FRINK Linked Data Fragments server

Matches in SemOpenAlex for { <https://semopenalex.org/work/W2008981610> ?p ?o ?g. }

• W2008981610 endingPage "42811" @default.
• W2008981610 startingPage "42807" @default.
• W2008981610 abstract "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 (Gt) binding was determined by monitoring MII-Gt complex formation spectrophotometrically. At 20 °C, in pH 7.5 isotonic buffer, the strongest MII-Gtbinding 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. 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 (Gt) binding was determined by monitoring MII-Gt complex formation spectrophotometrically. At 20 °C, in pH 7.5 isotonic buffer, the strongest MII-Gtbinding 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. docosahexanoic acid, or DHA docosapentaenoic acid, or DPA 1,2-didocosahexaenoyl-sn-glycero-3-phosphocholine 22:6PC, 1-stearoyl-2-docosahexaenoyl-sn-glycero-3-phosphocholine 18:1PC, 1-stearoyl-2-oleoyl-sn-glycero-3-phosphocholine transducin association constant for MII-Gt equilibrium constant for MI-MII in the absence of Gt observed equilibrium constant for MI-MII in the presence of Gt metarhodopsin I metarhodopsin II rod outer segment bleached rhodopsin phosphatidylserine phosphatidylethanolamine phosphatidylcholine The G protein-coupled motif is a fundamental mode of cell signaling, utilized in vision, taste, olfaction, and a variety of neurotransmitter systems. The receptors for these systems are integral membrane proteins, embedded in a lipid matrix. Neuronal and retinal tissues and the olfactory bulb contain high levels of then-3 polyunsaturated acyl chain derived from docosahexaenoic acid (22:6n-3)1 in their cell membrane phospholipids (1Stinson A.M. Wiegand R.D. Anderson R.E. Exp. Eye Res. 1991; 52: 213-218Crossref PubMed Scopus (94) Google Scholar, 2Salem N. Spiller G.A. Scala J. New Protective Roles for Selected Nutrients. Alan R. Liss Inc., New York1989: 109-228Google Scholar). Approximately 50% of the acyl chains in the phospholipids of the ROS disc membrane consist of 22:6n-3 (1Stinson A.M. Wiegand R.D. Anderson R.E. Exp. Eye Res. 1991; 52: 213-218Crossref PubMed Scopus (94) Google Scholar). The physiological significance of 22:6n-3 is demonstrated by the impaired visual response (3Birch E.E. Birch D.G. Hoffman D.R. Uauy R. Invest. Ophthalmol. Vis. Sci. 1992; 33: 3242-3253PubMed Google Scholar), learning deficits (2Salem N. Spiller G.A. Scala J. New Protective Roles for Selected Nutrients. Alan R. Liss Inc., New York1989: 109-228Google Scholar), loss of odor discrimination (4Greiner R.S. Moriguchi T. Hutton A. Slotnick B.M. Salem N. Lipids. 1999; 34 (suppl.): S239-S243Crossref PubMed Google Scholar), and reduced spatial learning (5Moriguchi T. Greiner R.S. Salem N. J. Neurochem. 2000; 75: 2563-2573Crossref PubMed Scopus (353) Google Scholar) associated with n-3 fatty acid deficiency. In all cases where acyl chain analysis was carried out, the 22:6n-3 content of membrane phospholipids was dramatically reduced in then-3-deficient animals where it was replaced by 22:5n-6 (5Moriguchi T. Greiner R.S. Salem N. J. Neurochem. 2000; 75: 2563-2573Crossref PubMed Scopus (353) Google Scholar). These findings suggest that the high levels of 22:6n-3 in membrane phospholipids play a critical role in various membrane-associated signaling pathways. A common thread in several of these processes is the ubiquitous motif of G protein-coupled signaling systems. However, molecular mechanisms linking 22:6n-3 phospholipids with essential physiological functions remain to be clarified. The study described herein aims to elucidate such mechanisms by investigating the effect of membrane lipid composition on G protein-coupled signal transduction.In G protein-coupled systems, the receptor activates an effector protein through the action of a G protein (6Helmreich E.J. Hofmann K.P. Biochim. Biophys. Acta. 1996; 1286: 285-322Crossref PubMed Scopus (127) Google Scholar). Receptors in this superfamily are integral membrane proteins made up of seven transmembrane