Mannose transporter of Escherichia coli. Backbone assignments and secondary structure of the IIA domain of the IIABMan subunit.

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.