Rapid Poster Presentation BACPATH 2019

Elucidating the Zn(II)-binding mechanism of Streptococcus pneumoniae AdcAII (#35)

Marina L Zupan 1 , Zhenyao Luo 2 3 4 , Victoria G Pederick 5 , Aimee Tan 1 , Jeffrey R Harmer 6 , Evelyne Deplazes 7 8 , Bostjan Kobe 2 3 4 , James C Paton 5 , Christopher A McDevitt 1
  1. Department of Microbiology and Immunology, The Peter Doherty Institute for Infection and Immunity, University of Melbourne, Melbourne, VIC, Australia
  2. School of Chemistry and Molecular Biosciences, The University of Queensland, Brisbane, QLD, Australia
  3. Australian Infectious Diseases Research Centre, The University of Queensland, Brisbane, QLD, Australia
  4. Institute for Molecular Bioscience, The University of Queensland, Brisbane, QLD, Australia
  5. Research Centre for Infectious Diseases, The University of Adelaide, Adelaide, SA, Australia
  6. Centre for Advanced Imaging, The University of Queensland, Brisbane, QLD, Australia
  7. School of Life Sciences, University of Technology Sydney, Sydney, NSW, Australia
  8. School of Pharmacy and Biomedical Sciences, Curtin Health Innovation Research Institute, Curtin University, Perth, Western Australia, Australia

Streptococcus pneumoniae is a globally significant human pathogen responsible for more than a million deaths annually. To colonise and persist within the host, the bacterium must acquire the transition metal ion zinc [Zn(II)], which is poorly abundant in the host environment. In S. pneumoniae, Zn(II) import is facilitated by the ATP-binding cassette transporter, AdcCB, and two Zn(II)-specific solute binding proteins (SBPs), AdcA and AdcAII. Although both proteins deliver Zn(II) to the AdcCB transporter, AdcAII has a greater role during initial infection and survival in response to Zn(II) starvation. Despite this, the molecular details of how AdcAII selectively acquires Zn(II) remain poorly understood. To date, our understanding of the Zn(II)-binding mechanism has been based solely on the crystal structure of Zn(II)-bound AdcAII, with an open, metal-free conformation remaining refractory to crystallographic approaches. As a consequence, the conformational changes that occur within AdcAII upon Zn(II)-binding remain unknown. Here, we overcame this challenge by individually mutating each of the four Zn(II)-coordinating residues and performing structural and biochemical analyses on the variant isoforms. Structural analyses revealed specific regions within the protein that underwent conformational changes via their direct coupling to each of the metal-binding residues. Quantitative metal-binding, metal ion affinity analyses and phenotypic assays revealed that two of the four coordinating residues had essential contributions to the Zn(II)-binding mechanism of AdcAII. Intriguingly, only one of these residues had a direct role in structural rearrangements within AdcAII. These analyses also revealed that AdcAII could interact with other first-row transition metal ions, in contrast to AdcA. Collectively, our structural, biophysical and microbiological data indicate that AdcAII employs a distinct mechanism of metal binding to other Zn(II)-specific SBPs. Elucidation of this mechanism will provide the structural and biochemical data required for future antimicrobial design strategies.