An Arabidopsis GRIP domain protein locates to the trans-Golgi and binds the small GTPase ARL1

TCA cycle impairment leads to PIN2 internalization and degradation via reduced MAB4 level and ARA6 components in Arabidopsis roots

The mitochondrial pyruvate dehydrogenase complex (PDC) plays a crucial role in linking the glycolysis pathway and the tricarboxylic acid (TCA) cycle. Previously, we reported that a mutation of MAB1, encoding an E1β subunit of PDC, affects the abundance of auxin efflux carriers PIN-FORMED proteins (PINs) via reduced recycling and enhanced degradation in vacuoles. Here, we further analyzed the effects of TCA cycle inhibition on vesicle trafficking using both the mab1-1 mutant and 3-BP, a TCA cycle inhibitor. Pharmacological and genetic impairment of the TCA cycle induced the aggregated components of ARA6, which is a plant-unique RAB5 GTPase that mediates endosomal trafficking to the plasma membrane. In addition, MAB4, which is an NPH3-like protein that inhibits PIN internalization from the plasma membrane, was severely reduced in 3-BP-treated roots and mab1-1. Furthermore, TCA cycle impairment led to the accumulation of reactive oxygen species in root tips, and treatment with H2O2 reduced MAB4 levels while increasing the internalization of PIN2 from the plasma membrane, and aggregated ARA6-positive compartments. These results suggest that TCA cycle impairment targets PIN proteins for degradation in the vacuole by disrupting both the MAB4-mediated block of internalization and the ARA6-mediated endocytic pathway.

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.

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

Small GTPases function by conformational switching ability between GDP- and GTP-bound states in rapid cell signaling events. The ADP-ribosylation factor (ARF) family is involved in vesicle trafficking. Though evolutionarily well conserved, little is known about ARF and ARF-like GTPases in plants. Here, we characterized biochemical properties and cellular localization of the essential small ARF-like GTPase TITAN 5/HALLIMASCH/ARL2/ARLC1 (hereafter termed TTN5) from Arabidopsis thaliana. Two TTN5 variants were included in the study with point mutations at conserved residues, suspected to be functional for nucleotide exchange and GTP hydrolysis, TTN5T30N and TTN5Q70L. We found that TTN5 had a very rapid intrinsic nucleotide exchange capacity with a conserved nucleotide switching mechanism. TTN5 acted as a non-classical small GTPase with a remarkably low GTP hydrolysis activity, suggesting it is likely present in GTP-loaded active form in the cell. We analyzed signals from yellow fluorescent protein (YFP)-tagged TTN5 and from in situ immunolocalization of hemagglutine-tagged HA3-TTN5 in Arabidopsis seedlings and in a transient expression system. Together with colocalization using endomembrane markers and pharmacological treatments the microscopic analysis suggests that TTN5 can be present at the plasma membrane and dynamically associated with membranes of vesicles, Golgi stacks and multivesicular bodies. While the TTN5Q70L variant showed similar GTPase activities and localization behavior as wild-type TTN5, the TTN5T30N mutant differed in some aspects. Hence, the unusual capacity of rapid nucleotide exchange activity of TTN5 is linked with cell membrane dynamics, likely associated with vesicle transport pathways in the endomembrane system.

Actomyosin and CSI1/POM2 cooperate to deliver cellulose synthase from Golgi to cortical microtubules in Arabidopsis

As one of the major components of plant cell walls, cellulose is crucial for plant growth and development. Cellulose is synthesized by cellulose synthase (CesA) complexes (CSCs), which are trafficked and delivered from the Golgi apparatus to the plasma membrane. How CesAs are released from Golgi remains largely unclear. In this study, we observed that STELLO (STL) family proteins localized at a group of small CesA-containing compartments called Small CesA compartments (SmaCCs) or microtubule-associated CesA compartments (MASCs). The STL-labeled SmaCCs/MASCs were directly derived from Golgi through a membrane-stretching process: membrane-patches of Golgi attached to cortical microtubules, which led to emergence of membrane-tails that finally ruptured to generate SmaCCs/MASCs associated with the cortical microtubules. While myosin propelled the movement of Golgi along actin filaments to stretch the tails, the CesA-microtubule linker protein, CSI1/POM2 was indispensable for the tight anchor of the membrane-tail ends at cortical microtubules. Together, our data reveal a non-canonical delivery route to the plasma membrane of a major enzyme complex in plant biology.

ADP-ribosylation factor D1 modulates Golgi morphology, cell plate formation, and plant growth in Arabidopsis

ADP-ribosylation factor (ARF) family proteins, one type of small guanine-nucleotide-binding (G) proteins, play a central role in regulating vesicular traffic and organelle structures in eukaryotes. The Arabidopsis (Arabidopsis thaliana) genome contains more than 21 ARF proteins, but relatively little is known about the functional heterogeneity of ARF homologs in plants. Here, we characterized the function of a unique ARF protein, ARFD1B, in Arabidopsis. ARFD1B exhibited both cytosol and punctate localization patterns, colocalizing with a Golgi marker in protoplasts and transgenic plants. Distinct from other ARF1 homologs, overexpression of a dominant-negative mutant form of ARFD1B did not alter the localization of the Golgi marker mannosidase I (ManI)-RFP in Arabidopsis cells. Interestingly, the ARFD1 artificial microRNA knockdown mutant arfd1 displayed a deleterious growth phenotype, while this phenotype was restored in complemented plants. Further, confocal imaging and transmission electron microscopy analyses of the arfd1 mutant revealed defective cell plate formation and abnormal Golgi morphology. Pull-down and liquid chromatography-tandem mass spectrometry analyses identified Coat Protein I (COPI) components as interacting partners of ARFD1B, and subsequent bimolecular fluorescence complementation, yeast (Saccharomyces cerevisiae) two-hybrid, and co-immunoprecipitation assays further confirmed these interactions. These results demonstrate that ARFD1 is required for cell plate formation, maintenance of Golgi morphology, and plant growth in Arabidopsis.

An Update on the Key Factors Required for Plant Golgi Structure Maintenance

Plant Golgi apparatus serves as the central station of the secretory pathway and is the site where protein modification and cell wall matrix polysaccharides synthesis occur. The polarized and stacked cisternal structure is a prerequisite for Golgi function. Our understanding of Golgi structure maintenance and trafficking are largely obtained from mammals and yeast, yet, plant Golgi has many different aspects. In this review, we summarize the key players in Golgi maintenance demonstrated by genetic studies in plants, which function in ER-Golgi, intra-Golgi and post-Golgi transport pathways. Among these, we emphasize on players in intra-Golgi trafficking.

