The Structure and Function of the Starch Utilization System in Bacteroides thetaiotaomicron.

Abstract

Degradation of polysaccharides is an important function performed by the human gut microbiota. Bacterial carbohydrate metabolism in the gut not only provides the host with a significant portion of their daily nutrients, but is also a major factor shaping the composition of the microbial community. The Bacteroidetes, one of the two dominant bacterial taxa in the human gut, degrade a large number of carbohydrates via expression of unique multi-protein complexes, each targeting a different glycan. The first such system described was the starch utilization system (Sus) in Bacteroides thetaiotaomicron (Bt), an eight protein system required for the bacterium to metabolize starch. Homologous “Sus-like” systems are found in the majority of gut Bacteroidetes with some species devoting up to 20% of their genome to encoding them. The Bt Sus is a model for glycan acquisition by the Bacteroidetes, and the work presented here addresses several important questions regarding the structure and function of individual Sus proteins as well as how these components function together to efficiently acquire and degrade the abundant dietary glycan starch. The crystal structures of two Sus outer-membrane proteins (OMPs), SusE and SusF, were solved revealing that they both contain multiple starch binding sites. In total the Sus OMPs (SusD,E,F and G) contain eight non-enzymatic starch binding sites that we demonstrate serve unique functions in starch catabolism. The SusD binding site is uniquely involved in initial sensing of available starch, leading to upregulation of the sus locus. Conversely, the SusE,F and G binding sites are important during starch catalysis, enhancing starch growth rate in a manner dependent on expression of the Bt polysaccharide capsule. We hypothesize these binding sites help overcome the barrier created by the bacterial capsule, which may obstruct access to starch. In vivo studies show that the Sus binding sites confer a fitness advantage to Bt on a starch-rich diet. Finally, we present the first single-molecule imaging studies performed with live Bt cells that provides evidence for a highly dynamic starch-induced Sus complex. These studies provide important insight into the mechanisms of carbohydrate metabolism by gut symbionts, a process that significantly affects human health.