ATP-Dependent formation of phosphatidylserine-rich vesicles from the endoplasmic reticulum of leek cells

The Endoplasmic Reticulum Is a Key Battleground between Phytoplasma Aggression and Host Plant Defense

Phytoplasmas are intracellular plant pathogens that heavily rely on host cell nutrients for survival and propagation due to their limited ability to synthesize essential substrates. The endoplasmic reticulum (ER), which plays a vital role in various cellular processes, including lipid and protein biosynthesis, is an attractive target for numerous intracellular pathogens to exploit. This study investigated the impact of potato purple top (PPT) phytoplasma infection on the ER in tomato plants. Abnormal accumulation of ER-resident proteins, disrupted ER network structures, and formation of protein aggregates in the phloem were observed using confocal microscopy and transmission electron microscopy, indicating a phytoplasma-infection-induced disturbance in ER homeostasis. The colocalization of phytoplasmas with the accumulated ER-resident proteins suggests an association between ER stress, unfolded protein response (UPR) induction, and phytoplasma infection and colonization, with the ER stress response likely contributing to the host plant's defense mechanisms. Quantitative real-time PCR revealed a negative correlation between ER stress/UPR activation and PPT phytoplasma titer, implying the involvement of UPR in curbing phytoplasma proliferation. Inducing ER stress and activating the UPR pathway effectively decreased phytoplasma titer, while suppressing the ER-resident protein, binding immunoglobulin protein (BiP) increased phytoplasma titer. These results highlight the ER as an intracellular battleground where phytoplasmas exploit host components for survival and multiplication, while host plants deploy defense mechanisms to counteract the invasion. Understanding the intricate interactions between phytoplasmas and plant hosts at the subcellular level, particularly within the ER, provides valuable insights for developing new strategies to control phytoplasma diseases.

Phosphatidylserine Is Required for the Normal Progression of Cell Plate Formation in Arabidopsis Root Meristems

Phosphatidylserine (PS) is involved in various cellular processes in yeast and animals. However, PS functions in plants remain unclear. In Arabidopsis, PS is relatively enriched in flower and root tissues, and the genetic disturbance of PS biosynthesis in phosphatidylserine synthase1 (PSS1)/ pss1 heterozygotes induces sporophytic and gametophytic defects during pollen maturation. This study functionally characterized PS in Arabidopsis roots and observed that pss1 seedlings exhibited a short-root phenotype by reducing the meristem size and cell elongation capacity. Confocal microscopy imaging analyses of PS with GFP-LactC2 and the endocytic activity with FM 4-64 revealed that although GFP-LactC2 (or PS) was localized in the plasma membrane and endocytic membranes, the lack of PS in pss1 roots did not affect the constitutive endocytosis. Instead, a fluorescence imaging analysis of the cytokinetic phases in the dividing zone of pss1-2 roots revealed a significant delay in telophase progression, requiring active cargo vesicle trafficking for cell plate formation. Confocal microscopy imaging analysis of transgenic GFP-LactC2 root cells with developing cell plates indicated that GFP-LactC2 was localized at the cell plate. Moreover, confocal microscopy images of transgenic pss1-2 and PSS1 roots expressing the cell plate-specific syntaxin construct ProKNOLLE:eGFP-KNOLLE showed abnormal cell plate development in pss1-2ProKNOLLE:eGFP-KNOLLE roots. These results suggested that PS is required for root cytokinesis, possibly because it helps mediate the cargo vesicular trafficking required for cell plate formation.

