Structural and functional studies of kappa M-conotoxin RIIIK interaction with Shaker-related potassium channels from trout fish (TSha1)

Despite the structural divergence of the peptides interacting with the voltage-gated potassium channels, all these peptides seem to share a dyad motif composed of a lysine and a hydrophobic amino acid residue. Neurotoxin peptides from the venomous cone snails (conotoxins) are highly specific against different voltage-gated ion channels. Recently, kappa M-conotoxin RIIIK (RIIIK) was described to block the Shaker potassium channel in a state-dependent manner. The trout TSha1 potassium channel, is the highest-affinity target of RIIIK yet identified. Interestingly, the 24-amino acid sequence of RIIIK contains three positively charged residues but no aromatic side chain. In the present study, we described the structural and functional parameters important for the binding of RIIIK with the TSha1 channels. An extensive mutational analysis was assessed with the aim to identify the functionally important residues of the toxin. The mutational analysis indicated that four amino acids (Leu1, Arg10, Lys18, and Arg19), which are located as a basic ring on the peptide, are essential for potassium channel binding, resembling that of the most recently described in some scorpion potassium antagonists. A mutant cycle analysis was used to identify the individual interactions of the residues important for RIIIK binding with selected amino acids in the pore region of TSha1. The docking model revealed a novel type of binding in which the toxin blocks the outer vestibule of the channel as a lid. This lid model is novel among potassium channel antagonists. Furthermore, it shows that RIIIK is the first conotoxin to block human Kv1.2 potassium channels, whereas the other members of the Kv1 family tested are not affected by the peptide. The Kv1.2 channel is known to be present in a variety of neurons, and in heart and smooth muscle. Therefore, RIIIK might be useful tool for studying the biophysical properties of these channels as well as their physiological functions