An 8-Å projected structure of FhuA, a "ligand-gated" channel of the Escherichia coli outer membrane

The structure of FhuA, a siderophore and phage receptor in the outer membrane of Escherichia coli, has been investigated by electron crystallography. Bidimensional crystals of hexahistidine-tagged FhuA protein solubilized in N,N-dimethyldodecylamine-N-oxide were produced after detergent removal with polystyrene beads. Frozen-hydrated crystals (unit cell dimensions of a = 124 Angstrom, b = 98 Angstrom, gamma = 90 degrees) exhibited a p22(1)2(1) plane group symmetry. A projection map at 8 Angstrom resolution showed the presence of dimeric ring-like structures with an elliptical shape (48 x 40 Angstrom). Each monomer was composed of a ring of densities with a radial width of 8-10 Angstrom corresponding to a cylinder of beta sheets. Few densities are present inside the barrel, leaving a central channel approximately 25 Angstrom in diameter. A projection map of FhuA at 15 Angstrom resolution, which was calculated from negatively stained preparations, demonstrated that most of the central channel was masked by extramembrane domains. This map also revealed an asymmetric distribution of extramembrane domains in FhuA, with large domains located mainly on one side of the molecule. Comparison with density maps derived from recent atomic structure allowed further interpretation of the electron microscopy projection structures with regard to long hydrophilic loops governing the selectivity and opening of the channel. (C) 1999 Academic Press.