ARF1 localizes to the golgi and the trans-golgi network

Endogenous Protein-Protein Interaction Network of the NPC Cholesterol Transporter 1 in the Cerebral Cortex

NPC intracellular cholesterol transporter 1 (NPC1) is a multipass, transmembrane glycoprotein mostly recognized for its key role in facilitating cholesterol efflux. Mutations in the NPC1 gene result in Niemann-Pick disease, type C (NPC), a fatal, lysosomal storage disease. Due to the progressively expanding implications of NPC1-related disorders, we investigated endogenous NPC1 protein-protein interactions in the mouse cortex and human-derived iPSCs neuronal models of the disease through coimmunoprecipitation-coupled with LC-MS based proteomics. The current study investigated protein-protein interactions specific to the wild-type and the most prevalent NPC1 mutation (NPC1I1061T) while filtering out any protein interactor identified in the Npc1-/- mouse model. Additionally, the results were matched across the two species to map the parallel interactome of wild-type and mutant NPC1I1061T. Most of the identified wild-type NPC1 interactors were related to cytoskeleton organization, synaptic vesicle activity, and translation. We found many putative NPC1 interactors not previously reported, including two SCAR/WAVE complex proteins that regulate ARP 2/3 complex actin nucleation and multiple membrane proteins important for neuronal activity at synapse. Moreover, we identified proteins important in trafficking specific to wild-type and mutant NPC1I1061T. Together, the findings are essential for a comprehensive understanding of NPC1 biological functions in addition to its classical role in sterol efflux.

Characterization of the small Arabidopsis thaliana GTPase and ADP-ribosylation factor-like 2 protein TITAN 5

Small GTPases switch between GDP- and GTP-bound states during cell signaling. The ADP-ribosylation factor (ARF) family of small GTPases is involved in vesicle trafficking. Although evolutionarily well conserved, little is known about ARF and ARF-like GTPases in plants. We characterized biochemical properties and cellular localization of the essential small ARF-like GTPase TITAN 5 (TTN5; also known as HALLIMASCH, ARL2 and ARLC1) from Arabidopsis thaliana, and two TTN5 proteins with point mutants in conserved residues, TTN5T30N and TTN5Q70L, that were expected to be unable to perform nucleotide exchange and GTP hydrolysis, respectively. TTN5 exhibited very rapid intrinsic nucleotide exchange and remarkably low GTP hydrolysis activity, functioning as a non-classical small GTPase being likely present in a GTP-loaded active form. We analyzed signals from YFP-TTN5 and HA3-TTN5 by in situ immunolocalization in Arabidopsis seedlings and through use of a transient expression system. Colocalization with endomembrane markers and pharmacological treatments suggests that TTN5 can be present at the plasma membrane and that it dynamically associates with membranes of vesicles, Golgi stacks and multivesicular bodies. Although TTN5Q70L mirrored wild-type TTN5 behavior, the TTN5T30N mutant differed in some aspects. Hence, the unusual rapid nucleotide exchange activity of TTN5 is linked with its membrane dynamics, and TTN5 likely has a role in vesicle transport within the endomembrane system.

Arabidopsis flippase ALA3 is required for adjustment of early subcellular trafficking in plant response to osmotic stress

To compensate for their sessile lifestyle, plants developed several responses to exogenous changes. One of the previously investigated and not yet fully understood adaptations occurs at the level of early subcellular trafficking, which needs to be rapidly adjusted to maintain cellular homeostasis and membrane integrity under osmotic stress conditions. To form a vesicle, the membrane needs to be deformed, which is ensured by multiple factors, including the activity of specific membrane proteins, such as flippases from the family of P4-ATPases. The membrane pumps actively translocate phospholipids from the exoplasmic/luminal to the cytoplasmic membrane leaflet to generate curvature, which might be coupled with recruitment of proteins involved in vesicle formation at specific sites of the donor membrane. We show that lack of the AMINOPHOSPHOLIPID ATPASE3 (ALA3) flippase activity caused defects at the plasma membrane and trans-Golgi network, resulting in altered endocytosis and secretion, processes relying on vesicle formation and movement. The mentioned cellular defects were translated into decreased intracellular trafficking flexibility failing to adjust the root growth on osmotic stress-eliciting media. In conclusion, we show that ALA3 cooperates with ARF-GEF BIG5/BEN1 and ARF1A1C/BEX1 in a similar regulatory pathway to vesicle formation, and together they are important for plant adaptation to osmotic stress.

