Purification and Characterization of Cellulose Synthase Activity
Cellulose synthase is the enzyme that performs cellulose biosynthesis. Purification
of this enzyme is a major objective for understanding its properties and in
determining its structure and mode of regulation. Cellulose synthase is a membrane
protein and like most membrane proteins its purification has eluded investigators
interested in isolating it. However, significant progress has been made in
purifying the cellulose synthase activity from
A. xylinum using the product
entrapment technique utilized for purification of the chitin synthase activity in
yeast (Lin and Brown, 1989). In
A. xylinum, using a combination of detergent
solubilization and product entrapment methods, two major polypeptide
bands were identified in the purified fraction. One of these polypeptides was
shown to selectively bind UDP-glucose, and this polypeptide was identified as the cellulose synthase catalytic subunit (Lin
et al., 1990). The other polypeptide was shown to bind the activator c-di-GMP (Mayer
et al., 1991). Sequence information
obtained from these polypeptides was useful in identifying the corresponding
genes from
A. xylinum (Saxena
et al., 1990, 1991). However, similar progress has
not been made with purifying the cellulose synthase activity in plants.
Laosinchai
(2002) used immunoaffinity techniques to purify cellulose synthase activity from
mung bean fractions that synthesized cellulose microfibrils
in vitro (Laosinchai,
2002). Unfortunately, sufficient amounts of the protein could not be isolated
for further characterization of this activity. The cellulose synthase activity
purified from
A. xylinum utilizes UDP-glucose as the substrate and is activated
by c-di-GMP. The cellulose synthase activity in plants is also shown to use
UDP-glucose as the substrate, but it is not activated by c-di-GMP. Instead, the
plant activity is influenced positively in the presence of cellobiose (Li and Brown,
1993). Although no requirement for a primer has been observed for cellulose
synthesis
in vitro using bacterial or plant extracts, a proposal for the requirement
of a sterol-glucoside primer has been made for cellulose synthesis in plants (Peng
et al., 2002). This proposal is based on the observation that cotton fiber membranes
synthesized sitosterol-cellodextrins (SCDs) from sitosterol-β-glucoside (SG) and
UDP-glucose under conditions that favor cellulose synthesis (Peng
et al., 2002).
As a result, this model invokes a number of other components besides cellulose
synthase and UDP-glucose, in a multistep reaction scheme, as opposed to the
single-step polymerization reaction that requires only cellulose synthase and
UDP-glucose. Since most of the experiments demonstrating
in vitro cellulose
synthesis do not suggest the requirement for a primer and no new evidence has
been provided in support of the multistep reaction scheme, the current view is
that polymerization of glucose residues from UDP-glucose occurs in a single-step
reaction catalyzed by the cellulose synthase.
Interestingly, many of the features of cellulose synthases from different
organisms are predicted from the derived amino sequences following identification
of the genes for cellulose synthases in these organisms.