Five Arabidopsis reticulon isoforms share endoplasmic reticulum location, topology, and membrane-shaping properties

Endomembrane and Golgi traffic in plant cells

Leaf epidermal cells make ideal specimens for the investigation of the plant secretory pathway in that it is relatively easy to tag with fluorescent proteins and visualize in vivo the various organelles of the pathway. A number of techniques can be employed to identify and study proteins within the endomembrane organelles and to study their dynamics and interactions. Here, we discuss the most commonly used approaches to express proteins within arabidopsis and tobacco leaves, the use of mutant screens to identify trafficking proteins, and the use of two in vivo techniques, Fluorescence recovery after photobleaching and Förster resonance energy transfer, to study protein dynamics in plant cells.

ERMO3/MVP1/GOLD36 is involved in a cell type-specific mechanism for maintaining ER morphology in Arabidopsis thaliana

The endoplasmic reticulum (ER) has a unique, network-like morphology. The ER structures are composed of tubules, cisternae, and three-way junctions. This morphology is highly conserved among eukaryotes, but the molecular mechanism that maintains ER morphology has not yet been elucidated. In addition, certain Brassicaceae plants develop a unique ER-derived organelle called the ER body. This organelle accumulates large amounts of PYK10, a β-glucosidase, but its physiological functions are still obscure. We aimed to identify a novel factor required for maintaining the morphology of the ER, including ER bodies, and employed a forward-genetic approach using transgenic Arabidopsis thaliana (GFP-h) with fluorescently-labeled ER. We isolated and investigated a mutant (designated endoplasmic reticulum morphology3, ermo3) with huge aggregates and abnormal punctate structures of ER. ERMO3 encodes a GDSL-lipase/esterase family protein, also known as MVP1. Here, we showed that, although ERMO3/MVP1/GOLD36 was expressed ubiquitously, the morphological defects of ermo3 were specifically seen in a certain type of cells where ER bodies developed. Coimmunoprecipitation analysis combined with mass spectrometry revealed that ERMO3/MVP1/GOLD36 interacts with the PYK10 complex, a huge protein complex that is thought to be important for ER body-related defense systems. We also found that the depletion of transcription factor NAI1, a master regulator for ER body formation, suppressed the formation of ER-aggregates in ermo3 cells, suggesting that NAI1 expression plays an important role in the abnormal aggregation of ER. Our results suggest that ERMO3/MVP1/GOLD36 is required for preventing ER and other organelles from abnormal aggregation and for maintaining proper ER morphology in a coordinated manner with NAI1.

Protein-protein and protein-membrane associations in the lignin pathway

Supramolecular organization of enzymes is proposed to orchestrate metabolic complexity and help channel intermediates in different pathways. Phenylpropanoid metabolism has to direct up to 30% of the carbon fixed by plants to the biosynthesis of lignin precursors. Effective coupling of the enzymes in the pathway thus seems to be required. Subcellular localization, mobility, protein-protein, and protein-membrane interactions of four consecutive enzymes around the main branch point leading to lignin precursors was investigated in leaf tissues of Nicotiana benthamiana and cells of Arabidopsis thaliana. CYP73A5 and CYP98A3, the two Arabidopsis cytochrome P450s (P450s) catalyzing para- and meta-hydroxylations of the phenolic ring of monolignols were found to colocalize in the endoplasmic reticulum (ER) and to form homo- and heteromers. They moved along with the fast remodeling plant ER, but their lateral diffusion on the ER surface was restricted, likely due to association with other ER proteins. The connecting soluble enzyme hydroxycinnamoyltransferase (HCT), was found partially associated with the ER. Both HCT and the 4-coumaroyl-CoA ligase relocalized closer to the membrane upon P450 expression. Fluorescence lifetime imaging microscopy supports P450 colocalization and interaction with the soluble proteins, enhanced by the expression of the partner proteins. Protein relocalization was further enhanced in tissues undergoing wound repair. CYP98A3 was the most effective in driving protein association.

A perturbation in glutathione biosynthesis disrupts endoplasmic reticulum morphology and secretory membrane traffic in Arabidopsis thaliana

To identify potentially novel and essential components of plant membrane trafficking mechanisms we performed a GFP-based forward genetic screen for seedling-lethal biosynthetic membrane trafficking mutants in Arabidopsis thaliana. Amongst these mutants, four recessive alleles of GSH2, which encodes glutathione synthase (GSH2), were recovered. Each allele was characterized by loss of the typical polygonal endoplasmic reticulum (ER) network and the accumulation of swollen ER-derived bodies which accumulated a soluble secretory marker. Since GSH2 is responsible for converting γ-glutamylcysteine (γ-EC) to glutathione (GSH) in the glutathione biosynthesis pathway, gsh2 mutants exhibited γ-EC hyperaccumulation and GSH deficiency. Redox-sensitive GFP revealed that gsh2 seedlings maintained redox poise in the cytoplasm but were more sensitive to oxidative challenge. Genetic and pharmacological evidence indicated that γ-EC accumulation rather than GSH deficiency was responsible for the perturbation of ER morphology. Use of soluble and membrane-bound ER markers suggested that the swollen ER bodies were derived from ER fusiform bodies. Despite the gross perturbation of ER morphology, gsh2 seedlings did not suffer from constitutive oxidative ER stress or lack of an unfolded protein response, and homozygotes for the weakest allele could be propagated. The link between glutathione biosynthesis and ER morphology and function is discussed.

