Hydroxyproline-O-glycosidic Linkage of the Plant Cell Wall Glycoprotein Extensin

Abstract

PRIMARY cell walls contain a protein, extensin, which is rich in trans 4-L-hydroxyproline^1–3. I suggested earlier that this protein could cross link wall polysaccharides⁴; if some of these cross linkages were labile, it would provide a chemical basis for changes in cell wall plasticity which are necessary for extension of plant cells. The recent isolation of several glycopeptides, rich in hydroxyproline from enzymatic digests of tomato cell walls⁵, confirmed the inference of a covalent carbohydrate–protein linkage in extensin. The chemical composition and properties of these glycopeptides also led me to the preliminary conclusion that attachment of the carbohydrate is by a glycosidic link through the hydroxyl group of hydroxyproline⁵. This rather unexpected possibility receives additional support from results presented in this communication, describing the isolation of hydroxyproline-O-glycosides from partial alkaline hydrolysates of tomato cell walls. These cell wall preparations account for more than 95 per cent of the hydroxyproline of the cells³. Most of this hydroxyproline (about 70 per cent) can be released as hydroxyproline glycosides. The method is based on the fact that glycosidic linkages are usually stable to those alkaline conditions which lead to rapid hydrolysis of peptide bonds⁶.