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   So far, the only experimental structural information on the nAChRs came
   from electron microscopy studies on Torpedo receptors^[55]18, ^[56]19,
   ^[57]20. These low–medium-resolution studies revealed the overall shape
   and dimensions of the receptor, a location of the ligand-binding site
   and the organization of the ion channel. The most recent 4.6 Ã…
   data^[58]21 showed acetylcholine-binding pockets surrounded by
   seven-stranded beta -sheet structures. This fits with circular
   dichroism measurements demonstrating that LBDs are predominantly beta
   -structured^[59]22, ^[60]23, ^[61]24. No high-resolution data are
   available yet and although soluble LBDs have been produced^[62]22,
   ^[63]23, ^[64]24, ^[65]25, ^[66]26, protein quantities were
   insufficient for high-resolution studies.
   [67]Top of page

 §3§ Acetylcholine-binding protein  §3§

   Acetylcholine-binding protein (AChBP) is a soluble protein found in the
   snail Lymnaea stagnalis^[68]27. It is produced and stored in glial
   cells, and released in an acetylcholine-dependent manner into the
   synaptic cleft, where it modulates synaptic transmission. Mature AChBP
   is 210 residues long and forms a stable homopentamer. It aligns with
   the N-terminal domains of pentameric LGICs ([69]Fig. 1) and lacks the
   transmembrane and intracellular domains present in the
   superfamily^[70]27. AChBP is most closely related to the alpha
   -subunits of the nAChRs ([71]Fig. 1). Nearly all residues that are
   conserved within the nAChR family are present in AChBP, including those
   that are relevant for ligand binding. Moreover, AChBP binds known nAChR
   agonists and competitive antagonists such as acetylcholine, nicotine,
   d-tubocurarine and alpha -bungarotoxin^[72]27. Therefore, AChBP can be
   used as an example of the N-terminal domain of an alpha -subunit
   of nAChRs. Here we report the crystal structure of the AChBP
   homopentamer at 2.7 Ã… resolution.

 §5§ [73] Figure 1: Sequence alignment of AChBP with pentameric ligand-gated
ion channels (LGICs). §5§

   [74]Figure 1 : Sequence alignment of AChBP with pentameric ligand-gated
   ion channels (LGICs). Unfortunately we are unable to provide accessible
   alternative text for this. If you require assistance to access this
   image, or to obtain a text description, please contact npg@nature.com

   The alignment shows only the N-terminal domain of the LGIC subunits and
   is based on a multi-sequence ClustalX^50 alignment of 92 full-length
   pentameric LGIC sequences. Alignments are shown from first to last
   AChBP residue (for example, starting at second residue of mature
   Torpedo alpha , beta italic gamma and delta subunits, ending approx 4
   residues into the first transmembrane domain^29). H: human, T.ca:
   Torpedo californica. Secondary structure elements ( alpha : alpha
   -helix, beta : beta -strand, eta : 3[10]-helix) are indicated in red
   above the sequence, in accordance with Fig. 3a. Precise beginnings and
   ends will change with higher resolution. AChBP shares 24% sequence
   identity with the ligand-binding domain (LBD) of human alpha [7] (shown
   in green), 20–24% with other nicotinic receptors and 15–18% with other
   LGICs. The residues conserved in the superfamily are shown in bold with
   grey background. Asterisk, beginning and end of the Cys loop. Colouring
   of interface residues at the plus (yellow) and minus side (light blue)
   shows the lack of sequence conservation in the subunit interface across
   the pentameric LGIC family. Nicotinic receptor ligand-binding residues
   on the principal (pink) and complementary (dark blue) side are
   indicated.
   [75]High resolution image and legend (140K)
   [76]Top of page

 §3§ Structure determination  §3§

   The crystal structure of AChBP was solved using weak Pb
   multiple-wavelength anomalous diffraction (MAD) data in two crystal
   forms. The electron density map was improved substantially by
   cross-crystal averaging of three crystal forms with 20, 10 and 5 copies
   of the protomer in the asymmetric unit ([77]Table 1). The structure was
   refined at 2.7 Ã… in space group P4[2]2[1]2, with one AChBP pentamer
   in the asymmetric unit. Refinement with partial five-fold
   non-crystallographic symmetry (NCS) restraints resulted in an R-factor
   of 26.4% (R[free] = 30%).