Phosphatidylserine Synthase from Salicornia europaea Is Involved in Plant Salt Tolerance by Regulating Plasma Membrane Stability

Salinity-induced lipid alterations have been reported in many plant species; however, how lipid biosynthesis and metabolism are regulated and how lipids work in plant salt tolerance are much less studied. Here, a constitutively much higher phosphatidylserine (PS) content in the plasma membrane (PM) was found in the euhalophyte Salicornia europaea than in Arabidopsis. A gene encoding PS synthase (PSS) was subsequently isolated from S. europaea, named SePSS, which was induced by salinity. Multiple alignments and phylogenetic analysis suggested that SePSS belongs to a base exchange-type PSS, which localises to the endoplasmic reticulum. Knockdown of SePSS in S. europaea suspension cells resulted in reduced PS content, decreased cell survival rate, and increased PM depolarization and K+ efflux under 400 or 800 mM NaCl. By contrast, the upregulation of SePSS leads to increased PS and phosphatidylethanolamine levels and enhanced salt tolerance in Arabidopsis, along with a lower accumulation of reactive oxygen species, less membrane injury, less PM depolarization and higher K+/Na+ in the transgenic lines than in wild-type (WT). These results suggest a positive correlation between PS levels and plant salt tolerance, and that SePSS participates in plant salt tolerance by regulating PS levels, hence PM potential and permeability, which help maintain ion homeostasis. Our work provides a potential strategy for improving plant growth under multiple stresses.

Getting membrane proteins on and off the shuttle bus between the endoplasmic reticulum and the Golgi complex

Secretory proteins exit the endoplasmic reticulum (ER) in coat protein complex II (COPII)-coated vesicles and then progress through the Golgi complex before delivery to their final destination. Soluble cargo can be recruited to ER exit sites by signal-mediated processes (cargo capture) or by bulk flow. For membrane proteins, a third mechanism, based on the interaction of their transmembrane domain (TMD) with lipid microdomains, must also be considered. In this Commentary, I review evidence in favor of the idea that partitioning of TMDs into bilayer domains that are endowed with distinct physico-chemical properties plays a pivotal role in the transport of membrane proteins within the early secretory pathway. The combination of such self-organizational phenomena with canonical intermolecular interactions is most likely to control the release of membrane proteins from the ER into the secretory pathway.

An investigation of the effect of membrane curvature on transmembrane-domain dependent protein sorting in lipid bilayers

Sorting of membrane proteins within the secretory pathway of eukaryotic cells is a complex process involving discrete sorting signals as well as physico-chemical properties of the transmembrane domain (TMD). Previous work demonstrated that tail-anchored (TA) protein sorting at the interface between the Endoplasmic Reticulum (ER) and the Golgi complex is exquisitely dependent on the length and hydrophobicity of the transmembrane domain, and suggested that an imbalance between TMD length and bilayer thickness (hydrophobic mismatch) could drive long TMD-containing proteins into curved membrane domains, including ER exit sites, with consequent export of the mismatched protein out of the ER. Here, we tested a possible role of curvature in TMD-dependent sorting in a model system consisting of Giant Unilamellar Vesicles (GUVs) from which narrow membrane tubes were pulled by micromanipulation. Fluorescent TA proteins differing in TMD length were incorporated into GUVs of uniform lipid composition or made of total ER lipids, and TMD-dependent sorting and diffusion, as well as the bending rigidity of bilayers made of microsomal lipids, were investigated. Long and short TMD-containing constructs were inserted with similar orientation, diffused equally rapidly in GUVs and in tubes pulled from GUVs, and no difference in their final distribution between planar and curved regions was detected. These results indicate that curvature alone is not sufficient to drive TMD-dependent sorting at the ER-Golgi interface, and set the basis for the investigation of the additional factors that must be required.

