Correct targeting of plant ARF GTPases relies on distinct protein domains

The role of lipid-modified proteins in cell wall synthesis and signaling

The plant cell wall is a complex and dynamic extracellular matrix. Plant primary cell walls are the first line of defense against pathogens and regulate cell expansion. Specialized cells deposit a secondary cell wall that provides support and permits water transport. The composition and organization of the cell wall varies between cell types and species, contributing to the extensibility, stiffness, and hydrophobicity required for its proper function. Recently, many of the proteins involved in the biosynthesis, maintenance, and remodeling of the cell wall have been identified as being post-translationally modified with lipids. These modifications exhibit diverse structures and attach to proteins at different sites, which defines the specific role played by each lipid modification. The introduction of relatively hydrophobic lipid moieties promotes the interaction of proteins with membranes and can act as sorting signals, allowing targeted delivery to the plasma membrane regions and secretion into the apoplast. Disruption of lipid modification results in aberrant deposition of cell wall components and defective cell wall remodeling in response to stresses, demonstrating the essential nature of these modifications. Although much is known about which proteins bear lipid modifications, many questions remain regarding the contribution of lipid-driven membrane domain localization and lipid heterogeneity to protein function in cell wall metabolism. In this update, we highlight the contribution of lipid modifications to proteins involved in the formation and maintenance of plant cell walls, with a focus on the addition of glycosylphosphatidylinositol anchors, N-myristoylation, prenylation, and S-acylation.

The putative myristoylome of Physcomitrium patens reveals conserved features of myristoylation in basal land plants

KEYMESSAGE

The putative myristoylome of moss P. patens opens an avenue for studying myristoylation substrates in non-canonical model plants. A myristoylation signal was shown sufficient for membrane targeting and useful for membrane dynamics visualization during cell growth. N-myristoylation (MYR) is one form of lipid modification catalyzed by N-myristoyltransferase that enables protein-membrane association. MYR is highly conserved in all eukaryotes. However, the study of MYR is limited to a few models such as yeasts, humans, and Arabidopsis. Here, using prediction tools, we report the characterization of the putative myristoylome of the moss Physcomitrium patens. We show that basal land plants display a similar signature of MYR to Arabidopsis and may have organism-specific substrates. Phylogenetically, MYR signals have mostly co-evolved with protein function but also exhibit variability in an organism-specific manner. We also demonstrate that the MYR motif of a moss brassinosteroid-signaling kinase is an efficient plasma membrane targeting signal and labels lipid-rich domains in tip-growing cells. Our results provide insights into the myristoylome in a basal land plant and lay the foundation for future studies on MYR and its roles in plant evolution.

ArfX2 GTPase Regulates Trafficking From the Trans-Golgi to Lysosomes and Is Necessary for Liver Abscess Formation in the Protozoan Parasite Entamoeba histolytica

Entamoeba histolytica is the causative agent of amoebic dysentery and liver abscess in humans. The parasitic lifestyle and the virulence of the protist require elaborate biological processes, including vesicular traffic and stress management against a variety of reactive oxygen and nitrogen species produced by the host immune response. Although the mechanisms for intracellular traffic of representative virulence factors have been investigated at molecular levels, it remains poorly understood whether and how intracellular traffic is involved in the defense against reactive oxygen and nitrogen species. Here, we demonstrate that EhArfX2, one of the Arf family of GTPases known to be involved in the regulation of vesicular traffic, was identified by comparative transcriptomic analysis of two isogenic strains: an animal-passaged highly virulent HM-1:IMSS Cl6 and in vitro maintained attenuated avirulent strain. EhArfX2 was identified as one of the most highly upregulated genes in the highly virulent strain. EhArfX2 was localized to small vesicle-like structures and largely colocalized with the marker for the trans-Golgi network SNARE, EhYkt6, but neither with the endoplasmic reticulum (ER)-resident chaperon, EhBip, nor the cis-Golgi SNARE, EhSed5, and Golgi-luminal galactosyl transferase, EhGalT. Expression of the dominant-active mutant form of EhArfX2 caused an increase in the number of lysosomes, while expression of the dominant-negative mutant led to a defect in lysosome formation and cysteine protease transport to lysosomes. Expression of the dominant-negative mutant in the virulent E. histolytica strain caused a reduction of the size of liver abscesses in a hamster model. This defect in liver abscess formation was likely at least partially attributed to reduced resistance to nitrosative, but not oxidative stress in vitro. These results showed that the EhArfX2-mediated traffic is necessary for the nitrosative stress response and virulence in the host.

