Solution Structure of α-Conotoxin ImI by¹H Nuclear Magnetic Resonance

Abstract

alpha-Conotoxin ImI derives from the venom of Conus imperialis and is the first and only small-peptide ligand that selectively binds to the neuronal alpha7 homopentameric subtype of the nicotinic acetylcholine receptor (nAChR). This receptor subtype is a possible drug target for several neurological disorders. The cysteines are connected in the pairs Cys2-Cys8 and Cys3-Cys12. To date it is the only alpha-conotoxin with a 4/3 residue spacing between the cysteines. The structure of ImI has been determined by 1H NMR spectroscopy in aqueous solution. The NMR structure is of high quality, with a backbone pairwise rmsd of 0.34 A for a family of 19 structures, and comprises primarily a series of nested beta turns. Addition of organic solvent does not perturb the solution structure. The first eight residues of ImI are identical to the larger, but related, conotoxin EpI and adopt a similar structure, despite a truncated second loop. Residues important for binding of ImI to the alpha7 nAChR are all clustered on one face of the molecule. Once further binding data for EpI and ImI are available, the ImI structure will allow for design of novel alpha7 nAChR-specific agonists and antagonists with a wide range of potential pharmaceutical applications.

α-Selenoconotoxins, a New Class of Potent α7 Neuronal Nicotinic Receptor Antagonists

“…This was anticipated given that correct pairing was driven by the differences in redox potential between cysteine and selenocysteine and that the formation of a mixed sulfide/selenide bond is generally unfavored. Moreover, the correct pairing of selenocysteine residues in the two diselenocysteine bond-containing [Sec 2,3,8,12 ]ImI was achieved as the major product without the use of regioselective pairing of selenocysteine residues (24). This suggests that formation of the diselenide bonds is a cooperative process with the correct fold driven by similar factors that influence the cysteine-rich homolog.…”

A New Level of Conotoxin Diversity, a Non-native Disulfide Bond Connectivity in α-Conotoxin AuIB Reduces Structural Definition but Increases Biological Activity

“…This structure is reminiscent of ImI, shown in Fig. 6c, where the first loop is identical to members of the 4/7 family but the second loop is significantly altered (13).…”

“…There are no solution structures reported for these molecules, but ImI has the same first loop as EpI, and four independent solution structures of ImI have been reported. An N-terminal 3 10 helical motif is reported in just one of the four ImI NMR structures (16), and no other NMR structures of ␣-conotoxins report this element of secondary structure (9,(11)(12)(13)(14)(15)17). In the structure described here a 3 10 helix is detected in 6 of 20 AuIB structures.…”

Analgesic α-Conotoxins Vc1.1 and Rg1A Inhibit N-Type Calcium Channels in Rat Sensory Neurons via GABABReceptor Activation

“…The -conotoxins CVID and MVIIA were prepared as described previously (Lewis et al, 2000). vc1a,RgIA,ImI,MII,BuIA and [A10L]PnIA, were synthesized as reported previously (Gehrmann et al, 1999;Hogg et al, 1999;Blanchfield et al, 2003;Clark et al, 2006Clark et al, , 2008Jin et al, 2007) and provided as a stock concentration in H 2 O of ϳ1 mM and used at a final concentration of 0.1 nM to 1 M. Reduced Vc1.1 was prepared by incubation with dithiothreitol (2 mM; 1 h; room temperature) or alkylated with iodoacetamide in 0.1 M ammonium acetate buffer.…”