RNAseq Analysis of FABP4 Knockout Mouse Hippocampal Transcriptome Suggests a Role for WNT/β-Catenin in Preventing Obesity-Induced Cognitive Impairment

Microglial fatty-acid binding protein 4 (FABP4) is a regulator of neuroinflammation. We hypothesized that the link between lipid metabolism and inflammation indicates a role for FABP4 in regulating high fat diet (HFD)-induced cognitive decline. We have previously shown that obese FABP4 knockout mice exhibit decreased neuroinflammation and cognitive decline. FABP4 knockout and wild type mice were fed 60% HFD for 12 weeks starting at 15 weeks old. Hippocampal tissue was dissected and RNA-seq was performed to measure differentially expressed transcripts. Reactome molecular pathway analysis was utilized to examine differentially expressed pathways. Results showed that HFD-fed FABP4 knockout mice have a hippocampal transcriptome consistent with neuroprotection, including associations with decreased proinflammatory signaling, ER stress, apoptosis, and cognitive decline. This is accompanied by an increase in transcripts upregulating neurogenesis, synaptic plasticity, long-term potentiation, and spatial working memory. Pathway analysis revealed that mice lacking FABP4 had changes in metabolic function that support reduction in oxidative stress and inflammation, and improved energy homeostasis and cognitive function. Analysis suggested a role for WNT/β-Catenin signaling in the protection against insulin resistance, alleviating neuroinflammation and cognitive decline. Collectively, our work shows that FABP4 represents a potential target in alleviating HFD-induced neuroinflammation and cognitive decline and suggests a role for WNT/β-Catenin in this protection.

Small extracellular vesicles released from germinated kiwi pollen (pollensomes) present characteristics similar to mammalian exosomes and carry a plant homolog of ALIX

Introduction

In the last decade, it has been discovered that allergen-bearing extracellular nanovesicles, termed "pollensomes", are released by pollen during germination. These extracellular vesicles (EVs) may play an important role in pollen-pistil interaction during fertilization, stabilizing the secreted bioactive molecules and allowing long-distance signaling. However, the molecular composition and the biological role of these EVs are still unclear. The present study had two main aims: (I) to clarify whether pollen germination is needed to release pollensomes, or if they can be secreted also in high humidity conditions; and (II) to investigate the molecular features of pollensomes following the most recent guidelines for EVs isolation and identification.

Methods

To do so, pollensomes were isolated from hydrated and germinated kiwi (Actinidia chinensis Planch.) pollen, and characterized using imaging techniques, immunoblotting, and proteomics.

Results

These analyses revealed that only germinated kiwi pollen released detectable concentrations of nanoparticles compatible with small EVs for shape and protein content. Moreover, a plant homolog of ALIX, which is a well-recognized and accepted marker of small EVs and exosomes in mammals, was found in pollensomes.

Discussion

The presence of this protein, along with other proteins involved in endocytosis, is consistent with the hypothesis that pollensomes could comprehend a prominent subpopulation of plant exosome-like vesicles.

Systematic analysis of specific and nonspecific auxin effects on endocytosis and trafficking

The phytohormone auxin and its directional transport through tissues are intensively studied. However, a mechanistic understanding of auxin-mediated feedback on endocytosis and polar distribution of PIN auxin transporters remains limited due to contradictory observations and interpretations. Here, we used state-of-the-art methods to reexamine the auxin effects on PIN endocytic trafficking. We used high auxin concentrations or longer treatments versus lower concentrations and shorter treatments of natural indole-3-acetic acid (IAA) and synthetic naphthalene acetic acid (NAA) auxins to distinguish between specific and nonspecific effects. Longer treatments of both auxins interfere with Brefeldin A-mediated intracellular PIN2 accumulation and also with general aggregation of endomembrane compartments. NAA treatment decreased the internalization of the endocytic tracer dye, FM4-64; however, NAA treatment also affected the number, distribution, and compartment identity of the early endosome/trans-Golgi network, rendering the FM4-64 endocytic assays at high NAA concentrations unreliable. To circumvent these nonspecific effects of NAA and IAA affecting the endomembrane system, we opted for alternative approaches visualizing the endocytic events directly at the plasma membrane (PM). Using total internal reflection fluorescence microscopy, we saw no significant effects of IAA or NAA treatments on the incidence and dynamics of clathrin foci, implying that these treatments do not affect the overall endocytosis rate. However, both NAA and IAA at low concentrations rapidly and specifically promoted endocytosis of photo-converted PIN2 from the PM. These analyses identify a specific effect of NAA and IAA on PIN2 endocytosis, thus, contributing to its polarity maintenance and furthermore illustrate that high auxin levels have nonspecific effects on trafficking and endomembrane compartments.

The Inhibitor Endosidin 4 Targets SEC7 Domain-Type ARF GTPase Exchange Factors and Interferes with Subcellular Trafficking in Eukaryotes

The trafficking of subcellular cargos in eukaryotic cells crucially depends on vesicle budding, a process mediated by ARF-GEFs (ADP-ribosylation factor guanine nucleotide exchange factors). In plants, ARF-GEFs play essential roles in endocytosis, vacuolar trafficking, recycling, secretion, and polar trafficking. Moreover, they are important for plant development, mainly through controlling the polar subcellular localization of PIN-FORMED transporters of the plant hormone auxin. Here, using a chemical genetics screen in Arabidopsis thaliana, we identified Endosidin 4 (ES4), an inhibitor of eukaryotic ARF-GEFs. ES4 acts similarly to and synergistically with the established ARF-GEF inhibitor Brefeldin A and has broad effects on intracellular trafficking, including endocytosis, exocytosis, and vacuolar targeting. Additionally, Arabidopsis and yeast (Saccharomyces cerevisiae) mutants defective in ARF-GEF show altered sensitivity to ES4. ES4 interferes with the activation-based membrane association of the ARF1 GTPases, but not of their mutant variants that are activated independently of ARF-GEF activity. Biochemical approaches and docking simulations confirmed that ES4 specifically targets the SEC7 domain-containing ARF-GEFs. These observations collectively identify ES4 as a chemical tool enabling the study of ARF-GEF-mediated processes, including ARF-GEF-mediated plant development.