Opaque1 encodes a myosin XI motor protein that is required for endoplasmic reticulum motility and protein body formation in maize endosperm

Myosins are encoded by multigene families and are involved in many basic biological processes. However, their functions in plants remain poorly understood. Here, we report the functional characterization of maize (Zea mays) opaque1 (o1), which encodes a myosin XI protein. o1 is a classic maize seed mutant with an opaque endosperm phenotype but a normal zein protein content. Compared with the wild type, o1 endosperm cells display dilated endoplasmic reticulum (ER) structures and an increased number of smaller, misshapen protein bodies. The O1 gene was isolated by map-based cloning and was shown to encode a member of the plant myosin XI family (myosin XI-I). In endosperm cells, the O1 protein is associated with rough ER and protein bodies. Overexpression of the O1 tail domain (the C-terminal 644 amino acids) significantly inhibited ER streaming in tobacco (Nicotiana benthamiana) cells. Yeast two-hybrid analysis suggested an association between O1 and the ER through a heat shock protein 70-interacting protein. In summary, this study indicated that O1 influences protein body biogenesis by affecting ER morphology and motility, ultimately affecting endosperm texture.

The INs and OUTs of pattern recognition receptors at the cell surface

Pattern recognition receptors (PRRs) enable plants to sense non-self molecules displayed by microbes to mount proper defense responses or establish symbiosis. In recent years the importance of PRR subcellular trafficking to plant immunity has become apparent. PRRs traffic through the endoplasmatic reticulum (ER) and the Golgi apparatus to the plasma membrane, where they recognize their cognate ligands. At the plasma membrane, PRRs can be recycled or internalized via endocytic pathways. By using genetic and biochemical tools in combination with bioimaging, the trafficking pathways and their role in PRR perception of microbial molecules are now being revealed.

The quest for four-dimensional imaging in plant cell biology: it's just a matter of time

BACKGROUND

Analysis of plant cell dynamics over time, or four-dimensional imaging (4-DI), represents a major goal of plant science. The ability to resolve structures in the third dimension within the cell or tissue during developmental events or in response to environmental or experimental stresses (i.e. 4-DI) is critical to our understanding of gene expression, post-expression modulations of macromolecules and sub-cellular system interactions.

SCOPE

Microscopy-based technologies have been profoundly integral to this type of investigation, and new and refined microscopy technologies now allow for the visualization of cell dynamics with unprecedented resolution, contrast and experimental versatility. However, certain realities of light and electron microscopy, choice of specimen and specimen preparation techniques limit the scope of readily attaining 4-DI. Today, the plant microscopist must use a combinatorial strategy whereby multiple microscopy-based investigations are used. Modern fluorescence, confocal laser scanning, transmission electron and scanning electron microscopy provide effective conduits for synthesizing data detailing live cell dynamics and highly resolved snapshots of specific cell structures that will ultimately lead to 4-DI. This review provides a synopsis of such technologies available.

ERES (ER exit sites) and the "secretory unit concept"

The higher plant Golgi apparatus consists of hundreds of individual Golgi stacks which move along the cortical ER, propelled by the actomysin system. Anterograde and retrograde transport between the endoplasmic reticulum (ER) and the plant Golgi occurs over a narrow interface (around 500 nm) and is generally considered to be mediated by COP-coated vesicles. Previously, ER exit sites (ERES) have been identified on the basis of to localization of transiently expressed COPII-coat proteins. As a consequence it has been held that ERES in higher plants are intimately associated with Golgi stacks, and that both move together as an integrated structure: the "secretory unit". Using a new COPII marker, as well as YFP-SEC24 (a bona fide COPII coat protein), we have made observations on tobacco leaf epidermis at high resolution in the CLSM. Our data clearly shows that COPII fluorescence is associated with the Golgi stacks rather than the surface of the ER and probably represents the temporary accumulation of COPII vesicles in the Golgi matrix prior to fusion with the cis-Golgi cisternae. We have calculated the numbers of COPII vesicles which would be required to provide a typical Golgi-associated COPII-fluorescent signal as being much less than 20. We have discussed the consequences of this and question the continued usage of the term "secretory unit".

Role of host reticulon proteins in rearranging membranes for positive-strand RNA virus replication

Positive-strand RNA [(+)RNA] viruses are responsible for numerous human, animal, and plant diseases. Because of the limiting coding capacity of (+)RNA viruses, their replication requires a complex orchestration of interactions between the viral genome, viral proteins and exploited host factors. To replicate their genomic RNAs, (+)RNA viruses induce membrane rearrangements that create membrane-linked RNA replication compartments. Along with substantial advances on the ultrastructure of the membrane-bound RNA replication compartments, recent results have shed light into the role that host factors play in rearranging these membranes. This review focuses on recent insights that have driven a new understanding of the role that the membrane-shaping host reticulon homology domain proteins (RHPs) play in facilitating the replication of various (+)RNA viruses.

Progressive sheet-to-tubule transformation is a general mechanism for endoplasmic reticulum partitioning in dividing mammalian cells

During mitosis, ER network reorganization can lead to packing of the ER into tight concentric layers at the cell cortex and occurs in tandem with rounding of the cell. Morphometric and 3D EM analysis shows that in addition to reorganization, ER sheets undergo transformation toward more fenestrated and tubular forms before anaphase in mammalian cells.

