Supramolecular Organization of Rod Outer Segment Membrane: New Rhodopsin Dimer Interface and Insights from the β2Ar-Gs Complex

Abstract

How the components of the G protein-coupled receptor (GPCR) “signalosome” assemble and function in the membrane bilayer is not known. Nevertheless the highly symmetrical organization of visual receptors (rhodopsin, rho) in rows-of-dimers suggested by AFM images of the rod cell disk membrane has a simple and direct consequence: the exposure of one unique surface of the receptor to the lipid phase of the membrane. This organization defines the encounter complex between rho and the G protein (transducin, Gt), and the orientation of rho in the dimer is therefore determinant. We have previously identified both computationally[1] and experimentally[2] a stable rho dimer involving TM1/TM2 and H8. Here we describe the investigation of alternative receptor binding modes leading to the identification of a new interface involving TM5/TM6. PMF analysis of this interface using the MARTINI coarse grain model indicates a similar order of stability as the TM1/TM2/H8 interface. However it involves a significant conformational change of a loop as compared to the ground state structure we originally used (PDB:1U19). This finding underlines the importance of loop flexibility in membrane protein interactions. Interestingly, when dimers of rho based on this new interface are arranged in rows-of-dimers, Gt placed on rho following the arrangement recently described for Gs on the β2AR correctly embed the lipid anchors of Gtα and Gtγ in the lipid phase. We also discuss the pros and cons of the different models of rho dimers for the visual phototransduction machinery, in the context of the rows-of-dimers arrangement, the functional implications of such supramolecular organization, the binding of Gt to rho and the structural, biochemical and physiological data available and the potential relevance for other GPCRs.[1] Periole, et al.(2012)JAmChemSoc134,10959-10965.[2] Knepp, et al.(2012)Biochemistry51,1819-1821.