Release of complexed xyloglucan endotransglycosylase (XET) from plant cell walls by a transglycosylation reaction with xyloglucan-derived oligosaccharides

Incubation of isolated NaCl-washed cell walls from epicotyls of pea (Pisum sativum) and nasturtium (Tropaeolum majus) with solutions of various oligosaccharides released among others the cell wall marker enzyme xyloglucan endotransglycosylase (XET, EC 2.4.1.207). The greatest release of XET occurred upon incubation of the cell walls with xyloglucan-derived oligosaccharides (XGOS, DP 7-9). Concomitantly, reduced radioactive nonasaccharide 〚3H〛-XLLGol (Gal2.Xyl3.Glc3.〚1-3H〛-glucitol) was incorporated into the cell walls. Subsequent hydrolysis of the radioactively labelled cell walls with Trichoderma cellulase liberated XGOS-alditols, DP 7-9 as the sole radioactive products indicating that 〚3H〛-XLLGol was incorporated into the cell wall xyloglucan by transglycosylation, as an entity. Oligosaccharides of cello-, chito- and/or oligoglucurono-series were much less effective than XGOS but a substantial liberation of XET and other proteins from plant cell walls could be achieved by the nucleophile 0.1 M imidazole. The specific release of the cell wall-associated XET activity by incubation with xyloglucan-derived oligosaccharides and the simultaneous incorporation of the tritiated xyloglucan nonasaccharide en bloc into the cell walls indicates that XET is present in the cell walls in form of a competent glycosyl-enzyme complex which decomposes by transglycosylation of its glycan moiety to added xyloglucan-oligosaccharide acceptors. This finding suggests a new concept for the regulation of activity of cell wall-associated glycanases/transglycosylases: they exist in plant cell walls in a transiently latent state as covalent glycosyl-enzyme complexes and are active only when suitable glycosyl acceptors become available.