Involved in the uptake of iron in complex with ferrichrome, a hydroxamate-type siderophore. Binds and transports ferrichrome-iron across the outer membrane (PubMed:1089064, PubMed:2066336). In addition to its role in ferrichrome-iron transport, transports the antibiotic albomycin, which is a structural analog of ferrichrome, and acts as a receptor for colicin M, microcin J25 and bacteriophages T1, T5, phi80 and UC-1 (PubMed:1089064, PubMed:353030, PubMed:2066336, PubMed:8617231). The energy source, which is required for all FhuA functions except infection by phage T5, is provided by the inner membrane TonB system (PubMed:353030, PubMed:9353297, PubMed:12427941). Binding of ferrichrome or colicin M enhances the interaction between FhuA and TonB (PubMed:9353297). TonB activates FhuA through interaction with the beta-barrel (PubMed:12427941). Monomer (PubMed:8916906, PubMed:9856937, PubMed:9865695). Interacts with TonB (PubMed:9353297, PubMed:12427941, PubMed:18653801). Induced 1.6-fold by hydroxyurea. Has two distinct conformations in the presence and absence of ferrichrome. The globular N-terminal domain acts a plug that closes the channel formed by the beta-barrel. Binding of ferrichrome at the cell surface induces a conformational change in FhuA, but does not open the channel. Structural changes are propagated and amplified across the plug, and may facilitate binding of FhuA to TonB (PubMed:9865695, PubMed:9353297). TonB binding promotes conformational changes in outer surface-exposed loops of FhuA (PubMed:18653801). Phage T5 is a TonB-independent ligand, and its binding to FhuA results in the formation of high conductance ion channels (PubMed:8617231, PubMed:9353297). Belongs to the TonB-dependent receptor family.