The ER Lumen

The desmotubule is now accepted to be a tightly furled membrane, running through the plasmodesma, in many cases appearing as an electron-dense rod (Lopez-Saez et al. 1966 Gunning and Hughes 1976 Overall et al. 1982 Olesen 1979 Gunning and Overall 1983 Robinson-Beers and Evert 1991 Ding et al. 1992b Waigmann et al. 1997). There is a layer of proteinaceous material either integral to, or closely associated with, the desmotubule. Ding and co-workers (1992b) have shown that the wall of the...

Modifications to Plasmodesmatal Structure

Potato Transverse Section

In importing tobacco leaf tissue, plasmodesmata between most cell types are simple and lack a central cavity (Ding et al. 1992b). However, occasionally, Fig. 2 a Transverse section through simple plasmodesmata connecting potato tuber parenchyma cells, showing the desmotubule (DT) with a central, electron-dense rod, the cytoplasmic sleeve (CS) and the plasma membrane (PM). Bar 20 nm. b Longitudinal section through simple plasmodesmata between the aleurone layer (Al) and the starchy endosperm...

Role of ER in the Formation of Plasmodesmata

In order to allow the differential functioning of cell types and tissues, the plant symplasm behaves as a system of operational subunits termed sym-plasmic domains (Erwee and Goodwin 1985 for reviews see Robards and Lucas 1990 Lucas et al. 1993 McLean et al. 1997 Ding et al. 1999 Ehlers and van Bel 1999 Lucas 1999). It is now recognised that the amount of sym-plasmic connectivity at different cell interfaces, although not identical, is precisely controlled (Ehlers and van Bel 1999) with the...

Golgi Independent Vacuolar Transport of NonKDELTailed Proteases

In addition to KDEL-tailed proteases, Arabidopsis RD21, a papain-type protease, and yVPE, an asparaginyl endopeptidase, are known to be vacuolar proteases which are transported to vacuoles in a Golgi-independent manner. RD21 and yVPE are localized in spindle-shaped vesicles, which are derived from ER ranging in size from 0.1-10 m, and the vesicles, termed ER-bodies, are known to fuse with vacuoles via a Golgi apparatus-independent pathway as for the KDEL-vesicles (Hayashi et al. 2001...

Calcium and Internal ER Regulation

It is well established that calcium works as an important cytosolic signal. In addition, the levels of calcium may be a potent switch for internal processes in various oganelles (Corbett and Michalak 2000 Rizzuto et al. 2004). The release and subsequent uptake of calcium during signalling events, creates continuous fluctuations of the available calcium in the organelle (Yu and Hinkle 2000). In animal cells, changes in the free ER calcium levels may control a variety of processes, e.g. protein...

Cytochrome P450 Enzymes

The cytochrome P450s are a large enzyme superfamily in plants. They are named after the 450-nm absorption maximum of the carbon monoxide-bound form of the protein. The P450 superfamily shows a great deal of amino acid sequence diversity, with some members having as little as 20 amino acid identity with each other. However, key amino acid residues and structures involved in binding the heme cofactor required for activity are conserved (Graham and Petersen 1999). Most plant P450s are likely to...

References

Alder NN, Shen Y, Brodsky J, Hendershot LM, Johnson AE (2005) The molecular mechanisms underlying BiP-mediated gating of the Sec61 translocon of the endoplasmic reticulum. J Cell Biol 168 389-399 Altpeter F, Popelka JC, Wieser H (2004) Stable expression of 1Dx5 and 1Dy10 high-molecular-weight glutenin subunit genes in transgenic rye drastically increases the polymeric glutelin fraction in rye flour. Plant Mol Biol 54 783-792 Alvim FC, Carolino SM, Cascardo JC, Nunes CC, Martinez CA, Otoni WC,...

Molecular Size Exclusion Limit

Open plasmodesmata allow the diffusive exchange of metabolites and small molecules (Tyree 1970 Tucker et al. 1989). The size of molecules able to pass through plasmodesmata determines the size exclusion limit (SEL) of the pore but should more accurately be referred to as the molecular exclusion limit (MEL) when it is measured in terms of the mass, rather than the physical dimensions, of the molecules involved. On the basis of investigations using predominantly fluorescently labelled dextrans...

