Ecoli packs a punch

Schematic of an E.coli membrane

The cells of E.coli bacteria are surrounded by a protective membrane. In this membrane are proteins (OmpF) which allow food into the cell. The antibacterial toxin Colicin N uses OmpF to penetrate into the cell and kill it. Neutron science has given us a picture of how this happens.
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Neutron scattering experiments have given a new insight into how E. coli bacteria, often associated with food poisoning, kill each other in the evolutionary competition for food and space. This breakthrough could be fundamental in developing new ways to treat illnesses such as food poisoning or meningitis.

Commonly found in the intestines of humans and animals, E. coli is normally considered to be a ‘helpful’ bacterium that aids digestion.  However, it can also cause vomiting and diarrhoea, and can be a serious illness for young children, the elderly and those with vulnerable immune systems. 

Discovering how antibacterial proteins attack harmful bacteria is important for establishing new methods of drug delivery. Antibacterial proteins often have to travel across a waterproof (hydrophobic) cell membrane to reach their target. Neutron reflectivity and small angle neutron scattering (SANS) have been used to help identify the mechanism by which the antibacterial proteins, colicins, travel across the hydrophobic membrane of E.coli.

Colicins are toxic proteins, secreted by E.coli to kill other E.coli, in a bid to reduce competition. Just one colicin can be enough to kill an E.coli, a bacterium that is about 400,000 times heavier than the protein itself. The E.coli cell membrane is a hydrophobic bilayer, which can be difficult to penetrate. In order to reach their target, colicins must overcome this barrier. Colicin N (ColN) is a large pore- forming colicin, which kills the target cell by punching a hole through its inner membrane. Neutron scattering experiments were used to decipher exactly how this protein passes through the membrane. Prior evidence suggests that ColN hijacks the outer membrane protein, “OmpF”, which sits in the lipid layer of the membrane, but how ColN interacts with OmpF was still a bit of a mystery.

Complementary neutron scattering methods revealed the initial steps in ColN translocation. “Neutron scattering techniques were able to show us the insertion of Colicin N into the hydrophobic membrane. Using neutrons allowed us to get a side view of the process, which is important when following proteins across a barrier” said Jeremy Lakey, Professor of Structural Biosciences at the University of Newcastle. Jeremy and his group are now conducting further studies, using the Inter and Sans2D instruments at ISIS, to monitor later stages of the ColN translocation process. This will enable more extensive elucidation of ColN’s journey through the bacterial cell membrane. This research, funded by the Wellcome Trust, could have important implications in antibacterial therapy. Colicins are expressed in E.coli and related proteins are found in cholera and plague- causing bacteria. Although colicins are too large to inject into humans, if scientists are able to understand how they get across the E.coli cell membrane, it may be possible to design a system whereby smaller antibacterial proteins can overcome this hydrophobic barrier.

Technical details

Neutron reflectivity and SANS have been used to investigate the interaction of ColN with OmpF. SANS was used to help visualise the ColN-OmpF complex and the individual proteins in greater detail, whilst neutron reflectivity followed the insertion of ColN into lipid monolayers in real time. The results showed that the insertion was much faster in the presence of OmpF. The combined results demonstrated that OmpF can induce the unfolding and penetration of ColN into the hydrophobic membrane via the protein-lipid interface. Therefore, the protein- lipid interface may be the route that ColN takes into the cell.

Amandeep Hundal

Research date: November 2011

Further Information

Jeremy Lakey, Professor of Structural Biochemistry, Institute for Cell and Molecular Biosciences, the University of Newcastle

Jeremy.lakey@ncl.ac.uk

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