An alpha-carbon template for the transmembrane helices in the rhodopsin family of G-protein-coupled receptors

Chloroethylclonidine and 2-aminoethyl methanethiosulfonate recognize two different conformations of the human alpha(2A)-adrenergic receptor

The substituted cysteine-accessibility method and two sulfhydryl-specific reagents, the methane-thiosulfonate derivative 2-aminoethyl methanethiosulfonate (MTSEA) and the alpha(2)-adrenergic receptor (alpha(2)-AR) agonist chloroethylclonidine (CEC), were used to determine the relative accessibility of engineered cysteines in the fifth transmembrane domain of the human alpha(2A)-AR (Halpha2A). The second-order rate constants for the reaction of the receptor with MTSEA and CEC were determined with the wild type Halpha2A (cysteine at position 201) and receptor mutants containing accessible cysteines at other positions within the binding-site crevice (positions 197, 200, and 204). The rate of reaction of CEC was similar to that of MTSEA at residues Cys-197, Cys-201, and Cys-204. The rate of reaction of CEC with Cys-200, however, was more than 5 times that of MTSEA, suggesting that these compounds may interact with two different receptor conformations. MTSEA, having no recognition specificity for the receptor, likely reacts with the predominant inactive receptor conformation (R), whereas the agonist CEC may stabilize and react preferentially with the active receptor conformation (R*). This hypothesis was consistent with three-dimensional receptor-ligand models, which further suggest that alpha(2A)-AR activation may involve the clockwise rotation of transmembrane domain 5.

Mutation of a highly conserved aspartic acid in the beta2 adrenergic receptor: constitutive activation, structural instability, and conformational rearrangement of transmembrane segment 6

Movements of transmembrane segments (TMs) 3 and 6 play a key role in activation of G protein-coupled receptors. However, the underlying molecular processes that govern these movements, and accordingly control receptor activation, remain unclear. To elucidate the importance of the conserved aspartic acid (Asp-130) in the Asp-Arg-Tyr motif of the beta2 adrenergic receptor (beta2AR), we mutated this residue to asparagine (D130N) to mimic its protonated state, and to alanine (D130A) to fully remove the functionality of the side chain. Both mutants displayed evidence of constitutive receptor activation. In COS-7 cells expressing either D130N or D130A, basal levels of cAMP accumulation were clearly elevated compared with cells expressing the wild-type beta2AR. Incubation of COS-7 cell membranes or purified receptor at 37 degrees C revealed also a marked structural instability of both mutant receptors, suggesting that stabilizing intramolecular constraints had been disrupted. Moreover, we obtained evidence for a conformational rearrangement by mutation of Asp-130. In D130N, a cysteine in TM 6, Cys-285, which is not accessible in the wild-type beta2AR, became accessible to methanethiosulfonate ethylammonium, a charged, sulfhydryl-reactive reagent. This is consistent with a counterclockwise rotation or tilting of TM 6 and provides for the first time structural evidence linking charge-neutralizing mutations of the aspartic acid in the DRY motif to the overall conformational state of the receptor. We propose that protonation of the aspartic acid leads to release of constraining intramolecular interactions, resulting in movements of TM 6 and, thus, conversion of the receptor to the active state.

Activation mechanism of human oxytocin receptor: a combined study of experimental and computer-simulated mutagenesis

The aim of this study was to investigate the molecular changes associated with the transition of the human oxytocin receptor from its inactive to its active states. Mutation of the conserved arginine of the glutamate/aspartate-arginine-tyrosine motif located in the second intracellular domain gave rise to the first known constitutively active oxytocin receptor (R137A), whereas mutation of the aspartic acid located in the second transmembrane domain led to an inactive receptor (D85A). The structural features of the constitutively active and inactive receptor mutants were compared with those of the wild type in its free and agonist-bound states. The results suggest that, although differently triggered, the activation process induced by the agonist and the activating mutation are characterized by the opening of a solvent exposed site formed by the 2nd intracellular loop, the cytosolic extension of helix 5, and the 3rd intracellular loop; on the contrary, the D85A mutation prevents oxytocin from triggering the opening of a cytosolic site. On the basis of these findings, we hypothesize that this cytosolic crevice plays an important role in G protein recognition. Finally, comparative analysis of the free- and agonist-bound forms of the wild-type oxytocin receptor and alpha1B adrenergic receptor suggests that the highly conserved polar amino acids and the seven helices play similar mechanistic roles in the different G protein-coupled receptors.

Structural organization of G-protein-coupled receptors

Atomic-resolution structures of the transmembrane 7-alpha-helical domains of 26 G-protein-coupled receptors (GPCRs) (including opsins, cationic amine, melatonin, purine, chemokine, opioid, and glycoprotein hormone receptors and two related proteins, retinochrome and Duffy erythrocyte antigen) were calculated by distance geometry using interhelical hydrogen bonds formed by various proteins from the family and collectively applied as distance constraints, as described previously [Pogozheva et al., Biophys. J., 70 (1997) 1963]. The main structural features of the calculated GPCR models are described and illustrated by examples. Some of the features reflect physical interactions that are responsible for the structural stability of the transmembrane alpha-bundle: the formation of extensive networks of interhelical H-bonds and sulfur-aromatic clusters that are spatially organized as 'polarity gradients'; the close packing of side-chains throughout the transmembrane domain; and the formation of interhelical disulfide bonds in some receptors and a plausible Zn2+ binding center in retinochrome. Other features of the models are related to biological function and evolution of GPCRs: the formation of a common 'minicore' of 43 evolutionarily conserved residues; a multitude of correlated replacements throughout the transmembrane domain; an Na(+)-binding site in some receptors, and excellent complementarity of receptor binding pockets to many structurally dissimilar, conformationally constrained ligands, such as retinal, cyclic opioid peptides, and cationic amine ligands. The calculated models are in good agreement with numerous experimental data.

Structural features of the C-terminal domain of bovine rhodopsin: a site-directed spin-labeling study

Eleven single-cysteine substitution mutants have been prepared in the sequence 325-340 of rhodopsin, corresponding to the C-terminal domain. Each of the cysteine mutants was modified with a selective nitroxide reagent to introduce a spin-labeled side chain. The electron paramagnetic resonance spectra of the labeled proteins were analyzed in terms of side chain dynamics. At all sites, the spectra reflected the presence of two populations of different mobility, although one was always dominant. The mobility of the dominant population increased in a regular fashion from the palmitoylation sites at 322C and 323C to the C-terminus, where the spectra resembled those of an unfolded protein. This apparent mobility gradient is only slightly affected in mutants lacking the palmitoyl groups, suggesting that they are not responsible for physically anchoring the C-terminal peptide at one end. Binding of a monoclonal antibody to its epitope at the C-terminus dramatically reduces the mobility of nearby residues, creating a local mobility gradient opposite that in the absence of the antibody. These results indicate that the C-terminal domain of rhodopsin, beyond the palmitoylation sites, is highly disordered and dynamic, resembling an unfolded peptide tethered at one end.

