Structural characterization of an α-glucosaccharide-binding protein from Paenibacillus sp. str. FPU-7

Paenibacillus sp. str. FPU-7 (P. FPU-7), a chitinolytic bacterium, efficiently degrades chitin and uses the solute-binding proteins (SBPs) NagB1 and NagB2 on the cell surface to facilitate the uptake and intracellular transport of chitooligosaccharides. SBPs are essential components of the bacterial carbohydrate transport system and play key roles in carbon source acquisition. However, they exhibit diverse substrate specificities with ligands yet to be identified. Predicting SBP ligands solely from amino acid sequences remains a significant challenge. In addition to encoding NagB1 and NagB2, the genome of P. FPU-7 encodes several SBPs that are potentially involved in carbohydrate import. In the present study, we identified a novel SBP, designated PsMBP, in the P. FPU-7 genome and confirmed its mRNA expression. Genes adjacent to psmbp encode transmembrane domains, suggesting that PsMBP functions as part of an ABC transporter complex. We also characterized the sugar-binding specificity of PsMBP using biochemical analysis. PsMBP exhibited binding affinities for various α-glucosaccharides, including maltose, trehalose, isomaltose, sucrose, maltotriose, and maltotetraose. In particular, it showed high binding affinity for both maltose and trehalose. Furthermore, we determined the crystal structure of PsMBP in its ligand-free form and in complexes with different saccharides at resolutions of 1.4-2.1 Å. The structures revealed the molecular basis for α-glucosaccharide recognition by PsMBP. Overall, our findings advance the understanding of bacterial carbohydrate transport mechanisms and provide a foundation for developing efficient transport systems and new microbial biotechnological applications.