Neutron reflectivity studies of bacterial membranes, peptides and proteins

Abstract

This thesis uses neutron and x-ray reflectivity to measure the interfacial structures of three molecular components associated with bacteria. Firstly, the way in which the membrane targeting sequence of a cell division protein interacts with monolayer models for the inner leaflet of the inner membrane of bacteria was measured at the air-water interface. Secondly, the influence of lipopolysaccharide on a monolayer model for the outer leaflet of the outer membrane of Gram-negative bacteria was measured at the air-water interface, as well as how this lipopolysaccharide interacts with an antimicrobial peptide. Finally, the structure of a layer of protein found at the surface of a Gram-positive biofilm was measured at the air-water interface. Binding of the membrane targeting sequence of the MinD protein (MinD-mts) to the inner leaflet of the cytoplasmic membrane is thought to be key for bacterial cell division. Modelling this membrane as a monolayer at the air-water interface, it was found that the insertion of the MinD-mts increased with decreasing lateral pressure within the monolayer, as well as with increasing unsaturation of the lipid components, and the incorporation of cardiolipin into the monolayer. Lipopolysaccharide (LPS) is the major component of Gram-negative outer membranes, such as Escherichia coli, and can be considered as having three structural components: lipid A, a core oligosaccharide, and a variable polysaccharide chain. By incorporating LPS into a model membrane at the air-water interface, it was found that the polysaccharide chain undergoes conformational changes depending on the area per molecule. The effect of the antimicrobial peptide Pexiganan on the structure of this LPS layer was also determined, and was found to insert into the polysaccharide chain layer, but have no impact on the conformation of the chains. In nature, many bacteria live within a biofilm structure. A critical component of the Gram-positive Bacillus subtilis biofilm is a surface active amphipathic protein called BslA, which gives rise to the formation of the highly hydrophobic surface of the biofilm. The kinetics of this film formation, its thickness, and the lateral packing of the protein at the air-water interface, were measured using both neutron and x-ray reflectivity. It was found that a native BslA protein consistently formed the same structural film, whilst the structure of films formed by mutant proteins depended on the conditions under which the film was formed.