systems, this suggests that formins might have a shared (ancestral or convergent) role at cell-cell junctions.

Microtubules play a role in trafficking prevacuolar compartments to vacuoles in tobacco pollen tubes

Fine regulation of exocytosis and endocytosis plays a basic role in pollen tube growth. Excess plasma membrane secreted during pollen tube elongation is known to be retrieved by endocytosis and partially reused in secretory pathways through the Golgi apparatus. Dissection of endocytosis has enabled distinct degradation pathways to be identified in tobacco pollen tubes and has shown that microtubules influence the transport of plasma membrane internalized in the tip region to vacuoles. Here, we used different drugs affecting the polymerization state of microtubules together with SYP21, a marker of prevacuolar compartments, to characterize trafficking of prevacuolar compartments in Nicotiana tabacum pollen tubes. Ultrastructural and biochemical analysis showed that microtubules bind SYP21-positive microsomes. Transient transformation of pollen tubes with LAT52-YFP-SYP21 revealed that microtubules play a key role in the delivery of prevacuolar compartments to tubular vacuoles.

Use Endosidin2 to Study Plant Exocytosis and Vacuolar Trafficking

Membrane trafficking is essential for multiple aspects of plant growth and development. Some small molecules have been widely used to study the mechanisms of membrane trafficking in plants. For example, short-term treatment with small molecules combined with live cell imaging has been shown to be very useful in understanding the dynamic processes of membrane trafficking. Small molecule Endosidin2 (ES2) has been found to target Exo70 protein and inhibit exocytosis and promote vacuolar trafficking in plants. Here we describe the method of using short-term ES2 treatment combined with live cell imaging to study plant exocytosis and vacuolar trafficking in Arabidopsis seedlings expressing a cargo protein PIN2:GFP.

The Use of Drugs in the Study of Vacuole Morphology and Trafficking to the Vacuole in Arabidopsis thaliana

Chemical compounds are useful to perturb biological functions in the same way as classical genetic approaches take advantage of mutations at the DNA level to perturb gene function. The use of bioactive chemicals currently called chemical genetic is especially valuable for cell biology. Chemical genetic approaches allow perturbations of cellular processes post-germination in a given time window controlling the severity of the effect by modifying or modulating the dose and/or the period of the treatment. Additionally, compounds can be applied directly to different mutants and translational fluorescent reporters/marker lines, expanding the repertoire of experimental setups addressing cell biology research. In this chapter, we describe standard protocols to visualize vacuole morphology and trafficking to the vacuole and the use of bioactive compounds as a proxy to study these biological processes.

Merging roads: chemical tools and cell biology to study unconventional protein secretion

The endomembrane trafficking network is highly complex and dynamic, with both conventional and so-called unconventional routes which are in essence recently discovered pathways that are poorly understood in plants. One approach to dissecting endomembrane pathways that we have pioneered is the use of chemical biology. Classical genetic manipulations often deal with indirect pleiotropic phenotypes resulting from the perturbation of key players of the trafficking routes. Many of these difficulties can be circumvented using small molecules to modify or disrupt the function or localization of key proteins regulating these pathways. In this review, we summarize how small molecules have been used as probes to define these pathways, and how they could be used to increase current knowledge of unconventional protein secretion pathways.

The endoplasmic reticulum is a hub to sort proteins toward unconventional traffic pathways and endosymbiotic organelles

The discovery that much of the extracellular proteome in eukaryotic cells consists of proteins lacking a signal peptide, which cannot therefore enter the secretory pathway, has led to the identification of alternative protein secretion routes bypassing the Golgi apparatus. However, proteins harboring a signal peptide for translocation into the endoplasmic reticulum can also be transported along these alternative routes, which are still far from being well elucidated in terms of the molecular machineries and subcellular/intermediate compartments involved. In this review, we first try to provide a definition of all the unconventional protein secretion pathways in eukaryotic cells, as those pathways followed by proteins directed to an 'external space' bypassing the Golgi, where 'external space' refers to the extracellular space plus the lumen of the secretory route compartments and the inner space of mitochondria and plastids. Then, we discuss the role of the endoplasmic reticulum in sorting proteins toward unconventional traffic pathways in plants. In this regard, various unconventional pathways exporting proteins from the endoplasmic reticulum to the vacuole, plasma membrane, apoplast, mitochondria, and plastids are described, including the short routes followed by the proteins resident in the endoplasmic reticulum.

