Tài liệu Báo cáo khoa học: Structure-activity relationships of a-conotoxins targeting neuronal

MINIREVIEW

Structure-activity relationships of a-conotoxins targeting neuronal

nicotinic acetylcholine receptors

Emma L. Millard, Norelle L. Daly and David J. Craik

Institute for Molecular Bioscience, University of Queensland, Brisbane, QLD, Australia

a-Conotoxins that target the neuronal nicotinic acetylcho￾line receptor have a range of potential therapeutic applica￾tions and are valuable probes for examining receptor

subtype selectivity. The three-dimensional structures of

about half of the known neuronal specific a-conotoxins have

now been determined and have a consensus fold containing a

helical region braced by two conserved disulfide bonds.

These disulfide bonds define the two-loop framework char￾acteristic for a-conotoxins, CCXmCXnC, where loop 1

comprises four residues (m ¼ 4) and loop 2 between three

and seven residues (n ¼ 3, 6 or 7). Structural studies, par￾ticularly using NMR spectroscopy have provided an insight

into the role and spatial location of residues implicated in

receptor binding and biological activity.

Keywords: NMR; peptide; X-ray crystallography.

Introduction

As outlined in other articles in this series, the a-conotoxins

have a range of potential therapeutic applications and have

proved to be valuable pharmacological tools based on their

ability to selectively inhibit the nicotinic acetylcholine

receptor (nAChR) [1–3]. The focus of this review is on

the three-dimensional structures of a-conotoxins and the

progress made towards dissecting the features involved in

receptor subtype selectivity. In particular, a-conotoxins

targeting neuronal rather than muscle nAChRs will be

discussed. Muscle specific a-conotoxins have been covered

in other more general reviews [4–6]. There is much current

interest in various neuronal receptor subtypes implicated in

diverse neurological disorders such as Alzheimer’s disease

and epilepsy [7–9], and in the regulation of small-cell lung

carcinoma [10,11].

The sequences, subtype selectivity and potency of

a-conotoxins targeting neuronal nAChRs are given in

Table 1, together with information on their structural

characterization. The cysteine residues and disulfide con￾nectivity are invariant throughout these sequences and

define a two-loop framework, CCXmCXnC (Xm and Xn

refer to the number of noncysteine residues), where the

loops correspond to the residues between successive cysteine

residues. The number of residues in the two loops (m/n) is

used to group the a-conotoxins into different frameworks.

ImI and ImII have a 4/3 framework and the other peptides

in Table 1 contain either a 4/6 or 4/7 framework. It is

interesting to note that although the majority of 4/6 and 4/7

a-conotoxins are selective for neuronal nAChRs, conotoxin

EI contains a 4/7 framework but binds to the muscle-type

nAChR [12].

The sequence conservation of the a-conotoxins extends

beyond the cysteine residues, with a Ser and Pro in loop 1

being highly conserved. However, there is a significant degree

of sequence variation in the remaining residues, particularly

in loop 2. It is this sequence diversity that provides the

exquisite selectivity that a-conotoxins display for various

nAChR subtypes (Table 1). Structures of neuronally active

a-conotoxins, in conjunction with activity studies, have

provided clues to understanding the complexity involved in

binding to the nAChR. A summary of this structural

information and the insights into structure-activity relation￾ships of a-conotoxins is presented in this review.

Structural features of a-conotoxins

The three-dimensional structures of a-conotoxins have

been determined, primarily using NMR spectroscopy. It

is unusual for such small peptides to crystallize but a few

a-conotoxins have been amenable to analysis with X-ray

crystallography. To date no neuronally active conotoxins

have been structurally characterized using both techniques,

however, the neuromuscularly active conotoxin GI has been

studied using both methods and the structures overlay very

closely [13–15].

Despite the small size of a-conotoxins they have well￾defined structures with a characteristic overall fold. With the

structures of more than half of the known neuronally active

a-conotoxins determined it is possible to determine the

consensus structural features. These features involve

restraints imposed by the conserved disulfide connectivity

and a helical region centred around Cys III. The helix

typically encompasses residues 5–12. A comparison of the

known structures is given in Fig. 1 with the three framework

classes presented separately for clarity. It is clear that the

backbone fold of loop 1 is highly conserved, including the

first turn of the helix. The major differences, as might be

Correspondence to D. Craik, Institute for Molecular Bioscience,

University of Queensland, Brisbane, QLD, 4072, Australia.

Fax: + 61 73346 2029, Tel.: + 61 73346 2019,

E-mail: [email protected]

Abbreviation: nAChR, nicotinic acetylcholine receptor.

(Received 22 January 2004, revised 19 March 2004,

accepted 6 April 2004)

Eur. J. Biochem. 271, 2320–2326 (2004)
FEBS 2004 doi:10.1111/j.1432-1033.2004.04148.x