A hot spot for the interaction of gating modifier toxins with voltage-dependent ion channels

The gating modifier toxins are a large family of protein toxins that modify either activation or inactivation of voltage-gated ion channels. omega -Aga-IVA is a gating modifier toxin from spider venom that inhibits voltage-gated Ca2+ channels by shifting activation to more depolarized voltages. We identified two Glu residues near the COOH-terminal edge of S3 in the alpha (1A) Ca2+ channel (one in repeat I and the other in repeat IV) that align with Glu residues previously implicated in for-cling the binding sites for gating modifier toxins on K+ and Na+ channels. We found that mutation of the Glu residue in repeat I of the Ca2+ channel had no significant effect on inhibition by omega -Aga-IVA, whereas the equivalent mutation of the Glu in repeat TV disrupted inhibition by the toxin. These results suggest that the COOH-terminal end of 53 within repeat IV contributes to forming a receptor for omega -Aga-IVA. The strong predictive value of previous mapping studies for K+ and Na+ channel toxins argues for a conserved binding motif for gating modifier toxins within the voltage-sensing domains of voltage-gated ion channels.