Biosynthesis of Xyloglucan in Suspension-cultured Soybean Cells. Evidence that the Enzyme System of Xyloglucan Synthesis does not Contain β-1,4-Glucan 4-β-D-Glucosyltransferase Activity (EC 2.4.1.12)

Abstract

Xyloglucan 4-β-D-glucosyltransferase, an enzyme responsible for the formation of the xyloglucan backbone, in a particulate preparation of soybean cells has been compared with β-1,4-glucan 4-β-D-glucosyltransferase of the same origin. The following observations indicate that the enzyme system of xyloglucan synthesis does not contain β-1,4-glucan 4-β-D-glucosyltransferase activity, although both enzymes transfer the glucosyl residue from UDP-glucose to form the β-1,4-glucosidic linkage: 1. The incorporation of [¹⁴C]glucose into xyloglucan depended on the presence of UDP-xylose in the incubation mixture. 2. No measurable amount of radioactivity was incorporated from UDP-[¹⁴C]xylose into the cello-oligosaccharides, although the incorporation of [¹⁴C]xylose into xyloglucan depended on the presence of UDP-glucose in the incubation mixture (Hayashi and Matsuda 1981b). 3. The activity of xyloglucan 4-β-D-glucosyltransferase was stimulated more strongly by Mn²⁺ than by Mg²⁺, whereas Mg²⁺ was the most active stimulator for the activity of β-1,4-glucan 4-β-D-glucosyltransferase. 4. An addition of GDP-glucose (100 μM) to the incubation mixture inhibited the activity of xyloglucan 4-β-D-glucosyltransferase by 17%, whereas the activity of β-1,4-glucan 4-β-D-glucosyltransferase was inhibited 56% under the same conditions. 5. Irpex exo-cellulase did not hydrolyze the xyloglucan synthesized in vitro. 6. The β-1,4-glucan synthesized in vitro was not a branched xyloglucan because it gave no 2,3-di-O-methyl glucose derivative on methylation analysis. 7. Pulse-chase experiments indicated that the β-1,4-glucan was not transformed into the xyloglucan. The subcellular distribution of the xyloglucan synthase, however, was similar to that of the β-1,4-glucan synthase (Golgi-located 1,4-β-D-glucan 4-β-D-glucosyltransferase). Thus, it appears that the latter enzyme is located at a site close to xyloglucan synthase and is set aside for the assembly of these polysaccharides into the plant cell surface.