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 endotrans-glycosylase (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 [H-3]-XLLGol (Gal(2)-Xyl(3).Glc(3.)[1-H-3]-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 [H-3]-X-LLGol 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. (C) 2001 Editions scientifiques et medicales Elsevier SAS.