Variations in receptor site‐3 on rat brain and insect sodium channels highlighted by binding of a funnel‐web spider δ‐atracotoxin

δ‐Atracotoxins (δ‐ACTXs) from Australian funnel‐web spiders differ structurally from scorpion α‐toxins (ScαTx) but similarly slow sodium current inactivation and compete for their binding to sodium channels at receptor site‐3. Characterization of the binding of 125I‐labelled δ‐ACTX‐Hv1a to various sodium channels reveals a decrease in affinity for depolarized (0 mV; Kd=6.5 ± 1.4 nm) vs.polarized (−55 mV; Kd=0.6 ± 0.2 nm) rat brain synaptosomes. The increased Kd under depolarized conditions correlates with a 4.3‐fold reduction in the association rate and a 1.8‐increase in the dissociation rate. In comparison, ScαTx binding affinity decreased 33‐fold under depolarized conditions due to a 48‐fold reduction in the association rate. The binding of 125I‐labelled δ‐ACTX‐Hv1a to rat brain synaptosomes is inhibited competitively by classical ScαTxs and allosterically by brevetoxin‐1, similar to ScαTx binding. However, in contrast with classical ScαTxs, 125I‐labelled δ‐ACTX‐Hv1a binds with high affinity to cockroach Na+ channels (Kd=0.42 ± 0.1 nm) and is displaced by the ScαTx, LqhαIT, a well‐defined ligand of insect sodium channel receptor site‐3. However, δ‐ACTX‐Hv1a exhibits a surprisingly low binding affinity to locust sodium channels. Thus, unlike ScαTxs, which are capable of differentiating between mammalian and insect sodium channels, δ‐ACTXs differentiate between various insect sodium channels but bind with similar high affinity to rat brain and cockroach channels. Structural comparison of δ‐ACTX‐Hv1a to ScαTxs suggests a similar putative bioactive surface but a ‘slimmer’ overall shape of the spider toxin. A slimmer shape may ease the interaction with the cockroach and mammalian receptor site‐3 and facilitate its association with different conformations of the rat brain receptor, correlated with closed/open and slow‐inactivated channel states.