The International Journal of Biochemistry & Cell Biology 32 (2000) 1017–1028
www.elsevier.com/locate/ijbcb
Review
a-Conotoxins
Hugo R. Arias *, Michael P. Blanton
Departments of Pharmacology and Anesthesiology, School of Medicine, Texas Tech Uni6ersity Health Sciences Center, Lubbock,
TX 79430, USA
Received 16 May 2000; accepted 2 August 2000
Abstract
a-Conotoxins (a-CgTxs) are a family of Cys-enriched peptides found in several marine snails from the genus Conus.
These small peptides behave pharmacologically as competitive antagonists of the nicotinic acetylcholine receptor
(AChR). The data indicate that (1) a-CgTxs are able to discriminate between muscle- and neuronal-type AChRs and
even among distinct AChR subtypes; (2) the binding sites for a-CgTxs are located, like other cholinergic ligands, at
the interface of a and non-a subunits (g, d, and o for the muscle-type AChR, and b for several neuronal-type AChRs);
(3) some a-CgTxs differentiate the high- from the low-affinity binding site found on either a/non-a subunit interface;
and that (4) specific residues in the cholinergic binding site are energetically coupled with their corresponding pairs
in the toxin stabilizing the a-CgTx-AChR complex. The a-CgTxs have proven to be excellent probes for studying the
structure and function of the AChR family. © 2000 Elsevier Science Ltd. All rights reserved.
Keywords: a-Conotoxins; Competitive antagonists; Nicotinic acetylcholine receptors
Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1018
2. Structure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1018
3. Synthesis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1020
Abbre6iations: AChR, nicotinic acetylcholine receptor; 5-HT, 5-hydroxytryptamine; 5-HT3R, 5-hydroxytryptamine type 3 receptor; NMDA, N-methyl-D-aspartate; a-BTx, a-bungarotoxin; a-CgTx, a-conotoxin; aA-CgTx, aA-conotoxin; m-CgTx, m-conotoxin;
v-CgTx, v-conotoxin; k-CgTx, k-conotoxin; d-CgTx, d-conotoxin; mO-CgTx, mO-conotoxin; s-CgTx, s-conotoxin.
* Corresponding author. Tel.: + 1-806-7432425, ext.: 244; fax: + 1-806-7432744.
E-mail address: phrhra@ttuhsc.edu (H.R. Arias).
1357-2725/00/$ - see front matter © 2000 Elsevier Science Ltd. All rights reserved.
PII: S 1 3 5 7 - 2 7 2 5 ( 0 0 ) 0 0 0 5 1 - 0
1018
H.R. Arias, M.P. Blanton / The International Journal of Biochemistry & Cell Biology 32 (2000) 1017–1028
4. Biological function . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1023
5. Possible medical applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1025
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1025
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1025
1. Introduction
From an evolutionary point of view, snails
from the genus Conus are among the most successful marine invertebrates (reviewed in [1]). The
genus Conus is formed by over 500 venomous
species. These marine mollusks prey on other
marine species by means of a very large number
of small peptides (10– 50 amino acids in length)
with specific pharmacological activities. Due to
the remarkably fast interspecific divergence of
peptide sequences, each Conus species has a repertoire of 50– 200 different peptides (reviewed in
[2]). These peptides, called conotoxins, affect the
functioning of different voltage-gated ion channels and neurotransmitter-gated receptors (reviewed in [3,4]). Based on the molecular form, the
approximately 50 000 conotoxins can be grouped
into a minimum of seven superfamilies; the A[e.g. a-conotoxins (a-CgTxs), aA-, and kACgTxs], M- (e.g. m- and c-CgTxs), O- (e.g. v-, k-,
d-, and mO-CgTxs), P- (e.g. Spasmodic peptide;
[5]), R-, S- (e.g. s-CgTxs), and T-superfamilies
(e.g. tx5a, p5a, au5a, and au5b; [6]) (reviewed in
[4,7]). This review will focus on the structure and
pharmacological activity of only the conotoxin
family (e.g. a-CgTx and aA-CgTxs) which specifically bind to several nicotinic acetylcholine receptors (AChRs) (reviewed in [4,8]).
