AtVPS45 complex formation at the trans-Golgi network

The plant Golgi apparatus--going with the flow

The plant Golgi apparatus is composed of many separate stacks of cisternae which are often associated with the endoplasmic reticulum and which in many cell types are motile. In this review, we discuss the latest data on the molecular regulation of Golgi function. The concept of the Golgi as a distinct organelle is challenged and the possibility of a continuum between the endoplasmic reticulum and Golgi is proposed.

VAN3 ARF-GAP-mediated vesicle transport is involved in leaf vascular network formation

Within the leaf of an angiosperm, the vascular system is constructed in a complex network pattern called venation. The formation of this vein pattern has been widely studied as a paradigm of tissue pattern formation in plants. To elucidate the molecular mechanism controlling the vein patterning process, we previously isolated Arabidopsis mutants van1 to van7, which show a discontinuous vein pattern. Here we report the phenotypic analysis of the van3 mutant in relation to auxin signaling and polar transport, and the molecular characterization of the VAN3 gene and protein. Double mutant analyses with pin1, emb30-7/gn and mp, and physiological analyses using the auxin-inducible marker DR5::GUS and an auxin transport inhibitor indicated that VAN3 may be involved in auxin signal transduction, but not in polar auxin transport. Positional cloning identified VAN3 as a gene that encodes an adenosine diphosphate (ADP)-ribosylation factor-guanosine triphosphatase (GTPase) activating protein (ARF-GAP). It resembles animal ACAPs and contains four domains: a BAR (BIN/amphiphysin/RVS) domain, a pleckstrin homology (PH) domain, an ARF-GAP domain and an ankyrin (ANK)-repeat domain. Recombinant VAN3 protein showed GTPase-activating activity and a specific affinity for phosphatidylinositols. This protein can self-associate through the N-terminal BAR domain in the yeast two-hybrid system. Subcellular localization analysis by double staining for Venus-tagged VAN3 and several green-fluorescent-protein-tagged intracellular markers indicated that VAN3 is located in a subpopulation of the trans-Golgi network (TGN). Our results indicate that the expression of this gene is induced by auxin and positively regulated by VAN3 itself, and that a specific ACAP type of ARF-GAP functions in vein pattern formation by regulating auxin signaling via a TGN-mediated vesicle transport system.

Conversion of Functional Specificity in Qb-SNARE VTI1 Homologues of Arabidopsis

In higher multicellular eukaryotes, highly specialized membrane structures or membrane trafficking events are required for supporting various physiological functions. SNAREs (soluble NSF attachment protein receptors) play an important role in specific membrane fusions. These protein receptors are assigned to subgroubs (Qa-, Qb-, Qc-, and R-SNARE) according to their specific SNARE structural motif. A specific set of Qa-, Qb-, and Qc-SNAREs, located on the target membrane, interact with R-SNARE on the vesicle to form a tight complex, leading to membrane fusion. The zig-1 mutant of Arabidopsis lacking Qb-SNARE VTI11 shows little shoot gravitropism and abnormal stem morphology. VTI11 and its homolog VTI12 exhibit partially overlapping but distinct intracellular localization and have different biological functions in plants. Little is known about how SNAREs are targeted to specific organelles, even though their functions and specific localization are closely linked. Here, we report that a novel mutation in VTI12 (zip1) was found as a dominant suppressor of zig-1. The zip1 mutation gave VTI12 the ability to function as VTI11 by changing both the specificity of SNARE complex formation and its intracellular localization. One amino acid substitution drastically altered VTI12, allowing it to suppress abnormalities of higher order physiological functions such as gravitropism and morphology. The zip1 mutation may be an indication of the flexibility in plant cell function afforded by gene duplication, particularly among the VTI11 genes and their recently diverged orthologs.

MEMBRANE TRAFFICKING IN PLANTS

Plant membrane trafficking shares many features with other eukaryotic organisms, including the machinery for vesicle formation and fusion. However, the plant endomembrane system lacks an ER-Golgi intermediate compartment, has numerous Golgi stacks and several types of vacuoles, and forms a transient compartment during cell division. ER-Golgi trafficking involves bulk flow and efficient recycling of H/KDEL-bearing proteins. Sorting in the Golgi stacks separates bulk flow to the plasma membrane from receptor-mediated trafficking to the lytic vacuole. Cargo for the protein storage vacuole is delivered from the endoplasmic reticulum (ER), cis-Golgi, and trans-Golgi. Endocytosis includes recycling of plasma membrane proteins from early endosomes. Late endosomes appear identical with the multivesiculate prevacuolar compartment that lies on the Golgi-vacuole trafficking pathway. In dividing cells, homotypic fusion of Golgi-derived vesicles forms the cell plate, which expands laterally by targeted vesicle fusion at its margin, eventually fusing with the plasma membrane.

