Regulation of retinal transducin by C-terminal peptides of rhodopsin

Determination of ligand-receptor interactions of cholecystokinin by nuclear magnetic resonance

To date high resolution structural studies of G protein coupled receptors, with the exception of rhodopsin, have not been feasible using conventional spectroscopic techniques. To overcome these difficulties, the structural features of partial or intact domains of GPCRs have been studied by nuclear magnetic resonance spectroscopy and X-ray crystallography. Here, we describe the structural characterization of receptor domains from the cholecystokinin 1 and 2 receptors and the elucidation of intermolecular interactions between the extracellular receptor domains and CCK-8 by solution state nmr.

Structural studies on rhodopsin

Bovine rhodopsin is the prototypical G protein coupled receptor (GPCR). It was the first GPCR to be obtained in quantity and studied in detail. It is also the first GPCR for which detailed three dimensional structural information has been obtained. Reviewed here are the experiments leading up to the high resolution structure determination of rhodopsin and the most recent structural information on the activation and stability of this integral membrane protein.

Peptide hormone binding to G-protein-coupled receptors: structural characterization via NMR techniques

G-protein-coupled receptors (GPCRs) allow cells to respond to calcium, hormones, and neurotransmitters. Not surprisingly, they currently make up the largest family of validated drug targets. Rational drug design for molecular regulators targeting GPCRs has been limited to theoretical-based computational approaches. X-ray crystallography of intact GPCRs has provided the topological orientation of the seven transmembrane helices, but limited structural information of the extracellular and intracellular loops and protein termini. In this review we detail an NMR-based approach which provides the high-resolution structural features on the extracellular domains of GPCRs and the ligand/receptor complexes formed upon titration of the peptide hormone. The results provide important contact points and a high-resolution description of the ligand/receptor interactions, which may be useful for the rational design of therapeutic agents targeting GPCRs. Recent results from our investigation of the cholecystokinin peptide hormone system are used to highlight this approach.

Characterization of CB1 cannabinoid receptors using receptor peptide fragments and site-directed antibodies

The mechanism by which CB1 cannabinoid receptors are coupled to the Gi/Go class of G proteins was studied. A peptide representing the juxtamembrane carboxyl terminus robustly stimulated guanosine-5'-O-(3-thio)triphosphate binding. Peptides simulating subdomains of the third intracellular loop (IL3) activated minimally when present alone but produced additive effects when present in combination. Peptides representing the amino-side IL3 and the juxtamembrane carboxyl terminus autonomously inhibited adenylate cyclase, and this response was not significantly augmented or inhibited by peptides representing other intracellular domains. Site-directed antipeptide antibodies developed against the domains of the amino terminus, first extracellular loop, amino-side IL3, and juxtamembrane carboxyl terminus of CB1 receptors failed to influence binding of [3H]CP-55940. However, IgG raised against the amino-side IL3 diminished the agonist-dependent inhibition of adenylate cyclase. These experiments suggest that the juxtamembrane carboxyl terminus is critical for G protein activation by CB1 cannabinoid receptors and that the amino-side IL3 also may interact with Gi proteins leading to inhibition of adenylate cyclase.

Three-dimensional structure of the cytoplasmic face of the G protein receptor rhodopsin

Rhodopsin is a G protein receptor from a many-membered family of membrane receptors. No high-resolution structure exists for any member of this family due to the insolubility of membrane proteins and the difficulty in crystallizing membrane proteins. Two new approaches to the structure of rhodopsin are described that circumvent these limitations: (1) individual solution structures of the four cytoplasmic domains of rhodopsin are fitted with the transmembrane domain; (2) the solution structure of a complex of the four cytoplasmic domains is determined from nuclear magnetic resonance data. The two structures are similar. To test the validity of these structures, specific site-to-site distances measured on intact membrane-bound rhodopsin are compared to the same distances on the structures reported here. Excellent agreement is obtained. Furthermore, the agreement is obtained with distances measured on the activated form of teh receptor and not with distances on the dark-adapted form of rhodopsin. This approach may prove to have general applicability for the determination of the structure for membrane proteins.

Structure of the carboxy-terminal domain of bovine rhodopsin

The biologically active carboxy-terminal peptide of the G-protein receptor, rhodopsin, forms a compact structure, suggesting that it is a structural domain in this integral membrane protein. The disposition of serines explains receptor kinase specificity.

