Muscarinic acetylcholine receptors: structure and function

Nervous System Membrane Proteins

~

Muscarinic acetylcholine receptors: structure and function Edward C. Hulme, Eleonora Kurtenbach and Carol A. M. Curtis Division of Physical Biochemistry, National Institute for Medical Research, Mill Hill, London NW7 IAA, U.K.

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Introduction Muscarinic acetylcholine receptors (mAChRs) are physiologically important members of the class of G-protein-coupled receptors. These transmembrane proteins exert their intracellular effects by the selective activation of members of a set of heterotrimeric GTP-binding (G-) proteins associated with the inner surface of the cell membrane [l]. The consensus view of the activation mechanism envisages a cascade process incorporating the steps briefly summarized below: (i) Binding of the agonist to the receptor induces or stabilizes a conformationally distinct activated state of the receptor. (ii) The receptor-agonist complex catalyses the release of GDP from the a-subunit of the cognate G-protein, and stimulates the binding of GTP, whose concentration exceeds that of GDP in the intracellular milieu. Catalysis proceeds through a transition state in which the receptor binding site attains a very high affinity locked state, whilst the GTP binding site of the G-protein a-subunit passes through a readily exchangeable open state. This transition state complex breaks down rapidly to liberate the activated, GTP-liganded G-protein, and to regenerate the agonist-receptor complex, which can engage in further catalytic cycles. (iii) The activated G-protein heterotrimer has been proposed to dissociate, freeing the a-subunit from the By-subunit complex (although this view has been questioned [2]). The a-subunit-GTP complex in turn acts catalytically to activate or inhibit effector molecules. In the case of the mAChRs, the repertoire includes the inhibition of adenylate cyclase, the stimulation of phosphoinositide bisphosphate-specific phospholipase C, stimulation of a certain class of potassium channels, possible effects on other ionic conductances and on phospholipase A, activity (see [3] for a review). (iv) The catalytic activity of the activated G-protein a-subunit is terminated by its intrinsic GTPase activity, possibly stimulated by interaction with other molecules.

Abbreviations used: mAChR, muscarinic acetylcholine receptor; G-protein, GTP-binding-protein; TM, transmembrane; SDM, site-directed mutagenesis; PrBCM,

propylbenzilylcholine mustard.

(v) In the case of the receptor, phosphorylation of the agonist-receptor complex by one or more specific receptor kinases, followed by interaction with arrestin-like molecules, is likely to play a major role in desensitizing and down-regulating the response [4]. Five distinct mammalian mAChR sequences, ml-m5, have now been determined by cDNA or genomic cloning [31. These have been supplemented by avian and Drosophila sequences [S, 61. Like other G-coupled receptors, the mAChRs are thought to conform to a generic protein fold, in which seven transmembrane (TM) sequences are joined by alternating extracellular and intracellular loops. This general arrangement has been confirmed in a painstaking immunological study of the topography of the B-adrenergic receptor [7]. Sequence analysis of the mAChRs, particularly of the second and third intracellular loops, has correlated primary sequence differences with G-protein specificity, subgrouping the sequences into m l , m3 and m5, which couple preferentially to G-proteins mediating phosphoinositide metabolism, and m2 and m4, which are relatively selective for the adenylate cyclase-inhibitory G, protein(s). Analysis of the structural determinants of G-protein coupling is proving susceptible to analysis by sitedirected mutagenesis (SDM), and the construction of chimaeric receptors [S-lo]. The discovery of amino acids important in ligand binding, and in stabilization of the protein fold, and thus of the binding site, is being sought both by protein labelling and sequencing experiments [ll-1.51 and by SDM [16]. In this short manuscript we will attempt to outline an approach which may help us to put this information into a structural context, allowing the rational evaluation and testing of hypotheses, and the planning of further experiments.