The Arabidopsis rab5 homologs rha1 and ara7 localize to the prevacuolar compartment

Rha1, an Arabidopsis Rab5 homolog, plays a critical role in vacuolar trafficking in plant cells. In this study, we investigated the localization of Rha1 and Ara7, two Arabidopsis proteins that have highly similar amino acid sequence homology to Rab5 in animal cells. Both Ara7 and Rha1 gave a punctate staining pattern and colocalized when transiently expressed as GFP- (green fluorescent protein) or small epitope-tagged forms in Arabidopsis protoplasts. In protoplasts, transiently expressed Rha1 and Ara7 colocalized with AtPEP12p and VSR(At-1), two proteins that are known to be present at the prevacuolar compartment (PVC). Furthermore, endogenous Rha1 also gave a punctate staining pattern and colocalized with AtPEP12p to the PVC. Mutations in the first and second GTP-binding motifs alter the localizations of GFP: Rha1[S24N] in the cytosol and Rha1[Q69L] in the tonoplast of the central vacuole. Also, mutations in the effector domain and the prenylation site inhibit membrane association of Rha1. Based on these results, we propose that Rha1 and Ara7 localize to the PVC and that GTP-binding motifs as well as the effector domain are important for localization of Rha1 to the PVC.

Sorting inhibitors (Sortins): Chemical compounds to study vacuolar sorting in Arabidopsis

Chemical genomics is an interdisciplinary approach that unites the power of chemical screens and genomics strategies to dissect biological processes such as endomembrane trafficking. We have taken advantage of the evolutionary conservation between plants and Saccharomyces cerevisiae to identify such chemicals. Using S. cerevisiae, we screened a library of diverse chemical structures for compounds that induce the secretion of carboxypeptidase Y, which is normally targeted to the vacuole. Among 4,800 chemicals screened, 14 compounds, termed sorting inhibitors (Sortins), were identified that stimulated secretion in yeast. In Arabidopsis seedlings, application of Sortin1 and -2 led to reversible defects in vacuole biogenesis and root development. Sortin1 was found to redirect the vacuolar destination of plant carboxypeptidase Y and other proteins in Arabidopsis suspension cells and cause these proteins to be secreted. Sortin1 treatment of whole Arabidopsis seedlings also resulted in carboxypeptidase Y secretion, indicating that the drug has a similar mode of action in cells and intact plants. We have demonstrated that screening of a simple eukaryote, in which vacuolar biogenesis is not essential, can be a powerful tool to find chemicals that interfere with vacuolar delivery of proteins in plants, where vacuole biogenesis is essential. Our studies were done by using a sublethal dose of Sortin1, demonstrating the powerful ability of the chemical to control the induced phenotype in a manner that would be difficult to achieve using conventional genetics.

The Arabidopsis Rab GTPase RabA4b localizes to the tips of growing root hair cells

Spatial and temporal control of cell wall deposition plays a unique and critical role during growth and development in plants. To characterize membrane trafficking pathways involved in these processes, we have examined the function of a plant Rab GTPase, RabA4b, during polarized expansion in developing root hair cells. Whereas a small fraction of RabA4b cofractionated with Golgi membrane marker proteins, the majority of this protein labeled a unique membrane compartment that did not cofractionate with the previously characterized trans-Golgi network syntaxin proteins SYP41 and SYP51. An enhanced yellow fluorescent protein (EYFP)-RabA4b fusion protein specifically localizes to the tips of growing root hair cells in Arabidopsis thaliana. Tip-localized EYFP-RabA4b disappears in mature root hair cells that have stopped expanding, and polar localization of the EYFP-RabA4b is disrupted by latrunculin B treatment. Loss of tip localization of EYFP-RabA4b was correlated with inhibition of expansion; upon washout of the inhibitor, root hair expansion recovered only after tip localization of the EYFP-RabA4b compartments was reestablished. Furthermore, in mutants with defective root hair morphology, EYFP-RabA4b was improperly localized or was absent from the tips of root hair cells. We propose that RabA4b regulates membrane trafficking through a compartment involved in the polarized secretion of cell wall components in plant cells.

