Membrane Curvature Revisited-the Archetype of Rhodopsin Studied by Time-Resolved Electronic Spectroscopy.

G-protein-coupled receptors (GPCRs) comprise the largest and most pharmacologically targeted membrane protein family. Here, we used the visual receptor rhodopsin as an archetype for understanding the influences of membrane lipids on conformational changes involved in GPCR activation. Visual rhodopsin was recombined with lipids varying in their degree of acyl chain unsaturation and polar headgroup size using 1-palmitoyl-2-oleoyl-sn-glycero- and 1,2-dioleoyl-sn-glycerophospholipids with phosphocholine (PC) or phosphoethanolamine (PE) headgroups. The receptor activation profile following light excitation was measured using time-resolved UV-visible spectroscopy. We discovered that more saturated POPC lipids back shifted the equilibrium to the inactive state, while the small-headgroup, highly unsaturated DOPE lipids favored formation of the active state. Increasing unsaturation or decreasing headgroup size have similar effects that combine to yield control of rhodopsin activation. Membrane effects persist even after electrostatic influences of charged groups at the membrane are taken into account, and necessitate the involvement of factors beyond changes in proteolipid solvation energy. Hence, we consider a balance of curvature free energy with hydrophobic matching and demonstrate how our data support a flexible surface model (FSM) of lipid-protein coupling interactions. The FSM is based on the Helfrich formulation of membrane bending energy as previously first applied to lipid-protein interactions. Membrane elasticity and curvature strain are induced by lateral pressure imbalances between the constituent lipids and drive key physiological processes at the membrane level. Spontaneous negative monolayer curvature towards water is mediated by unsaturated, small-headgroup lipids and couples directly to GPCR activation upon light absorption by rhodopsin. For the first time to our knowledge, we demonstrate this modulation in both the equilibrium and pre-equilibrium evolving states using a time-resolved approach.