The Coordinated KNR6-AGAP-ARF1 Complex Modulates Vegetative and Reproductive Traits by Participating in Vesicle Trafficking in Maize

The KERNEL NUMBER PER ROW6 (KNR6)-mediated phosphorylation of an adenosine diphosphate ribosylation factor (Arf) GTPase-activating protein (AGAP) forms a key regulatory module for the numbers of spikelets and kernels in the ear inflorescences of maize (Zea mays L.). However, the action mechanism of the KNR6–AGAP module remains poorly understood. Here, we characterized the AGAP-recruited complex and its roles in maize cellular physiology and agronomically important traits. AGAP and its two interacting Arf GTPase1 (ARF1) members preferentially localized to the Golgi apparatus. The loss-of-function AGAP mutant produced by CRISPR/Cas9 resulted in defective Golgi apparatus with thin and compact cisternae, together with delayed internalization and repressed vesicle agglomeration, leading to defective inflorescences and roots, and dwarfed plants with small leaves. The weak agap mutant was phenotypically similar to knr6, showing short ears with fewer kernels. AGAP interacted with KNR6, and a double mutant produced shorter inflorescence meristems and mature ears than the single agap and knr6 mutants. We hypothesized that the coordinated KNR6–AGAP–ARF1 complex modulates vegetative and reproductive traits by participating in vesicle trafficking in maize. Our findings provide a novel mechanistic insight into the regulation of inflorescence development, and ear length and kernel number, in maize.

The Small GTPase Superfamily in Plants: A Conserved Regulatory Module with Novel Functions

Small GTP-binding proteins represent a highly conserved signaling module in eukaryotes that regulates diverse cellular processes such as signal transduction, cytoskeletal organization and cell polarity, cell proliferation and differentiation, intracellular membrane trafficking and transport vesicle formation, and nucleocytoplasmic transport. These proteins function as molecular switches that cycle between active and inactive states, and this cycle is linked to GTP binding and hydrolysis. In this review, the roles of the plant complement of small GTP-binding proteins in these cellular processes are described, as well as accessory proteins that control their activity, and current understanding of the functions of individual members of these families in plants-with a focus on the model organism Arabidopsis-is presented. Some potential novel roles of these GTPases in plants, relative to their established roles in yeast and/or animal systems, are also discussed.

Homologous Proteins of the Manganese Transporter PAM71 Are Localized in the Golgi Apparatus and Endoplasmic Reticulum

Chloroplast manganese transporter 1 (CMT1) and photosynthesis-affected mutant 71 (PAM71) are two membrane proteins that function sequentially to mediate the passage of manganese across the chloroplast envelope and the thylakoid membrane. CMT1 and PAM71 belong to a small five-member protein family in Arabidopsis thaliana. The other three, photosynthesis-affected mutant 71 like 3 (PML3), PML4 and PML5 are not predicted to reside in chloroplast membranes. In this study, the subcellular localization of PML3:GFP, PML4:GFP and PML5:GFP was determined using transient and stable expression assays. PML3:GFP localizes to the Golgi apparatus, whereas PML4:GFP and PML5:GFP are found in the endoplasmic reticulum. We also examined patterns of PML3, PML4 and PML5 promoter activity. Although the precise expression pattern of each promoter was unique, all three genes were expressed in the leaf vasculature and in roots. Greenhouse grown single mutants pml3, pml4, pml5 and the pml4/pml5 double mutant did not exhibit growth defects, however an inspection of the root growth revealed a difference between pml3 and the other genotypes, including wild-type, in 500 µM manganese growth conditions. Strikingly, overexpression of PML3 resulted in a stunted growth phenotype. Putative functions of PML3, PML4 and PML5 are discussed in light of what is known about PAM71 and CMT1.

