Direct Photolabeling with [P]UDP-Glucose for Identification of a Subunit of Cotton Fiber Callose Synthase

The knowns and unknowns of callose biosynthesis in terrestrial plants

Callose, a linear (1,3)-β-glucan, is an indispensable carbohydrate polymer required for plant growth and development. Advances in biochemical, genetic, and genomic tools, along with specific antibodies, have significantly enhanced our understanding of callose biosynthesis. As additional components of the callose synthase machinery emerge, the elucidation of molecular biosynthetic mechanisms is expected to follow. Short-term objectives involve defining the stoichiometry and turnover rates of callose synthase subunits. Long-term goals include generating recombinant callose synthases to elucidate their biochemical properties and molecular mechanisms, potentially culminating in the determination of callose synthase three-dimensional structure. This review delves into the structures and intricate molecular processes underlying callose biosynthesis, emphasizing regulatory elements and assembly mechanisms.

Callose synthesis during reproductive development in monocotyledonous and dicotyledonous plants

Callose, a linear β-1,3-glucan molecule, plays important roles in a variety of processes in angiosperms, including development and the response to biotic and abiotic stress. Despite the importance of callose deposition, our understanding of the roles of callose in rice reproductive development and the regulation of callose biosynthesis is limited. GLUCAN SYNTHASE-LIKE genes encode callose synthases (GSLs), which function in the production of callose at diverse sites in plants. Studies have shown that callose participated in plant reproductive development, and that the timely deposition and degradation of callose were essential for normal male gametophyte development. In this mini-review, we described conserved sequences found in GSL family proteins from monocotyledonous (Oryza sativa and Zea mays) and dicotyledonous (Arabidopsis thaliana and Glycine max) plants. We also describe the latest findings on callose biosynthesis and deposition during reproductive development and discuss future challenges in unraveling the mechanism of callose synthesis and deposition in higher plants.

Sucrose synthase is an integral component of the cellulose synthesis machinery

Cellulose synthesis in plants is believed to be carried out by the plasma membrane-associated rosette structure which can be observed by electron microscopy. Despite decade-long speculation, it had not been demonstrated whether the rosette is the site of catalytic activity of cellulose synthesis. To determine the relationship between this structure and cellulose synthesis, we successfully isolated detergent-insoluble rosettes from the plasma membrane of bean epicotyls. However, the purified rosettes did not possess cellulose synthesis activity in vitro. Conversely, detergent-soluble granular particles of approximately 9.5-10 nm diameter were also isolated and exhibited UDP-glucose binding activity and possessed beta-1,4-glucan (cellulose) synthesis activity in vitro. The particle, referred to as the catalytic unit of cellulose synthesis, was enriched with a 78 kDa polypeptide which was verified as sucrose synthase like by mass spectrometry and immunoblotting. The catalytic units were able to bind to the rosettes and retained the cellulose synthesis activity in the presence of UDP-glucose or sucrose plus UDP when supplemented with magnesium. The incorporation of the catalytic unit into the rosette structure was confirmed by immunogold labeling with anti-sucrose synthase antibodies under an electron microscope. Our results suggest that the plasma membrane-associated rosette anchors the catalytic unit of cellulose synthesis to form the functional cellulose synthesis machinery.

Gene-rich islands for fiber development in the cotton genome

Cotton fiber is an economically important seed trichome and the world's leading natural fiber used in the manufacture of textiles. As a step toward elucidating the genomic organization and distribution of gene networks responsible for cotton fiber development, we investigated the distribution of fiber genes in the cotton genome. Results revealed the presence of gene-rich islands for fiber genes with a biased distribution in the tetraploid cotton (Gossypium hirsutum L.) genome that was also linked to discrete fiber developmental stages based on expression profiles. There were 3 fiber gene-rich islands associated with fiber initiation on chromosome 5, 3 islands for the early to middle elongation stage on chromosome 10, 3 islands for the middle to late elongation stage on chromosome 14, and 1 island on chromosome 15 for secondary cell wall deposition, for a total of 10 fiber gene-rich islands. Clustering of functionally related gene clusters in the cotton genome displaying similar transcriptional regulation indicates an organizational hierarchy with significant implications for the genetic enhancement of particular fiber quality traits. The relationship between gene-island distribution and functional expression profiling suggests for the first time the existence of functional coupling gene clusters in the cotton genome.