 §5§ [78] Table 1: Data collection statistics §5§

   [79]Table 1 - Data collection statistics

   [80]Full table
   [81]Top of page

 §3§ The AChBP pentamer  §3§

   The AChBP homopentamer, when viewed along the five-fold axis, resembles
   a windmill toy, with petal-like protomers ([82]Fig. 2a). When viewed
   perpendicular to the five-fold axis it forms a 62-Ã…-high cylinder
   ([83]Fig. 2b), with a diameter of 80 Ã…. The diameter of the central
   hole is approx 18 Ã…, between side chains. These dimensions are in good
   agreement with the N-terminal domain in the Torpedo nAChR electron
   microscopy data^[84]21. In the pentamer the only subunit contacts are
   dimer interfaces, of which each protomer has two, the plus side and the
   minus side. We refer to the A (plus)–B (minus) interface as an example
   for the five equivalent interfaces AB, BC, CD, DE and EA ([85]Fig. 2).

 §5§ [86] Figure 2: The pentameric structure of AChBP. §5§

   [87]Figure 2 : The pentameric structure of AChBP. Unfortunately we are
   unable to provide accessible alternative text for this. If you require
   assistance to access this image, or to obtain a text description,
   please contact npg@nature.com

   a, In this representation each protomer has a different colour.
   Subunits are labelled anti-clockwise, with A–B, B–C, C–D, D–E and E–A
   forming the plus and minus interface side, with the principal and
   complementary ligand-binding sites, respectively (ball-and-stick
   representation). b, Viewing the AChBP pentamer perpendicular to the
   five-fold axis. The equatorially located ligand-binding site
   (ball-and-stick representation) is highlighted only in the A (yellow)–B
   (blue) interface.
   [88]High resolution image and legend (165K)
   [89]Top of page

 §3§ The AChBP protomer  §3§

   Each AChBP protomer is a single domain protein, asymmetric in shape,
   with a size of around 62 times 47 times 34 Ã…^3 ([90]Fig. 3a). It
   consists of an N-terminal alpha -helix, two short 3[10] helices and a
   core of ten beta -strands, which form a beta -sandwich. The order of
   beta -strands conforms to a modified immunoglobulin (Ig)
   topology^[91]28 ([92]Fig. 3b) with an extra beta -hairpin (f'–f") and
   an extra strand (b'). These additional strands introduce two 'Greek
   key' folding motifs. An Ig-like topology had been predicted for
   nAChRs^[93]2, ^[94]29, but the location of the ligand-binding site was
   missed, owing to the presence of extra beta -strands. Compared with the
   classical Ig-fold^[95]28, the AChBP beta -strands are considerably
   twisted, with the beta -sheets rotated against each other, resulting in
   two separate hydrophobic cores. Thus the three-dimensional fold does
   not resemble other Ig-like proteins and comparison^[96]30 with the
   protein database did not result in a significant match to any known
   structure.

 §5§ [97] Figure 3: Overview of the AChBP protomer structure. §5§

   [98]Figure 3 : Overview of the AChBP protomer structure. Unfortunately
   we are unable to provide accessible alternative text for this. If you
   require assistance to access this image, or to obtain a text
   description, please contact npg@nature.com

   a, Stereo representation of the AChBP protomer as viewed from outside
   the pentameric ring. This ribbon representation is coloured as a
   rainbow gradient, from blue (N terminus) to red (C terminus).
   Disulphide bridges are indicated in green ball-and-stick
   representation. In a complete ion channel the N terminus would point
   towards the synaptic cleft and the C terminus would enter the membrane
   at the bottom, continuing into the first transmembrane domain. b,
   Topology diagram of the AChBP protomer. For comparison with Ig-folds
   the strands have been labelled a–g, showing the additional strand (b')
   and hairpin (f'–f"). In this structure, strands have been labelled beta
   1– beta 10 with loops (or turns) L1–L10 preceding each strand with the
   same number. The beta 5 strand is broken ( beta 5– beta 5') with
   internal loop L5'; beta 6 also has a small break, but it is shown
   continuously (see Fig. 1). The precise beginnings and ends of strands
   may change slightly with increasing resolution, but the topology seen
   here will be highly conserved across the entire family of pentameric
   LGICs. S: disulphide bridge.
   [99]High resolution image and legend (38K)
   [100]Top of page

 §3§ Positioning of functional regions  §3§

   In the structure, the N and C termini are located at 'top' and 'bottom'
   of the pentamer, respectively. In the ion channels the transmembrane
   domains are at the C-terminal end of the LBD, at the 'bottom' of the
   AChBP structure ([101]Figs 2b and [102]3), starting directly at the end
   of beta -strand beta 10.