Orientation of the transmembrane helices of the mAChRs There are no sequence-homologous proteins of known structure to guide attempts to model the three-dimensional structures of the G-coupled receptors. However, spectroscopic studies on the rhodopsins, which are G-coupled photoreceptor proteins, suggest that a suitable model for the T M protein fold may be provided by the photoactivated

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Biochemical Society Transactions

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proton pump bacteriorhodopsin [ 171. The structure of this seven TM helical bundle protein has recently been solved at a resolution of 3.5-6.0 A by highresolution electron microscopy [ 181. Fortunately, additional and more general structural rules have been adduced from analysis of the structure of both the bacteriorhodopsin family and other TM proteins, particularly the bacterial photosynthetic reaction centre, which has been solved at 2.3 A resolution by X-ray crystallography (see [21] and refs. therein). The TM regions of these proteins can be thought of as an everted version of the structure of water-soluble proteins in the sense that the core residues tend to have polar side chains, while the surface residues, which face the lipid bilayer, carry apolar side chains. However, both soluble globular proteins and TM proteins share the common property that the surface residues are subject to fewer bonding constraints than the core residues and are consequently more susceptible to genetic drift. In the case of TM helices, it is, therefore, a general prediction that the surface helices should exhibit a conserved, inward-pointing face, and a less well conserved outward-pointing face. The search for such signatures is amenable to variability analysis, an approach which was pioneered by Eisenberg and co-workers [20] and which successfully predicts surface residues in the photosynthetic reaction centre TM helices [ 19, 211. This form of analysis has been applied to the G-coupled receptors as a group by Donnelly et al. [22]. We have used it more specifically to make a working prediction of the structure and orientation of the TM sequences of the mAChRs. The calculation was performed as follows. (i) The sequences of the mAChR TM sequence were aligned using the program Multal [23]. Twelve sequences were included in the calculation, namely m l (rat, pig, human), m2 (rat, pig, human), m3 (rat, pig), m4 (rat, human), m5 (rat, human) and m (Drosophila). (ii) The index of variation, defined as the number of different amino acid types occurring at each position in the aligned TM helices, was determined. (iii) The Fourier transform power spectrum p(w) was calculated as follows:

5,

I

N

c

P(o)=(j = I

\2

( y - Qcos(jw)

I

where N is the number of residues in the TM sequence, w is the angular rotation per residue

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around the helix axis, and is the mean value of 5 for the entire TM helix. In the case of a standard a-helix with a conserved face and a non-conserved face, 40)is expected to peak at 100". The Fourier transform power spectra of the variability profiles of the mAChR TM helices are shown in Fig. 1. Each of the TM sequences gave a peak at values of about loo", consistent with an ahelical conformation. The actual maxima varied from 91" for TM6 to 105" for TM2. In the case of TM1, TM4, TM6 and TM7, the power spectral peak was particularly clear and unambiguous; in each case the ratio of the area under the peak (over a 30" range) to the area under the entire profile (ly) was greater than 2.0, strongly indicating that these TM sequences form surface a-helices with one face more conserved than the other. In the case of TM5, the profile was slightly more complex, but there was still a pronounced peak at approx. loo', and the value of was greater than 3.0, consistent with an amphipathic a-helix. In the case of TM3, ly was smaller, but the a-helical prediction remained stronger than any alternative. In the case of TM2, however, the power spectrum plot was broad and complex in the region of loo", and the maximum of the profile was at approx. 17o", a prediction which, at face value, could be taken as indicative of a Bstructure [ZO]. An alternative explanation is that TM2, which contains very highly conserved sequences, may be a core helix, somewhat buried within the structure of the helical bundle, and may therefore be subject to stronger constraints. The above calculations were repeated using amino acid polarity instead of variability as described by Eisenberg et al. [20] and by Cornette et al. [24]. In this case only TM7 gave a clear and unambiguous helical prediction (see last panel in Fig. 1); in fact the power spectrum was very similar to that of the variation index. The other helices also yielded peaks at approx. 100". These were too small to be taken in isolation as strongly indicative of amphipathic properties, but were, nevertheless, compatible with the calculations from sequence variation. We have used the results of the above calculations to improve our qualitative model of the TM region of the mAChRs [3, 121. The sequence of the TM regions of the rat m l mAChR is shown in Fig. 2(a). The resulting oriented helix model of the TM region is shown in Fig. 2(b). The connectivity and positioning of the helices is modelled on the bacteriorhodopsin structure 6 [ 181 so that helices which adjoin one another in the sequence lie antiparallel. The direction of the vector of maximum


Fig. I

Fourier transform power spectra of the variability profiles of the TM regions of the muscarinic receptors I35

The last panel shows the power spectrum of the hydropathy profile of TM7, calculated using the PRIFT polarity set of Cornette et a/. [24]. The arrows mark the region of each profile used for the calculation of the normalized peak area.

