Crystal structure of ferric citrate transporter FecA in the unliganded form

Experimental Data Snapshot

  • Resolution: 2.50 Å
  • R-Value Free: 0.259 
  • R-Value Work: 0.223 
  • R-Value Observed: 0.223 

Starting Model: experimental
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Structural evidence for iron-free citrate and ferric citrate binding to the TonB-dependent outer membrane transporter FecA

Yue, W.W.Grizot, S.Buchanan, S.K.

(2003) J Mol Biol 332: 353-368

  • DOI: https://doi.org/10.1016/s0022-2836(03)00855-6
  • Primary Citation of Related Structures:  
    1PNZ, 1PO0, 1PO3

  • PubMed Abstract: 

    Escherichia coli possesses a TonB-dependent transport system, which exploits the iron-binding capacity of citrate and its natural abundance. Here, we describe three structures of the outer membrane ferric citrate transporter FecA: unliganded and complexed with iron-free or diferric dicitrate. We show the structural mechanism for discrimination between the iron-free and ferric siderophore: the binding of diferric dicitrate, but not iron-free dicitrate alone, causes major conformational rearrangements in the transporter. The structure of FecA bound with iron-free dicitrate represents the first structure of a TonB-dependent transporter bound with an iron-free siderophore. Binding of diferric dicitrate to FecA results in changes in the orientation of the two citrate ions relative to each other and in their interactions with FecA, compared to the binding of iron-free dicitrate. The changes in ligand binding are accompanied by conformational changes in three areas of FecA: two extracellular loops, one plug domain loop and the periplasmic TonB-box motif. The positional and conformational changes in the siderophore and transporter initiate two independent events: ferric citrate transport into the periplasm and transcription induction of the fecABCDE transport genes. From these data, we propose a two-step ligand recognition event: FecA binds iron-free dicitrate in the non-productive state or first step, followed by siderophore displacement to form the transport-competent, diferric dicitrate-bound state in the second step.

  • Organizational Affiliation

    National Institute of Diabetes and Digestive and Kidney Diseases, National Institutes of Health, Bethesda, MD 20892, USA.

Find similar proteins by:  (by identity cutoff)  |  3D Structure
Entity ID: 1
MoleculeChains Sequence LengthOrganismDetailsImage
Iron(III) dicitrate transport protein fecA precursor751Escherichia coliMutation(s): 0 
Gene Names: FECA OR B4291
Membrane Entity: Yes 
Find proteins for P13036 (Escherichia coli (strain K12))
Explore P13036 
Go to UniProtKB:  P13036
Entity Groups  
Sequence Clusters30% Identity50% Identity70% Identity90% Identity95% Identity100% Identity
UniProt GroupP13036
Sequence Annotations
  • Reference Sequence
Experimental Data & Validation

Experimental Data

  • Resolution: 2.50 Å
  • R-Value Free: 0.259 
  • R-Value Work: 0.223 
  • R-Value Observed: 0.223 
  • Space Group: P 21 21 2
Unit Cell:
Length ( Å )Angle ( ˚ )
a = 117.05α = 90
b = 88.459β = 90
c = 95.3γ = 90
Software Package:
Software NamePurpose
HKL-2000data reduction
SCALEPACKdata scaling

Structure Validation

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Entry History 

Deposition Data

Revision History  (Full details and data files)

  • Version 1.0: 2003-11-18
    Type: Initial release
  • Version 1.1: 2008-04-29
    Changes: Version format compliance
  • Version 1.2: 2011-07-13
    Changes: Version format compliance
  • Version 1.3: 2023-08-16
    Changes: Data collection, Database references, Refinement description