Effects of carboxyl-terminal truncation on the stability and G protein-coupling activity of bovine rhodopsin

A number of studies have suggested that G protein-coupled receptors possess domains within the carboxyl terminus that are important for the catalytic activation of G proteins. To define these regions, truncation mutants were generated in the cDNA of bovine rhodopsin, the receptor responsible for visual signal transduction in the retinal rod cell. The mutants were expressed in HEK-293 cells and analyzed for their ability to bind the chromophore, 11-cis-retinal, and for activating Gt, the G protein of the rod cell regulated by rhodopsin. Removal of 38 carboxyl-terminal amino acids resulted in the production of a mutant (K311 stop) that does not bind 11-cis-retinal, has an abnormal pattern of glycosylation, and does not catalyze light-dependent binding of GTP gamma S to Gt, suggesting that it is unable to fold properly during biogenesis. However, a truncation mutant with only five additional amino acids (C316stop) coupled normally to Gt, using membranes from transfected cells, despite the fact that it lacked the "fourth cytoplasmic loop" formed by palmitoylation of cysteines-322 and -323. When C316stop is extracted from the membrane with detergent, only a fraction is able to bind 11-cis-retinal, but the fraction that binds retinal activates Gt normally. In contrast, detergent-solubilized wild-type rhodopsin and K325stop (a truncation mutant with the longest carboxyl terminus) both bind retinal and activate Gt normally. These data suggest that the proximal region of the carboxyl terminus is critical for the proper folding and stability of the rhodopsin molecule and that amino acids Cys316 to Ala348 are not necessary for the activation of Gt.

In vitro mutagenesis and the search for structure-function relationships among G protein-coupled receptors

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Acylation and glycosylation of rhodopsin in the rd mouse

Retinas of 9-10-day-old rd and control mice were incubated for 2 hr with [14C]leucine along with either tritiated palmitic acid or galactose to investigate the acylation or glycosylation, respectively, of rhodopsin. Although other laboratories have reported that phosphorylation of rhodopsin is not detectable in rd retinas, the two post-translational modifications of rhodopsin investigated in the present work are detectable. The rod outer segments (ROS) were separated from the retinal debris containing the rough endoplasmic reticulum (RER) of photoreceptor cells by vortexing and then by linear sucrose gradients. The rhodopsin from the RER was purified by affinity chromatography and gel electrophoresis. In the acylation studies, the mean ratio of palmitate to leucine in the rd mouse was nearly twice that of controls (11.73 +/- 2.84 v. 6.81 +/- 1.04). Possible explanations for the disparity between the two groups could include: (1) a diminished internal pool size of the fatty acid; or (2) acylation of amino acids such as serine or threonine which normally are not acylated in rhodopsin. Treatment of purified rhodopsin with 1 M hydroxylamine released similar amounts of palmitate from the rd mice and controls. Hence, the higher ratio of palmitate to leucine in rd mice is apparently due to a diminished internal pool size. In the glycosylation studies, the ratio of galactose to leucine was very similar between rd mice and controls, 1.7 +/- 0.43 v. 2.47 +/- 0.74. Protein content and specific activity were determined for the crude ROS preparations and for the remaining retinal debris. Although the amount of ROS protein differed significantly between the two groups, the specific activities did not.(ABSTRACT TRUNCATED AT 250 WORDS)

Structure/function relationship of proteins belonging to the family of receptors coupled to GTP-binding proteins

The structural properties of a number of proteins belonging to the family of receptors coupled to GTP binding proteins are discussed in relation to their function. The structure of the ligand binding site and of the regions involved in coupling to the G proteins are analyzed mainly for the adrenergic and muscarinic cholinergic receptors, for which site-directed mutagenesis and chimaeric constructions have been studied. The structure of the genes are compared and the presence of various regulatory elements is discussed in relation to control of expression. Mechanism of desensitization and internalization, while mostly studied for the beta 2-adrenergic receptor, are proposed to be generally applicable to all G-protein-coupled receptors.