Single-cysteine substitution mutants at amino acid positions 306-321 in rhodopsin, the sequence between the cytoplasmic end of helix VII and the palmitoylation sites: sulfhydryl reactivity and transducin activation reveal a tertiary structure

As sensors for structure at the cytoplasmic face of rhodopsin, single-cysteine substitution mutants have been previously studied in the regions connecting helices III and IV and helices V and VI. In this paper we report on single-cysteine substitution mutants at amino acid positions 306-321, comprising the cytoplasmic sequence between helix VII and the palmitoylation sites in rhodopsin. The cysteine opsin mutants were expressed in COS-1 cells and on treatment with 11-cis-retinal all formed the characteristic rhodopsin chromophore. Cysteines at positions 306-316 and 319 reacted in the dark with the thiol-specific reagent 4, 4'-dithiodipyridine (4-PDS) but showed a wide variation in reactivity. Cysteines at positions 317, 318, 320, and 321 showed no reaction with 4-PDS. The mutants on illumination also showed wide variations in activating GT. The mutant Y306C showed almost no GT activation, I307C and N310C were poor, and the activity of the mutants M309C, F313C, and M317C was also reduced relative to WT. The results suggest that the region comprising amino acids 306-321 is a part of a tertiary structure and that specific amino acids in this region on light-activation participate in the interaction with GT.

Structural features and light-dependent changes in the sequence 59-75 connecting helices I and II in rhodopsin: a site-directed spin-labeling study

Twenty-one single-cysteine substitution mutants were prepared in the sequence 56-75 between transmembrane helices I and II at the cytoplasmic surface of bovine rhodopsin. Each mutant was reacted with a sulfhydryl-specific reagent to produce a nitroxide side chain. The electron paramagnetic resonance of the labeled proteins in dodecyl maltoside solution was analyzed to provide the relative mobility and accessibility of the nitroxide side chain to both polar and nonpolar paramagnetic reagents. The results indicate that the hydrophobic-water interface of the micelle intersects helices I and II near residues 64 and 71, respectively. Thus, the sequence 64-71 is in the aqueous phase, while 56-63 and 72-75 lie in the transmembrane helices I and II, respectively. The lipid-facing surfaces on transmembrane helices I and II near the cytoplasmic surface correspond to approximately 180 degrees and 90 degrees of arc on the helical surfaces, respectively. Photoactivation of rhodopsin produced changes in structure in the region investigated, primarily around helix II. However, these changes are much smaller than those noted by spin labels in helix VI (Altenbach, C., Yang, K., Farrens, D., Farahbakhsh, Z., Khorana, H. G., and Hubbell, W. L. (1996) Biochemistry 35, 12470).

Structural features and light-dependent changes in the sequence 306-322 extending from helix VII to the palmitoylation sites in rhodopsin: a site-directed spin-labeling study

Sixteen single-cysteine substitution mutants of rhodopsin were prepared in the sequence 306-321 which begins in transmembrane helix VII and ends at the palmitoylation sites at 322C and 323C. The substituted cysteine residues were modified with a selective reagent to generate a nitroxide side chain, and the electron paramagnetic resonance spectrum of each spin-labeled mutant was analyzed in terms of residue accessibility and mobility. The periodic behavior of these parameters along the sequence indicated that residues 306-314 were in a regular alpha-helical conformation representing the end of helix VII. This helix apparently extends about 1.5 turns above the surface of the membrane, with one face in strong tertiary interaction with the core of the protein. For the segment 315-321, substituted cysteine residues at 317, 318, 320, and 321 had low reactivity with the spin-label reagent. This segment has the most extensive tertiary interactions yet observed in the rhodopsin extra-membrane sequences at the cytoplasmic surface. Previous studies showed the spontaneous formation of a disulfide bond between cysteine residues at 65 and 316. This result indicates that at least some of the tertiary contacts made in the 315-321 segment are with the sequence connecting transmembrane helices I and II. Photoactivation of rhodopsin produces changes in structure detected by spin labels at 306, 313, and 316. The changes at 313 can be accounted for by movements in the adjacent helix VI.

Similar structures and shared switch mechanisms of the beta2-adrenoceptor and the parathyroid hormone receptor. Zn(II) bridges between helices III and VI block activation

The seven transmembrane helices of serpentine receptors comprise a conserved switch that relays signals from extracellular stimuli to heterotrimeric G proteins on the cytoplasmic face of the membrane. By substituting histidines for residues at the cytoplasmic ends of helices III and VI in retinal rhodopsin, we engineered a metal-binding site whose occupancy by Zn(II) prevented the receptor from activating a retinal G protein, Gt (Sheikh, S. P., Zvyaga, T. A. , Lichtarge, O., Sakmar, T. P., and Bourne, H. R. (1996) Nature 383, 347-350). Now we report engineering of metal-binding sites bridging the cytoplasmic ends of these two helices in two other serpentine receptors, the beta2-adrenoreceptor and the parathyroid hormone receptor; occupancy of the metal-binding site by Zn(II) markedly impairs the ability of each receptor to mediate ligand-dependent activation of Gs, the stimulatory regulator of adenylyl cyclase. We infer that these two receptors share with rhodopsin a common three-dimensional architecture and an activation switch that requires movement, relative to one another, of helices III and VI; these inferences are surprising in the case of the parathyroid hormone receptor, a receptor that contains seven stretches of hydrophobic sequence but whose amino acid sequence otherwise shows no apparent similarity to those of receptors in the rhodopsin family. These findings highlight the evolutionary conservation of the switch mechanism of serpentine receptors and help to constrain models of how the switch works.

Use of a disulfide cross-linking strategy to study muscarinic receptor structure and mechanisms of activation

To gain insight into the molecular architecture of the cytoplasmic surface of G protein-coupled receptors, we have developed a disulfide cross-linking strategy using the m3 muscarinic receptor as a model system. To facilitate the interpretation of disulfide cross-linking data, we initially generated a mutant m3 muscarinic receptor (referred to as m3'(3C)-Xa) in which most native Cys residues had been deleted or substituted with Ala or Ser (remaining Cys residues Cys-140, Cys-220, and Cys-532) and in which the central portion of the third intracellular loop had been replaced with a factor Xa cleavage site. Radioligand binding and second messenger assays showed that the m3'(3C)-Xa mutant receptor was fully functional. In the next step, pairs of Cys residues were reintroduced into the m3'(3C)-Xa construct, thus generating 10 double Cys mutant receptors. All 10 mutant receptors contained a Cys residue at position 169 at the beginning of the second intracellular loop and a second Cys within the C-terminal portion of the third intracellular loop, at positions 484-493. Radioligand binding studies and phosphatidylinositol assays indicated that all double Cys mutant receptors were properly folded. Membrane lysates prepared from COS-7 cells transfected with the different mutant receptor constructs were incubated with factor Xa protease and the oxidizing agent Cu(II)-(1,10-phenanthroline)3, and the formation of intramolecular disulfide bonds between juxtaposed Cys residues was monitored by using a combined immunoprecipitation/immunoblotting strategy. To our surprise, efficient disulfide cross-linking was observed with 8 of the 10 double Cys mutant receptors studied (Cys-169/Cys-484 to Cys-491), suggesting that the intracellular m3 receptor surface is characterized by pronounced backbone fluctuations. Moreover, [35S]guanosine 5'-3-O-(thio)triphosphate binding assays indicated that the formation of intramolecular disulfide cross-links prevented or strongly inhibited receptor-mediated G protein activation, suggesting that the highly dynamic character of the cytoplasmic receptor surface is a prerequisite for efficient receptor-G protein interactions. This is the first study using a disulfide mapping strategy to examine the three-dimensional structure of a hormone-activated G protein-coupled receptor.