The endoplasmic reticulum (ER) is both structurally and functionally complex, consisting of a dynamic network of interconnected sheets and tubules. To achieve a more comprehensive view of ER organization in interphase and mitotic cells and to address a discrepancy in the field (i.e., whether ER sheets persist, or are transformed to tubules, during mitosis), we analyzed the ER in four different mammalian cell lines using live-cell imaging, high-resolution electron microscopy, and three dimensional electron microscopy. In interphase cells, we found great variation in network organization and sheet structures among different cell lines. In mitotic cells, we show that the ER undergoes both spatial reorganization and structural transformation of sheets toward more fenestrated and tubular forms. However, the extent of spatial reorganization and sheet-to-tubule transformation varies among cell lines. Fenestration and tubulation of the ER correlates with a reduced number of membrane-bound ribosomes.

Reticulon-like-1, the Drosophila orthologue of the hereditary spastic paraplegia gene reticulon 2, is required for organization of endoplasmic reticulum and of distal motor axons

Several causative genes for hereditary spastic paraplegia encode proteins with intramembrane hairpin loops that contribute to the curvature of the endoplasmic reticulum (ER), but the relevance of this function to axonal degeneration is not understood. One of these genes is reticulon2. In contrast to mammals, Drosophila has only one widely expressed reticulon orthologue, Rtnl1, and we therefore used Drosophila to test its importance for ER organization and axonal function. Rtnl1 distribution overlapped with that of the ER, but in contrast to the rough ER, was enriched in axons. The loss of Rtnl1 led to the expansion of the rough or sheet ER in larval epidermis and elevated levels of ER stress. It also caused abnormalities specifically within distal portions of longer motor axons and in their presynaptic terminals, including disruption of the smooth ER (SER), the microtubule cytoskeleton and mitochondria. In contrast, proximal axon portions appeared unaffected. Our results provide direct evidence for reticulon function in the organization of the SER in distal longer axons, and support a model in which spastic paraplegia can be caused by impairment of axonal the SER. Our data provide a route to further understanding of both the role of the SER in axons and the pathological consequences of the impairment of this compartment.

Arabidopsis senescence-associated protein DMP1 is involved in membrane remodeling of the ER and tonoplast

BACKGROUND

Arabidopsis DMP1 was discovered in a genome-wide screen for senescence-associated membrane proteins. DMP1 is a member of a novel plant-specific membrane protein family of unknown function. In rosette leaves DMP1 expression increases from very low background level several 100fold during senescence progression.

RESULTS

Expression of AtDMP1 fused to eGFP in Nicotiana benthamiana triggers a complex process of succeeding membrane remodeling events affecting the structure of the endoplasmic reticulum (ER) and the vacuole. Induction of spherical structures ("bulbs"), changes in the architecture of the ER from tubular to cisternal elements, expansion of smooth ER, formation of crystalloid ER, and emergence of vacuolar membrane sheets and foamy membrane structures inside the vacuole are proceeding in this order. In some cells it can be observed that the process culminates in cell death after breakdown of the entire ER network and the vacuole. The integrity of the plasma membrane, nucleus and Golgi vesicles are retained until this stage. In Arabidopsis thaliana plants expressing AtDMP1-eGFP by the 35S promoter massive ER and vacuole vesiculation is observed during the latest steps of leaf senescence, whereas earlier in development ER and vacuole morphology are not perturbed. Expression by the native DMP1 promoter visualizes formation of aggregates termed "boluses" in the ER membranes and vesiculation of the entire ER network, which precedes disintegration of the central vacuole during the latest stage of senescence in siliques, rosette and cauline leaves and in darkened rosette leaves. In roots tips, DMP1 is strongly expressed in the cortex undergoing vacuole biogenesis.

CONCLUSIONS

Our data suggest that DMP1 is directly or indirectly involved in membrane fission during breakdown of the ER and the tonoplast during leaf senescence and in membrane fusion during vacuole biogenesis in roots. We propose that these properties of DMP1, exacerbated by transient overexpression, may cause or contribute to the dramatic membrane remodeling events which lead to cell death in infiltrated tobacco leaves.

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 reticulons: guardians of the structure and function of the endoplasmic reticulum

The endoplasmic reticulum (ER) consists of the nuclear envelope and a peripheral network of tubules and membrane sheets. The tubules are shaped by a specific class of curvature stabilizing proteins, the reticulons and DP1; however it is still unclear how the sheets are assembled. The ER is the cellular compartment responsible for secretory and membrane protein synthesis. The reducing conditions of ER lead to the intra/inter-chain formation of new disulphide bonds into polypeptides during protein folding assessed by enzymatic or spontaneous reactions. Moreover, ER represents the main intracellular calcium storage site and it plays an important role in calcium signaling that impacts many cellular processes. Accordingly, the maintenance of ER function represents an essential condition for the cell, and ER morphology constitutes an important prerogative of it. Furthermore, it is well known that ER undergoes prominent shape transitions during events such as cell division and differentiation. Thus, maintaining the correct ER structure is an essential feature for cellular physiology. Now, it is known that proper ER-associated proteins play a fundamental role in ER tubules formation. Among these ER-shaping proteins are the reticulons (RTN), which are acquiring a relevant position. In fact, beyond the structural role of reticulons, in very recent years new and deeper functional implications of these proteins are emerging in relation to their involvement in several cellular processes.

