Biosynthesis and structure of glycosyl diglycerides, steryl glucosides, and acylated steryl glucosides

Cellulose biosynthesis in plants: from genes to rosettes

Modern techniques of gene cloning have identified the CesA genes as encoding the probable catalytic subunits of the plant CelS, the cellulose synthase enzyme complex visualized in the plasma membrane as rosettes. At least 10 CesA isoforms exist in Arabidopsis and have been shown by mutant analyses to play distinct role/s in the cellulose synthesis process. Functional specialization within this family includes differences in gene expression, regulation and, possibly, catalytic function. Current data points towards some CesA isoforms potentially being responsible for initiation or elongation of the recently identified sterol beta-glucoside primer within different cell types, e.g. those undergoing either primary or secondary wall cellulose synthesis. Different CesA isoforms may also play distinct roles within the rosette, and there is some circumstantial evidence that CesA genes may encode the catalytic subunit of the mixed linkage glucan synthase or callose synthase. Various other proteins such as the Korrigan endocellulase, sucrose synthase, cytoskeletal components, Rac13, redox proteins and a lipid transfer protein have been implicated to be involved in synthesizing cellulose but, apart from CesAs, only Korrigan has been definitively linked with cellulose synthesis. These proteins should prove valuable in identifying additional CelS components.

Sitosterol-beta-glucoside as primer for cellulose synthesis in plants

Cellulose synthesis in plants requires beta-1,4-glucan chain initiation, elongation, and termination. The process of chain elongation is likely to be distinct from the process of chain initiation. We demonstrate that a CesA glucosyltransferase initiates glucan polymerization by using sitosterol-beta-glucoside (SG) as primer. Cotton fiber membranes synthesize sitosterol-cellodextrins (SCDs) from SG and uridine 5'-diphosphate-glucose (UDP-Glc) under conditions that also favor cellulose synthesis. The cellulase encoded by the Korrigan (Kor) gene, required for cellulose synthesis in plants, may function to cleave SG from the growing polymer chain.

Gas chromatography/mass spectrometry demonstration of steryl glycosides in eucalypt wood, Kraft pulp and process liquids

The occurrence of steryl glycosides (SG) and acyl steryl glycosides (ASG) in eucalypt (Eucalyptus globulus) wood has been investigated. These compounds were analyzed as their trimethylsilyl ethers by gas chromatography/mass spectrometry (GC/MS) using a 15 m length high-temperature capillary column with a thin film, and identified on basis of their mass spectra and relative retention times comparing with those of authentic standards. Significant amounts of SG were identified in eucalypt wood whilst only traces of ASG could be detected. Eucalypt SG and ASG occur in the pyranoside form, which is readily distinguishable from the furanoside configuration by the mass spectra of their trimethylsilyl derivatives. The sterol part of the SG and ASG consisted of sitosterol, being sitosteryl 3beta-D-glucopyranoside and sitosteryl (6'-O-palmitoyl)-3beta-D-glucopyranoside, the major SG and ASG found in E. globulus wood. The presence of SG and ASG was also investigated after kraft cooking by analyzing unbleached pulp and process water samples. The GC/MS results also revealed the presence of sitosteryl 3beta-D-glucopyranoside in these samples. By contrast, no ASG could be detected. Therefore, we have shown that SG survive the kraft cooking and can be found at least partly intact after pulping, being a possible cause for pitch deposits together with free and esterified sterols.

Ultrastructure and chemistry of soluble and polymeric lipids in cell walls from seed coats and fibres of Gossypium species

