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).