The β-1,3-Glucan Synthase and Lessons from in vitro β-1,3-Glucan Synthesis
To understand the biochemical machinery required for cellulose synthesis in
plants, it is necessary to demonstrate in vitro
synthesis of cellulose using plant
extracts. Unfortunately, much to the dismay of most researchers studying cellulose
biosynthesis, the major in vitro
polysaccharide product synthesized from
plant extracts using UDP-glucose as the precursor was and is still found to be
callose, the β-1,3-glucan first reported from mung bean extracts by Feingold and
colleagues in 1958 (Feingold et al.
, 1958). Observing the synthesis of this polysaccharide
in place of cellulose has been both frustrating and invigorating as it brings
up a number of very interesting questions, many of which have not been fully
During normal development, cellulose is found in all plant cells, whereas
callose generally is synthesized in response to wounding, physiological stress,
or infection, and is a component of the cell plate in dividing cells apart from being
present in specialized cells. As such, enzymes for synthesis of this polysaccharide
are not expected to be active most of the time. The general explanation to account
for the large amount of in vitro
synthesis of callose as opposed to cellulose using
plant extracts is that this occurs in response to the wounding or stress of the
cells during cell breakage. Using antibodies against β-1,4-glucan synthase and
β-1,3-glucan synthase, Nakashima et al.
(2003) recently demonstrated that the
activation of β-1,3-glucan synthase upon wounding may be dependent on the
degradation of β-1,4-glucan synthases by specific proteases (Nakashima et al.
2003). However, under appropriate conditions in the presence of UDP-glucose,
plant extracts synthesize both callose and cellulose, and the optimal conditions
required for synthesis of these two polysaccharides have been shown to be only
slightly different. Whether the same enzyme catalyzes the synthesis of both
callose and cellulose has been debated for a number of years, but so far no
conclusive evidence is available in support of either the one enzyme-two polysaccharides
or the one enzyme-one polysaccharide synthesis with respect to these
two polysaccharides. Although it has been possible to separate the major
cellulose-synthesizing and callose synthesizing activities by native gel electrophoresis,
the polypeptide composition in these two fractions could not be completely
analyzed (Kudlicka and Brown, 1997). Interestingly, relatively much more is
known about the identity of the catalytic subunit of cellulose synthase as compared
to the nature of the catalytic subunit of callose synthase. This is true, in spite
of the fact that genes required for synthesis of β-1,3-glucans have been identified
in yeast, and similar genes have been identified in a number of plants (Cui et al.
2001; Doblin et al.
, 2001; Hong et al.
, 2001; Li et al.
, 2003). Surprisingly, the proteins
encoded by these genes do not show similarity to any known glycosyltransferase,
much less the cellulose synthases. These proteins are classified as 1,3-β-D-glucan
synthases and have been placed in family 48 of glycosyltransferases (http://afmb.
cnrs-mrs.fr/CAZY/). In plants, genes encoding this protein form a gene family,
and in Arabidopsis
10 members are identified in this gene family.
Since synthesis of β-1,3-glucans occurs much more readily when plant extracts
are used in vitro
, many more studies have reported on characterization of the
conditions for β-1,3-glucan synthase activity and its purification from a variety of
plants. As an example, optimal conditions for in vitro
synthesis of β-1,3 glucan
were defined by the presence in the reaction mixture of 50 mM
3-(N-morpholino) propanesulfonic acid (MOPS) buffer, pH 6.8, 1 mM UDPglucose,
8 mM Ca2+
, and 20 mM cellobiose (Lai-Kee-Him et al.
, 2001). Similar
conditions, in the presence or absence of Mg2+
in the reaction
mix, have also
been shown to be optimal for the synthesis of cellulose using plant extracts
(Colombani et al.
, 2004). Since both callose synthase and cellulose synthase are
membrane proteins, the choice and concentration of detergents used during
extraction of the proteins have been found to be very crucial in obtaining high
specific activity of both callose synthase and cellulose synthase from plant
extracts. Incorporating a variety of techniques, Dhugga and Ray (1994) purified the β-1,3-glucan synthase activity 5,500-fold from pea homogenates and found
two polypeptides that copurified with the enzyme activity (Dhugga and Ray,
1994). Unfortunately, the identity of these proteins could not be determined,
although one of these polypeptides was shown to bind to UDP-glucose. In related
sets of experiments, Kudlicka and Brown (1997) demonstrated separation of the
callose synthase and cellulose synthase activities in digitonin-solubilized mung
bean membranes using gel electrophoresis under nondenaturing conditions
(Kudlicka and Brown, 1997). The polypeptide composition in the two fractions
was analyzed by SDS-PAGE, and while three similar sized polypeptides were
observed in both activities, polypeptides unique to each activity were also
observed. However, the characterization of these polypeptides did not provide
any further information regarding the similarities or differences between the two
enzyme activities. As mentioned in this section, many of the studies for in vitro
synthesis of callose were applicable to in vitro
synthesis of cellulose using plant
extracts. Interestingly, conclusive demonstrations of cellulose synthesis in vitro
using plant extracts had to do more with utilizing a greater variety of techniques
for product characterization than with development of novel assay methods.
