This is sensor domain found in bacterial periplasmic binding proteins, several of which are involved in iron and cobalamin transport [1,2,3]. Among its ligands are siderophores bacillibactin, enterobactin and iron(III)-hydroxamates (Matilla et. al., FEMS Microbiology Reviews, fuab043, 45, 2021, 1 https://doi.org/10.1093/femsre/fuab043).

1: Nucleotide sequences of the fecBCDE genes and locations of the proteins suggest a periplasmic-binding-protein-dependent transport mechanism for iron(III) dicitrate in Escherichia coli. Staudenmaier H, Van Hove B, Yaraghi Z, Braun V; J Bacteriol 1989;171:2626-2633. PMID:2651410

2: The structure of Escherichia coli BtuF and binding to its cognate ATP binding cassette transporter. Borths EL, Locher KP, Lee AT, Rees DC; Proc Natl Acad Sci U S A. 2002;99:16642-16647. PMID:12475936

3: The role of FhuD2 in iron(III)-hydroxamate transport in Staphylococcus aureus. Demonstration that FhuD2 binds iron(III)-hydroxamates but with minimal conformational change and implication of mutations on transport. Sebulsky MT, Shilton BH, Speziali CD, Heinrichs DE; J Biol Chem. 2003;278:49890-49900. PMID:14514690