Optimization of receptor-G protein coupling by bilayer lipid composition II: formation of metarhodopsin II-transducin complex.

The visual transduction system was used as a model to investigate the effects of membrane lipid composition on receptor-G protein coupling. Rhodopsin was reconstituted into large, unilamellar phospholipid vesicles with varying acyl chain unsaturation, with and without cholesterol. The association constant (K(a)) for metarhodopsin II (MII) and transducin (G(t)) binding was determined by monitoring MII-G(t) complex formation spectrophotometrically. At 20 degrees C, in pH 7.5 isotonic buffer, the strongest MII-G(t) binding was observed in 1-stearoyl-2-docosahexaenoyl-sn-glycero-3-phosphocholine (18:0,22:6PC), whereas the weakest binding was in 1-stearoyl-2-oleoyl-sn-glycero-3-phosphocholine (18:0,18:1PC) with 30 mol% cholesterol. Increasing acyl chain unsaturation from 18:0,18:1PC to 18:0,22:6PC resulted in a 3-fold increase in K(a). The inclusion of 30 mol% cholesterol in the membrane reduced K(a) in both 18:0,22:6PC and 18:0,18:1PC. These findings demonstrate that membrane compositions can alter the signaling cascade by changing protein-protein interactions occurring predominantly in the hydrophilic region of the proteins, external to the lipid bilayer. These findings, if extended to other members of the superfamily of G protein-coupled receptors, suggest that a loss in efficiency of receptor-G protein binding is a contributing factor to the loss of cognitive skills, odor and spatial discrimination, and visual function associated with n-3 fatty acid deficiency.