INFLUENCE OF PROTEIN/LIPID INTERACTIONS ON SIGNAL TRANSD

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G protein-coupled receptors are ubiquitous components of signal transduction systems. This project is designed to assess the role of polyunsaturated phospholipids in modulating G protein- coupled signal transduction and to elucidate of the mechanism of action of ethanol in these systems. The visual transduction pathway in the rod photoreceptor, a prototypical G protein- coupled system, is being used as a model system. The effect lipid composition on: the kinetics and extent of formation of metarhodopsin II (MII), the G protein activating form of rhodopsin; MII/G protein complex formation; the rate of G protein activation; cGMP phosphodiesterase (PDE) activation; and the GTPase activity of the G protein are being studied. Along with the functional measures in the transduction pathway, acyl chain packing properties of the phospholipid bilayer are being determined by use of time-resolved fluorescence spectroscopy. The isolation and reconstitution of rhodopsin into bilayers of defined lipid composition allows an evaluation of the role of phospholipid acyl chain composition. In the current studies, rhodopsin, G protein, and PDE were reconstituted into phospholipid bilayers composed of 16:0, 18:1 PC or 16:0, 22:6 PC and compared to native disk membrane to which the G protein and PDE were reassociated. At low stimulus levels, twice as much activated rhodopsin was required to obtain equal levels of the light stimulated PDE activity bilayers composed of 16:0, 18:1 PC relative to those composed of 16:0, 22:6 PC. These results demonstrate that the enhanced formation of activated rhodopsin observed in polyunsaturated phospholipids is propagated down the transduction pathway, resulting in higher activity levels in the effector enzyme, PDE. Many of the neurotransmitter receptors are members of the superfamily of G protein-coupled receptors. Thus reductions of polyunsaturated acyl chains in the phospholipids in retinal and neuronal tissue can be expected to result in below optimal receptor function. Our observations in the visual pathway indicate that some part of the observed cognitive and visual deficits in omega-3 deficient animals arises from impaired signaling systems, resulting from the loss of 22:6n3 chains in membrane phospholipids.