A single class of ARF GTPase activated by several pathway-specific ARF-GEFs regulates essential membrane traffic in Arabidopsis

In eukaryotes, GTP-bound ARF GTPases promote intracellular membrane traffic by mediating the recruitment of coat proteins, which in turn sort cargo proteins into the forming membrane vesicles. Mammals employ several classes of ARF GTPases which are activated by different ARF guanine-nucleotide exchange factors (ARF-GEFs). In contrast, flowering plants only encode evolutionarily conserved ARF1 GTPases (class I) but not the other classes II and III known from mammals, as suggested by phylogenetic analysis of ARF family members across the five major clades of eukaryotes. Instead, flowering plants express plant-specific putative ARF GTPases such as ARFA and ARFB, in addition to evolutionarily conserved ARF-LIKE (ARL) proteins. Here we show that all eight ARF-GEFs of Arabidopsis interact with the same ARF1 GTPase, whereas only a subset of post-Golgi ARF-GEFs also interacts with ARFA, as assayed by immunoprecipitation. Both ARF1 and ARFA were detected at the Golgi stacks and the trans-Golgi network (TGN) by both live-imaging with the confocal microscope and nano-gold labeling followed by EM analysis. ARFB representing another plant-specific putative ARF GTPase was detected at both the plasma membrane and the TGN. The activation-impaired form (T31N) of ARF1, but neither ARFA nor ARFB, interfered with development, although ARFA-T31N interfered, like ARF1-T31N, with the GDP-GTP exchange. Mutant plants lacking both ARFA and ARFB transcripts were viable, suggesting that ARF1 is sufficient for all essential trafficking pathways under laboratory conditions. Detailed imaging of molecular markers revealed that ARF1 mediated all known trafficking pathways whereas ARFA was not essential to any major pathway. In contrast, the hydrolysis-impaired form (Q71L) of both ARF1 and ARFA, but not ARFB, had deleterious effects on development and various trafficking pathways. However, the deleterious effects of ARFA-Q71L were abolished by ARFA-T31N inhibiting cognate ARF-GEFs, both in cis (ARFA-T31N,Q71L) and in trans (ARFA-T31N + ARFA-Q71L), suggesting indirect effects of ARFA-Q71L on ARF1-mediated trafficking. The deleterious effects of ARFA-Q71L were also suppressed by strong over-expression of ARF1, which was consistent with a subset of BIG1-4 ARF-GEFs interacting with both ARF1 and ARFA. Indeed, the SEC7 domain of BIG5 activated both ARF1 and ARFA whereas the SEC7 domain of BIG3 only activated ARF1. Furthermore, ARFA-T31N impaired root growth if ARF1-specific BIG3 was knocked out and only ARF1- and ARFA-activating BIG4 was functional. Activated ARF1 recruits different coat proteins to different endomembrane compartments, depending on its activation by different ARF-GEFs. Unlike ARF GTPases, ARF-GEFs not only localize at distinct compartments but also regulate specific trafficking pathways, suggesting that ARF-GEFs might play specific roles in traffic regulation beyond the activation of ARF1 by GDP-GTP exchange.

Specificity of plant membrane trafficking - ARFs, regulators and coat proteins

Approximately one-third of all eukaryotic proteins are delivered to their destination by trafficking within the endomembrane system. Such cargo proteins are incorporated into forming membrane vesicles on donor compartments and delivered to acceptor compartments by vesicle fusion. How cargo proteins are sorted into forming vesicles is still largely unknown. Here we review the roles of small GTPases of the ARF/SAR1 family, their regulators designated ARF guanine-nucleotide exchange factors (ARF-GEFs) and ARF GTPase-activating proteins (ARF-GAPs) as well as coat protein complexes during membrane vesicle formation. Although conserved across eukaryotes, these four functional groups of proteins display plant-specific modifications in composition, structure and function.

