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Calreticulin and the Endoplasmic Reticulum in Plant Cell Biology

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Calreticulin is ubiquitously expressed in plants. The plant homologue shares with its animal counterpart a similar structural organization and basic functioning. A wide range of developmental and environmental stimuli differentially affect the expression of calreticulin in plant cells, highlighting its importance in cell physiology. Nevertheless, current knowledge on calreticulin's relevance in plant physiology is rather limited compared with animal systems. The contribution of the endoplasmic reticulum to Ca2+ homeostasis and signalling, and the multifunctional role of calreticulin in plant cellular events are rapidly emerging areas of study in plant biology.


Plant calreticulin appeared late in the cell biology field. Early information on this protein involved biochemical characterization and DNA sequencing, mainly in different species of higher plants. It was rapidly established that calreticulin is ubiquitous in plant cells. All green organisms in the evolutionary tree, from algae to higher plants, express calreticulin. Within the complex body of higher plants, all cell types examined to date, both meristematic and mature, constitutively express calreticulin. Its relative abundance may be related to the greater extension of the endoplasmic reticulum in some specialized cells. Calreticulin is one of the most abundant proteins resident in the endoplasmic reticulum1 and it is highly stable with a relatively long lifetime (half-time about 26 h).2

In comparison with the rapid growth of information coming from the animal world, which stresses an increasingly complex role for calreticulin in cell physiology, the acquisition of knowledge regarding plant calreticulin is proceeding more slowly. It is conceivable that calreticulin in plant cells has the same functions as in all eukaryotic cells. It is less easy to assign to calreticulin any role exclusive to plants, and to correlate calreticulin with specific plant metabolism and behavior. This results, in part, from the lack of conclusive knockout experiments and from the lack of any strong evidence supporting a specific function for calreticulin in plants. Nevertheless, some insights are now emerging. In particular, the largely common involvement of Ca2+ as a second messenger in regulating the interactions of plants with their environment highlights the crucial participation of the endoplasmic reticulum in Ca2+ homeostasis and signalling, either together with, or alternatively to, the vacuole.

Characteristics of Plant Calreticulin

Plant calreticulin does not strictly follow the rule “one protein, one gene” assessed for its animal counterpart3 but is encoded by a small copy-number gene family.48 Its amino acid sequence and molecular structure are highly conserved among plants (about 80% similarity), whereas the similarity between animal and plant calreticulins is somewhat lower, at about 50%. The three domain organization and the overall biochemical characteristics, in particular the Ca2+-binding and lectin-binding properties, remain unchanged (castor bean calreticulin binds 15 mol Ca2+/mol protein).6 The major difference between animal and plant calreticulins is that potential N-glycosylation site(s) are actually occupied by glycan chain(s) in many plant species, and several potential phosphorylation consensus sequences for protein kinase CK2 (‘casein kinase-2’) are phosphorylated efficiently in plant calreticulin, in vitro.


A comparison of available calreticulin sequences indicates that several, but not all, plant calreticulins have consensus site(s) for N-glycosylation. The most conserved N-glycosylation site is located at position 32 in the N-domain. Additional N-glycosylation sites in the same region are present in Prunus armeniaca, Ricinus communis, Beta vulgaris, Nicotiana tabacum, Brassica napus and Arabidopsis thaliana. Moreover, calreticulin from Arabidopsis shows a third consensus site for N-glycosylation in the C-domain. Distinctively, in calreticulin from the algae Euglena and Chlamydomonas the N-glycosylation consensus sites are lacking. Evidence from N-glycan structural analyses,9,10 endoglycosidase H sensitivity,2,10 and Concanavalin A binding5,11,12 suggest that the N-glycans have a high mannose structure, compatible with localization of the protein in the endoplasmic reticulum. It is not known why some (but not all) plant calreticulins are N-glycosylated and what functional role can be assigned to the glycosylation. It may represent an additional property that favors calreticulin's folding during biosynthesis, and increases its stability.13 Clearly, the N-linked glycan chain(s) should not hinder calreticulin from acquiring its correct three-dimensional structure or from binding specific substrates at its lectin domain.14

The detectable presence, in Liriodendron tulipifera L. ovary, of calreticulin glycoforms bearing complex carbohydrate chains suggests that in this species calreticulin can travel up to the medial and trans-Golgi where the protein acquires specific sugar residues. Evidence for this traffic comes from immunodetection assays with anti-(1,2)xylose antibodies (Fig. 10.1; Faye and Fitchette-Lainè, personal communication). The monosaccharide composition of the N-linked glycan chains of L. tulipifera calreticulin has been recently investigated (Navazio et al. 2002, note added in proof ). Both tobacco2 and maize10 calreticulins have been shown to acquire competence for N-glycan maturation inside the Golgi compartment when treatment with brefeldin A induces redistribution of Golgi enzymes into the endoplasmic reticulum. These results indicate that calreticulin N-glycans are accessible to glycan-processing enzymes resident in the Golgi. The limited amount of data so far available does not allow an evaluation of the extent of the actual in vivo occurrence of complex glycan chain(s) on plant calreticulin.