helices and their respective connecting loops. In contrast, the G protein and effector proteins are generally peripheral proteins, bound to the membrane by a combination of an isoprenoid chain-lipid bilayer interactions (7Matsuda T. Takao T. Shimonishi Y. Murata M. Asano T. Yoshizawa T. Fukada Y. J. Biol. Chem. 1994; 269: 30358-30363Abstract Full Text PDF PubMed Google Scholar, 8Kisselev O.G. Ermolaeva M.V. Gautam N. J. Biol. Chem. 1994; 269: 21399-21402Abstract Full Text PDF PubMed Google Scholar) and electrostatic forces (9Seitz H.R. Heck M. Hofmann K.P. Alt T. Pellaud J. Seelig A. Biochemistry. 1999; 38: 7950-7960Crossref PubMed Scopus (47) Google Scholar). The receptor-binding site for the ligand is formed by the transmembrane helices and lies near the midpoint of the membrane; hence, the conformational changes accompanying receptor activation would be expected to have a dependence on the membrane lipid composition. In contrast, the interaction of the G protein with the receptor occurs primary external to the membrane bilayer (10Ernst O.P. Meyer C.K. Marin E.P. Henklein P. Fu W.Y. Sakmar T.P. Hofmann K.P. J. Biol. Chem. 2000; 275: 1937-1943Abstract Full Text Full Text PDF PubMed Scopus (148) Google Scholar, 11Farahbakhsh Z.T. Ridge K.D. Khorana H.G. Hubbell W.L. Biochemistry. 1995; 34: 8812-8819Crossref PubMed Scopus (185) Google Scholar). How the lipid composition might affect the interaction between receptor and G protein external to membrane bilayer is not clear.The visual transduction system is among the best characterized G protein-coupled signaling systems (12Litman B.J. Mitchell D.C. Lee A. Biomembranes. JAI Press, Greenwich, CT1996: 1-32Google Scholar) and is used as a model in these studies (13Hargrave P.A. McDowell J.H. FASEB J. 1992; 6: 2323-2331Crossref PubMed Scopus (232) Google Scholar, 14Sakmar T.P. Prog. Nucleic Acid Res. Mol. Biol. 1998; 59: 1-34Crossref PubMed Scopus (136) Google Scholar). Light absorption results in the generation of a rapid equilibrium between MI and MII (15Mathews R. Hubbard R. Brown P. Wald G. J. Gen. Physiol. 1963; 47: 215-222Crossref PubMed Scopus (428) Google Scholar), and the active conformation, MII, readily associates with Gt, forming the MII-Gtcomplex, which is relatively stable in the absence of GTP (16Hofmann K.P. Biochim. Biophys. Acta. 1985; 810: 278-281Crossref PubMed Scopus (35) Google Scholar). The interaction sites on MII involved in binding Gt are composed of three cytoplasmic loops formed by the peptide sequence connecting helices III and IV, V and VI, and a putative loop formed by amino acids 310–321, anchored in the bilayer by palmitate groups esterified to Cys-322 and Cys-323 (17Hamm H.E. J. Biol. Chem. 1998; 273: 669-672Abstract Full Text Full Text PDF PubMed Scopus (930) Google Scholar, 18Konig B. Arendt A. McDowell J.H. Kahlert M. Hargrave P.A. Hofmann K.P. Proc. Natl. Acad. Sci. U. S. A. 1989; 86: 6878-6882Crossref PubMed Scopus (337) Google Scholar, 19Franke R.R. Konig B. Sakmar T.P. Khorana H.G. Hofmann K.P. Science. 1990; 250: 123-125Crossref PubMed Scopus (302) Google Scholar). Recent structural studies of these loops indicate a level of secondary structure in the form of α-helices (20Palczewski K. Kumasaka T. Hori T. Behnke C.A. Motoshima H. Fox B.A. Le T., I Teller D.C. Okada T. Stenkamp R.E. Yamamoto M. Miyano M. Science. 2000; 289: 739-745Crossref PubMed Scopus (4991) Google Scholar). Gt is a trimeric protein consisting of Gα, Gβ, and Gγ subunits (17Hamm H.E. J. Biol. Chem. 1998; 273: 669-672Abstract Full Text Full Text PDF PubMed Scopus (930) Google Scholar). Studies using various peptides representing the putative binding region of Gt implicate two regions in the carboxyl-terminal region of Gα and a segment of Gγ as the interaction sites with MII (18Konig B. Arendt A. McDowell J.H. Kahlert M. Hargrave P.A. Hofmann K.P. Proc. Natl. Acad. Sci. U. S. A. 1989; 86: 6878-6882Crossref PubMed Scopus (337) Google Scholar, 19Franke R.R. Konig B. Sakmar T.P. Khorana H.G. Hofmann K.P. Science. 