Traffic jams affect plant development and signal transduction

Analysis of the Arabidopsis thaliana endomembrane system has shown that plant cell viability depends on a properly functioning vacuole and intact vesicular trafficking. The endomembrane system is also essential for various aspects of plant development and signal transduction. In this review, we discuss examples of these newly discovered roles for the endomembrane system in plants, and new experimental approaches and technologies that are based on high-throughput screens, which combine chemical genetics and automated confocal microscopy.

A new catch in the SNARE

Vesicle traffic underpins cell homeostasis, growth and development in plants. Traffic is facilitated by a superfamily of proteins known as SNAREs ( soluble N-ethylmaleimide-sensitive fusion protein attachment protein receptors) that interact to draw vesicle and target membrane surfaces together for fusion of the bilayers. Several recent findings now indicate that plant SNAREs might not be limited to the conventional 'housekeeping' activities commonly attributed to vesicle trafficking. In the past five years, six different SNAREs have been implicated in stomatal movements, gravisensing and pathogen resistance. These proteins almost certainly do contribute to specific membrane fusion events but they are also essential for signal transduction and response. Some SNAREs can modulate the activity of non-SNARE proteins, notably ion channels. Other examples might reflect SNARE interactions with different scaffolding and structural components of the cell.

Arabidopsis mu A-adaptin interacts with the tyrosine motif of the vacuolar sorting receptor VSR-PS1

In receptor-mediated transport pathways in mammalian cells, clathrin-coated vesicle (CCV) mu-adaptins are the main binding partners for the tyrosine sorting/internalization motif (YXXØ). We have analyzed the function of the mu A-adaptin, one of the five mu-adaptins from Arabidopsis thaliana, by pull-down assays and plasmon resonance measurements using its receptor-binding domain (RBD) fused to a histidine tag. We show that this adaptin is able to bind the consensus tyrosine motif YXXØ from the pea vacuolar sorting receptor (VSR)-PS1, as well as from the mammalian trans-Golgi network (TGN)38 protein. Moreover, the tyrosine residue was revealed to be crucial for binding of the complete cytoplasmic tail of VSR-PS1 to the plant mu A-adaptin. The trans-Golgi localization of the mu A-adaptin strongly suggests its involvement in Golgi- to vacuole-trafficking events.

Systematic Analysis of SNARE Molecules in Arabidopsis: Dissection of the post-Golgi Network in Plant Cells

In all eucaryotic cells, specific vesicle fusion during vesicular transport is mediated by membrane-associated proteins called SNAREs (soluble N-ethyl-maleimide sensitive factor attachment protein receptors). Sequence analysis identified a total of 54 SNARE genes (18 Qa-SNAREs/Syntaxins, 11 Qb-SNAREs, 8 Qc-SNAREs, 14 R-SNAREs/VAMPs and 3 SNAP-25) in the Arabidopsis genome. Almost all of them were ubiquitously expressed through out all tissues examined. A series of transient expression assays using green fluorescent protein (GFP) fused proteins revealed that most of the SNARE proteins were located on specific intracellular compartments: 6 in the endoplasmic reticulum, 9 in the Golgi apparatus, 4 in the trans-Golgi network (TGN), 2 in endosomes, 17 on the plasma membrane, 7 in both the prevacuolar compartment (PVC) and vacuoles, 2 in TGN/PVC/vacuoles, and 1 in TGN/PVC/plasma membrane. Some SNARE proteins showed multiple localization patterns in two or more different organelles, suggesting that these SNAREs shuttle between the organelles. Furthermore, the SYP41/SYP61-residing compartment, which was defined as the TGN, was not always located along with the Golgi apparatus, suggesting that this compartment is an independent organelle distinct from the Golgi apparatus. We propose possible combinations of SNARE proteins on all subcellular compartments, and suggest the complexity of the post-Golgi membrane traffic in higher plant cells.