Proteomic and biochemical evidence links the callose synthase in Nicotiana alata pollen tubes to the product of the NaGSL1 gene

The NaGSL1 gene has been proposed to encode the callose synthase (CalS) enzyme from Nicotiana alata pollen tubes based on its similarity to fungal 1,3-beta-glucan synthases and its high expression in pollen and pollen tubes. We have used a biochemical approach to link the NaGSL1 protein with CalS enzymic activity. The CalS enzyme from N. alata pollen tubes was enriched over 100-fold using membrane fractionation and product entrapment. A 220 kDa polypeptide, the correct molecular weight to be NaGSL1, was specifically detected by anti-GSL antibodies, was specifically enriched with CalS activity, and was the most abundant polypeptide in the CalS-enriched fraction. This polypeptide was positively identified as NaGSL1 using both MALDI-TOF MS and LC-ESI-MS/MS analysis of tryptic peptides. Other low-abundance polypeptides in the CalS-enriched fractions were identified by MALDI-TOF MS as deriving from a 103 kDa plasma membrane H+-ATPase and a 60 kDa beta-subunit of mitochondrial ATPase, both of which were deduced to be contaminants in the product-entrapped material. These analyses thus suggest that NaGSL1 is required for CalS activity, although other smaller (<30 kDa) or low-abundance proteins could also be involved.

Biosynthesis of (1-->3)-beta-D-glucan (callose) by detergent extracts of a microsomal fraction from Arabidopsis thaliana

The aim of this work was to develop a biochemical approach to study (1-->3)-beta-D-glucan (callose) biosynthesis using suspension cultures of Arabidopsis thaliana. Optimal conditions for in vitro synthesis of callose corresponded to an assay mixture containing 50 mM Mops buffer, pH 6.8, 1 mM UDP-glucose, 8 mM Ca2+ and 20 mM cellobiose. The enzyme was Ca2+-dependent, and addition of Mg2+ to the reaction mixture did not favour cellulose biosynthesis. Enzyme kinetics suggested the existence of positive homotropic cooperativity of (1-->3)-beta-D-glucan synthase for the substrate UDP-glucose, in agreement with the hypothesis that callose synthase consists of a multimeric complex containing several catalytic subunits. Detergents belonging to different families were tested for their ability to extract and preserve membrane-bound (1-->3)-beta-D-glucan synthase activity. Cryo-transmission electron microscopy experiments showed that n-octyl-beta-D-glucopyranoside allowed the production of micelle-like structures, whereas vesicles were obtained with Chaps and Zwittergent 3-12. The morphology and size of the (1-->3)-beta-D-glucans synthesized in vitro by fractions obtained with different detergents were affected by the nature of the detergent tested. These data suggest that the general organization of the glucan synthase complexes and the properties of the in vitro products are influenced by the detergent used for protein extraction. The reaction products synthesized by different detergent extracts were characterized by infrared spectroscopy, methylation analysis, 13C-NMR spectroscopy, electron microscopy and X-ray diffraction. These products were identified as linear (1-->3)-beta-D-glucans having a degree of polymerization higher than 100, a microfibrillar structure, and a low degree of crystallinity.

A cell plate-specific callose synthase and its interaction with phragmoplastin

Callose is synthesized on the forming cell plate and several other locations in the plant. We cloned an Arabidopsis cDNA encoding a callose synthase (CalS1) catalytic subunit. The CalS1 gene comprises 42 exons with 41 introns and is transcribed into a 6.0-kb mRNA. The deduced peptide, with an approximate molecular mass of 226 kD, showed sequence homology with the yeast 1,3-beta-glucan synthases and is distinct from plant cellulose synthases. CalS1 contains 16 predicted transmembrane helices with the N-terminal region and a large central loop facing the cytoplasm. CalS1 interacts with two cell plate--associated proteins, phragmoplastin and a novel UDP-glucose transferase that copurifies with the CalS complex. That CalS1 is a cell plate--specific enzyme is demonstrated by the observations that the green fluorescent protein--CalS1 fusion protein was localized at the growing cell plate, that expression of CalS1 in transgenic tobacco cells enhanced callose synthesis on the forming cell plate, and that these cell lines exhibited higher levels of CalS activity. These data also suggest that plant CalS may form a complex with UDP-glucose transferase to facilitate the transfer of substrate for callose synthesis.

