Cellulose Synthases and Related Enzymes

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Cellulose synthases are integral membrane proteins localized to the plasma membrane. Based on their amino acid sequence similarities, cellulose synthesases and other glycosyl- transferases are classified into 47 families. With the availability of DNA sequences and cell wall mutants, interest has been revived in cellulose synthases and other glycosyltransferases involved in the synthesis of the cell wall in plants.   Enzymes similar to the cellulase synthases are believed to function in the synthesis of polysaccharides with a beta-linked backbone.
In a review article in the December, 2000 issue of  Current Opinion in Plant Biology, Dr. I.M. Saxena and  Professor R.M. Brown at MCDB, UT, present the current picture of the state of knowledge about the proteins involved in the synthesis of cellulose and  and other polysaccharides.

A comparison of the amino acid sequence of the Acetobacterium xylinum cellulase sythases with other glycosyltransferases led to the identification of conserved residues in this class of proteins.  The first identification of a plant cellulase synthase gene was done from a cotton fiber based on the presence of these conserved residues. Interestingly, the proteins encoded by the cotton genes (now referred to as GhCesA-1 and GhCesA-2) show certain unique features characterized  by  additional sequences, not present in the A. xylinum sequence.  Sequence analysis predicts that the encoded cellulose synthase from cotton  is a transmembrane protein with 13 transmembrane helices.
Practically very little information on the biochemical nature of cellulose synthases (Ces) from plants is available; this is because, there is neither any method available to purify the  enzyme nor are there suitable assays for determining their activity. Sequence analysis of  beta-glycosyltransferases had earlier shown the presence of  domains A and B  in processive enzymes and only A in non-processive enzymes. While the function of domain A is clearly defined, that of  domain B is still unknown. In A. thaliana  ten complete CesA gene sequences have been identified. These genes are scattered on different chromosomes.

Assembly and function of cellulose-synthase subunits: The expression of two sets of genes has been observed during secondary wall formation in cotton fibers and in poplar xylem cells. During the formation of both primary and secondary cell wall, the simultaneous expression of two sets of genes have been observed suggesting that both the subunits are essential for crystalline cellulose-I formation. These observations suggest  that cellulose synthases of plant origin may be organized as dimers in the cellulose synthesizing complex, although it is difficult to figure out why dimerization would be necessary during the polymerization of a single glucan chain.
It is also envisaged that the transmembrane regions may be made of beta-sheets and they may contribute to  the association of the different cellulose synthases to form dimers. The beta-sheets may also take part in the transport of glucans chains across the plasma membrane. There are six particles constituting the  terminal complex (TC) rosette and each particle in turn consists of six catalytic subunits. Each subunit may require three dimers for its formation and they may be required for the assembly of six glucan chains.

A study of the RSW1 mutant, rsw1 (radial swelling) has revealed that  this mutation causes a loss of rosette structure rendering a change of cellulose from a crystalline state to a non- crystalline aggregate. With the advancement of techniques such as immunogold-labeling technique, it may be possible to prove the hypothesis that  the assembly of rosettes and the formation of cellulose-1 occurs as a result of the involvement of heteromeric cellulase synthase dimers.

In the end the authors  express their hope that a thorough analysis of the proteins involved in cellulose synthesis by means of recombinant methods may be possible and that knowledge of the structure  and organization of domains in this group of proteins will help us understand about the enzymes involved in the biosynthesis of cell-wall polysaccharides.


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