Physiological characterization of SusG, an outer membrane protein essential for starch utilization by Bacteroides thetaiotaomicron

Results from previous studies had suggested that Bacteroides thetaiotaomicron utilizes starch by binding the polysaccharide to the bacterial surface and subsequently degrading the polymer by using cell-associated enzymes. Most of the starch-degrading activity was localized to the periplasm, but a portion appeared to be membrane associated. This raised the possibility that some breakdown might occur in the outer membrane prior to exposure of the polysaccharide to the periplasmic polysaccharide-degrading enzymes. In this study, we show that SusG, an outer membrane protein which has been shown genetically to be essential for starch utilization, has enzymatic activity. Results of protease accessibility experiments support the hypothesis that SusG is exposed on the cell surface. Results of [(14)C]starch binding assays, however, show that SusG plays a negligible role in binding of starch to the cell surface. Consistent with this, SusG has a relatively high K(m) for starch and by itself is not sufficient to allow cells to grow on starch or to bind starch. Hence, the main role of SusG is to hydrolyze starch, but the binding of starch to the cell surface is evidently mediated by other proteins presumably interacting with SusG.