Structural Determinants of the Supramolecular Organization of G Protein-Coupled Receptors in Bilayers

The G protein-coupled receptor (GPCR) rhodopsin self-assembles into supramolecular structures in native bilayers, but the structural determinants of receptor oligomerization are not known. We carried out multiple self-assembly coarse-grained molecular dynamics (CGMD) simulations of model membranes containing up to 64 molecules of the visual receptor rhodopsin over time scales reaching 100 μs. The simulations show strong preferential interaction modes between receptors. Two primary modes of receptor−receptor interactions are consistent with umbrella sampling/potential of mean force (PMF) calculations as a function of the distance between a pair of receptors. The preferential interfaces, involving helices (H) 1/8, 4/5 and 5, present no energy barrier to forming a very stable receptor dimer. Most notably, the PMFs show that the preferred rhodopsin dimer exists in a tail-to-tail conformation, with the interface comprising transmembrane H1/H2 and amphipathic H8 at the extracellular and cytoplasmic surfaces, respectively. This dimer orientation is in line with earlier electron microscopy, X-ray, and cross-linking experiments of rhodopsin and other GPCRs. Less stable interfaces, involving H4 and H6, have a free energy barrier for desolvation (delipidation) of the interfaces and appear to be designed to stabilize “lubricated” (i.e., lipid-coated) dimers. The overall CGMD strategy used here is general and can be applied to study the homo- and heterodimerization of GPCRs and other transmembrane proteins. Systematic extension of the work will deepen our understanding of the forces involved in the membrane organization of integral membrane proteins.

Rhodopsin Forms a Dimer with Cytoplasmic Helix 8 Contacts in Native Membranes

G protein-coupled receptors form dimers and higher-order oligomers in membranes, but the precise mode of receptor−receptor interaction remains unknown. To probe the intradimeric proximity of helix 8 (H8), we conducted chemical cross-linking of endogenous cysteines in rhodopsin in disk membranes. We identified a Cys316−Cys316 cross-link using partial proteolysis and liquid chromatography with mass spectrometry. These results show that a symmetric dimer interface mediated by H1 and H8 contacts is present in native membranes.