STRUCTURE OF THE IIA DOMAIN OF THE MANNOSE TRANSPORTER FROM ESCHERICHIA-COLI AT 1.7-ANGSTROM RESOLUTION

The mannose transporter from Escherichia coli is a member of the phosp hoenolpyruvate-dependent phosphotransferase system. The multisubunit c omplex couples translocation across the bacterial inner membrane with phosphorylation of the solute. A functional fragment (IIA(Man), residu es 2 to 133) of the membrane-associated IIAB(Man) subunit of the manno se transporter was expressed as a selenomethionine protein, and the un phosphorylated molecule was crystallized and its structure solved by X -ray crystallography. The protein consists of a central five-stranded beta-sheet covered by helices on either face. The order of the seconda ry structure elements is (beta alpha)(4),alpha beta. Four beta-strands are arranged in a parallel manner with strand order 2134 and are link ed by helices forming right-handed cross-over connections. The fifth s trand that forms one edge of the sheet and runs antiparallel to the ot hers is swapped between the subunits of the dimeric structure. Helices D and E form a helical hairpin. Histidine 10, which is transiently ph osphorylated during catalysis, is located at the topologial switch-poi nt of the structure, close to the subunit interface. Its imidazole rin g is hydrogen bonded to the buried side-chain of Asp67. It is likely t hat Asp67 acts as a general base and thus increases the nucleophilicit y of the histidine, Modeling suggests that the covalently bound phosph oryl group would be stabilized by the macrodipole of helix C. Putative interactions between IIA(Man) and the histidine-containing phosphocar rier protein are discussed. (C) 1996 Academic Press Limited