The calmodulin-like proteins AtCML4 and AtCML5 are single-pass membrane proteins targeted to the endomembrane system by an N-terminal signal anchor sequence

Calmodulins (CaMs) are important mediators of Ca(2+) signals that are found ubiquitously in all eukaryotic organisms. Plants contain a unique family of calmodulin-like proteins (CMLs) that exhibit greater sequence variance compared to canonical CaMs. The Arabidopsis thaliana proteins AtCML4 and AtCML5 are members of CML subfamily VII and possess a CaM domain comprising the characteristic double pair of EF-hands, but they are distinguished from other members of this subfamily and from canonical CaMs by an N-terminal extension of their amino acid sequence. Transient expression of yellow fluorescent protein-tagged AtCML4 and AtCML5 under a 35S-promoter in Nicotiana benthamiana leaf cells revealed a spherical fluorescence pattern. This pattern was confirmed by transient expression in Arabidopsis protoplasts under the native promoter. Co-localization analyses with various endomembrane marker proteins suggest that AtCML4 and AtCML5 are localized to vesicular structures in the interphase between Golgi and the endosomal system. Further studies revealed AtCML5 to be a single-pass membrane protein that is targeted into the endomembrane system by an N-terminal signal anchor sequence. Self-assembly green fluorescent protein and protease protection assays support a topology with the CaM domain exposed to the cytosolic surface and not the lumen of the vesicles, indicating that AtCML5 could sense Ca(2+) signals in the cytosol. Phylogenetic analysis suggests that AtCML4 and AtCML5 are closely related paralogues originating from a duplication event within the Brassicaceae family. CML4/5-like proteins seem to be universally present in eudicots but are absent in some monocots. Together these results show that CML4/5-like proteins represent a flowering plant-specific subfamily of CMLs with a potential function in vesicle transport within the plant endomembrane system.

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.

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.

A crucial role of the RGS domain in trans-Golgi network export of AtRGS1 in the protein secretory pathway

The secretory pathway is responsible for the transport of newly synthesized transmembrane proteins from the endoplasmic reticulum to their destinations via the Golgi/trans-Golgi network (TGN). Cargo proteins at each station are actively sorted by specific sorting signals on the cargo and the corresponding coat complexes. Here, we used the Arabidopsis regulator of G-protein signaling (AtRGS1), which contains an N-terminal potentially sensing glucose seven-transmembrane domain and a C-terminal RGS domain, as a model to uncover sorting motifs required for its cell surface expression. Expression of wild-type and truncated or mutated AtRGS1 fluorescent fusion proteins identified two cysteine residues in the extracellular N-terminus that are essential for endoplasmic reticulum exit and/or correct folding of AtRGS1. The linker between the seven-transmembrane and RGS domains contains an endoplasmic reticulum export signal, whereas the C-terminus is dispensable for the plasma membrane expression of AtRGS1. Interestingly, deletion of the RGS domain results in Golgi/TGN localization of the truncated AtRGS1. Further analysis using site-directed mutagenesis showed that a tyrosine-based motif embedded in the RGS domain is essential for Golgi/TGN export of AtRGS1. These results reveal a new role for the RGS domain in regulating AtRGS1 trafficking from the Golgi/TGN to the plasma membrane and explain the interaction between the seven-transmembrane and RGS domains.

Recruitment of Arf1-GDP to Golgi by Glo3p-type ArfGAPs is crucial for golgi maintenance and plant growth