Enzymatic synthesis and purification of [(3)H]uridine diphosphate galacturonic acid for use in studying Golgi-localized transporters

Uridine 5'-diphosphate galacturonic acid (UDP-GalA) is a substrate for the galacturonosyltransferases that synthesize the three pectic polysaccharides homogalacturonan, rhamnogalacturonan I, and rhamnogalacturonan II. Pectin synthesis occurs in the Golgi and it is hypothesized that UDP-GalA is transported into the lumen of the Golgi by membrane-localized transporters. To study the transport and metabolism of UDP-GalA in the Golgi, UDP-GalA labeled in the uridine moiety is required. Here we present a high-yield method for the synthesis of [(3)H]UDP-GalA from [(3)H]UTP and Glc-1-P by sequential reactions catalyzed by UDP-Glc pyrophosphorylase, UDP-Glc dehydrogenase, and UDP-GlcA-4-epimerase and the separation of the reaction products over a Dionex CarboPac PA1 anion-exchange column using high-performance anion-exchange chromatography (HPAEC). Approximately half of the [(3)H]UTP was converted into [(3)H]UDP-GalA and the remaining 50% was recovered as [(3)H]UDP-GlcA. Both products were purified and the identity of the [(3)H]UDP-GalA was confirmed by its conversion into [(3)H]UDP-GlcA by UDP-GlcA-4-epimerase. The enzymatic synthesis of diverse nucleotide sugars radiolabeled in the nucleotide by the use of nucleotide-converting enzymes, combined with the high-resolution separation of the nucleotide sugars and their purification by HPAEC, can provide unique substrates required for the study of diverse nucleotide sugar transporters.

The mechanism of synthesis of a mixed-linkage (1-->3), (1-->4)beta-D-glucan in maize. Evidence for multiple sites of glucosyl transfer in the synthase complex

We examined the mechanism of synthesis in vitro of (1-->3), (1-->4)beta-D-glucan (beta-glucan), a growth-specific cell wall polysaccharide found in grasses and cereals. beta-Glucan is composed primarily of cellotriosyl and cellotetraosyl units linked by single (1-->3)beta-linkages. The ratio of cellotriosyl and cellotetraosyl units in the native polymer is strictly controlled at between 2 and 3 in all grasses, whereas the ratios of these units in beta-glucan formed in vitro vary from 1.5 with 5 &mgr;M UDP-glucose (Glc) to over 11 with 30 mM substrate. These results support a model in which three sites of glycosyl transfer occur within the synthase complex to produce the cellobiosyl-(1-->3)-D-glucosyl units. We propose that failure to fill one of the sites results in the iterative addition of one or more cellobiosyl units to produce the longer cellodextrin units in the polymer. Variations in the UDP-Glc concentration in excised maize (Zea mays) coleoptiles did not result in wide variations in the ratios of cellotriosyl and cellotetraosyl units in beta-glucan synthesized in vivo, indicating that other factors control delivery of UDP-Glc to the synthase. In maize sucrose synthase is enriched in Golgi membranes and plasma membranes and may be involved in the control of substrate delivery to beta-glucan synthase and cellulose synthase.