Pumping up the volume - vacuole biogenesis in Arabidopsis thaliana

Plant architecture follows the need to collect CO_2, solar energy, water and mineral nutrients via large surface areas. It is by the presence of a central vacuole that fills much of the cell volume that plants manage to grow at low metabolic cost. In addition vacuoles buffer the fluctuating supply of essential nutrients and help to detoxify the cytosol when plants are challenged by harmful molecules. Despite their large size and multiple important functions, our knowledge of vacuole biogenesis and the machinery underlying their amazing dynamics is still fragmentary. In this review, we try to reconcile past and present models for vacuole biogenesis with the current knowledge of multiple parallel vacuolar trafficking pathways and the molecular machineries driving membrane fusion and organelle shape.

Chemical Genetic Dissection of Membrane Trafficking

The plant endomembrane system is an extensively connected functional unit for exchanging material between compartments. Secretory and endocytic pathways allow dynamic trafficking of proteins, lipids, and other molecules, regulating a myriad of biological processes. Chemical genetics-the use of compounds to perturb biological processes in a fast, tunable, and transient manner-provides elegant tools for investigating this system. Here, we review how chemical genetics has helped to elucidate different aspects of membrane trafficking. We discuss different strategies for uncovering the modes of action of such compounds and their use in unraveling membrane trafficking regulators. We also discuss how the bioactive chemicals that are currently used as probes to interrogate endomembrane trafficking were discovered and analyze the results regarding membrane trafficking and pathway crosstalk. The integration of different expertises and the rational implementation of chemical genetic strategies will improve the identification of molecular mechanisms that drive intracellular trafficking and our understanding of how trafficking interfaces with plant physiology and development.

Annexins as Overlooked Regulators of Membrane Trafficking in Plant Cells

Annexins are an evolutionary conserved superfamily of proteins able to bind membrane phospholipids in a calcium-dependent manner. Their physiological roles are still being intensively examined and it seems that, despite their general structural similarity, individual proteins are specialized toward specific functions. However, due to their general ability to coordinate membranes in a calcium-sensitive fashion they are thought to participate in membrane flow. In this review, we present a summary of the current understanding of cellular transport in plant cells and consider the possible roles of annexins in different stages of vesicular transport.

Dual-targeting of Arabidopsis DMP1 isoforms to the tonoplast and the plasma membrane

The reports of dual-targeted proteins in plants have steadily increased over the past years. The vast majority of these proteins are soluble proteins distributed between compartments of the non-secretory pathway, predominantly chloroplasts and mitochondria. In contrast, dual-targeted transmembrane proteins, especially of the secretory pathway, are rare and the mechanisms leading to their differential targeting remain largely unknown. Here, we report dual-targeting of the Arabidopsis DUF679 Membrane Protein 1 (DMP1) to the tonoplast (TP) and the plasma membrane (PM). In Arabidopsis and tobacco two equally abundant DMP1 isoforms are synthesized by alternative translation initiation: a full length protein, DMP1.1, and a truncated one, DMP1.2, which lacks the N-terminal 19 amino acids including a TP-targeting dileucine motif. Accumulation of DMP1.1 and DMP1.2 in the TP and the PM, respectively, is Brefeldin A-sensitive, indicating transit via the Golgi. However, DMP1.2 interacts with DMP1.1, leading to extensive rerouting of DMP1.2 to the TP and “eclipsed” localization of DMP1.2 in the PM where it is barely visible by confocal laser scanning microscopy but clearly detectable by membrane fractionation. It is demonstrated that eGFP fusion to either DMP1 terminus can cause mistargeting artifacts: C-terminal fusion to DMP1.1 or DMP1.2 results in altered ER export and N-terminal fusion to DMP1.1 causes mistargeting to the PM, presumably by masking of the TP targeting signal. These results illustrate how the interplay of alternative translation initiation, presence or absence of targeting information and rerouting due to protein-protein interaction determines the ultimate distribution of a transmembrane protein between two membranes.

