How a G protein binds a membrane.

Heterotrimeric G proteins interact with receptors and effectors at the membrane-cytoplasm interface. Structures of soluble forms have not revealed how they interact with membranes. We have used electron crystallography to determine the structure in ice of a helical array of the photoreceptor G protein, transducin, bound to the surface of a tubular lipid bilayer. The protein binds to the membrane with a very small area of contact, restricted to two points, between the surface of the protein and the surface of the lipids. Fitting the x-ray structure into the membrane-bound structure reveals one membrane contact near the lipidated Ggamma C terminus and Galpha N terminus, and another near the Galpha C terminus. The narrowness of the tethers to the lipid bilayer provides flexibility for the protein to adopt multiple orientations on the membrane, and leaves most of the G protein surface area available for protein-protein interactions.