v,

;;Ld 100

100

Degrees

variation is shown pointing away from the centre of mass of the helical bundle. The angle of rotation was defined by the maximum value of q w ) , and rotation is clockwise for the odd-numbered helices and anti-clockwise for the even-numbered helices. In reality, deviations from the ideal a-helical angle of 100" may well be due to tilting of the a-helix within the membrane, or to dislocations in the helical structure caused by the presence of proline residues. However, from the viewpoint shown, which is normal to the membrane plane from the extracellular side, such deviations are assumed to be irrelevant. Nevertheless even small variations from 100" produce large cumulative effects when plotting helical wheel diagrams. The resulting model is similar, but not identical, to that of Donnelly et al. [22]. When the position of conserved residues is superimposed on the model, it is clear that nearly all of the amino acids lining the central cavity of the helical bundle, and at interhelical boundaries, are invariant; 72 of the 85 conserved residues in the TM region are found in these positions. There is also a strong correlation with amino acid side chain polarity, with 38 of the 50 amino acid side chains capable of hydrogen bond formation being found within the central core or at helix interfaces. In several instances, surface polar residues in the m l sequence are replaced by non-polar residues in other mAChR sequences.

Orientation of the TM helices by polarity, or by conservation, produces a very similar picture. The model thus makes physical sense. Additional constraints on the structure are provided by the existence of disulphide bonds in the extracellular domain of the mAChRs. The extracellular loops of the mAChR contain two pairs of conserved Cys residues. The first pair has homologues in all members of the G-coupled receptor family, with the exception of the mas oncogene [3]. We have shown that this pair of Cys residues is disulphide-bonded together [ 13, 141, linking the centre of the second extracellular loop to the top of TM3. According to the model, the orientation of the TM3 Cys residue is such that the second extracellular loop should be pulled across the top of the central core of the receptor. Evidence from peptide mapping studies is compatible with the presence of a second disulphide bond in the third extracellular loop of the mAChRs, but this is far from conclusive (E. Kurtenbach, C. A. M. Curtis & E. C. Hulme, unpublished work). Such a bond would be unique to the mAChRs.

Residues involved in ligand binding and receptor activation It is reasonably certain that the onium headgroup of

muscarinic antagonists interacts directly with a con-

1991


Fig. 2 Model of the TM region of mAChR

(a) The TM sequences of the rat ml mAChR, arranged in an antiparallel fashion, with the N-terminus of TMI at the top left. (b) The oriented helix model of the TM sequences of the m I mAChRs. The amino acids in each helix are numbered with respect to the N-terminal residue of that helix. The arrows indicate the direction of the vector of maximum variation. The conserved Asp in TM3 and the conserved Tyr in TM7 are picked out. Amino acids conserved in all of the mAChR sequences are shown in bold type. The direction of view is normal to the extracellular surface of the membrane.

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(4 Extracellular 1

2

3

4

5

6

7

lntracellular

served Asp residue, Asp-105 (ml sequence), corresponding to Asp-6 in TM3 (Fig. 2). The evidence for this assertion is based primarily upon site-directed affinity labelling studies using the antagonist analogue 3H-propylbenzilylcholinemustard (PrBCM), in which the onium headgroup is replaced by a chemically reactive aziridinium ring. We have shown by mapping and sequencing of labelled peptides derived from proteolytic and chemical cleavage of 3H-PrBCM-labelled purified mAChRs that the aziridinium moiety undergoes nucleophilic

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attack by the carboxylate side chain of Asp-105, forming an ester link [ 11, 141. There is evidence to suggest that both m l and m2 subtypes are alkylated in the same position [15]. Mutation of the Asp residue to Asn greatly reduces the binding affinity of the muscarinic antagonists [ 161. There is no guarantee that the headgroup of acetylcholine is bound identically to that of PrBCM, but the most economical hypothesis is that the interactions are similar, at least in the ground state of binding. This is the only hypothesis which makes


