The mannose transporter of Escherichia coli consists of two transmembrane and one peripheral protein subunit. The complex acts by a mechanism which couples translocation of the substrate with substrate phosphorylation. The peripheral IIABMan is a homodimer. The IIABMan monomer itself contains two domains which are linked by an Ala-Pro-rich hinge and which are both transiently phosphorylated at histidyl residues. The IIA and IIB domains can be separated by limited proteolysis. The IIA domain has a dimer molecular mass of 2 x 14 kDa. Almost complete 1H, 13C, and 15N NMR assignments of the backbone resonances of IIAMan have been achieved using 3D and 4D double-and triple-resonance techniques. Secondary structure elements were derived from NOE data. The IIA domain consists of a central beta-sheet of four parallel and one antiparallel strand (strand order 5 4 3 1 2) with helices on both sides of the sheet. The active-site His-10 is located in a loop at the C-terminus of beta-strand 1. This loop and the loop after strand 3 are at the topological switch point of the sheet.