In vitro glucan synthesis by membranes of celery petioles: the role of the membrane in determining the type of linkage formed.

Glucan synthesis was achieved with an in vitro membrane fraction from the petioles of celery (Apium graveolens). The optimum conditions for maximum synthesis were established. The Km and Vmax for the enzymic system were 1.0 mM and 0.19 microM min-1 mg protein-1, respectively. Mechanical damage to the membrane fraction altered the proportion of beta-(1----3) to beta-(1----4) glucosyl linkages that were synthesized. We suggest that cellulose synthesis (beta-(1----4)-linked glucan chains) is controlled by the availability of UDP-glucose at the plasma membrane surface in conjunction with an organized relationship between the synthase system and a specifically oriented glucosyl radical acting as an acceptor held on the membrane surface. An intact membrane is therefore necessary to direct synthesis for the beta-(1----4) bond by an enzyme that is capable of transglucosylation to the secondary alcoholic groups on C-2, C-3 or C-4 of the acceptor radical. The specificity of the system is controlled by the whole enzyme complex held on the membrane.