Cloning and characterization of AtRGP1. A reversibly autoglycosylated arabidopsis protein implicated in cell wall biosynthesis

Class 1 reversibly glycosylated polypeptides are plasmodesmal-associated proteins delivered to plasmodesmata via the golgi apparatus

SE-WAP41, a salt-extractable 41-kD wall-associated protein that is associated with walls of etiolated maize (Zea mays) seedlings and is recognized by an antiserum previously reported to label plasmodesmata and the Golgi, was cloned, sequenced, and found to be a class 1 reversibly glycosylated polypeptide ((C1)RGP). Protein gel blot analysis of cell fractions with an antiserum against recombinant SE-WAP41 showed it to be enriched in the wall fraction. RNA gel blot analysis along the mesocotyl developmental axis and during deetiolation demonstrates that high SE-WAP41 transcript levels correlate spatially and temporally with primary and secondary plasmodesmata (Pd) formation. All four of the Arabidopsis thaliana (C1)RGP proteins, when fused to green fluorescent protein (GFP) and transiently expressed in tobacco (Nicotiana tabacum) epidermal cells, display fluorescence patterns indicating they are Golgi- and plasmodesmal-associated proteins. Localization to the Golgi apparatus was verified by colocalization of transiently expressed AtRGP2 fused to cyan fluorescence protein together with a known Golgi marker, Golgi Nucleotide Sugar Transporter 1 fused to yellow fluorescent protein (GONST1:YFP). In transgenic tobacco, AtRGP2:GFP fluorescence is punctate, is present only in contact walls between cells, and colocalizes with aniline blue-stained callose present around Pd. In plasmolyzed cells, AtRGP2:GFP remains wall embedded, whereas GONST1:YFP cannot be found embedded in cell walls. This result implies that the targeting to Pd is not due to a default pathway for Golgi-localized fusion proteins but is specific to (C1)RGPs. Treatment with the Golgi disrupting drug Brefeldin A inhibits Pd labeling by AtRGP2:GFP. Integrating these data, we conclude that (C1)RGPs are plasmodesmal-associated proteins delivered to plasmodesmata via the Golgi apparatus.

The anthracnose resistance locus Co-4 of common bean is located on chromosome 3 and contains putative disease resistance-related genes

The broadest based resistance to anthracnose of common bean ( Phaseolus vulgaris L.) is conferred by the Co-4 locus. We sequenced a bacterial artificial chromosome clone harboring part of the Co-4 locus of the bean genotype Sprite and assembled a single contig of 106.5 kb for functional annotation. This region contained five copies of the COK-4 gene that encodes for a serine threonine kinase protein previously mapped to the Co-4 locus and 19 novel genes with no similarity to any previously identified genes of common bean. Several putative genes of the Co-4 locus seemed to be expressed as they matched perfectly with bean expressed sequence tags. The expression of the COK-4 genes was assessed by reverse transcription (RT)-PCR, and a single 850-bp cDNA fragment was sequenced and compared with the genomic sequences of the COK-4 homologs. Although the COK-4 cDNA was isolated from a different bean cultivar, it showed high similarity (95%) to the exons of genes BA17 and BA21, suggesting that they were expressed. In a phylogenetic tree including all currently available Pto-like sequences from Phaseolus species, the COK-4 homologs formed a single cluster with the Pto gene, whereas two sequences from P. coccineus and all sequences of P. vulgaris formed two closely related clusters. The Co-4 locus was physically mapped to the short arm of bean chromosome 3, which corresponds to linkage group B8. This study represents a first step in gaining an understanding of the genomic organization of an anthracnose resistance locus of common bean and provides molecular data for comparative analysis with other plant species.