Cellulose and Callose Biosynthesis in Higher Plants (I. Solubilization and Separation of (1->3)- and (1->4)-[beta]-Glucan Synthase Activities from Mung Bean)

(1->3)- and (1->4)-[beta]-glucan synthase activities from higher plants have been physically separated by gel electrophoresis in nondenaturing conditions. The two glucan synthases show different mobilities in native polyacrylamide gels. Further separation by sodium dodecyl sulfate-polyacrylamide gel electrophoresis revealed a different polypeptide composition in these synthases. Three polypeptides (64, 54, and 32 kD) seem to be common to both synthase activities, whereas two polypeptides (78 and 38 kD) are associated only with callose synthase activity. Twelve polypeptides (170, 136, 108, 96, 83, 72, 66, 60, 52, 48, 42, and 34 kD) appear to be specifically associated with cellulose synthase activity. The successful separation of (1->3)- and (1->-4)-[beta]-glucan synthase activities was based on the manipulation of digitonin concentrations used in the solubilization of membrane proteins. At low dipitomin concentrations (0.05 and 0.1%), the ratio of the cellulose to callose synthase activity was higher. At higher digitonin (0.5-1%) concentrations, the ratio of the callose to cellulose synthase activity was higher. Rosette-like particles with attached product were observed in samples taken from the top of the stacking gel, where only cellulose was synthesized. Smaller (nonrosette) particles were found in the running gel, where only callose was synthesized. These findings suggest that a higher level of subunit organization is required for in vitro cellulose synthesis in comparison with callose assembly.

The role of endoxyloglucan transferase in the organization of plant cell walls

The plant cell wall plays a central role in morphogenesis as well as responsiveness to environmental signals. Xyloglucans are the principal component of the plant cell wall matrix and serve as cross-links between cellulose microfibrils to form the cellulose-xyloglucan framework. Endoxyloglucan transferase (EXGT), which was isolated and characterized in 1992, is an enzyme that mediates molecular grafting reaction between xyloglucan molecules. Structural studies on cDNAs encoding EXGT and its related proteins have disclosed the ubiquitous presence in the plant kingdom of a large multigene family of xyloglucan-related proteins (XRPs). Each XRP functions as either hydrolase or transferase acting on xyloglucans and is considered to be responsible for rearrangement of the cellulose-xyloglucan framework, the processes essential for the construction, modification, and degradation of plant cell walls. Different XRP genes exhibit potentially different expression profiles with respect to tissue specificity and responsiveness to hormonal and mechanical signals. The molecular approach to individual XRP genes will open a new path for exploring the controlling mechanisms by which the plant cell wall is constructed and reformed during plant growth and development.

Higher plants contain homologs of the bacterial celA genes encoding the catalytic subunit of cellulose synthase

In spite of much effort, no one has succeeded in isolating and characterizing the enzyme(s) responsible for synthesis of cellulose, the major cell wall polymer of plants. We have characterized two cotton (Gossypium hirsutum) cDNA clones and identified one rice (Oryza sativa) cDNA that are homologs of the bacterial celA genes that encode the catalytic subunit of cellulose synthase. Three regions in the deduced amino acid sequences of the plant celA gene products are conserved with respect to the proteins encoded by bacterial celA genes. Within these conserved regions, there are four highly conserved subdomains previously suggested to be critical for catalysis and/or binding of the substrate UDP-glucose (UDP-Glc). An overexpressed DNA segment of the cotton celA1 gene encodes a polypeptide fragment that spans these domains and binds UDP-Glc, while a similar fragment having one of these domains deleted does not. The plant celA genes show little homology at the N- and C-terminal regions and also contain two internal insertions of sequence, one conserved and one hypervariable, that are not found in the bacterial gene sequences. Cotton celA1 and celA2 genes are expressed at high levels during active secondary wall cellulose synthesis in developing cotton fibers. Genomic Southern blot analyses in cotton demonstrate that celA forms a small gene family.