Biogenesis and Function of Multivesicular Bodies in Plant Immunity

Multivesicular bodies (MVBs) are specialized endosomes that contain intraluminal vesicles generated from invagination and budding of the limiting membrane. In the endocytic pathway, MVBs are late endosomes whose content can be degraded through fusion with lysosomes/vacuoles or released into the extracellular space after fusion with the plasma membrane (PM). The proteins retained on the limiting membrane of MVBs are translocated to the membrane of lysosomes/vacuoles or delivered back to the PM. It has been long suspected that MVBs might fuse with the PM to form paramural bodies in plant cells, possibly leading to release of building blocks for deposition of papillae and antimicrobial molecules against invading pathogens. Over the past decade or so, major progress has been made in establishing the critical roles of MVBs and associated membrane trafficking in pathogen recognition, defense signaling, and deployment of defense-related molecules during plant immune responses. Regulatory proteins and signaling pathways associated with induced biogenesis and trafficking of MVBs during plant immune responses have also been identified and characterized. Recent successful isolation of plant extracellular vesicles and proteomic profiling of their content have provided additional support for the roles of MVBs in plant-pathogen interactions. In this review, we summarize the important progress and discuss how MVBs, particularly through routing of cellular components to different destinations, contribute to the complex network of plant immune system.

Plant exosomes: using an unconventional exit to prevent pathogen entry?

The ability to ward off filamentous pathogens, such as powdery mildew fungi, is one of the best studied examples of membrane trafficking-dependent disease resistance in plants. Here, papilla formation at the site of attack is essential for the pre-invasive immunity, whereas the encasement can hamper disease post-invasively. Exosomes containing antifungal peptides and small RNAs are thought to play a vital role in forming papillae and encasements that block fungal growth. While exosomes are well described in mammals, and have been shown to play important roles in cell-cell communication regulating development and disease, their function is not well-known in plants. In this review, we focus on some of the recent discoveries on plant exosomes and try to link this information with our current understanding of how plants use this form of unconventional secretion to acquire this durable and effective form of resistance.

Trans-Golgi network/early endosome: a central sorting station for cargo proteins in plant immunity

In plants, the trans-Golgi network (TGN) functionally overlaps with the early endosome (EE), serving as a central sorting hub to direct newly synthesized and endocytosed cargo to the cell surface or vacuole. Here, we focus on the emerging role of the TGN/EE in sorting of immune cargo proteins for effective plant immunity against pathogenic bacteria and fungi. Specific vesicle coat and regulatory components at the TGN/EE ensure that immune cargoes are correctly sorted and transported to the location of their cellular functions. Our understanding of the identity of immune cargoes and the underlying cellular mechanisms regulating their sorting are still rudimentary, but this knowledge is essential to understanding the physiological contribution of the TGN/EE to effective immune responses.

Small GTPases in plant biotic interactions

The superfamily of small monomeric GTPases originated in a common ancestor of eukaryotic multicellular organisms and, since then, it has evolved independently in each lineage to cope with the environmental challenges imposed by their different life styles. Members of the small GTPase family function in the control of vesicle trafficking, cytoskeleton rearrangements and signaling during crucial biological processes, such as cell growth and responses to environmental cues. In this review, we discuss the emerging roles of these small GTPases in the pathogenic and symbiotic interactions established by plants with microorganisms present in their nearest environment, in which membrane trafficking is crucial along the different steps of the interaction, from recognition and signal transduction to nutrient exchange.

Ethylene Regulates Differential Growth via BIG ARF-GEF-Dependent Post-Golgi Secretory Trafficking in Arabidopsis

During early seedling development, the shoot apical meristem is protected from damage as the seedling emerges from soil by the formation of apical hook. Hook formation requires differential growth across the epidermis below the meristem in the hypocotyl. The plant hormones ethylene and auxin play key roles during apical hook development by controlling differential growth. We provide genetic and cell biological evidence for the role of ADP-ribosylation factor 1 (ARF1)-GTPase and its effector ARF-guanine-exchange factors (GEFs) of the Brefeldin A-inhibited GEF (BIG) family and GNOM in ethylene- and auxin-mediated control of hook development. We show that ARF-GEF GNOM acts early, whereas BIG ARF-GEFs act at a later stage of apical hook development. We show that the localization of ARF1 and BIG4 at the trans-Golgi network (TGN) depends on ECHIDNA (ECH), a plant homolog of yeast Triacylglycerol lipase (TLG2/SYP4) interacting protein Tgl2-Vesicle Protein 23 (TVP23). BIGs together with ECH and ARF1 mediate the secretion of AUX1 influx carrier to the plasma membrane from the TGN during hook development and defects in BIG or ARF1 result in insensitivity to ethylene. Thus, our data indicate a division of labor within the ARF-GEF family in mediating differential growth with GNOM acting during the formation phase whereas BIGs act during the hook maintenance phase downstream of plant hormone ethylene.

