Plant cell extensin

The ability of PO to interact with the acetyl residues of chitin allows us to compare them with monovalent lectins (i.e. extensins) which when binding with hemicellulose are only affected in a medium with a high ionic strength (Brownleader et al., 2006). As a rule, POs are bound with the plant cell wall and act as its modifiers. Some POs can form complexes with an extensin of cell walls (Brownleader et al., 2006). Consequently, chitin-specific sites that are capable of interacting with polysaccharides exist in the molecules of PO, and these sites can resemble the membrane receptor binding sites or else be similar to the domains of heparinbinding proteins (Kim et al., 2001). 

Cellulose microfibrils make up the basic framework of the primary wall of young plant cells (3), where they form a complex network with other polysaccharides. The linking polysaccharides include hemicellulose, which is a mixture of predominantly neutral heterogly-cans (xylans, xyloglucans, arabinogalactans, etc.). Hemicellulose associates with the cellulose fibrils via noncovalent interactions. These complexes are connected by neutral and acidic pectins, which typically contain galac-turonic acid. Finally, a collagen-related protein, extensin, is also involved in the formation of primary walls. 

In animal structural proteins in vivo, the only known dimer of tyrosine is dityrosine (40,41) in the extensin of plant cell walls, in contrast, the only dimer formed in vivo is isodityrosine (16). How is the coupling of tyrosine in plants confined to the formation of isodityrosine There is nothing unique about the local environment of the tyrosine residues in (pure) extensin, since... 

Role of Neighboring Polysaccharide Molecules in Determining the Orientation of Tyrosine Residues During Coupling. These considerations suggest a third possible explanation for the exclusive formation of isodityrosine in the plant cell wall in vivo that the neighboring structural molecules of the wall constrain extensin to prevent dityrosine formation. This would mean that the biologically relevant substrate for peroxidase in the plant cell wall is not naked extensin but extensin complexed with another wall component, possibly an acidic polysaccharide to which the extensin would bind ionically. 

Collagen-like triple helices also occur within other proteins. One of these is protein Clq, a component of the complement system of blood (Chapter 31). This protein interacts with antibodies to trigger a major aspect of the immune response. Clq has six subunits, each made up of three different polypeptide chains of about 200 residues apiece. Beginning a few residues from the N termini, there are over 80 residues in each chain with collagen-like sequences. The three chains apparently form a triple helix within each subunit. However, the C-terminal portions are globular in nature.200 Collagen-like tails also are present on some forms of the enzyme acetylcholinesterase (see Chapter 12C,10). Tire extensins of plant cell walls contain 4-hydroxyproline and evidently have a structure... 

The plant cell wall comprises layers of cellulose ml-croflbrlls embedded within a matrix of hemicellulose, pectin, extensin, and other less abundant molecules. 

Coldman, M.H.S., M. Pezzotti, J. Seurinck, and C. Marian Developmental expression of tobacco pistil-specific genes encoding novel extensin-like proteins The Plant Cell 4 (1992) 1041-1051. 

E.S. Deimisa Chitinase, P-l,3-glucanase, osmotin, and extensin are expressed in tobacco explants during flower formation The Plant Cell 2 (1990) 673-684. 

Lamport, D.T.A. (1973) Glycopeptide of Extensin. O-D-Galactosyl Serine and O-L-Arabinosyl Hydroxyproline in Biogenesis of Plant Cell Wall Polysaccharides, Proceedings of Symposium 1972 (F.A. Loewus, ed). Academic Press, New York, pp. 149-164... 

C. is the main component of the plant cell wall. Certain plant fibers, such as cotton, hemp, flax and jute, are almost pure C.Wood, in contrast, is only 40-60 % C. In the cell wall, the C. is arrang in microfibrils which are 100-300 A wide and about half as thick. The C. chains are parallel and stabilized by interchain hydrogen bonding. The microfibrils are embedded in a matrix of other polysaccharides, including pectin, hemicelluloses and lignin, and small amounts of a protein, extensin. 

