Assignment of voltage-gated potassium channel blocking activity to κ-KTx1.3, a non-toxic homologue of κ-hefutoxin-1, from Heterometrus spinifer venom

Abstract

A new family of weak K+ channel toxins (designated ?-KTx) with a novel "bi-helical" scaffold has recently been characterized from Heterometrus fulvipes (Scorpionidae) venom. Based on the presence of the minimum functional dyad (Y5 and K19), ?-hefutoxin-1 (?-KTx1.1) was investigated and found to block Kv 1.2 (IC50 ~40 卩 and Kv 1.3 (IC50 ~150 卩 channels. In the present study, ?-KTx1.3, that shares ~60% identity with ?-hefutoxin 1, has been isolated from Heterometrus spinifer venom. Interestingly, despite the presence of the functional dyad (Y5 and K19), ?-KTx1.3 failed to reproduce the K+ channel blocking activity of ?-hefutoxin-1. Since the dyad lysine in ?-KTx1.3 was flanked by another lysine (K20), it was hypothesized that this additional positive charge could hinder the critical electrostatic interactions known to occur between the dyad lysine and the Kv 1 channel selectivity filter. Hence, mutants of ?-KTx1.3, substituting K20 with a neutral (K20A) or a negatively (K20E) or another positively (K20R) charged amino acid were synthesized. ?-KTx1.3 K20E, in congruence with ?-hefutoxin 1 with respect to subtype selectivity and affinity, produced blockade of Kv 1.2 (IC50 = 36.8 ᠴ.9 卩 and Kv 1.3 (IC50 = 53.7 ᠶ.7 卩 but not Kv 1.1 channels. ?-KTx1.3 K20A produced blockade of both Kv 1.2 (IC50 = 36.9 ᠴ.9 卩 and Kv 1.3 (IC50 = 115.7 ᠷ.3 卩 and in addition, acquired affinity for Kv 1.1 channels (IC50 = 110.7 ᠷ.7 卩. ?-KTx1.3 K20R failed to produce any blockade on the channel subtypes tested. These data suggest that the presence of an additional charged residue in a position adjacent to the dyad lysine impedes the functional block of Kv 1 channels produced by ?-KTx1.3.