Physiological Characterization of SusG, an Outer Membrane Protein Essential for Starch Utilization by Bacteroides thetaiotaomicron

Abstract

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 [14C]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 Km 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.

The genome and surface proteome of Capnocytophaga canimorsus reveal a key role of glycan foraging systems in host glycoproteins deglycosylation

“…They are described as secreted catalytic enzymes while Sus-like systems are multiprotein complexes associated with the outer membrane of Gram-negative bacteria. They can bind and degrade specific substrates and presumably operate an energy-dependent import of the processed molecules (Shipman et al, 1999;Cho and Salyers, 2001;Schauer et al, 2008;Martens et al, 2009). In conclusion, the genome of Cc5 does not encode any classical virulence system (e.g.…”

Functional characterization of a gene locus from an uncultured gut Bacteroides conferring xylo‐oligosaccharides utilization to Escherichia coli

“…Bacterial lipoproteins are membrane proteins present both in Gram-negative and Gram-positive bacteria. In Bacteroidetes some lipoproteins, including the hydrolase (SusG, GH13) from SUS systems are known to be transported to the outer surface of the outer membrane (Shipman et al, 1999). In E. coli the lipoproteins are anchored either to the inner or to the outer membrane, and oriented towards the periplasmic space (Tokuda and Matsuyama, 2004).…”

Structural basis for the flexible recognition of α‐glucan substrates by Bacteroides thetaiotaomicron SusG

“…4 SusG is a GH13 amylase that is central to starch utilization by the human gut commensal Bacteroides thetaiotaomicron. 5 SusG displays substrate flexibility for diverse a-glucans, including amylopectin, pullulan, and cyclodextrins, yet it invariably hydrolyzes a1,4-linkages. 6 Our previous work revealed that SusG displays an atypical domain organization among GH13 members, which in addition to the canonical A, B, and C domains, includes the insertion of a unique carbohydrate binding module (CBM) family 58 into the middle of the B domain which is located between b3 and a3 of the catalytic A domain.…”