Controlling GPCR Rhodopsin Function by Small, Physiologically Relevant Changes in Bilayer Hydrophobic Thickness

Abstract

It was established previously that the G protein-coupled membrane receptor rhodopsin has transmembrane helices which match a hydrophobic bilayer thickness of 27±1 A. Here we demonstrate that small changes of bilayer thickness of ±2 A about that match point translate in the considerable changes of rhodopsin activation measured as the metarhodopsin I (MI)/metarhodopsin II (MII) equilibrium. We observed a biphasic behavior of the MI/MII equilibrium, with a sharp decline towards MI from 25-27 A followed by a rapid increase of MII from 27-29 A. Results are qualitatively identical for thickness changes induced by mixing of 16:0-16:1 PC and 18:0-18:1 PC, or 16:1-16:1 PC and 18:1-18:1 PC, or addition of 0-30 mol% cholesterol to 16:0-16:1 PC. The biphasic behavior was observed regardless of lipids used to alter bilayer hydrophobic thickness suggesting a relationship between small changes in hydrophobic thickness and rhodopsin function. It strongly favors an explanation based on a change of elastic stresses in lipid bilayers upon the transition from negative curvature in lipid monolayers near the protein below 27 A hydrophobic thickness to positive monolayer curvature above the match point. A continuum elastic model of the membrane, including the effect of lipid monolayer curvature near the protein, predicts membrane mediated clustering of rhodopsin and stabilization of the MI photointermediate at the matching point. Small, physiologically relevant changes in cholesterol content of bilayers with a thickness in the physiologically relevant range do drastically down- or up regulate the amount of MII which is the state that activates G protein.