   In muscle type nAChRs, the main immunogenic region (MIR), comprising
   residues alpha [1]67– alpha [1]76, acts as an epitope in the autoimmune
   disease myasthenia gravis^[103]31. Although the MIR-related region in
   AChBP (residues 65–72) has no sequence homology with the alpha
   [1]-subunit, its location in a highly accessible position in loop L3 at
   the 'top' of the pentamer agrees with the expected accessibility for
   this region ([104]Fig. 3a). It also fits with electron microscopy
   studies that located the MIR at the distal end of the receptor relative
   to the membrane^[105]20.

   The central pore of the pentamer is very hydrophilic, lined with
   charged residues. On each AChBP promoter, a large pocket is visible,
   accessible from the central pore. Each pocket, framed at the entrance
   by beta -strands ( beta 3, beta 4, beta 5 and beta 5') ([106]Fig. 3a),
   is uncharged and mainly hydrophobic. This region probably corresponds
   to the tunnel framed by twisted beta -strands that was observed in the
   alpha [1]-subunit of the Torpedo receptor at 4.6 Ã… resolution^[107]21.

   There is a different cavity at each interface between the subunits.
   These cavities are lined by residues, which were biochemically shown to
   be involved in ligand binding in nAChRs^[108]2, ^[109]5, ^[110]6,
   ^[111]7, ^[112]8, ^[113]9, ^[114]10, ^[115]11, ^[116]12, ^[117]13,
   ^[118]14, ^[119]15, ^[120]16. These cavities are mostly buried from the
   solvent, and located close to the outside of the pentameric ring
   ([121]Fig. 2a). When viewed perpendicular to the five-fold axis, they
   are roughly equatorially positioned, about 30 Ã… away from the C termini
   ([122]Fig. 2b), conforming to the expected location of the Torpedo
   receptor ligand-binding site, as determined by labelling^[123]32 and
   electron microscopy studies^[124]18. We have concluded that these
   cavities are the ligand-binding sites.
   [125]Top of page

 §3§ The ligand-binding site  §3§

   Each ligand-binding site is found in a cleft formed by a series of
   loops from the principal face of one subunit and a series of beta
   -strands from the complementary face of an adjacent subunit ([126]Fig.
   4). The principal side on the plus side of the AB interface consists of
   residues coming from loop A (Tyr A89), loop B (Trp A143, A145) and loop
   C (Tyr A185, the double cysteine A187–A188 and Tyr A192) ([127]Fig.
   4c). The complementary part of this binding side is formed by beta
   -strands in protomer B contributing loop D (Trp B53, Gln B55), loop E
   (Arg B104, Val B106, Leu B112 and Met B114) and loop F (Tyr B164)
   ([128]Fig. 4d). Four of the aromatic residues form the bottom half of
   the cavity (Tyr A89, Tyr A185, Tyr B164 and Trp B53). The walls are
   formed by Tyr A192, Trp A143, the main chain of A145, the side chains
   of Met B114 and Gln B55, and the vicinal disulphide
   (Cys A187–Cys A188). The hydrophobic parts of Arg B104, Val B106 and
   Leu B112 form the top of the binding site ([129]Fig. 4a).

 §5§ [130] Figure 4: The ligand-binding site. §5§

   [131]Figure 4 : The ligand-binding site. Unfortunately we are unable to
   provide accessible alternative text for this. If you require assistance
   to access this image, or to obtain a text description, please contact
   npg@nature.com

   a, Stereo representation of the ligand-binding site in ball-and-stick
   representation, showing the contribution of the principal 'loops':
   A (Tyr A89/ alpha [1] Tyr 93, yellow), B (Trp A143/ alpha [1] Trp 149,
   dark yellow) and C (Tyr A185/ alpha [1] Tyr 190, Cys A187/ alpha [1]
   Cys 192, Cys A188/ alpha [1] Cys 193, Tyr A192/ alpha [1] Tyr 198,
   orange), and the complementary 'loops' D (Trp B53/ italic gamma Trp 55,
   Gln B55/ italic gamma Glu 57, violet), E (Arg B104/ italic gamma
   Leu 109, Val B106/ italic gamma Tyr 111, Leu B112/ italic gamma
   Tyr 117, Met B114/ italic gamma Leu 119, light blue) and F (Tyr B164,
   blue). b, Stereo view of the electron density map displaying a HEPES
   buffer molecule in the ligand-binding site. This experimental density
   (contoured at 1  sigma ) is derived from cross-crystal averaging. c,
   Location of the principal ligand-binding residues (colours as in a,
   orientation as in Fig. 2b). d, Location of the complementary
   ligand-binding residues (colours as in a, orientation as in Fig. 2b).
   Note that loops D, E and F are all on beta -strands.
   [132]High resolution image and legend (89K)

