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. Does not show effect on crabs. May correspond to the Bg30.66b fraction with a molecular mass of 4593.5 Da (with residues Gly and Thr at the N-terminus of the fragment (assigned by homology)). Belongs to the sea anemone type 3 (BDS) potassium channel toxin family.