ADP-ribosylation factor1 (Arf1), a member of the small GTP-binding proteins, plays a pivotal role in protein trafficking to multiple organelles. In its GDP-bound form, Arf1 is recruited from the cytosol to organelle membranes, where it functions in vesicle-mediated protein trafficking. However, the mechanism of Arf1-GDP recruitment remains unknown. Here, we provide evidence that two Glo3p-type Arf GTPase-activating proteins (ArfGAPs), ArfGAP domain8 (AGD8) and AGD9, are involved in the recruitment of Arf1-GDP to the Golgi apparatus in Arabidopsis (Arabidopsis thaliana). RNA interference plants expressing low levels of AGD8 and AGD9 exhibited abnormal Golgi morphology, inhibition of protein trafficking, and arrest of plant growth and development. In RNA interference plants, Arf1 was poorly recruited to the Golgi apparatus. Conversely, high levels of AGD8 and AGD9 induced Arf1 accumulation at the Golgi and suppressed Golgi disruption and inhibition of vacuolar trafficking that was caused by overexpression of AGD7. Based on these results, we propose that the Glo3p-type ArfGAPs AGD8 and AGD9 recruit Arf1-GDP from the cytosol to the Golgi for Arf1-mediated protein trafficking, which is essential for plant development and growth.

Heat shock response in photosynthetic organisms: membrane and lipid connections

The ability of photosynthetic organisms to adapt to increases in environmental temperatures is becoming more important with climate change. Heat stress is known to induce heat-shock proteins (HSPs) many of which act as chaperones. Traditionally, it has been thought that protein denaturation acts as a trigger for HSP induction. However, increasing evidence has shown that many stress events cause HSP induction without commensurate protein denaturation. This has led to the membrane sensor hypothesis where the membrane's physical and structural properties play an initiating role in the heat shock response. In this review, we discuss heat-induced modulation of the membrane's physical state and changes to these properties which can be brought about by interaction with HSPs. Heat stress also leads to changes in lipid-based signaling cascades and alterations in calcium transport and availability. Such observations emphasize the importance of membranes and their lipids in the heat shock response and provide a new perspective for guiding further studies into the mechanisms that mediate cellular and organismal responses to heat stress.

Molecular characterization and expression analysis of cDNAs encoding four Rab and two Arf GTPases in the latex of Hevea brasiliensis

In plants, Rab and Arf GTPases are key regulators of vesicle trafficking. To investigate whether these small GTPases (SG) play a role in the regulation of the regeneration of latex (the cytoplasm of the rubber-producing laticifer cell) in Hevea brasiliensis (Hevea hereafter), full-length cDNAs that encode four HbRab and two HbArf GTPases were cloned. The four HbRab proteins showed specific GTP-binding activity when expressed in Escherichia coli. Transcript expression of the six SG genes was investigated by real-time RT-PCR. All genes revealed to be expressed in each of the six Hevea tissues examined, but the expression patterns were different. Four genes, HbArf1, HbRab2, HbRab3 and HbRab4, displayed a preferential expression in latex. The expression of all genes was upregulated by the act of latex exploitation (tapping), and HbRab1 had the highest level of upregulation. Wounding markedly upregulated the expression of two SG genes (HbRab1 and HbArf2), and exogenous methyl jasmonate upregulated all six SG genes. Wounding might upregulate the expression of HbRab1 and HbArf2 through a jasmonic acid-mediated signaling pathway. None of the genes were markedly upregulated by Ethrel (an ethylene releaser and latex stimulator); instead, HbArf2 and HbRab4 were downregulated significantly after a 24 h treatment with Ethrel. This paper gives the first description of Rab and Arf GTPases in Hevea and provides clues for their involvement, HbRab1 in particular, in latex regeneration.

ARF family G proteins and their regulators: roles in membrane transport, development and disease

The ADP-ribosylation factor (ARF) family of guanine-nucleotide-binding (G) proteins, including the ARF proteins, ARF-like (ARL) proteins and SAR1, regulates membrane traffic and organelle structure, and each family member is regulated through a cycle of GTP binding and GTP hydrolysis, which activate and inactivate, respectively, the G protein.

Traditionally, ARFs have been characterized for their immediate effects in the recruitment of coat proteins to drive cargo sorting, the recruitment of enzymes that can alter membrane lipid composition and the regulation of cytoskeletal factors. Now, new roles for ARFs have been discovered at the Golgi complex, for example in driving lipid transport. ARL proteins are also being increasingly linked to coordination of trafficking with cytoskeletal processes, for example during ciliogenesis.