AtFTCD-L, a trans-Golgi network localized protein, modulates root growth of Arabidopsis in high-concentration agar culture medium

AtFTCD-L protein is localized on the TGN vesicles in Arabidopsis root cap cells. AtFTCD-L mutation resulted in slow root growth of Arabidopsis in high-concentration agar culture medium. Arabidopsis formiminotransferase cyclodeaminase-like protein (AtFTCD-L) in Arabidopsis is homologous to the formiminotransferase cyclodeaminase (FTCD) protein in animal cells. However, the localization and function of AtFTCD-L remain unknown in Arabidopsis. In this study, we generated and analyzed a deletion mutant of AtFTCD-L with a T-DNA insertion. We found that the growth of Arabidopsis roots with the T-DNA insertion mutation in AtFTCD-L was slower than that of wild-type roots when grown in high-concentration 1/2 MS agar culture medium. AtFTCD-L-GFP could restore the ftcd-l mutant phenotype. In addition, the AtFTCD-L protein was localized on the trans-Golgi network (TGN) vesicles in Arabidopsis root cap cells. Fluorescence recovery after photobleaching (FRAP) experiment using Arabidopsis pollen-specific receptor-like kinase-GFP (AtPRK1-GFP) stably transformed plants showed that the deficiency of AtFTCD-L protein in Arabidopsis led to slower secretion in the root cap peripheral cells. The AtFTCD-L protein deficiency also resulted in a significantly reduced monosaccharides content in the culture medium. Based on the above results, we speculate that the AtFTCD-L protein may be involved in sorting and/or transportation of TGN vesicles in root cap peripheral cells, thereby regulating the extracellular secretion of mucilage components in the root cap.

Living on the edge: the role of Atgolgin-84A at the plant ER-Golgi interface

The plant Golgi apparatus is responsible for the processing of proteins received from the endoplasmic reticulum (ER) and their distribution to multiple destinations within the cell. Golgi matrix components, such as golgins, have been identified and suggested to function as putative tethering factors to mediate the physical connections between Golgi bodies and the ER network. Golgins are proteins anchored to the Golgi membrane by the C-terminus either through transmembrane domains or interaction with small regulatory GTPases. The golgin N-terminus contains long coiled-coil domains, which consist of a number of α-helices wrapped around each other to form a structure similar to a rope being made from several strands, reaching into the cytoplasm. In animal cells, golgins are also implicated in specific recognition of cargo at the Golgi.Here, we investigate the plant golgin Atgolgin-84A for its subcellular localization and potential role as a tethering factor at the ER-Golgi interface. For this, fluorescent fusions of Atgolgin-84A and an Atgolgin-84A truncation lacking the coiled-coil domains (Atgolgin-84AΔ1-557) were transiently expressed in tobacco leaf epidermal cells and imaged using high-resolution confocal microscopy. We show that Atgolgin-84A localizes to a pre-cis-Golgi compartment that is also labelled by one of the COPII proteins as well as by the tether protein AtCASP. Upon overexpression of Atgolgin-84A or its deletion mutant, transport between the ER and Golgi bodies is impaired and cargo proteins are redirected to the vacuole. LAY DESCRIPTION: The Golgi apparatus is a specialised compartment found in mammalian and plant cells. It is the post office of the cell and packages proteins into small membrane boxes for transport to their destination in the cell. The plant Golgi apparatus consist of many separate Golgi bodies and is responsible for the processing of proteins received from the endoplasmic reticulum (ER) and their distribution to multiple destinations within the cell. Specialised proteins called golgins have been suggested to tether Golgi bodies and the ER. Here we investigate the plant golgin Atgolgin-84A for its exact within the Golgi body and its potential role as a tethering factor at the ER-Golgi interface. For this, we have fused Atgolgin-84A with a fluorescent protein from jellyfish and we are producing this combination in tobacco leaf cells. This allows us to see the protein using laser microscopy. We show that Atgolgin-84A localises to a compartment between the ER and Golgi that is also labelled by the tether protein AtCASP. When Atgolgin-84A is produced in high amounts in the cell, transport between the ER and Golgi bodies is inhibited and proteins are redirected to the vacuole.

The change of gravity vector induces short-term phosphoproteomic alterations in Arabidopsis

Plants can sense the gravitational force. When plants perceive a change in this natural force, they tend to reorient their organs with respect to the direction of the gravity vector, i.e., the shoot stem curves up. In the present study, we performed a 4C quantitative phosphoproteomics to identify those altered protein phosphosites resulting from 150 s of reorientation of Arabidopsis plants on earth. A total of 5556 phosphopeptides were identified from the gravistimulated Arabidopsis. Quantification based on the ¹⁵N-stable isotope labeling in Arabidopsis (SILIA) and computational analysis of the extracted ion chromatogram (XIC) of phosphopeptides showed eight and five unique PTM peptide arrays (UPAs) being up- and down-regulated, respectively, by gravistimulation. Among the 13 plant reorientation-responsive protein groups, many are related to the cytoskeleton dynamic and plastid movement. Interestingly, the most gravistimulation-responsive phosphosites are three serine residues, S350, S376, and S410, of a blue light receptor Phototropin 1 (PHOT1). The immunoblots experiment confirmed that the change of gravity vector indeed affected the phosphorylation level of S410 in PHOT1. The functional role of PHOT1 in gravitropic response was further validated with gravicurvature measurement in the darkness of both the loss-of-function double mutant phot1phot2 and its complementary transgenic plant PHOT1/phot1phot2. SIGNIFICANCE: The organs of sessile organisms, plants, are able to move in response to environmental stimuli, such as gravity vector, touch, light, water, or nutrients, which is termed tropism. For instance, the bending of plant shoots to the light source is called phototropism. Since all plants growing on earth are continuously exposed to the gravitational field, plants receive the mechanical signal elicited by the gravity vector change and convert it into plant morphogenesis, growth, and development. Past studies have resulted in various hypotheses for gravisensing, but our knowledge about how the signal of gravity force is transduced in plant cells is still minimal. In the present study, we performed a SILIA-based 4C quantitative phosphoproteomics on 150-s gravistimulated Arabidopsis seedlings to explore the phosphoproteins involved in the gravitropic response. Our data demonstrated that such a short-term reorientation of Arabidopsis caused changes in phosphorylation of cytoskeleton structural proteins like Chloroplast Unusual Positioning1 (CHUP1), Patellin3 (PATL3), and Plastid Movement Impaired2 (PMI2), as well as the blue light receptor Phototropin1 (PHOT1). These results suggested that protein phosphorylation plays a crucial role in gravisignaling, and two primary tropic responses of plants, gravitropism and phototropism, may share some common components and signaling pathways. We expect that the phosphoproteins detected from this study will facilitate the subsequent molecular and cellular studies on the mechanism underlying the signal transduction in plant gravitropic response.

