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.

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Last modified 27 October 2005.
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