1990; 250: 123-125Crossref PubMed Scopus (302) Google Scholar, 21Franke R.R. Sakmar T.P. Graham R.M. Khorana H.G. J. Biol. Chem. 1992; 267: 14767-14774Abstract Full Text PDF PubMed Google Scholar). The interactions appear to be mainly hydrophilic in nature, because the interaction sites are located external to the membrane bilayer.In this study, rhodopsin was reconstituted into large, unilamellar vesicles containing either di22:6PC, 18:0,22:6PC, or 18:0,18:1PC with and/or without 30 mol% cholesterol and the association constants of MII-Gt formation in these lipids were determined. Our results show that acyl chain unsaturation and cholesterol in the membrane significantly alter the MII-Gt coupling. Because the visual signaling system is the prototype member in the superfamily of G-protein coupled signaling systems, our findings of the effect of lipid composition and cholesterol on receptor-G protein coupling should serve as a general demonstration of the modulation of cell signaling efficiency by membrane composition.RESULTSThe MI-MII equilibrium and the association of MII with Gt can be readily monitored through changes in the absorption spectra of these photointermediates. Examples of the effect of two different lipid compositions on the MI-MII equilibrium and MII-Gt complex formation are shown in Fig.1. The spectra for rhodopsin reconstituted in vesicles consisting of a highly unsaturated 18:0,22:6PC are shown in Fig. 1 A, whereas those in a monounsaturated 18:0,18:1PC mixed with 30 mol% cholesterol are shown in Fig. 1 B. In the absence of Gt, the spectra in Fig. 1 A contained two absorption bands centered about 385 and 480 nm, associated with the MII and MI photointermediates, respectively. In Fig. 1 A (open circles), the MI and MII peaks are approximately equal, whereas the MII peak was greatly reduced accompanied by a large increase in the MI peak in Fig. 1 B (open circles). The presence of Gt caused an enhancement of the MII peaks in both bilayer systems (Fig. 1,A and B, filled circles). This is the result of MII-Gt complex formation and the fact that formation of this complex does not alter the spectral properties of MII.The spectral contribution of the bands with absorption peaks centered at 480 nm and 385 nm in Fig. 1 (A and B) were deconvolved into contributions as a result of MI and MII in the absence of Gt and MI and (MII + MII-Gt) in the presence of Gt, as shown by dashed and solid curves, respectively. It is clear that the amount of MII or (MII + MII-Gt) formed was greater in 18:0,22:6PC relative to 18:0,18:1PC with 30 mol% cholesterol, demonstrating the role of lipid composition in modulating the formation of MII and MII-Gt. The calculated values of K eq−G and K eq+G in 18:0,22:6PC (Fig.1 A) were 1.01 ± 0.02 and 2.22 ± 0.14, respectively, whereas K eq−Gand K eq+G in 18:0,18:1PC with 30 mol% cholesterol (Fig. 1 B) were 0.19 ± 0.04 and 0.35 ± 0.04, respectively.Both K eq−G and K eq+G varied by more than a factor of 5 over the range of bilayer compositions examined in this study, as shown in Fig. 2. In the absence of Gt, K eq−Gfollowed the order of di22:6PC > 18:0,22:6PC > 18:0,22:6PC + 30 mol% cholesterol ≈ 18:0,18:1PC > 18:0,18:1PC + 30 mol% cholesterol. This is consistent with previous findings (25Litman B.J. Mitchell D.C. Lipids. 1996; 31 (suppl.): S193-S197Crossref PubMed Google Scholar, 26Brown M.F. Chem. Phys. Lipids. 1994; 73: 159-180Crossref PubMed Scopus (368) Google Scholar, 27O'Brien D.F. Costa L.F. Ott R.A. Biochemistry. 1977; 16: 1295-1303Crossref PubMed Scopus (121) Google Scholar, 28Mitchell D.C. Straume M. Miller J.L. Litman B.J. Biochemistry. 1990; 29: 9143-9149Crossref PubMed Scopus (151) Google Scholar, 29Mitchell D.C. Straume M. Litman B.J. Biochemistry. 