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

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 Fe²⁺ or Fe³⁺. Rather, MntA binds a variety of other metal ions, including Mn²⁺, Zn²⁺, Cd²⁺, Co²⁺, and Ni²⁺ with affinities ranging from 10^-6 to 10^-8 M. Binding of Zn²⁺ and Co²⁺ have a pronounced thermo-stabilizing effect on MntA, with Mn²⁺ having a milder effect. The thermodynamic stability of MntA, competition experiments, and metal binding and release experiments all suggest that Mn²⁺ 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.