Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Organ-on-a-chip wikipedia , lookup
Signal transduction wikipedia , lookup
Cell culture wikipedia , lookup
Cellular differentiation wikipedia , lookup
Extracellular matrix wikipedia , lookup
Endomembrane system wikipedia , lookup
Cell growth wikipedia , lookup
Programmed cell death wikipedia , lookup
414 Opinion TRENDS in Plant Science Vol.6 No.9 September 2001 Pectin methylesterases: cell wall enzymes with important roles in plant physiology show that PMEs can be analysed at the tissue level using new methods such as microanalysis on 25-µm cryosections6,7 or a gel diffusion assay8,9. These studies have also shown the importance of the acidic PMEs in plant physiology6,9. Acidic PMEs are barely detectable among proteins extracted from isolated cell walls, whereas they are abundant in proteins eluted directly from tissues10 or from infiltrated organs11. Because acidic isoforms are probably only weakly adsorbed onto the cell wall components, soluble-protein extraction procedures increase their recovery dramatically6. These observations suggest that some of the failure to detect acidic isoforms in higher plants might be related to the experimental conditions used for protein extraction. Fabienne Micheli Pectin methylesterases: a multigene family Pectin methylesterases catalyse the demethylesterification of cell wall polygalacturonans. In dicot plants, these ubiquitous cell wall enzymes are involved in important developmental processes including cellular adhesion and stem elongation. Here, I highlight recent studies that challenge the accepted views of the mechanism and function of pectin methylesterases, including the co-secretion of pectins and pectin methylesterases into the apoplasm, new action patterns of mature pectin methylesterases and a possible function of the pro regions of pectin methylesterases as intramolecular chaperones. The plant cell wall is an intricate structure involved in the determination of cell size and shape, growth and development, intercellular communication, and interaction with the environment1. The primary cell wall is largely composed of polysaccharides (cellulose, hemicelluloses and pectins), enzymes and structural proteins. Pectins are a highly heterogeneous group of polymers that includes homogalacturonans and rhamnogalacturonans I and II (Ref. 2). Pectins form ~35% of the dry weight of dicot cell walls. Pectins are also abundant in gymnosperm cell walls, but less so in the walls of grasses3. It is widely accepted that pectins are polymerized in the cis Golgi, methylesterified in the medial Golgi and substituted with side chains in the trans Golgi cisternae4. Pectins are secreted into the wall as highly methylesterified forms. Subsequently, they can then be modified by pectinases such as pectin methylesterases (PMEs), which catalyse the demethylesterification of homogalacturonans releasing acidic pectins and methanol (Fig. 1). Biochemical study of pectin methylesterases Fabienne Micheli Laboratoire de Biologie Moléculaire des Relations Plantes–Microorganismes, INRA–CNRS, BP27, 31326 Castanet-Tolosan Cedex, France. e-mail: [email protected] Historically, the first PME analyses were made on PME isoforms purified from plant cell walls using standard conditions of extraction, assay and electrophoretic migration from large amounts of material5. Most of these purified PME isoforms have neutral or alkaline isoelectric points (pIs), and only a few studies have revealed the presence of acidic pectin methylesterases5. However, recent studies http://plants.trends.com In the past five years, it has been shown that the several PME isoforms detected in cell walls are encoded by a multigene family12,13. Recently, the systematic sequencing of the Arabidopsis genome has greatly contributed to the identification of the 67 PME-related genes in this species14. The PME genes encode pre-pro-proteins that have peptide motifs considered to be signatures of PMEs. The pre-region or signal peptide is required for protein targeting to the endoplasmic reticulum. The pro-PME is secreted to the apoplasm via the cis, medial and trans Golgi cisternae, and the trans Golgi network, and only the mature part of the PME (without the pro region) is found in the cell wall. According