1992; 31: 662-670Crossref PubMed Scopus (145) Google Scholar) that showed that the reduced acyl chain unsaturation and the presence of cholesterol reduce the equilibrium concentration of MII. The presence of Gt increased the apparent amount of MII formed in all samples, as indicated by the values of K eq+G. This results from the formation of the MII-Gt complex. The trend for K eq+G with bilayer composition followed that of K eq−G.Figure 2Effect of lipid composition onK eq−G (open bars) andK eq+G (diagonal bars). These values were derived from equilibrium measurements in the absence and presence of 1.0 μm Gt as shown in Fig. 1 at 20 °C, pH 7.5. K eq−G is defined as the ratio of [MII]/[MI], whereas K eq+G is defined as the ratio of ([MII]+[MII-Gt])/[MI].View Large Image Figure ViewerDownload (PPT)Values of [Gt]free, [MII-Gt], and [MII] were calculated from K eq−G and K eq+G according to EquationsEq. 3, Eq. 4, Eq. 5. A series of measurements made with increasing ratios of Gt to MII was used to produce the binding profiles of MII to Gt. Example plots of [MII-Gt]/[MII]total versus[Gt]free in two lipid compositions are shown in Fig. 3. Increased concentrations of Gt resulted in an increase amount of MII-Gtcomplex formation. However, the slopes in the binding plots were rather different, reflecting dissimilar binding constants in the two lipid bilayers. Analysis of the data according to Equation 2 gaveK a of 1.5 × 107m−1 for 18:0,22:6PC vesicles andK a of 2.5 × 106m−1 for 18:0,18:1PC vesicles containing 30 mol% cholesterol.Figure 3Lipid dependence of MII-Gtbinding. The smooth lines are best fits of Equation 2 to the data.A and B, 18:0,22:6PC (A) and 18:0,18:1PC (B) with 30 mol% cholesterol at 20 °C, pH 7.5.View Large Image Figure ViewerDownload (PPT)Both acyl chain unsaturation and cholesterol content modulated the binding of MII to Gt as shown in Fig.4. Two key observations may be drawn from the values of K a. 1) The increase in acyl chain unsaturation going from 18:0,18:1PC to 18:0,22:6PC resulted in a 3-fold enhancement in K a, whereas further increase in unsaturation going to di22:6PC resulted in a slight reduction inK a relative to 18:0,22:6PC. 2) Cholesterol reducedK a in both monounsaturated 18:0,18:1PC and highly unsaturated 18:0,22:6PC. The K a values in 18:0,18:1PC + 30 mol% cholesterol and 18:0,22:6PC + 30 mol% cholesterol are 2.5 × 106m−1 and 4.4 × 106m−1, respectively, showing a somewhat smaller effect in the polyunsaturated bilayer.Figure 4Effect of lipid composition onK a of MII-Gt. The values are derived from data similar to that shown in Fig. 3 at 20 °C, pH 7.5.View Large Image Figure ViewerDownload (PPT)DISCUSSIONPrevious studies have demonstrated that the formation of the active conformation of the G protein-coupled receptor rhodopsin, MII, is dependent on the membrane lipid composition (23;25–29), consistent with the present results regarding the lipid dependence of K eq−G. A primary finding of this study is that MII-Gt complex formation, the initial amplification step in the visual cascade, is also modulated by the phospholipid acyl chain and cholesterol composition of the membrane. Increased acyl chain unsaturation and decreased level of cholesterol resulted in a higher affinity of MII to Gt. One characteristic of the native disc membrane is that ∼50% of the total acyl chains are made of 22:6n-3, which is similar to that in 18:0,22:6PC. The reported K a in native disc is on the order of 107m−1 (30Kuhn H. Bennett N. Michel-Villaz M. Chabre M. Proc. Natl. Acad. Sci. U. S. A. 1981; 78: 6873-6877Crossref PubMed Scopus (184) Google Scholar, 31Parkes J.H. Gibson S.K. Liebman P.A. Biochemistry. 1999; 38: 6862-6878Crossref PubMed Scopus (30) Google Scholar), whereas K a in 18:0,22:6PC reconstituted vesicles is 1.5 × 107m−1. The agreement between these values, despite the differences in phospholipid headgroup composition, indicates the important role of 22:6n-3 in modulating the coupling of rhodopsin to Gt.Increased phospholipid acyl chain unsaturation was shown to increase the formation of MII (27O'Brien D.F. Costa L.F. Ott R.A. Biochemistry. 1977; 16: 1295-1303Crossref PubMed Scopus (121) Google Scholar, 29Mitchell D.C. Straume M. Litman B.J. Biochemistry. 1992; 31: 662-670Crossref PubMed Scopus (145) Google Scholar), whereas increased cholesterol concentration decreases MII (28Mitchell D.C. Straume M. Miller J.L. Litman B.J. Biochemistry. 