The trans-Golgi Network and the Golgi Stacks Behave Independently During Regeneration After Brefeldin A Treatment in Tobacco BY-2 Cells

The trans-Golgi network (TGN) plays an essential role in intracellular membrane trafficking. In plant cells, recent live-cell imaging studies have revealed the dynamic behavior of the TGN independent from the Golgi apparatus. In order to better understand the relationships between the two organelles, we examined their dynamic responses to the reagent brefeldin A (BFA) and their recovery after BFA removal. Golgi markers responded to BFA similarly over a range of concentrations, whereas the behavior of the TGN was BFA concentration dependent. The TGN formed aggregates at high concentrations of BFA; however, TGN proteins relocalized to numerous small vesicular structures dispersed throughout the cytoplasm at lower BFA concentrations. During recovery from weak BFA treatment, the TGN started to regenerate earlier than the completion of the Golgi. The regeneration of the two organelles proceeded independently of each other for a while, and eventually was completed by their association. Our data suggest that there is some degree of autonomy for the regeneration of the TGN and the Golgi in tobacco BY-2 cells.

The monomeric GTPase RabA2 is required for progression and maintenance of membrane integrity of infection threads during root nodule symbiosis

Progression of the infection canal that conducts rhizobia to the nodule primordium requires a functional Rab GTPase located in Golgi/trans-Golgi that also participate in root hair polar growth. Common bean (Phaseolus vulgaris) symbiotically associates with its partner Rhizobium etli, resulting in the formation of root nitrogen-fixing nodules. Compatible bacteria can reach cortical cells in a tightly regulated infection process, in which the specific recognition of signal molecules is a key step to select the symbiotic partner. In this work, we show that RabA2, a monomeric GTPase from common bean, is required for the progression of the infection canal, referred to as the infection thread (IT), toward the cortical cells. Expression of miss-regulated mutant variants of RabA2 resulted in an increased number of abortive infection events, including bursting of ITs and a reduction in the number of nodules. Nodules formed in these plants were small and contained infected cells with disrupted symbiosome membranes, indicating either early senescence of these cells or defects in the formation of the symbiosome membrane during bacterial release. RabA2 localized to mobile vesicles around the IT, but mutations that affect GTP hydrolysis or GTP/GDP exchange modified this localization. Colocalization of RabA2 with ArfA1 and a Golgi marker indicates that RabA2 localizes in Golgi stacks and the trans-Golgi network. Our results suggest that RabA2 is part of the vesicle transport events required to maintain the integrity of the membrane during IT progression.

The Qb-SNARE Memb11 interacts specifically with Arf1 in the Golgi apparatus of Arabidopsis thaliana

The SNARE (soluble N-ethylmaleimide-sensitive factor attachment protein receptor) proteins are critical for the function of the secretory pathway. The SNARE Memb11 is involved in membrane trafficking at the ER-Golgi interface. The aim of the work was to decipher molecular mechanisms acting in Memb11-mediated ER-Golgi traffic. In mammalian cells, the orthologue of Memb11 (membrin) is potentially involved in the recruitment of the GTPase Arf1 at the Golgi membrane. However molecular mechanisms associated to Memb11 remain unknown in plants. Memb11 was detected mainly at the cis-Golgi and co-immunoprecipitated with Arf1, suggesting that Arf1 may interact with Memb11. This interaction of Memb11 with Arf1 at the Golgi was confirmed by in vivo BiFC (Bimolecular Fluorescence Complementation) experiments. This interaction was found to be specific to Memb11 as compared to either Memb12 or Sec22. Using a structural bioinformatic approach, several sequences in the N-ter part of Memb11 were hypothesized to be critical for this interaction and were tested by BiFC on corresponding mutants. Finally, by using both in vitro and in vivo approaches, we determined that only the GDP-bound form of Arf1 interacts with Memb11. Together, our results indicate that Memb11 interacts with the GDP-bound form of Arf1 in the Golgi apparatus.

UDP-D-galactose synthesis by UDP-glucose 4-epimerase 4 is required for organization of the trans-Golgi network/early endosome in Arabidopsis thaliana root epidermal cells