There is particular interest in the mechanisms that control recruitment of the ARF guanine nucleotide exchange factors (GEFs) that mediate GTP binding to ARFs and, in the case of the cytohesin (also known as ARNO) GEF, membrane recruitment is coupled to relief of autoinhibition. GEFs such as cytohesin may also participate in a cascade of activation between particular pairs of ARFs.

Traditionally, G protein signalling has been viewed as a linear pathway, with the GDP-bound form of an ARF protein being inactive; however, more recent studies have highlighted novel roles for these GDP-bound forms and have also shown that GEFs and GTPase-activating proteins (GAPs) themselves can engage in distinct signalling responses through scaffolding functions.

The ADP-ribosylation factor (ARF) and ARF-like (ARL) family of G proteins, which are known to regulate membrane traffic and organelle structure, are emerging as regulators of diverse processes, including lipid and cytoskeletal transport. Although traditionally viewed as part of a linear signalling pathway, ARFs and their regulators must now be considered to exist within functional networks, in which both the 'inactive' ARF and the regulators themselves can mediate distinct effects.

Members of the ADP-ribosylation factor (ARF) family of guanine-nucleotide-binding (G) proteins, including the ARF-like (ARL) proteins and SAR1, regulate membrane traffic and organelle structure by recruiting cargo-sorting coat proteins, modulating membrane lipid composition, and interacting with regulators of other G proteins. New roles of ARF and ARL proteins are emerging, including novel functions at the Golgi complex and in cilia formation. Their function is under tight spatial control, which is mediated by guanine nucleotide exchange factors (GEFs) and GTPase-activating proteins (GAPs) that catalyse GTP exchange and hydrolysis, respectively. Important advances are being gained in our understanding of the functional networks that are formed not only by the GEFs and GAPs themselves but also by the inactive forms of the ARF proteins.

A TSPO-related protein localizes to the early secretory pathway in Arabidopsis, but is targeted to mitochondria when expressed in yeast

AtTSPO is a TspO/MBR domain-protein potentially involved in multiple stress regulation in Arabidopsis. As in most angiosperms, AtTSPO is encoded by a single, intronless gene. Expression of AtTSPO is tightly regulated both at the transcriptional and post-translational levels. It has been shown previously that overexpression of AtTSPO in plant cell can be detrimental, and the protein was detected in the endoplasmic reticulum (ER) and Golgi stacks, contrasting with previous findings and suggesting a mitochondrial subcellular localization for this protein. To ascertain these findings, immunocytochemistry and ABA induction were used to demonstrate that, in plant cells, physiological levels of AtTSPO colocalized with AtArf1, a mainly Golgi-localized protein in plant cells. In addition, fluorescent protein-tagged AtTSPO was targeted to the secretory pathway and did not colocalize with MitoTracker-labelled mitochondria. These results suggest that the polytopic membrane protein AtTSPO is cotranslationally targeted to the ER in plant cells and accumulates in the Trans-Golgi Network. Heterologous expression of AtTSPO in Saccharomyces cerevisiae, yeast devoid of TSPO-related protein, resulted in growth defects. However, subcellular fractionation and immunoprecipitation experiments showed that AtTSPO was targeted to mitochondria where it colocalized and interacted with the outer mitochondrial membrane porin VDAC1p, reminiscent of the subcellular localization and activity of mammalian translocator protein 18 kDa TSPO. The evolutionarily divergent AtTSPO appears therefore to be switching its sorting mode in a species-dependent manner, an uncommon peculiarity for a polytopic membrane protein in eukaryotic cells. These results are discussed in relation to the recognition and organelle targeting mechanisms of polytopic membrane proteins in eukaryotic cells.

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.

