The ABC transporter (ATP-binding-cassette transporter) OpuA is one of five membrane transport systems in Bacillus subtilis that mediate osmoprotection by importing compatible solutes. Just like all bacterial and archaeal ABC transporters that catalyse the import of substrates, OpuA (where Opu is osmoprotectant uptake) is composed of an ATPase subunit (OpuAA), a transmembrane subunit (OpuAB) and an extracellular substrate-binding protein (OpuAC). In contrast with many well-known ABC-ATPases, OpuAA is composed not only of a catalytic and a helical domain but also of an accessory domain located at its C-terminus. The paradigm of such an architecture is MalK, the ABC-ATPase of the maltose importer of Escherichia coli, for which detailed structural and functional information is available. In the present study, we have applied solution FRET (Förster resonance energy transfer) techniques using two single cysteine mutants to obtain initial structural information on the architecture of the OpuAA dimer in solution. Analysing our results in detail and comparing them with the existing MalK structures revealed that the catalytic and helical domains adopted an arrangement similar to those of MalK, whereas profound differences in the three-dimensional orientation of the accessory domain, which contains two CBS (cystathionine beta-synthetase) domains, were observed. These results shed new light on the role of this accessory domain present in a certain subset of ABC-ATPase in the fine-tuning of three-dimensional structure and biological function.