C5a receptor activation. Genetic identification of critical residues in four transmembrane helices

Hormones and sensory stimuli activate serpentine receptors, transmembrane switches that relay signals to heterotrimeric guanine nucleotide-binding proteins (G proteins). To understand the switch mechanism, we subjected 93 amino acids in transmembrane helices III, V, VI, and VII of the human chemoattractant C5a receptor to random saturation mutagenesis. A yeast selection identified 121 functioning mutant receptors, containing a total of 523 amino acid substitutions. Conserved hydrophobic residues are located on helix surfaces that face other helices in a modeled seven-helix bundle (Baldwin, J. M., Schertler, G. F., and Unger, V. M. (1997) J. Mol. Biol. 272, 144-164), whereas surfaces predicted to contact the surrounding lipid tolerate many substitutions. Our analysis identified 25 amino acid positions resistant to nonconservative substitutions. These appear to comprise two distinct components of the receptor switch, a surface at or near the extracellular membrane interface and a core cluster in the cytoplasmic half of the bundle. Twenty-one of the 121 mutant receptors exhibit constitutive activity. Amino acids substitutions in these activated receptors predominate in helices III and VI; other activating mutations truncate the receptor near the extracellular end of helix VI. These results identify key elements of a general mechanism for the serpentine receptor switch.

Phosphorylation-independent inhibition of parathyroid hormone receptor signaling by G protein-coupled receptor kinases

Homologous desensitization of G protein-coupled receptors is thought to occur in several steps: binding of G protein-coupled receptor kinases (GRKs) to receptors, receptor phosphorylation, kinase dissociation, and finally binding of beta-arrestins to phosphorylated receptors. It generally is assumed that only the last step inhibits receptor signaling. Investigating the parathyroid hormone (PTH) receptor --> inositol phosphate pathway, we report here that GRKs can inhibit receptor signaling already under nonphosphorylating conditions. GRKs phosphorylated the PTH receptor in membranes and in intact cells; the order of efficacy was GRK2>GRK3>GRK5. Transient transfection of GRKs with the PTH receptor into COS-1 cells inhibited PTH-stimulated inositol phosphate generation. Such an inhibition also was seen with the kinase-negative mutant GRK2-K220R and also for a C-terminal truncation mutant of the PTH receptor that could not be phosphorylated. Several lines of evidence indicated that this phosphorylation-independent inhibition was exerted by an interaction between GRKs and receptors: (a) this inhibition was not mimicked by proteins binding to G proteins, phosducin, and GRK2 C terminus, (b) GRKs caused an agonist-dependent inhibition (= desensitization) of receptor-stimulated G protein GTPase-activity (this effect also was seen with the kinase-inactive GRK2-mutant and the phosphorylation-deficient receptor mutant), and (c) GRKs bound directly to the PTH receptor. These data suggest that signaling by the PTH receptor already is inhibited by the first step of homologous desensitization, the binding of GRKs to the receptors.

Direct identification of the agonist binding site in the human brain cholecystokininB receptor

In investigating the agonist binding site of the human brain cholecystokininB receptor (CCKBR), we employed the direct protein chemical approach using a photoreactive tritiated analogue of sulfated cholecystokinin octapeptide, which contains the p-benzoylbenzoyl moiety at the N-terminus, followed by purification of the affinity-labeled receptor to homogeneity. This probe bound specifically, saturably, and with high affinity (KD = 1.2 nM) to the CCKBR and has full agonistic activity. As the starting material for receptor purification, we used stably transfected HEK 293 cells overexpressing functional CCKBR. Covalent labeling of the WGA-lectin-enriched receptor revealed a 70-80 kDa glycoprotein with a protein core of about 50 kDa. Identification of the agonist binding site was achieved by the application of subsequent chemical and enzymatical cleavage to the purified receptor. A radiolabeled peptide was identified by Edman degradation amino acid sequence analysis combined with MALDI-TOF mass spectrometry. The position of the radioactive probe within the identified peptide was determined using combined tandem electrospray mass spectrometry and peptide mapping. The probe was covalently attached within the sequence L52ELAIRITLY61 that represents the transition between the N-terminal domain and predicted transmembrane domain 1. Using this interaction as a constraint to orientate the ligand within the putative receptor binding site, a model of the CCK-8s-occupied CCKBR was constructed. The hormone was found to be placed in a binding pocket built from both extracellular and transmembrane domains of CCKBR with its N-terminus mainly interacting with residues Arg57 and Tyr61.

The variable and conserved interfaces of modeled olfactory receptor proteins

The accumulation of hundreds of olfactory receptor (OR) sequences, along with the recent availability of detailed models of other G-protein-coupled receptors, allows us to analyze the OR amino acid variability patterns in a structural context. A Fourier analysis of 197 multiply aligned olfactory receptor sequences showed an alpha-helical periodicity in the variability profile. This was particularly pronounced in the more variable transmembranal segments 3, 4, and 5. Rhodopsin-based homology modeling demonstrated that the inferred variable helical faces largely point to the interior of the receptor barrel. We propose that a set of 17 hypervariable residues, which point to the barrel interior and are more extracellularly disposed, constitute the odorant complementarity determining regions. While 12 of these residues coincide with established ligand-binding contact positions in other G-protein-coupled receptors, the rest are suggested to form an olfactory-unique aspect of the binding pocket. Highly conserved olfactory receptor-specific sequence motifs, found in the second and third intracellular loops, may comprise the G-protein recognition epitope. The prediction of olfactory receptor functional sites provides concrete suggestions of site-directed mutagenesis experiments for altering ligand and G-protein specificity.

Binding of arrestin to cytoplasmic loop mutants of bovine rhodopsin

The binding of arrestin to rhodopsin is a multistep process that begins when arrestin interacts with the phosphorylated C terminus of rhodopsin. This interaction appears to induce a conformational change in arrestin that exposes a high-affinity binding site for rhodopsin. Several studies in which synthetic peptides were used have suggested that sites on the rhodopsin cytoplasmic loops are involved in this interaction. However, the precise amino acids on rhodopsin that participate in this interaction are unknown. This study addresses the role of specific amino acids in the cytoplasmic loops of rhodopsin in binding arrestin through the use of site-directed mutagenesis and direct binding assays. A series of alanine mutants within the three cytoplasmic loops of rhodopsin were expressed in HEK-293 cells, reconstituted with 11-cis-retinal, prephosphorylated with rhodopsin kinase, and examined for their ability to bind in vitro-translated, 35S-labeled arrestin. Mutations at Asn-73 in loop I as well as at Pro-142 and Met-143 in loop II resulted in dramatic decreases in the level of arrestin binding, whereas the level of phosphorylation by rhodopsin kinase was similar to that of wild-type rhodopsin. The results indicate that these amino acids play a significant role in arrestin binding.

