Variability among insect sodium channels revealed by selective neurotoxins (original) (raw)
Related papers
Biochemistry, 1992
Site-directed antibodies corresponding to conserved putative extracellular segments of sodium channels, coupled with binding studies of radiolabeled insect-selective scorpion neurotoxins, were employed to clarify the relationship between the toxins' receptor sites and the insect sodium channel. (1) The depressant insect toxin LqhIT2 was shown to possess two noninteracting binding sites in locust neuronal membranes: a high-affinity (KD, = 0.9 f 0.6 nM) and low-capacity (Bmax, = 0.1 f 0.07 pmol/mg) binding site as well as a low-affinity (KD, = 185 f 13 nM) and high-capacity (Bmaxz = 10.0 f 0.6 pmol/mg) binding site. The high-affinity site serves as a target for binding competition by the excitatory insect toxin AaIT. The binding of LqhIT2 was significantly inhibited in a dose-dependent manner by each of four site-directed antibodies. The binding inhibition resulted from reduction in the number of binding sites. (4) The antibodymediated inhibition of [1251]AaIT binding differs from that of LqhIT2: three out of the four antibodies which inhibited LqhIT2 binding only partially affected AaIT binding. Two antibodies, one corresponding to extracellular and one to intracellular segments of the channel, did not affect the binding of either toxin. These data suggest that the receptors to the depressant and excitatory insect toxins (a) comprise an integral part of the insect sodium channel, (b) are formed by segments of external loops in domains I, 111, and IV of the sodium channel, and (c) are localized in close proximity but are not identical in spite of the competitive interaction between these toxins.
Binding of an α scorpion toxin to insect sodium channels is not dependent on membrane potential
FEBS Letters, 1993
The insect-specific LqhaIT toxin resembles a scorpion toxins affecting mammals by its amino acid sequence and effects on sodium conductance. The present study reveals that LqhaIT does not bind to rat brain membranes and possesses in locust neuronal membranes a single class of high affinity (& = 1.06 f 0.15 nM) and low capacity (B,,,, = 0.7 f 0.19 pmol/mg protein) binding sites. The latter are: (1) distinct from binding sites of other sodium channel neurotoxins; (2) inhibited by sea anemone toxin II; (3) cooperatively interacting with veratridine; (4) not dependent on membrane potential, in contrast to the binding sites of a toxins in vertebrate systems. These data suggest the occurrence of (a) conformationalstructural differences between insect and mammal sodium channels and (b) the animal group specificity and pharmacological importance of the u scorpion toxins.
Several scorpion toxins have been shown to exert their neuro- toxic effects by a direct interaction with voltage-dependent sodium channels. Both classical scorpion a-toxins such as Lqh II from Leiurus quiquestratus hebraeus and a-like toxins as toxin III from the same scorpion (Lqh III) competitively interact for binding on receptor site 3 of insect sodium channels. Con- versely, Lqh III, which is highly toxic in mammalian brain, re- veals no specific binding to sodium channels of rat brain syn- aptosomes and displaces the binding of Lqh II only at high concentration. The contrast between the low-affinity interaction and the high toxicity of Lqh III indicates that Lqh III binding sites distinct from those present in synaptosomes must exist in the brain. In agreement, electrophysiological experiments per- formed on acute rat hippocampal slices revealed that Lqh III strongly affects the inactivation of voltage-gated sodium chan- nels recorded either in current or voltage clamp, wherea...
European Journal of Neuroscience, 1999
α-Like toxins, a unique group designated among the scorpion α-toxin class that inhibit sodium channel inactivation, are highly toxic to mice but do not compete for α-toxin binding to receptor site 3 on rat brain sodium channels. We analysed the sequence of a new α-like toxin, which was also highly active on insects, and studied its action and binding on both mammalian and insect sodium channels. Action of the α-like toxin on isolated cockroach axon is similar to that of an α-toxin, and the radioactive toxin binds with a high affinity to insect sodium channels. Other sodium channel neurotoxins interact competitively or allosterically with the insect α-like toxin receptor site, similarly to α-toxins, suggesting that the α-like toxin receptor site is closely related to receptor site 3.
