Constitutive activation of opsin: influence of charge at position 134 and size at position 296

The role of Asp578 in maintaining the inactive conformation of the human lutropin/choriogonadotropin receptor

A constitutively activating mutation encoding Asp578-->Gly in transmembrane helix 6 of the lutropin/choriogonadotropin receptor (LHR) is the most common cause of gonadotropin-independent, male-limited precocious puberty. This mutant LHR produces a 4.5-fold increase in basal cAMP when expressed in COS-7 cells. To better understand the normal role of Asp578 in the LHR we studied the effect of seven other amino acid substitutions at this position. No agonist binding or response was detected with the Asp578-->Pro mutant. Agonist binding affinity was unaffected by the other substitutions and estimated receptor concentrations ranged from 11 to 184% of wild type. Substitution of Asp578 with Asn, a similarly sized, uncharged residue, did not produce agonist-independent activation. In contrast, replacement with Glu, Ser, or Leu caused 4. 9-5.6-fold stimulation of basal cAMP. Substitution with Tyr (8.5-fold) or Phe (7.5-fold) had a greater activating effect. Only the Tyr, Phe, and Leu mutants showed constitutive activation of the inositol phosphate pathway. Our data suggest that it is the ability of the Asp578 side chain to serve as a properly positioned hydrogen bond acceptor, rather than its negative charge, that is important for stabilizing the inactive state of the LHR. A bulky aromatic side chain at position 578 may further destabilize the inactive receptor conformation.

Modulation of GDP release from transducin by the conserved Glu134-Arg135 sequence in rhodopsin

A superfamily of seven-transmembrane helix receptors catalyzes GDP release from heterotrimeric guanine nucleotide-binding proteins (G proteins) to initiate the intracellular signaling cascade. The photoreceptor rhodopsin is a prototypical member of the superfamily that activates the retinal G protein transducin (Gt). The cytoplasmic domain of rhodopsin binds and activates Gt, but residues that stimulate GDP release from Gt have not been identified until now. We show here that the abnormal signal transduction phenotypes of several different mutations affecting the highly conserved Glu134-Arg135 charge pair result from alteration of the GDP release step in the Gt activation cascade. We propose that Glu134 and Arg135 constitute the site that directly provides the signal from rhodopsin to activate GDP release from Gt. Because the Glu/Asp-Arg sequence occurs at a topologically identical location in most of the seven-transmembrane helix receptors, we propose that these residues constitute a switch for signal transfer.

Mechanisms of opsin activation

Rhodopsin is constrained in an inactive conformation by interactions with 11-cis-retinal including formation of a protonated Schiff base with Lys296. Upon photoisomerization, major structural rearrangements that involve protonation of the active site Glu113 and cytoplasmic acidic residues, including Glu134, lead to the formation of the active form of the receptor, metarhodopsin II b, which decays to opsin. However, an activated receptor may be generated without illumination by addition of all-trans-retinal or its analogues to opsin, as measured in this study by the increased phosphorylation of opsin by rhodopsin kinase. The potency of stimulation depended on the chemical and isomeric nature of the analogues and the length of the polyene chain with all-trans-C17 aldehyde and all-trans-retinal being the most active and trans-C12 aldehyde being the least active. Certain cis-isomers, 11-cis-13-demethyl-retinal and 9-cis-C17 aldehyde, were also active. Most of the retinal analogues tested did not regenerate a spectrally identifiable pigment, and many were incapable of Schiff base formation (ketone, stable oximes, and Schiff base-derivatives of retinal). Thus, receptor activation resulted from formation of non-covalent complexes with opsin. pH titrations suggested that an equilibrium exists between partially active (protonated) and inactive (deprotonated) forms of opsin. These findings are consistent with a model in which protonation of one or more cytoplasmic carboxyl groups of opsin is essential for activity. Upon addition of retinoids, the partially active conformation of opsin is converted to a more active intermediate similar to metarhodopsin II b. The model provides an understanding of the structural requirements for opsin activation and an interpretation of the observed activities of natural and experimental opsin mutants.

