Potently and selectively inhibits voltage-gated potassium channels Kv11/KCNH/ERG. Acts as a gating-modifier toxin that shifts the voltage-dependence of ERG activation in the positive direction and suppresses its current amplitudes elicited by strong depolarizing pulses that maximally activate the channels. Belongs to the sea anemone type 3 (BDS) potassium channel toxin family.