Ten isoenzymes of xyloglucan endotransglycosylase from plant cell walls select and cleave the donor substrate stochastically

To map the preferred cleavage sites of xyloglucan endotransglycosylases (XE Ts; EC 2.4.1.207) along the donor substrate chain, we incubated the enzymes with tamarind (Tamarindus indica) xyloglucan (donor substrate; approximate to 205 kDa; 21 muM) plus the nonasaccharide [H-3]XLLGol (Gal(2). Xyl(3). G lc(3).[H-3]glucitol; acceptor substrate; 0.6 muM). After short incubation t imes, to minimize multiple cleavages, the size of the H-3-labelled transgly cosylation products (determined by gel-permeation chromatography) indicated the positions of the cleavage sites relative to the non-reducing terminus of the donor. There was very little difference between the size profiles of the products formed by any of ten XETs tested [one native XET purified fro m cauliflower (Brassica oleracea) florets, four native XET isoenzymes purif ied from etiolated mung-bean (Phaseolus aureus) shoots, native XETs purifie d from Lentil (Lens culinaris) and nasturtium (Tropaeolum majus) seeds, and three insect-cell-produced thalecress (Arabidopsis thaliana) XETs (EXGT, T CH4 and MERI-5)]. All such product profiles showed a good ht to a model in which the enzyme chooses its donor substrate independently of size and atta cks it, once only, at a randomly selected cleavage site. The results theref ore do not support the hypothesis that different XET isoenzymes are adapted to produce longer or shorter products such as might favour either the effi cient integration of new xyloglucan into the cell wall or the re-structurin g of old xyloglucan within an expanding wall.