SEMINAR: Bayliss Seminar Series

Many pathogenic bacteria produce outer polysaccharide capsules which play a major role in disease progression. The capsules bestow protection against host immune responses such as opsonisation, phagocytosis and complement mediated lysis. 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 then exported to the exterior where they are anchored to the outer membrane. This export is thought to be mediated by a large protein complex that spans the cellular envelope. There are two different systems used by bacteria to do this. The first, the ‘Wzy system,’ has been studied to some extent while the second ‘ATP Binding Cassette (ABC)-transporter dependent’ system still remains largely unstudied.

My project aims to characterise three proteins from an ABC-transporter dependent system; CtrA, CtrC and CtrD (Figure 1). This will be achieved through structural studies (X-ray crystallography and cryo-electron microscopy) and biophysical techniques (binding assays and dynamic light scattering). CtrA is located in the outer membrane and is predicted to form a multimeric alpha helical barrel. Such a fold has been seen in only one other outer membrane protein. The elucidation of the three dimensional structure of CtrA would prove indispensable to our understanding of outer membrane protein science. CtrC and CtrD collectively form an ABC-transporter with CtrC acting as the transmembrane domain and CtrD the nucleotide exchange factor. Examining the interactions between these proteins will provide insight into the initiation of the capsular polysaccharide export.