The AChR is the prototype of a superfamily of
ion channel-coupled receptors which are gated by
specific neurotransmitters. This superfamily also
includes the type A g-aminobutyric acid, glycine,
and type 3 5-hydroxytryptamine (5-HT) receptors
(reviewed in [8,9]). There are two main AChR
types, the muscle- and neuronal-type. The muscletype AChR is a pentamer comprised of two a1
subunits, one b1, one d and, one g or o subunit,
depending on whether the receptor is in an embry-
onic or adult stage, respectively. The a1 subunits
contain two adjacent cysteines at position 192 and
193 (sequence number from Torpedo AChR),
which are involved in the recognition and binding
of cholinergic agonists and competitive antagonists. Based on the presence or the absence of
these two cysteines, the neuronal-type AChR subunit classes are designated a (when they contain
both cysteines) or b (when they do not). To date,
eight a subunits (a2– a9) and three b subunits
(b2– b4) have been identified. The a7, a8, and a9
polypeptides are the only subunits capable of
forming homo-oligomeric ion channels. Interestingly, the two a subunits display non-equivalence
for the binding of several cholinergic agonists and
competitive antagonists in both muscle- and neuronal-type AChRs. In this regard, a-CgTxs have
become one of the most powerful tools to support
this non-equivalence (reviewed in [8,9]).
2. Structure
The initial purification and chemical characterization of an a-CgTx was performed by Olivera
and co-workers in 1981 [10]. Since then, a great
deal of structural information has been obtained.
Most of them exhibit four Cys residues in the
following conserved arrangements; CCX3CX5C
(the a3/5 subfamily), CCX4CX3C (the a4/3 subfamily), and CCX4CX7C (the a4/7 subfamily). Each
alternate Cys pair forms a disulfide loop (i.e.
loops I and II, respectively). In each case, the X
represents the number of amino acids between the
Cys residues. The only representative of the a3/5/3
subfamily, which has three Cys bonds, is a-CgTx
SII. The spacing between disulfide bonds is an
important determinant of backbone structure.
Additionally, a conserved Pro residue is found
H.R. Arias, M.P. Blanton / The International Journal of Biochemistry & Cell Biology 32 (2000) 1017–1028
between the second and the third Cys in almost all
a-CgTxs that have been characterized to date.
The exception is the aA-CgTx subfamily (e.g.
aA-PIVA, aA-EIVA, and aA-EIVB), which has a
core sequence that is very different than the other
1019
subfamilies [11– 13]. Table 1 shows the primary
and secondary structural features of some a- and
aA-CgTxs as examples of each subfamily.
The three-dimensional structures of several aCgTxs have been determined using either X-ray
Fig. 1. Surface models of a-CgTxs PnIA (A) and MII (B) and backbone representations of a-CgTxs PnIA (C), MII (D), and GI (E)
(taken from [16], with permission). Hydrophobic, polar, positive-charged, and negative-charged residues are displayed in purple,
yellow, red, and blue, respectively. Backbone conformations of a-CgTxs PnIA (C), MII (D), and GI (E) are shown in ribbons.
Surface distributions in dots with the same color codes. Arrows indicate carboxyl (blue) and amino (yellow) terminal residues.
1020
H.R. Arias, M.P. Blanton / The International Journal of Biochemistry & Cell Biology 32 (2000) 1017–1028
crystallography or NMR spectroscopy [11,14 – 26].
These studies provide support for the idea that
these small polypeptides achieve their conformational stability by means of disulfide bonding.
