A novel conotoxin from conus betulinus, κ-BtX, unique in cysteine pattern and in function as a specific BK channel modulator

Abstract

A novel conotoxin, K-conotoxin (κ-BtX), has been purified and characterized from the venom of a worm-hunting cone snail, Conus betulinus. The toxin, with four disulfide bonds, shares no sequence homology with any other conotoxins. Based on a partial amino acid sequence, its cDNA was cloned and sequenced. The deduced sequence consists of a 26-residue putative signal peptide, a 31-residue mature toxin, and a 13-residue extra peptide at the C terminus. The extra peptide is cleaved off by proteinase post-processing. All three Glu residues are γ-carboxylated, one of the two Pro residues is hydroxylated at position 27, and its C-terminal residue is Pro-amidated. The monoisotopic mass of the toxin is 3569.0 Da. Electrophysiological experiments show that: 1) among voltage-gated channels, κ-BtX is a specific modulator of K+ channels; 2) among the K channels, κ-BtX specifically up-modulates the Ca2+- and voltage-sensitive BK channels (252 ± 47%); 3) its EC50 is 0.7 nM with a single binding site (Hill = 0.88); 4) the time constant of wash-out is 8.3 s; and 5) κ-BtX has no effect on single channel conductance, but increases the open probability of BK channels. It is concluded that κ-BtX is a novel specific biotoxin against BK channels.