Endomembrane organization is essential for cell physiology. We previously identified an Arabidopsis thaliana mutant in which a plasma membrane (PM) marker GFP-NIP5;1 and trans-Golgi network/early endosome (TGN/EE) markers were accumulated in intracellular aggregates in epidermal cells of the root elongation zone. The mutant was identified as an allele of UDP-glucose epimerase 4 (UGE4)/root hair defective 1/root epidermal bulgar 1, which was previously described as a mutant with swollen root epidermal cells and has an altered sugar composition in cell wall polysaccharides. Importantly, these defects including aggregate formation were restored by supplementation of D-galactose in the medium. These results suggested that UDP-D-galactose synthesis by UGE4 is important for endomembrane organization in addition to cell wall structure. Here, we further investigated the nature of the aggregates using various markers of endomembrane compartments and BOR1-GFP, which traffics from PM to vacuole in response to high-B supply. The markers of multi-vesicular bodies/late endosomes (MVB/LEs) and BOR1-GFP were strongly accumulated in the intracellular aggregates, while those of the endoplasmic reticulum (ER), the vacuolar membrane, and the Golgi were only slightly affected in the uge4 mutant. The abnormal localizations of these markers in the uge4 mutant differed from the effects of inhibitors of actin and microtubule polymerization, although they also affected endomembrane organization. Furthermore, electron microscopy analysis revealed accumulation of abnormal high-electron-density vesicles in elongating epidermal cells. The abnormal vesicles were often associated or interconnected with TGN/EEs and contained ADP-ribosylation factor 1, which is usually localized to the Golgi and the TGN/EEs. On the other hand, structures of the ER, Golgi apparatus, and MVB/LEs were apparently normal in uge4 cells. Together, our data indicate the importance of UDP-D-galactose synthesis by UGE4 for the organization and function of endomembranes, especially TGN/EEs, which are a sorting station of the secretory and vacuolar pathways.

Formation and maintenance of the Golgi apparatus in plant cells

The Golgi apparatus plays essential roles in intracellular trafficking, protein and lipid modification, and polysaccharide synthesis in eukaryotic cells. It is well known for its unique stacked structure, which is conserved among most eukaryotes. However, the mechanisms of biogenesis and maintenance of the structure, which are deeply related to ER-Golgi and intra-Golgi transport systems, have long been mysterious. Now having extremely powerful microscopic technologies developed for live-cell imaging, the plant Golgi apparatus provides an ideal system to resolve the question. The plant Golgi apparatus has unique features that are not conserved in other kingdoms, which will also give new insights into the Golgi functions in plant life. In this review, we will summarize the features of the plant Golgi apparatus and transport mechanisms around it, with a focus on recent advances in Golgi biogenesis by live imaging of plants cells.

Arabinogalactan glycosyltransferases target to a unique subcellular compartment that may function in unconventional secretion in plants

We report that fluorescently tagged arabinogalactan glycosyltransferases target not only the Golgi apparatus but also uncharacterized smaller compartments when transiently expressed in Nicotiana benthamiana. Approximately 80% of AtGALT31A [Arabidopsis thaliana galactosyltransferase from family 31 (At1g32930)] was found in the small compartments, of which, 45 and 40% of AtGALT29A [Arabidopsis thaliana galactosyltransferase from family 29 (At1g08280)] and AtGlcAT14A [Arabidopsis thaliana glucuronosyltransferase from family 14 (At5g39990)] colocalized with AtGALT31A, respectively; in contrast, N-glycosylation enzymes rarely colocalized (3-18%), implicating a role of the small compartments in a part of arabinogalactan (O-glycan) biosynthesis rather than N-glycan processing. The dual localization of AtGALT31A was also observed for fluorescently tagged AtGALT31A stably expressed in an Arabidopsis atgalt31a mutant background. Further, site-directed mutagenesis of a phosphorylation site of AtGALT29A (Y144) increased the frequency of the protein being targeted to the AtGALT31A-localized small compartments, suggesting a role of Y144 in subcellular targeting. The AtGALT31A localized to the small compartments were colocalized with neither SYP61 (syntaxin of plants 61), a marker for trans-Golgi network (TGN), nor FM4-64-stained endosomes. However, 41% colocalized with EXO70E2 (Arabidopsis thaliana exocyst protein Exo70 homolog 2), a marker for exocyst-positive organelles, and least affected by Brefeldin A and Wortmannin. Taken together, AtGALT31A localized to small compartments that are distinct from the Golgi apparatus, the SYP61-localized TGN, FM4-64-stained endosomes and Wortmannin-vacuolated prevacuolar compartments, but may be part of an unconventional protein secretory pathway represented by EXO70E2 in plants.

Trans-Golgi network-located AP1 gamma adaptins mediate dileucine motif-directed vacuolar targeting in Arabidopsis

Membrane proteins on the tonoplast are indispensible for vacuolar functions in plants. However, how these proteins are transported to the vacuole and how they become separated from plasma membrane proteins remain largely unknown. In this study, we used Arabidopsis thaliana vacuolar ion transporter1 (VIT1) as a reporter to study the mechanisms of tonoplast targeting. We showed that VIT1 reached the tonoplast through a pathway involving the endoplasmic reticulum (ER), Golgi, trans-Golgi network (TGN), prevacuolar compartment, and tonoplast. VIT1 contains a putative N-terminal dihydrophobic type ER export signal, and its N terminus has a conserved dileucine motif (EKQTLL), which is responsible for tonoplast targeting. In vitro peptide binding assays with synthetic VIT1 N terminus identified adaptor protein complex-1 (AP1) subunits that interacted with the dileucine motif. A deficiency of AP1 gamma adaptins in Arabidopsis cells caused relocation of tonoplast proteins containing the dileucine motif, such as VIT1 and inositol transporter1, to the plasma membrane. The dileucine motif also effectively rerouted the plasma membrane protein SCAMP1 to the tonoplast. Together with subcellular localization studies showing that AP1 gamma adaptins localize to the TGN, we propose that the AP1 complex on the TGN mediates tonoplast targeting of membrane proteins with the dileucine motif.

