Membrane Curvature Effects On Rhodopsin Activation Investigated By Time-Resolved Electronic Spectroscopy

Lipid bilayer composition is a known regulator of physiological processes at the membrane, in contrast to the inert solvent view offered by the standard fluid mosaic model. While lipid-protein coupling interactions are gaining acceptance, the exact mechanisms of these interactions have sustained debate. Here, we recombined the retinal light receptor rhodopsin, a canonical G-protein-coupled receptor (GPCR), into different lipid membrane environments. We applied microsecond time-resolved electronic (UV-visible) spectroscopy to quantify the metastable activation equilibrium of rhodopsin following photoexcitation near physiological temperature. The evolution of the fully active metarhodopsin-II (MII) state from inactive metarhodopsin-I (MI) was monitored in a variety of lipid membranes, including DOPE, DOPC, POPC, and native rod disk membranes. Decreasing lipid headgroup size and increasing lipid acyl chain unsaturation universally forward shifted the metarhodopsin equilibrium to the active MII state, with the native membranes favoring the greatest MII fraction. Our results support a flexible surface model (FSM), in which lateral pressure imbalances among the lipids modulate the membrane free energy as it adopts various curvatures [1]. Longitudinal expansion of rhodopsin during its activation couples to changes in the monolayer curvature. Small-headgroup, unsaturated lipids such as DOPE favor a negative intrinsic curvature (towards water), which drives the activation of rhodopsin forward to the expanded MII state. Unlike hydrophobic mismatch forces or specific chemical interactions between lipids and rhodopsin, the FSM accounts for the experimental effects of headgroup size and acyl chain unsaturation [2]. Our results necessitate a reworking of the conventional fluid mosaic model to highlight the influence of mesoscopic, mechanical influences of the lipid bilayer soft matter on cellular membrane function. References: [1] M.F. Brown (2017) Annu. Rev. Biophys. 46, 379-410. [2] A.V. Botelho et al. (2006) Biophys. J. 91, 4464-4477.