AGD5 is a GTPase-activating protein at the trans-Golgi network

ARF-GTPases are important proteins that control membrane trafficking events. Their activity is largely influenced by the interplay between guanine nucleotide exchange factors (GEFs) and GTPase-activating proteins (GAPs), which facilitate the activation or inactivation of ARF-GTPases, respectively. There are 15 predicted proteins that contain an ARF-GAP domain within the Arabidopsis thaliana genome, and these are classified as ARF-GAP domain (AGD) proteins. The function and subcellular distribution of AGDs, including the ability to activate ARF-GTPases in vivo, that remain largely uncharacterized to date. Here we show that AGD5 is localised to the trans-Golgi network (TGN), where it co-localises with ARF1, a crucial GTPase that is involved in membrane trafficking and which was previously shown to be distributed on Golgi and post-Golgi structures of unknown nature. Taking advantage of the in vivo AGD5-ARF1 interaction at the TGN, we show that mutation of an arginine residue that is critical for ARF-GAP activity of AGD5 leads to longer residence of ARF1 on the membranes, as expected if GTP hydrolysis on ARF1 was impaired due to a defective GAP. Our results establish the nature of the post-Golgi compartments in which ARF1 localises, as well as identifying the role of AGD5 in vivo as a TGN-localised GAP. Furthermore, in vitro experiments established the promiscuous interaction between AGD5 and the plasma membrane-localised ADP ribosylation factor B (ARFB), confirming that ARF-GAP specificity for ARF-GTPases within the cell environment may be spatially regulated.

The multivesicular body-localized GTPase ARFA1b/1c is important for callose deposition and ROR2 syntaxin-dependent preinvasive basal defense in barley

Host cell vesicle traffic is essential for the interplay between plants and microbes. ADP-ribosylation factor (ARF) GTPases are required for vesicle budding, and we studied the role of these enzymes to identify important vesicle transport pathways in the plant-powdery mildew interaction. A combination of transient-induced gene silencing and transient expression of inactive forms of ARF GTPases provided evidence that barley (Hordeum vulgare) ARFA1b/1c function is important for preinvasive penetration resistance against powdery mildew, manifested by formation of a cell wall apposition, named a papilla. Mutant studies indicated that the plasma membrane-localized REQUIRED FOR MLO-SPECIFIED RESISTANCE2 (ROR2) syntaxin, also important for penetration resistance, and ARFA1b/1c function in the same vesicle transport pathway. This was substantiated by a requirement of ARFA1b/1c for ROR2 accumulation in the papilla. ARFA1b/1c is localized to multivesicular bodies, providing a functional link between ROR2 and these organelles in penetration resistance. During Blumeria graminis f sp hordei penetration attempts, ARFA1b/1c-positive multivesicular bodies assemble near the penetration site hours prior to the earliest detection of callose in papillae. Moreover, we showed that ARFA1b/1c is required for callose deposition in papillae and that the papilla structure is established independently of ARFA1b/1c. This raises the possibility that callose is loaded into papillae via multivesicular bodies, rather than being synthesized directly into this cell wall apposition.

Identification and differential expression dynamics of peach small GTPases encoding genes during fruit development and ripening

The function of monomeric GTPases of the RAS superfamily in fruit development and ripening has been partially characterized. Here the identification of peach (Prunus persica) small GTPases of the RAS superfamily expressed in fruit and the characterization of their expression profiles during fruit development are described. Extensive searches on expressed sequence tag (EST) databases led to the selection of a total of 24 genes from peach encoding proteins with significant similarity to Arabidopsis small GTPases. Sequence similarity analyses and identification of conserved motifs, diagnostic of specific RAS families and subfamilies, enabled bona fide assignment of fourteen PpRAB, seven PpARF/ARL/SAR, two PpROP and one PpRAN GTPases. Transcriptional expression profiles of peach monomeric GTPases, analysed by real-time quantitative reverse transcription-PCR, were obtained for mesocarp samples, collected in two consecutive years. Reproducible patterns of expression could be identified for five peach RAB-encoding genes (PpRABA1-1, PpRABA2, PpRABD2-1, PpRABD2-2, and PpRABC2), two ARFs (PpARFA1-1 and PpARLB1), and two ROPs (PpROP3 and PpROP4). Interestingly, the transient transcriptional up-regulation of PpARF genes and of PpRAB genes of the A and D clades, putatively controlling the exocytic delivery of cell wall components and modifying enzymes, appeared to coincide with peaks of growth speed and sugar accumulation and with the final phases of ripening. To our knowledge, this is the first description of the co-ordinated differential expression of a set of genes encoding small GTPases of the ARF and RAB families which takes place during key moments of fruit development and maturation.