Cellular mechanisms of callose deposition in response to fungal infection or chemical damage

Cellular mechanisms of callose deposition induced in cowpea (Vigna unguiculata) leaves by the living cowpea rust fungus (Uromyces vignae), boric acid, or the fungus killed by polyoxin D, were investigated by the use of chemical inhibitors. Effects of the inhibitors were observed in both a resistant and a susceptible cowpea cultivar. The effect of inhibitors differed depending on the type of callose-inducing stimulus and cultivar used. Inhibitors of transcription (actinomycin D) and protein synthesis (blasticidin S, cycloheximide) lowered the incidence of fungus-induced callose deposits in both cultivars. Inhibitors of protein synthesis also reduced deposits induced by boric acid or fungal death. Callose deposition induced by the living fungus in the resistant cultivar was reduced by inhibitors of protein glycosylation (tunicamycin, deoxynojirimycin) and microfilament function (cytochalasins B and E), but these inhibitors had no effect on callose deposition in the susceptible cultivar or on chemical or fungal death-induced deposition in either cultivar. No reduction in callose deposits was observed in plants treated with inhibitors of Golgi-associated vesicle transfer (brefeldin A, monensin) or microtubule polymerization (colchicine, oryzalin). The results suggest that the cellular processes involved in callose deposition differ with differing triggering stimuli and that callose deposition triggered by the living fungus in a resistant host cultivar is not a typical wound or damage response. Keywords: callose, infection, inhibitors, resistance, rust fungi.

A membrane-associated form of sucrose synthase and its potential role in synthesis of cellulose and callose in plants

Sucrose synthase (SuSy; EC 2.4.1.13; sucrose + UDP reversible UDPglucose + fructose) has always been studied as a cytoplasmic enzyme in plant cells where it serves to degrade sucrose and provide carbon for respiration and synthesis of cell wall polysaccharides and starch. We report here that at least half of the total SuSy of developing cotton fibers (Gossypium hirsutum) is tightly associated with the plasma membrane. Therefore, this form of SuSy might serve to channel carbon directly from sucrose to cellulose and/or callose synthases in the plasma membrane. By using detached and permeabilized cotton fibers, we show that carbon from sucrose can be converted at high rates to both cellulose and callose. Synthesis of cellulose or callose is favored by addition of EGTA or calcium and cellobiose, respectively. These findings contrast with the traditional observation that when UDPglucose is used as substrate in vitro, callose is the major product synthesized. Immunolocalization studies show that SuSy can be localized at the fiber surface in patterns consistent with the deposition of cellulose or callose. Thus, these results support a model in which SuSy exists in a complex with the beta-glucan synthases and serves to channel carbon from sucrose to glucan.

Genes encoding small GTP-binding proteins analogous to mammalian rac are preferentially expressed in developing cotton fibers

In animals, the small GTP-binding proteins, Rac and Rho, of the ras superfamily participate in the signal transduction pathway that regulates the organization of the actin cytoskeleton. We report here on the characterization of two distinct cDNA clones isolated from a cotton fiber cDNA library that code for homologs of animal Rac proteins. Using gene-specific probes, we have determined that amphidiploid cotton contains two genes that code for each of the two Rac proteins, designated Rac13 and Rac9, respectively. The gene for Rac13 shows highly enhanced expression in developing cotton fibers, with maximal expression occurring at the time of transition between primary and secondary wall synthesis. This is also the time at which reorganization of the cytoskeleton occurs, and thus the pattern of expression of Rac13 is consistent with its possible role, analogous to animal Rac, in the signal transduction pathway that controls cytoskeletal organization.

Purification of 1,3-beta-D-glucan synthase activity from pea tissue. Two polypeptides of 55 kDa and 70 kDa copurify with enzyme activity