The Desaturases of the ER

In the ER, oleic acid (18 1) is first desaturated to linoleic acid (18 2) by the action of oleate desaturase (FAD2) (Okuley et al. 1994). FAD2 desaturates oleic acid esterified to both the sn-1 and - 2 positions of PtdCho (Sperling et al. 1993). Linoleate, esterified to PtdCho maybe further desaturated to linolenate (18 3) by linoleate desaturase (FAD3) (Arondel et al. 1992). Both of the ER desaturases use cytochrome b5 and cytochrome b5 reductase to transfer electrons from nicotinamide adenine...

What is a BiP Ligand

BiP will not only associate to nascent polypeptides but also may retain affinity for certain folding intermediates that have been fully translocated, although it may yet have to acquire their final conformation or possibly assemble with other subunits to form complexes. How BiP binds to its ligands has been investigated with synthetic peptides, and a role of hydrophobic residues has been established (Blond-Elguindi et al. 1993a,b). This has led to the working hypothesis that BiP binds to...

RNA Localization in Animals and the Role of the ER

RNA localization is recognized as an important process in controlling the synthesis of proteins at specific sites within the cell. More than 100 messenger RNAs are now known to be targeted in a wide variety of eukaryotic cells. This process is essential for cell fate determination in yeast (Chartrand et al. 2001), during early vertebrate development (Bashurullar et al. 1998 Palacios and Johnston 2001), in polar cell growth of somatic cells (Ainger et al. 1997 Carson et al. 1998 Shestakova et...

Transport Through the Cytoplasmic Sleeve

It is now generally accepted that the majority of molecular transport through plasmodesmata takes place via the cytoplasmic sleeve, through the helical channels between the plasma membrane and the desmotubule. This of course includes molecules moving in association with the ER. In many cases it is assumed that molecules are moving through the cytoplasmic sleeve without requiring any specific interaction with components of the pore, i.e. non-selectively (Schulz 1999 Zambryski and Crawford 2000...

ER Bodies that Accumulate KDELTailed glucosidase

Green fluorescent protein (GFP) from luminescent jellyfish allows us to visualize various organelles in living cells and in real time. GFP fusion proteins have provided new insight into novel subcellular compartments and their dynamic changes. For in vivo observation of the ER, GFP should be fused with a signal peptide at the N-terminus and with an ER retention signal (H(K)DEL, Lys His-Asp-Glu-Leu) at the C-terminus. When the ER-targeted GFP is expressed in Arabidopsis, a number of bright...

Synthesis of Very LongChain Fatty Acids in the ER

Plants contain very-long-chain fatty acids (VLCFA), which are characterised by chain lengths of 20 or more carbon atoms and are either saturated or mo-nounsaturated. VLCFAs form components of epidermal lipids, sphingolipids or seed storage lipids. VLCFA are synthesised from 18 0- or 18 1-CoA by a process of elongation whereby two carbon atoms are added to the carboxyl end of the acyl chain. The elongase complex comprises four dissociable subunits, each possessing a unique enzymatic activity...

The ER Within Plasmodesmata

Oparka Scottish Crop Research Institute, Invergowrie, Dundee DD2 5DA, UK kwrigh scri.sari.ac.uk, kopark scri.sari.ac.uk Abstract The endoplasmic reticulum (ER) is an essential component of plasmodesmata, the membrane-lined pores that interconnect plant cells. The desmotubule which traverses the centre of a plasmodesma is formed from, and continuous with, the cortical ER. Whilst the exact role of the ER is only now being characterised, it is recognised that the ER...

ERES in Higher Plants

Vesiculation profiles at the ER in thin sections have only rarely been recorded in the literature dealing with higher plant ultrastructure (e.g. Craig and Stae-helin 1988 Staehelin 1997 Ritzenthaler et al. 2002 see also Fig. 4), suggesting that ERES in this cell type are less frequent than in mammalian cells. It could be that this is related to the fact that plants generally secrete less protein than animal cells. On the other hand, in those situations where large amounts of secreted or...

Control of Brassinosteroid Biosynthesis and Metabolism

There is evidence for a feedback regulation pathway from active BR on the expression of the BR biosynthetic enzymes (Bancos et al. 2002). In Arabidopsis, expression of the mRNAs of CYP90B1, CYP90C1, CYP90D1, CYP85A1 and CYP95A2 is increased in the BR-deficient mutant cpd (blocked at the CYP90A1-catalysed step of the BR biosynthesis pathway), and CYP90A1 expression is increased in the cbb3 mutant (cabbage3, an EMS mutant in CYP90A which does not affect CYP90A mRNA expression). In all cases...