Regulation of self-glycosylation of reversibly glycosylated polypeptides from Solanum tuberosum

Reversibly glycosylated polypeptides (RGPs) belong to a family of self-glycosylating proteins believed to be involved in plant polysaccharide synthesis. The precise function of these enzymes remains to be elucidated. Our results showed that the RGP 38-kDa subunit is phosphorylated in potato extracts (Solanum tuberosum L.). An increase in the self-glycosylation of Solanum tuberosum RGP (StRGP) 38-kDa subunit was observed after alkaline phosphatase (AP) treatment. Our results suggest that phosphorylation of StRGP appears to regulate its self-glycosylation. It was determined that when the StRGP reaction was carried out in the presence of UDP-[(14)C]Glc as the sugar donor and then 1 mM UDP was added in a chase-out experiment, radioactive UDP-Glc was obtained indicating that StRGP reaction seems to be reversible. The anomeric configuration of transferred sugars to StRGP protein was also studied.

Subcellular localization and topology of beta(1-->4)galactosyltransferase that elongates beta(1-->4)galactan side chains in rhamnogalacturonan I in potato

The subcellular localization and topology of rhamnogalacturonan I (RG-I) beta(1-->4)galactosyltransferase(s) (beta[1-->4]GalTs) from potato ( Solanum tuberosum L.) were investigated. Using two-step discontinuous sucrose step gradients, galactosyltransferase (GalT) activity that synthesized 70%-methanol-insoluble products from UDP-[(14)C]Gal was detected in both the 0.5 M sucrose fraction and the 0.25/1.1 M sucrose interface. The former fraction contained mainly soluble proteins and the latter was enriched in Golgi vesicles that contained most of the UDPase activity, a Golgi marker. By gel-filtration analysis, products of 180-2000 Da were found in the soluble fraction, whereas in the Golgi-enriched fraction the products were larger than 80 kDa and could be digested with rhamnogalacturonan lyase and beta(1,4) endogalactanase to yield smaller rhamnogalacturonan oligomers, galactobiose and galactose. The endogalactanase requires beta(1-->4)galactans with at least three galactosyl residues for cleavage, indicating that the enzyme(s) present in the 0.25/1.1 M Suc interface transferred one or more galactosyl residues to pre-existing beta(1-->4)galactans producing RG-I side chains in total longer than a trimer. Thus, the beta(1-->4)GalT activity that elongates beta(1-->4)-linked galactan on RG-I was located in the Golgi apparatus. This beta(1-->4)GalT activity was not reduced after treatment of the Golgi vesicles with proteinase, but approximately 75% of the activity was lost after treatment with proteinase in the presence of Triton X-100. In addition, the beta(1-->4)GalT activity was recovered in the detergent phase after treatment of Golgi vesicles with Triton X-114. Taken together, these observations supported the view that the RG-I beta(1-->4)GalT that elongates beta(1-->4)galactan was mainly located in the Golgi apparatus and integrated into the membrane with its catalytic site facing the lumen.

Macromolecular Transport and Signaling Through Plasmodesmata

Plasmodesmata (Pd) are channels in the plant cell wall that in conjunction with associated phloem form an intercellular communication network that supports the cell-to-cell and long-distance trafficking of a wide spectrum of endogenous proteins and ribonucleoprotein complexes. The trafficking of such macromolecules is of importance in the orchestration of non-cell autonomous developmental and physiological processes. Plant viruses encode movement proteins (MPs) that subvert this communication network to facilitate the spread of infection. These viral proteins thus represent excellent experimental keys for exploring the mechanisms involved in intercellular trafficking and communication via Pd.

Proteomics of Medicago truncatula seed development establishes the time frame of diverse metabolic processes related to reserve accumulation

