Oral Presentation BACPATH 2017

Molecular insights into zinc acquisition by Streptococcus pneumoniae. (#18)

Jacqueline Morey 1 , Zhenyao Luo 2 , Evelyne Deplazes 2 , Jeffrey Harmer 3 , Alina Motygullina 3 , Bart Eijkelkamp 1 , Bostjan Kobe 2 , Megan O'Mara 4 , Rafael Counago 2 , Alastair McEwan 2 , James Paton 1 , Christopher McDevitt 1
  1. Research Centre for Infectious Diseases, University of Adelaide, Adelaide, SA
  2. School of Chemistry and Molecular Biosciences, University of Queensland, Brisbane, QLD
  3. Centre for Advanced Imaging, University of Queensland, Brisbane, QLD
  4. Research School of Chemistry, Australian National University, Canberra, ACT

Streptococcus pneumoniae (the pneumococcus) is a significant human pathogen that causes more than one million deaths every year. Efficient acquisition of the transition metal zinc from the host environment is essential for pneumococcal viability and virulence. This is facilitated by the ATP-binding cassette permease AdcCBA, wherein the solute-binding protein (SBP) AdcA delivers zinc to the transmembrane importer AdcCB. Unusually, AdcA is comprised of two domains: an N-terminal canonical SBP domain (AdcANterm) and a C-terminal domain that shares homology with the Gram-negative zinc chaperone ZinT (AdcACterm). Additionally, AdcA features a highly mobile histidine-rich loop, the function of which is unknown. We have previously shown that both domains contain a high-affinity zinc-binding pocket, although the precise mechanism by which AdcA binds zinc remains unclear. Here, we present a comprehensive characterisation of the zinc-binding mechanism of AdcA. We show that AdcANterm is necessary for pneumococcal growth in zinc-limited media, while AdcACterm and the His-rich loop are not required. These data suggest that AdcACterm and the His-rich loop are accessory components for zinc uptake, while AdcANterm is critical for interaction with AdcCB. We then combined high-resolution structural analyses with spectroscopic techniques to investigate the zinc-binding mechanism of AdcANterm in greater detail. Intriguingly, zinc coordination was associated with localised changes in mobility, although no large structural rearrangements were observed. These changes in mobility are likely to be important for the ability of AdcANterm to interact with AdcCB, as a putative docking residue was identified within one of the variably mobile regions. This residue was then shown to be important for the function of AdcA in zinc uptake, supporting its potential role in mediating the interaction with AdcCB. Collectively, these findings provide the first detailed ligand-binding mechanism of a zinc-specific SBP, and enable crucial insights into how the pneumococcus acquires sufficient zinc in the host environment.