AIMS: To outline the current state of knowledge and discuss the evolution of various viewpoints put forth to explain the mechanism of cellulose biosynthesis. SCOPE: Understanding the mechanism of ...cellulose biosynthesis is one of the major challenges in plant biology. The simplicity in the chemical structure of cellulose belies the complexities that are associated with the synthesis and assembly of this polysaccharide. Assembly of cellulose microfibrils in most organisms is visualized as a multi-step process involving a number of proteins with the key protein being the cellulose synthase catalytic sub-unit. Although genes encoding this protein have been identified in almost all cellulose synthesizing organisms, it has been a challenge in general, and more specifically in vascular plants, to demonstrate cellulose synthase activity in vitro. The assembly of glucan chains into cellulose microfibrils of specific dimensions, viewed as a spontaneous process, necessitates the assembly of synthesizing sites unique to most groups of organisms. The steps of polymerization (requiring the specific arrangement and activity of the cellulose synthase catalytic sub-units) and crystallization (directed self-assembly of glucan chains) are certainly interlinked in the formation of cellulose microfibrils. Mutants affected in cellulose biosynthesis have been identified in vascular plants. Studies on these mutants and herbicide-treated plants suggest an interesting link between the steps of polymerization and crystallization during cellulose biosynthesis. CONCLUSIONS: With the identification of a large number of genes encoding cellulose synthases and cellulose synthase-like proteins in vascular plants and the supposed role of a number of other proteins in cellulose biosynthesis, a complete understanding of this process will necessitate a wider variety of research tools and approaches than was thought to be required a few years back.
The ability to synthesize cellulose by Asaia bogorensis, a member of the acetic acid bacteria, was studied in two substrains, AJ and JCM. Although both strains have identical 16S rDNA sequence, only ...the AJ strain formed a solid pellicle at the air–liquid interface in static culture medium, and we analyzed this pellicle using a variety of techniques. In the presence of cellulase, glucose and cellobiose were released from the pellicle suggesting that it is made of cellulose. Field emission electron microscopy allowed the visualization of a 3D knitted structure with ultrafine microfibrils (approximately 5–20 nm in width) in cellulose from A. bogorensis compared with the 40–100 nm wide microfibrils observed in cellulose isolated from Gluconacetobacter xylinus, suggesting differences in the mechanism of cellulose biosynthesis or organization of cellulose synthesizing sites in these two related bacterial species. Identifying these differences will lead to a better understanding of cellulose biosynthesis in bacteria.
A combined structural and functional model of the catalytic region of cellulose synthase is presented as a prototype for the action of processive β-glycosyltransferases and other ...glycosyltransferases. A 285 amino acid segment of the
Acetobacter xylinum cellulose synthase containing all the conserved residues in the globular region was subjected to protein modeling using the genetic algorithm. This region folds into a single large domain with a topology exhibiting a mixed alpha/beta structure. The predicted structure serves as a topological outline for the structure of this processive β-glycosyltransferase. By incorporating new site-directed mutagenesis data and comparative analysis of the conserved aspartic acid residues and the QXXRW motif we deduce a number of functional implications based on the structure. This includes location of the UDP–glucose substrate-binding cavity, suggestions for the catalytic processing including positions of conserved and catalytic residues, secondary structure arrangement and domain organization. Comparisons to cellulose synthases from higher plants (genetic algorithm based model for cotton CelA1), data from neural network predictions (PHD), and to the recently experimentally determined structures of the non-processive SpsA and β4-galactosyltransferase retest and further validate our structure-function description of this glycosyltransferase.
A model of the globular region of cellulose synthase shows a topology with a mixed α/β structure and location of the conserved amino acid residues that are required for enzyme activity.