   All residues in the binding site have been identified by photoaffinity
   labelling and mutagenesis studies^[133]6, ^[134]7, ^[135]8, ^[136]9,
   ^[137]10, ^[138]11, ^[139]12, ^[140]13, ^[141]14, ^[142]15, ^[143]16.
   Although the side chain of His A145 is pointing away from the cavity,
   its main chain is involved in the binding site. One residue identified
   by labelling studies^[144]6, Trp A82 ( alpha [1] Trp 86), is involved
   in hydrophobic core formation and located far from the pocket.
   Therefore it probably does not participate directly in ligand binding.
   Additional residues may be involved in binding large ligands such as
   toxins^[145]13. Otherwise, the AChBP ligand-binding site entirely
   confirms the available biochemical and mutational data on nAChRs. The
   structure, however, reveals for the first time how these residues are
   positioned with respect to each other, and will provide a valuable tool
   for drug design studies.

   All observed residues are conserved between known ligand-binding
   subunits of nicotinic receptors except the loop F Tyr B164 residue. In
   our structure the loop F region has an unusual conformation, but as it
   is relatively weakly resolved, its precise analysis is difficult. The
   loop F region has low sequence conservation in the nicotinic family
   ([146]Fig. 1), and in other superfamily members it may have a different
   conformation, providing different residues to the binding site. Such
   changes could lead to variations in affinity, for example by changing
   the size of the ligand-binding site or its access route.

   Close to the loop F region, the electron density was interpreted as a
   calcium ion, because the crystals were grown in the presence of Ca^2+.
   The cation has Asp B161, Asp B175 and the main chain of B176 as
   ligands. This putative Ca^2+ ion may help to orientate Tyr B164 and
   could thus be involved in ligand binding. This agrees with observations
   that the residue equivalent to Asp B161 in muscle/Torpedo subunits (
   italic gamma Asp 174/ delta Asp 180) is important in ligand
   binding^[147]14, ^[148]15. Additionally, calcium-binding sites that
   enhance the response to agonist binding have been identified in a
   homologous region (residue range 161–172) of the neuronal alpha
   [7]-receptor^[149]33.

   The most likely access routes to the ligand-binding sites are from
   above or below the double-cysteine-containing loop C ([150]Fig. 4a).
   This region buries the ligand-binding site from the solvent, preventing
   access from the outside. Access of some of the larger ligands, such as
   d-tubocurarine, would require the binding site to be opened up, for
   example by movement of the beta -hairpin beta 9– beta 10 in the C-loop
   region. Access from the central pore has been suggested^[151]21, but
   this would require major structural rearrangements at the interface,
   which are less likely.

   From the location of the ligand-binding site, we can draw conclusions
   about the arrangement of ligand-binding alpha [1]-subunits with respect
   to their complementary partners in the Torpedo and muscle receptors. It
   has been suggested that the alpha [1] italic gamma and alpha [1] delta
   interfaces occur in a clockwise alpha [1] italic gamma alpha [1] delta
   beta [1] arrangement when looking towards the membrane^[152]34. Our
   structure would favour an anticlockwise alpha [1] italic gamma alpha
   [1]delta beta [1] arrangement, as the principal ligand-binding site is
   located on the anticlockwise side of its complementary partner
   ([153]Fig. 2).
   [154]Top of page

