Two crystal structures of the cytoplasmic molybdate-binding protein ModG suggest a novel cooperative binding mechanism and provide insights into ligand-binding specificity

J Mol Biol. 2001 May 18;308(5):1063-79. doi: 10.1006/jmbi.2001.4636.


The X-ray structures of the cytoplasmic molybdate-binding protein ModG from Azotobacter vinelandii in two different crystal forms have been determined. For such a small protein it is remarkably complex. Each 14.3 kDa subunit contains two small beta-barrel domains, which display an OB-fold motif, also seen in the related structure of ModE, a molybdenum-dependent transcriptional regulator, and very recently in the Mop protein that, like ModG, has been implicated in molybdenum homeostasis within the cell. In contrast to earlier speculation, the functional unit of ModG is actually not a dimer (as in ModE), but a trimer capable of binding a total of eight molybdate molecules that are distributed between two disparate types of site. All the binding sites are located at subunit interfaces, with one type lying on a crystallographic 3-fold axis, whilst the other lies between pairs of subunits. The two types of site are linked by short hydrogen bond networks that may suggest a cooperative binding mechanism. A superposition of two subunits of the ModG trimer on the apo-ModE dimer allows the probable locations of the molybdate-binding sites of the latter to be assigned. Through structural comparisons with other oxyanion-binding proteins, including Mop and ModE, it is possible to speculate about ligand-binding affinities, selectivity and evolution.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Aldehyde Oxidoreductases / chemistry
  • Aldehyde Oxidoreductases / metabolism
  • Amino Acid Sequence
  • Anions / metabolism
  • Apoproteins / chemistry
  • Apoproteins / metabolism
  • Azotobacter vinelandii / chemistry*
  • Bacterial Proteins*
  • Binding Sites
  • Carrier Proteins / chemistry*
  • Carrier Proteins / metabolism*
  • Crystallography, X-Ray
  • Cytoplasm / chemistry*
  • Dimerization
  • Evolution, Molecular
  • Hydrogen Bonding
  • Intracellular Signaling Peptides and Proteins
  • Ligands
  • Models, Molecular
  • Molecular Sequence Data
  • Molybdenum / metabolism*
  • Protein Structure, Quaternary
  • Protein Structure, Secondary
  • Protein Structure, Tertiary
  • Protein Subunits
  • Sequence Alignment
  • Static Electricity
  • Substrate Specificity
  • Transcription Factors / chemistry
  • Transcription Factors / metabolism


  • Anions
  • Apoproteins
  • Bacterial Proteins
  • Carrier Proteins
  • Intracellular Signaling Peptides and Proteins
  • Ligands
  • ModE protein, bacteria
  • ModG protein, Azotobacter vinelandii
  • Protein Subunits
  • Transcription Factors
  • molybdate
  • Molybdenum
  • Aldehyde Oxidoreductases
  • Mop protein, Desulfovibrio gigas

Associated data

  • PDB/1H9J
  • PDB/1H9K
  • PDB/1H9M