Role in cellulose synthesis

Much of our understanding of the TCs has, in fact, been obtained from genetic and microscopic analyses. Interestingly, genetic analysis of cellulose-deficient mutants in plants led to the proposal that the rosette TC contains three different nonredundant cellulose synthases, and mutation in any one of these results in a defect in the assembly of cellulose microfibrils (Taylor et al. 2003). In addition, the cellulose synthases identified for cellulose synthesis in the primary cell wall are different from the cellulose synthases required for cellulose synthesis in the secondary cell wall (Robert et al. 2004). In spite of the failure to localize any other protein except the cellulose synthase to the rosette TC, other proteins have been predicted by mutant analysis to be associated with the complex and these may have a direct or indirect role in cellulose synthesis. 

The dolichol-linked D-mannosyl, D-glucosyl, and 2-acetamido-2-deoxy-D-glucosyl residues are used in the glycosylation of proteins and in the biosynthesis of D-m an nan.2,35 GlcNAc-PP-Dol may also play a role in the synthesis of glycosaminoglycans. Reaction 4 is the first step in the formation of lipid-linked precursors for cellulose.46... 

Significantly, the ordered pattern of fibril deposition in the secondary wall of Micrasterias367 was shown to be derived from the structure of the complexes located in the plasma membrane. The results indicated that the widest fibrils, those in the center of a band, are formed by the longest rows of rosettes, those in the center of arosette array. The shorter rows of rosettes within an array give rise to narrower fibrils. This proportionality between the width of a secondary cellulose fibril and the number of rosettes involved in its formation provides strong evidence that the rosette structure plays a significant role in the synthesis of cellulose fibrils. 

The isolation of mutants deficient in secondary cell wall cellulose synthesis has led to the identification of a number of genes essential for cellulose production. The identification of these genes and the discovery that three CesA proteins are involved in the same protein complex has answered some of the questions regarding the complexity of the cellulose synthase complex. That fact that all three proteins are required for the correct assembly and targeting of the complex to the plasma membrane will allow us to further dissect the roles of these and other proteins in cellulose synthesis. In addition, the identification of a protein that is involved in both primary and secondary cell wall cellulose synthesis may allow us to identify other common components between the primary and secondary cell wall cellulose synthesizing machinery. 

A PERSPECTIVE ON THE ASSEMBLY OF CELLULOSE-SYNTHESIZING COMPLEXES POSSIBLE ROLE OF KORRIGAN AND MICROTUBULES IN CELLULOSE SYNTHESIS IN PLANTS... 

Emons, A.M.C. 1991. Role of particle rosettes and terminal globules in cellulose synthesis. In Haigler C.H. and Weimer P.J. (eds.) Biosynthesis and biodegradation of cellulose. Marcel Dekker, New York, pp. 71-98. 

ICimura, S. and Itoh, T. 1995. Evidence for the role of glomerulocyte in cellulose synthesis in the tunicate, Metandrocarpa uedai. Protoplasma 186 24-33. 

Our electron microscopy observations have revealed some of the roles of cell organellae involved in biosynthesis of cell wall components (i) the plasma membrane is the site of cellulose synthesis. This supports the proposal that terminal and rosette complexes at the plasma membrane are responsible for cellulose synthesis, (ii) The Golgi-bodies and small circular vesicles derived from the r-ER s are involved in the biosynthesis and/or transport of the hemicelluloses. Our investigations, however, could not distinguish between what type of cell organellae contained what kind of hemicelluloses, and how these polymers were processed in the organellae. 

Ever since the discovery, by Ledbetter and Porter,88 of microtubules below the surface of the plasma membrane, suggestions have been made that these structures play some role in microfibril orientation. The suggestion arose because of two observations that (I) the orientation of microtubules has very frequently, but not always, been observed to be parallel to the orientation of the microfibrils most recently synthesized, and (2) agents, such as colchicine, that disrupt microtubules interfere with the orientation, but not the synthesis, of cellulose microfibrils. The literature pertaining to these studies has been well reviewed by Robinson,4 Schnepf and coworkers,89 Hepler and Palevitz,90 and Heath.91 In sum, the present evidence seems to favor some role for microtubules in orientation in some cases, such as the studies on guard cells by Palevitz and Hepler,92 and a series of papers on Oocystis by Robinson and coworkers,84,93-95 the case for micro-... 

A considerably stronger case can be made for a role for UDP-glucose as precursor to cellulose. The first convincing report of in vitro synthesis of alkali-insoluble (l- 4)-j8-D-glucan from UDP-glucose... 

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