Subunit arrangement of cholera toxin in solution and bound to receptor-containing model membranes.

Quasi-elastic laser light scattering (QLS) is used to study the translational frictional properties of cholera toxin and its complex with ganglioside Gm1 receptor containing phospholipid vesicles. These properties are compared to theoretically calculated values for model structures composed of spherical subunits in order to assess the structural configuration of the toxin and its binding geometry on membrane surfaces. The structure for the toxin that best fits the experimental results consists of the five B subunits arranged radially about an elongated A subunit, which extends well above the plane of the B subunits. Binding of cholera toxin to Gm1-containing model membranes results in a complex in which the B subunits are absorbed on the surface while the A subunit penetrates the membrane bilayer.