Influences of membrane biophysical properties on the Metarhodopsin I to Metarhodopsin II transition in visual excitation

Abstract

Current biophysical studies of membrane proteins are centered on the relation of their structures to key biological functions of membranes in terms of lipid-protein interactions. The conformational transition of rhodopsin from Metarhodopsin I to Metarhodopsin II (Meta I-Meta II) is the triggering event for the visual process. Meta II is the activated form of the visual receptor and binds a signal transducing G protein (transducin), followed by two amplification stages which lead to generation of a visual nerve impulse. Herein, flash photolysis and surface plasmon resonance (SPR) spectroscopy techniques have been used to monitor the Meta I-Meta II transition of rhodopsin in various membrane recombinants. The flash photolysis experiments clearly show a substantial shift to the left of the Meta I-Meta II equilibrium for rhodopsin in egg phosphatidylcholine recombinant membranes. Investigation of the influences on rhodopsin function by non-lamellar forming lipids reveals a characteristic relationship between the Gibbs free energy change for the Meta I-Meta II equilibrium of rhodopsin and the intrinsic curvature of the lipid bilayer. Complementary SPR studies suggest a protrusion of the protein at the activated Meta II state which may be associated with exposure of recognition sites for the signal transducing G protein on the cytoplasmic surface of rhodopsin. All the experimental results obtained here are consistent with the hypothesis of a new flexible surface biomembrane model. The Meta II state is favored by a negative spontaneous curvature of the bilayer, corresponding to an imbalance of the lateral forces within the polar head groups and acyl chains. The mean curvature of membrane bilayer in the Meta II state reflects the small spontaneous curvature of the lipid bilayer in the vicinity of protein. Relief of the lipid curvature frustration in the Meta II state energetically couples the lipids to the photoexcitation of rhodopsin. Consideration of the various energetic contributions suggests the bilayer curvature free energy provides a reservoir of work in the modulation of rhodopsin function in the visual process. These studies that biophysical properties of the liquid-crystalline lipid bilayer are important in relation to protein function and may be relevant to the biomedical investigations of visual dysfunction.