Structure and function of G protein coupled receptors

The G protein coupled receptors (GPC-Rs) comprise a large superfamily of genes encoding numerous receptors which all show common structural features, e.g., seven putative membrane spanning domains. Their biological functions are extremely diverse, ranging from vision and olfaction to neuronal and endocrine signaling. The GPC-Rs couple via multiple G proteins to a growing number of recognized second messenger pathway, e.g., cAMP and phosphatidyl inositol turnover. This review summarizes our current knowledge of the molecular mechanisms by which the GPC-Rs activate second messenger systems, and it addresses their regulation and structure.

The localization and timing of post-translational modifications of rat rhodopsin

Rat retinas were labeled by incubation with, or intravitreal injection of, [14C]leucine along with tritiated palmitic acid, glucosamine or galactose. At selected intervals, subcellular fractions were prepared on linear sucrose gradients and rhodopsin was extracted and purified by affinity chromatography and gel electrophoresis. Time courses revealed that leucine rapidly and transiently labeled the rhodopsin in the rough endoplasmic reticulum (RER), with a maximum at 1.5 hr post-injection. Subsequently, the rod outer segments (ROS) contained the labeled rhodopsin, with the ROS labeling maximally at 6-12 hr. Palmitate labeling followed the same pattern but was subject to a delay, presumably because of a large intracellular pool of the fatty acid. With palmitate the RER rhodopsin was not maximally labeled until 12 hr. The acylation of rhodopsin takes place in the RER sometime after the polypeptide has been translated but before transport to the Golgi. Glucosamine labeling was also delayed because of intracellular pools of the sugar or its metabolic derivatives. But because of secondary glycosylation in the Golgi, the rhodopsin in the ROS also labeled maximally with glucosamine at about 6 hr. Administration of [3H]galactose resulted in the labeling of rhodopsin both in vivo and in vitro, in part possibly because of its conversion to mannose and subsequent insertion into the core oligosaccharide on the RER. However, in the ROS the ratio of tritium, derived from [3H]galactose, to [14C]leucine decreased by a factor of 2 between 6 and 24 hr post-injection. Moreover, between 6 and 12 hr post-injection, labeled rhodopsin molecules in the ROS underwent a shift in mobility on gels indicative of trimming to a lower molecular weight. Thus some sugar residues may be added to the rhodopsin in the inner segment and removed in the ROS.

Adrenergic receptors. Models for regulation of signal transduction processes

Adrenergic receptors are prototypic models for the study of the relations between structure and function of G protein-coupled receptors. Each receptor is encoded by a distinct gene. These receptors are integral membrane proteins with several striking structural features. They consist of a single subunit containing seven stretches of 20-28 hydrophobic amino acids that represent potential membrane-spanning alpha-helixes. Many of these receptors share considerable amino acid sequence homology, particularly in the transmembrane domains. All of these macromolecules share other similarities that include one or more potential sites of extracellular N-linked glycosylation near the amino terminus and several potential sites of regulatory phosphorylation that are located intracellularly. By using a variety of techniques, it has been demonstrated that various regions of the receptor molecules are critical for different receptor functions. The seven transmembrane regions of the receptors appear to form a ligand-binding pocket. Cysteine residues in the extracellular domains may stabilize the ligand-binding pocket by participating in disulfide bonds. The cytoplasmic domains contain regions capable of interacting with G proteins and various kinases and are therefore important in such processes as signal transduction, receptor-G protein coupling, receptor sequestration, and down-regulation. Finally, regions of these macromolecules may undergo posttranslational modifications important in the regulation of receptor function. Our understanding of these complex relations is constantly evolving and much work remains to be done. Greater understanding of the basic mechanisms involved in G protein-coupled, receptor-mediated signal transduction may provide leads into the nature of certain pathophysiological states.

The sequence of human retinal S-antigen reveals similarities with alpha-transducin

The complete amino acid sequence of human retinal S-antigen (48 kDa protein), a retinal protein involved in the visual process has been determined by cDNA sequencing. The largest cDNA was 1590 base pairs (bp) and it contained an entire coding sequence. The similarity of nucleotide sequence between the human and bovine is approximately 80%. The predicted amino acid sequence indicates that human S-antigen has 405 residues and its molecular mass is 45050 Da. The amino acid sequence homology between human and bovine is 81%. There is no overall sequence similarity between S-antigen and other proteins listed in the National Biomedical Research Foundation (NBRF) protein data base. However, local regions of sequence homology with alpha-transducin (T alpha) are apparent including the putative rhodopsin binding and phosphoryl binding sites. In addition, human S-antigen has sequences identical to bovine uveitopathogenic sites, indicating that some types of human uveitis may in part be related to the animal model of experimental autoimmune uveitis (EAU).