PHOSPHATIDYLSERINE SYNTHASE1 is required for microspore development in Arabidopsis thaliana

Phosphatidylserine (PS) has many important biological roles, but little is known about its role in plants, partly because of its low abundance. We show here that PS is enriched in Arabidopsis floral tissues and that genetic disruption of PS biosynthesis decreased heterozygote fertility due to inhibition of pollen maturation. At1g15110, designated PSS1, encodes a base-exchange-type PS synthase. Escherichia coli cells expressing PSS1 accumulated PS in the presence of l-serine at 23°C. Promoter-GUS assays showed PSS1 expression in developing anther pollen and tapetum. A few seeds with pss1-1 and pss1-2 knockout alleles escaped embryonic lethality but developed into sterile dwarf mutant plants. These plants contained no PS, verifying that PSS1 is essential for PS biosynthesis. Reciprocal crossing revealed reduced pss1 transmission via male gametophytes, predicting a rate of 61.6%pss1-1 pollen defects in PSS1/pss1-1 plants. Alexander's staining of inseparable qrt1-1 PSS1/pss1-1 quartets revealed a rate of 42% having three or four dead pollen grains, suggesting sporophytic pss1-1 cell death effects. Analysis with the nuclear stain 4',6-diamidino-2-phenylindole (DAPI) showed that all tetrads from PSS1/pss1-1 anthers retain their nuclei, whereas unicellular microspores were sometimes anucleate. Transgenic Arabidopsis expressing a GFP-LactC2 construct that binds PS revealed vesicular staining in tetrads and bicellular microspores and nuclear membrane staining in unicellular microspores. Hence, distribution and/or transport of PS across membranes were dynamically regulated in pollen microspores. However, among unicellular microspores from PSS1/pss1-2 GFP-LactC2 plants, all anucleate microspores showed little GFP-LactC2 fluorescence, suggesting that pss1-2 microspores are more sensitive to sporophytic defects or show partial gametophytic defects.

Static retention of the lumenal monotopic membrane protein torsinA in the endoplasmic reticulum

TorsinA is a membrane-associated enzyme in the endoplasmic reticulum (ER) lumen that is mutated in DYT1 dystonia. How it remains in the ER has been unclear. We report that a hydrophobic N-terminal domain (NTD) directs static retention of torsinA within the ER by excluding it from ER exit sites, as has been previously reported for short transmembrane domains (TMDs). We show that despite the NTD's physicochemical similarity to TMDs, it does not traverse the membrane, defining torsinA as a lumenal monotopic membrane protein and requiring a new paradigm to explain retention. ER retention and membrane association are perturbed by a subset of nonconservative mutations to the NTD, suggesting that a helical structure with defined orientation in the membrane is required. TorsinA preferentially enriches in ER sheets, as might be expected for a lumenal monotopic membrane protein. We propose that the principle of membrane-based protein sorting extends to monotopic membrane proteins, and identify other proteins including the monotopic lumenal enzyme cyclooxygenase 1 (prostaglandin H synthase 1) that share this mechanism of retention with torsinA.

Transmembrane domain-dependent partitioning of membrane proteins within the endoplasmic reticulum

The length and hydrophobicity of the transmembrane domain (TMD) play an important role in the sorting of membrane proteins within the secretory pathway; however, the relative contributions of protein-protein and protein-lipid interactions to this phenomenon are currently not understood. To investigate the mechanism of TMD-dependent sorting, we used the following two C tail-anchored fluorescent proteins (FPs), which differ only in TMD length: FP-17, which is anchored to the endoplasmic reticulum (ER) membrane by 17 uncharged residues, and FP-22, which is driven to the plasma membrane by its 22-residue-long TMD. Before export of FP-22, the two constructs, although freely diffusible, were seen to distribute differently between ER tubules and sheets. Analyses in temperature-blocked cells revealed that FP-17 is excluded from ER exit sites, whereas FP-22 is recruited to them, although it remains freely exchangeable with the surrounding reticulum. Thus, physicochemical features of the TMD influence sorting of membrane proteins both within the ER and at the ER-Golgi boundary by simple receptor-independent mechanisms based on partitioning.