In Arabidopsis, the spatial and dynamic organization of the endoplasmic reticulum and Golgi apparatus is influenced by the integrity of the C-terminal domain of RHD3, a non-essential GTPase

The mechanisms underlying the organization and dynamics of plant endomembranes are largely unknown. Arabidopsis RHD3, a distant member of the dynamin superfamily, has recently been implicated in plant ER morphology and Golgi movement through analyses of dominant-negative mutants of the putative GTPase domain in a heterologous system. Whether RHD3 is indispensable for ER architecture and what role regions other than the putative GTPase domain play in RHD3 function are unanswered questions. Here we characterized an EMS mutant, gom8, with disrupted Golgi movement and positioning and compromised ER shape and dynamics. gom8 mapped to a missense mutation in the RHD3 hairpin loop domain, causing accumulation of the mutant protein into large structures, a markedly different distribution compared with wild-type RHD3 over the ER network. Despite the GOM8 distribution, tubules fused in the peripheral ER of the gom8 mutant. These data imply that integrity of the hairpin region is important for the subcellular distribution of RHD3, and that reduced availability of RHD3 over the ER can cause ER morphology defects, but does not prevent peripheral fusion between tubules. This was confirmed by evidence that gom8 was phenocopied in an RHD3 null background. Furthermore, we established that the region encompassing the RHD3 hairpin domain and the C-terminal cytosolic domain is necessary for RHD3 function. We conclude that RHD3 is important in ER morphology, but is dispensable for peripheral ER tubulation in an endogenous context, and that its activity relies on the C-terminal region in addition to the GTPase domain.

The Gp78 ubiquitin ligase: probing endoplasmic reticulum complexity

The endoplasmic reticulum (ER) has been classically divided, based on electron microscopy analysis, into parallel ribosome-studded rough ER sheets and a tubular smooth ER network. Recent studies have identified molecular constituents of the ER, the reticulons and DP1, that drive ER tubule formation and whose expression determines expression of ER sheets and tubules and thereby rough and smooth ER. However, segregation of the ER into only two domains remains simplistic and multiple functionally distinct ER domains necessarily exist. In this review, we will discuss the sub-organization of the ER in different domains focusing on the localization and role of the gp78 ubiquitin ligase in the mitochondria-associated smooth ER and on the evidence for a quality control ERAD domain.

Protein-induced modulation of chloroplast membrane morphology

Organelles are surrounded by membranes with a distinct lipid and protein composition. While it is well established that lipids affect protein functioning and vice versa, it has been only recently suggested that elevated membrane protein concentrations may affect the shape and organization of membranes. We therefore analyzed the effects of high chloroplast envelope protein concentrations on membrane structures using an in vivo approach with protoplasts. Transient expression of outer envelope proteins or protein domains such as CHUP1-TM-GFP, outer envelope protein of 7 kDa-GFP, or outer envelope protein of 24 kDa-GFP at high levels led to the formation of punctate, circular, and tubular membrane protrusions. Expression of inner membrane proteins such as translocase of inner chloroplast membrane 20, isoform II (Tic20-II)-GFP led to membrane protrusions including invaginations. Using increasing amounts of DNA for transfection, we could show that the frequency, size, and intensity of these protrusions increased with protein concentration. The membrane deformations were absent after cycloheximide treatment. Co-expression of CHUP1-TM-Cherry and Tic20-II-GFP led to membrane protrusions of various shapes and sizes including some stromule-like structures, for which several functions have been proposed. Interestingly, some structures seemed to contain both proteins, while others seem to contain one protein exclusively, indicating that outer and inner envelope dynamics might be regulated independently. While it was more difficult to investigate the effects of high expression levels of membrane proteins on mitochondrial membrane shapes using confocal imaging, it was striking that the expression of the outer membrane protein Tom20 led to more elongate mitochondria. We discuss that the effect of protein concentrations on membrane structure is possibly caused by an imbalance in the lipid to protein ratio and may be involved in a signaling pathway regulating membrane biogenesis. Finally, the observed phenomenon provides a valuable experimental approach to investigate the relationship between lipid synthesis and membrane protein expression in future studies.

Hijack it, change it: how do plant viruses utilize the host secretory pathway for efficient viral replication and spread?

The secretory pathway of eukaryotic cells has an elaborated set of endomembrane compartments involved in the synthesis, modification, and sorting of proteins and lipids. The secretory pathway in plant cells shares many features with that in other eukaryotic cells but also has distinct characteristics important for fundamental cell and developmental processes and for proper immune responses. Recently, there has been evidence that the remodeling of this pathway, and often the formation of viral-induced organelles, play an important role in viral replication and spread. The modification of the host secretory pathway seems to be a common feature among most single-stranded positive ss(+)RNA and even some DNA viruses. In this review, we will present the recent advances in the understanding of the organization and dynamics of the plant secretory pathway and the molecular regulation of membrane trafficking in the pathway. We will also discuss how different plant viruses may interact with the host secretory pathway for their efficient replication and spread, with a focus on tobacco mosaic virus and turnip mosaic virus.

Plasmodesmata - membrane tunnels with attitude

Plasmodesmata are doors in the rigid cell wall. In multicellular tissues, they allow the passage of molecules needed to create physiological gradients and, by closure, symplastic boundaries, which are necessary for the fundamental processes of plant growth, development and defence. Despite this central role in plant growth our knowledge of their contribution has been hindered by difficulties in biochemical and molecular characterisation. Recent advances in proteomic, biochemical, cell biological and genetic analysis of their structure and function is showing that plasmodesmata are plastic yet highly regulated structures. They require the perception of small molecule signals (such as reactive oxygen species) to activate local changes in the cell wall that place physical constraints on the channel. This article reviews recent evidence that highlights the roles of the membrane subcomponents both as structural elements and as environments for resident signalling molecules.

FrontiERs: movers and shapers of the higher plant cortical endoplasmic reticulum

The endoplasmic reticulum (ER) in higher plants performs many important functions, yet our understanding of how its intricate network shape and dynamics relate to function is very limited. Recent work has begun to unpick key molecular players in the generation of the pleomorphic, highly dynamic ER network structure that pervades the entire cytoplasm. ER movement is acto-myosin dependent. ER shape is dependent on RHD3 (Root Hair Defective 3) and a family of proteins called reticulons. The major challenge that lies ahead is understanding how factors that control ER shape and movement are regulated and how this relates to the numerous functions of the ER.