Isolated transmembrane helices arranged across a membrane: computational studies

A computational procedure for predicting the arrangement of an isolated helical fragment across a membrane was developed. The procedure places the transmembrane helical segment into a model triple-phase system 'water-octanol-water'; pulls the segment through the membrane, varying its 'global' position as a rigid body; optimizes the intrahelical and solvation energies in each global position by 'local' coordinates (dihedral angles of side chains); and selects the lowest energy global position for the segment. The procedure was applied to 45 transmembrane helices from the photosynthetic reaction center from Rhodopseudomonas viridis, cytochrome c oxidase from Paracoccus denitrificans and bacteriorhodopsin. In two thirds of the helical fragments considered, the procedure has predicted the vertical shifts of the fragments across the membrane with an accuracy of -0.15 +/- 3.12 residues compared with the experimental data. The accuracy for the remaining 15 fragments was 2.17 +/- 3.07 residues, which is about half of a helix turn. The procedure predicts the actual membrane boundaries of transmembrane helical fragments with greater accuracy than existing statistical methods. At the same time, the procedure overestimates the tilt values for the helical fragments.

Intron splice sites of Papilio glaucus PglRh3 corroborate insect opsin phylogeny

Full-length cDNA clones encoding the PglRh3 opsin from the tiger swallowtail butterfly Papilio glaucus were isolated from cDNA synthesized from adult head tissue total RNA. This cDNA consists of 1679 nucleotides and contains a single open reading frame predicted to be 379 amino acids in length. PCR amplification of genomic DNA with primers spanning the coding region yielded a single 2760bp fragment which was sequenced. The PglRh3 gene has nine exons and eight introns, four of which are in unique locations relative to the positions of introns in other known insect opsin sequences. Phylogenetic analyses of amino acid and nucleotide sequence data places PglRh3 within a clade of insect visual pigments thought to be sensitive to long wavelengths of light. The genomic structure of PglRh3 is the first characterized from a member of this opsin clade. Three PglRh3 intron positions are shared with Drosophila Rh1, and one of these is also shared with Drosophila Rh2. By contrast, none of the known intron locations in a clade of anciently diverged ultraviolet- and blue-sensitive visual pigments are shared by P. glaucus PglRh3, Drosophila Rh1 or Rh2. The placement of introns within opsin genes therefore independently supports the clustering of a putatively long-wavelength-sensitive clade with a clade of blue-green-sensitive visual pigments.

The functional topography of transmembrane domain 3 of the M1 muscarinic acetylcholine receptor, revealed by scanning mutagenesis

Alanine-scanning mutagenesis has been applied to residues 100-121 in transmembrane domain 3 of the M1 muscarinic acetylcholine receptor. This study complements a previous investigation of the triad Asp122-Arg123-Tyr124 (Lu, Z-L., Curtis, C. A., Jones, P. G., Pavia, J., and Hulme, E. C. (1997) Mol. Pharmacol. 51, 234-241). The results demonstrate the alpha-helical secondary structure of the domain and suggest its orientation with respect to the other transmembrane domains. The C-terminal part of the helix appears to be largely buried within the receptor structure. On its surface, there is a patch of three residues, Val113, Leu116, and Ser120, which may form intramolecular contacts that help to stabilize the inactive ground state of the receptor. Mutagenic disruption of these increased agonist affinity and signaling efficacy. In two cases (L116A and S120A), this led to constitutive activation of the receptor. Parallel to the helix axis and spanning the whole transmembrane region, a distinct strip of residues on one face of transmembrane domain 3 forms intermolecular (acetylcholine-receptor, receptor-G protein) or intrareceptor bonds that contribute to the activated state. The binding of acetylcholine may destabilize the first set of contacts while favoring the formation of the second.

Structure-function analysis of muscarinic receptors and their associated G proteins

Each member of the muscarinic receptor family (M1-M5) can interact only with a limited subset of the many structurally closely related heterotrimeric G proteins expressed within a cell. To understand how this selectivity is achieved at a molecular level, we have used the G(i/0)-coupled M2 and the Gq/11-coupled M3 muscarinic receptors as model systems. We developed a genetic strategy involving the coexpression of wild type or mutant muscarinic receptors with hybrid or mutant G protein alpha subunits to identify specific, functionally relevant receptor/G protein contact sites. This approach led to the identification of N- and C-terminal amino acids on alpha(q) and alpha(i) that are critical for maintaining proper receptor/G protein coupling. Moreover, several receptor sites were identified that are likely to be contacted by these functionally critical G alpha residues. To gain deeper insight into muscarinic receptor structure, we recently developed a cysteine disulfide cross-linking strategy, using the M3 muscarinic receptor as a model system. Among other structural modifications, this approach involves the removal of most native cysteine residues by site-directed mutagenesis, the insertion of three factor Xa cleavage sites into the third intracellular loop, and systematic 'reintroduction' of pairs of cysteine residues. Following treatment of receptor-containing membrane preparations with factor Xa and oxidizing agents, disulfide cross-linked products can be identified by immunoprecipitation and immunoblotting studies. This approach should greatly advance our knowledge of the molecular architecture of muscarinic and other G protein-coupled receptors.

Structure and function in rhodopsin: further elucidation of the role of the intradiscal cysteines, Cys-110, -185, and -187, in rhodopsin folding and function

The disulfide bond between Cys-110 and Cys-187 in the intradiscal domain is required for correct folding in vivo and function of mammalian rhodopsin. Misfolding in rhodopsin, characterized by the loss of ability to bind 11-cis-retinal, has been shown to be caused by an intradiscal disulfide bond different from the above native disulfide bond. Further, naturally occurring single mutations of the intradiscal cysteines (C110F, C110Y, and C187Y) are associated with retinitis pigmentosa (RP). To elucidate further the role of every one of the three intradiscal cysteines, mutants containing single-cysteine replacements by alanine residues and the above three RP mutants have been studied. We find that C110A, C110F, and C110Y all form a disulfide bond between C185 and C187 and cause loss of retinal binding. C185A allows the formation of a C110-C187 disulfide bond, with wild-type-like rhodopsin phenotype. C187A forms a disulfide bond between C110 and C185 and binds retinal, and the pigment formed has markedly altered bleaching behavior. However, the opsin from the RP mutant C187Y forms no rhodopsin chromophore.

NMR of drugs and ligands bound to membrane receptors

NMR methods are now able to give detailed structural, dynamic and electronic information about drugs and ligands while constrained at their site of action in membrane-embedded receptors, information which is essential for mechanistic descriptions of their action and design of new ligands. Using solid state NMR methods, a peptic ulcer drug analogue has been described at atomic resolution (to +/- 0.3 A between two atoms) at its site of action in the gastric H+/K+-ATPase, and the aromaticity of the agonist binding site of the nicotinic acetylcholine receptor has been demonstrated, with both targets in functionally competent membranes under conditions similar to those used in screening assays. G-protein-coupled receptor ligands and prosthetic groups are also being resolved using NMR methods.