Although a-CgTxs are apparently very rigid structures, two or more interconvertible conformers
may exist in solution (reviewed in [4]). The overall
shapes of the different a-CgTx subfamilies are as
follow: the a4/7 subfamily is more rectangular, the
a3/5 subfamily is more triangular, whereas the aA
subfamily has been described as an iron. The
structural comparison of several a-CgTxs specific
for neuronal-type AChRs, such as a-CgTx ImI,
PnIA, PnIB, MII, and EpI (Table 3), exhibit
remarkable similarity in local backbone conformations and relative solvent-accessible surface areas [21]. A model of the tertiary structure of
a-CgTxs PnIA, MII, and GI is shown in Fig. 1
(taken from [16]). The backbones of the a-CgTxs
PnIA and MII structures (panels C and D, respectively) are very similar. Nevertheless, the surfaces
of both the polypeptides are unique. The protrud-
ing Tyr residue on the surface of the a-CgTx
PnIA structure (panel C, in purple) contrasts with
the flat hydrophobic surface of a-CgTx MII
(panel D, residues in purple). Furthermore,
charged residues are exposed on the a-CgTx MII
surface (panel D, negative residues in blue and
positive residues in red), whereas they are not
exposed on the a-CgTx PnIA surface (panel C).
In addition, both the backbone and the surface of
a-CgTx GI (panel E), which is specific for muscletype AChRs (see Table 2), diverge significantly
from the other PnIA and MII a-CgTxs.
3. Synthesis
The Conus venom is biosynthesized and stored
in the venom duct of the venom apparatus (reviewed in [27]). When the snail is hunting prey,
the venom, by contraction of the muscular venom
bulb, is delivered through a harpoon-like radula
tooth located in the proboscis.
Table 1
Primary and secondary structure of each conotoxin subfamily
a-Conotoxin
Conus species
Subfamily
Primarya and secondaryb structure
MI
Conus magus
a3/5
GRCCHPACGKNYSCc
PnIA
Conus pennaceus
?
?
?
?
Conus imperialis
?
a4/7
GCCSLPPCAANNPDYdCc
a4/3
GCCSDPRCAWRCc
?
Conus striatus
a3/5/3
?
Conus purpurascens
aA
[56]
?
?
?
GCCCNPACGPNYGCCGTSCS
?
aA-PIVA
[55]
?
?
SII
[30]
?
?
ImI
References
[57]
?
?
?
GCCGSYONAACHOCSCKDROSYCGc
?
?
?
[13]
?
a
The sequence alignment of the different a-CgTxs is shown in the standard one-letter amino acid code. The letter O represents
trans-4-hydroxyproline.
b
a-Conotoxin subfamilies a3/5, a4/7, and a4/3 are formed by two disulfide bonds, while the aA-CgTx subfamily and a-CgTx SII
(a3/5/3 subfamily) have three disulfide bonds.
c
Amidated COOH-terminus. Instead, a-CgTx SII presents a free COOH-terminus.
d
Sulfated residue.
H.R. Arias, M.P. Blanton / The International Journal of Biochemistry & Cell Biology 32 (2000) 1017–1028
1021
Table 2
a-Conotoxin specificity for different muscle-type nicotinic acetylcholine receptors
a-Conotoxin
AChR Source
a
IC50
References
(subunit interface)
MI
BC3H-1
Mouse
Torpedo
Human (a2bod)
BC3H-1
GI
Mouse
Torpedo
CnIA
SI
Human (a2bod)
Frog
Fish (Eigenmannia)
Torpedo