Direct interaction of baculovirus capsid proteins VP39 and EXON0 with kinesin-1 in insect cells determined by fluorescence resonance energy transfer-fluorescence lifetime imaging microscopy

Autographa californica multiple nucleopolyhedrovirus (AcMNPV) replicates in the nucleus of insect cells to produce nucleocapsids, which are transported from the nucleus to the plasma membrane for budding through GP64-enriched areas to form budded viruses. However, little is known about the anterograde trafficking of baculovirus nucleocapsids in insect cells. Preliminary confocal scanning laser microscopy studies showed that enhanced green fluorescent protein (EGFP)-tagged nucleocapsids and capsid proteins aligned and colocalized with the peripheral microtubules of virus-infected insect cells. A colchicine inhibition assay of virus-infected insect cells showed a significant reduction in budded virus production, providing further evidence for the involvement of microtubules and suggesting a possible role of kinesin in baculovirus anterograde trafficking. We investigated the interaction between AcMNPV nucleocapsids and kinesin-1 with fluorescence resonance energy transfer-fluorescence lifetime imaging microscopy (FRET-FLIM) and show for the first time that AcMNPV capsid proteins VP39 and EXON0, but not Orf1629, interact with the tetratricopeptide repeat (TPR) domain of kinesin. The excited-state fluorescence lifetime of EGFP fused to VP39 or EXON0 was quenched from 2.4 ± 1 ns to 2.1 ± 1 ns by monomeric fluorescent protein (mDsRed) fused to TPR (mDsRed-TPR). However, the excited-state fluorescence lifetime of an EGFP fusion of Orf1629 remained unquenched by mDsRed-TPR. These data indicate that kinesin-1 plays an important role in the anterograde trafficking of baculovirus in insect cells.

The ER in 3D: a multifunctional dynamic membrane network

The endoplasmic reticulum (ER) is a large, singular, membrane-bound organelle that has an elaborate 3D structure with a diversity of structural domains. It contains regions that are flat and cisternal, ones that are highly curved and tubular, and others adapted to form contacts with nearly every other organelle and with the plasma membrane. The 3D structure of the ER is determined by both integral ER membrane proteins and by interactions with the cytoskeleton. In this review, we describe some of the factors that are known to regulate ER structure and discuss how this structural organization and the dynamic nature of the ER membrane network allow it to perform its many different functions.

Arabidopsis RTNLB1 and RTNLB2 Reticulon-like proteins regulate intracellular trafficking and activity of the FLS2 immune receptor

Receptors localized at the plasma membrane are critical for the recognition of pathogens. The molecular determinants that regulate receptor transport to the plasma membrane are poorly understood. In a screen for proteins that interact with the FLAGELIN-SENSITIVE2 (FLS2) receptor using Arabidopsis thaliana protein microarrays, we identified the reticulon-like protein RTNLB1. We showed that FLS2 interacts in vivo with both RTNLB1 and its homolog RTNLB2 and that a Ser-rich region in the N-terminal tail of RTNLB1 is critical for the interaction with FLS2. Transgenic plants that lack RTNLB1 and RTNLB2 (rtnlb1 rtnlb2) or overexpress RTNLB1 (RTNLB1ox) exhibit reduced activation of FLS2-dependent signaling and increased susceptibility to pathogens. In both rtnlb1 rtnlb2 and RTNLB1ox, FLS2 accumulation at the plasma membrane was significantly affected compared with the wild type. Transient overexpression of RTNLB1 led to FLS2 retention in the endoplasmic reticulum (ER) and affected FLS2 glycosylation but not FLS2 stability. Removal of the critical N-terminal Ser-rich region or either of the two Tyr-dependent sorting motifs from RTNLB1 causes partial reversion of the negative effects of excess RTNLB1 on FLS2 transport out of the ER and accumulation at the membrane. The results are consistent with a model whereby RTNLB1 and RTNLB2 regulate the transport of newly synthesized FLS2 to the plasma membrane.

Laser stimulation of the chloroplast/endoplasmic reticulum nexus in tobacco transiently produces protein aggregates (boluses) within the endoplasmic reticulum and stimulates local ER remodeling

Does the ER subdomain that associates with the chloroplast in vivo, hereafter referred to as the chloroplast/ER nexus, play a role in protein flow within the ER? In studies of tobacco cells either constitutively or transiently expressing ER-retained luminal, GFP-HDEL, or trans-membrane, YFP-RHD3, fluorescent fusion proteins, brief 405-nm (3-6-mW) laser stimulation of the nexus causes a qualitative difference in the movement and behavior of proteins in the ER. Photostimulating the nexus produces fluorescent protein punctate aggregates (boluses) within the lumen and membrane of the ER. The aggregation propagates through the membrane network throughout the cell, but within minutes can revert to normal, with disaggregation propagating back toward the originally photostimulated nexus. In the meantime, the ER grows and anastomoses around the chloroplast, forming a dense cisternal and tubular network. If this network is again photostimulated, bolus formation does not recur and, if the photostimulation results in photobleaching, fluorescence recovery after photobleaching occurs as it would typically in areas away from the nexus. Bolus propagation is not mediated by the actin cytoskeleton, but can be reversed by pre-conditioning the cells for 30  min with high, 40-45°C, temperature (heat stress). Because it is not reversed with heat stress, the reorganization of the ER at the nexus following photostimulation is a separate event.