The TSH receptor and thyroid diseases

Recent advances in the understanding of the molecular biology of the TSH receptor have had a considerable impact on several aspects of thyroidology. The identification and functional characterization of mutations in the TSH receptor gene which constitutively activate the TSH receptor in the absence of its ligand provide an explanation for the molecular mechanism which is most likely responsible for the majority of the hyperfunctioning thyroid adenomas. Moreover, these constitutively activating mutations also cause a new form of familial hyperthyroidism: non-autoimmune autosomal dominant hyperthyroidism and also sporadic cases of congenital non-autoimmune hyperthyroidism. TSH receptor mutations which cause a reduced sensitivity to TSH have been identified as the cause of non-autoimmune congenital hypothyroidism. TSH receptor mRNA variants have been found in thyroid associated ophthalmopathy. If protein expression for these variants can be demonstrated, this finding could advance our understanding of thyroid associated ophthalmopathy. The ability to produce large quantities of TSH receptor protein in bacteria has led to the generation of more sophisticated assays for TSH receptor antibodies and enabled the generation of an animal model for thyroid autoimmunity.

Automated method for modeling seven-helix transmembrane receptors from experimental data

A rule-based automated method is presented for modeling the structures of the seven transmembrane helices of G-protein-coupled receptors. The structures are generated by using a simulated annealing Monte Carlo procedure that positions and orients rigid helices to satisfy structural restraints. The restraints are derived from analysis of experimental information from biophysical studies on native and mutant proteins, from analysis of the sequences of related proteins, and from theoretical considerations of protein structure. Calculations are presented for two systems. The method was validated through calculations using appropriate experimental information for bacteriorhodopsin, which produced a model structure with a root mean square (rms) deviation of 1.87 A from the structure determined by electron microscopy. Calculations are also presented using experimental and theoretical information available for bovine rhodopsin to assign the helices to a projection density map and to produce a model of bovine rhodopsin that can be used as a template for modeling other G-protein-coupled receptors.

Photoactivated state of rhodopsin and how it can form

A variety of spectroscopic and biochemical studies of the photoreceptor rhodopsin have revealed conformation changes which occur upon its photoactivation. Assignment of these molecular alterations to specific regions in the receptor has been attempted by studying native opsin regenerated with synthetic retinal analogs or recombinant opsins regenerated with 11-cis retinal. We propose a model for the photoactivation mechanism which defines 'off' and 'on' states for individual molecular groups. These groups have been identified to undergo structural alterations during photoactivation. Analysis of mutant pigments in which specific groups are locked into their respective 'on' or 'off' states provides a framework to identify determinants of the active conformation as well as the minimal number of intramolecular transitions to switch to this conformation. The simple model proposed for the active-state of rhodopsin can be compared to structural models of its ground-state to localize chromophore-protein interactions that may be important in the photoactivation mechanism.

Effect of carboxyl mutations on functional properties of bovine rhodopsin

Bovine rod rhodopsin and membrane-carboxyl group mutants are expressed using the recombinant baculovirus expression system. Biosynthesis of wild-type and the mutant D83N is normal. The mutations E122L and E134D/R affect glycosylation and translocation. After regeneration, purification and reconstitution in retina lipids a wild-type photosensitive pigment with spectral and photolytic properties identical to native bovine rod rhodopsin is generated. Only the mutations D83N and E122L affect the spectral properties and then only slightly. All mutations induce a shift in the Meta I<==>Meta II equilibrium towards Meta I (E134D/R) or Meta II (D83N, E122L). FT-IR analysis shows that the mutation E134D/R does not significantly affect the carboxyl-vibration region but, in particular in the case of E134R, affects secondary structural changes upon Meta II formation. E122L also has an effect on secondary structural changes and in addition eliminates a negative band at 1728 cm-1. The mutation D83N removes a pair of negative/positive bands from the carboxyl-vibration region, indicating that Asp83 stays protonated upon formation of Meta II but undergoes a change in hydrogen bonding.

