A macromolecule consisting of a protein backbone along which short oligosaccharide branches are attached at intervals. The most important plant glycoprotein is extensin, a structural component of cell walls. The pro-tein backbone of extensin is unusual in that it contains a large number of *hydroxyproline
residues (about 30% of the total amino acid residues). The hydroxyproline serves to provide attachment points for the carbohydrate chains - in extensin these side chains are tetrasaccharides of arabinose.