We utilized a proteomic approach to investigate seed development in Medicago truncatula, cv Jemalong, line J5 at specific stages of seed filling corresponding to the acquisition of germination capacity and protein deposition. One hundred twenty proteins differing in kinetics of appearance were subjected to matrix-assisted laser desorption ionization time of flight mass spectrometry. These analyses provided peptide mass fingerprint data that identified 84 of them. Some of these proteins had previously been shown to accumulate during seed development in legumes (e.g. legumins, vicilins, convicilins, and lipoxygenases), confirming the validity of M. truncatula as a model for analysis of legume seed filling. The study also revealed proteins presumably involved in cell division during embryogenesis (beta-tubulin and annexin). Their abundance decreased before the accumulation of the major storage protein families, which itself occurs in a specific temporal order: vicilins (14 d after pollination [DAP]), legumins (16 DAP), and convicilins (18 DAP). Furthermore, the study showed an accumulation of enzymes of carbon metabolism (e.g. sucrose synthase, starch synthase) and of proteins involved in embryonic photosynthesis (e.g. chlorophyll a/b binding), which may play a role in providing cofactors for protein/lipid synthesis or for CO2 refixation during seed filling. Correlated with the reserve deposition phase was the accumulation of proteins associated with cell expansion (actin 7 and reversibly glycosylated polypeptide) and of components of the precursor accumulating vesicles, which give rise to a trypsin inhibitor on maturation. Finally, we revealed a differential accumulation of enzymes involved in methionine metabolism (S-adenosyl-methionine synthetase and S-adenosylhomo-cysteine hydrolase) and propose a role for these enzymes in the transition from a highly active to a quiescent state during seed development.

Characterization of UDP-glucose:protein transglucosylase genes from potato

Many plant autocatalytic glycosyltransferases are implicated in plant polysaccharide biosynthesis. Cloning of cDNAs encoding potato (Solanum tuberosum L.) UDP-Glc:protein transglucosylase (UPTG, EC 2.4.1.112) and expression of the cDNA clone E11 in Escherichia coli have been previously reported. Here, we studied the functional expression of a second cDNA of the enzyme (E2 clone). Northern blots analysis, with specific cDNA probes for the two UPTG isoforms, showed a differential expression pattern of mRNA levels in different potato tissues. Moreover, both UPTG recombinant enzymes showed different kinetic parameters. The recombinant protein encoded by E2 clone has an apparent Imax for UDP-Xyl and UDP-Gal, significantly higher than for UDP-Glc. The Km values for UDP-Glc were 0.45-0.71 microM and the values for UDP-Xyl and UDP-Gal were slightly higher than that of the UDP-Glc (1.2-2.71 microM) for both UPTG recombinant enzymes. The present study revealed further evidence for the proposed role of UPTG in the synthesis of cell wall polysaccharide. It was found a correlation between UPTG transcript levels and the growing state of the tissues in which there was an active synthesis of cell wall components. Southern blot analysis indicates that at least three genes encoding UPTG are present in potato genome. Phylogenetic analysis of both UPTG recombinant proteins showed that they are members of the RGP subfamilies from dicots.

Arabinoxylan biosynthesis in wheat. Characterization of arabinosyltransferase activity in Golgi membranes

Arabinoxylan arabinosyltransferase (AX-AraT) activity was investigated using microsomes and Golgi vesicles isolated from wheat (Triticum aestivum) seedlings. Incubation of microsomes with UDP-[(14)C]-beta-L-arabinopyranose resulted in incorporation of radioactivity into two different products, although most of the radioactivity was present in xylose (Xyl), indicating a high degree of UDP-arabinose (Ara) epimerization. In isolated Golgi vesicles, the epimerization was negligible, and incubation with UDP-[(14)C]Ara resulted in formation of a product that could be solubilized with proteinase K. In contrast, when Golgi vesicles were incubated with UDP-[(14)C]Ara in the presence of unlabeled UDP-Xyl, the product obtained could be solubilized with xylanase, whereas proteinase K had no effect. Thus, the AX-AraT is dependent on the synthesis of unsubstituted xylan acting as acceptor. Further analysis of the radiolabeled product formed in the presence of unlabeled UDP-Xyl revealed that it had an apparent molecular mass of approximately 500 kD. Furthermore, the total incorporation of [(14)C]Ara was dependent on the time of incubation and the amount of Golgi protein used. AX-AraT activity had a pH optimum at 6, and required the presence of divalent cations, Mn(2+) being the most efficient. In the absence of UDP-Xyl, a single arabinosylated protein with an apparent molecular mass of 40 kD was radiolabeled. The [(14)C]Ara labeling became reversible by adding unlabeled UDP-Xyl to the reaction medium. The possible role of this protein in arabinoxylan biosynthesis is discussed.

