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Extensins are a family of flexuous, rodlike, hydroxyproline-rich glycoproteins (HRGPs) of the plant cell wall. [1] [2] They are highly abundant proteins. There are around 20 extensins in Arabidopsis thaliana. They form crosslinked networks in the young cell wall.
Hydroxyproline rich glycoproteins (HRGPs) are also found in plant cell walls. [9] These hydroxyprolines serve as the attachment points for glycan chains which are added as post-translational modifications. [9]
They are members of the wider class of hydroxyproline (Hyp)-rich cell wall glycoproteins, a large and diverse group of glycosylated wall proteins. AGPs have been reported in a wide range of higher plants in seeds, roots, stems, leaves and inflorescences. AGPs account for only a small portion of the cell wall, usually no more than 1% of dry mass ...
Additionally, structural proteins (1-5%) are found in most plant cell walls; they are classified as hydroxyproline-rich glycoproteins (HRGP), arabinogalactan proteins (AGP), glycine-rich proteins (GRPs), and proline-rich proteins (PRPs). Each class of glycoprotein is defined by a characteristic, highly repetitive protein sequence.
In 2001, biologically active hydroxyproline-rich glycopeptides were isolated from tobacco which activated the production of protease inhibitors in a similar way to systemin in tomatoes. [1] Although they are structurally unrelated to systemins, their similar function resulted in them being named hydroxyproline-rich systemins (HypSys).
Adult plant resistance (APR) is a specialist term referring to quantitative resistance that is not effective in the seedling stage but is effective throughout many remaining plant growth stages. [ 43 ] [ 44 ] [ 42 ] The difference between adult plant resistance and seedling resistance is especially important in annual crops . [ 45 ]
One example of glycoproteins found in the body is mucins, which are secreted in the mucus of the respiratory and digestive tracts. The sugars when attached to mucins give them considerable water-holding capacity and also make them resistant to proteolysis by digestive enzymes. Glycoproteins are important for white blood cell recognition.
In protein, hydroxyproline is incorporated into protein by hydroxylation of proline. Pipecolic acid, a heavier analog of proline, is found in efrapeptin. Sarcosine is a N-methylized glycine so its methyl group is used in many biochemical reactions. Azetidine-2-carboxylic acid, which is a smaller homolog of proline in plants.