Neutron and X-ray scattering studies of cholera toxin interactions with lipid monolayers at the air-liquid interface

Using neutron/X-ray reflectivity and X-ray grazing incidence diffraction (GID), we have characterized the structure of mixed DPPE:GM1 lipid monolayers before and during the binding of cholera toxin (CTAB5) or its B subunit (CTB5). Structural parameters such as the density and thickness of the lipid layer, extension of the GM1 oligosaccharide headgroup, and orientation and position of the protein upon binding are reported. Both CTAB5 and CTB5 were measured to have ∼50% coverage when bound to the lipid monolayer. X-ray GID experiments show that both the lipid monolayer and the cholera toxin layer are crystalline. The effects of X-ray beam damage have been assessed and the monolayer/toxin structure does not change with time after protein binding has saturated.