to recent data from the systematic sequencing of the Arabidopsis genome14, PME genes can be divided into two classes. Genes in the first class contain only two or three introns and a long pro region, and genes in the other class contain five or six introns and a short or nonexistent pro region; these two classes have been called type I and type II, respectively (L. Richard, pers. commun.). The type II sequences have a structure close to that of the PMEs identified in phytopathogenic organisms (bacteria, fungi) and are involved in cell wall soaking during plant infection. Such data force us to consider the role and development of the PME pro region. An intramolecular chaperone? Because it has been observed that cell-wall-extracted PMEs do not have a pro region15 (Fig. 2c), cleavage of the pro region from the mature PME might occur either early on, before excretion of the mature PME into the apoplasm (Fig. 2a), or later (Fig. 2b). In the first case, the pro region might be degraded (Fig. 2d) or play a role either inside the cell (Fig. 2e) or in the apoplasm (Fig. 2f ). Although the role of the pro region is not known, several hypotheses are possible. It might play a role: (1) in the biological function of PMEs; (2) in targeting PMEs towards the cell wall; (3) as an intramolecular chaperone, allowing conformational folding of the mature part of the PME (Ref. 16); or (4) as an inhibitor of the enzyme activity 1360-1385/01/$ – see front matter © 2001 Elsevier Science Ltd. All rights reserved. PII: S1360-1385(01)02045-3 Opinion Fig. 1. Demethylesterification of pectins by pectin methylesterases (PME). TRENDS in Plant Science Vol.6 No.9 September 2001 COOCH3 O O 415 COOH OH O COOCH3 O OH OH 2 H2O 2 CH3OH O O OH O PME OH COOH O O OH OH O OH TRENDS in Plant Science carried out by the mature PME. However, at present there is no evidence that the pro region is correlated with the suggested roles (2) and (4). Although experimental evidence to support the view that the pro regions of PMEs are intramolecular chaperones is lacking, the arguments in Table 1 could be applied to PMEs, supporting the hypothetical role of the PME pro region in inhibiting the mature part of the protein and protein folding. Moreover, it can be hypothesized that the pro region could inhibit the mature part during its secretion to the apoplasm to prevent premature demethylesterification of pectins before their insertion in the cell wall (Fig. 3). This point is particularly interesting because the isoforms involved in microsporogenesis and pollen tube growth are type II PMEs related to bacterial PMEs (Refs 17,18). Comparison of PME action patterns between pollen, Pro-PME cis Medial Golgi trans (d) (a) (a) (e) (b) ? (f) Cytoplasm (c) ? Cell wall TRENDS in Plant Science Fig. 2. Hypotheses for pectin methylesterase (PME) excretion into the apoplasm and maturation of the protein. The cleavage of the pro region (red) from the mature PME (green) occurred either early on, before excretion of the mature PME into the apoplasm (a,c), or afterwards (b). In the first case, the pro region could be degraded (d) or play a role either inside the cell (e) or in the apoplasm (f). http://plants.trends.com during pollen tube growth, and phytopathogenic bacteria, during plant infection, show that both PMEs were involved in plant cell wall soaking19. This action pattern during ‘invasion’ could characterize PMEs that do not have a pro region. Indeed, this hypothesis agrees with the fact that the inhibition of PME activity was not necessary during protein excretion by bacteria, because they did not secrete pectins. Mode of action of mature pectin methylesterases After their integration into the cell wall, mature PMEs could have different modes of action. For 20 years, the commonly accepted hypothesis concerning the mode of action of PMEs on homogalacturonans was that they could act either randomly (as in fungi) or linearly (as in plants) along the chain of pectins20. When PMEs act randomly on homogalacturonans, the demethylesterification releases protons that promote the action of endopolygalacturonases21 and contribute to cell wall loosening. When PMEs act linearly on homogalacturonans, PMEs give rise to blocks of free carboxyl groups that could interact with Ca2+, so creating