Glucosylation activity and complex formation of two classes of reversibly glycosylated polypeptides

Reversibly glycosylated polypeptides (RGPs) have been implicated in polysaccharide biosynthesis. In plants, these proteins may function, for example, in cell wall synthesis and/or in synthesis of starch. We have isolated wheat (Triticum aestivum) and rice (Oryza sativa) Rgp cDNA clones to study the function of RGPs. Sequence comparisons showed the existence of two classes of RGP proteins, designated RGP1 and RGP2. Glucosylation activity of RGP1 and RGP2 from wheat and rice was studied. After separate expression of Rgp1 and Rgp2 in Escherichia coli or yeast (Saccharomyces cerevisiae), only RGP1 showed self-glucosylation. In Superose 12 fractions from wheat endosperm extract, a polypeptide with a molecular mass of about 40 kD is glucosylated by UDP-glucose. Transgenic tobacco (Nicotiana tabacum) plants, overexpressing either wheat Rgp1 or Rgp2, were generated. Subsequent glucosylation assays revealed that in RGP1-containing tobacco extracts as well as in RGP2-containing tobacco extracts UDP-glucose is incorporated, indicating that an RGP2-containing complex is active. Gel filtration experiments with wheat endosperm extracts and extracts from transgenic tobacco plants, overexpressing either wheat Rgp1 or Rgp2, showed the presence of RGP1 and RGP2 in high-molecular mass complexes. Yeast two-hybrid studies indicated that RGP1 and RGP2 form homo- and heterodimers. Screening of a cDNA library using the yeast two-hybrid system and purification of the complex by an antibody affinity column did not reveal the presence of other proteins in the RGP complexes. Taken together, these results suggest the presence of active RGP1 and RGP2 homo- and heteromultimers in wheat endosperm.

Isolation of a cotton RGP gene: a homolog of reversibly glycosylated polypeptide highly expressed during fiber development

A full-length cDNA encoding putative reversibly glycosylated polypeptide (RGP) was cloned from cotton fiber cells using differential display combined with rapid amplification of the cDNA ends. The gene, designated GhRGP1, contains an open reading frame of 1080 bp encoding a protein of 359 amino acids which has 78-86% identity with other plant RGPs. Northern blot analysis showed that the gene is preferentially expressed in fiber cells and its transcripts are abundant both at the primary cell wall elongation stage and at the later stage of secondary cell thickening, suggesting that GhRGP1 may be involved in non-cellulosic polysaccharide biosynthesis of the plant cell wall.

Beta-D-glycan synthases and the CesA gene family: lessons to be learned from the mixed-linkage (1-->3),(1-->4)beta-D-glucan synthase

Cellulose synthase genes (CesAs) encode a broad range of processive glycosyltransferases that synthesize (1-->4)beta-D-glycosyl units. The proteins predicted to be encoded by these genes contain up to eight membrane-spanning domains and four 'U-motifs' with conserved aspartate residues and a QxxRW motif that are essential for substrate binding and catalysis. In higher plants, the domain structure includes two plant-specific regions, one that is relatively conserved and a second, so-called 'hypervariable region' (HVR). Analysis of the phylogenetic relationships among members of the CesA multi-gene families from two grass species, Oryza sativa and Zea mays, with Arabidopsis thaliana and other dicotyledonous species reveals that the CesA genes cluster into several distinct sub-classes. Whereas some sub-classes are populated by CesAs from all species, two sub-classes are populated solely by CesAs from grass species. The sub-class identity is primarily defined by the HVR, and the sequence in this region does not vary substantially among members of the same sub-class. Hence, we suggest that the region is more aptly termed a 'class-specific region' (CSR). Several motifs containing cysteine, basic, acidic and aromatic residues indicate that the CSR may function in substrate binding specificity and catalysis. Similar motifs are conserved in bacterial cellulose synthases, the Dictyostelium discoideum cellulose synthase, and other processive glycosyltransferases involved in the synthesis of non-cellulosic polymers with (1-->4)beta-linked backbones, including chitin, heparan, and hyaluronan. These analyses re-open the question whether all the CesA genes encode cellulose synthases or whether some of the sub-class members may encode other non-cellulosic (1-->4)beta-glycan synthases in plants. For example, the mixed-linkage (1-->3)(1-->4)beta-D-glucan synthase is found specifically in grasses and possesses many features more similar to those of cellulose synthase than to those of other beta-linked cross-linking glycans. In this respect, the enzymatic properties of the mixed-linkage beta-glucan synthases not only provide special insight into the mechanisms of (1-->4)beta-glycan synthesis but may also uncover the genes that encode the synthases themselves.