A sophisticated, differentiated Golgi in the ancestor of eukaryotes

BACKGROUND

The Golgi apparatus is a central meeting point for the endocytic and exocytic systems in eukaryotic cells, and the organelle's dysfunction results in human disease. Its characteristic morphology of multiple differentiated compartments organized into stacked flattened cisternae is one of the most recognizable features of modern eukaryotic cells, and yet how this is maintained is not well understood. The Golgi is also an ancient aspect of eukaryotes, but the extent and nature of its complexity in the ancestor of eukaryotes is unclear. Various proteins have roles in organizing the Golgi, chief among them being the golgins.

RESULTS

We address Golgi evolution by analyzing genome sequences from organisms which have lost stacked cisternae as a feature of their Golgi and those that have not. Using genomics and immunomicroscopy, we first identify Golgi in the anaerobic amoeba Mastigamoeba balamuthi. We then searched 87 genomes spanning eukaryotic diversity for presence of the most prominent proteins implicated in Golgi structure, focusing on golgins. We show some candidates as animal specific and others as ancestral to eukaryotes.

CONCLUSIONS

None of the proteins examined show a phyletic distribution that correlates with the morphology of stacked cisternae, suggesting the possibility of stacking as an emergent property. Strikingly, however, the combination of golgins conserved among diverse eukaryotes allows for the most detailed reconstruction of the organelle to date, showing a sophisticated Golgi with differentiated compartments and trafficking pathways in the common eukaryotic ancestor.

Transient co-expression with three O-glycosylation enzymes allows production of GalNAc-O-glycosylated Granulocyte-Colony Stimulating Factor in N. benthamiana

BACKGROUND

Expression of economically relevant proteins in alternative expression platforms, especially plant expression platforms, has gained significant interest in recent years. A special interest in working with plants as bioreactors for the production of pharmaceutical proteins is related to low production costs, product safety and quality. Among the different properties that plants can also offer for the production of recombinant proteins, protein glycosylation is crucial since it may have an impact on pharmaceutical functionality and/or stability.

RESULTS

The pharmaceutical glycoprotein human Granulocyte-Colony Stimulating Factor was transiently expressed in Nicotiana benthamiana plants and subjected to mammalian-specific mucin-type O-glycosylation by co-expressing the pharmaceutical protein together with the glycosylation machinery responsible for such post-translational modification.

CONCLUSIONS

The pharmaceutical glycoprotein human Granulocyte-Colony Stimulating Factor can be expressed in N. benthamiana plants via agroinfiltration with its native mammalian-specific mucin-type O-glycosylation.

Stacks off tracks: a role for the golgin AtCASP in plant endoplasmic reticulum-Golgi apparatus tethering

The plant Golgi apparatus modifies and sorts incoming proteins from the endoplasmic reticulum (ER) and synthesizes cell wall matrix material. Plant cells possess numerous motile Golgi bodies, which are connected to the ER by yet to be identified tethering factors. Previous studies indicated a role for cis-Golgi plant golgins, which are long coiled-coil domain proteins anchored to Golgi membranes, in Golgi biogenesis. Here we show a tethering role for the golgin AtCASP at the ER-Golgi interface. Using live-cell imaging, Golgi body dynamics were compared in Arabidopsis thaliana leaf epidermal cells expressing fluorescently tagged AtCASP, a truncated AtCASP-ΔCC lacking the coiled-coil domains, and the Golgi marker STtmd. Golgi body speed and displacement were significantly reduced in AtCASP-ΔCC lines. Using a dual-colour optical trapping system and a TIRF-tweezer system, individual Golgi bodies were captured in planta. Golgi bodies in AtCASP-ΔCC lines were easier to trap and the ER-Golgi connection was more easily disrupted. Occasionally, the ER tubule followed a trapped Golgi body with a gap, indicating the presence of other tethering factors. Our work confirms that the intimate ER-Golgi association can be disrupted or weakened by expression of truncated AtCASP-ΔCC and suggests that this connection is most likely maintained by a golgin-mediated tethering complex.

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 golgin coiled-coil proteins capture different types of transport carriers via distinct N-terminal motifs

Background

The internal organization of cells depends on mechanisms to ensure that transport carriers, such as vesicles, fuse only with the correct destination organelle. Several types of proteins have been proposed to confer specificity to this process, and we have recently shown that a set of coiled-coil proteins on the Golgi, called golgins, are able to capture specific classes of carriers when relocated to an ectopic location.

Results

Mapping of six different golgins reveals that, in each case, a short 20–50 residue region is necessary and sufficient to capture specific carriers. In all six of GMAP-210, golgin-84, TMF, golgin-97, golgin-245, and GCC88, this region is located at the extreme N-terminus of the protein. The vesicle-capturing regions of GMAP-210, golgin-84, and TMF capture intra-Golgi vesicles and share some sequence features, suggesting that they act in a related, if distinct, manner. In the case of GMAP-210, this shared feature is in addition to a previously characterized “amphipathic lipid-packing sensor” motif that can capture highly curved membranes, with the two motifs being apparently involved in capturing distinct types of vesicles. Of the three GRIP domain golgins that capture endosome-to-Golgi carriers, golgin-97 and golgin-245 share a closely related capture motif, whereas that in GCC88 is distinct, suggesting that it works by a different mechanism and raising the possibility that the three golgins capture different classes of endosome-derived carriers that share many cargos but have distinct features for recognition at the Golgi.

Conclusions

For six different golgins, the capture of carriers is mediated by a short region at the N-terminus of the protein. There appear to be at least four different types of motif, consistent with specific golgins capturing specific classes of carrier and implying the existence of distinct receptors present on each of these different carrier classes.