a pectate gel4. Because the action of endopolygalacturonases in such a gel is limited, this action pattern of PMEs contributes to cell wall stiffening. Because acidic PMEs were thought to be essentially confined to fungi, the simplest hypothesis was that random demethylesterification depended on acidic PMEs, whereas linear demethylesterification depended on alkaline PMEs. However, more recent studies have shown that PME activity also depends on pH and the initial degree of methylesterification of the pectins. Some isoforms can act randomly at acidic pH but linearly at alkaline pH. And, at a given pH, some isoforms are more effective than others on highly methylesterified pectins22,23. Moreover, PME activity is enhanced by cations: trivalent cations are more effective than bivalent cations, which are themselves more effective than monovalent cations24. Depending on their concentration, cations might also modify the affinity of PMEs for their substrate. In conclusion, the action pattern of mature PMEs is regulated in the cell wall by numerous factors that both reveal the complexity of the enzymes and challenge the simplistic hypothesis that divides the PMEs into two groups: ‘alkaline, linear demethylesterification’ and ‘acidic, random demethylesterification’ (Fig. 4). Opinion 416 TRENDS in Plant Science Vol.6 No.9 September 2001 Table 1. Comparison of the characteristics of intramolecular chaperones and pectin methylesterasesa Function Intramolecular chaperone Pectin methylesterases (PMEs)b Essential requirement for protein folding Yes ? Catalytic activity (in vivo reusability) No It has not been shown that PME activity is provided from the pro region. All active microsequenced PMEs were mature proteins49,50. ATP requirement No ? Interaction with the folded protein Competitive inhibitor The pro region could inhibit the mature part during its secretion to the apoplasm to prevent premature pectin demethylesterification before their insertion in the cell wall. Specificity for mediating folding Highly specific Pro regions of PMEs do not have high homologies and are variable in size and sequence according to the isoform, suggesting specificity of the pro region with respect to the corresponding mature part. Folding mechanism Numerous mechanisms depending on the protein and the conditions in which the folding is made ? Released via a proteolytic cleavage Comparison between polypeptidic sequences of cell wall extracted PMEs, and the open-reading-frame of the corresponding gene shows that the proteolytic cleavage of the pro region is made close to the RR(K)LL motif conserved in every plant PME (Ref. 50). This motif is highly conserved with the cleavage site (RRKRR) found in some intramolecular chaperones from animals51. Association with the protein to be folded Covalently attached to the N-terminus The mature part of PMEs is covalently attached to the pro region. aData from Ref. 16. marks indicate data not known. bQuestions Regulation of pectin methylesterase activity Pectins Pro-PME In addition to the possible role of the pro region as an inhibitor of PME activity during the secretory pathway, and to the regulation of the pectin demethylesterification in the cell wall, it has been shown that there are inhibitors of PME activity in the cell wall25,26. PME activity is also regulated by hormones. Auxin-induced PME activity increases cell wall extension and, as a result, water absorption by the cell27,28. Some contradictory results have been obtained about the role of abscisic acid (ABA) on PME regulation. For example, although ABA enhanced PME activity in tomato seeds8, it inhibited PME activity during seed germination in yellow cedar (Chamaecyparis nootkatensis)9. Moreover, in these cedar seeds, gibberellic acid (GA3) had a stimulatory effect on PME activity9. cis Medial Golgi trans ? ? Structure of pectin methylesterases Cytoplasm Cell wall Regulation (e.g. pH, ions) TRENDS in Plant Science Fig. 3. Hypothesis of co-secretion of the pectins and the pectin methylesterases (PME) into the apoplasm. In this scheme, the pro region (red) could inhibit the mature part (green) during its secretion to the apoplasm to prevent premature demethylesterification of pectins before their insertion into the cell wall. Methylesterified galacturonic