G protein-coupled receptor structure and function: the impact of disease-causing mutations

Just as the discovery of 'inborn errors of metabolism' in humans contributed to our basic understanding of normal enzymatic pathways, so can genetic defects in signal transduction help to elucidate the functions normally subserved by different GPCR pathways. Identification and characterization of naturally occurring GPCR mutations not only has inherent value in understanding the molecular basis of disease, but can also accelerate progress in understanding the fundamental mechanisms involved in GPCR synthesis, transport to the membrane, ligand binding, activation and deactivation.

Characterization of rhodopsin mutants that bind transducin but fail to induce GTP nucleotide uptake. Classification of mutant pigments by fluorescence, nucleotide release, and flash-induced light-scattering assays

The photoreceptor rhodopsin is a seven-transmembrane helix receptor that activates the G protein transducin in response to light. Several site-directed rhodopsin mutants have been reported to be defective in transducin activation. Two of these mutants bound transducin in response to light, but failed to release the bound transducin in the presence of GTP (Franke, R. R., König, B., Sakmar, T. P., Khorana, H. G., and Hofmann, K. P. (1990) Science 250, 123-125). The present study was carried out to determine the nucleotide-binding state of transducin as it interacts with rhodopsin mutants. Five mutant bovine opsin genes were prepared by site-specific mutagenesis. Three mutant genes had deletions from one cytoplasmic loop each: AB delta 70-71; CD delta 143-150; and EF delta 237-249. Two additional loop CD mutant genes were prepared: E134R/R135E had a reversal of a conserved charge pair, and CD r140-152 had a 13-amino acid sequence replaced by a sequence derived from the amino-terminal tail. Three types of assays were carried out: 1) a fluorescence assay of photoactivated rhodopsin (R*)-dependent guanosine 5'-O-(3-thiotriphosphate) uptake by transducin, 2) an assay of R*-dependent release of labeled GDP from the alpha-subunit of transducin holoenzyme (Gt alpha).GDP, and 3) a light-scattering assay of R*.Gt complex formation and dissociation. We show that the mutant pigments, which are able to bind transducin in a light-dependent manner but lack the ability to activate transducin, most likely form R*.Gt alpha beta gamma.GDP complexes that are impaired in GDP release.

Genetic heterogeneity of constitutively activating mutations of the human luteinizing hormone receptor in familial male-limited precocious puberty

Genomic DNA from 32 unrelated families with male-limited precocious puberty was examined for the previously described Asp-578-->Gly, Met-571-->Ile, and Thr-577-->Ile mutations in transmembrane helix 6 of the human luteinizing hormone receptor (hLHR). Twenty-eight families had the inherited form of the disorder, and of these, 24 were found to have the Asp-578-->Gly mutation. Four additional mutations were found among the remaining four families with the inherited form and in four sporadic cases of the disorder: an A-->C transversion resulting in substitution of leucine for Ile-542 in the fifth transmembrane helix, an A-->G transition resulting in substitution of glycine for Asp-564 in the third cytoplasmic loop, a G-->T transversion resulting in substitution of tyrosine for Asp-578 in the sixth transmembrane helix, and a T-->C transition resulting in substitution of arginine for Cys-581 in the sixth transmembrane helix. Human embryonic kidney cells transfected with cDNAs for each of the mutant hLHRs, created by PCR-based mutagenesis of the wild-type hLHR cDNA, exhibited increased levels of basal cAMP production in the absence of agonist, indicating constitutive activation of the mutation hLHRs. Three of the additional mutations had specific features: Ile-542-->Leu and Cys-581-->Arg appeared ligand-unresponsive, whereas Asp-578-->Tyr appeared to correlate genotype with phenotype. We conclude that the region spanning nt 1624-1741 of exon 11 is a hotspot for heterogeneous point mutations that constitutively activate the hLHR and cause male-limited precocious puberty.

