Oral Presentation Lorne Infection and Immunity 2021

The molecular basis for zinc uptake via Streptococcus pneumoniae AdcAII (#24)

Marina Zupan 1 , Zhenyao Luo 2 3 4 , Aimee Tan 1 , Victoria Pederick 5 , Katherine Ganio 1 , Evelyne Deplazes 6 7 , Bostjan Kobe 2 3 4 , Christopher 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. School of Pharmacy and Biomedical Sciences, Curtin Health Innovation Research Institute, Curtin University, Perth, Western Australia, Australia
  7. School of Life Sciences, University of Technology Sydney, Sydney, NSW, Australia

Streptococcus pneumoniae is a globally significant human pathogen that scavenges essential zinc [Zn(II)] ions from the host during colonization and infection. This is achieved by the ATP-binding cassette transporter, AdcCB, and two solute-binding proteins (SBPs), AdcA and AdcAII. Although both SBPs are required for full virulence of S. pneumoniae, AdcAII plays a greater role during the early stages of infection and under zinc limitation. However, the molecular details of how AdcAII acquires Zn(II) ions remain poorly defined. This can be attributed to the inability of crystallographic approaches to determine a high-resolution structure of ligand-free AdcAII. Here, we overcame this issue by systematically mutating each of the four Zn(II)-coordinating residues and performing structural and biochemical analyses on the variant isoforms. Structural analyses of Zn(II)-bound AdcAII variant proteins revealed how specific regions within the SBP undergo conformational changes via their direct coupling to each of the metal-binding residues. Quantitative in vitro metal-binding assays, combined with affinity determination and phenotypic analyses, revealed the relative contribution of each coordinating residue to the Zn(II)-binding mechanism. These analyses also revealed that in contrast to AdcA, AdcAII is permissive for interacting with other first-row transition metal ions. Intriguingly, the impact of mutant adcAII alleles on the growth of S. pneumoniae did not generally correlate with SBP affinity, but was instead consistent with the degree of structural perturbation exhibited in mutant AdcAII proteins. Taken together, our data show, for the first time, that SBP conformation rather than affinity is the primary determinant of efficacious Zn(II) uptake in S. pneumoniae. Collectively, our data reveal a novel metal-binding mechanism for AdcAII and highlight how ligand affinity and protein conformational changes are coupled within ligand-receptor proteins. These mechanistic insights provide a foundation for novel antimicrobial design to disrupt this process in bacterial metal-receptor proteins.