An Exo2 derivative affects ER and Golgi morphology and vacuolar sorting in a tissue-specific manner in arabidopsis

We screened a panel of compounds derived from Exo2 - a drug that perturbs post-Golgi compartments and trafficking in mammalian cells - for their effect on the secretory pathway in Arabidopsis root epidermal cells. While Exo2 and most related compounds had no significant effect, one Exo2 derivative, named LG8, induced severe morphological alterations in both the Golgi (at high concentrations) and the endoplasmic reticulum (ER). LG8 causes the ER to form foci of interconnecting tubules, which at the ultrastructural level appear similar to those previously reported in Arabidopsis roots after treatment with the herbicide oryzalin. In cotyledonary leaves, LG8 causes redistribution of a trans Golgi network (TGN) marker to the vacuole. LG8 affects the anterograde secretory pathway by inducing secretion of vacuolar cargo and preventing the brassinosteroid receptor BRI1 from reaching the plasma membrane. Uptake and arrival at the TGN of the endocytic marker FM4-64 is not affected. Unlike the ADP ribosylation factor-GTP exchange factor (ARF-GEF) inhibitor brefeldin A (BFA), LG8 affects these post-Golgi events without causing the formation of BFA bodies. Up to concentrations of 50 µm, the effects of LG8 are reversible.

Dynamic trafficking of wheat γ-gliadin and of its structural domains in tobacco cells, studied with fluorescent protein fusions

Prolamins, the main storage proteins of wheat seeds, are synthesized and retained in the endoplasmic reticulum (ER) of the endosperm cells, where they accumulate in protein bodies (PBs) and are then exported to the storage vacuole. The mechanisms leading to these events are unresolved. To investigate this unconventional trafficking pathway, wheat γ-gliadin and its isolated repeated N-terminal and cysteine-rich C-terminal domains were fused to fluorescent proteins and expressed in tobacco leaf epidermal cells. The results indicated that γ-gliadin and both isolated domains were able to be retained and accumulated as protein body-like structures (PBLS) in the ER, suggesting that tandem repeats are not the only sequence involved in γ-gliadin ER retention and PBLS formation. The high actin-dependent mobility of γ-gliadin PBLS is also reported, and it is demonstrated that most of them do not co-localize with Golgi body or pre-vacuolar compartment markers. Both γ-gliadin domains are found in the same PBLS when co-expressed, which is most probably due to their ability to interact with each other, as indicated by the yeast two-hybrid and FRET-FLIM experiments. Moreover, when stably expressed in BY-2 cells, green fluorescent protein (GFP) fusions to γ-gliadin and its isolated domains were retained in the ER for several days before being exported to the vacuole in a Golgi-dependent manner, and degraded, leading to the release of the GFP 'core'. Taken together, the results show that tobacco cells are a convenient model to study the atypical wheat prolamin trafficking with fluorescent protein fusions.

Balancing ER dynamics: shaping, bending, severing, and mending membranes

The endoplasmic reticulum is a multifunctional organelle composed of functionally and morphologically distinct domains. These include the relatively planar nuclear envelope and the peripheral ER, a network of sheet-like cisternae interconnected with tubules that spread throughout the cytoplasm. The ER is highly dynamic and the shape of its domains as well as their relative content are in constant flux. The multiple forces driving these morphological changes depend on the interaction between the ER and microtubules, membrane fusion and fission events and the action of proteins capable of actively shaping membranes. The interplay between these forces is ultimately responsible for the dynamic morphology of the ER, which in turn is crucial for properly executing the varied functions of this organelle.

A 3D analysis of yeast ER structure reveals how ER domains are organized by membrane curvature

Electron tomography of continuous ER domains during budding shows that reticulons and Yop1 stabilize rather than generate membrane curvature in this organelle.

We analyzed the structure of yeast endoplasmic reticulum (ER) during six sequential stages of budding by electron tomography to reveal a three-dimensional portrait of ER organization during inheritance at a nanometer resolution. We have determined the distribution, dimensions, and ribosome densities of structurally distinct but continuous ER domains during multiple stages of budding with and without the tubule-shaping proteins, reticulons (Rtns) and Yop1. In wild-type cells, the peripheral ER contains cytoplasmic cisternae, many tubules, and a large plasma membrane (PM)–associated ER domain that consists of both tubules and fenestrated cisternae. In the absence of Rtn/Yop1, all three domains lose membrane curvature, ER ribosome density changes, and the amount of PM-associated ER increases dramatically. Deletion of Rtns/Yop1 does not, however, prevent bloated ER tubules from being pulled from the mother cisterna into the bud and strongly suggests that Rtns/Yop1 stabilize/maintain rather than generate membrane curvature at all peripheral ER domains in yeast.

Arabidopsis RHD3 mediates the generation of the tubular ER network and is required for Golgi distribution and motility in plant cells

In plant cells, the endoplasmic reticulum (ER) and Golgi apparatus form a unique system in which single Golgi stacks are motile and in close association with the underlying ER tubules. Arabidopsis has three RHD3 (ROOT HAIR DEFECTIVE 3) isoforms that are analogous to the mammalian atlastin GTPases involved in shaping ER tubules. We used live-cell imaging, genetic complementation, split ubiquitin assays and western blot analyses in Arabidopsis and tobacco to show that RHD3 mediates the generation of the tubular ER network and is required for the distribution and motility of Golgi stacks in root and leaf epidermal cells. We established that RHD3 forms homotypic interactions at ER punctae. In addition, the activity of RHD3 on the tubular ER is specifically correlated with the cellular distribution and motility of Golgi stacks because ER to Golgi as well as Golgi to plasma membrane transport was not affected by RHD3 mutations in the conserved GDP/GTP motifs. We found a possible partial redundancy within the RHD3 isoforms in Arabidopsis. However, yeast Sey1p, a functional atlastin homologue, and RHD3 are not interchangeable in complementing the respective loss-of-function mutants, suggesting that the molecular mechanisms controlling ER tubular morphology might not be entirely conserved among eukaryotic lineages.