The VTI family of SNARE proteins is necessary for plant viability and mediates different protein transport pathways

The Arabidopsis genome contains a family of v-SNAREs: VTI11, VTI12, and VTI13. Only VTI11 and VTI12 are expressed at appreciable levels. Although these two proteins are 60% identical, they complement different transport pathways when expressed in the yeast vti1 mutant. VTI11 was identified recently as the mutated gene in the shoot gravitropic mutant zig. Here, we show that the vti11 zig mutant has defects in vascular patterning and auxin transport. An Arabidopsis T-DNA insertion mutant, vti12, had a normal phenotype under nutrient-rich growth conditions. However, under nutrient-poor conditions, vti12 showed an accelerated senescence phenotype, suggesting that VTI12 may play a role in the plant autophagy pathway. VTI11 and VTI12 also were able to substitute for each other in their respective SNARE complexes, and a double-mutant cross between zig and vti12 was embryo lethal. These results suggest that some VTI1 protein was necessary for plant viability and that the two proteins were partially functionally redundant.

Vesicular cycling mechanisms that control auxin transport polarity

The polar transport of auxin controls many important plant growth and developmental processes. The polarity of auxin movement has long been suggested to be mediated by asymmetric distribution of auxin transport proteins, yet, until recently, little was known about the mechanisms that establish protein asymmetry in auxin-transporting cells. Now, a recent paper provides significant insight into the mechanism by which the GNOM protein controls the cycling of an auxin efflux carrier protein, PIN1, between the endosome and the plasma membrane. The dynamic movement of auxin transport proteins between internal compartments and the plasma membrane suggests mechanisms for alterations in auxin transport polarity in response to changing developmental or environmental regulation.

The AtC-VPS protein complex is localized to the tonoplast and the prevacuolar compartment in arabidopsis

Plant cells contain several types of vacuoles with specialized functions. Although the biogenesis of these organelles is well understood at the morphological level, the machinery involved in plant vacuole formation is largely unknown. We have recently identified an Arabidopsis mutant, vcl1, that is deficient in vacuolar formation. VCL1 is homologous to a protein that regulates membrane fusion at the tonoplast in yeast. On the basis of these observations, VCL1 is predicted to play a direct role in vacuolar biogenesis and vesicular trafficking to the vacuole in plants. In this work, we show that VCL1 forms a complex with AtVPS11 and AtVPS33 in vivo. These two proteins are homologues of proteins that have a well-characterized role in membrane fusion at the tonoplast in yeast. VCL1, AtVPS11, and AtVPS33 are membrane-associated and cofractionate with tonoplast and denser endomembrane markers in subcellular fractionation experiments. Consistent with this, VCL1, AtVPS11, and AtVPS33 are found on the tonoplast and the prevacuolar compartment (PVC) by immunoelectron microscopy. We also show that a VCL1-containing complex includes SYP2-type syntaxins and is most likely involved in membrane fusion on both the PVC and tonoplast in vivo. VCL1, AtVPS11, and AtVPS33 are the first components of the vacuolar biogenesis machinery to be identified in plants.

The Arabidopsis GNOM ARF-GEF mediates endosomal recycling, auxin transport, and auxin-dependent plant growth

Exchange factors for ARF GTPases (ARF-GEFs) regulate vesicle trafficking in a variety of organisms. The Arabidopsis protein GNOM is a brefeldin A (BFA) sensitive ARF-GEF that is required for the proper polar localization of PIN1, a candidate transporter of the plant hormone auxin. Mutations in GNOM lead to developmental defects that resemble those caused by interfering with auxin transport. Both PIN1 localization and auxin transport are also sensitive to BFA. In this paper, we show that GNOM localizes to endosomes and is required for their structural integrity. We engineered a BFA-resistant version of GNOM. In plants harboring this fully functional GNOM variant, PIN1 localization and auxin transport are no longer sensitive to BFA, while trafficking of other proteins is still affected by the drug. Our results demonstrate that GNOM is required for the recycling of auxin transport components and suggest that ARF-GEFs regulate specific endosomal trafficking pathways.

Physical methods

Despite the advent of high-output, recombinant DNA-based screening strategies, many important protein-protein interactions in the plant cell have been and still are revealed using co-sedimentation, affinity chromatography and other affinity techniques, co-immunoprecipitation and cross-linking. The advantages of these techniques, the care that should be taken interpreting the data obtained and the possible ways to overcome pitfalls are illustrated.

