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Publication Abstract from PubMed

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 on amino acid sequence alone. Here, we performed a structure-function analysis of Sas20 and determined that it features two discrete starch-binding domains separated by a flexible linker. We show Sas20 domain 1 contains an N-terminal beta-sandwich followed by a cluster of alpha-helices, and the non-reducing end of maltooligosaccharides can be captured between these structural features. Furthermore, the crystal structure of a close homolog of Sas20 domain 2 revealed a unique bi-lobed starch-binding groove that targets the helical alpha1,4-linked glycan chains found in amorphous regions of amylopectin and crystalline regions of amylose within starch granules. Affinity PAGE and isothermal titration calorimetry demonstrated that both domains bind maltoheptaose and soluble starch with relatively high affinity (Kd </= 20 muM) but exhibit limited or no binding to cyclodextrins. Finally, 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. Taken together, we conclude that Sas20 binds distinct features within the starch granule, facilitating the ability of R. bromii to hydrolyze dietary resistant starch.

Sas20 is a highly flexible starch-binding protein in the Ruminococcus bromii cell-surface amylosome.,Cerqueira FM, Photenhauer AL, Doden HL, Brown AN, Abdel-Hamid AM, Morais S, Bayer EA, Wawrzak Z, Cann I, Ridlon JM, Hopkins J, Koropatkin NM J Biol Chem. 2022 Apr 1:101896. doi: 10.1016/j.jbc.2022.101896. PMID:35378131^[1]

From MEDLINE®/PubMed®, a database of the U.S. National Library of Medicine.