Xyloglucan glucosyltransferase in Golgi membranes from Pisum sativum (pea).

Cell membranes from etiolated Pisum sativum (pea) tissues were separated by ultracentrifugation on linear sucrose density gradients and assayed for membrane marker and glycosyltransferase activity. Membrane fractions were shown to incorporate glucose from UDP-D-[14C]glucose into polysaccharides with glycosyl linkages consistent with synthesis of xyloglucan. A combined assay using g.c., radiogas proportional counting and m.s. was employed to determine the identities of 14C-labelled glycosyl residues and the glycosyl linkages between them. In glucan synthase I assays, membrane fractions enriched for Golgi membranes showed 14C incorporation into 4- and 4,6-glucose residues, with minor incorporation into 3-glucose residues. In glucan synthase II assays, all 14C incorporation was into 3- and 3,4-glucose. There was a shift in glycosyl linkage of 14C incorporation from predominantly 4-glucose at low UDP-glucose concentration to predominantly 3- and 3,4-glucose at high UDP-glucose concentrations. Mn2+ stimulated incorporation of radioactivity into 4,6-glucose residues characteristic of xyloglucan polysaccharides. Addition of exogenous UDP-xylose to assay mixtures stimulated incorporation into 4,6-glucose, with a maximum at 15 microM UDP-xylose.