Vacuolar Sorting Receptor on the Membrane of PAC Vesicles

Storage protein precursors are selectively taken into the PAC vesicles in two ways one is by aggregate sorting and the other is through receptor-dependent sorting. The former sorting mechanism is advantageous to maturing seeds that actively synthesize a large quantity of storage proteins as described above. The latter ensures proper delivery of the storage proteins by avoiding the mis-sorting of escaped molecules. What molecular mechanism is involved in the uptake of storage protein precursors...

The ER A Stress Responsive Compartment

Unfavourable radiation, water stress, severe temperature changes, wounding, and attacks by pathogens are possible environmental strain situations that plants have to cope with. The ER organization of affected cells may alter if the parameters involved in the signal transmitting cascade, like the proton and or Ca2+ concentration, change. Morphological variations, in principle, concern a dynamic shift between the two major ER modifications, tubules, and lamellar sheets, frequently preceded or...

DiAcidic Sequence Dxeexe Motifs

Export motifs based on acidic residues were initially characterized in studies of the transport of the vesicular stomatitis virus glycoprotein (VSV-G), which has served as a model secretory protein in studies on folding and export from the ER. VSV-G, a type I transmembrane protein that traffics to the cell surface, is abundantly expressed in VSV-infected cells and concentrated into ER-derived transport vesicles (Nishimura and Balch 1997 Doms et al. 1998). VSV-G possesses a cytoplasmically...

The Dependence of RNA Transport on Zip Codes

The cis elements or zip codes (Singer 1993) responsible for prolamine RNA targeting to the PB-ER were defined by studying the localization of RNAs coded by a series of GFP-prolamine cDNA genes in transgenic rice endosperm cells (Hamada et al. 2003b). GFP RNAs, which normally are localized to the cisternal ER, are redirected to the PB-ER when prolamine RNA sequences are attached to the 3'UTR. Deletion analysis of the prolamine sequences showed that two zip codes are required for PB-ER targeting....

Vacuolar Proteases and Their Trafficking Pathways Along the Endomembrane System

A major function of plant vacuoles is protein degradation and processing, and many vacuolar proteases have been identified as mediators for these proteolytic events. Recent proteomic analysis of vacuoles isolated from Ara-bidopsis rosette leaves indicated that at least 23 kinds of proteases, including cysteine, serine and aspartic proteases, exist in vacuoles (Carter et al. 2004). Papain-type proteases (EC3.4.22), which possess a cysteine residue at the active site forming a catalytic triad and...

The Cytoplasmic Sleeve

The plasma membrane within a plasmodesma is lined on its cytoplasmic face with electron-dense structures (Botha et al. 1993 Ding et al. 1992b Tilney et al. 1991). These globular particles are connected by elongated spokes to the particles embedded in the wall of the desmotubule (Fig. 1) and may regulate transport by expanding or contracting the cytoplasmic sleeve (Ding et al. 1992b Overall and Blackman 1996). Although the diameter of a plasmo-desma channel is 20-50 nm and the desmotubule is...

Role of Sari in Secretion

The small GTP-binding protein Sarlp is a crucial player in the formation of COPII-coated vesicles and in the uncoating of the vesicle. It is a small protein of about 20 kDa without any lipid modification. On the basis of the analysis of the yeast protein, the following scheme for the Sar1 GTPase cycle has been established (Lee et al. 2004 Watanabe and Riezman 2004). Firstly, the GTP-form of Sarlp is generated by the action of Sec12p, which is an integral membrane protein and functions as a...

Brassinosteroid Biosynthesis and Catabolism

The brassinosteroids (BRs) are a group of compounds closely related to the mammalian steroid hormones. While the existence of BRs had been known for some years, their functions in growth and development have only become clear following molecular genetics studies in Arabidopsis. Mutants in BR biosynthesis suggest that this hormone has important roles in cell elongation, xylem differentiation and photomorphogenesis. The BR biosynthesis pathway is a continuation of the sterol biosynthesis pathway...

ER Export Signals and Cargo Recognition

In order to be included into COPII-coated vesicles, transmembrane cargo proteins must interact with one or more components of the coat. Therefore, domains of Sari and the Sec23-Sec24 dimer should be available for binding to integral membrane cargo proteins (Bi et al. 2002). Indeed, different types of signals in the cytosolic tails of these proteins have been shown to bind to Sari-Sec23-Sec24 pre-budding complexes (Aridor et al. i998 Kuehn et al. i998), and therefore are held to act as ER export...

