Bifidobacterium longum is a symbiotic human gut bacterium that has a degradation system for β-arabinooligosaccharides, which are present in the hydroxyproline-rich glycoproteins of edible plants. Whereas microbial degradation systems for α-linked arabinofuranosyl carbohydrates have been extensively studied, little is understood about the degradation systems targeting β-linked arabinofuranosyl carbohydrates. We functionally and structurally analyzed a substrate-binding protein (SBP) of a putative ABC transporter (BLLJ_0208) in the β-arabinooligosaccharide degradation system. Thermal shift assays and isothermal titration calorimetry revealed that the SBP specifically bound Araf-β1,2-Araf (β-Ara2 ) with a Kd of 0.150 μm, but did not bind L-arabinose or methyl-β-Ara2 . Therefore, the SBP was termed β-arabinobiose-binding protein (BABP). Crystal structures of BABP complexed with β-Ara2 were determined at resolutions of up to 1.78 Å. The findings showed that β-Ara2 was bound to BABP within a short tunnel between two lobes as an α-anomeric form at its reducing end. BABP forms extensive interactions with β-Ara2 , and its binding mode was unique among SBPs. A molecular dynamics simulation revealed that the closed conformation of substrate-bound BABP is stable, whereas substrate-free form can adopt a fully open and two distinct semi-open states. The importer system specific for β-Ara2 may contribute to microbial survival in biological niches with limited amounts of digestible carbohydrates. DATABASE: Atomic coordinates and structure factors (codes 6LCE and 6LCF) have been deposited in the Protein Data Bank (http://wwpdb.org/).
Keywords: ABC transporter; human gut bacterium; isothermal titration calorimetry; molecular dynamics simulation; substrate-binding protein.
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