Escherichia coli has an asymmetric outer membrane which is an important barrier for antibiotics. However the toxin colicin N can cross this in seconds. The work at ISIS and ILL set out to see if the protein goes through its receptor OmpF or down the outside. The experiments revealed that the colicin bound to the outside of OmpF (right) and this protein-lipid interface may be a weak link in the defences of E coli.
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Neutron radiation can reveal the structure of cell membranes
Congratulations to Jeremy Lakey whose article,Seeing into Bacteria was recently published in Catalyst Magazine.
Prof. Jeremy Lakey and his research team studied how the membranes of disease-causing bacteria work using neutron scattering at both the Rutherford Appleton Laboratory, UK and the Institut Laue Langevin (ILL), Grenoble, France.
Bacteria are largely divided into two groups termed Gram-positive and Gram-negative because of the different colours they display under the microscope after being treated with a stain developed by Christian Gram in the 19th century. The Gram-negative cells owe their staining behaviour to the presence of an extra ‘outer’ membrane which they uniquely possess.
The cells thus resemble a medieval castle with an outer wall, a courtyard and an inner keep, whereas most cells are more like a house with just an outer wall. In spite of this, gram-negative bacterial cells are still far smaller than animal cells and this makes structural studies difficult. Through the use of the neutron facilities at ISIS and ILL, Prof. Lakey and his team were able to obtain a side view of colicin ( a protein that kills E coli)during its penetration into complex bacterial membranes which electron microscopy or X-ray imaging couldn’t provide.
Neutrons are sensitive to the nucleus of the atoms they interact with and can differentiate between elements, as well as isotopes of the same element. This means neutrons are efficient at looking at hydrogen, which biological materials are full of, and can tell the difference between hydrogen and its isotope, deuterium.
Prof. Lakey's team made a model outer membrane using proteins and lipids and then used neutron scattering techniques at ISIS to study the internal structure of the membrane. By using lipids and proteins labelled with deuterium they were able to show that adding a single protein called ‘outer membrane protein F’ (OmpF) enables colicin to penetrate into the membrane. This is important because bacteria which don’t have OmpF are not killed by colicin. The colicin only penetrates 3 nm into the membrane, but this is enough to get past the protective barrier. Interestingly, colicin appears to stretch as it binds to OmpF, and this led to a further investigation with the use of the ILL facilities.
By mixing OmpF labelled with deuterium with normal hydrogen containing colicin, Prof. Lakey and his team were able to solve the structure of the colicin-OmpF complex at ILL. This showed that colicin unfolds into a longer protein and binds to the outside of the OmpF protein where it finds a route across the membrane layer into the cell.
These studies have shown that the junction between protein and lipid in the bacterial membrane is where the protective barrier is at its weakest. Colicin proteins can slip between the cracks in the castle walls and so are efficient at evading the defence mechanisms. They show an amazing ability to penetrate the layers of defensive molecules, which has inspires the design of other antibiotics which could penetrate bacteria using a similar route into the target cell.
Prof Jeremy Lakey , Newcastle University
Research Date: October 2012
J Lakey, Catalyst: Secondary Science Review, October 2012 Volume 23, Issue 1, pages 16-18
Research date: October 2012
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