EVIDENCE FOR A UDP-GLUCOSE TRANSPORTER IN GOLGI APPARATUS-DERIVED VESICLES FROM PEA AND ITS POSSIBLE ROLE IN POLYSACCHARIDE BIOSYNTHESIS

The Golgi apparatus in plant cells is involved in hemicellulose and pe ctin biosynthesis. While it is known that glucan synthase I is respons ible for the formation of beta-1-4-linked glucose (Glc) polymers and u ses UDP-Glc as a substrate, very little is known about the topography of reactions leading to the biosynthesis of polysaccharides in this or ganelle. We isolated from pea (Pisum sativum) stems a fraction highly enriched in Golgi apparatus-derived vesicles that are sealed and have the same topographical orientation that the membranes have in vivo. Us ing these vesicles and UDP-Glc, we reconstituted polysaccharide biosyn thesis in vitro and found evidence for a luminal location of the activ e site of glucan synthase I. In addition, we identified a UDP-Glc tran sport activity, which is likely to be involved in supplying substrate for glucan synthase I. We found that UDP-Glc transport is protein medi ated. Moreover, our results suggest that UDP-Glc transport is coupled to the exit of a luminal uridine-containing nucleotide via an antiport er mechanism. We suggest that UDP-Glc is transported into the lumen of Golgi and that Glc is transferred to a polysaccharide chain, whereas the nucleotide moiety leaves the vesicle by an antiporter mechanism.