We have determined the three-dimensional structure
of the potassium channel inhibitor HsTX1, using nuclear
magnetic resonance and molecular modeling. This protein
belongs to the scorpion short toxin family, which essentially
contains potassium channel blockers of 29 to 39 amino acids
and three disulfide bridges. It is highly active on voltage-gated
Kv1.3 potassium channels. Furthermore, it has the particularity
to possess a fourth disulfide bridge. We show that HsTX1
has a fold similar to that of the three-disulfide–bridged
toxins and conserves the hydrophobic core found in the
scorpion short toxins. Thus, the fourth bridge has no influence
on the global conformation of HsTX1. Most residues spatially
analogous to those interacting with voltage-gated potassium
channels in the three-disulfide–bridged toxins are
conserved in HsTX1. Thus, we propose that Tyr21, Lys23,
Met25, and Asn26 are involved in the biological activity
of HsTX1. As an additional positively charged residue is
always spatially close to the aromatic residue in toxins
blocking the voltage-gated potassium channels, and as previous
mutagenesis experiments have shown the critical role played
by the C-terminus in HsTX1, we suggest that Arg33 is also
important for the activity of the four disulfide-bridged
toxin. Docking calculations confirm that, if Lys23 and
Met25 interact with the GYGDMH motif of Kv1.3, Arg33 can
contact Asp386 and, thus, play the role of the additional
positively charged residue of the toxin functional site.
This original configuration of the binding site of HsTX1
for Kv1.3, if confirmed experimentally, offers new structural
possibilities for the construction of a molecule blocking
the voltage-gated potassium channels.