138. Brown, Jr. R. M. 1990. Algae as tools
in studying the biosynthesis of cellulose, nature's most abundant
macromolecule. In: Experimental Phycology l: Cell
walls and Surfaces, Reproduction,
Photosynthesis Ed:Wiessner, Robinson, and Starr. Springer-Verlag
Berlin pp 20-39.
138. Introduction
The most dominant polysaccharide of the
cell wall is cellulose. The universal distribution of this natural
polymer among procaryotic and eucaryotic organisms attests to
its ancient evolutionary history. Not only is cellulose found
among photosynthetic and protistan cells, it is present in animals
such as the Ascidians (Wardrop, 1970). Furthermore, levels of
elevated cellulose synthesis have been suggested in humans with
the disease scleroderma (Hall, t al, 1960). The algae have been
prominent organisms of study among eucaryotic organisms because
of their great diversity of structure and cellular organization,
Chloroplast morphology (Gibbs, 1981), organization of the mitotic
apparatus (Pickett-Heaps, 1972) , cell wall structure and composition
(Preston, 1974), flagellar apparatus (Stewart and Matter, 1978),
and reproduction have provided a wealth of information on algal
phylogeny (Stewart and Matter, 1982); however, very few studies
have concentrated on evolutionary and phylogenetic aspects of
cell walls, let alone cellulose , mainly because the cellulose
synthases had never been observed or isolated. In·1976,
the first successful application of freeze fracture demonstrated
the structure of a cellulose synthase complex in Oocystis apiculata
(Brown and Montezinos, 1976). These membrane associated structures,
called terminal synthesizing complexes (=TCs), were found at the
growing tip of microfibril impressions on the E-fracture face
of the plasma membrane. Earlier, Roelofsen (1958) had predicted
that an organized terminal enzyme complex would be found; however,
the shape and geometry could not be predicted at that time. In
1964, Preston proposed the ordered granule hypothesis for the
cellulose synthase complex. This study was based on observations
of remnants of organized particle subunits associated with the
innermost wall of Chaetomorpha. During this time, the
freeze fracture technique was becoming widely used , and organized
particle complexes were found in the plasma membranes, particularly
abundant in yeasts (Moor and Muhlethaler, 1963). Thus, a logical
extension of the possibility of such an organization could be
made for the cellulose synthesizing complex. Interestingly,
TCs were not observed in cells which had been chemically fixed
or treated with glycerol or cryoprotectants. The breakthrough
in finding TCs came when Brown and Montezinos demonstrated that
rapid direct freezing of living cells yielded organized particle
structures associated with the tips of microfibrils. Since 1976,
TCs have been found in more than 14 algal genera, and presently,
a distinct pattern of TC structure is beginning to emerge which
provides some insight into the substantial diversity of cellulose
microfibril synthesis among the algae. In this presentation,
the fundamental TC diversity among the algae will be described.
Because TC variation is the greatest among the algae, it also
provides hints at the relationship between TC organization and
microfibril shape, molecular weight of the cellulose, and crystallization.
Phylogenetic relationships based on TC structure can be correlated
with other structural and biochemical evidence. This presentation
will conclude with a proposed evolutionary history of cellulose
biogenesis.