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Arabinogalactan protein (AGPs)
Arabinogalactan protein is a broad term applied to diverse class of cell surface glycoproteins present in plant cell walls. It is heavily glycosylated, with only 2-10% comprising the protein region.The protein family has been earlier reported to contain O-linked glycans, whereas recent efforts employing mass spectrometry have revealed the presence of N-linked glycans as well within this protein family isolated from elongating cotton fiber cells.
Structure of Arabinogalactan Protein (AGPs)
AGPs contains protein backbone of varied length (5-30kDa) with N-terminal secretory peptide followed by AGP, Fasciclin domains and a C-terminal glycosylphosphatidylinositol (GPI) lipid anchor site. In some plant cells, the length of the mature protein backbone is only 10-13 residues long and they are therefore called as Arabinogalactan peptides. The protein backbone contains domain rich in hydroxyproline/proline, serine, alanine and glycine amino acids. The repeated occurrence of Alanine/Serine/Threonine-Proline stretch (glycomodules) and the presence of hydroxyproline suggests the sites for O-linked glycosylation and arabinogalactan modification. The O-linked glycan chains predominantly contain galactose, arabinose, rhamnose, mannose, galacturonic acid and/or glucuronic acids and have a relatively high degree of polymerization, with a structure similar to arabinogalactan II.
Biological roles of AGPs
AGPs are known to be involved in many biological processes like cell division, cell elongation, cell death, pattern formation, maintenance of post embryonic pattern, vascular, gametophyte development, signalling, root-microbe interaction etc.
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