Increasing Cellulose Synthase Activity in vitro and Utilizing More
Techniques for Product Characterization
Techniques to identify and characterize the cellulose product have played a crucial
role in determining cellulose synthesis in vitro
. Interestingly, many of the criteria
used by Glaser in 1958 for in vitro
cellulose production using bacterial extracts are
still used for characterizing the cellulose product and determining the cellulose
synthase activity, namely incorporation of 14C-glucose from UDP-14C-glucose
into a hot alkali-insoluble fraction (Glaser, 1958). The product was further characterized
by acid hydrolysis and/or enzymatic analysis using cellulases. Although less
than 1% of the glucose fromUDP-glucose was incorporated into the alkali-insoluble
fraction in the in vitro
reaction, the product was characterized as cellulose.
A major breakthrough in understanding cellulose biosynthesis in A. xylinum
and increasing cellulose synthase activity in bacterial extracts came with the
identification of cyclic di-guanosine monophosphate (c-di-GMP) as an allosteric
activator of cellulose synthase (Ross et al.
, 1986). This nucleotide is now recognized
to be a regulator of many more bacterial functions than previously thought
(D’Argenio and Miller, 2004). The addition of c-di-GMP in reaction mixtures using
bacterial extracts led to a remarkable increase in incorporation of glucose from
UDP-glucose into a cellulose product.
In another development, the in vitro
product using bacterial extracts for the
first time was visualized by electron microscopy, and this product was shown to
bind to gold-labeled cellobiohydrolase providing evidence that this product is
cellulose (Lin et al.
, 1985). The in vitro
product obtained using A. xylinum
membrane was furthermore shown to be cellulose II (Bureau and Brown, 1987).
The capability to synthesize large amounts of the in vitro
product was crucial in
performing X-ray diffraction, sugar analysis, linkage analysis and molecular
weight analysis to demonstrate conclusively that the product was cellulose
(Bureau and Brown, 1987).
Many of these techniques were later utilized by Okuda et al.
(1993) using cotton
fiber extracts to demonstrate the in vitro
production of cellulose II (Okuda et al.
Additionally, the incorporation of glucose from UDP-glucose into an Updegraff
reagent-resistant fraction was included to be a stricter criterion for the cellulose
product. Although no activator comparable to c-di-GMP was identified for activation
of the cellulose synthase fromplant tissues, a number of nucleotides were found
to increase the in vitro
cellulose synthase activity (Li and Brown, 1993). Overall, the
success in demonstrating cellulose synthesis in vitro
is ascribed to the choice of
plant tissue (cotton fibers), method of extraction, and the ability to synthesize
large amounts of the in vitro
product for characterization. Although cellulose was
synthesized in vitro
using plant extracts, the major product was still β-1,3 glucan,
and this could be distinguished from cellulose using electron microscopy.
In later studies, using a variety of detergents, Kudlicka et al.
(1995) was able to
demonstrate not only an increase in the amount of cellulose synthesized in vitro
but also the production of cellulose I using plant extracts (Kudlicka et al.
Lai-Kee-Him et al.
(2002) used detergent solubilized microsomal fractions from
suspension-cultured cells of blackberry (Rubus fruticosus
) for in vitro
synthesis (Lai-Kee-Him et al.
, 2002). These investigators found that the detergents
Brij 58 and taurocholate were effective in solubilizing membrane proteins that
allowed synthesis of both cellulose and callose given UDP-glucose as the
substrate. Roughly 20% of the in vitro
product was cellulose with taurocholate
as the detergent, and no Mg2+
was required. The cellulose product was characterized
by methylation analysis, electron microscopy, electron and X-ray synchrotron
diffractions, and resistance to Updegraff reagent. Cellulose microfibrils were
obtained in vitro
, and they had the same dimensions as microfibrils isolated from
primary cell walls. However, the cellulose diffracted as cellulose IVI, a disorganized
form of cellulose I that is formed when the fibrillar material contains
crystalline domains that are too narrow or too disorganized to be considered
real cellulose I crystals (Lai-Kee-Him et al.
In related studies, but using immunoaffinity purified cellulose synthase from
mung bean hypocotyls, Laosinchai (2002) also demonstrated the in vitro
of cellulose microfibrils (Laosinchai, 2002).