Figure 1. Immunodetection with anti-spinach calreticulin (A) and anti-β(1,2)xylose (B) antibodies on protein extracts from Liriodendron tulipifera ovary (lane 1, 20 μg), spinach leaves (lane 2, 0.

Figure 1

Immunodetection with anti-spinach calreticulin (A) and anti-β(1,2)xylose (B) antibodies on protein extracts from Liriodendron tulipifera ovary (lane 1, 20 μg), spinach leaves (lane 2, 0.5 μg) and bean seeds (lane 3, 50 μg). (more...)


It has been demonstrated that calreticulin from spinach leaves and L. tulipifera ovary is phosphorylated, in vitro, by both exogenous and endogenous protein kinase CK2. The optimal consensus sites for phosphorylation by CK2 are located mainly in the C-terminus.15 Unpublished observations from our group indicate a similar behavior by calreticulin from tobacco and carrot suspension cultured cells and tobacco pollen tubes. Under the same experimental conditions, calreticulin from Euglena16 and Chlamydomonas17 is not a substrate for CK2; in both these algal calreticulins the potential phosphorylation sites are hindered by either basic or proline residues in proximity to the phospho-acceptor residue. Results to date suggest that phosphorylation by CK2 is limited to calreticulin in higher plants. Currently, there is no evidence for in vivo phosphorylation of calreticulin by CK2, and CK2 has not been localized within the plant endoplasmic reticulum. However, Cala18 has reported that in insect cells the reticuloplasmin GRP94, which is a substrate for CK2 in vitro, is also phosphorylated by this kinase in vivo. His data support the possibility that a CK2 isoform or a CK2-like protein is localized in the endoplasmic reticulum, strengthening the suggestion that phosphorylation of reticuloplasmins by CK2 may have physiological significance.

The possible relevance of calreticulin phosphorylation by CK2 (if confirmed in vivo) is at the moment purely speculative. Phosphorylation events could control and modulate the biological activity of the protein and/or be involved in the complex cellular signalling network. Droillard et al19 have demonstrated that the phosphorylation of tobacco calreticulin is modulated both in vitro and in vivo during signalling induced by elicitors such as cell wall pectic fragments. However, they did not identify which protein kinase(s) is responsible for the phosphorylation of calreticulin. In vitro experimental evidence indicates that phosphorylation of calreticulin by CK2 is significantly reduced at Ca2+ concentrations which nearly fully saturate the binding capacity of calreticulin without affecting normal CK2 activity (Baldan et al unpublished results). These results suggest that the Ca2+-binding activity of calreticulin could be negatively regulated through possible conformational control of its C-terminal tail, where both the phosphorylation sites and the low affinity, high capacity Ca2+-binding sites are located.

Intracellular Localization of Calreticulin

The retention of calreticulin in the endoplasmic reticulum largely depends on the C-terminal specific retention/retrieval signal. All plant calreticulins so far cloned contain the HDEL sequence, with the exception of Euglena calreticulin which has KDEL.16 In plant cells, as in yeast and mammalian cells, proteins that are resident in the endoplasmic reticulum can exit and recycle back via the K/HDEL-dependent retrieval mechanism, which is mediated by the membrane-bound ERD2 receptor.20 Recently, calreticulin has been detected in COPI-coated vesicles,21 confirming that an efficient mechanism for the retrieval of endoplasmic reticulum proteins from the Golgi compartment functions in plant cells. Apparently calreticulin can become competent for export from the endoplasmic reticulum, since a form of calreticulin, minus the HDEL motif, is transported in a COPII-dependent anterograde pathway.22 The ability of reticuloplasmins to interact and complex with other proteins resident in the endoplasmic reticulum, forming a large network, is considered to be partially responsible for their retention in this compartment. Calreticulin has been found in association with BiP in tobacco cells. These proteins form stable complexes with different molecular weights, in a Ca2+-independent way, probably with the participation of other reticuloplasmins.23