KMS1 and KMS2, two plant endoplasmic reticulum proteins involved in the early secretory pathway

We have identified two endoplasmic reticulum (ER)-associated Arabidopsis proteins, KMS1 and KMS2, which are conserved among most species. Fluorescent protein fusions of KMS1 localised to the ER in plant cells, and over-expression induced the formation of a membrane structure, identified as ER whorls by electron microscopy. Hydrophobicity analysis suggested that KMS1 and KMS2 are integral membrane proteins bearing six transmembrane domains. Membrane protein topology was assessed by a redox-based topology assay (ReTA) with redox-sensitive GFP and confirmed by a protease protection assay. A major loop domain between transmembrane domains 2 and 3, plus the N- and C-termini were found on the cytosolic side of the ER. A C-terminal di(tri)-lysine motif is involved in retrieval of KMS1 and deletion led to a reduction of the GFP-KMS1 signal in the ER. Over-expression of KMS1/KMS2 truncations perturbed ER and Golgi morphology and similar effects were also seen when KMS1/KMS2 were knocked-down by RNA interference. Microscopy and biochemical experiments suggested that expression of KMS1/KMS2 truncations inhibited ER to Golgi protein transport.

The nuclear envelope in the plant cell cycle: structure, function and regulation

BACKGROUND

Higher plants are, like animals, organisms in which successful completion of the cell cycle requires the breakdown and reformation of the nuclear envelope in a highly controlled manner. Interestingly, however, while the structures and processes appear similar, there are remarkable differences in protein composition and function between plants and animals.

SCOPE

Recent characterization of integral and associated components of the plant nuclear envelope has been instrumental in understanding its functions and behaviour. It is clear that protein interactions at the nuclear envelope are central to many processes in interphase and dividing cells and that the nuclear envelope has a key role in structural and regulatory events.

CONCLUSION

Dissecting the mechanisms of nuclear envelope breakdown and reformation in plants is necessary before a better understanding of the functions of nuclear envelope components during the cell cycle can be gained.

Myosin XI-dependent formation of tubular structures from endoplasmic reticulum isolated from tobacco cultured BY-2 cells

The reticular network of the endoplasmic reticulum (ER) consists of tubular and lamellar elements and is arranged in the cortical region of plant cells. This network constantly shows shape change and remodeling motion. Tubular ER structures were formed when GTP was added to the ER vesicles isolated from tobacco (Nicotiana tabacum) cultured BY-2 cells expressing ER-localized green fluorescent protein. The hydrolysis of GTP during ER tubule formation was higher than that under conditions in which ER tubule formation was not induced. Furthermore, a shearing force, such as the flow of liquid, was needed for the elongation/extension of the ER tubule. The shearing force was assumed to correspond to the force generated by the actomyosin system in vivo. To confirm this hypothesis, the S12 fraction was prepared, which contained both cytosol and microsome fractions, including two classes of myosins, XI (175-kD myosin) and VIII (BY-2 myosin VIII-1), and ER-localized green fluorescent protein vesicles. The ER tubules and their mesh-like structures were arranged in the S12 fraction efficiently by the addition of ATP, GTP, and exogenous filamentous actin. The tubule formation was significantly inhibited by the depletion of 175-kD myosin from the S12 fraction but not BY-2 myosin VIII-1. Furthermore, a recombinant carboxyl-terminal tail region of 175-kD myosin also suppressed ER tubule formation. The tips of tubules moved along filamentous actin during tubule elongation. These results indicated that the motive force generated by the actomyosin system contributes to the formation of ER tubules, suggesting that myosin XI is responsible not only for the transport of ER in cytoplasm but also for the reticular organization of cortical ER.

Arabidopsis plasmodesmal proteome

The multicellular nature of plants requires that cells should communicate in order to coordinate essential functions. This is achieved in part by molecular flux through pores in the cell wall, called plasmodesmata. We describe the proteomic analysis of plasmodesmata purified from the walls of Arabidopsis suspension cells. Isolated plasmodesmata were seen as membrane-rich structures largely devoid of immunoreactive markers for the plasma membrane, endoplasmic reticulum and cytoplasmic components. Using nano-liquid chromatography and an Orbitrap ion-trap tandem mass spectrometer, 1341 proteins were identified. We refer to this list as the plasmodesmata- or PD-proteome. Relative to other cell wall proteomes, the PD-proteome is depleted in wall proteins and enriched for membrane proteins, but still has a significant number (35%) of putative cytoplasmic contaminants, probably reflecting the sensitivity of the proteomic detection system. To validate the PD-proteome we searched for known plasmodesmal proteins and used molecular and cell biological techniques to identify novel putative plasmodesmal proteins from a small subset of candidates. The PD-proteome contained known plasmodesmal proteins and some inferred plasmodesmal proteins, based upon sequence or functional homology with examples identified in different plant systems. Many of these had a membrane association reflecting the membranous nature of isolated structures. Exploiting this connection we analysed a sample of the abundant receptor-like class of membrane proteins and a small random selection of other membrane proteins for their ability to target plasmodesmata as fluorescently-tagged fusion proteins. From 15 candidates we identified three receptor-like kinases, a tetraspanin and a protein of unknown function as novel potential plasmodesmal proteins. Together with published work, these data suggest that the membranous elements in plasmodesmata may be rich in receptor-like functions, and they validate the content of the PD-proteome as a valuable resource for the further uncovering of the structure and function of plasmodesmata as key components in cell-to-cell communication in plants.