Incorporation of Modified Fatty Acids from Membrane Phospholipids into TAG

De novo synthesised oleate may be modified by desaturation reactions catalysed by FAD2 and FAD3 or other modifying enzymes depending on whether the species accumulates unusual fatty acids (Sect. 3.2). The 18 2 and 18 3, or otherwise modified fatty acids may then be transferred from PtdCho and the acyl-CoA pool (Fig. 1, step 7) by a freely reversible exchange reaction catalysed by acyl-CoA lyso-PtdCho acyltransferase (LPC-AT) the basic biochemistry is reviewed in (Frentzen 1993). The released...

The Specialised Case of Pore Plasmodesma Units

Plasmodesmata connecting companion cells (CC) to sieve elements (SE) are specifically modified to form specialised structures known as pore-plasmodesma units (PPU) (van Bel 1996) (Fig. 2d). The ER within these PPU is believed to play a vital role in the survival of SE and functioning of the phloem. The SE-CC complexes form the functional units of phloem and are specialised for the long-distance flow of solutes. Both cells originate from an unequal division of the SE-CC mother cell. Soon after...

The Individual Members

Signal Peptidase and the Removal of Signal Peptides The enzyme signal peptidase is possibly the first member of the ER helper community encountered by nascent secretory polypeptides when entering the ER. The enzyme removes co-translationally the signal peptide present at the N-terminus of soluble and type I membrane secretory proteins. The reasonable hypothesis that signal peptide removal is essential for correct protein folding is supported by studies on two natural mutations in the signal...

ERES in Mammalian Cells

In mammalian cells, especially cultured cells whose secretion modus is primarily of the constitutive type ERES are distributed uniformly over the surface of the ER (Hammond and Glick 2000 Stephens et al. 2000). In contrast to the Golgi apparatus which is typically perinuclear, the ER in these cells is mainly located in the cell cortex with the consequence that ER to Golgi trafficking in these cells has a long-distance component. A typical feature of mammalian cells is therefore the ERGIC (ER to...

Source and Transfer of Peroxisomal Membrane Phospholipids

Most of the data on the lipid composition of peroxisomal membranes comes from studies with plant peroxisomes (see Trelease 2002, and references therein). Phosphatidylcholine (PC) and phosphatidylethanolamine (PE) are the most common phospholipids in plant peroxisomal membranes. Interestingly, nonpolar lipids are also prevalent in plant membranes. Statements are often made that PC and PE are the most prevalent phospholipids in mammalian and yeast peroxisomal membranes, but studies supporting...

Transport Within the Desmotubule Lumen

In many cases plasmodesmata have been observed to be occluded by the proteins embedded in the desmotubule membrane, so it is envisaged that only a single water molecule could pass (Overall et al. 1982). However, other plasmodesmata have been observed to have an open desmotubule lumen. It appears that the open lumen in plasmodesmata of some trichomes may be a particular adaptation to their secretory role. In chickpea (Cicer arietinum) trichomes there appears to be a specialised system of...

Secondary Plasmodesmata

By definition, secondary plasmodesmata are formed de novo across existing cell walls (Ehlers and Kollmann 2001). They cannot be distinguished from primary plasmodesmata on the basis solely of their structure, since they also may display simple or branched architectures (Ding et al. 1992b), and therefore can only be identified unambiguously by their location. At the time of their formation some secondary plasmodesmata, like primary plasmodes-mata, have a simple structure that may subsequently be...

Primary Plasmodesmata

Biogenesis Plasmodesmata

Based on where and how they are formed, plasmodesmata are categorised as either primary or secondary. Primary plasmodesmata form during cytokinesis at sites where ER tubules cross the phragmoplast of a dividing cell (Fig. 2e) (Hepler 1982 Staehelin and Hepler 1996). ER tubules, surrounded by cytoplasm, are trapped among the fusing Golgi vesicles that deliver wall material to the developing cell plate (Fig. 3a). As the cytoplasmic bridges become increasingly constricted they develop into...

The ER and Plant Hormones

CSIRO Plant Industry, GPO Box 1600, 2601 Canberra,ACT, Australia chris.helliwell csiro.au Abstract A large number of reactions in hormone biosynthesis and catabolism pathways are located in the endoplasmic reticulum (ER). These reactions are catalysed by cytochrome P450s, a large family of enzymes involved in many metabolic pathways in plants. As well as being involved in hormone synthesis and inactivation, many of the P450s involved in hormone biology are likely to be under regulation both to...