sense of the 1:l correlation which exists between the presence of an Asp residue in TM3, and the presence of a cationic amine headgroup in the corresponding physiological agonist. It is supported by the ability of aziridinium analogues of acetylcholine to alkylate and block mAChRs in a manner similar to that of PrBCM ([25] and refs. therein). According to the oriented helix model of the mAChRs, Asp-105 faces the inner side of the helical bundle, within the outer leaflet of the lipid bilayer. This places the ligand headgroup at the same depth within the structure as the retinal chromophore in rhodopsin. This being so, it is interesting to note that the residue which corresponds to the retinalattachment Lys in the rhodopsins, namely Tyr-408 (Tyr-7 in TM7), is also inward-facing in the oriented helic model. There appears to be a strong correlation between the presence of a Tyr residue in TM7, and an Asp residue in TM3. There are no sequences which have the Asp but not the Tyr, and only one which has the Tyr but not the Asp; the latter is a putative G-coupled receptor sequence of unknown function recently discovered in the human cytomegalovirus genome [26]. In view of the evidence which exists for the involvement of TM7 as well as TM3 in ligand binding, we have proposed the possible existence of a hydrogen bond between the phenolic OH group of Tyr-408 and the carboxylate side chain of Asp-105 [3, 121. We are testing this hypothesis by SDM of the residues involved to alter or delete the presumed hydrogen bond. It is clear that the interactions which agonists make with the conformationally altered activated state of the receptor must be different from those which are formed in the ground state. Interactions with the excited state are most likely to involve the side chains of the ligands, and it may be these bonds, rather than those made by the headgroup, which are the primary determinants of the pharmacological distinctions between the amine ligands. A point of departure in the search for amino acids which may contribute to pharmacologically productive hydrogen-bonding interactions with the acetyl side chain of acetylcholine is to focus o n residues with polar side chains which are conserved in the mAChR TM regions, but are not found in other cationic amine receptor sequences. There are approximately 14 candidate residues in the mAChR sequences. Superimposition of these on the oriented helix model narrows down the field to approx. six amino acids located, with one exception, on the conserved, inward-facing surfaces of TM helices 5,

6 and 7. All of these residues are within the outer leaflet of the phospholipid bilayer, on the same level as Asp-105. These residues are primary targets for mutagenesis studies. In conclusion, by using the three-dimensional structure of bacteriorhodopsin as a guide to the protein fold, and applying Fourier transform analysis to search for periodicity within the TM regions of the mAChR sequences, we have been able to construct a working model of the intramembrane portion of the receptor structure. This region contains the ligand binding site. The model allows a sensible interpretation of the known features of muscarinic receptor structure, and should prove capable of refinement as more information becomes available.

1. Freissmuth, M., Casey, P. J. & Gilman, A. G. (1989) FASEB J. 3,2125-2131 2. Marbach, I.. Bar-Sinai, A., Minich, M. & Levitzki, A. (1990) J. Biol. Chem. 265.9999-10004 3. Hulme, E. C., Birdsall, N. J. M. & Buckley, N. J. (1990) Annu. Rev. Pharmacol.Toxicol. 30,633-673 4. Lohse, M. J.. Benovic, J. L., Codina, J., Caron, M. G. & Letkowitz, R. J. (1990) Science 248, 1547-1550 5. Tietje, K. M., Goldman, P. S. & Nathanson, N. M. (1990) J. Biol. Chem. 265,2828-2834 6. Onai, T., FitzGerald, M. G., Arakawa, S. Gocayne, J. D., Urquhart, D. A,, Hall, L. M., Fraser, C. M., McCombie, W. R. & Venter, J. C. (1989) FEBS Lett. 255,219-255 7. Wong, H.-S., Lipfert, I,., Malbon, C. C. & Rahouth, S. (1989) J. Biol. Chem. 264,14424-14431 8. Kubo, T., Bujo, H., Akiba, I., Nakai, J., Mishina, M. & Numa. S. (1988) FEBS Lett. 241,119-125 9. Wess, J , Brann, M. R. & Bonner, T. I. (1989) FEBS Lett. 258, 133-136 10. Wong, S. K.-F., Parker, E. M. & Ross, E. M. (1990) J. Biol. Chem. 265,6219-6224 11. Curtis, C. A. M., Wheatley, M., Sansal, S.,Birdsall, N. J. M.. Eveleigh, P., Pedder, E. K., Poyner, D. & Hulme, E. C. (1989) J. Biol. Chem. 264,489-495 12. Hulme, E. C., Curtis, C. A. M., Wheatley, M., Aitken, A. & Harris, A. C. (1989) Trends Pharmacol. Sci. 10, 22-25 13. Kurtenbach, E., Pedder, E. K., Curtis, C. A. M. & Hulme, E. C. (1990) Biochem. SOC. Trans. 18, 442-443 14. Kurtenbach. E., Curtis, C. A. M., Pedder, E. K.. Aitken, A., Harris, A. C. M. & Hulme, E. C. (1990) J. Biol. Chem. 265,13702-13708 15. Uchiyama, H., Ohara, K., Haga, K., Haga, T. & Ichiyama, A. (1990) J. Neurochem. 54,1870-1881 16. Fraser, C. M., Wang, C.-I>., Robinson, D. A,, Gocayne.J. D. & Venter, J. C. (1989) Mol. Pharmacol. 36,840-847