Hydrophobic-domain-dependent protein-protein interactions mediate the localization of GPAT enzymes to ER subdomains

The endoplasmic reticulum (ER) is a dynamic organelle that consists of numerous regions or 'subdomains' that have discrete morphological features and functional properties. Although it is generally accepted that these subdomains differ in their protein and perhaps lipid compositions, a clear understanding of how they are assembled and maintained has not been well established. We previously demonstrated that two diacylglycerol acyltransferase enzymes (DGAT1 and DGAT2) from tung tree (Vernicia fordii) were located in different subdomains of ER, but the mechanisms responsible for protein targeting to these subdomains were not elucidated. Here we extend these studies by describing two glycerol-3-phosphate acyltransferase-like (GPAT) enzymes from tung tree, GPAT8 and GPAT9, that both colocalize with DGAT2 in the same ER subdomains. Measurement of protein-protein interactions using the split-ubiquitin assay revealed that GPAT8 interacts with itself, GPAT9 and DGAT2, but not with DGAT1. Furthermore, mutational analysis of GPAT8 revealed that the protein's first predicted hydrophobic region, which contains an amphipathic helix-like motif, is required for interaction with DGAT2 and for DGAT2-dependent colocalization in ER subdomains. Taken together, these results suggest that the regulation and organization of ER subdomains is mediated at least in part by higher-ordered, hydrophobic-domain-dependent homo- and hetero-oligomeric protein-protein interactions.

Plasmodesmata viewed as specialised membrane adhesion sites

A significant amount of work has been expended to identify the elusive components of plasmodesmata (PD) to help understand their structure, as well as how proteins are targeted to them. This review focuses on the role that lipid membranes may play in defining PD both structurally and as subcellular targeting addresses. Parallels are drawn to findings in other areas of research which focus on the lateral segregation of membrane domains and the generation of three-dimensional organellar shapes from flat lipid bilayers. We conclude that consideration of the protein-lipid interactions in cell biological studies of PD components and PD-targeted proteins may yield new insights into some of the many open questions regarding these unique structures.

Mechanisms determining the morphology of the peripheral ER

The endoplasmic reticulum (ER) consists of the nuclear envelope and a peripheral network of tubules and membrane sheets. The tubules are shaped by the curvature-stabilizing proteins reticulons and DP1/Yop1p, but how the sheets are formed is unclear. Here, we identify several sheet-enriched membrane proteins in the mammalian ER, including proteins that translocate and modify newly synthesized polypeptides, as well as coiled-coil membrane proteins that are highly upregulated in cells with proliferated ER sheets, all of which are localized by membrane-bound polysomes. These results indicate that sheets and tubules correspond to rough and smooth ER, respectively. One of the coiled-coil proteins, Climp63, serves as a "luminal ER spacer" and forms sheets when overexpressed. More universally, however, sheet formation appears to involve the reticulons and DP1/Yop1p, which localize to sheet edges and whose abundance determines the ratio of sheets to tubules. These proteins may generate sheets by stabilizing the high curvature of edges.

Reticulon short hairpin transmembrane domains are used to shape ER tubules

Reticulons are integral membrane proteins that partition into and shape the tubular endoplasmic reticulum (ER). We propose that reticulons use a membrane insertion mechanism to generate regions of high membrane curvature in the ER. A reticulon contains two short hairpin transmembrane domains (TMDs), which could generate membrane curvature by increasing the area of the cytoplasmic leaflet. Here, we test whether the short length of these hairpin TMDs is required for reticulon membrane-shaping functions in mammalian cells. We lengthened the TMDs of reticulon 4 to resemble a typical bi-pass TMD that spans both leaflets. We find that TMD mutants oligomerize like wild type (wt), however, they are not immobilized, do not partition into tubules, do not constrict tubules and no longer suppress peripheral ER cisternae. Therefore, short hairpin TMD length is required for reticulon protein partitioning and membrane-shaping functions. Another membrane protein with a short hairpin TMD is caveolin. We show that an ER-retained caveolin construct also partitions within the ER in a manner that is dependent on it containing a short hairpin TMD. These data suggest that a short hairpin TMD may be a general feature used by membrane-shaping proteins to partition into and shape regions of high membrane curvature.

Transmembrane domain length is responsible for the ability of a plant reticulon to shape endoplasmic reticulum tubules in vivo

Reticulons are integral endoplasmic reticulum (ER) membrane proteins that have the ability to shape the ER into tubules. It has been hypothesized that their unusually long conserved hydrophobic regions cause reticulons to assume a wedge-like topology that induces membrane curvature. Here we provide proof of this hypothesis. When over-expressed, an Arabidopsis thaliana reticulon (RTNLB13) localized to, and induced constrictions in, cortical ER tubules. Ectopic expression of RTNLB13 was sufficient to induce ER tubulation in an Arabidopsis mutant (pah1 pah2) whose ER membrane is mostly present in a sheet-like form. By sequential shortening of the four transmembrane domains (TMDs) of RTNLB13, we show that the length of the transmembrane regions is directly correlated with the ability of RTNLB13 to induce membrane tubulation and to form low-mobility complexes within the ER membrane. We also show that full-length TMDs are necessary for the ability of RTNLB13 to reside in the ER membrane.