How a G Protein Binds a Membrane

Zhixian Zhang,Thomas J Melia,Feng He,Ching Yuan,Amy Mcgough,Michael F Schmid,Theodore G Wensel

doi:10.1074/jbc.m403404200

Zhixian Zhang, Thomas J Melia + Show 5 more

Open Access

https://doi.org/10.1074/jbc.m403404200

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Abstract

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.

Highlights

There have been numerous studies of G protein-membrane interactions using biochemical, functional, and mutagenic approaches
Restriction of Membrane Contacts to Two Narrow Surfaces— Perhaps the most surprising result from the reconstruction is that the area of contact between protein and lipid is quite small, involving only 2% of the protein surface
The structure revealed by electron crystallography indicates very few direct contacts between the lipid surface and the protein, and the regions near the membrane surface are predominantly of negative charge (Fig. 6B)

Summary

How a G Protein Binds a Membrane*

The only G protein structure in which a lipid modification is present and visible is the complex of G␤␥t with phosducin, in which the farnesyl group is tucked into the ␤-propeller blades of G␤t [17], and which has greatly diminished affinity for membranes Another region thought to be near the membrane is the C terminus of the ␣ subunit, because of its critical role in binding to receptors [18, 19]. Because of the intrinsic difficulty of incorporating a membrane-protein interface into three-dimensional crystals suitable for x-ray crystallography, we have used the approach of electron crystallography to determine the structure of the G protein-membrane complex In this approach the protein is allowed to assemble into an ordered lattice on the surface of a lipid bilayer, and electron microscopy of samples preserved in vitreous ice is used to obtain images for structural analysis. In addition to revealing the G protein orientation on the membrane surface, these results suggest that electron crystallography using helical protein lattices assembled on lipid tubules may be a generally applicable approach for determining membrane orientations for peripheral proteins

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DISCUSSION