Binds specifically to voltage-gated sodium channels (Nav) (site 3), thereby delaying their inactivation. This toxin has the highest affinity of all anemone toxins for the mammalian sodium channel, whereas its paralog Anthopleurin-A retains the greatest capacity to discriminate between cardiac (Nav1.5/SCN5A) and neuronal sodium channels (PubMed:8916901). When tested electrophysiologically, this toxin exhibits a high affinity for multiple sodium channels with a 50-fold preference for rat cardiac (Nav1.5/SCN5A) over neuronal channels (0.1 nM versus 5 nM). When tested by ion flux, the affinities are similar and appear to have higher affinity (9 nM versus 22 nM) (PubMed:8276803, PubMed:7612595). The residue Lys-37 of this toxin has been shown to interact with channel Nav1.5 (residue Asp-1612 in rat and Asp-1610 in human), which is located in the DIV S3-S4 linker (corresponding to channel site 3) (PubMed:9417050, PubMed:24898004). Selectively modifies sodium channel inactivation from the open state with little effect on channel activation or on inactivation from closed states (By similarity). Does not display phospholipid-binding activities, suggesting that the domain IV S3-S4 linker is located at the extracellular surface and not buried in the phospholipid bilayer (PubMed:15632158). Belongs to the sea anemone sodium channel inhibitory toxin family. Type I subfamily. Extended N-terminus.