Vesicle traffic in the endomembrane system: a tale of COPs, Rabs and SNAREs

Recent years have seen remarkable progress in our understanding of the endomembrane system of plants. A large number of genes and proteins that are involved in membrane exchange between the different compartments of this system have been identified on the basis of their similarity to animal and yeast homologs. These proteins indicate that the endomembrane system in plants functions in essentially the same way as those in other eukaryotes. However, a growing number of examples demonstrate that the dynamic interplay between membrane-exchange proteins can be regulated differently in plant cells. Novel tools and a better understanding of the molecular effects of the inhibitor brefeldin A are helping to unravel these plant-specific adaptations.

Characterization of AtCDC48. Evidence for multiple membrane fusion mechanisms at the plane of cell division in plants

The components of the cellular machinery that accomplish the various complex and dynamic membrane fusion events that occur at the division plane during plant cytokinesis, including assembly of the cell plate, are not fully understood. The most well-characterized component, KNOLLE, a cell plate-specific soluble N-ethylmaleimide-sensitive fusion protein (NSF)-attachment protein receptor (SNARE), is a membrane fusion machine component required for plant cytokinesis. Here, we show the plant ortholog of Cdc48p/p97, AtCDC48, colocalizes at the division plane in dividing Arabidopsis cells with KNOLLE and another SNARE, the plant ortholog of syntaxin 5, SYP31. In contrast to KNOLLE, SYP31 resides in defined punctate membrane structures during interphase and is targeted during cytokinesis to the division plane. In vitro-binding studies demonstrate that AtCDC48 specifically interacts in an ATP-dependent manner with SYP31 but not with KNOLLE. In contrast, we show that KNOLLE assembles in vitro into a large approximately 20S complex in an Sec18p/NSF-dependent manner. These results suggest that there are at least two distinct membrane fusion pathways involving Cdc48p/p97 and Sec18p/NSF that operate at the division plane to mediate plant cytokinesis. Models for the role of AtCDC48 and SYP31 at the division plane will be discussed.

Protein secretion in plants: from the trans-Golgi network to the outer space

Functional analysis of exocytosis in yeast and animal cells has led to the identification of conserved elements and mechanisms of the trafficking machinery over the last decade. Although functional studies of protein secretion in plants are still fairly limited, the Arabidopsis genome sequence provides an opportunity to identify key players of vesicle trafficking that are conserved across the eukaryotic kingdoms. Here, we review and add to recent genome analyses of trafficking components and highlight some plant-specific modifications of the common eukaryotic machinery. Furthermore, we discuss the evidence for targeted, polarised secretion in plant cells, and speculate about possible underlying cargo sorting processes at the trans-Golgi network and endosomes, based on what is known in animals and yeast.

How Tlg2p/syntaxin 16 'snares' Vps45

Soluble N-ethylmaleimide sensitive factor-attachment protein receptors (SNAREs) and Sec1p/Munc18-homologs (SM proteins) play key roles in intracellular membrane fusion. The SNAREs form tight four-helix bundles (core complexes) that bring the membranes together, but it is unclear how this activity is coupled to SM protein function. Studies of the yeast trans-Golgi network (TGN)/endosomal SNARE complex, which includes the syntaxin-like SNARE Tlg2p, have suggested that its assembly requires activation by binding of the SM protein Vps45p to the cytoplasmic region of Tlg2p folded into a closed conformation. Nuclear magnetic resonance and biochemical experiments now show that Tlg2p and Pep12p, a late- endosomal syntaxin that interacts functionally but not directly with Vps45p, have a domain structure characteristic of syntaxins but do not adopt a closed conformation. Tlg2p binds tightly to Vps45p via a short N-terminal peptide motif that is absent in Pep12p. The Tlg2p/Vps45p binding mode is shared by the mammalian syntaxin 16, confirming that it is a Tlg2p homolog, and resembles the mode of interaction between the SM protein Sly1p and the syntaxins Ufe1p and Sed5p. Thus, this mechanism represents the most widespread mode of coupling between syntaxins and SM proteins.

