Abstract
Membrane-derived oligosaccharides are branched, substituted beta-glucans localized in the periplasmic space of Escherichia coli and other Gram-negative bacteria. The biosynthesis of membrane-derived oligosaccharides and of analogous periplasmic oligosaccharides found in plant bacteria is of particular interest because it is subject to strict osmotic regulation [Miller, K.J., Kennedy, E.P., and Reinhold, V.N. (1986) Science 231, 48-51]. An enzyme system catalyzing the synthesis of the (beta 1-2)-linked glucan backbone of E. coli membrane-derived oligosaccharides from UDP-glucose requires both a membrane component and a cytosolic protein termed transglucosylation factor. The factor has now been purified to apparent homogeneity and has been found to be identical to acyl carrier protein (ACP), the phosphopantetheine-containing protein of low molecular weight that has long been known to be essential for fatty acid synthesis in E. coli and other organisms. Both are small, heat-stable, highly anionic proteins with identical chromatographic and electrophoretic behavior. ACP of the highest purity has an activity in the transglucosylation system indistinguishable from that of the protein independently purified as transglucosylation factor. Antibody raised against pure ACP completely inhibits transglucosylation activity; this inhibition is overcome by titration of the antibody with either ACP or transglucosylation factor. These findings provide evidence for an essential function of ACP unrelated to the biosynthesis of lipid.