1990; 29: 9143-9149Crossref PubMed Scopus (151) Google Scholar). These findings have been linked to the specific packing properties of polyunsaturated acyl chains and the effect of cholesterol on these packing properties (29Mitchell D.C. Straume M. Litman B.J. Biochemistry. 1992; 31: 662-670Crossref PubMed Scopus (145) Google Scholar). Current evidence indicates that MII-Gt interactions involve the three hydrophilic loops on the cytoplasmic surface of rhodopsin with regions in the carboxyl-terminal region of Gt, placing the interaction surfaces external to the bilayer. The dependence of the extent of MII-Gt complex formation on the phospholipid acyl chain composition demonstrates that membrane lipid composition can not only play a role in modulating the level of MII formation, but it has a marked effect on the coupling of an integral membrane protein receptor to a peripherally bound G protein. Hence, the acyl chain packing in the hydrophobic region of the bilayer can affect interactions thought to occur primarily in the hydrophilic region of integral and peripheral membrane proteins.Our results demonstrate that acyl chain composition and cholesterol content modulate the coupling step of Gt to MII. To understand how lipid composition may modulate MII-Gtinteractions, it is necessary to consider the molecular events associated with MII-Gt binding. The formation of the MII-Gt complex involves a diffusional search of MII and Gt for each other on the membrane surface and subsequent productive collisions leading to binding. Varying phospholipid acyl chain composition and cholesterol content can alter membrane properties in a number of ways. 1) Acyl chain packing properties can affect the rotation and diffusion of rhodopsin in the membrane. 2) The lateral diffusion and association of Gt on the membrane can be changed. Gt is associated with the membrane through an isoprenoid linkage. Acyl chain packing may affect the orientation of Gt in the bilayer making MII-Gt collisions less productive in terms of complex formation. 3) Increased acyl chain saturation inhibits the formation of MII because the outward movement of helices during MII formation may be hindered in a more rigid lipid environment resulting in reduced MII-Gt complex formation. In addition, the sensitivity to acyl chain and cholesterol content may indicate a greater role of protein-protein interactions within the hydrophobic portion of the membrane than were considered previously. In a separate study, the effect of lipid composition on the kinetics of MII and MII-Gt formation was studied using flash photolysis (32Mitchell D.C. Niu S. Litman B.J. J. Biol. Chem. 2001; 276: 42801-42806Abstract Full Text Full Text PDF PubMed Scopus (107) Google Scholar). We found that the kinetics of MII formation, which is a unimolecular reaction, exhibited relatively mild dependence on bilayer composition, whereas the kinetics of MII-Gt formation was greatly diminished by the presence of cholesterol and more saturated lipids. These findings support the role of lipid composition in modulating the diffusional coupling of MII to Gt on membrane surface.Visual signaling is initiated from rhodopsin and propagated along the visual cascade through a series of coupled steps. In this study we have demonstrated that the initial steps, which are rhodopsin activation and MII-Gt coupling, are modulated by lipid composition and cholesterol. The net effect of bilayer composition on visual transduction can be evaluated in terms of the yield of MII-Gt complex formation per bleached rhodopsin, [MII-Gt]/[Rh*]. The following equation was used for such calculation. [MII−Gt][Rh*]=m−m2−4·Ka2·Rh*·[Gt]2·Ka·[Rh*],Eq. 6 where [Gt] is the total concentration of Gt, and m=(1+1/Keq−G)+Ka·[Gt]+[Rh*]·Ka.Eq. 7 Equation 6 is derived from the following equations. Keq−G=[MII]/[MI]Eq. 8 Ka=[MII­Gt]/([MII]·([Gt]−[MII­Gt]))Eq. 9 [Rh*]=[MI]+[MII]+[MII­Gt]Eq. 10 Under physiological conditions, the ratio of rhodopsin to Gt is ∼10 to 1 in ROS