An 8-angstrom projected structure of FhuA, a "ligand-gated" channel of theEscherichia coli outer membrane

The structure of FhuA, a siderophore and phage receptor in the outer membra ne 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 = 1 24 Angstrom, b = 98 Angstrom, gamma = 90 degrees) exhibited a p22(1)2(1) pl ane group symmetry. A projection map at 8 Angstrom resolution showed the pr esence of dimeric ring-like structures with an elliptical shape (48 x 40 An gstrom). Each monomer was composed of a ring of densities with a radial wid th of 8-10 Angstrom corresponding to a cylinder of beta sheets. Few densiti es are present inside the barrel, leaving a central channel approximately 2 5 Angstrom in diameter. A projection map of FhuA at 15 Angstrom resolution, which was calculated from negatively stained preparations, demonstrated th at most of the central channel was masked by extramembrane domains. This ma p 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 int erpretation of the electron microscopy projection structures with regard to long hydrophilic loops governing the selectivity and opening of the channe l. (C) 1999 Academic Press.