Journal of Biological Chemistry, 1996
Sodium channels posses receptor sites for many neurotoxins, of which several groups were shown to inhibit sodium current inactivation. Receptor sites that bind ␣and ␣-like scorpion toxins are of particular interest since neurotoxin binding at these extracellular regions can affect the inactivation process at intramembranal segments of the channel. We examined, for the first time, the interaction of different scorpion neurotoxins, all affecting sodium current inactivation and toxic to mammals, with ␣-scorpion toxin receptor sites on both mammalian and insect sodium channels. As specific probes for rat and insect sodium channels, we used the radiolabeled ␣-scorpion toxins AaH II and Lqh␣IT, the most active ␣-toxins on mammals and insect, respectively. We demonstrate that the different scorpion toxins may be classified to several groups, according to their in vivo and in vitro activity on mammalian and insect sodium channels. Analysis of competitive binding interaction reveal that each group may occupy a distinct receptor site on sodium channels. The ␣-mammal scorpion toxins and the anti-insect Lqh␣IT bind to homologous but not identical receptor sites on both rat brain and insect sodium channels. Sea anemone toxin ATX II, previously considered to share receptor site 3 with ␣-scorpion toxins, is suggested to bind to a partially overlapping receptor site with both AaH II and Lqh␣IT. Competitive binding interactions with other scorpion toxins suggest the presence of a putative additional receptor site on sodium channels, which may bind a unique group of these scorpion toxins (Bom III and IV), active on both mammals and insects. We suggest the presence of a cluster of receptor sites for scorpion toxins that inhibit sodium current inactivation, which is very similar on insect and rat brain sodium channels, in spite of the structural and pharmacological differences between them. The sea anemone toxin ATX II is also suggested to bind within this cluster.
Toxicon, 2007
Receptor site-3 on voltage-gated sodium channels is targeted by a variety of structurally distinct toxins from scorpions, sea anemones, and spiders whose typical action is the inhibition of sodium current inactivation. This site interacts allosterically with other topologically distinct receptors that bind alkaloids, lypophilic polyether toxins, pyrethroids, and site-4 scorpion toxins. These features suggest that design of insecticides with specificity for site-3 might be rewarding due to the positive cooperativity with other toxins or insecticidal agents. Yet, despite the central role of scorpion a-toxins in envenomation and their vast use in the study of channel functions, molecular details on site-3 are scarce. Scorpion a-toxins vary greatly in preference for sodium channels of insects and mammals, and some of them are highly active on insects. This implies that despite its commonality, receptor site-3 varies on insect vs. mammalian channels, and that elucidation of these differences could potentially be exploited for manipulation of toxin preference. This review provides current perspectives on (i) the classification of scorpion a-toxins, (ii) their mode of interaction with sodium channels and pharmacological divergence, (iii) molecular details on their bioactive surfaces and differences associated with preference for channel subtypes, as well as (iv) a summary of the present knowledge about elements involved in constituting receptor site-3. These details, combined with the variations in allosteric interactions between site-3 and the other receptor sites on insect and mammalian sodium channels, may be useful in new strategies of insect control and future design of anti-insect selective ligands. r
Invertebrate Neuroscience, 1997
Voltage-sensitive sodium channels are responsible for the generation of electrical signals in most excitable tissues and serve as specific targets for many neurotoxins. At least seven distinct classes of neurotoxins have been designated on the basis of physiological activity and competitive binding studies. Although the characterization of the neurotoxin receptor sites was predominantly performed using vertebrate excitable preparations, insect neuronal membranes were shown to possess similar receptor sites. We have demonstrated that the two mutually competing antiinsect excitatory and depressant scorpion toxins, previously suggested to occupy the same receptor site, bind to two distinct receptors on insect sodium channels. The latter provides a new approach to their combined use in insect control strategy. Although the sodium channel receptor sites are topologically separated, there are strong allosteric interactions among them. We have shown that the lipid-soluble sodium channel activators, veratridine and brevetoxin, reveal divergent allosteric modulation on scorpion a-toxins binding at homologous receptor sites on mammalian and insect sodium channels. The differences suggest a functionally important structural distinction between these channel subtypes. The differential allosteric modulation may provide a new approach to increase selective activity of pesticides on target organisms by simultaneous application of allosterically interacting drugs, designed on the basis of the selective toxins. Thus, a comparative study of neurotoxin receptor sites on mammalian and invertebrate sodium channels may elucidate the structural features involved in the binding and activity of the various neurotoxins, and may offer new targets and approaches to the development of highly selective pesticides.