Cell wall accumulation of fluorescent proteins derived from a trans-Golgi cisternal membrane marker and paramural bodies in interdigitated Arabidopsis leaf epidermal cells

In most dicotyledonous plants, leaf epidermal pavement cells develop jigsaw puzzle-like shapes during cell expansion. The rapid growth and complicated cell shape of pavement cells is suggested to be achieved by targeted exocytosis that is coordinated with cytoskeletal rearrangement to provide plasma membrane and/or cell wall materials for lobe development during their morphogenesis. Therefore, visualization of membrane trafficking in leaf pavement cells should contribute an understanding of the mechanism of plant cell morphogenesis. To reveal membrane trafficking in pavement cells, we observed monomeric red fluorescent protein-tagged rat sialyl transferases, which are markers of trans-Golgi cisternal membranes, in the leaf epidermis of Arabidopsis thaliana. Quantitative fluorescence imaging techniques and immunoelectron microscopic observations revealed that accumulation of the red fluorescent protein occurred mostly in the curved regions of pavement cell borders and guard cell ends during leaf expansion. Transmission electron microscopy observations revealed that apoplastic vesicular membrane structures called paramural bodies were more frequent beneath the curved cell wall regions of interdigitated pavement cells and guard cell ends in young leaf epidermis. In addition, pharmacological studies showed that perturbations in membrane trafficking resulted in simple cell shapes. These results suggested possible heterogeneity of the curved regions of plasma membranes, implying a relationship with pavement cell morphogenesis.

Golgi-localized STELLO proteins regulate the assembly and trafficking of cellulose synthase complexes in Arabidopsis

As the most abundant biopolymer on Earth, cellulose is a key structural component of the plant cell wall. Cellulose is produced at the plasma membrane by cellulose synthase (CesA) complexes (CSCs), which are assembled in the endomembrane system and trafficked to the plasma membrane. While several proteins that affect CesA activity have been identified, components that regulate CSC assembly and trafficking remain unknown. Here we show that STELLO1 and 2 are Golgi-localized proteins that can interact with CesAs and control cellulose quantity. In the absence of STELLO function, the spatial distribution within the Golgi, secretion and activity of the CSCs are impaired indicating a central role of the STELLO proteins in CSC assembly. Point mutations in the predicted catalytic domains of the STELLO proteins indicate that they are glycosyltransferases facing the Golgi lumen. Hence, we have uncovered proteins that regulate CSC assembly in the plant Golgi apparatus.

Probing protein targeting to plasmodesmata using fluorescence recovery after photo-bleaching

Fluorescence recovery after photo-bleaching (FRAP) involves the irreversible bleaching of a fluorescent protein within a specific area of the cell using a high-intensity laser. The recovery of fluorescence represents the movement of new protein into this area and can therefore be used to investigate factors involved in this movement. Here we describe a FRAP method to investigate the effect of a range of pharmacological agents on the targeting of Tobacco mosaic virus movement protein to plasmodesmata.

The C-TERMINUS of AtGRIP is crucial for its self-association and for targeting to Golgi stacks in Arabidopsis

Background

In animals and fungi, dimerization is crucial for targeting GRIP domain proteins to the Golgi apparatus. Only one gene in the Arabidopsis genome, AtGRIP, codes for a GRIP domain protein. It remains unclear whether AtGRIP has properties similar to those of GRIP domain proteins.

Results

In this study, western blot and yeast two-hybrid analyses indicated that AtGRIPs could form a parallel homodimer. In addition, yeast two-hybrid analysis indicated that AtGRIP_aa711–753, AtGRIP_aa711–766 and AtGRIP_aa711–776 did not interact with themselves, but the intact GRIP domain with the AtGRIP C-terminus did. Confocal microscopy showed that only an intact GRIP domain with an AtGRIP C-terminus could localize to the Golgi stacks in Arabidopsis leaf protoplasts. A BLAST analysis showed that the C-terminus of GRIP proteins was conserved in the plant kingdom. Mutagenesis and yeast two-hybrid analyses showed that the L742 of AtGRIP contributed to dimerization and was crucial for Golgi localization.

Conclusions

These results indicate that the C-terminus of GRIP proteins is essential for self-association and for targeting of Golgi stacks in plant cells. We suggest that several properties of GRIP proteins differ between plant and animal cells.

The Arf family G protein Arl1 is required for secretory granule biogenesis in Drosophila

The small G protein Arf like 1 (Arl1) is found at the Golgi complex, and its GTP-bound form recruits several effectors to the Golgi including GRIP-domain-containing coiled-coil proteins, and the Arf1 exchange factors Big1 and Big2. To investigate the role of Arl1, we have characterised a loss-of-function mutant of the Drosophila Arl1 orthologue. The gene is essential, and examination of clones of cells lacking Arl1 shows that it is required for recruitment of three of the four GRIP domain golgins to the Golgi, with Drosophila GCC185 being less dependent on Arl1. At a functional level, Arl1 is essential for formation of secretory granules in the larval salivary gland. When Arl1 is missing, Golgi are still present but there is a dispersal of adaptor protein 1 (AP-1), a clathrin adaptor that requires Arf1 for its membrane recruitment and which is known to be required for secretory granule biogenesis. Arl1 does not appear to be required for AP-1 recruitment in all tissues, suggesting that it is crucially required to enhance Arf1 activation at the trans-Golgi in particular tissues.

A simple and fast protocol for the protein complex immunoprecipitation (Co-IP) of effector: host protein complexes

Plant pathogens are responsible for enormous damage in natural and cultured ecosystems. One strategy most pathogenic organisms follow is the secretion of effector proteins that manipulate the host immune system to suppress defense responses. There is considerable interest in finding host targets of pathogen effectors as this helps to shape our understanding of how those proteins work in planta. The presented protocol describes a protein complex immunoprecipitation method aimed at verifying protein-protein interactions derived from protein complementation assays like Yeast-two-Hybrid.

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.

Mon2 is a negative regulator of the monomeric G protein, Arl1

Using site-directed mutants of ARL1 predicted to alter nucleotide binding, we examined phenotypes associated with the loss of ARL1 , including effects on membrane traffic and K (+) homeostasis. The GTP-restricted allele, ARL[Q72L] , complemented the membrane traffic phenotype (CPY secretion), but not the K (+) homeostasis phenotypes (sensitivity to hygromycin B, steady-state levels of K (+) , and accumulation of (86) Rb (+) ), while the XTP-restricted mutant, ARL1[D130N] , complemented the ion phenotypes, but not the membrane traffic phenotype. A GDP-restricted allele, ARL1[T32N] , did not effectively complement either phenotype. These results are consistent with a model in which Arl1 has three different conformations in vivo. We also explored the relationship between ARL1 and MON2 using the synthetic lethal phenotype exhibited by these two genes and demonstrated that MON2 is a negative regulator of the GTP-restricted allele of ARL1 , ARL1[Q72L] . Finally, we constructed several new alleles predicted to alter binding of Arl1 to the sole GRIP domain containing protein in yeast, Imh1, and found that ARL1[F52G] and ARL1[Y82G] were unable to complement the loss of ARL1 with respect to either the membrane traffic or K (+) homeostasis phenotypes. Our study expands understanding of the roles of Arl1 in vivo.