Beta-D-glycan synthases and the CesA gene family: lessons to be learned from the mixed-linkage (1-->3),(1-->4)beta-D-glucan synthase

Cellulose synthase genes (CesAs) encode a broad range of processive glycosyltransferases that synthesize (1-->4)beta-D-glycosyl units. The proteins predicted to be encoded by these genes contain up to eight membrane-spanning domains and four 'U-motifs' with conserved aspartate residues and a QxxRW motif that are essential for substrate binding and catalysis. In higher plants, the domain structure includes two plant-specific regions, one that is relatively conserved and a second, so-called 'hypervariable region' (HVR). Analysis of the phylogenetic relationships among members of the CesA multi-gene families from two grass species, Oryza sativa and Zea mays, with Arabidopsis thaliana and other dicotyledonous species reveals that the CesA genes cluster into several distinct sub-classes. Whereas some sub-classes are populated by CesAs from all species, two sub-classes are populated solely by CesAs from grass species. The sub-class identity is primarily defined by the HVR, and the sequence in this region does not vary substantially among members of the same sub-class. Hence, we suggest that the region is more aptly termed a 'class-specific region' (CSR). Several motifs containing cysteine, basic, acidic and aromatic residues indicate that the CSR may function in substrate binding specificity and catalysis. Similar motifs are conserved in bacterial cellulose synthases, the Dictyostelium discoideum cellulose synthase, and other processive glycosyltransferases involved in the synthesis of non-cellulosic polymers with (1-->4)beta-linked backbones, including chitin, heparan, and hyaluronan. These analyses re-open the question whether all the CesA genes encode cellulose synthases or whether some of the sub-class members may encode other non-cellulosic (1-->4)beta-glycan synthases in plants. For example, the mixed-linkage (1-->3)(1-->4)beta-D-glucan synthase is found specifically in grasses and possesses many features more similar to those of cellulose synthase than to those of other beta-linked cross-linking glycans. In this respect, the enzymatic properties of the mixed-linkage beta-glucan synthases not only provide special insight into the mechanisms of (1-->4)beta-glycan synthesis but may also uncover the genes that encode the synthases themselves.

Cell wall biosynthesis: glycan containing oligomers in developing cotton fibers, cotton fabric, wood and paper

A series of oligomeric glycans can be extracted from the cell walls of developing cotton fibers with weak acid. Glycans that produce similar profiles on high pH anion chromatography with pulsed amperometric detection (HPAEC-PAD) are also found in a protein complex extracted from developing fibers and in amorphous aggregates found in association with immature fibers in developing, but not in mature cotton bolls. The quantity and composition of the glycans recovered from the carbohydrate-protein complex varies significantly with the time of day when the bolls are harvested. This diurnal variation is consistent with the hypothesis that secondary cell walls are deposited primarily at night. Incubation of re-hydrated cotton fibers in the presence of exogenous oligosaccharides, myo-inositol and glycerol substantially alters the apparent quantity of the oligomers extracted from the fibers. The same and similar glycans have also been extracted from cotton fabric, marine algae, various paper products and wood. While many of the oligomers isolated from the various cellulose sources display the same peaks by HPAEC-PAD, the specific number of oligomers and their relative quantities appear unique for each source of cellulosic material. Oligomeric glycans, as described in the preceding, are present in all cellulose sources that have been investigated. Their relative abundance changes in response to source, stage of development and other physiological variables. We hypothesize that the glycans are intermediates in the biological assembly of cellulose, and that their incorporation in cellulose is mediated by physicochemical and enzymatic mechanisms.

