Streptococcus pneumoniae, also known as the pneumococcus, is the world’s foremost bacterial pathogen. Pneumococcal disease is a leading cause of death in the elderly and children under 5. In Australia, it is of particular concern amongst Aboriginal and Torres Strait Islander communities. Pneumococcal surface adhesin A (PsaA) is a major virulence factor, which is located on the cell-surface of all known pneumococcal serotypes that, is essential for host colonization and mediating disease. PsaA has a demonstrated dual role as a metal binding protein that delivers manganese (Mn2+) to the PsaC permease domain of its cognate ABC importer; and as a surface adhesin that binds to the E-cadherin receptor of human nasopharyngeal cells. PsaA is thus a promising target for antimicrobial therapeutics aimed at treating pneumococcal disease.
The recently published crystal structure of PsaA in the metal-free (apo) conformation has laid the groundwork for rational design of antimicrobial drugs that target PsaA. However, a crystal structure is only a static snapshot that cannot characterise the spatio-temporal motion of the protein in solution. To address this issue we carried out molecular dynamics (MD) simulations in conjunction with electron paramagnetic resonance (EPR) spectroscopy experiments. Five different cysteine mutants of the apo PsaA protein were labelled with the thiol-reactive spin label MTSL, and continuous-wave EPR spectra (CW-EPR) were collected. In addition, the MTSL-labelled proteins were modelled for 200 ns each using unrestrained MD simulations. From the combined results the relative mobility of different protein domains was determined. A comparison of the data form MD simulations and CW-EPR and an analysis of the structure and dynamics of the PsaA protein in solution with respect to the proposed metal-binding mechanism will be presented.