Electron-microscopic examination in conjunction with extraction procedures and chemical analysis have confirmed that a suberin-like lipid biopolymer is located within the concentric polylamellate layers found in the secondary cell walls of green cotton fibres (Gossypium hirsutum cv. green lint). A polymer of similar ultrastructure and chemical constitution also occurs mainly in the secondary seed-coat walls of the outer epidermis of both green and white varieties of G. hirsutum. The suberins composed of predominantly C22 compounds are, however, markedly different from those present in the periderms of the same plants; these comprise mainly C16 and C18 compounds. Long-chain 1-alkanols (C26-C36) and alkanoic acids (C16-C36) are the principal components of the wax from white fibres but these lipid classes comprise a much smaller proportion of that from green fibres. unidentified highmolecular-weight compounds were the major constituents of the green-fibre was extract which also contains a number of yellow-green pigments, probably flavonoid in nature. These pigments are thought to be associated with the ultrahistochemical reaction with silver proteinate that was observed only in the green-fibre cell walls. A total of 16 wild and cultivated cotton species were examined with the electron microscope for the presence of suberin. The outer seed-coat epidermis of all the examined species but only the fibres of the wild ones were found to be suberized. Among the analysed mutants of fibre colour in G. hirsutum only the gene Lg (green lint) seemed to be associated with suberin.

Endogenous glycosyltransferases glucosylate lipids in flagella of Euglena

Flagella, intact deflagellated cells and isolated cell surfaces of the unicell , Euglena were separately assayed for glycosyltransferase activity by incubating these fractions with uridine diphosphate-[3H]glucose and isolating radiolabeled products. Most of the label was incorporated into lipophilic products, soluble in chloroform/methanol, which could be separated via thin layer chromatography or LH-60 chromatography into four distinct classes. The most polar of these products was extracted from flagella and purified by column chromatography for use as an in vitro substrate to identify flagella-associated glycosyltransferases. After flagella were treated with the detergent CHAPS , a soluble fraction was removed that was capable of glycosylation in solution. The glycosyltransferase(s) responsible for this activity were further enriched on sucrose or fructose gradients and ultimately identified on acrylamide gels through the combined use of nondenaturing gels, dial-[3H]uridine diphosphate binding, and fluorography. The enzyme had an apparent monomer molecular weight of 32,000 and consisted of four or fewer subunits. The occurrence of endogenous glycosyltransferase(s) in flagella suggests that modifications and/or assembly of the flagella surface can take place in situ in this organism.

Sterol glucosylation by the plasma membrane from cotyledons of Phaseolus aureus

Membrane fractions were isolated from dark grown cotyledons of Phaseolus auneus by differential and sucrose density gradient centrifugation. Endoplasmic reticulum-, Golgi apparatus- and plasma membrane-rich fractions were identified by their respective enzymic activities and tested for their ability to transfer glucose from UDP-glucose to endogenous sterols to form steryl glucosides. The glucosyltransferase activity was shown to be located mainly at the plasma membrane.

Steryl glycoside biosynthesis

The existence of steroid glycosides has been known for many years and, more recently, their derivatives have been described. Steroid glycosides have been isolated from a number of organisms, however, the largest number of these compounds are found in plants. As to glycoside biosynthesis, the sterols are the most extensively studied steroid group. Of the sterols, only the 4‐demethyl sterols have been isolated as glycosides. The glycosidic bond formation is mediated through nucleotide sugars, and UDP‐glucose appears to be the most active glycosyl donor. In cell‐free studies, the pH of the incubation medium is quite critical and depends on the tissue under investigation, but generally the optimum is near pH 7.0. Formation of steryl glycosides is particulate in nature and is stimulated by ATP, Ca2+, and Mg2+. Similar results are obtained, regardless whether the sterol or the sugar moiety is labeled. Formation of acylsteryl glycosides could occur via two pathways: through the acylation of steryl glycosides or through the transfer of an acylglycosyl unit to a sterol moiety. Results from in vitro experiments suggest that acylsteryl glycoside formation occurs via steryl glycosides. Two acyl transfer reactions have been demonstrated; one is microsomal in nature and involves phosphatidylethanolamine, while the other reaction involves a soluble enzyme and requires galactolipids. In vivo experiments, however, indicate that a second pathway may also exist. If cholesterol‐4‐14C is used as substrate, a highly radioactive component can be isolated which is readily converted to acylsteryl glycoside, but not to free sterol or steryl glycoside. It is suggested that this component is an intermediate in acylsteryl glycoside biocynthesis. At present, the nature of the component is unknown. It is quite stable, and acid hydrolysis produces free sterol. Saponification produces two products which in thin layer chromatograms closely resemble acylsteryl glycoside.