Tales of tethers and tentacles: golgins in plants

As plant Golgi bodies move through the cell along the actin cytoskeleton, they face the need to maintain their polarized stack structure whilst receiving, processing and distributing protein cargo destined for secretion. Structural proteins, or Golgi matrix proteins, help to hold cisternae together and tethering factors direct cargo carriers to the correct target membranes. This review focuses on golgins, a protein family containing long coiled-coil regions, summarizes their known functions in animal cells and highlights recent findings about plant golgins and their putative roles in the plant secretory pathway.

Fluorescent protein-based technologies: shedding new light on the plant endomembrane system

Without doubt, GFP and spectral derivatives have revolutionized the way biologists approach their journey toward the discovery of how plant cells function. It is fascinating that in its early days GFP was used merely for localization studies, but as time progressed researchers successfully explored new avenues to push the power of GFP technology to reach new and exciting research frontiers. This has had a profound impact on the way we can now study complex and dynamic systems such as plant endomembranes. Here we briefly describe some of the approaches where GFP has revolutionized in vivo studies of protein distribution and dynamics and focus on two emerging approaches for the application of GFP technology in plant endomembranes, namely optical tweezers and forward genetics approaches, which are based either on the light or on genetic manipulation of secretory organelles to gain insights on the factors that control their activities and integrity.

The golgin coiled-coil proteins of the Golgi apparatus

A number of long coiled-coil proteins are present on the Golgi. Often referred to as "golgins," they are well conserved in evolution and at least five are likely to have been present in the last common ancestor of all eukaryotes. Individual golgins are found in different parts of the Golgi stack, and they are typically anchored to the membrane at their carboxyl termini by a transmembrane domain or by binding a small GTPase. They appear to have roles in membrane traffic and Golgi structure, but their precise function is in most cases unclear. Many have binding sites for Rab family GTPases along their length, and this has led to the suggestion that the golgins act collectively to form a tentacular matrix that surrounds the Golgi to capture Rab-coated membranes in the vicinity of the stack. Such a collective role might explain the lack of cell lethality seen following loss of some of the genes in human familial conditions or mouse models.

Imaging fluorescently tagged Phytophthora effector proteins inside infected plant tissue

Assays to determine the role of pathogen effectors within an infected plant cell are yielding valuable information about which host processes are targeted to allow successful pathogen colonization. However, this does not necessarily inform on the cellular location of these interactions, or if these effector-virulence target interactions occur only in the presence of the pathogen. Here, we describe techniques to allow the subcellular localization of pathogen effectors inside infected plant cells or tissues, based largely on infiltration of plant tissue by Agrobacterium tumefaciens and its delivery of DNA encoding fluorescent protein-tagged effectors, and subsequent confocal microscopy.

Sequential depletion and acquisition of proteins during Golgi stack disassembly and reformation

Herein, we report the stepwise transport of multiple plant Golgi membrane markers during disassembly of the Golgi apparatus in tobacco leaf epidermal cells in response to the induced expression of the GTP-locked Sar1p or Brefeldin A (BFA), and reassembly on BFA washout. The distribution of fluorescent Golgi-resident N-glycan processing enzymes and matrix proteins (golgins) with specific cis-trans-Golgi sub-locations was followed by confocal microscopy during disassembly and reassembly. The first event during Golgi disassembly was the loss of trans-Golgi enzymes and golgins from Golgi membranes, followed by a sequential redistribution of medial and cis-Golgi enzymes into the endoplasmic reticulum (ER), whilst golgins were relocated to the ER or cytoplasm. This event was confirmed by fractionation and immuno-blotting. The sequential redistribution of Golgi components in a trans-cis sequence may highlight a novel retrograde trafficking pathway between the trans-Golgi and the ER in plants. Release of Golgi markers from the ER upon BFA washout occurred in the opposite sequence, with cis-matrix proteins labelling Golgi-like structures before cis/medial enzymes. Trans-enzyme location was preceded by trans-matrix proteins being recruited back to Golgi membranes. Our results show that Golgi disassembly and reassembly occur in a highly ordered fashion in plants.

The N-terminal domain of PMTV TGB1 movement protein is required for nucleolar localization, microtubule association, and long-distance movement

The triple-gene-block (TGB)1 protein of Potato mop-top virus (PMTV) was fused to fluorescent proteins and expressed in epidermal cells of Nicotiana benthamiana under the control of the 35S promoter. TGB1 fluorescence was observed in the cytoplasm, nucleus, and nucleolus and occasionally associated with microtubules. When expressed from a modified virus (PMTV.YFP-TGB1) which formed local lesions but was not competent for systemic movement, yellow fluorescent protein (YFP)-TGB1 labeled plasmodesmata in cells at the leading edge of the lesion and plasmodesmata, microtubules, nuclei, and nucleoli in cells immediately behind the leading edge. Deletion of 84 amino acids from the N-terminus of unlabeled TGB1 within the PMTV genome abolished movement of viral RNA to noninoculated leaves. When the same deletion was introduced into PMTV.YFP-TGB1, labeling of microtubules and nucleoli was abolished. The N-terminal 84 amino acids of TGB1 were fused to green fluorescent protein (GFP) and expressed in epidermal cells where GFP localized strongly to the nucleolus (not seen with unfused GFP), indicating that these amino acids contain a nucleolar localization signal; the fusion protein did not label microtubules. This is the first report of nucleolar and microtubule association of a TGB movement protein. The results suggest that PMTV TGB1 requires interaction with nuclear components and, possibly, microtubules for long-distance movement of viral RNA.

MAG4/Atp115 is a golgi-localized tethering factor that mediates efficient anterograde transport in Arabidopsis

Seed storage proteins are synthesized on rough endoplasmic reticulum (ER) in a precursor form and then are transported to protein storage vacuoles (PSVs) where they are converted to their mature form. To understand the mechanisms by which storage proteins are transported, we screened Arabidopsis maigo mutants to identify those that abnormally accumulate storage protein precursors. Here we describe a new maigo mutant, maigo 4 (mag4), that abnormally accumulates the precursors of two major storage proteins, 12S globulin and 2S albumin, in dry seeds. Electron microscopy revealed that mag4 seed cells abnormally develop a large number of novel structures that exhibit a highly electron-dense core. Some of these structures were surrounded by ribosomes. Immunogold analysis suggests that the electron-dense core is an aggregate of 2S albumin precursors and that 12S globulins are localized around the core. The MAG4 gene was identified as At3g27530, and the MAG4 protein has domains homologous to those found in bovine vesicular transport factor p115. MAG4 molecules were concentrated at cis-Golgi stacks. Our findings suggest that MAG4 functions in the transport of storage protein precursors from the ER to the Golgi complex in plants. In addition, the mag4 mutant exhibits a dwarf phenotype, suggesting that MAG4 is involved in both the transport of storage proteins and in plant growth and development.