Conformational changes in rhodopsin. Movement of helix f detected by site-specific chemical labeling and fluorescence spectroscopy

A recent proposal for the formation of functionally active rhodopsin has placed critical importance on a movement of one of its transmembrane helices (Farrens, D. L., Altenbach, C., Yang, K., Hubbell, W. L., and Khorana, H. G. (1996) Science 274, 768-770). We investigated this hypothesis using a series of eight rhodopsin mutants containing single reactive cysteine residues in the region (helix F) where movement was previously detected. The cysteine mutants were studied in two ways, by measuring their reactivity to a cysteine-specific reagent (PyMPO-maleimide), and by labeling the cysteines with a fluorescent label (monobromobimane) followed by fluorescence spectroscopic analysis. The chemical reactivity data showed sequence-specific variations in reactivity for the mutants in the dark state, resulting in a pattern suggestive of an alpha helix. Interestingly, only upon photoactivation to the MII form did residues found on the inner "face" of this helix react with the PyMPO-maleimide. The ability of the dark state mutants to react with the fluorescent label monobromobimane followed a similar pattern. Furthermore, fluorescence measurements indicate that a bimane label on the inner face of the helix (at V250C) detects changes in the polarity of its environment and accessibility to a fluorescence quenching agent upon MII formation. Viewed together, the data provide further direct evidence that rhodopsin activation involves a conformational change at helix F.

Use of a sandwich enzyme-linked immunosorbent assay strategy to study mechanisms of G protein-coupled receptor assembly

All G protein-coupled receptors are predicted to consist of a bundle of seven transmembrane helices (I-VII) that are connected by various extracellular and intracellular loops. At present, little is known about the molecular interactions that are critical for the proper assembly of the transmembrane receptor core. To address this issue, we took advantage of the ability of coexpressed N- and C-terminal m3 muscarinic receptor fragments to form functional receptor complexes (Schöneberg, T., Liu, J., and Wess, J. (1995) J. Biol. Chem. 270, 18000-18006). As a model system, we used two polypeptides, referred to as m3-trunk and m3-tail, that were generated by "splitting" the m3 muscarinic receptor within the third intracellular loop. We initially demonstrated, by employing a sandwich enzyme-linked immunosorbent assay strategy, that the two receptor fragments directly associate with each other when coexpressed in COS-7 cells. Additional studies with N- and C-terminal fragments derived from other G protein-coupled receptors showed that fragment association was highly receptor-specific. In subsequent experiments, the sandwich enzyme-linked immunosorbent assay system was used to identify amino acids that are required for proper fragment (receptor) assembly. Point mutations were introduced into m3-trunk or m3-tail, and the ability of these mutations to interfere with efficient fragment assembly was examined. These studies showed that three highly conserved proline residues (located in transmembrane helices V, VI, and VII) are essential for proper fragment association (receptor assembly). Interestingly, incubation with classical muscarinic agonists and antagonists or allosteric ligands led to significant increases in the efficiency of fragment association (particularly upon substitution of the conserved proline residues), indicating that all of these ligands can act as "anchors" between the m3-trunk and m3-tail fragments. The approach described here should be generally applicable to gain deeper insight into the molecular mechanisms governing G protein-coupled receptor structure and assembly.

Effects of phosphorylation on the structure of the G-protein receptor rhodopsin

Upon activation by light, rhodopsin is subject to phosphorylation by rhodopsin kinase at serine and threonine residues in the carboxyl terminal region of the protein. A 19 amino acid peptide that corresponds to the carboxyl terminal end of rhodopsin (residues 330-348) and contains these phosphorylation sites was synthesized. The structure of this peptide was determined using two-dimensional proton NMR. The structure of this peptide was similar to that determined for this region in peptides corresponding to the carboxyl 33 and 43 amino acids of rhodopsin. The effect of phosphorylation on the structure of the carboxyl terminal domain of rhodopsin was determined by solving the solution structures of the peptide containing residues 330-348 with phosphorylation at one (residue 343), three (residues 343, 338, and 334) and seven residues (residues 334, 335, 336, 338, 340, 342, 343). These data indicate that the major structural change occurs upon phosphorylation of the first residue, and that an additional structural change occurs with seven phosphates.

Molecular modelling studies on G protein-coupled receptors: from sequence to structure?

In pharmaceutical research G protein-coupled receptors (GPCR) emerged as a superfamily of prominent drug targets. Extensive protein sequence analyses on GPCRs revealed a common protein topology consisting of a membrane-spanning seven-helix bundle, which is believed to accommodate the binding site for low-molecular weight ligands. Enormous efforts are undertaken to generate GPCR structural models by means of molecular modelling, since these have already been shown to aid the process of lead structure finding and optimisation in that they provide atomistic models for structure-based drug design approaches. One of the most critical steps in modelling the transmembrane domains of GPCRs is the assignment of the putative transmembrane sequence stretches from multiple protein sequence alignment analyses. This study focuses on the comparative evaluation of protein sequence analysis tools, such as periodicity analyses, multiple sequence analyses or directional helix descriptors, especially developed for modelling the 7TM domains of GPCRs. In this context we will demonstrate that from application of different methods contradictory results can be obtained for the identification of the putative transmembrane sequence stretches of peptide-binding GPCRs, as exemplified with a comprehensive protein sequence analysis study based on the most prominent members of that receptor class (angiotensin II, CCK/gastrin, interleukin 8, endothelin, etc.).

Molecular basis of receptor/G-protein-coupling selectivity

Molecular cloning studies have shown that G-protein-coupled receptors form one of the largest protein families found in nature, and it is estimated that approximately 1000 different such receptors exist in mammals. Characteristically, when activated by the appropriate ligand, an individual receptor can recognize and activate only a limited set of the many structurally closely related heterotrimeric G-proteins expressed within a cell. To understand how this selectivity is achieved at a molecular level has become the focus of an ever increasing number of laboratories. This review provides an overview of recent structural, molecular genetic, biochemical, and biophysical studies that have led to novel insights into the molecular mechanisms governing receptor-mediated G-protein activation and receptor/G-protein coupling selectivity.

