Cellulose synthase

BURTON, R.A., GIBEAUT, D.M., BACIC, A., FINDLAY, K., ROBERTS, K., HAMILTON, A., BAULCOMBE, D.C., FINCHER, G.B., Virus-induced silencing of a plant cellulose synthase gene, Plant Cell., 2000,12,691-705. 

CELLULOSE POLYSULFATASE CELLULOSE SYNTHASE CELSIUS TEMPERATURE CENNAMO PLOT ISO MECHANISMS Center of chirality,... 

The terminal complex hypothesis proposes that the cellulose synthesizing enzyme complex can be visualized with electron microscopy. Terminal complex is the name given to collections of plasma membrane particles thought to represent the cellulose synthase. While direct evidence is still not available to support this hypothesis, the amount of indirect supporting evidence has grown dramatically in the past few years. The relationship between terminal complexes, cellulose physical structure and the biochemical events of cellulose biosynthesis will be discussed. 

The terminal complex hypothesis proposes that structural manifestations of the cellulose synthase enzyme complex can be visualized with the freeze fracture specimen preparation technique for electron microscopy. These... 

Direct microscopic evidence demonstrating that terminal complex particles are cellulose synthesizing enzymes is not currently available and will await the production of antibodies against cellulose synthase following its isolation and purification. However, the proposal that terminal complexes are part of the cellulose synthase complex is increasingly becoming accepted... 

PF had been proposed as the terminal complex (23) and associated pores were reported on the outer membrane EF (24). Due to their proximity to the site of cellulose ribbon extrusion from the cell surface, these structures were assumed to be responsible for cellulose synthesis. A model was advanced in which cellulose synthase was localized on the outer membrane, which invoked adhesion sites between the outer and plasma membranes as a mechanism to explain the transfer of uridine-diphosphoryl-glucose (UDPG) from the cytoplasm to the cellulose synthases (25,26). However, when the outer and plasma membranes of Acetobacter were isolated separately by density-gradient centrifugation, the cellulose synthase activity was localized only in the plasma membrane fraction (27). Therefore, the linear structures observed on the Acetobacter outer membrane, while they may be associated in some manner with cellulose biosynthesis, are probably not the cellulose synthase terminal complexes. Since no ultrastructural evidence for adhesion sites between the outer and plasma membranes has been presented, a thorough investigation of the mechanism of / (1-4) glucan chain translocation from the cytoplasmic membrane to the outer membrane in Acetobacter xylinvm is now in order. 

It now appears that cellulose I is not exclusively the native polymorph present in all organisms. The results reported originally by Sisson (61), which provided evidence that cellulose II was the native polymorph present in Halicystis (Ulvophyceae) cell walls, were recently reinvestigated and confirmed (62). Additionally, cellulose II producing mutants of Aceiobacier have been isolated and analyzed with x-ray and low-dose electron diffraction (63). When cellotetraose is induced to crystallize in solution it forms a structure which has been used as a model compound approximating the crystallographic nature of cellulose II based on x-ray diffraction, electron diffraction and CP-MAS 13C NMR evidence (64). Significantly, in all cases where Aceiobacier cellulose synthase in vitro activity has been reported,... 

Isolation and sequencing of the cellulose synthase gene(s) has not been accomplished yet however, DNA from Acetobacter xylinum containing this gene(s) was cloned into broad host-range plasmid vectors (82). These vectors were mobilized into Pel- mutants to test for complementation. To date, this approach has not produced a pellicle-forming transconjugant from a Pel- mutant of Acetobacter (82). The direct correlation between cellulose production and presence of plasmid DNA in Acetobacter has been reported... 

The concentrations of cellulose in potato cell walls were modified by up- and down-regulating four potato cellulose S5mthase genes (StCESAl, 2, 3 and 4) known to be involved in encoding cellulose synthases in cells making primary walls (Oomen et al., 2004). It was found that... 