acids are represented in blue and demethylesterified galacturonic acids in yellow. http://plants.trends.com A better understanding of the pectin–PME interaction could be obtained by overexpression followed by analysis of the structure of PMEs using X-ray crystallography. Thus, PMEs from phytopathogenic bacteria and fungi have been produced in heterologous prokaryotic29 and eukaryotic30 systems. The lack of data showing higher plant PME overexpression shows that, during export towards the apoplasm, plant PME isoenzymes probably undergo organism-specific post-translational processing that is necessary for their structural and functional integrity. As a consequence, the functional characterization of the PME-related genes identified to date is generally difficult and requires alternative methods based on the overexpression of the genes in heterologous plant systems31. Opinion Fig. 4. Modes of action of pectin methylesterases (PMEs). Depending on the cell wall properties, the mature PMEs (green) can act randomly (a), promoting the action of pH-dependent cell wall hydrolases such as endopolygalacturonases (PG) and contributing to cell wall loosening, or can act linearly (b), giving rise to blocks of free carboxyl groups that interact with bivalent ions (Ca2+), so ridigifying the cell wall. Methylesterified galacturonic acids are represented in blue and demethylesterified galacturonic acids in yellow. TRENDS in Plant Science Vol.6 No.9 September 2001 417 Cytoplasm Cell wall ? (a) Ca2+ Regulation by pH, ions, pectin methylesterification degree PG (–) (+) (b) Ca2+ PG (+) (–) Ca2+ Ca2+ Ca2+ Ca2+ Ca2+ Ca2+ Ca2+ Ca2+ Ca2+ Ca2+ Cell wall loosening Cell wall rigidification TRENDS in Plant Science In view of the difficulty of overexpressing a plant PME gene in a heterologous system, the structure of a plant PME protein has been predicted by in silico methods32. This model, established by comparison with the three-dimensional structure of a crystallized pectin lyase from Aspergillus niger33, revealed that the PME is a β-helix protein that is characterized by secondary elements of parallel β sheets coiled into a large righthanded cylinder (Fig. 5). This model is closest to the http://plants.trends.com structures observed for other pectinases that have already been analysed crystallographically34–36 and has recently been confirmed by the preparation of crystals of bacterial PME (Ref. 37). Ubiquitous enzymes involved in many physiological processes Several studies have shown a strong correlation between PME activity or PME gene expression and 418 Fig. 5. Homology model of the three-dimensional structure of a plant pectin methylesterase. This is a β-helix protein characterized by secondary elements of parallel β sheets (yellow) coiled into a large righthanded cylinder and by a loop with α helix (red). Also shown are four highly conserved tryptophan residues (TRP, light-green) and four strictly conserved tyrosine residues (TYR, white), which are probably involved in binding the substrate. Three conserved phenylalanine residues (PHE, pink) are shown within the molecule, which are important in forming the hydrophobic core of the parallel β helix32. Acknowledgements I thank Nigel Chaffey (University of Bristol, UK) for critical reading of the manuscript. I am grateful to Renée Goldberg, Luc Richard and Marianne Bordenave (Université Pierre et Marie Curie, France) for helpful discussions during the past four years. Research support was provided by the commission of the European Communities, Agriculture and Fisheries (FAIR) specific RTD programme (CT 98-3972). It does not necessarily reflect the Commission’s views and in no way anticipates the Commission’s future policy in this area. Opinion TRENDS in Plant Science Vol.6 No.9 September 2001 Pectin methylesterases in cellular adhesion During separation of the border cells of the root cap of pea, PME activity increases and is correlated with an increase in the amount of acidic pectin and a decrease in cell wall pH (Ref. 45). This study was performed using an antisense transgenic plant transformed with a PME gene (rcpme1) obtained by screening a root cDNA library46. Analysis of transgenic plants showed that rcpme1 expression is required for the maintenance of extracellular pH, elongation of