In a cellulose-producing bacterium Gluconacetobacter xylinus, endo-β-1,4-glucanase and β-glucosidase are suggested to have an important role in cellulose biosynthesis, and genes encoding these ...enzymes are present adjacent to the cellulose synthesis operon. However, in other cellulose-producing bacteria including Asaia bogorensis, β-glucosidase gene is not found next to the cellulose synthesis operon, and their cellulose productivities are lower than that of G. xylinus. To investigate whether A. bogorensis produces a similar type of β-glucosidase, crude enzyme fractions were prepared from G. xylinus and A. bogorensis. The hydrolytic activities toward p-nitrophenyl-β-D-glycosides were compared. In A. bogorensis, the hydrolytic activities were found in the cytosolic fraction, but the fraction did not indicate an ability to produce gentiobiose that is suggested to be an inducer for expression of the endo-β-1,4-glucanase gene in G. xylinus. We conclude that the mechanism of cellulose biosynthesis regulated by endo-β-1,4-glucanase may be different in A. bogorensis and G. xylinus.
Analysis of cellulose biosynthesis using molecular approaches has been successful in identifying genes in many cellulose-producing organisms, yet the mechanism of cellulose biosynthesis still remains ...to be understood. We are interested in developing the moss Physcomitrella patens as a useful system for the study of cellulose biosynthesis. This moss affords a number of advantages including a haploid dominated gametophyte and a very high efficiency of homologous recombination in its nuclear DNA for constructing gene knockouts. In addition, P. patens has only a primary cell wall unlike Arabidopsis thaliana, which has both a primary and a secondary cell wall. We identified two full-length cellulose synthase (CesA) genes of P. patens, PpCesA6 and PpCesA7 from an EST database and have analyzed the genomic sequences. PpCesA6 and PpCesA7 show high similarity to each other, both at the cDNA and genomic DNA levels. Single and double knockouts of PpCesA6 and PpCesA7 were generated and screened for phenotypic changes. While the PpCesA6 and PpCesA7 single knockouts did not show any obvious phenotypic differences from the wild-type, the double knockout had significantly reduced stem length. These results suggest that PpCesA6 and PpCesA7 probably have a very similar role in cellulose biosynthesis and their functions may be redundant. Additionally, their roles may overlap with the other P. patens CesAs as observed for CesAs involved in primary cell wall biosynthesis in A. thaliana.
Morphology changes in bacterial cellulose produced by Acetobacter xylinum ATCC23769 were observed in the presence of β-glucodisaccharides such as gentiobiose and cellobiose. Endo-β-1,4-glucanase ...activity in culture broth was higher than that in the absence of those sugars. So we have investigated the properties of endo-β-1,4-glucanase (AEG) produced by this bacterium. This enzyme could hydrolyze water-soluble cellulose such as CMC, hydroxyethyl cellulose and cellodextrin, and decreased the viscosity of the substrate solution. On the other hand, AEG could not produce any soluble sugars from water-insoluble cellulose such as Avicel and bacterial cellulose. These properties were completely different from endo-glucanase from fungi. AEG hydrolyzed cellohexaose and produced cellobiose, cellotriose and cellotetraose, but in the presence of bacterial cellulose, the soluble sugars produced from cellohexaose disappeared in the reaction mixture. It is suggested that AEG might have transglycosyl activity, though it belongs to glycosidase family 8. It is proposed that this activity is closely related to cellulose synthesis.
Acetobacter xylinum ATCC23769 produces not only cellulose but also various oligosaccharides during cell growth. These oligosaccharides accumulated and increased gradually up to about half the amount ...of cellulose with the increase of endo-1,4-alpha-glucanases activity in culture broth. These oligosaccharides were identified cello-oligosaccharides, gentiobiose and rhamnose, which were constituent sugars of acetan. It is suggested that they are the degradation products from acetan, as enzymes prepared from culture broth hydrolvzed acetan rather than cellulose.
Cellulose synthases and related enzymes Saxena, Inder M; Brown Jr, R Malcolm
Current Opinion in Plant Biology,
12/2000, Letnik:
3, Številka:
6
Book Review, Journal Article
Recenzirano
The discovery of a large number of genes encoding cellulose synthases and related glycosyltransferases in plants has led to a renewed interest in the biosynthesis of cell-wall polysaccharides. A ...number of approaches, including virus-induced gene silencing have proven useful in the functional analysis of these genes. X-ray analysis of the structures of a few glycosyltransferases has led to the identification and confirmation of the role of conserved residues within this group of enzymes. Analysis of related enzymes has provided useful information on the possible domain organization of cellulose synthases and the requirement for at least two separate glycosyltransferase activities in the processive synthesis of sugar chains.
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