NPSN11 is a cell plate-associated SNARE protein that interacts with the syntaxin KNOLLE

SNAREs are important components of the vesicle trafficking machinery in eukaryotic cells. In plants, SNAREs have been found to play a variety of roles in the development and physiology of the whole organism. Here, we describe the identification and characterization of a novel plant-specific SNARE, NPSN11, a member of a closely related small gene family in Arabidopsis. NSPN11 is highly expressed in actively dividing cells. In a subcellular fractionation experiment, NSPN11 cofractionates with the cytokinesis-specific syntaxin, KNOLLE, which is required for the formation of the cell plate. By immunofluorescence microscopy, NSPN11 was localized to the cell plate in dividing cells. Consistent with the localization studies, NSPN11 was found to interact with KNOLLE. Our results suggest that NPSN11 is another component of the membrane trafficking and fusion machinery involved in cell plate formation.

Toward understanding vesicle traffic and the guard cell model

Vesicular trafficking and membrane fusion are integral to cell growth and development with SNARE proteins, RabGTPases and their associates implicated in membrane fusion and secretion throughout the plant endomembrane system. Although the overall pattern of function is similar to that of animals and yeast, many aspects of endomembrane organization and vesicle trafficking appear unique to plants, for example, the control of cell and vacuolar expansion, asymmetric growth and cell division. However, the dominant membrane trafficking pathways have yet to be defined. Comparative genomics provide important information about vesicle trafficking elements but assigning biological roles based on sequence similarities is extremely difficult. Cellular and genetic approaches are reviewed here that have allowed visualization of vesicle trafficking in plants, including capacitance and dye methods, imaging and marker techniques, protein interactions and reverse genetics. Stomatal guard cells are discussed as cell models for identifying vesicle trafficking pathways and evidence points to a role for vesicle trafficking in stomatal function. For plants generally, kinetic analyses and biochemical studies suggest that several different pools of vesicles, and possibly different mechanisms for delivery, are available for vesicle traffic between endomembrane compartments and the plasma membrane. Characterizing these pathways, their functions and controls provides a major challenge for the future.

The Tlg SNARE complex is required for TGN homotypic fusion

Using a new assay for membrane fusion between late Golgi/endosomal compartments, we have reconstituted a rapid, robust homotypic fusion reaction between membranes containing Kex2p and Ste13p, two enzymes resident in the yeast trans-Golgi network (TGN). Fusion was temperature, ATP, and cytosol dependent. It was inhibited by dilution, Ca+2 chelation, N-ethylmaleimide, and detergent. Coimmunoisolation confirmed that the reaction resulted in cointegration of the two enzymes into the same bilayer. Antibody inhibition experiments coupled with antigen competition indicated a requirement for soluble NSF attachment protein receptor (SNARE) proteins Tlg1p, Tlg2p, and Vti1p in this reaction. Membrane fusion also required the rab protein Vps21p. Vps21p was sufficient if present on either the Kex2p or Ste13p membranes alone, indicative of an inherent symmetry in the reaction. These results identify roles for a Tlg SNARE complex composed of Tlg1p, Tlg2p, Vti1p, and the rab Vps21p in this previously uncharacterized homotypic TGN fusion reaction.

Interactions between syntaxins identify at least five SNARE complexes within the Golgi/prevacuolar system of the Arabidopsis cell

The syntaxin family of soluble N-ethyl maleimide sensitive factor adaptor protein receptors (SNAREs) is known to play an important role in the fusion of transport vesicles with specific organelles. Twenty-four syntaxins are encoded in the genome of the model plant Arabidopsis thaliana. These 24 genes are found in 10 gene families and have been reclassified as syntaxins of plants (SYPs). Some of these gene families have been previously characterized, with the SYP2-type syntaxins being found in the prevacuolar compartment (PVC) and the SYP4-type syntaxins on the trans-Golgi network (TGN). Here we report on two previously uncharacterized syntaxin groups. The SYP5 group is encoded by a two-member gene family, whereas SYP61 is a single gene. Both types of syntaxins are localized to multiple compartments of the endomembrane system, including the TGN and the PVC. These two groups of syntaxins form SNARE complexes with each other, and with other Arabidopsis SNAREs. On the TGN, SYP61 forms complexes with the SNARE VTI12 and either SYP41 or SYP42. SYP51 and SYP61 interact with each other and with VTI12, most likely also on the TGN. On the PVC, a SYP5-type syntaxin interacts specifically with a SYP2-type syntaxin, as well as the SNARE VTI11, forming a SNARE complex likely involved in TGN-to-PVC trafficking.