There are some indications that in plant cells endoplasmic reticulum domains can be specifically enriched in calreticulin. Recently, using immunocytochemistry in maize roots, calreticulin has been localized to the plasmodesmata, gateable cell-to-cell cytoplasmic channels which are unique to the plant body.24,25 These structures, frequently grouped into pit fields, span the cell wall, are lined by the plasma membrane and contain a cytoplasmic sleeve, coaxial to a central endoplasmic reticulum strand (desmotubule). A cytoplasmic continuum between adjacent cells is established through the plasmodesmata, and cell-to-cell communications are made possible. Endoplasmic reticulum is also continuous between cells, but along the entire length of the plasmodesmata the endoplasmic reticulum membranes are appressed and the lumen eliminated: the desmotubule is essentially a solid strand of lipids. Transport through the plasmodesmata is supposed to be regulated by an actin-myosin-based mechanism. The enrichment, in calreticulin, of the endoplasmic reticulum elements that are associated with the plasmodesmata, and its co-localization with myosin VIII, is suggestive that calreticulin participates in regulating plasmodesmal gating through modulation of local Ca2+ levels.25 Further support for this notion comes from the demonstration that Ca2+ is involved in the regulation of plasmodesmata permeability: the elevation of cytosolic Ca2+ concentration ([Ca2+]cyt) that results from cold shock26 or mastoparan27 induces a rapid closure of plasmodesmata. In tip-growing cells, such as pollen tubes and root hairs, a tip-high gradient of [Ca2+]cyt is generated and maintained during polarized cell growth. The gradient is regulated by influx of Ca2+ through channels located on the apical plasma membrane and by Ca2+ sequestration in endoplasmic reticulum elements that act as internal Ca2+ buffering stores. Calreticulin has been found to accumulate in the apical zone of maize growing root hairs28 and of Petunia pollen tubes,29 where the endoplasmic reticulum is very abundant and densely arranged.

Localization of calreticulin outside the endoplasmic reticulum has been reported in plant cells, specifically in the Golgi compartment (with a high abundance), in several small patches on the plasma membrane of Nicotiana plumbaginifolia protoplasts,30 and in protein bodies/protein storage vacuoles of rice endosperm cells.31 Calreticulin is totally absent from the vacuole, the major Ca2+ store in plant cells.32

Inducible Expression of Calreticulin

In plants, a number of different stimuli have been found to increase endogenous basal levels of expression of both calreticulin mRNA and protein. Moreover, calreticulin is regulated at the transcriptional level during different developmental stages of the life cycle.

The first suggestion that calreticulin may be important during fertilization events came from Chen et al4 who observed increased expression of the calreticulin gene in barley ovaries one day after pollination and during the early stages of embryogenesis. In subsequent developmental steps, the level of calreticulin mRNA returns to that in unpollinated ovaries. Similar data have been obtained in tobacco,1 maize,33,34 Arabidopsis thaliana,7 Ricinus communis6 and N. plumbaginifolia.30 The high levels of calreticulin observed in maize cells after fertilization, and in the immature embryos and floral tissues of tobacco, Arabidopsis and Ricinus, highlights modulation of calreticulin expression during plant reproduction. Northern blot assays have shown elevated expression of calreticulin mRNA during the early developmental stages of somatic embryos and ovules after fertilization of N. plumbaginifolia.30 Moreover, the unicellular chlorophyte Chlamydomonas reinhardtii shows enhanced levels of both calreticulin transcripts and protein in gametes compared with vegetative cells.17 It is unknown how calreticulin participates in plant reproductive processes, but cytosolic Ca2+ fluxes recognized to occur during animal reproduction enable the suggestion that calreticulin may act as a Ca2+-buffer in regulating [Ca2+]cyt. Indeed, during gamete differentiation Chlamydomonas cells accumulate Ca2+ in intracellular stores35 and maize sperm cells exposed to Ca2+ rapidly internalize the ion.34 Thus, Ca2+ seems to be necessary for gamete activation. Interestingly, during the differentiation of Chlamydomonas gametes it is possible to distinguish a pre-gamete phase when the cells are not able to mate (induced by a withdrawal of the nitrogen source), in which over-expression of calreticulin occurs with a simultaneous increase in BiP mRNA and protein. In the mature gametes, despite a further increase in expression of calreticulin, BiP expression remains constant. 17 The finding that a transient cytosolic Ca2+ increase triggers plant post-fertilization phases36 and that this corresponds with an up-regulation of the calreticulin gene, suggests important role(s) for calreticulin in both pre-fertilization and post-fertilization events in plant sexual reproduction.