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17. Findlay. J. B. C. & Pappin. D. J. C. (1 986)Biochem. J. 238,625-642 18. Henderson, R., Baldwin, J. M., Ceska, T. A., Zemlin, F., Beckmann, E. & Downing, K. H. (1990)J. Mol. Biol. 213,899-920 19. Rees, D.C., DeAntonio, L. & Eisenberg, D. (1989) Science 245,s10-513 20. Eisenberg, D.,Weiss, R. M. & Terwilliger, T. C. (1984)Proc. Natl. Acad. Sci. U.S.A. 81, 140-144 21. Rees, D. C.. Komiya, H., Yeates, T. O., Allen, J. P. & Feher, G. (1989)Annu. Rev. Biochem. 58,607-633 22. Donnelly, D., Johnson, M. S., Blundell, T. L. & Saunders,J. (1989)FEBS Lett. 251,109-116

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Received 30 August 1990

Antibodies to receptors by an auto-anti-idiotypic strategy B. F. Erlanger Department of Microbiology, Columbia University, New York, NY I oO32,U.S.A. Although Lindenman [l] was one of the fitrst to show that a paratope-specific antibody could mimic an antigen, the concept of ‘internal images’ comes from Jeme’s proposal of an idiotypic network [Z], more particularly from three postulates that underly the network concept. They are: (1) antibodies can recognize any foreign or self antigen; (2) antibody molecules have immunogenic idiotopes (i.e. variable region epitopes); and (3) idiotopes on an antibody molecule can mimic (i.e. be internal images 09 any foreign or self antigen. These postulates lead to the conclusion that the idiotypic network can be exploited to produce antibodies to ‘all’ antigens and that, from among the anti-idiotypic antibodies, immunoglobulin molecules can be isolated that will mimic ‘any’ antigen. Our first experience with the network was inadvertent and resulted from some experiments with a potent new photochromic agonist of the nicotinic acetylcholine receptor (AChR) developed in our laboratory, trans-3,3’-bis[ a-(trimethylammonio)methyl]azobenzene (BisQ) [3, 41. Its Cisisomer, to which it is converted by light of 320 nm, was inactive. trans-BisQ is a structurally constrained molecule. Unlike a flexible molecule, which can Abbreviations used: AChR, nicotinic acetylcholine receptor; BisQ, truns-3,3’-bis[a-(trimethylammonio)methyl]azobenzene; TSH. thyrotropin; L-PIA, N“-(L-phenylisopropy1)adenosine; CHA, N”-cyclohexyladenosine; NECA. 5’-N-ethylcarboxamidoadenosine;IC,,, concentration producing 50% inhibition; h, human; b, bovine; CDR, complementarity-determining region; MR, aldosterone (mineralocorticoid)receptor.

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assume many conformations in solution, BisQ has few, if any, degrees of freedom. Hence, its activity as a physiological effector molecule would depend upon whether its constrained conformation is complementary to that of the binding site of AChR The high activity of trans-BisQ as an agonist implied complementarity with AChR in its activated state. Macromolecules that bind the same ligand but have different biological functions can show homology in their binding specificities [ S , 61. We were therefore intrigued by the possibility that antibodies to BisQ would have binding specificities similar to that of AChR in its activated state. We found that they did [7]. It was only a small step in the thought process to ask whether anti-anti-BisQ would bind AChR Rabbits were immunized with specifically purified anti-BisQ antibodies. The rabbits produced anti-idiotypic antibodies (anti-anti-BisQ) that bound AChR. Moreover, they developed signs of myasthenia gravis, a disease that is characterized by the appearance of anti-AChR antibodies in the circulation [7,8]. During these experiments we also detected anti-anti-anti-BisQ antibodies, and this led us to investigate the possibility of producing anti-idiotypic anti-receptor antibodies directly by immunization with ligand, rather than with anti-ligand. We chose to continue our experiments with AChR to investigate this strategy, which we termed an autoanti-idiotypic strategy. We also chose the monoclonal route because complications caused by idiotype-anti-idiotype interactions in sera would be avoided.