Insights into the role of specific lipids in the formation and delivery of lipid microdomains to the plasma membrane of plant cells

The existence of sphingolipid- and sterol-enriched microdomains, known as lipid rafts, in the plasma membrane (PM) of eukaryotic cells is well documented. To obtain more insight into the lipid molecular species required for the formation of microdomains in plants, we have isolated detergent (Triton X-100)-resistant membranes (DRMs) from the PM of Arabidopsis (Arabidopsis thaliana) and leek (Allium porrum) seedlings as well as from Arabidopsis cell cultures. Here, we show that all DRM preparations are enriched in sterols, sterylglucosides, and glucosylceramides (GluCer) and depleted in glycerophospholipids. The GluCer of DRMs from leek seedlings contain hydroxypalmitic acid. We investigated the role of sterols in DRM formation along the secretory pathway in leek seedlings. We present evidence for the presence of DRMs in both the PM and the Golgi apparatus but not in the endoplasmic reticulum. In leek seedlings treated with fenpropimorph, a sterol biosynthesis inhibitor, the usual Delta(5)-sterols are replaced by 9beta,19-cyclopropylsterols. In these plants, sterols and hydroxypalmitic acid-containing GluCer do not reach the PM, and most DRMs are recovered from the Golgi apparatus, indicating that Delta(5)-sterols and GluCer play a crucial role in lipid microdomain formation and delivery to the PM. In addition, DRM formation in Arabidopsis cells is shown to depend on the unsaturation degree of fatty acyl chains as evidenced by the dramatic decrease in the amount of DRMs prepared from the Arabidopsis mutants, fad2 and Fad3+, affected in their fatty acid desaturases.

Two tail-anchored protein variants, differing in transmembrane domain length and intracellular sorting, interact differently with lipids

C-tail-anchored (TA) proteins often require a transmembrane domain of moderate hydrophobicity to maintain their endoplasmic reticulum residence, but the suggested role of protein-lipid interactions in this phenomenon has not been established. Here, we studied the interaction of TA proteins with lipids by differential scanning calorimetry by using a model system consisting of liposomes embedding either of two forms of cytochrome b(5): the endoplasmic reticulum-resident wild-type (b(5)wt) and a mutant thereof (b(5)ext), that contains five extra nonpolar amino acids in its transmembrane domain and, therefore, reaches the plasma membrane. The proteins were incorporated into liposomes of palmitoyl-oleyl-phosphatidylcholine (POPC) or POPC mixed with either distearoyl-phosphatidylserine (DSPS), palmitoyl-oleyl-phosphatidylserine (POPS), distearoyl-phosphatidylcholine (DSPC), or C16-ceramide (CER). POPC liposomes displayed a single thermotropic transition centered at -3.4 degrees C. When present, the second lipid formed a domain within the POPC bilayer, as indicated by the appearance of an additional peak. This peak was centered at temperatures close to 0 degrees C in the case of liposomes containing 10% CER, DSPS, and POPS and at 23 degrees C in the case of DSPC, likely reflecting a higher degree of molecular packing for DSPC domains. In DSPS/POPC, POPS/POPC, or CER/POPC, but not in DSPC/POPC liposomes, the insertion of b(5)wt increased, whereas b(5)ext decreased, the relative contribution to the total enthalpy of the higher temperature, phase-separated component. These results were confirmed with fluorescence measurements by using pyrene-labeled phospholipids. The dissimilar interaction with lipids of these two differently localized TA proteins could have implications for their intracellular sorting.

Evidence for a different metabolism of PC and PE in shoots and roots

We investigated phosphatidylcholine (PC) and phosphatidylethanolamine (PE) labelling in shoots and roots from leek plantlets, maize seedlings and Arabidopsis thaliana through the incorporation of radiolabelled acetate. Regardless of the pathway followed in shoots, PC labelling was always higher than PE labelling. However, we obtained an opposite situation in leek and A. thaliana roots since PC labelling was much lower than PE labelling. Several hypotheses to explain the origin(s) of these discrepancies between roots and shoots were tested. Among them, neither the level of the respective AAPT activities, nor specific regulations of PC biosynthesis through the mRNA levels of several enzymes (choline citidylyltransferase (CCT), ethanolamine citidylyltransferase (ECT), phosphoethanolamine methyltransferase (PEAMT)), nor the fatty acyl chain composition of PC, PE, and diacylglycerol, were responsible for the differences observed between PC and PE metabolism in roots and shoots. Finally, we investigated the acylation of PC and PE in vitro in both shoots and roots of A. thaliana seedlings, and demonstrated that some specific remodelling of PC and PE by acylation was responsible for the differences in labelling observed in vivo.