Arabidopsis stromal 70-kDa heat shock proteins are essential for chloroplast development

70 kDa heat shock proteins (Hsp70s) act as molecular chaperones involved in essential cellular processes such as protein folding and protein transport across membranes. They also play a role in the cell's response to a wide range of stress conditions. The Arabidopsis family of Hsp70s homologues includes two highly conserved proteins, cpHsc70-1 and cpHsc70-2 which are both imported into chloroplasts (Su and Li in Plant Physiol 146:1231-1241, 2008). Here, we demonstrate that YFP-fusion proteins of both cpHsc70-1 and cpHsc70-2 are predominantly stromal, though low levels were detected in the thylakoid membrane. Both genes are ubiquitously expressed at high levels in both seedlings and adult plants. We further show that both cpHsc70-1 and cpHsc70-2 harbour ATPase activity which is essential for Hsp70 chaperone activity. A previously described T-DNA insertion line for cpHsc70-1 (DeltacpHsc70-1) has variegated cotyledons, malformed leaves, growth retardation, impaired root growth and sensitivity to heat shock treatment. In addition, under stress conditions, this mutant also exhibits unusual sepals, and malformed flowers and sucrose concentrations as low as 1% significantly impair growth. cpHsc70-1/cpHsc70-2 double-mutants are lethal. However, we demonstrate through co-suppression and artificial microRNA (amiRNA) approaches that transgenic plants with severely reduced levels of both genes have a white and stunted phenotype. Interestingly, chloroplasts in these plants have an unusual morphology and contain few or no thylakoid membranes. Our data show that cpHsc70-1 and cpHsc70-2 are essential ATPases, have overlapping roles and are required for normal plastid structure.

Biogenesis of the plant Golgi apparatus

It has long been assumed that the individual cisternal stacks that comprise the plant Golgi apparatus multiply by some kind of fission process. However, more recently, it has been demonstrated that the Golgi apparatus can be experimentally disassembled and the reformation process from the ER (endoplasmic reticulum) monitored sequentially using confocal fluorescence and electron microscopy. Some other evidence suggests that Golgi stacks may arise de novo in cells. In the present paper, we review some of the more recent findings on plant Golgi stack biogenesis and propose a new model for their growth de novo from ER exit sites.

BoPMEI1, a pollen-specific pectin methylesterase inhibitor, has an essential role in pollen tube growth

Pectin methylesterase (PME) is known to have important roles in pollen development and pollen tube growth. As pivotal regulatory factors in PME activity modulation, PME inhibitors (PMEIs) are thought to be key regulators of cell wall stability at the tip of the pollen tube. We report on the cloning and characterization of a novel B. oleracea PMEI gene, BoPMEI1. Heterologously expressed BoPMEI1 showed PMEI activity. RT-PCR studies of different tissues and promoter-GUS fusions confirmed that BoPMEI1 was specifically expressed in mature pollen grains and pollen tubes. Based on in vivo transient assays, we found that BoPMEI1 appears to be largely localized to the plasma membrane. Transgenic Arabidopsis plants expressing antisense BoPMEI1 under the control of the CaMV 35S promoter suppressed the expression of the orthologous gene At1g10770, which led to partial male sterility and decreased seed set by inhibition of pollen tube growth. Our study demonstrates the involvement of BoPMEI1 in pollen tube growth.

Golgi membrane dynamics after induction of a dominant-negative mutant Sar1 GTPase in tobacco

An inducible system has been established in Nicotiana tabacum plants allowing controlled expression of Sar1-GTP and thus the investigation of protein dynamics after inhibition of endoplasmic reticulum (ER) to Golgi transport. Complete Golgi disassembly and redistribution of Golgi markers into the ER was observed within 18-24h after induction. At the ultrastructural level Sar1-GTP expression led to a decrease in Golgi stack size followed by Golgi fragmentation and accumulation of vesicle remnants. Induction of Sar1-GTP resulted in redistribution of the green fluorescent protein (GFP)-tagged Arabidopsis golgins AtCASP and GC1 (golgin candidate 1, an Arabidopsis golgin 84 isoform) into the ER or cytoplasm, respectively. Additionally, both fusion proteins were observed in punctate structures, which co-located with a yellow fluorescent protein (YFP)-tagged version of Sar1-GTP. The Sar1-GTP-inducible system is compared with constitutive Sar1-GTP expression and brefeldin A treatment, and its potential for the study of the composition of ER exit sites and early cis-Golgi structures is discussed.

The rice alpha-amylase glycoprotein is targeted from the Golgi apparatus through the secretory pathway to the plastids

The well-characterized secretory glycoprotein, rice (Oryza sativa) alpha-amylase isoform I-1 (AmyI-1), was localized within the plastids and proved to be involved in the degradation of starch granules in the organelles of rice cells. In addition, a large portion of transiently expressed AmyI-1 fused to green fluorescent protein (AmyI-1-GFP) colocalized with a simultaneously expressed fluorescent plastid marker in onion (Allium cepa) epidermal cells. The plastid targeting of AmyI-1 was inhibited by both dominant-negative and constitutively active mutants of Arabidopsis thaliana ARF1 and Arabidopsis SAR1, which arrest endoplasmic reticulum-to-Golgi traffic. In cells expressing fluorescent trans-Golgi and plastid markers, these fluorescent markers frequently colocalized when coexpressed with AmyI-1. Three-dimensional time-lapse imaging and electron microscopy of high-pressure frozen/freeze-substituted cells demonstrated that contact of the Golgi-derived membrane vesicles with cargo and subsequent absorption into plastids occur within the cells. The transient expression of a series of C-terminal-truncated AmyI-1-GFP fusion proteins in the onion cell system showed that the region from Trp-301 to Gln-369 is necessary for plastid targeting of AmyI-1. Furthermore, the results obtained by site-directed mutations of Trp-302 and Gly-354, located on the surface and on opposite sides of the AmyI-1 protein, suggest that multiple surface regions are necessary for plastid targeting. Thus, Golgi-to-plastid traffic appears to be involved in the transport of glycoproteins to plastids and plastid targeting seems to be accomplished in a sorting signal-dependent manner.

