Interconversion of metarhodopsins I and II: a branched photointermediate decay model

Straume, Martin; Mitchell, Drake C.; Miller, James L.; Litman, Burton J.

doi:10.1021/bi00491a006

Cited by 114 publications

(100 citation statements)

References 27 publications

Supporting

Mentioning

Contrasting

Unclassified

Order By: Relevance

“…The meta II intermediate is favored at low pH and in lipids with unsaturated acyl chains (20). We compared the ability to convert rhodopsin to meta II by using native rod outer-segment membranes, rhodopsin reconstituted into unsaturated lipids, and rhodopsin solubilized in DM detergent micelles (data not shown).…”

Section: Resultsmentioning

confidence: 99%

Coupling of retinal isomerization to the activation of rhodopsin

Patel

Crocker²,

Eilers³

et al. 2004

Proc. Natl. Acad. Sci. U.S.A.

139

186

View full text Add to dashboard Cite

Activation of the visual pigment rhodopsin is caused by 11-cis to -trans isomerization of its retinal chromophore. High-resolution solid-state NMR measurements on both rhodopsin and the metarhodopsin II intermediate show how retinal isomerization disrupts helix interactions that lock the receptor off in the dark. We made 2D dipolar-assisted rotational resonance NMR measurements between 13 C-labels on the retinal chromophore and specific 13 C-labels on tyrosine, glycine, serine, and threonine in the retinal binding site of rhodopsin. The essential aspects of the isomerization trajectory are a large rotation of the C20 methyl group toward extracellular loop 2 and a 4-to 5-Å translation of the retinal chromophore toward transmembrane helix 5. The retinal-protein contacts observed in the active metarhodopsin II intermediate suggest a general activation mechanism for class A G proteincoupled receptors involving coupled motion of transmembrane helices 5, 6, and 7.G protein-coupled receptors (GPCRs) are a large superfamily of membrane receptors that have seven transmembrane (TM) helices and respond to a wide array of signaling ligands. Similarities in the sequences of these receptors have led to the idea that they share a common activation mechanism, whereas sequence diversity is correlated with the specificity of different receptors for different ligands and G proteins. With the exception of the visual pigment rhodopsin (1, 2), the structures of GPCRs are unknown. The rhodopsin crystal structure shows that the TM helices are locked in an inactive conformation by interhelical interactions involving conserved amino acids. The general model of GPCR activation that has emerged over the past few years is that ligand-binding, or retinal isomerization in the case of the visual pigments, disrupts these interactions and drives a rigid body movement of one or more of the TM helices (3, 4).Helix-helix interactions in rhodopsin are mediated by two sets of conserved amino acids. The highly conserved signature amino acids have sequence identities of Ͼ80% across the family of class A GPCRs. These amino acids are generally highly polar, aromatics, or proline. The group-conserved amino acids are small and͞or weakly polar (Gly, Ala, Ser, Thr, and Cys) (5). They have low individual sequence identities but are highly conserved (Ͼ80%) when considered as a group. These amino acids are involved in mediating close helix contacts and facilitating interhelical hydrogen bonding (6).The location of the group-conserved amino acids largely in the interfaces of TM helices H1-H4 of rhodopsin has suggested that these helices are locked in a stable structure that does not change significantly upon receptor activation (5). There are fewer group-conserved amino acids in H5-H7. H5 and H7 are unusual in containing conserved prolines that facilitate interactions with the H1-H4 core by exposing the i-4 backbone carbonyl for interhelical hydrogen bonding. H6 is unusual in that the TM region is composed of conserved aromatic and large hydrophobic amino acids ...

show abstract

Section: Resultsmentioning

confidence: 99%

Coupling of retinal isomerization to the activation of rhodopsin

Patel

Crocker²,

Eilers³

et al. 2004

Proc. Natl. Acad. Sci. U.S.A.

139

186

View full text Add to dashboard Cite

show abstract

“…The PDE activity is obtained from the shift in pH and is expressed as cGMP hydrolyzed (mM)/s. Rhodopsin activation was characterized by the MI-MII equilibrium constant, K eq , using a spectroscopic method (19,20).…”

Section: Methodsmentioning

confidence: 99%

“…The detailed kinetics of MII formation were extracted from the changes in absorbance observed at 380 nm in the absence of G t via analysis in terms of the microscopic rate constants of a branched photoreaction model (19). The kinetics of MII-G t formation were determined by analyzing absorbance changes at 380 nm obtained in the presence of G t in terms of the branched model plus a reaction between MII and G t to form the MII-G t complex.…”

Section: Methodsmentioning

confidence: 99%

Reduced G Protein-coupled Signaling Efficiency in Retinal Rod Outer Segments in Response to n-3 Fatty Acid Deficiency

