3D structure of kaliotoxin: is residue 34 a key for channel selectivity?

Kaliotoxin (KTX) is a natural peptide blocker of voltage-dependent K+ channels. The 3D structure of a truncated analogue of KTX (Fernandez et al. (1994) Biochemistry 33, 14256-14263) was determined by NMR spectroscopy and showed significant differences from structures established for other related scorpion toxins. A recent publication with the structure of the complete toxin (Aiyar et al. (1995) Neuron 15, 1169-1181) did not confirm these differences. In this communication we report NMR data for KTX at pH 3.0, 5.5 and 7.2 and the 3D structure obtained from data at pH = 5.5. Complete KTX displays a folding similar to that of other toxins with an alpha-helix and a beta-sheet linked by two disulphide bonds. The pKa of His 34 is anomalously low (4.7-5.2 depending on the buffer) owing to its interaction with two Lys residues (including the essential Lys 27), the charged N-terminus and the side chain of Met 29. Charged residues are placed symmetrically with respect to an axis that approximately coincides with one of the principal components of the moment of inertia of the toxin. His 34, which occupies a well-defined position between two conserved Cys, is located on the centre of a layer of charged groups. Positively and negatively charged residues are found at the same position in related toxins. It is suggested that electrostatic effects modulate the distances between positive charges in flexible side chains, contributing to the fine tuning of the selectivity toward different channel subclasses and that the approximate coincidence between the moment of inertia and the charge axis facilitate the approach of the toxin to the channel. The very low pKa of His 34 implies that it will be completely unprotonated at physiological pH.