From pea plasma membranes isolated by aqueous polymer two-phase partitioning we have purified 1,3-beta-D-glucan synthase [glucan synthase-II (GS-II) or callose synthase], an enzyme that several reports have suggested consists of between six and nine different subunits. The procedure involves (a) preliminary removal of peripheral proteins by 0.1% digitonin; (b) solubilization of GS-II with 0.5% digitonin; (c) precipitation of activity-irrelevant proteins from the digitonin extract by Ca2+, spermine and cellobiose, which are GS-II effectors needed in step (d); (d) product entrapment by formation of 1,3-beta-D-glucan from UDP-Glc by GS-II in the presence of the mentioned effectors, followed by centrifugal sedimentation of product micelles and elution of proteins therefrom with buffer; (e) preparative isoelectric focusing (IEF) of product-entrapped proteins; and (f) glycerol gradient centrifugation of the fractions of peak GS-II activity from IEF. The procedure yields 300-fold enrichment of GS-II specific activity over that in isolated plasma membranes, and 5500-fold over that in the original homogenate. Out of approximately six principal polypeptides that occur after the product entrapment step, the glycerol gradient GS-II activity peak contains only two major polypeptides, one of 55 kDa and another of 70 kDa, plus minor amounts of one or two others whose distribution and occurrence indicate are not responsible for GS-II activity. Antisera against either the 55-kDa or the 70-kDa polypeptide adsorb more than 60% of the GS-II activity from a product-entrapped preparation. After native gel electrophoresis, GS-II activity is associated with a single protein band of very large molecular mass, whose principal components are the 55-kDa and 70-kDa polypeptides, accompanied by minor amounts of a few other polypeptides most of which do not occur in enzyme preparations purified by the previously described procedure. The 55-kDa but not the 70-kDa component can be labeled by ultraviolet irradiation of the plasma membranes in the presence of [alpha-32P]UDP-Glc under GS-II assay conditions. It seems likely, therefore, that the 55-kDa and 70-kDa polypeptides form a large catalytic complex of which the 55-kDa component is the UDP-Glc-binding subunit.

Cotton fiber annexins: a potential role in the regulation of callose synthase

Cotton fibers contain a characteristic set of proteins which interact with plasma membranes in a Ca(2+)-dependent manner. The association of these proteins with the membrane is correlated with a reduced level of UDP-glucose: (1-->3)-beta-glucan (callose) synthase activity. Analysis of the proteins released from membranes by EDTA treatment shows that the most abundant proteins comprise a family of at least three polypeptides (p34) which resemble annexins. This resemblance includes similarity in size (about 34 kDa), sequence homology, Ca(2+)-dependent precipitation or interaction with the plasma membrane, and ability to serve as a substrate for phosphorylation by endogenous protein kinase(s) which also bind to the membranes in a Ca(2+)-dependent manner. A purified fraction of these annexins binds to, and inhibits, the activity of a partially purified cotton fiber callose synthase. These findings suggest that one possible function of annexin(s) in plants is to modulate the activity and/or localization of callose synthase.

[beta]-Glucan Synthesis in the Cotton Fiber (III. Identification of UDP-Glucose-Binding Subunits of [beta]-Glucan Synthases by Photoaffinity Labeling with [[beta]-32P]5[prime]-N3-UDP-Glucose

Using differential product entrapment and photolabeling under specifying conditions, we identifIed a 37-kD polypeptide as the best candidate among the UDP-glucose-binding polypeptides for the catalytic subunit of cotton (Gossypium hirsutum) cellulose synthase. This polypeptide is enriched by entrapment under conditions favoring [beta]-1,4-glucan synthesis, and it is magnesium dependent and sensitive to unlabeled UDP-glucose. A 52-kD polypeptide was identified as the most likely candidate for the catalytic subunit of [beta]-1,3-glucan synthase because this polypeptide is the most abundant protein in the entrapment fraction obtained under conditions favoring [beta]-1,3-glucan synthesis, is coincident with [beta]-1,3-glucan synthase activity, and is calcium dependent. The possible involvement of other polypeptides in the synthesis of [beta]-1,3-glucan is discussed.

Synthesis, assembly and function of plant cell wall macromolecules

The plant cell wall consists of a structurally intricate network of polysaccharide and protein whose biosynthesis, assembly and functions are still poorly understood. Recent research has shown how cell wall macromolecules, and fragments thereof, appear to be involved in processes such as cell growth, cell and tissue differentiation and the control of pathogenesis.

beta-Furfuryl-beta-Glucoside: An Endogenous Activator of Higher Plant UDP-Glucose:(1-3)-beta-Glucan Synthase : Biological Activity, Distribution, and in Vitro Synthesis