Identification of a polar region in transmembrane domain 6 that regulates the function of the G protein-coupled alpha-factor receptor

The alpha-factor pheromone receptor (Ste2p) of the yeast Saccharomyces cerevisiae belongs to the family of G protein-coupled receptors that contain seven transmembrane domains (TMDs). Because polar residues can influence receptor structure by forming intramolecular contacts between TMDs, we tested the role of the five polar amino acids in TMD6 of the alpha-factor receptor by mutating these residues to nonpolar leucine. Interestingly, a subset of these mutants showed increased affinity for ligand and constitutive receptor activity. The mutation of the most polar residue, Q253L, resulted in 25-fold increased affinity and a 5-fold-higher basal level of signaling that was equal to about 19% of the alpha-factor induced maximum signal. Mutation of the adjacent residue, S254L, caused weaker constitutive activity and a 5-fold increase in affinity. Comparison of nine different mutations affecting Ser254 showed that an S254F mutation caused higher constitutive activity, suggesting that a large hydrophobic amino acid residue at position 254 alters transmembrane helix packing. Thus, these studies indicate that Gln253 and Ser254 are likely to be involved in intramolecular interactions with other TMDs. Furthermore, Gln253 and Ser254 fall on one side of the transmembrane helix that is on the opposite side from residues that do not cause constitutive activity when mutated. These results suggest that Gln253 and Ser254 face inward toward the other TMDs and thus provide the first experimental evidence to suggest the orientation of a TMD in this receptor. Consistent with this, we identified two residues in TMD7 (Ser288 and Ser292) that are potential contact residues for Gln253 because mutations affecting these residues also cause constitutive activity. Altogether, these results identify a new domain of the alpha-factor receptor that regulates its ability to enter the activated conformation.

Conformational analysis and automated receptor docking of selective arylacetamide-based kappa-opioid agonists

The three-dimensional structure, dynamics, and binding modes of representative kappa-opioid agonists of the arylacetamide class (U50, 488; U69,593; U62,066; CI-977; ICI199,441; ICI197,067; BRL52,537; and BRL52,656) have been investigated using molecular modeling techniques. Systematic exploration of the conformational space of the ligand combined with molecular dynamics (MD) simulations in water revealed consistent conformational preferences for all the kappa-agonists in this series. The results were further compared with available X-ray and 1D- and 2D-NMR data to identify potential "lead" conformers for molecular docking. Ligand binding modes were initially determined using automated docking of two of the ligands (U50,488 and BRL52,537) to the kappa-opioid receptor. Extrapolation of the predicted binding mode to other members in this ligand series revealed similar docking preferences, with each ligand docked along the receptor helical axis. The binding modes were further refined using MD simulations of the receptor-ligand complexes. The results show a that salt bridge is formed between the amino proton of the ligands and the carboxylate group of Asp138 in TM3. This interaction most likely serves as a key anchoring point for the agonist association. Additional ligand contacts were noted with kappa-specific residues Ile294, Leu295, and Ala298, which may, in part, explain the kappa-selectivity in this series. In comparing the arylacetamides with opiate-based ligands, no evidence was found to link these classes through a common binding motif (except for the ion pair). The binding site model was also applied to explain the enantiomeric preference of U50,488 and to provide insight to the mu/kappa-selectivity of representative ligands in this series. Overall, the results provide a structure-based rationale for ligand recognition that is consistent both with site-directed mutagenesis experiments and structure-function relationship data.

A conserved tyrosine residue (Y601) in transmembrane domain 5 of the human thyrotropin receptor serves as a molecular switch to determine G-protein coupling

In the human thyroid, the wild-type thyrotropin receptor (TSHR) couples to adenylyl cyclase and phospholipase C and constitutively increases intracellular cAMP levels. The first human TSHR sequence submitted differs from subsequently cloned wild-type receptors by an exchange of a conserved Y residue within transmembrane domain 5 (TM5) for an H residue. We did not detect the Y601H mutant in 263 European individuals, but confirmed the homozygous occurrence of TSHR-Y601. Expression of TSHR-Y601H in COS-7 cells revealed a loss of constitutive cAMP production and selective lack of TSH-induced phosphoinositide hydrolysis, whereas agonist-induced cAMP formation remained unaltered. Analysis of several mutant receptors (Y601A, Y601D, Y601F, Y601K, Y601P, Y601S, Y601W, Y601Delta) did not show restoration of constitutive activity and dual signaling, thus suggesting a functional role of a properly spaced hydroxyl group at position 601. Molecular modeling revealed that the formation of a hydrogen bond between the hydroxyl group of Y601 in TM5 and the carbonyl oxygen of A623 in the peptide backbone of TM6 is critical for the receptor to adopt active conformations that impart wild-type signaling properties. Our findings indicate that multiple active receptor states underlie coupling of a G-protein-coupled receptor to different G-proteins.

Structure of rhodopsin

Two-dimensional crystals of rhodopsin were studied to determine the arrangement of the transmembrane alpha helices. A combination of electron cryo-microscopy, image processing and electron crystallography was used to extract amplitudes and phases from images, and a three-dimensional map to a resolution of 7.5 A was calculated. Density peaks for all seven transmembrane helices were observed and the helix axes for all seven helices could be estimated. Near the intracellular side, which interacts with the G protein transducin, we observed three layers of helices arranged differently from bacteriorhodopsin. The arrangement opens up towards the extracellular side forming a cavity that serves as the binding pocket for the retinal. This cavity is closed towards the intracellular side by the long and highly tilted helix 3, and must be closed towards the extracellular side by the loop linking helices 4 and 5 that is linked by a disulphide bridge to the extracellular end of helix 3.

Molecular basis of V2 vasopressin receptor/Gs coupling selectivity

The molecular mechanisms governing the coupling selectivity of G protein-coupled receptors activated by peptide ligands are not well understood. To shed light on this issue, we have used the Gq/11-linked V1a and the Gs-coupled V2 vasopressin peptide receptors as model systems. To explore the structural basis underlying the ability of the V2 receptor to selectively recognize Gs, we systematically substituted distinct V2 receptor segments (or single amino acids) into the V1a receptor and studied whether the resulting hybrid receptors gained the ability to mediate hormone-dependent cAMP production. This strategy appeared particularly attractive since hormone stimulation of the V1a receptor has virtually no effect on intracellular cAMP levels. Functional analysis of a large number of mutant receptors transiently expressed in COS-7 cells indicated that the presence of V2 receptor sequence at the N terminus of the third intracellular loop is critical for efficient activation of Gs. More detailed mutational analysis of this receptor region showed that two polar V2 receptor residues, Gln225 and Glu231, play key roles in Gs recognition. In addition, a short sequence at the N terminus of the cytoplasmic tail was found to make an important contribution to V2 receptor/Gs coupling selectivity. We also made the novel observation that the efficiency of V2 receptor/Gs coupling can be modulated by the length of the central portion of the third intracellular loop (rather than the specific amino acid sequence within this domain). These findings provide novel insights into the molecular mechanisms regulating peptide receptor/G protein coupling selectivity.

Characterisation of an improved two-dimensional p22121 crystal from bovine rhodopsin

Dialysis of rhodopsin isolated from bovine rod outer segments resulted in the formation of a new two-dimensional crystal form suitable for electron crystallography. The crystals obtained were tubular or single layers and showed p22121 symmetry (a=60.6(+/-0.8) A, b=86.3(+/-1.6) A). For the first time the size and order of the crystals allowed us to take electron diffraction patterns showing spots to a resolution of about 3.5 A. Images were recorded at liquid nitrogen temperature using a high voltage field emission electron microscope. Out of a large number of images 20 crystalline areas were selected and processed with the MRC image processing software. A projection structure of bovine rhodopsin to 5 A resolution was calculated using amplitudes and phases extracted from these images. The achieved resolution exceeds the resolution of all previously obtained structures of frog, bovine and squid rhodopsin crystals. In this map small differences are observed compared to the previous maps. Helix 5 seems to be even more highly tilted and between the arc-shaped feature and helix 5 a peak is present suggesting that helix 3 is prolonging this feature towards helix 5. These observations are in agreement with the latest model for the three-dimensional arrangement of rhodopsin. The resolution achieved as well as the availability of electron diffraction data suggest that there is a good possibility to collect data from tilted crystals and calculate an improved three-dimensional structure of rhodopsin.

