The relative stability of divalent first-row transition metal ion complexes, as defined by the Irving-Williams series, poses a fundamental chemical challenge for selectivity in bacterial metal ion acquisition. Here we show that although the substrate-binding protein of Streptococcus pneumoniae, PsaA, is finely attuned to bind its physiological substrate manganese, it can also bind a broad range of other divalent transition metal cations. By combining high-resolution structural data, metal-binding assays and mutational analyses, we show that the inability of open-state PsaA to satisfy the preferred coordination chemistry of manganese enables the protein to undergo the conformational changes required for cargo release to the Psa permease. This is specific for manganese ions, whereas zinc ions remain bound to PsaA. Collectively, these findings suggest a new ligand binding and release mechanism for PsaA and related substrate-binding proteins that facilitate specificity for divalent cations during competition from zinc ions, which are more abundant in biological systems.
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This work was supported by the Australian Research Council (ARC) grants DP0986578 to A.G.M. and DP120103957 to C.A.M. and the National Health and Medical Research Council (NHMRC) project grant 1022240 to C.A.M. and program grant 565526 to J.C.P., A.G.M. and B.K. M.L.O. holds an ARC Discovery Early Career Researcher Award. B.K. is a NHMRC Senior Research Fellow. J.C.P. is a NHMRC Senior Principal Research Fellow. We thank A.E. Mark for discussions.
The authors declare no competing financial interests.
Supplementary Text and Figures
Supplementary Results, Supplementary Tables 1–4, Supplementary Movie Legend and Supplementary Figures 1–6. (PDF 4603 kb)
The movie shows the conformational changes in PsaA as the cognate physiological ligand Mn2+ binds the protein, as revealed by the crystal structures of metal-free, open-state PsaA (PDB code 3ZK7); open-state Mn2+-PsaAD280N (PDB code 3ZKA); and closed-state Mn2+-PsaA (PDB code 3ZTT). Changes in secondary structure and backbone hydrogen bonding that occur in the flexible ('spring') helix of PsaA during binding are highlighted. The movie also shows the conformational differences between closed-state Mn2+-PsaA (PDB code 3ZTT) and closed-state Zn2+-PsaA (PDB code 1PSZ) (MOV 22844 kb)
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Couñago, R., Ween, M., Begg, S. et al. Imperfect coordination chemistry facilitates metal ion release in the Psa permease. Nat Chem Biol 10, 35–41 (2014). https://doi.org/10.1038/nchembio.1382
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