VACUOLELESS1 is an essential gene required for vacuole formation and morphogenesis in Arabidopsis

Most plant cells are characterized by the presence of a large central vacuole that in differentiated cells accounts for more than 90% of the total volume. We have undertaken a genetic screen to look for mutants that are affected in the formation of vacuoles in plants. In this study, we report that inactivation of the Arabidopsis gene VACUOLELESS1 (VCL1) blocks vacuole formation and alters the pattern of cell division orientation and cell elongation in the embryo. Consistent with a role in vacuole biogenesis, we show that VCL1 encodes the Arabidopsis ortholog of yeast Vps16p. In contrast to yeast mutants that lack a vacuolar compartment but are viable and morphologically normal, loss of the plant vacuole leads to aberrant morphogenesis and embryonic lethality.

A new dynamin-like protein, ADL6, is involved in trafficking from the trans-Golgi network to the central vacuole in Arabidopsis

Dynamin, a high-molecular-weight GTPase, plays a critical role in vesicle formation at the plasma membrane during endocytosis in animal cells. Here we report the identification of a new dynamin homolog in Arabidopsis named Arabidopsis dynamin-like 6 (ADL6). ADL6 is quite similar to dynamin I in its structural organization: a conserved GTPase domain at the N terminus, a pleckstrin homology domain at the center, and a Pro-rich motif at the C terminus. In the cell, a majority of ADL6 is associated with membranes. Immunohistochemistry and in vivo targeting experiments revealed that ADL6 is localized to the Golgi apparatus. Expression of the dominant negative mutant ADL6[K51E] in Arabidopsis protoplasts inhibited trafficking of cargo proteins destined for the lytic vacuole and caused them to accumulate at the trans-Golgi network. In contrast, expression of ADL6[K51E] did not affect trafficking of a cargo protein, H(+)-ATPase:green fluorescent protein, destined for the plasma membrane. These results suggest that ADL6 is involved in vesicle formation for vacuolar trafficking at the trans-Golgi network but not for trafficking to the plasma membrane in plant cells.

A new dynamin-like protein, ADL6, is involved in trafficking from the trans-Golgi network to the central vacuole in Arabidopsis

Dynamin, a high-molecular-weight GTPase, plays a critical role in vesicle formation at the plasma membrane during endocytosis in animal cells. Here we report the identification of a new dynamin homolog in Arabidopsis named Arabidopsis dynamin-like 6 (ADL6). ADL6 is quite similar to dynamin I in its structural organization: a conserved GTPase domain at the N terminus, a pleckstrin homology domain at the center, and a Pro-rich motif at the C terminus. In the cell, a majority of ADL6 is associated with membranes. Immunohistochemistry and in vivo targeting experiments revealed that ADL6 is localized to the Golgi apparatus. Expression of the dominant negative mutant ADL6[K51E] in Arabidopsis protoplasts inhibited trafficking of cargo proteins destined for the lytic vacuole and caused them to accumulate at the trans-Golgi network. In contrast, expression of ADL6[K51E] did not affect trafficking of a cargo protein, H(+)-ATPase:green fluorescent protein, destined for the plasma membrane. These results suggest that ADL6 is involved in vesicle formation for vacuolar trafficking at the trans-Golgi network but not for trafficking to the plasma membrane in plant cells.

A new dynamin-like protein, ADL6, is involved in trafficking from the trans-Golgi network to the central vacuole in Arabidopsis

Dynamin, a high-molecular-weight GTPase, plays a critical role in vesicle formation at the plasma membrane during endocytosis in animal cells. Here we report the identification of a new dynamin homolog in Arabidopsis named Arabidopsis dynamin-like 6 (ADL6). ADL6 is quite similar to dynamin I in its structural organization: a conserved GTPase domain at the N terminus, a pleckstrin homology domain at the center, and a Pro-rich motif at the C terminus. In the cell, a majority of ADL6 is associated with membranes. Immunohistochemistry and in vivo targeting experiments revealed that ADL6 is localized to the Golgi apparatus. Expression of the dominant negative mutant ADL6[K51E] in Arabidopsis protoplasts inhibited trafficking of cargo proteins destined for the lytic vacuole and caused them to accumulate at the trans-Golgi network. In contrast, expression of ADL6[K51E] did not affect trafficking of a cargo protein, H(+)-ATPase:green fluorescent protein, destined for the plasma membrane. These results suggest that ADL6 is involved in vesicle formation for vacuolar trafficking at the trans-Golgi network but not for trafficking to the plasma membrane in plant cells.

