title: Bacillus cereus iron uptake protein fishes out an unstable ferric citrate trimer.
abstract: Citrate is a common biomolecule that chelates Fe(III). Many bacteria and plants use ferric citrate to fulfill their nutritional requirement for iron. Only the Escherichia coli ferric citrate outer-membrane transport protein FecA has been characterized; little is known about other ferric citrate-binding proteins. Here we report a unique siderophore-binding protein from the gram-positive pathogenic bacterium Bacillus cereus that binds multinuclear ferric citrate complexes. We have demonstrated that B. cereus ATCC 14579 takes up (55)Fe radiolabeled ferric citrate and that a protein, BC_3466 [renamed FctC (ferric citrate-binding protein C)], binds ferric citrate. The dissociation constant (K(d)) of FctC at pH 7.4 with ferric citrate (molar ratio 1:50) is 2.6 nM. This is the tightest binding observed of any B. cereus siderophore-binding protein. Nano electrospray ionization-mass spectrometry (nano ESI-MS) analysis of FctC and ferric citrate complexes or citrate alone show that FctC binds diferric di-citrate, and triferric tricitrate, but does not bind ferric di-citrate, ferric monocitrate, or citrate alone. Significantly, the protein selectively binds triferric tricitrate even though this species is naturally present at very low equilibrium concentrations.
authors.affiliation: Department of Chemistry, University of California, Berkeley, CA 94720-1460, USA.
authors.forename: Tatsuya
authors.initials: T
authors.lastname: Fukushima
authors.forename: Allyson K
authors.initials: AK
authors.lastname: Sia
authors.forename: Benjamin E
authors.initials: BE
authors.lastname: Allred
authors.forename: Rita
authors.initials: R
authors.lastname: Nichiporuk
authors.forename: Zhongrui
authors.initials: Z
authors.lastname: Zhou
authors.forename: Ulla N
authors.initials: UN
authors.lastname: Andersen
authors.forename: Kenneth N
authors.initials: KN
authors.lastname: Raymond
mesh_terms: D001409:Bacillus cereus / Q000378:metabolism*; D001426:Bacterial Proteins / Q000378:metabolism*; D005290:Ferric Compounds / Q000378:metabolism / Q000493:pharmacokinetics*; D007504:Iron Radioisotopes / Q000493:pharmacokinetics*; D007553:Isotope Labeling; D013058:Mass Spectrometry; D015394:Molecular Structure; D011485:Protein Binding; D017262:Siderophores / Q000378:metabolism
chemical_list: D001426:Bacterial Proteins; D005290:Ferric Compounds; D007504:Iron Radioisotopes; D017262:Siderophores; C025314:ferric citrate