Maltodextrin transport in the extremely thermophilic, lignocellulose degrading bacterium Anaerocellum bescii (f. Caldicellulosiruptor bescii).

Sugar transport into microbial cells is a critical, yet understudied step in the conversion of lignocellulosic biomass to metabolic products. Anaerocellum bescii (formerly Caldicellulosiruptor bescii) is an extremely thermophilic, anaerobic bacterium that readily degrades the cellulose and hemicellulose components of lignocellulosic biomass into a diversity of oligosaccharide substrates. Despite significant understanding of how this microorganism degrades lignocellulose, the mechanisms underlying its highly efficient transport of the released oligosaccharides into the cell are comparatively underexplored. Here, we identify and characterize the ATP-binding cassette (ABC) transporters in A. bescii governing maltodextrin transport. Utilizing past transcriptomic studies on Anaerocellum and Caldicellulosiruptor species, we identify two maltodextrin transporters in A. bescii and express and purify their substrate-binding proteins (Athe_2310 and Athe_2574) for characterization. Using differential scanning calorimetry and isothermal titration calorimetry, we show that Athe_2310 strongly interacts with shorter maltodextrins, such as maltose and trehalose, with dissociation constants in the micromolar range, while Athe_2574 binds longer maltodextrins, with dissociation constants in the sub-micromolar range. Using a sequence-structure-function comparison approach combined with molecular modeling, we provide context for the specificity of each of these substrate-binding proteins. We propose that A. bescii utilizes orthogonal ABC transporters to uptake malto-oligosaccharides of different lengths to maximize transport efficiency.

Importance: Here, we reveal the biophysical and structural basis for oligosaccharide transport by two maltodextrin ATP-binding cassette (ABC) transporters in Anaerocellum bescii. This is the first biophysical characterization of carbohydrate uptake in this organism and establishes a workflow for characterizing other oligosaccharide transporters in A. bescii and similar biomass-degrading thermophiles of interest for lignocellulosic bioprocessing. By deciphering the mechanisms underlying high-affinity sugar uptake in A. bescii, we shed light on an underexplored step between extracellular lignocellulose degradation and intracellular conversion of sugars to metabolic products. This understanding will expand opportunities for harnessing sugar transport in thermophiles to reshape lignocellulose bioprocessing as part of a renewable bioeconomy.