SusG: A Unique Cell-Membrane-Associated α-Amylase from a Prominent Human Gut Symbiont Targets Complex Starch Molecules

SummarySusG is an α-amylase and part of a large protein complex on the outer surface of the bacterial cell and plays a major role in carbohydrate acquisition by the animal gut microbiota. Presented here, the atomic structure of SusG has an unusual extended, bilobed structure composed of amylase at one end and an unprecedented internal carbohydrate-binding motif at the other. Structural studies further demonstrate that the carbohydrate-binding motif binds maltooligosaccharide distal to, and on the opposite side of, the amylase catalytic site. SusG has an additional starch-binding site on the amylase domain immediately adjacent to the active cleft. Mutagenesis analysis demonstrates that these two additional starch-binding sites appear to play a role in catabolism of insoluble starch. However, elimination of these sites has only a limited effect, suggesting that they may have a more important role in product exchange with other Sus components.

► SusG is an outer membrane α-amylase with an extended bi-lobed structure
► One end is the α-amylase and the other is a novel starch-binding domain
► A second accessory starch-binding site lies immediately adjacent to the active site
► The starch-binding sites are likely key for interacting with other Sus proteins.