GacH is the solute binding protein (receptor) of the putative oligosaccharide ATP-binding cassette (ABC) transporter GacFG, encoded in the acarbose biosynthetic gene cluster (gac) from Streptomyces glaucescens GLA.O. In the context of the proposed function of acarbose (acarviosyl-1,4-maltose) as a 'carbophor', the transporter in complex with a yet to be identified ATPase subunit, is supposed to mediate the uptake of longer acarbose homologs as well as of acarbose for recycling purposes. Binding assays using isothermal titration calorimetry identified GacH as a maltose/maltodextrin binding protein with low affinity for acarbose but considerable binding activity for its homolog, acarviosyl-1,4-maltose-1,4-glucose-1,1-glucose (component 5C). In contrast, maltose-binding protein of Salmonella typhimurium (MalE) displays high-affinity acarbose binding. We determined the crystal structures of GacH in complex with acarbose, component 5C, and maltotetraose as well as in the unliganded form. As found for other solute receptors, the polypeptide chain of GacH is folded into two distinct domains (lobes) connected by a hinge, the interface between the lobes forming the substrate-binding pocket. GacH does not specifically bind the acarviosyl group but displays specificity towards binding of the maltose moiety in the inner part of its binding pocket. The crystal structure of acarbose-loaded MalE showed that two glucose units of acarbose are bound at the same region and position as maltose. A comparative analysis revealed that in GacH, acarbose is buried deeper into the binding pocket than in MalE by exactly one glucose ring shift, resulting in a total of 18 hydrogen bond interactions versus 21 for MalE(acarbose). Since the substrate specificity of ABC import systems is determined by the cognate binding protein, our results provide first biochemical and structural evidence for the proposed role of GacHFG in acarbose metabolism.