Functional analysis of GnRH receptor ligand binding using biotinylated GnRH derivatives

The objective of this study was to determine whether the gonadotrophin-releasing hormone (GnRH) ligand binds to the GnRH receptor (GnRH-R) with either the N- and C-termini or the beta-II turn pointing towards the cell. The functionality of GnRH and two biotinylated GnRH derivatives, biotin [D-Lys6]GnRH and biotin [Gln1]GnRH biotinylated at positions 6 and 1, respectively was assessed. Streptavidin was also used in combination with these peptides to investigate the effects of the steric hindrance caused by this molecule on ligand binding when bound to the biotin molecules at the two positions. GnRH bound to the receptor with high affinity, which was not affected by the addition of streptavidin. Both the biotinylated derivatives bound to the receptor though with lower affinities than GnRH. The biotin [D-Lys6]GnRH-streptavidin complex bound to the receptor albeit with lower affinity compared to biotin [D-Lys6]GnRH only, although it maintained its ability to cause receptor internalisation. The ability of the biotin [Gln1]GnRH to bind to the receptor was abolished in the presence of excess streptavidin. Both GnRH and biotin [D-Lys6]GnRH stimulated total inositol phosphate production whereas biotin [Gln1]GnRH exhibited GnRH antagonist activity. It appears that the small biotin molecule can be accommodated within the binding pore when attached to position 1 of the ligand but not when complexed to streptavidin. The fact that biotin [D-Lys6]GnRH maintains functionality when complexed to streptavidin while biotin [Gln1]GnRH does not, suggests that the N- and possibly the C-termini are required for receptor binding. Thus the most likely binding orientation for the ligand is with the N- and C-termini pointing inwards with the residue at position 6 pointing away from the binding site.

The second intracellular loop of the m5 muscarinic receptor is the switch which enables G-protein coupling

We have completed a systematic search of the intracellular loops of a muscarinic acetylcholine receptor for domains that govern G-protein coupling. A unique feature of the second intracellular (i2) loop was an ordered cluster of residues where diverse substitutions cause constitutive activation. A second group of residues in i2 was identified where mutations compromised receptor/G-protein coupling. The residues of each group alternate and are spaced three to four positions apart, suggesting an alpha-helical structure where these groups form opposing faces of the helix. We propose that the constitutively activating face normally constrains the receptor in the "off-state," while the other face couples G-proteins in the "on-state." Therefore, the i2 loop functions as the switch enabling G-protein activation.

Combined biophysical and biochemical information confirms arrangement of transmembrane helices visible from the three-dimensional map of frog rhodopsin

The electron density projection map of frog rhodopsin at 6 A resolution had been until recently the most direct evidence for the three-dimensional structure of a transmembrane domain of any G-protein-coupled receptor. Only three out of seven transmembrane helices are clearly defined, whilst the other four are hidden in a patch of unresolved electron density. A model of the seven-helix bundle has been created by generating positions and orientations for the four unresolved helices through performing a conformational search directed by structural restraints derived from other experimental data. These four helices are significantly tilted with respect to the membrane normal, and the cytosolic end of helix C is inserted between helices D and E. These calculations produce positions and orientations for these additional helices that are consistent with the recently published low-resolution three-dimensional map, and provide a template for more detailed modelling of rhodopsin structure and function.

Identification of a ligand-dependent switch within a muscarinic receptor

G-protein-coupled receptors spontaneously switch between active and inactive conformations. Agonists stabilize the active conformation, whereas antagonists stabilize the inactive conformation. In a systematic search for residues that participate in receptor function, several regions of the m5 muscarinic receptor were randomly mutated and tested for their functional properties. Mutations spanning one face of transmembrane 6 (TM6) were found to induce high levels of receptor activity in the absence of agonists (constitutive activity). The same face of TM6 contained several residues crucial for receptor activation by agonists and one residue identified as a contact site for both agonists and antagonists. In addition, one mutation induced agonist-like responses from the receptor when exposed to classical antagonists. These results suggest that TM6 is a switch that defines the activation state of the receptor, and that ligand interactions with TM6 stabilize the receptor in either an active or an inactive conformation.

Opioid receptor three-dimensional structures from distance geometry calculations with hydrogen bonding constraints

Three-dimensional structures of the transmembrane, seven alpha-helical domains and extracellular loops of delta, mu, and kappa opioid receptors, were calculated using the distance geometry algorithm, with hydrogen bonding constraints based on the previously developed general model of the transmembrane alpha-bundle for rhodopsin-like G-protein coupled receptors (Biophys. J. 1997. 70:1963). Each calculated opioid receptor structure has an extensive network of interhelical hydrogen bonds and a ligand-binding crevice that is partially covered by a beta-hairpin formed by the second extracellular loop. The binding cavities consist of an inner "conserved region" composed of 18 residues that are identical in delta, mu, and kappa opioid receptors, and a peripheral "variable region," composed of 19 residues that are different in delta, mu, and kappa subtypes and are responsible for the subtype specificity of various ligands. Sixteen delta-, mu-, or kappa-selective, conformationally constrained peptide and nonpeptide opioid agonists and antagonists and affinity labels were fit into the binding pockets of the opioid receptors. All ligands considered have a similar spatial arrangement in the receptors, with the tyramine moiety of alkaloids or Tyr1 of opioid peptides interacting with conserved residues in the bottom of the pocket and the tyramine N+ and OH groups forming ionic interactions or H-bonds with a conserved aspartate from helix III and a conserved histidine from helix VI, respectively. The central, conformationally constrained fragments of the opioids (the disulfide-bridged cycles of the peptides and various ring structures in the nonpeptide ligands) are oriented approximately perpendicular to the tyramine and directed toward the extracellular surface. The results obtained are qualitatively consistent with ligand affinities, cross-linking studies, and mutagenesis data.

A proposed structure for transmembrane segment 7 of G protein-coupled receptors incorporating an asn-Pro/Asp-Pro motif

Transmembrane segment (TMS) 7 has been shown to play an important role in the signal transduction function of G-protein-coupled receptors (GPCRs). Although transmembrane segments are most likely to adopt a helical structure, results from a variety of experimental studies involving TMS 7 are inconsistent with it being an ideal alpha-helix. Using results from a search of the structure database and extensive simulated annealing Monte Carlo runs with the new Conformational Memories method, we have identified the conserved (N/D)PxxY region of TMS 7 as the major determinant for deviation of TMS 7 from ideal helicity. The perturbation consists of an Asx turn and a flexible "hinge" region. The Conformational Memories procedure yielded a model structure of TMS 7 which, unlike an ideal alpha-helix, is capable of accommodating all of the experimentally derived geometrical criteria for the interactions of TMS 7 in the transmembrane bundle of GPCRs. In the context of the entire structure of a transmembrane bundle model for the 5HT2a receptor, the specific perturbation of TMS 7 by the NP sequence suggests a structural hypothesis for the pattern of amino acid conservation observed in TMS 1, 2, and 7 of GPCRs. The structure resulting from the incorporation of the (N/D)P motif satisfies fully the H-bonding capabilities of the 100% conserved polar residues in these TMSs, in agreement with results from mutagenesis experiments. The flexibility introduced by the specific structural perturbation produced by the (NP/DP) motif in TMS 7 is proposed to have a significant role in receptor activation.