The up-regulation of calreticulin expression is not limited to reproductive processes. An induction of calreticulin gene expression has been observed in proliferating and secreting tissues. 33,6,7 Significant accumulation of calreticulin transcripts has been found in meristematic regions such as root tips, nodes and leaf base,1,33,6 suggesting a possible role for calreticulin in plant cell division. Moreover, the high abundance of calreticulin mRNA in cells that are active in secretion has led to the hypothesis that calreticulin probably acts as a molecular chaperone in assisting the assembly of newly synthesized enzymes and/or secreted (glyco)proteins.1,6,7,30

The expression of the calreticulin gene in plant cells can be affected by different stresses. Denecke et al1 and Borisjuk et al30 demonstrated modulation of calreticulin expression by treatment with exogenous phytohormones. Barley aleurone cells treated with gibberellic acid have enhanced levels of the calreticulin transcript.1 N. plumbaginifolia cells show auxin-dependent changes in the amount of calreticulin,30 with increased protein expression in the presence of α-naphthaleneacetic acid and decreased transcript levels in the presence of 2,4-dichlorophenoxyacetic acid. Furthermore, studies on the regeneration of rice cultured suspension cells have shown increased transcription of the calreticulin gene related to the growth factors naphthaleneacetic acid and 6-benzyladenine.8 Interestingly, tunicamycin treatment of tobacco cells does not affect calreticulin expression, whereas BiP and PDI transcriptional levels are enhanced.1

Calreticulin has been implicated in signalling pathways specific to plants, such as the differential growth linked to the perception of gravity.37 Gravistimulation in maize plants has been found to induce several-fold increase in calreticulin and calmodulin transcripts, which preferentially accumulate in the stem pulvinus cells induced to respond to the gravity stimulus. An increased recruitment of calreticulin and calmodulin transcripts onto polyribosomes has also been observed, implying increased synthesis of these proteins and suggesting a role for them during the early stages of the gravity response.37 Pathogen attack causes plant defence responses which aim to combat the invader and prevent further invasion. This process occurs with the production of a range of defence-related proteins, most synthesized in the rough endoplasmic reticulum. It has been shown that expression of some lumenal proteins of the endoplasmic reticulum, including calreticulin, is induced during plant-pathogen interaction, probably as an early response necessary to enable the synthesis of pathogenesis-related proteins.1,38 Since both gravitropism and pathogen-activated signalling are known to be mediated by calcium ions,39,40 it is conceivable that the role played by calreticulin can be attributed to its potential to affect cellular Ca2+ homeostasis.

Altered growth conditions can interfere with endoplasmic reticulum functions leading to the up-regulation of genes encoding endoplasmic reticulum proteins. In maize roots, mannitol-induced osmotic stress and aluminum treatment both cause the deposition of callose at plasmodesmata pit fields with an increased expression of calreticulin at the sites of callose deposition.24,41 Calreticulin Ca2+ buffering and signalling might then be essential for structural and functional properties of plant cell plasmodesmata.

Overall, the spatial and temporal analyses of calreticulin's expression pattern highlight its importance as both a chaperone and a Ca2+-buffering and signalling protein.

Endoplasmic Reticulum in Plant Cell Physiology

Quality Control

As in all eukaryotic cells, plant endoplasmic reticulum provides a specialized environment promoting the folding, oxidation and oligomeric assembly of proteins. Plant endoplasmic reticulum is equipped with several folding enzymes, molecular chaperones, and folding sensors largely similar to those operating in all eukaryotes. The endoplasmic reticulum enables newly synthesized and properly folded proteins to access subsequent steps of the secretory pathway.