Unconventional Transport Routes of Soluble and Membrane Proteins and Their Role in Developmental Biology

Many proteins and cargoes in eukaryotic cells are secreted through the conventional secretory pathway that brings proteins and membranes from the endoplasmic reticulum to the plasma membrane, passing through various cell compartments, and then the extracellular space. The recent identification of an increasing number of leaderless secreted proteins bypassing the Golgi apparatus unveiled the existence of alternative protein secretion pathways. Moreover, other unconventional routes for secretion of soluble or transmembrane proteins with initial endoplasmic reticulum localization were identified. Furthermore, other proteins normally functioning in conventional membrane traffic or in the biogenesis of unique plant/fungi organelles or in plasmodesmata transport seem to be involved in unconventional secretory pathways. These alternative pathways are functionally related to biotic stress and development, and are becoming more and more important in cell biology studies in yeast, mammalian cells and in plants. The city of Lecce hosted specialists working on mammals, plants and microorganisms for the inaugural meeting on “Unconventional Protein and Membrane Traffic” (UPMT) during 4–7 October 2016. The main aim of the meeting was to include the highest number of topics, summarized in this report, related to the unconventional transport routes of protein and membranes.

The Central Vacuole of the Diatom Phaeodactylum tricornutum: Identification of New Vacuolar Membrane Proteins and of a Functional Di-leucine-based Targeting Motif

Diatoms are unicellular organisms evolved by secondary endosymbiosis. Although studied in many aspects, the functions of vacuolar-like structures of these organisms are rarely investigated. One of these structures is a dominant central vacuole-like compartment with a marbled phenotype, which is supposed to represent a chrysolaminarin-storing and carbohydrate mobilization compartment. However, other functions as well as targeting of proteins to this compartment are not shown experimentally. In order to study trafficking of membrane proteins to the vacuolar membrane, we scanned the genome for intrinsic vacuolar membrane proteins and used one representative for targeting studies. Our work led to the identification of several proteins located in the vacuolar membrane as well as the sub-compartmentalized localization of one protein. In addition, we show that a di-leucine-based motif is an important signal for correct targeting to the central vacuole of diatoms, like it is in plants.

Evolution of tonoplast P-ATPase transporters involved in vacuolar acidification

Petunia mutants (Petunia hybrida) with blue flowers defined a novel vacuolar proton pump consisting of two interacting P-ATPases, PH1 and PH5, that hyper-acidify the vacuoles of petal cells. PH5 is similar to plasma membrane H(+) P3A -ATPase, whereas PH1 is the only known eukaryoticP3B -ATPase. As there were no indications that this tonoplast pump is widespread in plants, we investigated the distribution and evolution of PH1 and PH5. We combined database mining and phylogenetic and synteny analyses of PH1- and PH5-like proteins from all kingdoms with functional analyses (mutant complementation and intracellular localization) of homologs from diverse angiosperms. We identified functional PH1 and PH5 homologs in divergent angiosperms. PH5 homologs evolved from plasma membrane P3A -ATPases, acquiring an N-terminal tonoplast-sorting sequence and new cellular function before angiosperms appeared. PH1 is widespread among seed plants and related proteins are found in some groups of bacteria and fungi and in one moss, but is absent in most algae, suggesting that its evolution involved several cases of gene loss and possibly horizontal transfer events. The distribution of PH1 and PH5 in the plant kingdom suggests that vacuolar acidification by P-ATPases appeared in gymnosperms before flowers. This implies that, next to flower color determination, vacuolar hyper-acidification is required for yet unknown processes.

The Arabidopsis tonoplast is almost devoid of glycoproteins with complex N-glycans, unlike the rat lysosomal membrane

The distribution of the N-glycoproteome in integral membrane proteins of the vacuolar membrane (tonoplast) or the plasma membrane of Arabidopsis thaliana and, for further comparison, of the Rattus norvegicus lysosomal and plasma membranes, was analyzed. In silico analysis showed that potential N-glycosylation sites are much less frequent in tonoplast proteins. Biochemical analysis of Arabidopsis subcellular fractions with the lectin concanavalin A, which recognizes mainly unmodified N-glycans, or with antiserum against Golgi-modified N-glycans confirmed the in silico results and showed that, unlike the plant plasma membrane, the tonoplast is almost or totally devoid of N-glycoproteins with Golgi-modified glycans. Lysosomes share with vacuoles the hydrolytic functions and the position along the secretory pathway; however, our results indicate that their membranes had a divergent evolution. We propose that protection against the luminal hydrolases that are abundant in inner hydrolytic compartments, which seems to have been achieved in many lysosomal membrane proteins by extensive N-glycosylation of the luminal domains, has instead been obtained in the vast majority of tonoplast proteins by limiting the length of such domains.