Fluorescence lifetime imaging of interactions between Golgi tethering factors and small GTPases in plants

Peripheral tethering factors bind to small GTPases in order to obtain their correct location within the Golgi apparatus. Using fluorescence resonance energy transfer (FRET) and fluorescence lifetime imaging microscopy (FLIM) we visualized interactions between Arabidopsis homologues of tethering factors and small GTPases at the Golgi stacks in planta. Co-expression of the coiled-coil proteins AtGRIP and golgin candidate 5 (GC5) [TATA element modulatory factor (TMF)] and the putative post-Golgi tethering factor AtVPS52 fused to green fluorescent protein (GFP) with mRFP (monomeric red fluorescent protein) fusions to the small GTPases AtRab-H1(b), AtRab-H1(c) and AtARL1 resulted in reduced GFP lifetimes compared to the control proteins. Interestingly, we observed differences in GFP quenching between the different protein combinations as well as selective quenching of GFP-AtVPS52-labelled structures. The data presented here indicate that the FRET-FLIM technique should prove invaluable in assessing protein interactions in living plant cells at the organelle level.

Grab a Golgi: laser trapping of Golgi bodies reveals in vivo interactions with the endoplasmic reticulum

In many vacuolate plant cells, individual Golgi bodies appear to be attached to tubules of the pleiomorphic cortical endoplasmic reticulum (ER) network. Such observations culminated in the controversial mobile secretory unit hypothesis to explain transport of cargo from the ER to Golgi via Golgi attached export sites. This proposes that individual Golgi bodies and an attached-ER exit machinery move over or with the surface of the ER whilst collecting cargo for secretion. By the application of infrared laser optical traps to individual Golgi bodies within living leaf cells, we show that individual Golgi bodies can be micromanipulated to reveal their association with the ER. Golgi bodies are physically attached to ER tubules and lateral displacement of individual Golgi bodies results in the rapid growth of the attached ER tubule. Remarkably, the ER network can be remodelled in living cells simply by movement of laser trapped Golgi dragging new ER tubules through the cytoplasm and new ER anchor sites can be established. Finally, we show that trapped Golgi ripped off the ER are 'sticky' and can be docked on to and attached to ER tubules, which will again show rapid growth whilst pulled by moving Golgi.

Comparative phylogenetic analysis of small GTP-binding genes of model legume plants and assessment of their roles in root nodules

Small GTP-binding genes play an essential regulatory role in a multitude of cellular processes such as vesicle-mediated intracellular trafficking, signal transduction, cytoskeletal organization, and cell division in plants and animals. Medicago truncatula and Lotus japonicus are important model plants for studying legume-specific biological processes such as nodulation. The publicly available online resources for these plants from websites such as http://www.ncbi.nih.gov, http://www.medicago.org, http://www.tigr.org, and related sites were searched to collect nucleotide sequences that encode GTP-binding protein homologues. A total of 460 small GTPase sequences from several legume species including Medicago and Lotus, Arabidopsis, human, and yeast were phyletically analysed to shed light on the evolution and functional characteristics of legume-specific homologues. One of the main emphases of this study was the elucidation of the possible involvement of some members of small GTPase homologues in the establishment and maintenance of symbiotic associations in root nodules of legumes. A high frequency of vesicle-mediated trafficking in nodules led to the idea of a probable subfunctionalization of some members of this family in legumes. As a result of the analyses, a group of 10 small GTPases that are likely to be mainly expressed in nodules was determined. The sequences determined as a result of this study could be used in more detailed molecular genetic analyses such as creation of RNA interference silencing mutants for further clarification of the role of GTPases in nodulation. This study will also assist in furthering our understanding of the evolutionary history of small GTPases in legume species.

The plant ER-Golgi interface

The interface between the endoplasmic reticulum (ER) and the Golgi apparatus is a critical junction in the secretory pathway mediating the transport of both soluble and membrane cargo between the two organelles. Such transport can be bidirectional and is mediated by coated membranes. In this review, we consider the organization and dynamics of this interface in plant cells, the putative structure of which has caused some controversy in the literature, and we speculate on the stages of Golgi biogenesis from the ER and the role of the Golgi and ER on each other's motility.

Rab and Arl GTPase family members cooperate in the localization of the golgin GCC185

GCC185 is a large coiled-coil protein at the trans Golgi network that is required for receipt of transport vesicles inbound from late endosomes and for anchoring noncentrosomal microtubules that emanate from the Golgi. Here, we demonstrate that recruitment of GCC185 to the Golgi is mediated by two Golgi-localized small GTPases of the Rab and Arl families. GCC185 binds Rab6, and mutation of residues needed for Rab binding abolishes Golgi localization. The crystal structure of Rab6 bound to the GCC185 Rab-binding domain reveals that Rab6 recognizes a two-fold symmetric surface on a coiled coil immediately adjacent to a C-terminal GRIP domain. Unexpectedly, Rab6 binding promotes association of Arl1 with the GRIP domain. We present a structure-derived model for dual GTPase membrane attachment that highlights the potential ability of Rab GTPases to reach binding partners at a significant distance from the membrane via their unstructured and membrane-anchored, hypervariable domains.

Correct targeting of plant ARF GTPases relies on distinct protein domains

Indispensable membrane trafficking events depend on the activity of conserved small guanosine triphosphatases (GTPases), anchored to individual organelle membranes. In plant cells, it is currently unknown how these proteins reach their correct target membranes and interact with their effectors. To address these important biological questions, we studied two members of the ADP ribosylation factor (ARF) GTPase family, ARF1 and ARFB, which are membrane anchored through the same N-terminal myristoyl group but to different target membranes. Specifically, we investigated how ARF1 is targeted to the Golgi and post-Golgi structures, whereas ARFB accumulates at the plasma membrane. While the subcellular localization of ARFB appears to depend on multiple domains including the C-terminal half of the GTPase, the correct targeting of ARF1 is dependent on two domains: an N-terminal ARF1 domain that is necessary for the targeting of the GTPase to membranes and a core domain carrying a conserved MxxE motif that influences the relative distribution of ARF1 between the Golgi and post-Golgi compartments. We also established that the N-terminal ARF1 domain alone was insufficient to maintain an interaction with membranes and that correct targeting is a protein-specific property that depends on the status of the GTP switch. Finally, an ARF1-ARFB chimera containing only the first 18 amino acids from ARF1 was shown to compete with ARF1 membrane binding loci. Although this chimera exhibited GTPase activity in vitro, it was unable to recruit coatomer, a known ARF1 effector, onto Golgi membranes. Our results suggest that the targeting of ARF GTPases to the correct membranes may not only depend on interactions with effectors but also relies on distinct protein domains and further binding partners on the Golgi surface.