Sugar-nucleotide-binding and autoglycosylating polypeptide(s) from nasturtium fruit: biochemical capacities and potential functions

Polypeptide assemblies cross-linked by S-S bonds (molecular mass>200 kDa) and single polypeptides folded with internal S-S cross-links (<41 kDa) have been detected by SDS/PAGE in particulate membranes and soluble extracts of developing cotyledons of nasturtium (Tropaeolum majus L.). When first prepared from fruit homogenates, these polypeptides were found to bind reversibly to UDP-Gal (labelled with [(14)C]Gal or [(3)H]uridine), and to co-precipitate specifically with added xyloglucan from solutions made with 67% ethanol. Initially, the bound UDP-[(14)C]Gal could be replaced (bumped) by adding excess UDP, or exchanged (chased) with UDP-Gal, -Glc, -Man or -Xyl. However, this capacity for turnover was lost during incubation in reaction media, or during SDS/PAGE under reducing conditions, even as the glycone moiety was conserved by autoglycosylation to form a stable 41 kDa polypeptide. Polyclonal antibodies raised to a similar product purified from Arabidopsis bound to all the labelled nasturtium polypeptides in immunoblotting tests. The antibodies also inhibited the binding of nasturtium polypeptides to UDP-Gal, the uptake of UDP-[(14)C]Gal into intact nasturtium membrane vesicles and the incorporation of [(14)C]Gal into nascent xyloglucan within these vesicles. This is the first direct evidence that these polypeptides facilitate the channelling of UDP-activated sugars from the cytoplasm through Golgi vesicle membranes to lumenal sites, where they can be used as substrates for glycosyltransferases to synthesize products such as xyloglucan.

Arabidopsis Sec21p and Sec23p homologs. Probable coat proteins of plant COP-coated vesicles

Intracellular protein transport between the endoplasmic reticulum (ER) and the Golgi apparatus and within the Golgi apparatus is facilitated by COP (coat protein)-coated vesicles. Their existence in plant cells has not yet been demonstrated, although the GTP-binding proteins required for coat formation have been identified. We have generated antisera against glutathione-S-transferase-fusion proteins prepared with cDNAs encoding the Arabidopsis Sec21p and Sec23p homologs (AtSec21p and AtSec23p, respectively). The former is a constituent of the COPI vesicle coatomer, and the latter is part of the Sec23/24p dimeric complex of the COPII vesicle coat. Cauliflower (Brassica oleracea) inflorescence homogenates were probed with these antibodies and demonstrated the presence of AtSec21p and AtSec23p antigens in both the cytosol and membrane fractions of the cell. The membrane-associated forms of both antigens can be solubilized by treatments typical for extrinsic proteins. The amounts of the cytosolic antigens relative to the membrane-bound forms increase after cold treatment, and the two antigens belong to different protein complexes with molecular sizes comparable to the corresponding nonplant coat proteins. Sucrose-density-gradient centrifugation of microsomal cell membranes from cauliflower suggests that, although AtSec23p seems to be preferentially associated with ER membranes, AtSec21p appears to be bound to both the ER and the Golgi membranes. This could be in agreement with the notion that COPII vesicles are formed at the ER, whereas COPI vesicles can be made by both Golgi and ER membranes. Both AtSec21p and AtSec23p antigens were detected on membranes equilibrating at sucrose densities equivalent to those typical for in vitro-induced COP vesicles from animal and yeast systems. Therefore, a further purification of the putative plant COP vesicles was undertaken.