Cell biology of the plant Golgi apparatus

The higher plant Golgi apparatus, comprising many individual stacks of membrane bounded cisternae, is one of the most enigmatic of the cytoplasmic organelles. Not only can the stacks receive material from the endoplasmic reticulum, process it and target it to the correct cellular destination, but they can also synthesise and export complex carbohydrates and lipids and most likely act as one end point of the endocytic pathway. In many cells such processing and sorting can take place while the stacks are moving within the cytoplasm and, remarkably, the organelle manages to retain its structural integrity. This review considers some of the latest data and views on transport both to and from the Golgi and the mechanisms by which such activity is regulated.

A 2H solid-state NMR spectroscopic investigation of biomimetic bicelles containing cholesterol and polyunsaturated phosphatidylcholine

Deuterium solid-state NMR spectroscopy was used to qualitatively study the effects of both 1-palmitoyl-2-linoleoyl-sn-glycero-3-phosphatidylcholine (PLiPC) and cholesterol on magnetically aligned phospholipid bilayers (bicelles) as a function of temperature utilizing the chain-perdeuterated probe 1,2-dimyristoyl-sn-glycero-3-phosphatidylcholine (DMPC-d54) in DMPC/dihexanoylPC (DHPC) phospholipid bilayers. The results demonstrate that polyunsaturated PC and cholesterol were successfully incorporated into DMPC/DHPC phospholipid bilayers, leading to a bicelle that will be useful for investigations of eukaryotic membrane protein-lipid interactions. The data indicate that polyunsaturated PC increases membrane fluidity and decreases the minimum magnetic alignment temperature for DMPC/DHPC bicelles. Conversely, the introduction of cholesterol into aligned DMPC/DHPC bilayers decreases fluidity in the membrane and increases the minimum temperature necessary to magnetically align the phospholipid bilayers. Finally, the addition of Tm3+ to magnetically aligned DMPC/DMPC-d54/PLiPC/DHPC bilayers doubles the quadrupolar splittings, indicating that this unique bicelle system can be aligned with the bilayer normal parallel to the static magnetic field.

Chromatographic purification of an insoluble histidine tag recombinant Ykt6p SNARE from Arabidopsis thaliana over-expressed in E. coli

In order to undertake in plant cell the study of the endoplasmic reticulum (ER)-Golgi apparatus (GA) protein and/or lipid vesicular transport pathway, expressed sequence tag (EST) coding for a homologue to the yeast soluble N-ethylmaleimide-sensitive factor attachment protein receptors (SNAREs) Ykt6p has been cloned in Arabidopsis thaliana by reverse transcription polymerase chain reaction (RT-PCR). The corresponding protein was over-expressed as a recombinant histidine-tag (his-tag) protein in E. coli. Starting from one litter of culture, an ultrasonic homogenization was performed for cell disruption and after centrifugation the Arabidopsis Ykt6p SNARE present in inclusion bodies in the pellet was solubilized. After centrifugation, the clarified feedstock obtained was injected onto an immobilized metal affinity chromatography (IMAC) in presence of 6 M guanidine and on-column refolding was performed. Folded and subsequently purified (94% purity) recombinant protein was obtained with 82% of recovery.