G-protein diseases furnish a model for the turn-on switch

How does a trimeric G protein on the inside of a cell membrane respond to activation by a transmembrane receptor? G-protein mutations in patients with hypertension and inherited endocrine disorders enhance or block signals from stimulated receptors. In combination with three-dimensional crystal structures and results from biochemical experiments, the phenotypes produced by these mutations suggest a model for the molecular activation mechanism that relays hormonal and sensory signals transmitted by many transmembrane receptors.

Structure-function analysis of muscarinic acetylcholine receptors

The structural basis underlying the G protein coupling selectivity of different muscarinic receptor subtypes was analyzed by using a combined molecular genetic/biochemical approach. These studies led to the identification of key residues on the receptors as well as the associated G proteins that are critically involved in determining proper receptor/G protein recognition. Mutational analysis of the m3 muscarinic receptor showed that most native cysteine residues are not required for productive receptor/G protein coupling. The putative extracellular disulfide bond was found to be essential for efficient trafficking of the receptor protein to the cell surface but not for receptor-mediated G protein activation.

Scanning mutagenesis of transmembrane domain 3 of the M1 muscarinic acetylcholine receptor

Scanning mutagenesis of transmembrane domain 3 of the M1 muscarinic acetylcholine receptor has revealed a highly-differentiated alpha-helical structure. Lipid-facing residues are distinguished from a patch of residues which selectively stabilise the ground state of the receptor, and from a band of amino acids extending the full length of the helix, which contribute to the active agonist-receptor-G protein complex. The most important residues are strongly conserved in the GPCR superfamily.

Spectroscopic evidence for interaction between transmembrane helices 3 and 5 in rhodopsin

Recent molecular models of rhodopsin (Rho) propose a specific interaction between transmembrane (TM) helices 3 and 5, which appears to be mediated by amino acid residues Glu122 and His211 on TM helices 3 and 5, respectively. To test this proposed interaction, four single-site histidine replacement mutants (H100N, H152N, H211N, and H211F), two single-site glutamic acid replacement mutants (E122Q and E122A), and three double-site replacement mutants (E122Q/H211F, E122Q/H211N, and E122A/H211F) of Rho were prepared. The expressed mutant pigments reconstituted into membranes were studied by FTIR difference spectroscopy addressing especially the transition to metarhodopsin I (MI). It is shown that the lipid environment influences bands typical of the MI state. Spectra of mutants with substituted Glu122 allowed assignments of the C=O stretch of protonated Glu122 in the dark state and in MI of Rho. Mutation of His211, but not of other histidine residues, affects these vibrational modes assigned to Glu122. In addition, replacements of His211 affect protein modes that are proposed to arise from a third, hydroxyl-bearing group, which also interacts with Glu122. These modes are influenced as well when Glu122 is replaced by Ala in mutant E122A but not when it is replaced by Gln in mutant E122Q. These results provide direct experimental evidence for an interaction between TM helices 3 and 5 in Rho, which is mediated by Glu122 and His211.

Heterologous expression of G-protein-coupled receptors: human opioid receptors under scrutiny

G-protein-coupled receptors whose topology shows seven transmembrane domains form the largest known family of receptors involved in higher organism signal transduction. Despite increasing knowledge on the functioning mechanisms of these receptors, almost no structural data are available but only a few models. Structural studies using a wide range of physical and biochemical techniques may require fairly large (up to several milligrams) amounts of purified protein. Since such quantities are not naturally available, overexpression is prerequisite. Heterologous expression systems are then assayed for maximal production of a protein facsimile. Heterologous systems may also provide interesting alternatives for receptor functional studies in a different cellular context. Opioid receptors will be used as an example to discuss aspects related to the choice and suitability of several different expression systems for the intended analysis of G-protein-coupled receptor properties. General implications will be outlined.

Ab initio modeling and molecular dynamics simulation of the alpha 1b-adrenergic receptor activation

This work describes the ab initio procedure employed to build an activation model for the alpha 1b-adrenergic receptor (alpha 1b-AR). The first version of the model was progressively modified and complicated by means of a many-step iterative procedure characterized by the employment of experimental validations of the model in each upgrading step. A combined simulated (molecular dynamics) and experimental mutagenesis approach was used to determine the structural and dynamic features characterizing the inactive and active states of alpha 1b-AR. The latest version of the model has been successfully challenged with respect to its ability to interpret and predict the functional properties of a large number of mutants. The iterative approach employed to describe alpha 1b-AR activation in terms of molecular structure and dynamics allows further complications of the model to allow prediction and interpretation of an ever-increasing number of experimental data.

Do transmembrane protein superfolds exist?

A reliable and widely used transmembrane protein structure prediction algorithm was applied to five representative genomic sequence data sets in order to re-examine the hypothesis that in contrast to globular proteins there are no favored transmembrane protein fold families. When the number of predicted membrane spanning segments and the topology of these segments is taken into account then definite biases are observed which suggest that certain transmembrane topologies are significantly more common than others.

Mechanism of spectral tuning in the dolphin visual pigments

The absorption maxima of both rod and cone visual pigments of the bottlenose dolphin (Tursiops truncatus) are blue-shifted relative to those of terrestrial mammals. A comparison of the sequence of the dolphin rod photopigment gene with that of the bovine rod suggests that, fo the 28 nonidentical amino acids, three amino acid substitutions at positions 83, 292, and 299 in the dolphin rod pigment are responsible for the 10 nm blue shift in absorption maxima. A similar comparison of the dolphin long-wavelength sensitive (LWS) cone photopigment gene with those of the human LWS cones suggests that a single substitution at position 292 (using the convention of rhodopsin numbering) in the dolphin LWS cone pigment results in a blue shift in absorption maxima. A mutagenesis study reveals that the combination of the three dolphin specific substitutions in the bovine rod pigment (83D to 83N, 292A to 292S, and 299A to 299S) causes a blue shift from the wild-type lambdamax of 499 nm to 389 nm. The single substitution in the dolphin LWS cone pigment (292S to 292A) causes a red shift from the wild-type lambdamax of 524 nm to 552 nm. The interactions of the three amino acids identified in the rod pigment with the chromophore may be a general mechanism for blue shifting in rod visual pigments. Furthermore, the single substitution in the dolphin LWS opsin gene is a novel mechanism of wavelength modulation in mammalian LWS pigments.