BiP The Midwife of the Secretory Pathway

It was discovered over a decade ago that reticuloplasmins (proteins residing in the ER lumen) were required for the completion of translocation (Nicchitta and Blobel 1993). One of these is BiP, shown to be essential for translocation of nascent proteins across the ER membrane in vivo (Vogel et al. 1990). BiP was later shown to carry out a crucial role in Sec61-mediated protein translocation (Brodsky et al. 1993 Brodsky and Schekman 1993 Lyman and Schekman 1995, 1997 Young et al. 2001). This...

Evidence for Copii Proteins in Plants

In plants, the analysis of COPII-mediated transport from the ER is a little behind that of yeast and mammals. Earlier complementation work of yeast sec12ts mutants with Arabidopsis cDNA was the starting point of the characterization of plant genes for COPII proteins (d'Enfert et al. 1992). Both Arabidopsis Sec12 and Sar1 cDNAs were identified in this study. Further identification and characterization of these proteins was carried out in planta (Bar-Peled and Raikhel 1997) or with yeast mutants...

Energy Consumption by BiP

ATP-mediated release suggests an energy-dependent release mechanism, and consistently, dominant negative BiP ATPase mutants that fail to release their ligands compromise proper protein folding (Hendershot et al. 1996). Lig-and binding is thought to be regulated by a communication between the ATPase domain and the peptide binding domain the presence of ATP in the N-terminal binding site produces rapid low-affinity binding to ligands, whereas the ADP-bound form exhibits high-affinity binding with...

Induction of PAC Vesicles in Vegetative Tissues

PAC vesicles are specific to maturing seeds, because they accumulate seed storage protein precursors. Unexpectedly, however, PAC vesicles can be induced in vegetative tissues (Hayashi et al. 1999). Figure 4a and b shows electron micrographs of the rosette leaves of transgenic Arabidopsis plants, which overexpress a fusion protein composed of a small subunit of pumpkin 2S albumin and a selectable marker enzyme, phosphinothricin acetyltrans-ferase. The leaves develop PAC vesicles together with...

Oleosins in Tapetum Cells and the Novel Organelle Tapetosome

The presence of oleosins in tapetum cells of anthers in Arabidopsis and Brassica was discovered a decade ago from unintended gene cloning results (deOliverira et al. 1993 Roberts et al. 1994). The finding was unexpected because tapetum cells were not known to contain OBs similar to those in seeds. Subsequently, these oleosins were found to be present in a novel, neutral lipid-containing organelle, which has been termed the tapetosome because of its unique presence in the tapetum of plants (Wu...

Brome Mosaic Virus

BMV is the type-member of the genus Bromovirus within the family Bro-moviridea. Its replication cycle has been carefully reviewed in recent years Noueiry and Ahlquist 2003 . Its genome consists of three positive-sense RNAs designated RNA 1, RNA 2, and RNA 3. All RNAs possess a cap structure at their 5' extremities and contain a conserved 200-nucleotide tRNA-like structure that can be acylated by tyrosine at their 3'ends Ahlquist 1992 Ahola and Ahlquist 1999 . The monocistronic RNAs 1 and 2...

DiAromatic or DiHydrophobic Motifs

Another type of transport ER exit signal for membrane cargo consists of a pair of bulky hydrophobic residues and have therefore been described as di-aromatic or di-hydrophobic motifs. An example of a protein bearing this type of signal is the membrane protein ERGIC53, a putative cargo receptor for some glycoproteins which cycle between the ER and Golgi compartments Appenzeller et al. 1999 . This type I transmembrane protein possesses a cytoplasmic tail sequence of 16 residues that is required...

ER Morphology and the Changing Demands of the Cell Cycle

While ultrastructural methods are suitable for studying the details of ER structure in all cell types, the use of fluorochromes such as the vital stain 3,3 -dihexyloxacarbocyanine iodide, DiOC6 3 Terasaki et al. 1984 and GFP technology Haseloff et al. 1997 are superior in visualizing dynamic changes in ER organization in three dimensions. Although DiOC6 3 and other fluorochromes appear to stain the great majority of different ER domains in plant cells, there is still no proof that all domains...

ER Morphology and the Cytoskeleton

Many metabolic pathways in plant cells are under strict spatial control, as is apparent from distinct organelle distributions which are guaranteed by the two major cytoskeletal elements microtubules MTs and actin filaments AFs . MTs and AFs function in organelle patterning and motility as relatively stiff tracks along which the organelles are transported via specific motor proteins like dynein or kinesin for MTs , and myosin for AFs . The controlled arrangement of the ER membrane meshwork...