The AP-3 adaptor complex mediates sorting of yeast and mammalian PQ-loop-family basic amino acid transporters to the vacuolar/lysosomal membrane

The limiting membrane of lysosomes in animal cells and that of the vacuole in yeast include a wide variety of transporters, but little is known about how these proteins reach their destination membrane. The mammalian PQLC2 protein catalyzes efflux of basic amino acids from the lysosome, and the similar Ypq1, −2, and −3 proteins of yeast perform an equivalent function at the vacuole. We here show that the Ypq proteins are delivered to the vacuolar membrane via the alkaline phosphatase (ALP) trafficking pathway, which requires the AP-3 adaptor complex. When traffic via this pathway is deficient, the Ypq proteins pass through endosomes from where Ypq1 and Ypq2 properly reach the vacuolar membrane whereas Ypq3 is missorted to the vacuolar lumen via the multivesicular body pathway. When produced in yeast, PQLC2 also reaches the vacuolar membrane via the ALP pathway, but tends to sort to the vacuolar lumen if AP-3 is defective. Finally, in HeLa cells, inhibiting the synthesis of an AP-3 subunit also impairs sorting of PQLC2 to lysosomes. Our results suggest the existence of a conserved AP-3-dependent trafficking pathway for proper delivery of basic amino acid exporters to the yeast vacuole and to lysosomes of human cells.

Molecular Composition of Plant Vacuoles: Important but Less Understood Regulations and Roles of Tonoplast Lipids

The vacuole is an essential organelle for plant growth and development. It is the location for the storage of nutrients; such as sugars and proteins; and other metabolic products. Understanding the mechanisms of vacuolar trafficking and molecule transport across the vacuolar membrane is of great importance in understanding basic plant development and cell biology and for crop quality improvement. Proteins play important roles in vacuolar trafficking; such proteins include Rab GTPase signaling proteins; cargo recognition receptors; and SNAREs (Soluble NSF Attachment Protein Receptors) that are involved in membrane fusion. Some vacuole membrane proteins also serve as the transporters or channels for transport across the tonoplast. Less understood but critical are the roles of lipids in vacuolar trafficking. In this review, we will first summarize molecular composition of plant vacuoles and we will then discuss our latest understanding on the role of lipids in plant vacuolar trafficking and a surprising connection to ribosome function through the study of ribosomal mutants.

Sugar transport across the plant vacuolar membrane: nature and regulation of carrier proteins

The ability of higher plants to store sugars is of crucial importance for plant development, adaption to endogenous or environmental cues and for the economic value of crop species. Sugar storage and accumulation, and its homeostasis in plant cells are managed by the vacuole. Although transport of sugars across the vacuolar membrane has been monitored for about four decades, the molecular entities of the transporters involved have been identified in the last 10 years only. Thus, it is just recently that our pictures of the transporters that channel the sugar load across the tonoplast have gained real shape. Here we describe the molecular nature and regulation of an important group of tonoplast sugar transporter (TST) allowing accumulation of sugars against large concentration gradients. In addition, we report on proton-driven tonoplast sugar exporters and on facilitators, which are also involved in balancing cytosolic and vacuolar sugar levels.

Evaluation of defective endosomal trafficking to the vacuole by monitoring seed storage proteins in Arabidopsis thaliana

Vacuolar proteins are synthesized as precursor forms in the endoplasmic reticulum and are sorted to the vacuole. In this chapter, we introduce two easy methods for the evaluation of vacuolar protein transport using Arabidopsis seeds. These methods are adequate to detect defects in vacuolar transport mediated by endosomes and other trafficking pathways as well. They include an immunoblot assay that monitors the abnormal accumulation of storage protein precursors, and an immunogold labeling assay that monitors the abnormal secretion of storage proteins. Each method facilitates the rapid identification of defects in the transport of endogenous vacuolar proteins in Arabidopsis mutants.

