Many pathogenic bacteria produce outer polysaccharide capsules which play a major role in disease progression. The capsule provides protection against host immune responses such as opsonisation, phagocytosis and complement mediated lysis1. Certain capsules also act to mask the bacterium from the host immune system through molecular mimicry. Despite the importance in disease progression, very little is known about the formation of the capsules.
The polysaccharide components of the capsule are synthesised within the cell and exported to the exterior by a large cell envelope spanning protein complex. This export is coordinated by one of two systems; the Wzy dependent system and the ABC-transporter dependent system. The Wzy dependent system has been characterised to some extent while the ABC-transporter dependent system remains largely unstudied2.
Neisseria meningitidis utilises an ABC-transporter dependent system involving four proteins; Capsule transporter A (CtrA), CtrB, CtrC and CtrD. CtrC and CtrD collectively form an ABC transporter that initiates export across the inner membrane. The translocation across the periplasm and peptidoglycan layer is then thought to be assisted by CtrB. Finally, CtrA completes the export across the cell envelope and the polysaccharides are anchored into the outer membrane. My project aims to elucidate the three dimensional structures of CtrA, CtrB, CtrC and CtrD. In addition, biophysical techniques will be used to determine the oligomeric state of each protein in a single complex, as well as the binding affinity between them.
Here I will present my progress on the recombinant expression, purification and biophysical analyses of CtrA, CtrB, CtrC and CtrD.