BC3H-1
Human (a2bod)
Torpedo
SIA
BC3H-1
Torpedo
EI
BC3H-1
Torpedo
BC3H-1
Torpedo
Torpedo
Mouse
SII
aA-EIVA
aA-EIVB
aA-PIVA
a
BC3H-1
Torpedo
Torpedo
High-affinity binding site
NM
Low-affinity binding site
mM
1.50 9 0.24 (ad)
0.42 9 0.15 (ad)
0.40 9 0.01b (a2bgd)
0.40 90.17b (abd)
1.4 9 0.1b (kinetic)
0.55 9 0.06 (a2bgd)
1.34 (abd)
5.3 (a2bd)
1.9 (ad)
12b (ad)
2.89 1.3 (ad)
2.69 1.0 (ag)
1209 10 (ag)
4.509 1.60 (ag)
1.69 1.4 (a2bgd)
32 9 2 (abg)
9 9 1 (ad)
4.9 9 1.9 (ad)
1.3 9 0.3 (ad)
20b (ad)
360940 (ag)
4.59 1.3 (ag)
41.39 8.2c (ag)
2 91 (ad)
22.0 9 1.1 (ag)
23.09 4.1 (ag)
50-100b
190
6809 160 (ad)
13009 430 (ad)
809 14 (ao)
7.70 9 0.14 (ad)
5909 40 (ag)
420
9.40 9 1.20 (ad)
0.41 9 0.09 (ad)
189 5 (abg)
18.39 0.93 (a2bgd)
11.7 (abg)
139 1.3 (a2bgd)
15 (ag)
7.8 9 1.3 (ag)
2.39 0.7 (ad)
2191 (ad)
0.489 0.10 (ad)
1.09 0.3 (a2bgd)
1.29 0.1 (abd)
0.099 0.01 (ao)
589 12 (ag)
609 1 (ag)
249 2 (ad)
0.099 0.02 (ad)
0.149 0.01 (ao)
2-4b
2209 45 (ag)
2909 110 (ag)
8.319 1.2 (ad)
0.17 9 0.04 (ag or ad)
16 9 1 (ag or ad)
1
2009 8.6 (ag)
\260 (ad)
0.28 9 0.03 (ag)
0.199 0.02 (ag)
18 9 6.6
8
[31]
[31]
[30]
[57]
[12]
[12]
17b
11b (a2bgd)
11b (abg)
32b (kinetic)
15b (abd)
37b (kinetic)
150910
18b
[12]
[12]
[13]
1
These values were obtained by inhibition of [125I]a-BTx binding except those determined by the following:
or fluorescence spectroscopy.
c
[30]
[31]
[12]
[12]
[12]
[42]
[42]
[39]
[32]
[58]
[33]
[30]
[45]
[31]
[40]
[40]
[48]
[30]
[43]
[58]
[45]
[43]
[59]
[48]
[56]
[56]
[19]
[30]
[43]
[48]
[44]
[45]
[57]
[30]
[45]
b
electrophysiology
1022
H.R. Arias, M.P. Blanton / The International Journal of Biochemistry & Cell Biology 32 (2000) 1017–1028
Table 3
a-Conotoxin specificity for different neuronal-type nicotinic acetylcholine receptors
a-Conotoxin
AChR Subtype
CnIA
MII
a7
a3b2
EpI
a4b2
a3b4
a3b4
PnIA
AuIB
PnIB
a3b2/a3b4
a7/?
a7
a7 (human)/5-HT3R (rat)
a3b2
a3b4
a3b4
a7
a7/?
a7
a7 (human)/5-HT3R (rat)
a3b2
a3b4
ImI
a7 (rat)
a7
a7
a7 (human)
a9
a3b4/a3b4a5
a3b4
Aplysia
a
IC50 nM
References
14 800 b
3.5
8.0 9 1.1
24.3 9 2.9c (synaptosomes)
17.3 9 0.1c (slices)
400
3000d (noradrenaline)
84 9 19d (adrenaline)
2109 30d (noradrenaline)
1.6
14
252
176 (Kd)
61 2009 1 100b
9.56
21 000–28 000d (noradrenaline)
500 9 140 (Kd)
750
20 000d (noradrenaline)
\7000
33
61.3
84.9 (Kd)
29 6009 600b
1970
700b
700d (noradrenaline)
1000d (adrenaline)
220
100
300d
86.2 9 1.2
2450 9 100b
1800
2500 91200e
\3000
47 (desensitizing Cl− response)
\20 000 (sustained Cl− response)
150 922 (cationic response)
[19]
[46]
[36]
[36]
[36]
[46]
[62]
[54]
[54]
[54]
[50]
[51]
[51]
[53]
[51]
[52]
[37]
[37]
[52]
[37]
[50]
[51]
[51]
[53]
[51]
[30]
[52]
[52]
[58]
[60]
[61]
[48]
[58]
[34]
[60]
[35]
a
These values were obtained by electrophysiological techniques except those determined by the following: b inhibition of
[125I]a-BTx binding, inhibition of agonist–induced, c dopamine, d catecholamine (e.g. adrenaline or noradrenaline), e 5-HT release.