membranes and in the range of 1 of 100,000 rhodopsins absorbs a photon, giving rise to a visual response. The effect of bilayer composition on [MII-Gt]/[Rh*] is clearly demonstrated in Fig. 5. Although ∼90% of bleached rhodopsin formed complex with Gt in 18:0,22:6PC and di22:6PC vesicles, only 60% of such complex is formed in 18:0,18:1PC vesicles. The presence of 30 mol% cholesterol in 18:0,22:6PC and 18:0,18:1PC resulted in ∼60 and 30% complex formation, respectively. It was recently reported that ∼10% of rhodopsin in the ROS disc membrane is contained in a detergent-resistant membrane fraction or lipid raft (33Seno K. Kishimoto M. Abe M. Higuchi Y. Mieda M. Owada Y. Yoshiyama W. Liu H. Hayashi F. J. Biol. Chem. 2001; 276: 20813-20816Abstract Full Text Full Text PDF PubMed Scopus (98) Google Scholar). If this raftlike phase exists under physiological conditions, and like other lipid rafts, it is rich in cholesterol and saturated acyl chains, the present results suggest that MII-Gt binding strength and kinetics would be reduced for this population of rhodopsin. In addition to phosphatidylcholine, the native disc membrane also contains ∼10% phosphatidylserine (PS) and ∼42% phosphatidylethanolamine (PE) (1Stinson A.M. Wiegand R.D. Anderson R.E. Exp. Eye Res. 1991; 52: 213-218Crossref PubMed Scopus (94) Google Scholar). PS will add a negative surface charge to the membrane, whereas PE will contribute an increased level of acyl chain packing order because of its higher melting point relative to PC. Our current data would suggest that the presence of PE would be somewhat inhibitory relative to MII-Gt complex formation. The effect of PS is yet to be determined.Figure 5Calculated yield of MII-Gt formed relative to the number of rhodopsin molecules that absorbed a photon at physiological light levels. [MII-Gt]/[Rh*] was calculated according to Equation 6 using experimentally determined values of K eq−G andK a. The concentrations of rhodopsin and Gt were set at 10 and 1 μm, respectively. The physiological bleach level for rhodopsin was assumed to be 1 of 100,000.View Large Image Figure ViewerDownload (PPT)A model, relating the biochemical events in the visual transduction pathway with the neural response, as measured by the electroretinograms, was recently published (34Leskov I.B. Klenchin V.A. Handy J.W. Whitlock G.G. Govardovskii V.I. Bownds M.D. Lamb T.D. Pugh E.N. Arshavsky V.Y. Neuron. 2000; 27: 525-537Abstract Full Text Full Text PDF PubMed Scopus (154) Google Scholar). In this model, the response at any time after a light stimulus is directly proportional to the concentration of activated rhodopsin molecules, i.e.[MII]. Here, we have determined that, at very low light levels, the fraction of MII-Gt complex to bleached rhodopsin, [MII-Gt/Rh*] depends on the lipid composition of the membrane, Fig. 5. If [MII-Gt/Rh*] is a measure of the fraction of bleached rhodopsin that can participate in activating Gt, then the factor in the equation for the response time at physiological light exposures needs to be corrected for variation in membrane lipid composition. Both the 22:6n-3-containing PCs examined in this study support nearly full participation of the bleached rhodopsin in Gt activation, Fig. 5. In contrast, the 18:1n-9- containing PC supports only 60% participation of the bleached rhodopsin in Gt activation, whereas the addition of 30 mol% of cholesterol reduces this to about 30%. Thus, the response time in the 22:6n-3-containing PC's would be expected to be faster relative to that observed in the 18:1n-9-containing PC by a factor of 1.67. In the case ofn-3 deficiency, 22:6n-3 is replaced by a lower level polyunsaturated, 22:5n-6, and a lag time is observed in the leading edge of the a-wave in the electroretinograms (35Neuringer M. Connor W.E. Van Petten C. Barstad L. J. Clin. Invest. 