Roles of membrane trafficking in plant cell wall dynamics

The cell wall is one of the characteristic components of plant cells. The cell wall composition differs among cell types and is modified in response to various environmental conditions. To properly generate and modify the cell wall, many proteins are transported to the plasma membrane or extracellular space through membrane trafficking, which is one of the key protein transport mechanisms in eukaryotic cells. Given the diverse composition and functions of the cell wall in plants, the transport of the cell wall components and proteins that are involved in cell wall-related events could be specialized for each cell type, i.e., the machinery for cell wall biogenesis, modification, and maintenance could be transported via different trafficking pathways. In this review, we summarize the recent progress in the current understanding of the roles and mechanisms of membrane trafficking in plant cells and focus on the biogenesis and regulation of the cell wall.

Vacuolar Sorting Receptor-Mediated Trafficking of Soluble Vacuolar Proteins in Plant Cells

Vacuoles are one of the most prominent organelles in plant cells, and they play various important roles, such as degradation of waste materials, storage of ions and metabolites, and maintaining turgor. During the past two decades, numerous advances have been made in understanding how proteins are specifically delivered to the vacuole. One of the most crucial steps in this process is specific sorting of soluble vacuolar proteins. Vacuolar sorting receptors (VSRs), which are type I membrane proteins, are involved in the sorting and packaging of soluble vacuolar proteins into transport vesicles with the help of various accessory proteins. To date, large amounts of data have led to the development of two different models describing VSR-mediated vacuolar trafficking that are radically different in multiple ways, particularly regarding the location of cargo binding to, and release from, the VSR and the types of carriers utilized. In this review, we summarize current literature aimed at elucidating VSR-mediated vacuolar trafficking and compare the two models with respect to the sorting signals of vacuolar proteins, as well as the molecular machinery involved in VSR-mediated vacuolar trafficking and its action mechanisms.

Lipid trafficking in plant cells

Plant cells contain unique organelles such as chloroplasts with an extensive photosynthetic membrane. In addition, specialized epidermal cells produce an extracellular cuticle composed primarily of lipids, and storage cells accumulate large amounts of storage lipids. As lipid assembly is associated only with discrete membranes or organelles, there is a need for extensive lipid trafficking within plant cells, more so in specialized cells and sometimes also in response to changing environmental conditions such as phosphate deprivation. Because of the complexity of plant lipid metabolism and the inherent recalcitrance of membrane lipid transporters, the mechanisms of lipid transport within plant cells are not yet fully understood. Recently, several new proteins have been implicated in different aspects of plant lipid trafficking. While these proteins provide only first insights into limited aspects of lipid transport phenomena in plant cells, they represent exciting opportunities for further studies.

Contribution of chitinase A's C-terminal vacuolar sorting determinant to the study of soluble protein compartmentation

Plant chitinases have been studied for their importance in the defense of crop plants from pathogen attacks and for their peculiar vacuolar sorting determinants. A peculiarity of the sequence of many family 19 chitinases is the presence of a C-terminal extension that seems to be important for their correct recognition by the vacuole sorting machinery. The 7 amino acids long C-terminal vacuolar sorting determinant (CtVSD) of tobacco chitinase A is necessary and sufficient for the transport to the vacuole. This VSD shares no homology with other CtVSDs such as the phaseolin’s tetrapeptide AFVY (AlaPheValTyr) and it is also sorted by different mechanisms. While a receptor for this signal has not yet been convincingly identified, the research using the chitinase CtVSD has been very informative, leading to the observation of phenomena otherwise difficult to observe such as the presence of separate vacuoles in differentiating cells and the existence of a Golgi-independent route to the vacuole. Thanks to these new insights in the endoplasmic reticulum (ER)-to-vacuole transport, GFPChi (Green Fluorescent Protein carrying the chitinase A CtVSD) and other markers based on chitinase signals will continue to help the investigation of vacuolar biogenesis in plants.