Cone snails generate novel polypeptide sequences by amino acid hypermutation (reviewed
in [2,7]). The synthesis of conotoxins can be compared with a combinatorial library search strategy. From studies with the O-superfamily,
conotoxins have been considered to be initially
translated as larger prepropeptide precursors 70–
120 amino acids in length with a single copy of
the toxin present at the C-terminal end. Other
neuropeptide precursors encode either multiple
H.R. Arias, M.P. Blanton / The International Journal of Biochemistry & Cell Biology 32 (2000) 1017–1028
copies of a specific peptide or several distinct toxins.
The high number of polypeptide structures observed in different cone snails that inhibit a
specific target are thought to have evolved by
hypermutation of the amino acids located closer to
the C-terminal. Exceptions are the Cys residues
located in the toxin proper. The rest of the precursor sequence remains highly conserved. The signal
sequence is the polypeptide region with the highest
level of sequence conservation. Between the signal
sequence and the mature toxin there exists an
intervening pro-region 40 amino acids in length
which exhibits a low mutation rate.
4. Biological function
The pioneering studies done in Dr Baldomero
Olivera’s laboratory provide the initial methodology for determining the biological activity of aCgTxs. Initial pharmacological data demonstrated
that, in general, the family of a-CgTxs behave as
competitive antagonists of the AChR (reviewed in
[7,8]). The a-CgTxs compete for the binding sites
of acetylcholine and cholinergic agonists. However,
a distinct conotoxin from Conus purpurascens,
called c-CgTx PIIIE, inhibits the functional activity of the AChR in a non-competitive manner [28].
The earliest studies showed that a-CgTxs inhibited
muscle-type AChRs. Table 2 shows the a-CTx
specificities for muscle-type AChRs from different
species. Nonequivalent binding of some a-CgTxs at
the two agonist/competitive antagonist binding
domains in Torpedo AChR [29– 31] is also summarized in Table 1. Although early studies focused on
a-CgTxs action in muscle-type AChRs, more recent
efforts have identified pharmacological effects of
these compounds in neuronal-type AChRs. Table
3 shows the a-CgTx specificities for different neuronal-type AChRs.
In order to determine which subunits of the
Torpedo AChR are involved in the a-CgTx binding
site, purified a-CgTx MI was crosslinked to the
AChR with bivalent succinimide reagents of different lengths [29]. With a 12-atom crosslinker, all the
four subunits were labeled, whereas a 4-atom
crosslinker labeled the b and g subunits. In addition, two azidosalicylate a-CgTx GIA derivatives
1023
were used for photoaffinity labeling of the AChR
[29]. These studies showed that depending on the
a-CgTx derivative used, the specifically labeled
AChR subunits were b and g, or d and g. However,
labeling of detergent-solubilized AChR was exclusive for residues 121 and 183 of the g subunit. The
p-benzoylphenylalanine derivative of a-CgTx GI
also labeled the a subunits [38].
In order to identify the determinants of a-CgTx
MI selectivity, Dr Steven Sine’s laboratory used
subunit chimeras and site-directed mutagenesis
[33,39]. From these studies, it was found that the
high affinity of subtype MI for the ad subunit
interface of mammalian AChRs is determined by
amino acids S36, Y113, and I178 from the d subunit,
while the low affinity for the ag interface is determined by residues K34, S111, Y117, L119, and F172 of
the g subunit. Since dY113 and gS111 are exchanged
for Arg and Tyr in the Torpedo AChR, these two
natural differences may account for the observed
site-specificity between the two species. This idea is
corroborated by the fact that the mutation dR113Y
in the a2bd2 or the mutation gY111R in a2bg2
Torpedo AChR results either in an enhancement of
or a decrease in a-CgTx MI affinity, respectively
[40]. The pairs gK34/dS36 and gF172/dI178, as primary determinants for a-CgTx MI selectivity in
mouse AChR, coincide with that for the selectivity
of carbamylcholine [41]. In contrast, neither the
gS111Y nor dY113S mutation affected carbamylcholine affinity, suggesting that agonists and at
least the a-CgTx MI subtype do not have identical
selectivity determinants. Residues gK34, gS111, and
gF172 contribute to loops D, F, and G, respectively,
of the agonist/competitive antagonist binding site
(reviewed in [8]). Since other residues from the a
subunit are considered to be involved in the agonist/competitive antagonist binding sites (reviewed
in [8,9]), Sugiyama et al. [42] examined the contribution of some of these residues to the binding of
a-CgTx MI. Mutations aY190F and aY189F do
not affect a-CgTx MI affinity, whereas removal of
aromaticity by exchanging these residues for Thr
has a marked influence on a-CgTx association. This
suggests that aromaticity may be required to stabilize the cationic peptide. The effect elicited by
substitutions of Y93 (loop A; see [8]) and D152
residue (loop B; see [8]) indicate that both peptide
and nonpeptidic ligands bind to the same site in
1024
H.R. Arias, M.P. Blanton / The International Journal of Biochemistry & Cell Biology 32 (2000) 1017–1028
the AChR, but unique though overlapping sets of
amino acids contribute to the binding domain. In
addition, substitution of Y12A on the MI toxin
dramatically reduces its affinity for the highaffinity site (ad) with little effect on toxin potency
at the low-affinity site (ag) [43]. This and additional data suggest that the orientation of residue
Y12 is important in the formation of the a-CgTx
MI-AChR complex.