Transducin activation by the bovine opsin apoprotein

The interaction of the bovine opsin apoprotein with transducin in rod outer segment membranes was investigated using a guanyl nucleotide exchange assay. In exhaustive binding experiments, opsin activates transducin, with half-maximal exchange activity occurring at 0.8 mol of opsin/mol of transducin. The opsin activity was light-insensitive, hydroxylamine-resistant, unaffected by stoichiometric concentrations of retinaloxime, and more heat-labile than rhodopsin. The t1/2 of transducin activation in the presence of excess opsin was 8.5 min, compared with 0.7 min for metarhodopsin (II). The second-order rate constants were determined to be 0.012 pmol of guanosine 5'-(gamma-thio)triphosphate (GTP gamma S) bound per min/nM opsin and 0.35 pmol of GTP gamma S bound per min/nM metarhodopsin (II). Opsin was able to activate more than one transducin, although there appeared to be a turnover-dependent inactivation of the apoprotein. Opsin showed a broad pH range (5.8-7.4) for optimal activity, with no activity in buffers of pH > 9, whereas metarhodopsin (II) exhibited activity at pH > 9. Regulation of opsin activity by stoichiometric amounts of retinal was observed, with inhibition by 11-cis-retinal and stimulation by all-trans-retinal. A model for opsin activity is proposed.

The function of a highly-conserved arginine residue in activation of the muscarinic M1 receptor

Arg123 in the rat muscarinic M1 receptor is part of the highly conserved triad Asp-Arg-Tyr found at the junction of transmembrane helix 3 with the second intracellular loop. Mutation of Arg123 to Lys, Ala, Leu, Glu and Gln had no effect on levels of receptor expression in COS-7 cells, or on affinities for the antagonist N-methylscopolamine. Acetylcholine stimulation of the Lys123 receptor evoked the same maximum phosphoinositide response as the wild type, although the potency was reduced six-fold, but mutation to other residues strongly disrupted receptor function. Mutation of Arg123 always decreased the ratio of the high affinity to the low affinity agonist binding constant, but the absolute effect on the latter varied from a 4-fold increase for the Lys123 to a small decrease for the Leu123 mutation. These results suggest that a positive charge at position 123 is central to the activation of G-proteins by the muscarinic M1 receptor.

Cloning of the rhodopsin-encoding gene from the rod-less lizard Anolis carolinensis

The rhodopsin (Rh) of the lizard Anolis carolinensis has only been detected in the pineal gland which controls circadian rhythms [Foster et al., J. Comp. Physiol. (1993) 33-45]. In the present study, we isolated and sequenced a genomic DNA clone for the A. carolinensis Rh gene. The deduced amino acid (aa) sequence revealed six A. carolinensis-specific replacements in conserved aa residues among vertebrate Rhs. Four out of these six replacements (S22N, N199H, E232A and T319M) reside outside the transmembrane (TM) domains. Some of these replacements might have been required for Rh to function solely in the pineal gland.

Germline mutations in the thyrotropin receptor gene cause non-autoimmune autosomal dominant hyperthyroidism

The thyrotropin receptor (TSHR), a member of the large family of G protein-coupled receptors, controls both the function and growth of thyroid cells via stimulation of adenylyl cyclase. We report two different mutations in the TSHR gene of affected members of two large pedigrees with non-autoimmune autosomal dominant hyperthyroidism (toxic thyroid hyperplasia), that involve residues in the third (Val509Ala) and seventh (Cys672Tyr) transmembrane segments. When expressed by transfection in COS-7 cells, the mutated receptors display a higher constitutive activation of adenylyl cyclase than wild type. This new disease entity is the germline counterpart of hyperfunctioning thyroid adenomas, in which different somatic mutations with similar functional characteristics have been demonstrated.

