Primary Alcohols Modulate the Activation of the G Protein-coupled Receptor Rhodopsin by a Lipid-mediated Mechanism

Abstract

The visual pigment rhodopsin is a prototypical member of the G protein-coupled receptor superfamily. In this study, we have investigated the effect of a series of n-alcohols on the formation of metarhodopsin II (MII), the photoactivated conformation of rhodopsin, which binds and activates transducin. When rhodopsin was photolyzed in the presence of several n-alcohols, increased MII formation was observed in the order ethanol > butanol > hexanol, whereas longer chain n-alcohols inhibited MII formation with decanol > octanol. The magnitude of the stimulatory effects was greater in a more highly unsaturated phospholipid. Alcohols, which enhanced MII formation also increased phospholipid acyl chain packing free volume, while those that decreased this bilayer property inhibited MII formation. An apparent discontinuity in the effect of these alcohols results when their potency is calculated in terms of the total aqueous alcohol concentration. In sharp contrast, a continuous variation in their behavior is observed, when their potency is calculated in terms of the amount of alcohol partitioned in the membrane. Our findings strongly support a lipid-mediated mechanism of action for alcohols on rhodopsin and, by analogy, for other G protein-coupled receptors.The mechanism of action of alcohols and general anesthetics is generally discussed in terms of two opposing mechanisms. The first involves an alteration of phospholipid bilayer properties by these agents, resulting in a modulation of membrane protein function, while the second is based on the direct interaction with membrane proteins (1, 2). The lipid mechanism was based originally on the observation (3, 4) that anesthetic potency correlated directly with the solubility of an anesthetic in olive oil. Additional support for the lipid mechanism came from the observation that acute exposure of membranes to ethanol resulted in a disordering of the phospholipid acyl chain packing (5). Subsequent experiments demonstrating that the activity of a soluble enzyme, firefly luciferase, could be inhibited by a diverse group of alcohols and general anesthetics (6) have focused attention on the protein binding hypothesis.Many of the recent studies aimed at elucidating the mechanism of action of alcohols and general anesthetics have dealt with the effect of these agents on ligand-gated channels, whose ligand binding site is in a portion of the protein external to the bilayer (1, 7-10). The superfamily of G protein-coupled receptors has received relatively little study. In these receptors, the ligand binding sites are formed by their transmembrane helical segments and lie at the median point of the bilayer. One of the best characterized members of this superfamily is rhodopsin, which triggers the visual transduction pathway in rod cells. A metastable equilibrium between MI 1 and MII (whereis established within milliseconds of photon absorption (11). MII binds and activates the visual G protein, transducin (12, 13). MII formation increases with higher levels of phospholipid acyl chain u...

Examining the Role of Membrane Lipid Composition in Determining the Ethanol Tolerance of Saccharomyces cerevisiae

“…Some investigators have concluded that it is the interaction of ethanol with the lipid membrane that induces structural changes in membrane-associated proteins (33), while others contend that it is the interaction of ethanol with the membrane proteins that is responsible for its effect (the interested reader should consult Mihic et al [34]). Investigations of the interaction of ethanol with membrane bilayers utilizing nuclear magnetic resonance (NMR) spectroscopy are in agreement with the former (35).…”

Light-induced Reorganization of Phospholipids in Rod Disc Membranes

“…Understanding of the role of membrane properties in signal transduction is, at the physicochemical level, still incomplete. However, with regard to photoreceptor function, there are two identified determinants, namely, the formation of the active intermediate and the transport of the hydrophobic retinal ligand.Rhodopsin's active Meta II conformation depends on the composition (10) and the physical properties of the lipid bilayer (11,12). Its formation goes along with a number of physicochemical alterations, including a considerable reaction volume, and changes of the interfacial potential, positive relative to the aqueous exterior of the disc, within a boundary region of the native isolated disc membrane near the cytoplasmic surface.…”

“…Rhodopsin's active Meta II conformation depends on the composition (10) and the physical properties of the lipid bilayer (11,12). Its formation goes along with a number of physicochemical alterations, including a considerable reaction volume, and changes of the interfacial potential, positive relative to the aqueous exterior of the disc, within a boundary region of the native isolated disc membrane near the cytoplasmic surface.…”

Activity of phosphatidylinositol transfer protein is sensitive to ethanol and membrane curvature

“…This has led some to question whether any of the functional effects of alcohols and anaesthetics are mediated by membrane lipids [47]. Persuasive evidence for lipid-mediated actions of ethanol has been presented but is limited [48][49][50]. The sensitivity to ethanol described here for PITP rivals or exceeds that of any previously described lipidmediated action of ethanol.…”