Arabidopsis LIP5, a positive regulator of multivesicular body biogenesis, is a critical target of pathogen-responsive MAPK cascade in plant basal defense

Multivesicular bodies (MVBs) play essential roles in many cellular processes. The MVB pathway requires reversible membrane association of the endosomal sorting complexes required for transports (ESCRTs) for sustained protein trafficking. Membrane dissociation of ESCRTs is catalyzed by the AAA ATPase SKD1, which is stimulated by LYST-INTERACTING PROTEIN 5 (LIP5). We report here that LIP5 is a target of pathogen-responsive mitogen-activated protein kinases (MPKs) and plays a critical role in plant basal resistance. Arabidopsis LIP5 interacts with MPK6 and MPK3 and is phosphorylated in vitro by activated MPK3 and MPK6 and in vivo upon expression of MPK3/6-activating NtMEK2^DD and pathogen infection. Disruption of LIP5 has little effects on flg22-, salicylic acid-induced defense responses but compromises basal resistance to Pseudomonas syringae. The critical role of LIP5 in plant basal resistance is dependent on its ability to interact with SKD1. Mutation of MPK phosphorylation sites in LIP5 does not affect interaction with SKD1 but reduces the stability and compromises the ability to complement the lip5 mutant phenotypes. Using the membrane-selective FM1–43 dye and transmission electron microscopy, we demonstrated that pathogen infection increases formation of both intracellular MVBs and exosome-like paramural vesicles situated between the plasma membrane and the cell wall in a largely LIP5-dependent manner. These results indicate that the MVB pathway is positively regulated by pathogen-responsive MPK3/6 through LIP5 phosphorylation and plays a critical role in plant immune system likely through relocalization of defense-related molecules.

Pathogen- and stress-responsive mitogen-activated protein kinases 3 and 6 (MPK3/6) cascade plays an important role in plant basal resistance to microbial pathogens. Here we showed that Arabidopsis MPK3 and MPK6 interact with and phosphorylate the LIP5 positive regulator of biogenesis of multivesicular bodies (MVBs), which are unique organelles containing small vesicles in their lumen. Disruption of LIP5 causes increased susceptibility to the bacterial pathogen Pseudomonas syringae. Compromised disease resistance of the lip5 mutants is associated with competent flg22- and salicylic acid-induced defense responses but compromised accumulation of intracellular MVBs and exosome-like paramural vesicles, which have previously been shown to be involved in the relocalization of defense-related molecules. Phosphorylation by MPK3/6 increases LIP5 stability, which is necessary for pathogen-induced MVB trafficking and basal disease resistance. Based on these results we conclude that the MVB pathway is positively regulated by pathogen-responsive MPK3/6 through LIP5 phosphorylation and plays a critical role in plant immune system probably through involvement in the relocalization of defense-related molecules.

Insights into the localization and function of the membrane trafficking regulator GNOM ARF-GEF at the Golgi apparatus in Arabidopsis

GNOM is one of the most characterized membrane trafficking regulators in plants, with crucial roles in development. GNOM encodes an ARF-guanine nucleotide exchange factor (ARF-GEF) that activates small GTPases of the ARF (ADP ribosylation factor) class to mediate vesicle budding at endomembranes. The crucial role of GNOM in recycling of PIN auxin transporters and other proteins to the plasma membrane was identified in studies using the ARF-GEF inhibitor brefeldin A (BFA). GNOM, the most prominent regulator of recycling in plants, has been proposed to act and localize at so far elusive recycling endosomes. Here, we report the GNOM localization in context of its cellular function in Arabidopsis thaliana. State-of-the-art imaging, pharmacological interference, and ultrastructure analysis show that GNOM predominantly localizes to Golgi apparatus. Super-resolution confocal live imaging microscopy identified GNOM and its closest homolog GNOM-like 1 at distinct subdomains on Golgi cisternae. Short-term BFA treatment stabilizes GNOM at the Golgi apparatus, whereas prolonged exposures results in GNOM translocation to trans-Golgi network (TGN)/early endosomes (EEs). Malformed TGN/EE in gnom mutants suggests a role for GNOM in maintaining TGN/EE function. Our results redefine the subcellular action of GNOM and reevaluate the identity and function of recycling endosomes in plants.