Homotypic vacuole fusion requires VTI11 and is regulated by phosphoinositides

Most plant cells contain a large central vacuole that is essential to maintain cellular turgor. We report a new mutant allele of VTI11 that implicates the SNARE protein VTI11 in homotypic fusion of protein storage and lytic vacuoles. Fusion of the multiple vacuoles present in vti11 mutants could be induced by treatment with Wortmannin and LY294002, which are inhibitors of Phosphatidylinositol 3-Kinase (PI3K). We provide evidence that Phosphatidylinositol 3-Phosphate (PtdIns(3)P) regulates vacuole fusion in vti11 mutants, and that fusion of these vacuoles requires intact microtubules and actin filaments. Finally, we show that Wortmannin also induced the fusion of guard cell vacuoles in fava beans, where vacuoles are naturally fragmented after ABA-induced stomata closure. These results suggest a ubiquitous role of phosphoinositides in vacuole fusion, both during the development of the large central vacuole and during the dynamic vacuole remodeling that occurs as part of stomata movements.

Membrane trafficking pathways and their roles in plant-microbe interactions

Membrane trafficking functions in the delivery of proteins that are newly synthesized in the endoplasmic reticulum (ER) to their final destinations, such as the plasma membrane (PM) and the vacuole, and in the internalization of extracellular components or PM-associated proteins for recycling or degradative regulation. These trafficking pathways play pivotal roles in the rapid responses to environmental stimuli such as challenges by microorganisms. In this review, we provide an overview of the current knowledge of plant membrane trafficking and its roles in plant-microbe interactions. Although there is little information regarding the mechanism of pathogenic modulation of plant membrane trafficking thus far, recent research has identified many membrane trafficking factors as possible targets of microbial modulation.

Multiple vacuoles in impaired tonoplast trafficking3 mutants are independent organelles

Plant vacuoles are essential and dynamic organelles, and mechanisms of vacuole biogenesis and fusion are not well characterized. We recently demonstrated that Wortmannin, an inhibitor of Phosphatidylinositol 3-Kinase (PI3K), induces the fusion of plant vacuoles both in roots of itt3/vti11 mutant alleles and in guard cells of wild type Arabidopsis and Fava bean. Here we used Fluorescence Recovery After Photobleaching (FRAP) to demonstrate that the vacuoles in itt3/vti11 are independent organelles. Furthermore, we used fluorescent protein reporters that bind specifically to Phosphatidylinositol 3-Phosphate (PtdIns(3)P) or PtdIns(4)P to show that Wortmannin treatments that induce the fusion of vti11 vacuoles result in the loss of PtdIns(3)P from cellular membranes. These results provided supporting evidence for a critical role of PtdIns(3)P in vacuole fusion in roots and guard cells.

ER and vacuoles: never been closer

The endoplasmic reticulum (ER) represents the gateway for intracellular trafficking of membrane proteins, soluble cargoes and lipids. In all eukaryotes, the best described mechanism of exiting the ER is via COPII-coated vesicles, which transport both membrane proteins and soluble cargoes to the cis-Golgi. The vacuole, together with the plasma membrane, is the most distal point of the secretory pathway, and many vacuolar proteins are transported from the ER through intermediate compartments. However, past results and recent findings demonstrate the presence of alternative transport routes from the ER towards the tonoplast, which are independent of Golgi- and post-Golgi trafficking. Moreover, the transport mechanism of the vacuolar proton pumps VHA-a3 and AVP1 challenges the current model of vacuole biogenesis, pointing to the endoplasmic reticulum for being the main membrane source for the biogenesis of the plant lytic compartment. This review gives an overview of the current knowledge on the transport routes towards the vacuole and discusses the possible mechanism of vacuole biogenesis in plants.

Protein body formation in the endoplasmic reticulum as an evolution of storage protein sorting to vacuoles: insights from maize γ-zein

The albumin and globulin seed storage proteins present in all plants accumulate in storage vacuoles. Prolamins, which are the major proteins in cereal seeds and are present only there, instead accumulate within the endoplasmic reticulum (ER) lumen as very large insoluble polymers termed protein bodies. Inter-chain disulfide bonds play a major role in polymerization and insolubility of many prolamins. The N-terminal domain of the maize prolamin 27 kD γ-zein is able to promote protein body formation when fused to other proteins and contains seven cysteine residues involved in inter-chain bonds. We show that progressive substitution of these amino acids with serine residues in full length γ-zein leads to similarly progressive increase in solubility and availability to traffic from the ER along the secretory pathway. Total substitution results in very efficient secretion, whereas the presence of a single cysteine is sufficient to promote partial sorting to the vacuole via a wortmannin-sensitive pathway, similar to the traffic pathway of vacuolar storage proteins. We propose that the mechanism leading to accumulation of prolamins in the ER is a further evolutionary step of the one responsible for accumulation in storage vacuoles.

