Sas20 is a highly flexible starch-binding protein in the Ruminococcus bromii cell-surface amylosome

Ruminococcus bromii is a keystone species in the human gut that has the rare ability to degrade dietary resistant starch (RS). This bacterium secretes a suite of starch-active proteins that work together within larger complexes called amylosomes that allow R. bromii to adhere to and degrade RS. Sas20 is one of the more abundant proteins assembled within amylosomes, but little could be predicted about its molecular features based upon amino acid sequence. Here, we perform a structure-function analysis of Sas20 which features two discrete starch-binding domains separated by a flexible linker. Sas20 domain 1 has an N-terminal β-sandwich followed by a cluster of α-helices and captures the non-reducing end of maltooligosaccharides between these structural features. The crystal structure of a close homolog of Sas20 domain 2 revealed a unique bilobed starch-binding groove that targets the helical 1,4-linked glycan chains found in amorphous regions of amylopectin and crystalline regions of amylose within starch granules. Affinity PAGE and isothermal titration calorimetry demonstrate both domains bind maltoheptaose and soluble starch with relatively high affinity (Kd 20 M) but exhibit limited or no binding to cyclodextrins. Small angle x-ray scattering analysis of the individual and combined domains support that these structures are highly flexible, which may allow the protein to adopt conformations that enhance its starch-targeting efficiency.