LAUSR

ABC importers are membrane proteins responsible for the transport of nutrients into the cells of prokaryotes. The structures of ABC importers vary, yet all contain four conserved domains: two nucleotide-binding domains (NBDs), which bind and hydrolyze ATP, and two transmembrane domains (TMDs), which help translocate the substrate. ABC importers are also dependent on an additional protein component, a high-affinity substrate binding protein (SBP) that specifically binds the target ligand for delivery to the appropriate ABC transporter. AbnE is an SBP belonging to the ABC importer for arabino-oligosaccharides in the Gram-positive thermophilic bacterium Geobacillus stearothermophilus. Using isothermal titration calorimetry (ITC), purified AbnE was shown to bind medium-sized arabino-oligosaccharides, in the range of arabino-biose (A2) to arabino-octaose (A8), all with Kd values in the nanomolar range. We describe herein the 3D structure of AbnE in its closed conformation, in complex with a wide range of arabino-oligosaccharide substrates. These structures provide the basis for the detailed structural analysis of the AbnE-sugar complexes, and together with complementary quantum chemical calculations, site specific mutagenesis and isothermal titration calorimetry (ITC) experiments, provide detailed insights into the AbnE-substrate interactions involved. Small angle X-ray scattering (SAXS) experiments and normal mode analysis (NMA) are used to study the conformational changes of AbnE, and taken together, these data suggest clues regarding its binding mode to the full transporter.