Glycoprotein extensins

The hydroxy-L-proline of the crude, mung-bean-wall preparation is not associated with the lectin259 this indicates a difference between the mung-bean-wall lectin and the lectin isolated from potato tubers. The latter is rich in hydroxy-L-proline, arabinose, and galactose260 in this respect, it shows features similar to those of the hydroxy-L-proline-rich glycoprotein ( extensin ) of the primary cell-wall, first reported by Lamport261 (see Section VII, 1). 

Fig. 8. — Partial Model of Primary Cell-Wall in Lupin Hypocotyl, Proposed by Monro and Coworkers.49 [The half of the Figure labeled (A) represents the extensin-hemicellulose network, and the half labeled (B) represents the separate, pectic network, which is believed not to involve the wall glycoprotein (extensin). Thus, the cellulose microfibrils (M) are separately cross-linked by two networks of polymers, the first (A) being composed of the wall glycoprotein and polysaccharide (probably hemicelluloses), and the second (B) being composed of the pectic polymers. These two networks have been separated in the Figure for clarity. This model is tentative and incomplete, as the nature of the linkages between the polymers in these two networks has not yet been identified. The...

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... 

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

The basic extensins. These are hydroxyproline-, lysine- and tyrosine-rich glycoproteins consisting of rigid molecular rods about 80 nm long (14,15), bearing short mono- to tetrasaccharide side-chains (2,14). When newly secreted they bind ionically to the acidic polysaccharides of the cell wall and can be extracted with cold salt solutions later they become much more resistant to salt-extraction and are said to be covalently bound, probably via dimerization of their tyrosine residues to form isodityrosine (15). 

It has not been clearly established whether extensin is a lectin, but such a lectin property might allow this glycoprotein to bind noncova-lently to polysaccharides within the primary-wall structure. 

Since the discovery of the arabinosyloxy-L-proline-rich glycoprotein, sometimes referred to as extensin, in the primary cell-wall by Lamport,41 and its negative correlation with growth,263 the cell-wall matrix has been regarded by some as an extensin-polysaccharide complex. 

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. 

E. Extraction and Analysis of Hydroxyproline (Hyp)-Rich Glycoprotein(s) (Extensin)... 

E. EXTRACTION AND ANALYSIS OF HYDROXYPROLINE (HYP)-RICH GLYCOPROTEIN(S) (EXTENSIN)... 

The extensins and allied glycoproteins are peculiar to plants and are distinguished by their content of 4-hydroxy-L-proline and L-arabinose, which are covalently linked by way of an alkali-stable O-glycosidic bond. In addition they also contain varying amounts of galactosyl residues, linked Oglyco-sidically to serine. 

Hydroxyproline. Glycoproteins containing sugars linked to hydroxyproline comprise those in which the sugar is arabinose as in plant extensin or galactose as in a protein found in the alga chlamidomonas. 

Varner, 1988). Extensin is secreted as single soluble monomeric hydroxyproline forms that are slowly insolubilized in the cell wall, probably via the oxidative formation of isodityrosine cross-links (Cooper and Varner, 1983 Fry, 1986a). Wall-catalyzed cross-linking of soluble extensin is inhibited by ascorbate, indicating that this reaction is dependent on an oxidative mechanism (Cooper and Varner, 1984). Thus, it is probable that a cell wall-bound peroxidase-ascorbate oxidase system controls the redox state at the wall, and hence the extensibility of cell wall, by controlling the cross-linking of wall glycoproteins. 

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