the cells within the root tip and for cell wall degradation leading to border cell separation. Pectin methylesterases in stem elongation physiological processes such as fruit maturation38, microsporogenesis and tube pollen growth39,40, cambial cell differentiation6,41, seed germination9 and hypocotyl elongation15. Interestingly, two recent studies have shown unequivocally that a PME is a host-cell receptor for the tobacco mosaic virus (TMV) movement protein42,43. Thus, the interaction between the virus movement protein and the PME is required for viral cell-to-cell movement through plasmodesmata. One hypothesis proposed is that binding of the TMV movement protein interferes with PME activity, altering the cell wall ion balance and consequently inducing changes in the permeability of the plasmodesmata43. Furthermore, studies using mutants permit a more specific characterization of the physiological involvement of PMEs in plant development, as illustrated below. Pectin methylesterases as a methanol source In 1998, a close correlation was reported between PME activity and levels of methanol in fruit tissues from both wild-type tomato and a PME antisense mutant, indicating that PME is on the primary biosynthetic pathway for methanol production in tomato fruit44. Because methanol oxidation to CO2 could result in the incorporation of methanol carbon into metabolites via the Calvin–Benson cycle, PMEs could play an appreciable, albeit indirect, role in the photosynthetic metabolism of the plant. References 1 Carpita, N.C. and Gibeaut, D.M. (1993) Structural models of primary cell walls in flowering plants: consistency of molecular structure with the physical properties of the wall during growth. Plant J. 3, 1–30 2 Perez, S. et al. (2000) The three-dimensional structures of the pectic polysaccharides. Plant Physiol. Biochem. 38, 37–55 3 Varner, J.E. and Lin, L.S. (1989) Plant cell wall architecture. Cell 56, 231–239 4 Goldberg, R. et al. (1996) Methyl-esterification, deesterification and gelation of pectins in the primary cell wall. In Pectins and Pectinases (Progress in Biotechnology) (Vol. 14) (Visser, J. and Voragen, A.G.J., eds), pp. 151–172, Elsevier Science 5 Bordenave, M. (1996) Analysis of pectin methyl esterases. In Plant Cell Wall Analysis (Modern http://plants.trends.com 6 7 8 9 Transgenic potato plants were transformed with a Petunia-inflata-derived cDNA encoding a PME in sense orientation under a constitutive promoter. This showed that the apex of the stem contained less PME activity than the wild type. Furthermore, during the early stages of development, stems of transgenic plants elongated more rapidly than those of the wild type47. Perspectives and future directions Recent advances in biochemistry and molecular biology have revealed the large number of PME isoforms, which supports the complexity of their roles in plant development. Determining these roles in planta will be helped by knowledge of the overall number of PME genes in Arabidopsis (which should enable us to overcome the difficulties posed by the existence of PME multigene families) and being able to choose molecular strategies that have been adapted for each gene study (e.g. specific regions of genes for expression analysis or creating antisense plants). Another important component of future analysis is overexpressing plant PMEs in a heterologous system, which remains a challenge. Such overexpression should enable us to obtain PME crystals as well as monospecific anti-PME antibodies raised against a single isoform or against different parts of the PME protein. Such antibodies should enable us to visualize the secretion pathway of pro and mature PMEs as well as to co-localize pectins48 and PMEs in the cell wall. Methods of Plant Analysis) (Vol. 17) (Linskens, H.F. and Jackson, J.F., eds), pp. 165–180, Springer-Verlag Micheli, F. et al. (2000) Radial distribution pattern of pectin methylesterases across the cambial region of hybrid aspen at activity and dormancy. Plant Physiol. 124, 191–199 Micheli, F. et al. (2001) Cell walls of woody tissues: cytochemical, biochemical and molecular analysis of pectins and pectin methylesterases. In Wood Formation in Trees: Cell and Molecular Biology Techniques (Chaffey, N.J., ed.), pp. 179–200, Harwood Academic Publishers Downie, B. et al. (1998) A gel diffusion assay for quantification of pectin methylesterase activity. Anal. Biochem. 