Niu

Mitchell

Lim

et al. 2004

Journal of Biological Chemistry

Self Cite

210

151

View full text Add to dashboard Cite

The fatty acid (FA) docosahexaenoic acid (DHA, 22: 6n-3) is highly enriched in membrane phospholipids of the central nervous system and retina. Loss of DHA because of n-3 FA deficiency leads to suboptimal function in learning, memory, olfactory-based discrimination, spatial learning, and visual acuity. G protein-coupled receptor (GPCR) signal transduction is a common signaling motif in these neuronal pathways. Here we investigated the effect of n-3 FA deficiency on GPCR signaling in retinal rod outer segment (ROS) membranes isolated from rats raised on n-3-adequate or -deficient diets. ROS membranes of second generation n-3 FA-deficient rats had ϳ80% less DHA than n-3-adequate rats. DHA was replaced by docosapentaenoic acid (22:5n-6), an n-6 FA. This replacement correlated with desensitization of visual signaling in n-3 FA-deficient ROS, as evidenced by reduced rhodopsin activation, rhodopsintransducin (G t ) coupling, cGMP phosphodiesterase activity, and slower formation of metarhodopsin II (MII) and the MII-G t complex relative to n-3 FA-adequate ROS. ROS membranes from n-3 FA-deficient rats exhibited a higher degree of phospholipid acyl chain order relative to n-3 FA-adequate rats. These findings reported here provide an explanation for the reduced amplitude and delayed response of the electroretinogram a-wave observed in n-3 FA deficiency in rodents and nonhuman primates. Because members of the GPCR family are widespread in signaling pathways in the nervous system, the effect of reduced GPCR signaling due to the loss of membrane DHA may serve as an explanation for the suboptimal neural signaling observed in n-3 FA deficiency.

show abstract

“…Equilibrium Absorbance Measurements-Absorbance spectra of MI-MII equilibrium mixtures ϳ3 s following a flash that bleached 15-20% of the rhodopsin were acquired with a Hewlett-Packard 8452A diode array spectrophotometer (0.2-s measurements yielded Ͻ0.3% bleach by measuring beam) (28). Individual MI and MII bands were resolved by using nonlinear least squares to fit the sum of two asymmetric Gaussian absorbance bands to difference spectra that had been corrected for the presence of unbleached rhodopsin (28).…”

Section: Methodsmentioning

confidence: 99%

“…Individual MI and MII bands were resolved by using nonlinear least squares to fit the sum of two asymmetric Gaussian absorbance bands to difference spectra that had been corrected for the presence of unbleached rhodopsin (28). The equilibrium constant for the MI to MII conversion is defined by K eq ϭ [MII]/ [MI].…”

Section: Methodsmentioning

confidence: 99%

Effect of Ethanol and Osmotic Stress on Receptor Conformation

Mitchell

Litman

2000

Journal of Biological Chemistry

Self Cite

View full text Add to dashboard Cite

The combined effects of ethanol and osmolytes on both the extent of formation of metarhodopsin II (MII), which binds and activates transducin, and on acyl chain packing were examined in rod outer segment disc membranes. The ethanol-induced increase in MII formation was amplified by the addition of neutral osmolytes. This enhancement was linear with osmolality. At 360 milliosmolal, the osmolality of human plasma, 50 mM ethanol was 2.7 times more potent than at 0 osmolality, demonstrating the importance of water activity in in vitro experiments dealing with ethanol potency. Ethanol disordered acyl chain packing, and increasing osmolality enhanced this acyl chain disordering. Prior osmotic stress data showed a release of 35 ؎ 2 water molecules upon MII formation. Ethanol increases this number to 49 water molecules, suggesting that ethanol replaces 15 additional water molecules upon MII formation. Amplification of ethanol effects on MII formation and acyl chain packing by osmolytes suggests that ethanol increases the equilibrium concentration of MII both by disordering acyl chain packing and by disrupting rhodopsin-water hydrogen bonds, demonstrating a direct effect of ethanol on rhodopsin. At physiologically relevant levels of osmolality and ethanol, about 90% of ethanol's effect is due to disordered acyl chain packing.One of the proposed modes of action of ethanol on biological macromolecules and membranes involves the replacement of hydrogen-bonded water by ethanol (1, 2). Based on the hydrogen bonding capability of both water and ethanol, it is postulated that they compete for hydrogen bonding sites on the surface of lipids and proteins. Thus, this action of ethanol is equally applicable to mechanisms of ethanol action involving both lipid-mediated and direct protein interactions. The general importance of changes in hydration in enzyme activity is widely acknowledged (3, 4), but there is little detailed knowledge regarding how replacement of water by ethanol in specific protein-water hydrogen bonds might alter protein conformational equilibria or function. In addition, the physical properties of the surrounding phospholipid bilayer are known to modulate membrane function. Ethanol-induced changes in phospholipid hydrogen bonding have been observed to change acyl chain packing in pure lipid bilayers (5-7). Thus, the function of integral membrane receptors could be especially susceptible to the disruption of hydrogen-bonded water by ethanol.Thorough examination of the interplay between ethanol and hydration in modulating membrane receptor function requires the ability to separately analyze the effects of ethanol on both protein structure and bilayer physical properties and to be able to correlate the observed changes. Previously, we examined the effects of ethanol (8) and a series of n-alkanols (9) on both the MI 1 -MII conformational equilibrium and acyl chain packing in the surrounding bilayer. In the present work, the effects of ethanol on the MI-MII equilibrium and acyl chain packing are reexamined as a functi...

show abstract

Interconversion of metarhodopsins I and II: a branched photointermediate decay model

Cited by 114 publications

References 27 publications

Coupling of retinal isomerization to the activation of rhodopsin

Coupling of retinal isomerization to the activation of rhodopsin

Reduced G Protein-coupled Signaling Efficiency in Retinal Rod Outer Segments in Response to n-3 Fatty Acid Deficiency

Effect of Ethanol and Osmotic Stress on Receptor Conformation

Contact Info

Product

Resources

About