A very efficient quality control system that inhibits export of incompletely folded or misfolded proteins from the endoplasmic reticulum is active in plants. For example, endoplasmic reticulum quality control is involved in the proper maturation of phaseolin, the vacuolar storage glycoprotein of the common bean. When correctly assembled in a trimeric form, phaseolin is targeted to the vacuole. However, a defectively assembled form of phaseolin remains confined to the endoplasmic reticulum, extensively associated with BiP, and is eventually degraded.42 Similar evidence comes from experiments with a mutated form of the pea storage protein vicilin,43 and with zein polypeptides expressed in transgenic plants.44

The unfolded protein response results in the transcriptional up-regulation of a set of endoplasmic reticulum chaperones, and some other target genes. This response may also be triggered in plants: a variety of stresses leading to the accumulation of misfolded proteins in the endoplasmic reticulum increase the transcription of the BiP gene. Furthermore, over-expression of BiP in tobacco cells mitigates the endoplasmic reticulum stress.45

Among endoplasmic reticulum chaperones that have been identified in plants, BiP is one of the best characterized:46 its function(s) under both normal growth conditions and endoplasmic reticulum stress are well documented. A possible role as a molecular chaperone has also been attributed, in several circumstances, to calreticulin (see above in this chapter). In fact, the activity of the non classical chaperones47 (calreticulin and its related partner calnexin) is linked to their lectin binding properties. Evidence for the calnexin/calreticulin cycle in glycoprotein folding, which is so well characterized in mammals,14 is only circumstantial in plant cells: assembly of phaseolin is affected by its degree of glycosylation, as shown by the faster assembly rate observed when glucose-trimming by endoplasmic reticulum glucosidases is inhibited.48

Plant Endoplasmic Reticulum As a Ca2+ Store

The plant cell has several potential sites for Ca2+ accumulation (Fig. 10.2), although not all of them can be considered to be rapidly exchangeable Ca2+ pools.

Figure 2. Calcium stores in plant cells.

Figure 2

Calcium stores in plant cells.

In the cell wall, Ca2+-binding sites are mainly located on the pectic polymers and the [Ca2+] is estimated to be in the millimolar range.49 The [Ca2+] in chloroplasts and in the nucleus is controlled independently of the Ca2+ level in the cytosol: both chloroplasts and the nucleus generate their own Ca2+ signals, which are expected to regulate Ca2+-dependent processes within the two compartments.50,51,52 Information about the contribution of plant mitochondria to the Ca2+ network is still very scarce. Nevertheless, the recent imaging in animal cells of close contacts between mitochondria and the endoplasmic reticulum53 opens up the possibility that even in plant cells the spatial distribution of these organelles may allow microdomains of Ca2+ sensing.

The vacuole has, so far, been considered as the main intracellular Ca2+ store because of its large volume and key role in ion homeostasis in the plant cell. Evidence for several Ca2+ transporters and Ca2+ release channels in the vacuolar membrane (for a review see ref. 54) has further reinforced appreciation of the vacuole as a major stimulus-releasable reservoir of Ca2+. In the vacuole, owing to the low pH (pH 3–6) of the vacuolar sap, the Ca2+ buffering role may be carried out by organic or inorganic ions, and/or by Ca2+-binding proteins with properties different from Ca2+-binding reticuloplasmins. A low affinity, high capacity Ca2+-binding protein has been recently characterized in radish vacuole; its deduced amino acid sequence does not show any significant similarity with either calreticulin or other Ca2+-binding proteins. In view of its properties, this protein can be considered as a good candidate for Ca2+ buffering in the vacuole.55

Alongside the vacuole, the endoplasmic reticulum is increasingly being seen as an intracellular Ca2+ store that plays a potentially important role in Ca2+ signalling in plants. Ca2+ ATPases, inhibited by cyclopiazonic acid but not by thapsigargin, and differentially regulated by calmodulin56,57 are located in plant endoplasmic reticulum membranes. Different classes of Ca2+ permeable channels have been reported in plant endoplasmic reticulum, i.e., voltage-gated58,59 and ligand-gated, activated by the pyridine nucleotide derivatives nicotinic acid adenine dinucleotide phosphate (NAADP)60 and cyclic ADP-ribose (cADPR)61 and, possibly, by inositol 1,4,5-trisphosphate (InsP3).62 The occurrence of multiple Ca2+ release pathways suggests that the endoplasmic reticulum is not just a Ca2+ repository for the plant cell, but can be implicated in cell signalling as a mobilizable Ca2+ store. In keeping with this, plant endoplasmic reticulum contains calreticulin, an effective Ca2+ buffer that may allow the transient storage of the ion and its prompt mobilization when Ca2+ release is triggered.