Biogenesis and cellular dynamics of aminoglycerophospholipids

Aminoglycerophospholipids phosphatidylserine (PtdSer), phosphatidylethanolamine (PtdEtn), and phosphatidylcholine (PtdCho) comprise about 80% of total cellular phospholipids in most cell types. While the major function of PtdCho in eukaryotes and PtdEtn in prokaryotes is that of bulk membrane lipids, PtdSer is a minor component and appears to play a more specialized role in the plasma membrane of eukaryotes, e.g., in cell recognition processes. All three aminoglycerophospholipid classes are essential in mammals, whereas prokaryotes and lower eukaryotes such as yeast appear to be more flexible regarding their aminoglycerophospholipid requirement. Since different subcellular compartments of eukaryotes, namely the endoplasmic reticulum and mitochondria, contribute to the biosynthetic sequence of aminoglycerophospholipid formation, intracellular transport, sorting, and specific function of these lipids in different organelles are of special interest.

Inhibition of the sterol pathway in leek seedlings impairs phosphatidylserine and glucosylceramide synthesis but triggers an accumulation of triacylglycerols

Like most higher plants, leek seedlings (Allium porrum L.) contain a mixture of Delta(5)-sterols in which sitosterol largely predominates. As previously reported (Plant Physiol., 117 (1998) 931), these compounds, which are synthesized at the endoplasmic reticulum level, were shown to be actively transported to the plasma membrane via a membrane-mediated process, together with phosphatidylserine (PS). In the present work, leek seedlings were allowed to germinate for 7 days in the presence of fenpropimorph, a sterol biosynthesis inhibitor. Such a treatment was found to trigger an almost complete replacement of the usual sterols by 9beta,19-cyclopropylsterols (mainly cycloeucalenol and 29-norcycloartenol). Extensive lipid analyses and labeling experiments with sodium [14C]acetate were performed to examine potential changes in the content and the rate of synthesis of the other lipid molecular species. The results indicate that the inhibition of the sterol pathway was accompanied by a severe decrease in PS and glucosylceramide synthesis as well as by a redirection of fatty acids toward the storage triacylglycerol pathway. Triacyglycerols are shown to accumulate concomitantly with a significant increase in intracellular lipid droplets in both aerial parts and roots of leek seedlings. Taken together, the present data emphasize that a coordinated regulation of the biosynthetic pathways of sterols and some specific lipid molecular species could take place during plant membrane biogenesis.

Effects of brefeldin A and nordihydroguaiaretic acid on endomembrane dynamics and lipid synthesis in plant cells

Effects of brefeldin A (BFA) and nordihydroguaiaretic acid (NDGA) on endomembrane structures and lipid synthesis were compared in maize root cells and tobacco Bright Yellow-2 cells. Immunofluorescence and electron microscopy studies showed that NDGA altered the structure and distribution of the endoplasmic reticulum (ER) within 1 h but not of the Golgi apparatus whereas, as shown previously, BFA altered that organization of the Golgi apparatus and, only subsequently, of the ER. Biochemical studies revealed that both drugs and especially BFA led to a strong inhibition of the phytosterol biosynthetic pathway: BFA led to accumulation of sterol precursors. The importance of phytosterols in membrane architecture and membrane trafficking is discussed.

The destination for single-pass membrane proteins is influenced markedly by the length of the hydrophobic domain

The tonoplast was proposed as a default destination of membrane-bound proteins without specific targeting signals. To investigate the nature of this targeting, we created type I fusion proteins with green fluorescent protein followed by the transmembrane domain of the human lysosomal protein LAMP1. We varied the length of the transmembrane domain from 23 to either 20 or 17 amino acids by deletion within the hydrophobic domain. The resulting chimeras, called TM23, TM20, and TM17, were expressed either transiently or stably in tobacco. TM23 clearly accumulated in the plasmalemma, as confirmed by immunoelectron microscopy. In contrast, TM17 clearly was retained in the endoplasmic reticulum, and TM20 accumulated in small mobile structures. The nature of the TM20-labeled compartments was investigated by coexpression with a marker localized mainly in the Golgi apparatus, AtERD2, fused to a yellow fluorescent protein. The strict colocalization of both fluorescent proteins indicated that TM20 accumulated in the Golgi apparatus. To further test the default destination of type I membrane proteins, green fluorescent protein was fused to the 19-amino acid transmembrane domain of the plant vacuolar sorting receptor BP-80. The resulting chimera also accumulated in the Golgi instead of in post-Golgi compartments, where native BP-80 localized. Additionally, when the transmembrane domain of BP-80 was lengthened to 22 amino acids, the reporter escaped the Golgi and accumulated in the plasma membrane. Thus, the tonoplast apparently is not a favored default destination for type I membrane proteins in plants. Moreover, the target membrane where the chimera concentrates is not unique and depends at least in part on the length of the membrane-spanning domain.