1984; 73: 272-276Crossref PubMed Scopus (504) Google Scholar). Decreased MII participation in MII-Gt complex formation would also contribute to lower signal amplitude, because fewer Gt proteins would be activated. Although, it is not anticipated that the difference between 22:6n-3 and 22:5n-6 will produce as great a lag time as is indicated for 18:1n-9, the observed lag time and reduced signal amplitude in n-3 deficiency relative to n-3-sufficient subjects is consistent with the dependence of the level of MII-Gt complex formation on the membrane lipid composition.The visual cascade, initiated by the light activation of rhodopsin, involves a series of protein-coupled reactions resulting in an amplified response. The first step in signal amplification in the visual pathway is the formation of the MII-Gt complex. The modulatory effect of bilayer acyl chain composition and cholesterol content on both the kinetics and extent of formation of the MII-Gt complex observed in this and the previous (32Mitchell D.C. Niu S. Litman B.J. J. Biol. Chem. 2001; 276: 42801-42806Abstract Full Text Full Text PDF PubMed Scopus (107) Google Scholar) study will have direct impact on the downstream steps of the visual cascade. Weakened MII-Gt interactions will results in reduced amplification and slower kinetics at the Gt activation step, which will propagate down the pathway to produce reduced activity of the effector protein, cGMP phosphodiesterase. These effects may well provide the molecular basis for the diminished amplitude and sensitivity (36Weisinger H.S. Vingrys A.J. Sinclair A.J. Lipids. 1996; 31: 65-70Crossref PubMed Scopus (96) Google Scholar, 37Weisinger H.S. Vingrys A.J. Bui B.V. Sinclair A.J. Invest. Ophthalmol. Vis. Sci. 1999; 40: 327-338PubMed Google Scholar) and lag time in the electroretinogram a-wave (35Neuringer M. Connor W.E. Van Petten C. Barstad L. J. Clin. Invest. 1984; 73: 272-276Crossref PubMed Scopus (504) Google Scholar) and the reduced visual acuity (38SanGiovanni J.P. Parra-Cabrera S. Colditz G.A. Berkey C.S. Dwyer J.T. Pediatrics. 2000; 105: 1292-1298Crossref PubMed Scopus (213) Google Scholar) associated with 22:6n-3 deficiency. Because of the similar signaling motif in other G protein-coupled signaling systems, the findings in this study should be generally applicable to other members in the G protein-coupled family, providing a molecular mechanism for the observed loss in cognitive skills (2Salem N. Spiller G.A. Scala J. New Protective Roles for Selected Nutrients. Alan R. Liss Inc., New York1989: 109-228Google Scholar), odor (4Greiner R.S. Moriguchi T. Hutton A. Slotnick B.M. Salem N. Lipids. 1999; 34 (suppl.): S239-S243Crossref PubMed Google Scholar), and spatial discrimination (5Moriguchi T. Greiner R.S. Salem N. J. Neurochem. 2000; 75: 2563-2573Crossref PubMed Scopus (353) Google Scholar) observed inn-3 fatty acid deficiency. The G protein-coupled motif is a fundamental mode of cell signaling, utilized in vision, taste, olfaction, and a variety of neurotransmitter systems. The receptors for these systems are integral membrane proteins, embedded in a lipid matrix. Neuronal and retinal tissues and the olfactory bulb contain high levels of then-" @default.
• W2008981610 created "2016-06-24" @default.
• W2008981610 creator A5018783217 @default.
• W2008981610 creator A5032181672 @default.
• W2008981610 creator A5082333968 @default.
• W2008981610 date "2001-11-01" @default.
• W2008981610 modified "2023-10-16" @default.
• W2008981610 title "Optimization of Receptor-G Protein Coupling by Bilayer Lipid Composition II" @default.
• W2008981610 cites W1482237901 @default.
• W2008981610 cites W1511498323 @default.
• W2008981610 cites W1566497779 @default.
• W2008981610 cites W1590595627 @default.
• W2008981610 cites W1890185838 @default.
• W2008981610 cites W1970384111 @default.
• W2008981610 cites W1976351696 @default.
• W2008981610 cites W1977297527 @default.
• W2008981610 cites W1980823970 @default.
• W2008981610 cites W1993343824 @default.
• W2008981610 cites W2006296579 @default.
• W2008981610 cites W2009259360 @default.
• W2008981610 cites W2020792265 @default.
• W2008981610 cites W2021414145 @default.
• W2008981610 cites W2037435604 @default.
• W2008981610 cites W2043427265 @default.
