Rhodopsin Activation is Modulated by Non-Specific Membrane Lipid-Protein Interactions

Abstract

Rhodopsin activation proceeds through an ensemble of conformational substates: Meta I ↔ Meta IIa ↔ Meta IIb ↔ Meta IIbH+ [1]. These substates are characterized by Schiff base deprotonation (Meta I), an outward tilt of helix H6 (Meta IIb), and protonation of Glu134 (Meta IIbH+). Lipid bilayer composition has been shown to affect rhodopsin activation, providing an opportunity to systematically investigate membrane protein-lipid bilayer interactions on the mesoscale [2]. To quantify lipid acyl chain effects on GPCR activation, rhodopsin was reconstituted in lipids with symmetric unsaturated acyl chains (DOPC) or asymmetric lipids with sn-1 saturated and sn-2 unsaturated acyl groups (POPC). Proteolipid recombinant membranes were studied by UV-visible and FTIR spectroscopy. Symmetric membrane lipids (DOPC) stabilize Meta IIa and render the normally weakly populated substate accessible to study. The Meta IIa substate is characterized by an opening of the Schiff base ionic lock, and an activation switch in a conserved water-mediated, hydrogen-bonded network involving helices H1/H2/H7 that is sensed by Asp83. Replacement of an unsaturated acyl chain with a saturated chain (POPC) increases the pKa value for the Meta I ↔ Meta II equilibrium, and consequently destabilizes the Meta IIa substate. Modulation of the bilayer curvature stress due to a negative monolayer spontaneous curvature (H0) (DOPC) contributes a mechanical force that shifts the Meta I ↔ Meta II equilibrium towards Meta II, leading to rhodopsin activation. The flexible surface model (FSM) explains how chemically non-specific interactions between membrane proteins and the lipid bilayer contribute to GPCR activation [3,4].[1] M. Mahalingam et al. (2008) PNAS 105:17795.[2] E. Zaitseva et al. (2010) JACS 132:4815.[3] A.V. Botelho et al. (2006) Biophys. J. 91:4464.[4] M.F. Brown (1994) Chem. Phys. Lipids 73:159.