Phosphatidylserine delivery to endoplasmic reticulum-derived vesicles of plant cells depends on two biosynthetic pathways

Vesicles formed from endoplasmic reticulum (ER) by a cell-free system of leek cells (Allium porrum) are enriched in phosphatidylserine (PS), especially species containing very long chain fatty acids (VLCFA, at least 20 carbon atoms). In plant cells, PS is formed either by PS synthase or the serine exchange enzyme, although it is not known which pathway(s) contribute(s) to PS delivery in the ER-derived vesicles (EV), nor to what extent this occurs. Taking advantage of a cell-free system, we have shown that PS enrichment originates mainly from the serine exchange enzyme which is the only pathway that synthesizes the VLCFA-PS species. On the other hand, both enzymes synthesize PS with long chain fatty acids (up to 18 carbon atoms), but these species are given to the EV by PS synthase.

One of the origins of plasma membrane phosphatidylserine in plant cells is a local synthesis by a serine exchange activity

In plant cells, as in animal cells, the endoplasmic reticulum (ER) is considered to be the major site of phospholipid synthesis, and it has been shown that phosphatidylserine (PS) reaches the plasma membrane via the vesicular ER-Golgi-plasma membrane pathway in leek cells. However, it has never been determined whether the plasma membrane of leek cells is able to synthesize PS. We have analyzed the distribution of PS synthesizing enzymes along the vesicular pathway. In ER, Golgi and plasma membrane fractions isolated from leek cells, we have measured the activity of the two biosynthetic pathways leading to the synthesis of PS, i.e. serine exchange and CTP cytidylyltransferase plus PS synthase. We have found a high serine exchange activity in the plasma membrane fraction, and then determined that this membrane is able to synthesize both long chain fatty acid- and very long chain fatty acid-containing PS. Therefore, the PS in the plasma membrane of leek cells has two different origins: the intracellular vesicular pathway from the ER and a local synthesis in the plasma membrane.

Endomembranes and vesicle trafficking

Over the past year extensive analyses of the accumulated data on the structural and functional organisation of the endomembrane system and vesicular trafficking in higher plants have shown it to be far more complex than previously anticipated. The availability of molecular tools combined with new opportunities to visualise endomembrane dynamics in vivo will allow better understanding of the fundamental processes underlying the complexity of endomembrane behaviour and vesicular trafficking.

Characterization of microsome-associated tobacco BY-2 centrins

Centrin - higher plants - MTOCs - microtubules nucleation In most eukaryotic cells, the Ca(2+)-binding protein centrin is associated with structured microtubule-organizing centers (MTOCs) such as centrosomes. In these cells, centrin either forms centrosome-associated contractile fibers, or is involved in centrosome biogenesis. Our aim was to investigate the functions of centrin in higher plant cells which do not contain centrosome-like MTOCs. We have cloned two tobacco BY-2 centrin cDNAs and we show that higher plant centrins define a phylogenetic group of proteins distinct from centrosome-associated centrins. In addition, tobacco centrins were found primarily associated with microsomes and did not colocalize with gamma-tubulin, a known MTOC marker. While the overall level of centrin did not vary during the cell cycle, centrin was prominently detected at the cell plate during telophase. Our results suggest that in tobacco, the major portion of centrin is not MTOC-associated and could be involved in the formation of the cell plate during cytokinesis.