Hydroxyproline-Rich Proteins

The high level of conservation of tyrosine and lysine residues suggests an important role for these amino acids and their post-translational derivatives in both adhesive and cohesive processes. As mentioned earlier, Waite has proposed that lysine and quinone residues are involved in protein crosslinking. Substitutions at other positions generally result in the presence of serine, threonine, proline (perhaps hydroxyproline), alanine, and isoleucine residues, with an emphasis on polar residues that can interact with most biological surfaces. The resulting protein is rich in the six amino acids tyrosine, lysine, alanine, serine, threonine and proline. Preliminary characterization of cDNA clones encoding... 

Proline is the only protein imino acid. In some proteins, proline is post-translationally hydroxylated to hydroxyproline. Such modifications occur to a large extent in hydroxyproline-rich glycoproteins (HPRG), which are involved in cell wall construction (Moore et al, 1991). Apart from the presence of proline in proteins, this amino acid also occurs in significant concentrations... 

Sadava, D., Walker, F., and Chrispeels, M. J., 1973, Hydroxyproline-rich cell wall protein (extensin) Biosynthesis and accumulation in growing pea epicotyls, Dev. Biol. 30 42-48. 

Free arabinose and galactose, which are often associated with hydroxyproline-rich proteins, are found in the free space of cell walls (132). Levels of proteins and free hydroxyproline are increased by ethylene treatment of pea (Fisum sativa) stem segments. However, the total amount of hydroxyproline in ethanol-insoluble polymers after extraction of the free space with water and Ca was not influenced by ethylene. Terry et al. (132) propose that the response to ethylene, which is now known to be derived from methionine, can be divided into two components. One requires changes in cellulose microfibrils of the cell wall, which result in reorientation of the plane of cell expansion. The other involves a change in the hemicellulosic xyloglucan, which inhibits extension growth of these cells. 

Hydroxyproline-rich glycoproteins are among the most prominent proteins in cell walls. The hydroxylation of proline is one of over 100 post-translational modifications that can occur on amino acid residues in protein (137). As a result of protein turnover (31) and cell stress, sometimes these amino acids may be found in extracellular matrices. 

Plants contain at least three classes of hydroxyproline-rich glycoproteins 1) the cell wall proteins, 2) the arabinogalactan proteins, and 3) lectins. According to Cooper and Varner (23) the hydroxyproline-rich glycoprotein of the cell wall of carrots is secreted from the cytoplasm as a soluble monomer that slowly becomes insolubilized. 

Several classes of proteins have been identified in plant walls and termed structural proteins , implying an absence of enzymatic function (reviewed in [15]). These include hydroxyproline-rich glycoproteins (HRGPs), proline-rich proteins (PRPs), glycine-rich proteins (GRPs) and arabinogalactan proteins (AGPs). Primary walls also contain numerous enzymes, and still other proteins with no known function [68]. 

Fragments of the hydroxyproline-rich protein obtained from the primary cell walls of dicots invariably contain arabinosyl and galactosyl... 

Single galactosyl residues are glycosidically attached to the serine hydroxyls of the hydroxyproline-rich cell wall proteins of suspension-cultured tomato cells (84, 85). This was shown by removing the arabinosides from the intact cell walls by acid hydrolysis (pH 1 for 1 hr at 100° C). The hydroxyproline-rich wall protein, with arabinosyl residues removed, is susceptible to proteolysis with trypsin. The resulting solubilized tryptides have been separated by cation exchange and gel filtration chromatography. The composition of the tryptides has been determined by amino acid analysis (85). Some of the tryptides have been sequenced by subtractive N-terminus identification and further partial acid hydrolysis (85). 

Each of the hydroxyproline-rich wall protein tryptides contains a pentapeptide of serine-(hydroxyproline)4, while most of the tryptides contain one or more galactosyl residues (85). One tryptide, which was found to contain 2 residues each of galactose and serine, was subjected to P-elimination by several different methods. The elimination procedures converted serine to alanine or cysteic acid with a concomitant release of free galactose (85). These results demonstrated the covalent attachment of a single galactosyl residue to each seryl residue in the... 

The covalent attachment of arabinose and galactose to the hydroxy-proline-rich proteins of primary cell walls is a generally accepted fact (83,85). However, the available evidence suggests that the hydroxyproline-rich glycoprotein is not covalently attached to any of the other cell wall polymers. The evidence does not rule out the possible existence of strong, non-covalent bonding between the hydroxyproline-rich glycoprotein and the other wall polymers. 

A hydroxyproline-rich glycoprotein is secreted by suspension-cultured sycamore cells into their culture medium (75). The carbohydrate component of the glycoprotein is an arabinogalactan. The structure of the arabinogalactan portion of this glycoprotein has been studied by methylation analysis and found to be structurally similar to a protein-free arabinogalactan which is also present in the culture medium of suspension-cultured sycamore cells (see Section IV F). 

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