Intimately associated with the cell wall carbohydrate and lignin framework is a network of proteins thought to impart structural integrity to the plant cell wall.
These structural proteins are enriched with glycoproteins containing hydroxypro-line, proline, and glycine. Another class of structural proteins called extensin, similar to collagen, consists of repeating serine-hydroxyproline and tyrosine-lysine-tyrosine sequences (Fig. 12.10). This structural protein plays an important architectural role cross-linking lignin to the carbohydrate network and may comprise up to 15% of the primary cell wall (Lodish et al., 2000). Most of the hydroxypro-lines are glycosylated with chains of three or four arabinose residues, and the serines are linked to galactose, making extensin about 65% carbohydrate. In addition to providing structural integrity to the plant cell wall, the unique amino acid and gly-cosation sequence may prevent microbial attack by presenting unrecognizable cleavage sites to proteolytic enzymes. Pathogens may gain access to the middle lamella through their ability to degrade extensin. The degradation of extensin may provide access for enzymes to further the attack of the polysaccharide and lignin framework of plant cell walls. Extensin most likely provides a source of N during decomposition. Another group of soil glycoproteins is found in the product identified as glomalin in soil. This fairly stable fungal product is produced by arbuscular mycorrhiza and is said to be involved with aggregation (Rillig, 2004).
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