Kaliotoxin (1-37) shows structural differences with related potassium channel blockers

The three-dimensional structure of kaliotoxin (1-37), KTX(1-37), a toxin from the scorpion Androctonus mauretanicus mauretanicus that blocks calcium-dependent potassium channels, has been determined by NMR. This toxin is homologous with other scorpion toxins such as charybdotoxin (ChTX) or iberiotoxin (IbTX) for which the structures are already known, but the presence of prolines in the expected alpha-helical region suggested that there may be some major difference in the structure of KTX that could be related to its different selectivity. Proline residues are also found in the homologous region of other scorpion toxins such as noxiustoxin or margatoxin. Our results indicate that KTX(1-37) contains the same sequence of secondary structure elements as ChTX but that the helical region is shorter and distorted due to the presence of two prolines. The distortion consists of a bending in the alpha-helix and in the presence of a 3(10) helix turn in the last three residues. Furthermore, the increased length of the extended structure preceding the helix favors a different packing of this part of the molecule with respect to the secondary structure elements. This change in folding modifies the accessibility of the conserved 27Lys which is known, from mutation studies, to be involved in channel blocking by ChTX.