Role of Primer and/or Intermediates During Cellulose Synthesis?
In straightforward terms, cellulose biosynthesis requires the enzyme cellulose synthase for catalyzing the polymerization of glucose residues from UDP-glucose into a β-1,4-linked glucan chain. This simple mechanism envisions direct polymerization without the need for any intermediates or a primer. Cellulose biosynthesis has been demonstrated in vitro using membrane and detergent-solubilized extracts from A. xylinum and a number of plants in the presence of only UDP-glucose (Kudlicka and Brown, 1997; Lai-Kee-Him et al., 2002; Lin and Brown, 1989; Okuda et al., 1993). The synthesis of cellulose in vitro with the minimal added components in the reaction mixture strongly supports the direct polymerization of glucose without any requirement for a primer. However, in the absence of purified cellulose synthases it is not possible to completely exclude the role of other proteins or components contributed by the membrane fraction or detergent extracts during cellulose synthesis. In 2002, Peng et al. proposed a model for cellulose biosynthesis in which they suggested that SG serves as a primer for synthesis of SCDs by CesA proteins (Peng et al., 2002). According to their model, a membrane-associated endoglucanase Kor (encoded by the Korrigan gene) cleaves SCDs giving rise to SG and cellodextrins (CDs). In the next step, the CDs undergo β-1,4-glucan chain elongation catalyzed by CesA proteins. The glucose moiety of SG is found to be attached via its reducing end to sitosterol and chain elongation in the first step is predicted to proceed from the nonreducing end. Based on this model, plants deficient in sitosterol are expected to show a severe phenotype due to impairment in cellulose synthesis (Peng et al., 2002). A number of mutants deficient in sitosterol content have been identified in Arabidopsis. However, dwf1/dim mutants of Arabidopsis that have a severe reduction in sitosterol content have been rescued to the wild type by brassinosteroid (BR) treatment suggesting that sitosterol may not have a major role in cellulose biosynthesis (Clouse, 2002). In the absence of any direct evidence for the role of sitosterol in cellulose biosynthesis, doubts have been raised regarding the proposed involvement of SG as a primer (Somerville et al., 2004).
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