Endoplasmic reticulum membranes form a dynamic, three-dimensional network, the distribution of which within the cell may fulfil localized requests for Ca2+. In plant cells the cortical endoplasmic reticulum, i.e., the endoplasmic reticulum underlying the plasma membrane, is highly developed and may function as a semi-immobile polygonal network along which movement of the Golgi stacks are driven by actin cables.63 Reuzeau et al64 have proposed that plant cortical endoplasmic reticulum physically attaches to the plasma membrane at adhesion sites through cytoskeletal proteins and transmembrane integrin-like proteins. Ion channels and signal receptors may also be clustered around these adhesion sites. Indeed, the close proximity and association between the cortical endoplasmic reticulum and the plasma membrane would allow ready access to signals emanating from the plasma membrane.

The source and/or location of Ca2+ signals helps to determine their specificity. In aequorin-transformed tobacco seedlings, signalling induced by cold shock triggers Ca2+ fluxes primarily at the plasma membrane, whereas mechanical stimulation involves elevations in [Ca2+] which derive from intracellular Ca2+ stores.65,66 The conclusive assessment of the direct participation of the endoplasmic reticulum in specific signal transduction pathways awaits accurate and reliable measurements of the Ca2+ concentration in the lumen of the endoplasmic reticulum ([Ca2+]ER) and of its variations during signalling.

Plant endoplasmic reticulum may be involved in the generation of Ca2+ oscillations in some specialized cell types, such as stomata guard cells and pollen tubes, in response to a wide range of stimuli.67 Repetitive Ca2+ release and Ca2+ re-uptake by the endoplasmic reticulum have been proposed to generate repetitive Ca2+ spikes in a unicellular green alga.68,69 In contrast to animal cells, there are only a few examples of Ca2+ waves in plants.54 In all cases, Ca2+ waves have been observed to propagate through regions containing endoplasmic reticulum but no large vacuoles, suggesting that the presence of a huge central vacuole in many plant cells may hamper propagation and detection of waves of elevated [Ca2+].70

Calreticulin and Ca2+ Signalling

The interrelationships between calreticulin and Ca2+ in the endoplasmic reticulum have been deeply investigated in animal cells. The emerging picture is that of a complex sensing-signalling network involving more than one role for calreticulin, including a lectin-like chaperone activity, interactions with other endoplasmic reticulum chaperones, regulation of [Ca2+]ER and participation in the endoplasmic reticulum signalling network.71 Nothing, or very little, is known about these issues in plant cells. Although the Ca2+ binding properties of plant calreticulin suggest a potential role in intracellular Ca2+ homeostasis, as in animal cells, conclusive evidence that calreticulin affects the Ca2+ status of the plant endoplasmic reticulum has only recently been obtained. Persson et al72 demonstrated that the over-expression of calreticulin in tobacco suspension cells affects the endoplasmic reticulum Ca2+ pool. Elevation of the calreticulin level, in microsomes enriched with endoplasmic reticulum membranes, resulted in increased ATP-dependent Ca2+ accumulation, and in increased Ca2+ release and Ca2+ retention after ionophore treatment. At present it is not known whether this effect is exerted via modulation of the activity of the endoplasmic reticulum Ca2+ ATPases and/or agonist-triggered Ca2+ channels, as shown in animal cells.73,74

Over-expression of calreticulin in planta enhances the survival of transgenic plants grown in a limiting, low Ca2+ medium.72 Furthermore, expression of the C-domain of calreticulin (targeted to the endoplasmic reticulum) in Arabidopsis enhances survival of seedlings on Ca2+-depleted medium75, supporting the hypothesis that the key factor helping cells to maintain their Ca2+ homeostasis under altered growth conditions is an increased Ca2+ buffering ability stemming from over-production of calreticulin.

Although calreticulin is highly conserved, constitutively present and ubiquitously distributed, the roles of the protein in plant cells have not been fully elucidated. However, despite the fundamental differences between plants and animals in their cellular organization, body plan and life style, a convergence of the physiological behavior of calreticulin as a multifunctional player in the eukaryotic kingdom is increasingly becoming apparent.

Note Added in Proof

The results concerning the characterization of glycan chains of L. tulipifera calreticulin, reported on page 96, have been recently published: Navazio L, Miuzzo M, Royle L et al. Monitoring endoplasmic reticulum-to-Golgi traffic of a plant calreticulin by protein glycosylation analysis. Biochemistry 2002; 41:14141–14149.


We are grateful to L. Faye (Mt. St. Aignan, France) for making available results on our collaborative work and to F. Meggio (Padova, Italy) for helpful discussion on calreticulin phosphorylation. Research in the authors' laboratory is supported by grants from Ministero dell' Universitß e della Ricerca Scientifica e Tecnologica.


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