Opsins with mutations at the site of chromophore attachment constitutively activate transducin but are not phosphorylated by rhodopsin kinase

More than 70 mutations in the gene encoding the visual pigment rhodopsin have been identified in patients with autosomal dominant retinitis pigmentosa. Most of these mutations are thought to interfere with proper folding of the membrane protein. However, families with a severe phenotype of retinitis pigmentosa have been identified and shown to carry a mutation at the site of chromophore attachment, Lys-296. This mutation disrupts the inactive conformation of opsin and results in a constitutively active protein that can activate the rod-specific GTP-binding protein, transducin, in the absence of light and in the absence of the chromophore 11-cis-retinal. It has been suggested that this mutant opsin molecule may cause rod degeneration by depletion of the components used to inactivate rhodopsin, such as rhodopsin kinase. In this work we test this idea by determining whether two constitutively active opsin mutants are phosphorylated by rhodopsin kinase. We found that opsin mutants where Lys-296 is replaced either by Glu (K296E) or by Gly (K296G) are not substrates of rhodopsin kinase in the absence of chromophore. However, when K296G is regenerated with a Schiff base complex of 11-cis-retinal and n-propylamine and exposed to illumination, phosphorylation of opsin occurs. These experiments suggest that in the rod photoreceptors of patients with retinitis pigmentosa carrying a mutation at Lys-296, there is persistent activation of the GTP-binding protein-mediated cascade. This may result in a situation that mimics long-term exposure to continuous illumination and results in the degeneration of photoreceptors.

Structure and function of receptors coupled to G proteins

Direct structural data on receptors coupled to G proteins were obtained last year in the form of a low resolution projection map of rhodopsin. A large number of receptor sequences have now been determined and detailed analysis of these has provided structural information about the receptors. New results from site-directed mutagenesis experiments can be examined in conjunction with the structural information from sequence analysis and the rhodopsin map. The identification of constitutively active mutated receptors has influenced our understanding of normal receptor equilibria.

Rhodopsin mutation G90D and a molecular mechanism for congenital night blindness

Mutations in the gene for the visual pigment rhodopsin cause retinitis pigmentosa (RP) and congenital night blindness. Inheritance of the diseases is generally autosomal dominant and about 40 different rhodopsin mutations have been documented. Although the cell death and retinal degeneration associated with RP have been suggested to result from improper folding and accumulation of the mutant proteins in rod photoreceptor cells, this may not account for the disease in all cases. For example, RP mutations at Lys 296, site of Schiff base linkage to the retinal chromophore, result in constitutive activation of the protein in vitro; that is, the mutants can catalytically activate the G protein transducin in the absence of chromophore and in the absence of light. Similarly, mutation of Ala 292-->Glu activates opsin in vitro and causes night blindness. We show here that the mutation Gly 90-->Asp (G90D) in the second transmembrane segment of rhodopsin, which causes congenital night blindness, also constitutively activates opsin. Furthermore, we show that Asp 90 can substitute for the Schiff base counterion, Glu 113, which is located in the third transmembrane segment of the protein. This demonstrates the proximity of Asp 90 and Lys 296 in the three-dimensional structure of rhodopsin and suggests that the constitutively activating mutations operate by a common molecular mechanism, disrupting a salt bridge between Lys 296 and the Schiff base counterion, Glu 113.

Photoactivated conformational changes in rhodopsin: a time-resolved spin label study

Rhodopsin has been selectively spin-labeled near the cytoplasmic termini of helices C and G. Photoactivation with a light flash induces an electron paramagnetic resonance spectral change in the millisecond time domain, coincident with the appearance of the active metarhodopsin II intermediate. The spectral change is consistent with a small movement near the cytoplasmic termination of the C helix and reverses upon formation of the MIII state. These results provide an important link between the optical changes associated with the retinal chromophore and protein conformational states.