In order to determine the existence of a linkage
relationship between mutations in a and d/g subunits, a mutant cycle analysis was employed [42].
From this study, a high coupling energy between
S36 and I178 of the d subunit was demonstrated. In
contrast, a relatively low linkage between residues
aY93/V188 and pairs gK34/dS36, gS111/dY113, and
gF172/dI178 is evident. Taking into account that
the energetic contributions of amino acids in the a
chain to a-CgTx MI association with the AChR
seem to be independent from the ones at the d/g
subunits, it is postulated that one of the surfaces
of the neurotoxin molecule interacts with the a
subunit, whereas the other surface interacts with
the d or the g subunit. In this regard, recent
conformational sudies using [H1]-NMR spectroscopy suggest that both the faces of the aCgTx GI are involved in the orientation of the
molecule within the ad subunit interface [25]. The
binding face of a-CgTx GI, a toxin closely related
to the MI subtype in structure, interacts by means
of residues C2, N4, P5, A6, and C7 (from loop I)
with the a1 subunit, whereas the selectivity face
comprising amino acids R9 and H10 (from loop II)
is oriented towards the d subunit (the subunit
forming the high-affinity a-CgTx GI locus in
mammalian AChRs). Residues R9 and H10 were
found to be responsible for the high differential
selectivity and affinity between both the cholinergic ligand binding sites [44,45]. The lack of effect
of the mutation P9 to the neutral residue Ala in
the a-CgTx SI suggests that the cationic group of
A9 in the a-CgTx GI plays a major role in ag
selectivity in the Torpedo receptor. The critical
difference between a-CgTx GI and SI has been
ascribed to position 9 (reviewed in [24]). The
importance of a cationic group for high selectivity
is further substantiated by the fact that mutations
on the a-CgTx MI at position K10, the ho-
mologous residue of A9, resulted in a loss of
selectivity [45].
Taking into account that neuronal receptors
containing the subunit composition a4b2, a2b2,
or a3b4 are more than 200-fold less sensitive to
a-CgTx MII than a3b2 AChRs, the determinants
of a-CgTx selectivity were identified using
chimeric subunits and subunits with single residue
substitutions [46]. Residues b2T59, a3K185, and
a3I188 were identified as specific determinants for
a-CgTx MII sensitivity. The amino acid a3K185
may electrostatically interact with E11 from aCgTx MII.
Regarding a-CgTx ImI specificity, the pairs
a7W55/a1R55, a7S59/a1Q59, and a7T77/a1K77 have
been considered as components conferring high
affinity binding to a7/5-HT3R compared with a1/
5-HT3R homooligomeric chimeras [47] (reviewed
in [8]). The third pair (a7T77/a1K77) may be considered as a new loop or an allosterically coupled
loop. Experiments performed in parallel show
that two regions in the a-CgTx ImI molecule are
essential for binding to the a7/5-HT3R chimera
[48]: a region comprising residues D5-P6-R7 in the
first loop and a second region in loop II formed
by W10. The fact that D5 functions as an N-terminal cap and P6 as a helix-initiator suggests that
the contribution of both the amino acids to binding may be due to their structural roles rather
than due to direct interaction with the AChR [21].
