Structural and thermodynamic analyses of a novel β‐1,2‐glucan binding mode in the ABC transporter solute‐binding protein Chy400_4166 from Chloroflexus aurantiacus

Résumé

β‐1,2‐Glucans are glucose polymers widely distributed in nature and play various physiological roles in the interactions between organisms such as pathogenicity and symbiosis. While various β‐1,2‐glucan‐degrading enzymes have been identified recently, transporters incorporating β‐1,2‐glucans are still poorly characterized. In this study, we have found a β‐1,2‐glucan binding protein of ABC transporter from Chloroflexus aurantiacus Y‐400‐fl, a filamentous anoxygenic phototrophic bacterium. The protein showed a clear affinity for linear β‐1,2‐glucan in the gel shift assay. Isothermal titration calorimetric analysis revealed high binding affinities for both linear and cyclic β‐1,2‐glucans, unlike for the barley β‐glucan. The recorded binding constants were high for the binding of the ABC transporter to β‐1,2‐glucans. The observed unfavorable negative entropy change may have resulted from conformational restraints upon complex formation. Complex structures with linear β‐1,2‐glucan and cyclic β‐1,2‐glucans with degrees of polymerization of 17–20 were obtained using X‐ray crystallography. Ten glucose units, designated A–J from the nonreducing end, were shared among the substrates in the complexes. Unit G is recognized by W74, W308, and D336, which are highly conserved residues within the phylogenetic group Chy400_4166. The substrate‐binding mode of Chy400_4166 is completely different from that of the β‐1,2‐glucooligosaccharide‐binding protein from Listeria innocua . The discovery of a new type of β‐1,2‐glucan‐related binding protein has expanded our understanding of the metabolism of β‐1,2‐glucans.