Another common post-translationaUy modified amino acid is the prohne derivative L-4-hydroxyproIine, (2S,4R)-4-hydroxyproline, also known as L-4-hydroxypyrrolidine-2-carboxyKc acid (abbreviated Hyp, 2-6), which is an important structural component of collagen, gelatine (about 12% of content) and the polypeptide (glycopeptide) of plant cell walls known by the trivial name extensin (see Section 4.5.1.3.1). Its content is low in most other proteins. The amount of 4-hydroxyproline in meat products... 

Lamport, D. T. A. Hydroxyproline-O-glycosidic Linkages of the Plant Cell Wall Glycoprotein Extensin. Nature 216, 1322 (1967). 

Some divalent cations such as Cu and Pb form very stable complexes with pectate, but are unlikely to be present at sufiScient concentration in the apoplast of plants to form a major fraction of the counterions associated with the pectic fraction in vivo. The Al ion may deserve closer examination, as it is certainly able to displace Ca from cell walls and reaches substantial concentrations in plant roots under some conditions [60,61]. aluminium is not usually considered to be freely translocated, however. Basic peptides with their negative charges spaced at a similar interval to galacturonans (0.43 nm or a small multiple thereof) can in principle have a very high afiBnity for pectate [62,63], but the extensins that are associated with the most insoluble pectic fractions [M] do not appear to have this type of structure. The possibility that the non-extractable pectic polymers in most cell walls are very strongly complexed with some cation other than Ca " cannot be ruled out, but there is little evidence to support it at present. 

In plants, ascorbate is required as a substrate for the enzyme ascorbate peroxidase, which converts H202 to water. The peroxide is generated from the 02 produced in photosynthesis, an unavoidable consequence of generating 02 in a compartment laden with powerful oxidation-reduction systems (Chapter 19). Ascorbate is a also a precursor of oxalate and tartrate in plants, and is involved in the hydroxylation of Pro residues in cell wall proteins called extensins. Ascorbate is found in all subcellular compartments of plants, at concentrations of 2 to 25 mM—which is why plants are such good sources of vitamin C. 

Although the principal cell wall components of plants are carbohydrates, proteins account for 5-10% of the mass.165 Predominant among these are glycoprotein extensins. Like collagen, they are rich in 4-hydroxyproline which is glycosylated with arabi-nose oligosaccharides and galactose (p. 181). Other... 

In Section III, it was mentioned that cell wall is a complex structure formed by different polysaccharides connected to glycoproteins. Hydroxy-L-proline-rich glycoproteins, such as extensin, have been found in almost all plants surveyed, and in some algae.203,281 A network of protein, pectic polymers, and xyloglucan, serving to cross-link the cellulose fibers of the cell wall, has been proposed.282,283 However, covalent links between the different components have not been demonstrated moreover, some of them can be extracted separately,284 and some associations may be artificial.285 Nevertheless, results are consistent with interactions through dipole-dipole (such as hydrogen bonds) or hydrophobic bonds. 

A FIGURE 6-33 Schematic representation of the cell wall of an onion. Cellulose and hemicellulose are arranged into at least three layers in a matrix of pectin polymers. The size of the polymers and their separations are drawn to scale. To simplify the diagram, most of the hemicellulose cross-links and other matrix constituents (e.g., extensin, lignin) are not shown. [Adapted from M. McCann and K. R. Roberts, 1991, in C. Lloyd, ed.. The Cytoskeletal Basis of Plant Growth and Form, Academic Press,... 

The extensins are highly insoluble components of cell walls of terrestrial plants and are considered to be important in the plastic extension of the typical cell wall of the land plants (Lamport, 1970, 1973). However, some of the related arabinoproteins are found elsewhere in the cell, and one of the best studied is the readily soluble lectin of the potato, Solanum tuberosum (Allen and Neuberger, 1973), which has a high content of carbohydrate. 

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