A MEMBRANE-ASSOCIATED FORM OF SUCROSE SYNTHASE AND ITS POTENTIAL ROLEIN SYNTHESIS OF CELLULOSE AND CALLOSE IN PLANTS

Sucrose synthase (SuSy; EC 2.4.1.13; sucrose + UDP reversible arrow UD Pglucose + fructose) has always been studied as a cytoplasmic enzyme i n plant cells where it serves to degrade sucrose and provide carbon fo r respiration and synthesis of cell wall polysaccharides and starch, W e report here that at least half of the total SuSy of developing cotto n fibers (Gossypium hirsutum) is tightly associated with the plasma me mbrane. Therefore, this form of SuSy might serve to channel carbon dir ectly from sucrose to cellulose and/or callose synthases in the plasma membrane. By using detached and permeabilized cotton fibers, we show that carbon from sucrose can be converted at high rates to both cellul ose and callose, Synthesis of cellulose or callose is favored by addit ion of EGTA or calcium and cellobiose, respectively, These findings co ntrast with the. traditional observation that when UDPglucose is used as substrate in vitro, callose is the major product synthesized. Immun olocalization studies show that SuSy can be localized at the fiber sur face in patterns consistent with the deposition of cellulose or callos e, Thus, these results support a model in which SuSy exists in a compl ex with the beta-glucan synthases and serves to channel carbon from su crose to glucan.