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   To our surprise we found features of bulky electron density that
   stacked onto Trp 143 in each ligand-binding site in the experimental
   cross-crystal averaged electron density ([155]Fig. 4b). We have
   assigned this to a HEPES
   (N-2-hydroxyethylpiperazine-N'-2-ethanesulphonic acid) buffer molecule,
   which contains a positively charged quaternary ammonium group and
   therefore has some similarity to known nicotinic receptor ligands. Its
   dissociation constant of 100 mM (data not shown) indicates that binding
   under crystallization conditions (100–150 mM) is possible at low
   occupancy. Although HEPES does not make any specific hydrogen bonds
   with the protein, it stacks with its quaternary ammonium onto Trp 143,
   making cation- pi interactions as expected for nicotinic
   agonists^[156]17, ^[157]35 ([158]Fig. 4b). It buries equal amounts of
   surface of the principal and complementary subunit. However, owing to
   low occupancy and limited resolution of the data, the precise
   orientation of the HEPES molecule cannot be definitely resolved. It has
   been suggested^[159]3 that the ligand-binding site of nAChRs could be
   similar to that of acetylcholinesterase (AChE). Although the size
   of the binding site is roughly similar in AChBP and AChE, the observed
   arrangement of aromatic residues is quite different. However, the
   stacking of the quaternary ammonium of HEPES, as far as it can be
   refined in the current AChBP structure, is similar to that of the
   quaternary ammonium of the decamethonium in AChE on Trp 84 (ref.
   [160]36).
   [161]Top of page

 §3§ Pentamer interface  §3§

   The subunit interface consists on the plus side entirely of loop
   regions (L1, L2, L4, L5, L7, L8 and L10), whereas the minus side mostly
   presents secondary structure elements to the interface ( alpha 1, beta
   1, beta 2, beta 3, beta 5, beta 6 and L9; [162]Fig. 5). Several
   residues are important for both ligand binding and pentamer formation.
   The interface buries a large surface area (2,700 Ã…^2), with a mainly
   uncharged character including only a single bifurcated salt bridge
   (Glu A149–Arg B3 and Arg B104). It is a very convoluted surface,
   indicating that shape complementarity may be important in pentamer
   formation ([163]Fig. 5a). The interface residues are not well conserved
   between subfamilies of the superfamily of pentameric ligand-gated ion
   channels ([164]Fig. 1). The residues change markedly between the
   different ion-channel subtypes, with for example changes from
   hydrophobic to charged and vice versa ([165]Fig. 1).

 §5§ [166] Figure 5: Dimer interface. §5§

   [167]Figure 5 : Dimer interface. Unfortunately we are unable to provide
   accessible alternative text for this. If you require assistance to
   access this image, or to obtain a text description, please contact
   npg@nature.com

   a, Stereo figure of the dimer interface. Representation of the
   interface residues (ball-and-stick) on a schematic secondary structure.
   The figure shows the plus face of subunit A (light yellow, orange
   interface residues) and the minus face of subunit B (light blue, pink
   interface side chains). b, Dimer interface interactions. Note that
   owing to the low conservation of these interfaces (Fig. 1) the actual
   interactions will not be conserved in any pentameric LGIC interface,
   but that in all receptors these topological regions are likely to form
   the interface.
   [168]High resolution image and legend (58K)
   [169]Top of page

 §3§ Ligand-gated ion channels  §3§

   The superfamily of ligand-gated ion channels has highly conserved LBDs,
   and the function and location of the conserved residues can now be
   analysed in the light of the structure. Most residues that are
   conserved in the superfamily ([170]Fig. 1) are hydrophobic and help to
   maintain the hydrophobic core of the protomer, grouped into
   three clusters ([171]Fig. 6). The first cluster is involved in packing
   of the N-terminal helix alpha 1 against the main framework of the
   protomer. The second cluster is situated in the upper half of the beta
   -core region. The third cluster is located at the lower end of the beta
   -sandwich ([172]Fig. 6). In the superfamily, two residues are conserved
   in the ligand-binding site. Only very few conserved residues are
   hydrophilic; two of these maintain the turns of a Greek key motif
   connecting strands beta 3, beta 5, beta 6 and beta 2, in which Asp 60
   stabilizes the N terminus of a small 3[10] helix and Gly 109 enables
   tight-turn formation. Conserved residues Asp 85 and Asn 90 are involved
   in packing of the beta -sheets. Asp 85 forms hydrogen bonds to the
   highly conserved Ser 142 and Thr 144, and residue Asn 90 brings
   together the main-chain oxygens of Ser 122 and Arg 137, enabling
   disulphide-bond formation of the nearby absolutely conserved disulphide
   bond (123–136). This disulphide bond is topologically equivalent to the
   'tyrosine cornerstone'^[173]37 found in Ig-like proteins, in which
   tyrosine links the two beta -sheets together, and a similar role is
   fulfilled by the disulphide bond in AChBP. This structural role for the
   disulphide bond explains why, in the Torpedo receptor, the
   Cys 128–Cys 142 bond is important for both preservation of subunit
   conformational stability^[174]38 and complete nAChR assembly^[175]39.
   As the conserved residues mainly contribute to the overall structure
   formation, it is clear that all pentameric LGIC N-terminal domains will
   have the same three-dimensional structure.