• W2008981610 cites W2044835337 @default.
• W2008981610 cites W2059751684 @default.
• W2008981610 cites W2060622317 @default.
• W2008981610 cites W2064284529 @default.
• W2008981610 cites W2064554902 @default.
• W2008981610 cites W2066109743 @default.
• W2008981610 cites W2067382519 @default.
• W2008981610 cites W2072550433 @default.
• W2008981610 cites W2090241067 @default.
• W2008981610 cites W2090912192 @default.
• W2008981610 cites W2104782686 @default.
• W2008981610 cites W2113183595 @default.
• W2008981610 cites W2114859465 @default.
• W2008981610 cites W2131019594 @default.
• W2008981610 cites W2149963584 @default.
• W2008981610 cites W2172278741 @default.
• W2008981610 cites W4251074911 @default.
• W2008981610 doi "https://doi.org/10.1074/jbc.m105778200" @default.
• W2008981610 hasPubMedId "https://pubmed.ncbi.nlm.nih.gov/11544259" @default.
• W2008981610 hasPublicationYear "2001" @default.
• W2008981610 type Work @default.
• W2008981610 sameAs 2008981610 @default.
• W2008981610 citedByCount "103" @default.
• W2008981610 countsByYear W20089816102012 @default.
• W2008981610 countsByYear W20089816102013 @default.
• W2008981610 countsByYear W20089816102014 @default.
• W2008981610 countsByYear W20089816102015 @default.
• W2008981610 countsByYear W20089816102016 @default.
• W2008981610 countsByYear W20089816102017 @default.
• W2008981610 countsByYear W20089816102018 @default.
• W2008981610 countsByYear W20089816102019 @default.
• W2008981610 countsByYear W20089816102020 @default.
• W2008981610 countsByYear W20089816102021 @default.
• W2008981610 countsByYear W20089816102022 @default.
• W2008981610 countsByYear W20089816102023 @default.
• W2008981610 crossrefType "journal-article" @default.
• W2008981610 hasAuthorship W2008981610A5018783217 @default.
• W2008981610 hasAuthorship W2008981610A5032181672 @default.
• W2008981610 hasAuthorship W2008981610A5082333968 @default.
• W2008981610 hasBestOaLocation W20089816101 @default.
• W2008981610 hasConcept C12554922 @default.
• W2008981610 hasConcept C131584629 @default.
• W2008981610 hasConcept C138885662 @default.
• W2008981610 hasConcept C185592680 @default.
• W2008981610 hasConcept C191897082 @default.
• W2008981610 hasConcept C192157962 @default.
• W2008981610 hasConcept C192562407 @default.
• W2008981610 hasConcept C39944091 @default.
• W2008981610 hasConcept C40231798 @default.
• W2008981610 hasConcept C41625074 @default.
• W2008981610 hasConcept C41895202 @default.
• W2008981610 hasConcept C55493867 @default.
• W2008981610 hasConcept C86803240 @default.
• W2008981610 hasConceptScore W2008981610C12554922 @default.
• W2008981610 hasConceptScore W2008981610C131584629 @default.
• W2008981610 hasConceptScore W2008981610C138885662 @default.
• W2008981610 hasConceptScore W2008981610C185592680 @default.
• W2008981610 hasConceptScore W2008981610C191897082 @default.
• W2008981610 hasConceptScore W2008981610C192157962 @default.
• W2008981610 hasConceptScore W2008981610C192562407 @default.
• W2008981610 hasConceptScore W2008981610C39944091 @default.
• W2008981610 hasConceptScore W2008981610C40231798 @default.
• W2008981610 hasConceptScore W2008981610C41625074 @default.
• W2008981610 hasConceptScore W2008981610C41895202 @default.
• W2008981610 hasConceptScore W2008981610C55493867 @default.
• W2008981610 hasConceptScore W2008981610C86803240 @default.
• W2008981610 hasIssue "46" @default.
• W2008981610 hasLocation W20089816101 @default.
• W2008981610 hasOpenAccess W2008981610 @default.
• W2008981610 hasPrimaryLocation W20089816101 @default.
• W2008981610 hasRelatedWork W1963901611 @default.
• W2008981610 hasRelatedWork W2031051143 @default.
• W2008981610 hasRelatedWork W2059795428 @default.
• W2008981610 hasRelatedWork W2061481432 @default.

• next