Membrane-bound form of the pore-forming domain of colicin A. A neutron scattering study

The ion-channel forming C-terminal fragment of colicin A binds to negatively charged lipid vesicles and provides an example of the insertion of a soluble protein into a lipid bilayer. The soluble structure is known and consists of a ten-helix bundle containing a hydrophobic helical hairpin. This fragment forms a well-defined complex with dimyristoylphosphatidyl-glycerol which is thus amenable to neutron scattering studies. Neutron scattering experiments in the Guinier range (low angles) provided the mass and the stoichiometry of the complex (290,000 (+/- 10,000) M(r), 8.2 (+/- 0.5)), in fair agreement with previous determinations. By varying the neutron scattering length density of the solvent with 2H2O/H2O mixtures and therefore the contrast of the different components, the radial distribution of the protein and of the lipids was determined. Finally, an attempt was made to fit various models to the wider angle scattering data. This study suggests that the pore-forming fragment of colicin A lies mostly at the surface of the membrane, with the lipids arranged in a bilayer organization.