264, 149–157 Ren, C. and Kermode, A.R. (2000) An increase in pectin methyl esterase activity accompanies 10 11 12 13 14 dormancy breakage and germination of yellow cedar seeds. Plant Physiol. 124, 231–242 Lin, T.P. et al. (1989) Purification and characterization of pectinesterase from Ficus awkeotsang Makino achenes. Plant Physiol. 91, 1445–1453 Bordenave, M. and Goldberg, R. (1994) Immobilized and free apoplastic pectin methylesterases in mung bean hypocotyl. Plant Physiol. 106, 1151–1156 Richard, L. et al. (1996) Clustered genes within the genome of Arabidopsis thaliana encoding pectin methylesterase-like enzymes. Gene 170, 207–211 Micheli, F. et al. (1998) Characterization of the pectin methylesterase-like gene AtPME3: a new member of a gene family comprising at least 12 genes in Arabidopsis thaliana. Gene 220, 13–20 Kaul, S. et al. (2000) Analysis of the genome sequence of the flowering plant Arabidopsis thaliana. Nature 408, 796–815 Opinion TRENDS in Plant Science Vol.6 No.9 September 2001 15 Bordenave, M. and Goldberg, R. (1993) Purification and characterization of pectin methylesterases from mung bean hypocotyl cell walls. Phytochemistry 33, 999–1003 16 Shinde, U. et al. (1995) Propeptide mediated protein folding: intramolecular chaperones. In Intramolecular Chaperones and Protein Folding (Shinde, U. and Inouye, M., eds), pp. 1–9, Molecular Biology Intelligence Unit, RG Landes Company, Springer, Heidelberg, Germany 17 Mu, J.H. et al. (1994) Characterization of a pollenexpressed gene encoding a putative pectin esterase of Petunia inflata. Plant Mol. Biol. 25, 539–544 18 Qiu, X. and Erickson, L. (1995) A pollen-specific cDNA (P65, Accession No. U28148) encoding a putative pectin esterase in Alfalfa1 (PGR95–094). Plant Physiol. 109, 1127 19 Lord, E. (2000) Adhesion and cell movement during pollination: cherchez la femme. Trends Plant Sci. 5, 368–373 20 Markovic, O. and Kohn, R. (1984) Mode of pectin deesterification by Trichoderma reesei pectinesterase. Experientia 40, 842–843 21 Moustacas, A.M. et al. (1991) Pectin methylesterase, metal ions and plant cell-wall extension. The role of metal ions in plant cell-wall extension. Biochem. J. 279, 351–354 22 Catoire, L. et al. (1998) Investigation of the action patterns of pectin methylesterase isoforms through kinetic analyses and NMR spectroscopy. Implications in cell wall expansion. J. Biol. Chem. 50, 33150–33156 23 Denès, J.M. et al. (2000) Different action pattern for apple pectin methylesterase at pH 7.0 and 4.5. Carbohydr. Res. 327, 385–393 24 Schmohl, N. et al. (2000) Pectin methylesterase modulates aluminium sensitivity in Zea mays and Solanum tuberosum. Physiol. Plant. 109, 419–427 25 MacMillan, G.P. and Perombelon, M.C.M. (1995) Purification and characterization of a high pI pectin methyl esterase isoenzyme and its inhibitor from tubers of Solanum tuberosum subsp. tuberosum cv. Katahdin. Physiol. Mol. Plant Pathol. 46, 413–427 26 Camardella, L. et al. (2000) Kiwi protein inhibitor of pectin methylesterase. Amino-acid sequence and structural importance of two disulfide bridges. Eur. J. Biochem. 267, 4561–4565 27 Brian, W. and Newcomb, E.H. (1954) Stimulation of pectin methylesterase activity 28 29 30 31 32 33 34 35 36 37 38 39 of cultured tobacco pith by indole acetic acid. Physiol. Plant 7, 290–297 Yoda, S. (1958) Auxin action and pectin enzyme. Bot. Mag. Tokyo 71, 207–213 Laurent, F. et al. (2000) Overproduction in Escherichia coli of the pectin methylesterase A from Erwinia chrysanthemi 3937: one-step purification, biochemical characterization, and production of polyclonal antibodies. Can. J. Microbiol. 46, 474–480 Christgau, S. et al. (1996) Pectin methyl esterase from Aspergillus aculeatus: expression cloning in yeast and characterization of the recombinant enzyme. Biochem. J. 319, 705–712 Gaffé, J. et al. (1997) Characterization and functional expression of a ubiquitously expressed tomato pectin methylesterase. Plant Physiol. 