Rod/cone dysplasia in Irish setters. Presence of an altered rhodopsin

On the basis of the amino acid sequence of bovine rhodopsin, a series of peptides from the C-terminus (Rhod-4 and Rhod-1) and external loops (Rhod-10) were synthesized. Rabbit antisera to these peptides recognize the rhodopsin molecule in whole retina from 8-week-old normal and affected rcdl (rod/cone-dysplasic) Irish setters (8- and 4-weeks-old). When the rhodopsin content was equalized by using a solid-phase radioimmunoassay, the reaction with anti-peptide antisera to the C-terminal octapeptide (residues 341-348) is severely decreased in the rcdl-dog retinas. The results of mixing experiments suggest that this is not due to proteolytic clipping of the rhodopsin C-terminus from the affected dogs. Treatment of retinas with 1.0 mM-NaF, a phosphatase inhibitor, or pretreatment with alkaline and acid phosphatases does alter the reaction of the rhodopsin with anti-rhodopsin antisera. This suggests that the decreased reaction of the affected rhodopsin with the anti-peptide antisera may partially result from differences in intrinsic rhodopsin phosphorylation. However, since the reaction of rcdl retinas cannot be restored to that of the normals, these results suggest that the rhodopsin molecule from the rcdl dogs may be structurally altered in other ways.

Modulation of retinal transducin and phosphodiesterase activities by synthetic peptides of the phosphodiesterase gamma-subunit

Synthetic peptides corresponding to various regions of the light-activated guanosine 3',5'-cyclic monophosphate phosphodiesterase (PDE) gamma-subunit (PDE gamma) from bovine retinal rod outer segments were synthesized and tested for their ability to inhibit PDE activity, and GTPase activity of transducin. One of these peptides, corresponding to PDE gamma residues 31-45, inhibited PDE activity and GTPase activity in a dose-dependent manner. The GTPase activity was inhibited by PDE gamma-3 non-competitively. This region of the PDE gamma subunit may be involved in the direct interaction of transducin and PDE alpha beta with PDE gamma.

The opsin family of proteins

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C-terminal peptides of rhodopsin. Determination of the optimum sequence for recognition of retinal transducin

In vertebrate retinal rod outer segments, transducin, a guanine-nucleotide-binding protein, mediates signal coupling between rhodopsin and cyclic GMP phosphodiesterase. Whereas the T alpha subunit (39 kDa) of transducin binds guanine nucleotides and is the activator of the phosphodiesterase, the T beta gamma subunits (35 and 10 kDa) may function to physically link T alpha with photolysed rhodopsin. We have previously reported that a site of binding of transducin is on the C-terminus of bovine rhodopsin. By using competition with synthetic peptides, the recognition region was localized to bovine opsin amino acid residues 317-339. Further studies are detailed which determine the boundaries of this binding site on rhodopsin, as well as some of the critical amino acids needed for transducin binding. These results suggest that the serine and threonine residues in the rhodopsin C-terminal peptides Rhod-1 and Rhod-3 are critical for reconstitution of transducin GTPase activity.

Reduced rhodopsin phosphorylation during retinal dystrophy

During inherited retinal dystrophy in Irish Setter dogs, decreased activity of cGMP phosphodiesterase (PDE) results in high cGMP levels and retinal degeneration (1-3). This defect could be in PDE itself, or in its interactions with other proteins of the rod outer segment. We report herein that when retinas from 8-week-old dogs were phosphorylated with gamma-32P-ATP, and separated on SDS-PAGE, phosphorylation of rd dog rhodopsin was reduced. When rd retinas were mixed with normal dog retinas, phosphorylation of the latter was inhibited. Since rd-mediated inhibition was prevented by 1 mM NaF, the results suggest that the cause of reduced rd phosphorylation is increased phosphatase activity. Together, these results demonstrate that decreased phosphorylation of rhodopsin due to increased phosphatase activity is a fundamental biochemical change which may partially account for the degenerative process and loss of visual acuity during inherited retinal dystrophy.