Early gene expression programs accompanying trans-differentiation of epidermal cells of Vicia faba cotyledons into transfer cells

Transfer cells (TCs) trans-differentiate from differentiated cells by developing extensive wall ingrowths that enhance plasma membrane transport of nutrients. Here, we investigated transcriptional changes accompanying induction of TC development in adaxial epidermal cells of cultured Vicia faba cotyledons. Global changes in gene expression revealed by cDNA-AFLP were compared between adaxial epidermal cells during induction (3 h) and subsequent building (24 h) of wall ingrowths, and in cells of adjoining storage parenchyma tissue, which do not form wall ingrowths. A total of 5795 transcript-derived fragments (TDFs) were detected; of these, 264 TDFs showed epidermal-specific changes in gene expression and a further 207 TDFs were differentially expressed in both epidermal and storage parenchyma cells. Genes involved in signalling (auxin/ethylene), metabolism (mitochondrial; storage product hydrolysis), cell division, vesicle trafficking and cell wall biosynthesis were specifically induced in epidermal TCs. Blockers of auxin action and vesicle trafficking inhibited ingrowth formation and marked increases in cell division accompanied TC development. Auxin and possibly ethylene signalling cascades induce epidermal cells of V. faba cotyledons to trans-differentiate into TCs. Trans-differentiation is initiated by rapid de-differentiation to a mitotic state accompanied by mitochondrial biogenesis driving storage product hydrolysis to fuel wall ingrowth formation orchestrated by a modified vesicle trafficking mechanism.

Post-Golgi traffic in plants

Secretory and endocytic traffic through the post-Golgi endomembrane system regulates the abundance of plasma-membrane proteins such as receptors, transporters and ion channels, modulating the ability of a cell to communicate with its neighbours and to adapt to a changing environment. The major post-Golgi compartments are numerous and appear to be similar to their counterparts in animals. However, endosomes are rather ill defined morphologically but seem to be involved in specific trafficking pathways. Many plasma-membrane proteins cycle constitutively via endosomal compartments. The trans-Golgi network (TGN) appears to be an early endosome where secretory and endocytic traffic meet. Endocytosed proteins that are to be degraded are targeted to the vacuole via the multivesiculate prevacuolar compartment (PVC) whereas cycling proteins pass through recycling endosomes. The trafficking machinery involves the same classes of proteins as in other eukaryotes. However, there are modifications that match the specifics of post-Golgi traffic in plants. Although plants lack epithelia, some plasma-membrane proteins are located on specific faces of the cell which reflects polarized traffic and influences the physiological performance of the tissue. Plants also differentiate highly polarized tip-growing cells in which post-Golgi traffic is adapted to very high rates of targeted exocytosis, endocytosis and recycling.

Intermediate organelles of the plant secretory pathway: identity and function

The secretory pathway of eukaryotic cells comprises a network of organelles that connects three large membranes, the plasma membrane, the vacuole and the endoplasmic reticulum. The Golgi apparatus and the various post-Golgi organelles that control vacuolar sorting, secretion and endocytosis can be regarded as intermediate organelles of the endocytic and biosynthetic routes. Many processes in the secretory pathway have evolved differently in plants and cannot be studied using yeast or mammalian cells as models. The best characterized organelles are the Golgi apparatus and the prevacuolar compartment, but recent work has shed light on the role of the trans Golgi network, which has to be regarded as a separate organelle in plants. In this study, we wish to highlight recent findings regarding the late secretory pathway and its crosstalk with the early secretory pathway as well as the endocytic route in plants. Recently published findings and suggested models are discussed within the context of known features of the equivalent pathway in other eukaryotes.