Targeting of tonoplast proteins to the vacuole

Vacuoles are essential for plant growth and development, and are dynamic compartments that require constant deposition of integral membrane proteins. These membrane proteins carry out many critical functions of the vacuole such as transporting ions and metabolites for vacuolar storage. Understanding the mechanisms for targeting proteins to the vacuolar membrane, or tonoplast, is important for developing novel applications for biotechnology. The mechanisms to target tonoplast proteins to the vacuole are quite complex. Multiple routes, including both Golgi-dependent and Golgi-independent mechanisms, have been implicated in tonoplast protein trafficking. A few endomembrane proteins that regulate this traffic at the level of the endoplasmic reticulum, the pre-vacuolar compartment and the tonoplast are now known. Recent reports indicate that the Golgi-dependent and independent pathways may merge at the level of the pre-vacuolar compartment. Finally, the small GTP-binding protein Rab7 and the SNARE protein SYP21 have been implicated in the traffic of tonoplast proteins from the pre-vacuolar compartment to the tonoplast. With multiple cargo proteins being analyzed under a variety of experimental systems, a clearer picture for targeting mechanisms for tonoplast proteins is starting to emerge.

The endoplasmic reticulum is the main membrane source for biogenesis of the lytic vacuole in Arabidopsis

Vacuoles are multifunctional organelles essential for the sessile lifestyle of plants. Despite their central functions in cell growth, storage, and detoxification, knowledge about mechanisms underlying their biogenesis and associated protein trafficking pathways remains limited. Here, we show that in meristematic cells of the Arabidopsis thaliana root, biogenesis of vacuoles as well as the trafficking of sterols and of two major tonoplast proteins, the vacuolar H(+)-pyrophosphatase and the vacuolar H(+)-adenosinetriphosphatase, occurs independently of endoplasmic reticulum (ER)-Golgi and post-Golgi trafficking. Instead, both pumps are found in provacuoles that structurally resemble autophagosomes but are not formed by the core autophagy machinery. Taken together, our results suggest that vacuole biogenesis and trafficking of tonoplast proteins and lipids can occur directly from the ER independent of Golgi function.

Unconventional pathways of secretory plant proteins from the endoplasmic reticulum to the vacuole bypassing the Golgi complex

Studies on the basic mechanisms that regulate vacuolar delivering of proteins synthesized in the endoplasmic reticulum (ER) have a great importance in plant cell biology. Indeed, many aspects of plant physiology are affected by this intracellular traffic, for example, germination or reaction to biotic stresses due to the accumulation of storage proteins in seeds or enzymes in vegetative tissues, respectively. Up to now, the Golgi complex has been considered the main hub in the sorting of vacuolar secretory proteins; those polypeptides able to reach their final destination without the aid of this organelle are regarded as exceptions to an established route. This mini-review aims to emphasize the existence of several Golgi-independent pathways involved in the trafficking of different types of vacuolar proteins.

Comparison of membrane targeting strategies for the accumulation of the human immunodeficiency virus p24 protein in transgenic tobacco

Membrane anchorage was tested as a strategy to accumulate recombinant proteins in transgenic plants. Transmembrane domains of different lengths and topology were fused to the cytosolic HIV antigen p24, to promote endoplasmic reticulum (ER) residence or traffic to distal compartments of the secretory pathway in transgenic tobacco. Fusions to a domain of the maize seed storage protein γ-zein were also expressed, as a reference strategy that leads to very high stability via the formation of large polymers in the ER lumen. Although all the membrane anchored constructs were less stable compared to the zein fusions, residence at the ER membrane either as a type I fusion (where the p24 sequence is luminal) or a tail-anchored fusion (where the p24 sequence is cytosolic) resulted in much higher stability than delivery to the plasma membrane or intermediate traffic compartments. Delivery to the tonoplast was never observed. The inclusion of a thrombin cleavage site allowed for the quantitative in vitro recovery of p24 from all constructs. These results point to the ER as suitable compartment for the accumulation of membrane-anchored recombinant proteins in plants.