FIGURE 20-30 Rosettes. The outside surface of the plant plasma membrane in a freeze-fractured sample, viewed here with electron microscopy, contains many hexagonal arrays of particles about 10 nm in diameter, believed to be composed of cellulose synthase molecules and associated enzymes. 

In addition to its catalytic subunit, cellulose synthase may have subunits that mediate extrusion of the polysaccharide chain (the pore subunit) and crystallization of the polysaccharide chains outside the cell... 

In one proposed model, cellulose synthase spans the plasma membrane and uses cytosolic UDP-glucose as the precursor for extracellular cellulose synthesis. In another, a membrane-bound form of sucrose synthase forms a complex with cellulose synthase, feeding UDP-glucose from sucrose directly into cell wall synthesis (Fig. 20-32). 

FIGURE 20-32 A plausible model for the structure of cellulose synthase. The enzyme complex includes a catalytic subunit with eight transmembrane segments and several other subunits that are presumed to act in threading cellulose chains through the catalytic site and out of the cell, and in the crystallization of 36 cellulose strands into the paracrystalline microfibrils shown in Figure 20-29. 

Cellulose synthesis takes place in terminal complexes (rosettes) in the plasma membrane. Each cellulose chain begins as a sitosterol dextrin formed inside the cell. It then flips to the outside, where the oligosaccharide portion is transferred to cellulose synthase in the rosette and is then extended. Each rosette produces 36 separate cellulose chains simultaneously and in parallel. The chains crystallize into one of the microfibrils that form the cell wall. 

Saxena, I.M. Brown, R.M., Jr. (2000) Cellulose synthases and related enzymes. Curr Opin. Plant Biol. 3, 523-531. 

The bacterial cellulose synthase from Acetobacter xylinum can be solubilized with detergents, and the resulting enzyme generates characteristic 1.7 ran cellulose fibrils (Fig. 20-4) from UDP-glucose.125/127-129 These are similar, but not identical, to the fibrils of cellulose I produced by intact bacteria.125 130 Each native fibril appears as a left-handed helix which may contain about nine parallel chains in a crystalline array. Three of these helices appear to coil together (Fig. 20-4) to form a larger 3.7-nm left-handed helical fibril. Similar fibrils are formed by plants. In both... 

Chitin. Like cellulose synthase, fungal chitin synthases are present in the plasma membrane and extrude microfibrils of chitin to the outside.147 150 In the fungus Mucor the majority of the chitin synthesized later has its N-acetyl groups removed hydrolytically to form the deacetylated polymer chitosan.151152 Chitin is also a major component of insect exoskeletons. For this reason, chitin synthase is an appropriate target enzyme for design of synthetic insecticides.153... 

It serves as a membrane anchor for the growing polysaccharide. We have already discussed one example in the hypothetical cellulose synthase mechanism of Fig. 20-5. For some polysaccharides the mechanism is better established. The synthetic cycles all resemble that of Fig. 20-5 and can be generalized as in Eq. 20-20. Here NDP-Glx is a suitable nucleotide disphosphate derivative of sugar Glx, and Z-Glx is the repeating unit of the polysaccharide formed by the action of glycosyl-transferases and other enzymes. 

The parallels among chitin, cellulose, and HA structures, all being j3 -chains of hexose polymers are reflected in the striking similarity in sequence between the HAS from vertebrates, cellulose synthases from plants, and chitin synthases from fungi. A primordial ancestral gene must have existed from which all of these enzymes evolved that are involved in the biosynthesis of all polymers that contain -glycoside linkages, an ancient j3-polysaccharide synthase. 

Pear, J. R., Kawagoe, Y., Schreckengost, W. E., Delmer, D. P, and Stalker, D. M., Higher plants contain homologs ofthe bacterial celA genes encoding the catalytic subunit of cellulose synthase. Proc Natl Acad Sci USA 1996,93 (22), 12637- 2. 

Appenzeller, L., Doblin, M., Barreiro, R., Wang, H., Niu, X., et al., Cellulose synthesis in maize Isolation and expression analysis ofthe cellulose synthase (CesA) gene family. Cellulose 2004, 11, 287-299. 

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