SNARE-Ware: The Role of SNARE-Domain Proteins in Plant Biology

In yeast and animal cells, members of the superfamily of N-ethylmaleimide-sensitive factor adaptor protein receptor (SNARE)-domain-containing proteins are key players in vesicle-associated membrane fusion events during transport processes between individual compartments of the endomembrane system, including exocytosis and endocytosis. Compared with genomes of other eukaryotes, genomes of monocotyledonous and dicotyledonous plants encode a surprisingly high number of SNARE proteins, suggesting vital roles for this protein class in higher plant species. Although to date it remains elusive whether plant SNARE proteins function like their yeast and animal counterparts, genetic screens have recently begun to unravel the variety of biological tasks in which plant SNAREs are involved. These duties involve fundamental processes such as cytokinesis, shoot gravitropism, pathogen defense, symbiosis, and abiotic stress responses, suggesting that SNAREs contribute essentially to many facets of plant biology.

Post-Golgi protein traffic in the plant secretory pathway

The Golgi apparatus in plants is organized as a multitude of individual stacks that are motile in the cytoplasm and in close association with the endoplasmic reticulum (ER) (Boevink et al. in Plant J 15:441-447, 1998). These stacks operate as a sorting centre for cargo molecules, providing modification and redirection to other organelles as appropriate. In the post-Golgi direction, these include vacuole and plasma membrane, and specialized transport routes to each are required to prevent mislocalization. Recent evidence in plant cells points to the existence of post-Golgi organelles that function as intermediate stations for efficient protein traffic, as well as to the influence of small GTPases such as Rabs and ARFs on post-Golgi trafficking. This review focuses on the latest findings on post-Golgi trafficking routes and on the involvement of GTPases and their effectors on the trafficking of proteins in the plant secretory pathway.

Functional anthology of intrinsic disorder. 2. Cellular components, domains, technical terms, developmental processes, and coding sequence diversities correlated with long disordered regions

Biologically active proteins without stable ordered structure (i.e., intrinsically disordered proteins) are attracting increased attention. Functional repertoires of ordered and disordered proteins are very different, and the ability to differentiate whether a given function is associated with intrinsic disorder or with a well-folded protein is crucial for modern protein science. However, there is a large gap between the number of proteins experimentally confirmed to be disordered and their actual number in nature. As a result, studies of functional properties of confirmed disordered proteins, while helpful in revealing the functional diversity of protein disorder, provide only a limited view. To overcome this problem, a bioinformatics approach for comprehensive study of functional roles of protein disorder was proposed in the first paper of this series (Xie, H.; Vucetic, S.; Iakoucheva, L. M.; Oldfield, C. J.; Dunker, A. K.; Obradovic, Z.; Uversky, V. N. Functional anthology of intrinsic disorder. 1. Biological processes and functions of proteins with long disordered regions. J. Proteome Res. 2007, 5, 1882-1898). Applying this novel approach to Swiss-Prot sequences and functional keywords, we found over 238 and 302 keywords to be strongly positively or negatively correlated, respectively, with long intrinsically disordered regions. This paper describes approximately 90 Swiss-Prot keywords attributed to the cellular components, domains, technical terms, developmental processes, and coding sequence diversities possessing strong positive and negative correlation with long disordered regions.

Targeting of TMV movement protein to plasmodesmata requires the actin/ER network: evidence from FRAP

Fluorescence recovery after photobleaching (FRAP) was used to study the mechanism by which fluorescent-protein-tagged movement protein (MP) of tobacco mosaic virus (TMV) is targeted to plasmodesmata (PD). The data show that fluorescence recovery in PD at the leading edge of an infection requires elements of the cortical actin/endoplasmic reticulum (ER) network and can occur in the absence of an intact microtubule (MT) cytoskeleton. Inhibitors of the actin cytoskeleton (latrunculin and cytochalasin) significantly inhibited MP targeting, while MT inhibitors (colchicine and oryzalin) did not. Application of sodium azide to infected cells implicated an active component of MP transfer to PD. Treatment of cells with Brefeldin A (BFA) at a concentration that caused reabsorption of the Golgi bodies into the ER (precluding secretion of viral MP) had no effect on MP targeting, while disruption of the cortical ER with higher concentrations of BFA caused significant inhibition. Our results support a model of TMV MP function in which targeting of MP to PD during infection is mediated by the actin/ER network.

New Insights into Membrane Trafficking and Protein Sorting

Protein transport in the secretory and endocytic pathways is a multistep process involving the generation of transport carriers loaded with defined sets of cargo, the shipment of the cargo-loaded transport carriers between compartments, and the specific fusion of these transport carriers with a target membrane. The regulation of these membrane-mediated processes involves a complex array of protein and lipid interactions. As the machinery and regulatory processes of membrane trafficking have been defined, it is increasingly apparent that membrane transport is intimately connected with a number of other cellular processes, such as quality control in the endoplasmic reticulum (ER), cytoskeletal dynamics, receptor signaling, and mitosis. The fidelity of membrane trafficking relies on the correct assembly of components on organelles. Recruitment of peripheral proteins plays a critical role in defining organelle identity and the establishment of membrane subdomains, essential for the regulation of vesicle transport. The molecular mechanisms for the biogenesis of membrane subdomains are also central to understanding how cargo is sorted and segregated and how different populations of transport carriers are generated. In this review we will focus on the emerging themes of organelle identity, membrane subdomains, regulation of Golgi trafficking, and advances in dissecting pathways in physiological systems.

Localization and domain characterization of Arabidopsis golgin candidates

Golgins are large coiled-coil proteins that play a role in tethering of vesicles to Golgi membranes and in maintaining the overall structure of the Golgi apparatus. Six Arabidopsis proteins with the structural characteristics of golgins were isolated and shown to locate to Golgi stacks when fused to GFP. Two of these golgin candidates (GC1 and GC2) possess C-terminal transmembrane (TM) domains with similarity to the TM domain of human golgin-84. The C-termini of two others (GC3/GDAP1 and GC4) contain conserved GRAB and GA1 domains that are also found in yeast Rud3p and human GMAP210. GC5 shares similarity with yeast Sgm1p and human TMF and GC6 with yeast Uso1p and human p115. When fused to GFP, the C-terminal domains of AtCASP and GC1 to GC6 localized to the Golgi, showing that they contain Golgi localization motifs. The N-termini, on the other hand, label the cytosol or nucleus. Immuno-gold labelling and co-expression with the cis Golgi Q-SNARE Memb11 resulted in a more detailed picture of the sub-Golgi location of some of these putative golgins. Using two independent assays it is further demonstrated that the interaction between GC5, the TMF homologue, and the Rab6 homologues is conserved in plants.