Part of the ABC transporter complex CysAWTP (TC 3.A.1.6.1) involved in sulfate/thiosulfate import. This protein specifically binds thiosulfate and is involved in its transmembrane transport. The complex is composed of two ATP-binding proteins (CysA), two transmembrane proteins (CysT and CysW) and a solute-binding protein (CysP). Belongs to the prokaryotic sulfate-binding protein family.