Effects of brefeldin A on the endomembrane system and germ tube formation of the tetraspore of Gelidium floridanum (Rhodophyta, Florideophyceae)

Gelidium floridanum W.R. Taylor tetraspores are units of dispersal and are responsible for substrate attachment. This study aimed to examine evidence of direct interaction between germ tube formation and Golgi activity during tetraspore germination of G. floridanum. After release, the tetraspores were incubated with brefeldin A (BFA) in concentrations of 4 and 8 μM over a 6 h period. The controls and treatments were analyzed with light, fluorescence (FM4-64 dye) and transmission electron microscopy. In the control samples, the Golgi bodies were responsible for germ tube formation. In contrast, BFA-treated samples were observed to inhibit spore adhesion and germ tube formation. These tetraspores also showed an increase in volume (≥30 μm width). BFA treatment also resulted in the disassembly of Golgi cisternae and the formation of vesiculated areas of the cytoplasm, blocking the secretion of protein and amorphous matrix polysaccharides. When stained with FM4-64, the control samples showed fluorescence in the apical region of the germ tube, but the treated samples showed an intense fluorescence throughout the cytoplasm. From these results, we can conclude that the germ tube is formed by the incorporation of vesicles derived from Golgi. Thus, vesicle secretion and Golgi organization are basic processes and essential in adhesion and tube formation. By blocking the secretion of protein and amorphous matrix polysaccharides, BFA treatment precluded tetraspore germination.

The complexity of vesicle transport factors in plants examined by orthology search

Vesicle transport is a central process to ensure protein and lipid distribution in eukaryotic cells. The current knowledge on the molecular components and mechanisms of this process is majorly based on studies in Saccharomyces cerevisiae and Arabidopsis thaliana, which revealed 240 different proteinaceous factors either experimentally proven or predicted to be involved in vesicle transport. In here, we performed an orthologue search using two different algorithms to identify the components of the secretory pathway in yeast and 14 plant genomes by using the 'core-set' of 240 factors as bait. We identified 4021 orthologues and (co-)orthologues in the discussed plant species accounting for components of COP-II, COP-I, Clathrin Coated Vesicles, Retromers and ESCRTs, Rab GTPases, Tethering factors and SNAREs. In plants, we observed a significantly higher number of (co-)orthologues than yeast, while only 8 tethering factors from yeast seem to be absent in the analyzed plant genomes. To link the identified (co-)orthologues to vesicle transport, the domain architecture of the proteins from yeast, genetic model plant A. thaliana and agriculturally relevant crop Solanum lycopersicum has been inspected. For the orthologous groups containing (co-)orthologues from yeast, A. thaliana and S. lycopersicum, we observed the same domain architecture for 79% (416/527) of the (co-)orthologues, which documents a very high conservation of this process. Further, publically available tissue-specific expression profiles for a subset of (co-)orthologues found in A. thaliana and S. lycopersicum suggest that some (co-)orthologues are involved in tissue-specific functions. Inspection of localization of the (co-)orthologues based on available proteome data or localization predictions lead to the assignment of plastid- as well as mitochondrial localized (co-)orthologues of vesicle transport factors and the relevance of this is discussed.

Molecular mechanisms of Sar/Arf GTPases in vesicular trafficking in yeast and plants

Small GTPase proteins play essential roles in the regulation of vesicular trafficking systems in eukaryotic cells. Two types of small GTPases, secretion-associated Ras-related protein (Sar) and ADP-ribosylation factor (Arf), act in the biogenesis of transport vesicles. Sar/Arf GTPases function as molecular switches by cycling between active, GTP-bound and inactive, GDP-bound forms, catalyzed by guanine nucleotide exchange factors and GTPase-activating proteins, respectively. Activated Sar/Arf GTPases undergo a conformational change, exposing the N-terminal amphipathic α-helix for insertion into membranes. The process triggers the recruitment and assembly of coat proteins to the membranes, followed by coated vesicle formation and scission. In higher plants, Sar/Arf GTPases also play pivotal roles in maintaining the dynamic identity of organelles in the secretory pathway. Sar1 protein strictly controls anterograde transport from the endoplasmic reticulum (ER) through the recruitment of plant COPII coat components onto membranes. COPII vesicle transport is responsible for the organization of highly conserved polygonal ER networks. In contrast, Arf proteins contribute to the regulation of multiple trafficking routes, including transport through the Golgi complex and endocytic transport. These transport systems have diversified in the plant kingdom independently and exhibit several plant-specific features with respect to Golgi organization, endocytic cycling, cell polarity and cytokinesis. The functional diversification of vesicular trafficking systems ensures the multicellular development of higher plants. This review focuses on the current knowledge of Sar/Arf GTPases, highlighting the molecular details of GTPase regulation in vesicle formation in yeast and advances in knowledge of the characteristics of vesicle trafficking in plants.

Transcytosis shuts the door for an unwanted guest

Penetration resistance is a well-described plant defense process, in which SOLUBLE N-ETHYLMALEIMIDE-SENSITIVE-FACTOR ATTACHMENT RECEPTOR (SNARE) proteins have essential roles in membrane fusion processes. Strong focal accumulation of these proteins at the site of attack by powdery mildew fungi has been considered important for their function. However, recent insight indicates that transcytosis, leading to the formation of exosomes, has an important role in this defense and, furthermore, that strong accumulation of these SNARE proteins with the exosomes is biologically irrelevant. These findings alter the established function of SNAREs in penetration resistance; therefore, in this opinion, we propose that PEN1 and its SNARE partners function on an endosome in their control of penetration resistance.