CYTOKINESIS AND BUILDING OF THE CELL PLATE IN PLANTS

Cytokinesis in plant cells is more complex than in animals, as it involves building a cell plate as the final step in generating two cells. The cell plate is built in the center of phragmoplast by fusion of Golgi-derived vesicles. This step imposes an architectural problem where ballooning of the fused structures has to be avoided to create a plate instead. This is apparently achieved by squeezing the vesicles into dumbbell-shaped vesicle-tubule-vesicle (VTV) structures with the help of phragmoplastin, a homolog of dynamin. These structures are fused at their ends in a star-shaped body creating a tubulovesicular "honeycomb-like" structure sandwiched between the positive ends of the phragmoplast microtubules. This review summarizes our current understanding of various mechanisms involved in budding-off of Golgi vesicles, delivery and fusion of vesicles to initiate cell plate, and the synthesis of polysaccharides at the forming cell plate. Little is known about the molecular mechanisms involved in determining the site, direction, and the point of attachment of the growing cell plate with the parental cell wall. These gaps may be filled soon, as many genes that have been identified by mutations are analyzed and functions of their products are deciphered.

Mobile factories: Golgi dynamics in plant cells

The plant Golgi apparatus plays a central role in the synthesis of cell wall material and the modification and sorting of proteins destined for the cell surface and vacuoles. Earlier perceptions of this organelle were shaped by static transmission electron micrographs and by its biosynthetic functions. However, it has become increasingly clear that many Golgi activities can only be understood in the context of its dynamic organization. Significant new insights have been gained recently into the molecules that mediate this dynamic behavior, and how this machinery differs between plants and animals or yeast. Most notable is the discovery that plant Golgi stacks can actively move through the cytoplasm along actin filaments, an observation that has major implications for trafficking to, through and from this organelle.

Disruption of individual members of Arabidopsis syntaxin gene families indicates each has essential functions

Syntaxins are a large group of proteins found in all eukaryotes involved in the fusion of transport vesicles to target membranes. Twenty-four syntaxins grouped into 10 gene families are found in the model plant Arabidopsis thaliana, each group containing one to five paralogous members. The Arabidopsis SYP2 and SYP4 gene families contain three members each that share 60 to 80% protein sequence identity. Gene disruptions of the yeast (Saccharomyces cerevisiae) orthologs of the SYP2 and SYP4 gene families (Pep12p and Tlg2p, respectively) indicate that these syntaxins are not essential for growth in yeast. However, we have isolated and characterized gene disruptions in two genes from each family, finding that disruption of individual syntaxins from these families is lethal in the male gametophyte of Arabidopsis. Complementation of the syp21-1 gene disruption with its cognate transgene indicated that the lethality is linked to the loss of the single syntaxin gene. Thus, it is clear that each syntaxin in the SYP2 and SYP4 families serves an essential nonredundant function.

The Arabidopsis genome. An abundance of soluble N-ethylmaleimide-sensitive factor adaptor protein receptors

Many factors have been characterized as essential for vesicle trafficking, including a number of proteins commonly referred to as soluble N-ethylmaleimide-sensitive factor adaptor protein receptor (SNARE) components. The Arabidopsis genome contains a remarkable number of SNAREs. In general, the vesicle fusion machinery appears highly conserved. However, whereas some classes of yeast and mammalian genes appear to be lacking in Arabidopsis, this small plant genome has gene families not found in other eukaryotes. Very little is known about the precise function of plant SNAREs. By contrast, the intracellular localization of and interactions between a large number of plant SNAREs have been determined, and these data are discussed in light of the phylogenetic analysis.

Unique features of the plant vacuolar sorting machinery

Multiple types of vacuoles can exist within the same plant cell, and different vesicle-trafficking pathways transport proteins to each of them. Recent work has identified proteins unique to each vacuole type, and the transport pathways have begun to be elucidated. Plant trafficking proteins are usually encoded by small gene families, the different members of which have distinct functions in the endomembrane system.