 §5§ [176] Figure 6: Conservation in the pentameric LGIC superfamily. §5§

   [177]Figure 6 : Conservation in the pentameric LGIC superfamily.
   Unfortunately we are unable to provide accessible alternative text for
   this. If you require assistance to access this image, or to obtain a
   text description, please contact npg@nature.com

   Conserved residues are indicated as viewed from the central pore.
   Hydrophobic cluster I (red): residues 6, 10, 63, 65, 71, 81, 105, 111;
   Cluster II (green): residues 20, 27, 29, 58, 82, 84, 86, 88, 140, 150,
   152, 195; Cluster III (pink): residues 33, 35, 38, 41, 48, 52, 123,
   125, 136, 138, 165, 171, 173, 199, 201. The hydrophilic conserved
   residues (dark blue): Asp 60, Asp 85, Asn 90, Gly 109, Lys 203.
   Conserved residues in the ligand-binding site (light blue): His 145,
   Tyr 192, or close by: Ala 87. Very few conserved residues are at the
   surface. Residues conserved between pentameric LGICs but not AChBP are
   indicated by yellow main chain (14, 19, 170 and the Cys-loop124–135).
   [178]High resolution image and legend (66K)

   Contrary to the above residues, the Cys loop ([179]Fig. 3a) is well
   conserved in the pentameric LGIC family but not in AChBP ([180]Fig. 1,
   residues 129–141). It is hydrophobic in the receptors, whereas it is
   hydrophilic in AChBP. The Cys loop is located at the bottom (membrane)
   side of the protein, close to the dimer interface. This position, and
   its hydrophobicity in the LGIC family, implies that it could interact
   with the membrane or with the transmembrane region of the receptors,
   functions that are absent in AChBP.

   Finally, the interface region is among the least conserved regions in
   the superfamily ([181]Fig. 1). Pentamer formation does not apparently
   impose very stringent evolutionary requirements on these domains. The
   high level of structural conservation, however, implies that the same
   topological regions form these interface contacts in all superfamily
   members ([182]Fig. 5b). But in these interfaces different combinations
   of subunits will have different interactions, possibly creating
   variations in the precise allosteric contacts and movements in the
   various subclasses of these ion channels.
   [183]Top of page

 §3§ Activation mechanism  §3§

   The location of the ligand-binding site is conserved among pentameric
   LGIC receptors^[184]1 but the actual ligand-binding residues vary,
   creating specificity for different ligands. However, all ligand-gated
   ion channel LBDs have intrinsically the same function: the activation
   of a membrane pore. Indeed, a nicotinic receptor LBD is capable of
   activating a serotonin membrane channel in a chimaeric
   receptor^[185]40. Thus, the essential activation mechanism is conserved
   across the superfamily. One option is that activation takes place by
   direct rotation of the protomer through a pivoting point at the
   ligand-binding site. But for any other mechanism, the location of the
   conserved residues implies that transmission of a signal from the
   binding pocket to the transmembrane domain takes place within the
   protomer and not through the interface region.

   Within the protomer, several possible regions could transmit activation
   signals: changes in loop C, induced by ligand binding, could be
   transmitted directly into the transmembrane domain through the beta 9–
   beta 10 beta -hairpin; alternatively, large structural changes in the
   beta -sheet regions could be transmitting the signal through the
   conserved Cys loop, acting directly on the membrane part of the
   pentameric LGICs. The movement observed at 9 Ã… for the Torpedo nAChR
   upon agonist binding^[186]19 fits well with the latter suggestion. We
   see a twisted beta -sandwich, with two distinct hydrophobic cores, and
   it is possible that these cores move with respect to each other upon
   ligand binding. The effect of any of these activation mechanisms in the
   protomer will then be modulated by the varying subunit interfaces in
   the different subtypes of the receptor, allowing intricate specificity
   in neuronal signal transmission.
   [187]Top of page

 §3§ Conclusions  §3§

   This crystal structure shows that molluscan AChBP is a homologue of the
   pentameric LGIC superfamily ligand-binding domains. It confirms the
   predicted Ig-topology, the location of the binding site at the subunit
   interface, the position of the MIR and the extensive data on the
   nicotinic ligand-binding residues.