In a recent paper (P Ohana, DP Delmer, JC Steffens, DE Matthews, R Mayer, M Benziman [1991] J Biol Chem 266: 13472-13475), we described the purification and structural characterization of beta-furfuryl-beta-glucoside (FG), an endogenous activator of plant UDP-glucose:(1-->3)-beta-glucan (callose) synthase. In the present report, we provide evidence that FG specifically stimulates callose synthase. The effects of FG on the kinetic properties of callose synthase were studied, and we ascertained that FG, or at least a very similar compound, is present in other plant systems. Chemically synthesized alpha-furfuryl-beta-glucoside also stimulates callose synthase, exhibiting a slightly higher K(a) of 80 micromolar, compared with 50 micromolar for FG. In addition, we have identified and partially characterized an enzyme that catalyzes the synthesis of FG using beta-furfuryl alcohol and UDP-glucose as substrates. A model for the regulation of callose synthesis in vivo, involving changes in intracellular compartmentation of FG and Ca(2+), is proposed.

Rapid Enrichment of CHAPS-Solubilized UDP-Glucose: (1,3)-beta-Glucan (Callose) Synthase from Beta vulgaris L. by Product Entrapment : Entrapment Mechanisms and Polypeptide Characterization

Rapid enrichment of CHAPS-solubilized UDP-glucose:(1,3)-beta-glucan (callose) synthase from storage tissue of red beet (Beta vulgaris L.) is obtained when the preparation is incubated with an enzyme assay mixture, then centrifuged and the enzyme released from the callose pellet with a buffer containing EDTA and CHAPS (20-fold purification relative to microsomes). When centrifuged at high speed (80,000g), the enzyme can also be pelleted in the absence of substrate (UDP-Glc) or synthesis of callose, due to nonspecific aggregation of proteins caused by excess cations and insufficient detergent in the assay buffer. True time-dependent and substrate-dependent product-entrapment of callose synthase is obtained by low-speed centrifugation (7,000-11,000g) of enzyme incubated in reaction mixtures containing low levels of cations (0.5 millimolar Mg(2+), 1 millimolar Ca(2+)) and sufficient detergent (0.02% digitonin, 0.12% CHAPS), together with cellobiose, buffer, and UDP-Glc. Entrapment conditions, therefore, are a compromise between preventing nonspecific precipitation of proteins and permitting sufficient enzyme activity for callose synthesis. Further enrichment of the enzyme released from the callose pellet was not obtained by rate-zonal glycerol gradient centrifugation, although its sedimentation rate was greatly enhanced by inclusion of divalent cations in the gradient. Preparations were markedly cleaner when product-entrapment was conducted on enzyme solubilized from plasma membranes isolated by aqueous two-phase partitioning rather than by gradient centrifugation. Product-entrapped preparations consistently contained polypeptides or groups of closely-migrating polypeptides at molecular masses of 92, 83, 70, 57, 43, 35, 31/29, and 27 kilodaltons. This polypeptide profile is in accordance with the findings of other callose synthase enrichment studies using a variety of tissue sources, and is consistent with the existence of a multi-subunit enzyme complex.

Evidence for a cyclic diguanylic acid-dependent cellulose synthase in plants

Because numerous attempts to detect an activity for a cellulose synthase in plants have failed, we have taken a different approach toward detecting polypeptides involved in this process. The uniqueness of the structure and function of cyclic diguanylic acid (c-di-GMP) as an activator of the cellulose synthase of the bacterium Acetobacter xylinum makes it an attractive probe to use in a search for a c-di-GMP receptor that might be involved in the process in plants. Direct photolabeling with 32P-c-di-GMP has been used, therefore, to identify in plants two membrane polypeptides of 83 and 48 kD derived from cotton fibers that possess properties consistent with their being components of a c-di-GMP-dependent cellulose synthase. Based upon several criteria, the 48-kD species is proposed to be derived by proteolytic cleavage of the 83-kD polypeptide. Both polypeptides bind c-di-GMP with high affinity and specificity and show antigenic relatedness to the bacterial cellulose synthase, and the N-terminal sequence of the 48-kD polypeptide also indicates relatedness to the bacterial synthase. Ability to detect both cotton fiber polypeptides by photolabeling increases markedly in extracts derived from fibers entering the active phase of secondary wall cellulose synthesis. These results provide a basis for future work aimed at identifying and characterizing genes involved in cellulose synthesis in plants.