A structural view of molecular events in the life-cycle of a Bordetella effector

BteA is a T3SS effector in Bordetella pertussis, the etiological agent of pertussis and related mammalian respiratory diseases. The BteA N-terminal domain binds its cognate chaperone BtcA, and targets the protein to PIP₂-containing lipid raft microdomains within the eukaryotic host cell. We use a combination of structural magnetic resonance and biophysical methods to determine the molecular basis for BteA interactions along the T3SS pathway. The crystal structure of a functional N-terminal 131-residue domain was determined to be an elongated helical bundle capped by a critical ‘tip’ motif. Using lipoprotein nanodiscs and soluble phosphatidylinositols allowed nuclear magnetic resonance (NMR) to map the membrane-targeting site to a contiguous BteA basic surface comprised of a helix-loop-helix motif, with regiospecificity for the PI(4,5)P₂ isomer, whereas the tip region plays a structural role. This was confirmed using an array of biophysical approaches including SAXS and differential scanning calorimetry. NMR also identified a chaperone-induced global unfolding of the effector involving the membrane-targeting region. True to the globular/non-globular hypothesis of chaperone-binding, both NMR and electron paramagnetic resonance (EPR) pinpoint effector domains that interact with the chaperone and suggest a model for the complex and its role in effector injection. Our results offer a structural view of the Bordetella effector along the T3SS pathway, highlighting its interaction modes with the chaperone and host membranes leading to cytotoxicity, and may have implications for understanding similar pathways in other human pathogens.