Scorpion toxins that affect sodium channel (NaCh) gating in excitable cells are divided into a-and b-classes. Whereas a-toxins have been found in scorpions throughout the world, anti-mammalian b-toxins have been assigned, thus far, to ÔNew WorldÕ scorpions while anti-insect selective b-toxins (depressant and excitatory) have been described only in the ÔOld WorldÕ. 2This distribution suggested that diversification of b-toxins into distinct pharmacological groups occurred after the separation of the continents, 150 million years ago. We have characterized a unique toxin, Lqhb1, from the ÔOld WorldÕ scorpion, Leiurus quinquestriatus hebraeus, that resembles in sequence and activity both ÔNew WorldÕ b-toxins as well as ÔOld WorldÕ depressant toxins. Lqhb1 competes, with apparent high affinity, with anti-insect and anti-mammalian b-toxins for binding to cockroach and rat brain synaptosomes, respectively. Surprisingly, Lqhb1 also competes with an anti-mammalian a-toxin on binding to rat brain NaChs. Analysis of Lqhb1 effects on rat brain and Drosophila Para NaChs expressed in Xenopus oocytes revealed a shift in the voltage-dependence of activation to more negative membrane potentials and a reduction in sodium peak currents in a manner typifying b-toxin activity. Moreover, Lqhb1 resembles b-toxins by having a weak effect on cardiac NaChs and a marked effect on rat brain and skeletal muscle NaChs 3. These multifaceted features suggest that Lqhb1 may represent an ancestral b-toxin group in ÔOld WorldÕ scorpions that gave rise, after the separation of the continents, to depressant toxins in ÔOld WorldÕ scorpions and to various b-toxin subgroups in ÔNew WorldÕ scorpions.Keywords: scorpion toxins; sodium channel subtypes; toxin diversification.Scorpion Ôlong chainÕ toxins affecting voltage-gated sodium channels (NaCh) are polypeptides of 61-76 amino acids long that traditionally are divided between two major classes, a and b, according to their physiological effects on channel gating and their binding properties [1][2][3]. a-Toxins slow sodium channel inactivation upon binding to a homologous cluster of binding sites named receptor site-3, and are subdivided into distinct groups according to their potency for mammalian and insect receptors and their affinity for sodium channel subtypes [2,[4][5][6][7]. a-Toxins predominate in the venom of Buthidae scorpions of the ÔOld WorldÕ (Africa and Asia), but some representatives have been also identified in ÔNew WorldÕ (America) scorpions [1]. b-Toxins shift the activation voltage of sodium channels to more negative membrane potentials upon binding to receptor site-4 [2,7,8], and vary greatly in their effects on various animals. Css2 and Css4 (from Centruroides suffusus suffusus) show specificity for mammals [1]; Cll1 (from C. limpidus limpidus), Cn5, and Cn11 (from C. noxius) are highly effective on crustaceans [9-12]; Ts7 and Tst1 (from Tityus serrulatus and T. stigmurus), and Tbs1 and Tbs2 (from T. bahiensis) are highly effective on both insects and mammals [1,[12][13][14][15][16]...