The structural role of P6 was recently corroborated by mutagenesis studies [49]. Subsequent
thermodynamic mutant cycle analyses demonstrated the existence of a dominant pairwise interaction between a-CgTx ImI R7 and a7Y195
(located in loop C; reviewed in [8]), and multiple
weak interactions between a-CgTx ImI D5 and
W149, Y151, and G153 of a7 (which are all located
in loop B; reviewed in [8]), and between a-CgTx
ImI W10 and a7T77 (which is probably located in
a new loop) and a7N111 (located in loop F; reviewed in [8]) [49].
Although a-CgTxs PnIA and PnIB differ in
only two amino acids (A10L and N11S, which are
located on the helix face exposed to the solvent
residues), the PnIA subtype preferentially inhibits
the a3b2 AChR, whereas a-CgTx PnIB is selective
for the a7 receptor [50] (Table 3). The fact that
H.R. Arias, M.P. Blanton / The International Journal of Biochemistry & Cell Biology 32 (2000) 1017–1028
the a-CgTx PnIA A10L mutant binds with higher
affinity to the a7 receptor suggests that position
10 is important for the observed selectivity [50,51].
These studies also suggest that both A10 and N11
in a-CgTx PnIA independently interact with the
a3b2 AChR whereas L10 in a-CgTx PnIB seems
to be the only structural requirement for the
binding to either a7 or a3b2 subtype [51], as well
as for the inhibition of the nicotine-evoked catecholamine release [52]. Subsequent thermodynamic mutant cycle analyses demonstrated the
existence of a dominant interaction between aCgTx PnIB L10 and a7W149 (located in loop B;
reviewed in [8]), and weaker interactions between
a-CgTx PnIB P6 and a7W149, and between both
P6 and P7 of a-CgTx PnIB and a7Y93 (located in
loop A; reviewed in [8]) [53]. The evidence from
mutational experiments also suggests that the
binding site for a-CgTx PnIA [50] or for a-CgTx
PnIB [53] on the a7 receptor is different from the
a-CgTx ImI site [49].
5. Possible medical applications
Nicotinic acetylcholine receptors appear to be
important for a number of neurophysiological
processes including cognition, learning, and memory. In addition, this receptor family has been
implicated in the pathophysiology of several neuropsychiatric disorders including Alzheimer’s and
Parkinson’s disease, schizophrenia, Tourette’s
syndrome, nocturnal frontal lobe epilepsy, as well
as nicotine addiction, myasthenia gravis, and various congenital myasthenic syndromes (reviewed in
[63]). Thus, the identification of a ligand with high
specificity for certain AChR subtype will be of
great importance in the development of new drugs
with potential medical uses. In the future, a-CTxs
might be used as therapeutic agents in the treatment of some of the above mentioned diseases. In
this regard, additional Conus toxins not discussed
in this review are being examined for possible
clinical use. For example, conantokin-R which
inhibits the N-methyl-D-aspartate (NMDA)-type
glutamate receptor might have use as an anticonvulsant agent [64]. The v-CTx MVIIA, which is
highly specific for the voltage-gated calcium chan-
1025
nels containing the a1B subunit, is being used in
clinical trials for the treatment of certain chronic
pain syndromes (e.g. intractable pain resulting
from cancer, traumatic nerve demage, or amputation) and it has also proved to be useful as a
neuroprotector of cerebral ischemia provoked by
stroke, cardiac arrest, or head trauma (reviewed
in [65]). Finally, an immunoprecipitation assay
with [125I]v-CTx is used to diagnose the Lambert – Eaton myastenic syndrome (reviewed in
[65]), an autoimmune disease in which antibodies
recognize endogenous calcium channels.
Acknowledgements
This work was supported in part by NINDS
Grant R29 NS35786 from the National Institutes
of Health (to M.P. Blanton). We thank Dr Tina
Machu for her critical reading of the manuscript.
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