Xyloglucan from the walls of Rosa cells that had been cultured on [C-12]- or [C-13]-glucose formed bands in caesium trifluoroacetate with mean buoyant densities of 1.575 or 1.616 g/ml respectively. Incubation of a mixture of [C-13,H-3]xyloglucan and [C-12,H-1]xyloglucan in the presence of xyloglucan endotransglycosylase (XET) activity caused the mean buoyant density of the radioactive material to decrease, indicating that interpolymeric transglycosylation could be detected in vitro. We used two C-13/H-3-dual-labelling protocols to look for interpolymeric transglycosylation in vivo. In protocol A, [C-13]glucose-grown Rosa cells were transferred into [C-12]glucose medium 6 h after a approximate to 2 h pulse of L-[1-H-3]arabinose (which radiolabels the xylose residues of xyloglucan). The mean buoyant density of the wall-bound [H-3]xyloglucan decreased during the following 7 days in culture, This indicates that, during or after the wall-binding of newly synthesized [C-12,H-1]xyloglucan, it became covalently attached to previously wall-bound [C-13,H-3]xyloglucan. In protocol B, [C-12]glycerol- or [C-12]glucose-grown Rosa cells were transferred into [C-13]glucose medium, 20 or 60 min before a approximate to 2h pulse of [H-3]arabinose. The buoyant density of the earliest wall-bound [H-3]xyloglucan showed that it had a C-12/C-13 ratio of similar to 1:1. This indicates that, during (or, implausibly, before) wall-binding, the newly synthesized [C-13,H-3]xyloglucan became covalently attached to previously synthesized [C-12]xyloglucan. During the following 7 days in culture, the mean buoyant density of the [H-3]xyloglucan increased, showing that later-synthesized [C-13,H-1]xyloglucan can be covalently attached to previously wall-bound [C-12,C-13,H-3]xyloglucan. The only known mechanism by which segments of xyloglucans could become covalently attached to each other in the cell wall is by interpolymeric transglycosylation catalysed by XET, We conclude that XET-catalysed interpolymeric transglycosylation accompanies, and probably causes, the integration of newly secreted xyloglucan into the cell-wall architecture.
|Number of pages||10|
|Publication status||Published - 1 Nov 1997|