MTV1 and MTV4 encode plant-specific ENTH and ARF GAP proteins that mediate clathrin-dependent trafficking of vacuolar cargo from the trans-Golgi network

Many soluble proteins transit through the trans-Golgi network (TGN) and the prevacuolar compartment (PVC) en route to the vacuole, but our mechanistic understanding of this vectorial trafficking step in plants is limited. In particular, it is unknown whether clathrin-coated vesicles (CCVs) participate in this transport step. Through a screen for modified transport to the vacuole (mtv) mutants that secrete the vacuolar protein VAC2, we identified MTV1, which encodes an epsin N-terminal homology protein, and MTV4, which encodes the ADP ribosylation factor GTPase-activating protein nevershed/AGD5. MTV1 and NEV/AGD5 have overlapping expression patterns and interact genetically to transport vacuolar cargo and promote plant growth, but they have no apparent roles in protein secretion or endocytosis. MTV1 and NEV/AGD5 colocalize with clathrin at the TGN and are incorporated into CCVs. Importantly, mtv1 nev/agd5 double mutants show altered subcellular distribution of CCV cargo exported from the TGN. Moreover, MTV1 binds clathrin in vitro, and NEV/AGD5 associates in vivo with clathrin, directly linking these proteins to CCV formation. These results indicate that MTV1 and NEV/AGD5 are key effectors for CCV-mediated trafficking of vacuolar proteins from the TGN to the PVC in plants.

Golgi traffic and integrity depend on N-myristoyl transferase-1 in Arabidopsis

N-myristoylation is a crucial irreversible eukaryotic lipid modification allowing a key subset of proteins to be targeted at the periphery of specific membrane compartments. Eukaryotes have conserved N-myristoylation enzymes, involving one or two N-myristoyltransferases (NMT1 and NMT2), among which NMT1 is the major enzyme. In the postembryonic developmental stages, defects in NMT1 lead to aberrant cell polarity, flower differentiation, fruit maturation, and innate immunity; however, no specific NMT1 target responsible for such deficiencies has hitherto been identified. Using a confocal microscopy forward genetics screen for the identification of Arabidopsis thaliana secretory mutants, we isolated STINGY, a recessive mutant with defective Golgi traffic and integrity. We mapped STINGY to a substitution at position 160 of Arabidopsis NMT1 (NMT1A160T). In vitro kinetic studies with purified NMT1A160T enzyme revealed a significant reduction in its activity due to a remarkable decrease in affinity for both myristoyl-CoA and peptide substrates. We show here that this recessive mutation is responsible for the alteration of Golgi traffic and integrity by predominantly affecting the Golgi membrane/cytosol partitioning of ADP-ribosylation factor proteins. Our results provide important functional insight into N-myristoylation in plants by ascribing postembryonic functions of Arabidopsis NMT1 that involve regulation of the functional and morphological integrity of the plant endomembranes.

Dynamic behavior of clathrin in Arabidopsis thaliana unveiled by live imaging

Clathrin-coated vesicles (CCV) are necessary for selective transport events, including receptor-mediated endocytosis on the plasma membrane and cargo molecule sorting in the trans-Golgi network (TGN). Components involved in CCV formation include clathrin heavy and light chains and several adaptor proteins that are conserved among plants. Clathrin-dependent endocytosis has been shown to play an integral part in plant endocytosis. However, little information is known about clathrin dynamics in living plant cells. In this study, we have visualized clathrin in Arabidopsis thaliana by tagging clathrin light chain with green fluorescent protein (CLC-GFP). Quantitative evaluations of colocalization demonstrate that the majority of CLC-GFP is localized to the TGN, and a minor population is associated with multivesicular endosomes and the Golgi trans-cisternae. Live imaging further demonstrated the presence of highly dynamic clathrin-positive tubules and vesicles, which appeared to mediate interactions between the TGNs. CLC-GFP is also targeted to cell plates and the plasma membrane. Although CLC-GFP colocalizes with a dynamin isoform at the plasma membrane, these proteins exhibit distinct distributions at newly forming cell plates. This finding indicates independent functions of CLC (clathrin light chains) and dynamin during the formation of cell plates. We have also found that brefeldin A and wortmannin treatment causes distinctly different alterations in the dynamics and distribution of clathrin-coated domains at the plasma membrane. This could account for the different effects of these drugs on plant endocytosis.

Membrane traffic and fusion at post-Golgi compartments

Complete sequencing of the Arabidopsis genome a decade ago has facilitated the functional analysis of various biological processes including membrane traffic by which many proteins are delivered to their sites of action and turnover. In particular, membrane traffic between post-Golgi compartments plays an important role in cell signaling, taking care of receptor-ligand interaction and inactivation, which requires secretion, endocytosis, and recycling or targeting to the vacuole for degradation. Here, we discuss recent studies that address the identity of post-Golgi compartments, the machinery involved in traffic and fusion or functionally characterized cargo proteins that are delivered to or pass through post-Golgi compartments. We also provide an outlook on future challenges in this area of research.