FIG. 1.

Unfolded FhuA β-barrel as inserted into the outer membrane (OM). Loops L1 to L11 and turns T1 to T11 connect the 22 antiparallel β-strands. The amino acid residues represented as circles are those present in loops and turns, and those represented as squares are residues contained in β-strands. Black squares indicate the ferrichrome binding sites in the β-barrel. The two disulfide bridges are also marked in black. The numbers indicate the amino acid residues of mature FhuA. Modified from a figure in reference 4 with permission of the publisher.