The mechanism of synthesis of a mixed-linkage (1 -> 3),(1 -> 4)beta-D-glucan in maize. Evidence for multiple sites of glucosyl transfer in the synthase complex

We examined the mechanism of synthesis in vitro of (1 --> 3),(1 --> 4) beta -D-glucan (beta-glucan), a growth-specific cell wall polysaccharide found i n grasses and cereals, beta-Glucan is composed primarily of cellotriosyl an d cellotetraosyl units linked by single (1 --> 3) beta-linkages. The ratio of cellotriosyl and cellotetraosyl units in the native polymer is strictly controlled at between 2 and 3 in all grasses, whereas the ratios of these u nits in beta-glucan formed in vitro vary from 1.5 with 5 mu M UDP-glucose ( Glc) to over 11 with 30 mM substrate. These results support a model in whic h three sites of glycosyl transfer occur within the synthase complex to pro duce the cellobiosyl-(1 --> 3)-D-glucosyl units. We propose that failure to fill one of the sites results in the iterative addition of one or more cel lobiosyl units to produce the longer cellodextrin units in the polymer. Var iations in the UDP-Glc concentration in excised maize (Zea mays) coleoptile s did not result in wide variations in the ratios of cellotriosyl and cello tetraosyl units in beta-glucan synthesized in vivo, indicating that other f actors control delivery of UDP-Glc to the synthase. In maize sucrose syntha se is enriched in Golgi membranes and plasma membranes and may be involved in the control of substrate delivery to beta-glucan synthase and cellulose synthase.