Metal binding spectrum and model structure of the Bacillus anthracis virulence determinant MntA

Metallomics. 2015 Oct;7(10):1407-19. doi: 10.1039/c5mt00100e. Epub 2015 Jun 24.


The potentially lethal human pathogen Bacillus anthracis expresses a putative metal import system, MntBCA, which belongs to the large family of ABC transporters. MntBCA is essential for virulence of Bacillus anthracis: deletion of MntA, the system's substrate binding protein, yields a completely non-virulent strain. Here we determined the metal binding spectrum of MntA. In contrast to what can be inferred from growth complementation studies we find no evidence that MntA binds Fe(2+) or Fe(3+). Rather, MntA binds a variety of other metal ions, including Mn(2+), Zn(2+), Cd(2+), Co(2+), and Ni(2+) with affinities ranging from 10(-6) to 10(-8) M. Binding of Zn(2+) and Co(2+) have a pronounced thermo-stabilizing effect on MntA, with Mn(2+) having a milder effect. The thermodynamic stability of MntA, competition experiments, and metal binding and release experiments all suggest that Mn(2+) is the metal that is likely transported by MntBCA and is therefore the limiting factor for virulence of Bacillus anthracis. A homology-model of MntA shows a single, highly conserved metal binding site, with four residues that participate in metal coordination: two histidines, a glutamate, and an aspartate. The metals bind to this site in a mutually exclusive manner, yet surprisingly, mutational analysis shows that for proper coordination each metal requires a different subset of these four residues. ConSurf evolutionary analysis and structural comparison of MntA and its homologues suggest that substrate binding proteins (SBPs) of metal ions use a pair of highly conserved prolines to interact with their cognate ABC transporters. This proline pair is found exclusively in ABC import systems of metal ions.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Bacillus anthracis / metabolism*
  • Bacillus anthracis / pathogenicity*
  • Bacterial Proteins / chemistry
  • Bacterial Proteins / metabolism*
  • Cadmium / metabolism
  • Cobalt / metabolism
  • Manganese / metabolism
  • Metals / metabolism*
  • Nickel / metabolism
  • Protein Structure, Secondary
  • Protein Structure, Tertiary
  • Virulence
  • Zinc / metabolism


  • Bacterial Proteins
  • Metals
  • Cadmium
  • Cobalt
  • Manganese
  • Nickel
  • Zinc