114, 1547–1556 Goodenough, P. and Carter, C.E. (2000) The structure of pectin methylesterase: the relevance of parallel β helix proteins to xylem differentiation. In Cell and Molecular Biology of Wood Formation (Savidge, R. et al., eds), pp. 305–314, BIOS Scientific Publishers, Oxford, UK Mayans, O. et al. (1997) Two crystal structures of pectin lyase A from Aspergillus reveal a pH driven conformational change and striking divergence in the substrate-binding clefts of pectin and pectate lyases. Structure 5, 677–689 Pickersgill, R. et al. (1994) The structure of Bacillus subtilis pectate lyase in complex with calcium. Nat. Struct. Biol. 1, 717–723 Peterson, T.N. et al. (1997) The crystal structure of rhamnogalacturonase A from Aspergillus aculeatus: a right-handed parallel β-helix. Structure 5, 533–544 Pickersgill, R. et al. (1998) Crystal structure of polygalacturonase from Erwinia carotovora ssp. carotovora. J. Biol. Chem. 223, 24660–24664 Jenkins, J. et al. (2001) Three-dimensional structure of Erwinia chrysanthemi pectin methylesterase reveals a novel esterase active site. J. Mol. Biol. 305, 951–960 Kagan-Zur, V. et al. (1995) Differential regulation of polygalacturonase and pectin methylesterase gene expression during and after heat stress in ripening tomato (Lycopersicon esculentum Mill.) fruits. Plant Mol. Biol. 29, 1101–1110 Wakeley, P.R. et al. (1998) A maize pectin methylesterase-like gene, ZmC5, specifically expressed in pollen. Plant Mol. Biol. 37, 187–192 419 40 Futamura, N. et al. (2000) Male flower-specific expression of genes for polygalacturonase, pectin methylesterase and β-1,3-glucanase in a dioecious willow (Salix gilgliana Seemen). Plant Cell Physiol. 41, 16–26 41 Guglielmino, N. et al. (1997) Pectin immunolocalization and calcium visualization in differentiating derivatives from poplar cambium. Protoplasma 199, 151–160 42 Dorokhov, Y.L. et al. (1999) A novel function for a ubiquitous plant enzyme pectin methylesterase: the host-cell receptor for the tobacco mosaic virus movement protein. FEBS Lett. 461, 223–228 43 Chen, M. et al. (2000) Interaction between the tobacco mosaic virus movement protein and host cell pectin methylesterases is required for viral cell-to-cell movement. EMBO J. 19, 913–920 44 Frenkel, C. et al. (1998) Pectin methylesterase regulates methanol and ethanol accumulation in ripening tomato (Lycopersicon esculentum) fruit. J. Biol. Chem. 273, 4293–4295 45 Stephenson, M.B. and Hawes, M.C. (1994) Correlation of pectin methylesterase activity in root caps of pea with root border cell separation. Plant Physiol. 106, 739–745 46 Wen, F. et al. (1999) Effect of pectin methylesterase gene expression on pea root development. Plant Cell 11, 1129–1140 47 Pilling, J. et al. (2000) Expression of Petunia inflata pectin methyl esterase in Solanum tuberosum L. enhances stem elongation and modifies cation distribution. Planta 210, 391–399 48 Knox, J.P. (1997) The use of antibodies to study the architecture and developmental regulation of plant cell walls. Int. Rev. Cytol. 171, 79–120 49 Markovic, O. and Jörnvall, H. (1986) Pectinesterase. The primary structure of tomato enzyme. Eur. J. Biochem. 158, 455–462 50 Bordenave, M. et al. (1996) Pectin methylesterase isoforms from Vigna radiata hypocotyl cell walls: kinetic properties and molecular cloning of a cDNA encoding the most alkaline isoform. Plant Mol. Biol. 31, 1039–1049 51 Seidah, N.G. (1995) The mammalian family of subtilisin/kexin-like pro-protein convertases. In Intramolecular Chaperones and Protein Folding (Shinde, U. and Inouye, M., eds), pp. 181–203, Molecular Biology Intelligence Unit, RG Landes Company, Springer, Heidelberg, Germany Selected Trends pre-prints now available! Did you know that a selection of pre-print articles from the Trends journals are featured in every bi-weekly issue of our online sister title HMS Beagle (http://hmsbeagle.com) – and that access is free on BioMedNet.com? The pre-prints cover a broad range of interesting topics, including recent coverage of the human genome publications and rapid response articles to innovative research in cell biology, genetics, microbiology and all the life sciences. Recent articles include: Ubiquitin: more than just a signal for protein degradation, Cezary Wojcik Pre-printed from Trends in Cell Biology Plant microtubule-associated proteins: the HEAT is off in temperature-sensitive mor 1, Patrick J. Hussey and Timothy J. Hawkins Pre-printed from Trends in Plant Science http://plants.trends.com