title: Conversion of a scorpion toxin agonist into an antagonist highlights an acidic residue involved in voltage sensor trapping during activation of neuronal Na+ channels.
abstract: Gating modifiers constitute a large group of polypeptide toxins that interact with the voltage-sensing module of ion channels. Among them, scorpion beta-toxins induce a negative shift in the voltage dependence of sodium channel activation. To explain their effect, a "voltage sensor trapping" model has been proposed in which the voltage sensor of domain-II (DIIS4) is trapped in an outward, activated position by a prebound beta-toxin upon membrane depolarization. Whereas toxin effect on channel activation was enhanced upon neutralization of the two outermost arginines in DIIS4, toxin residues involved in sensor trapping have not been identified. Using the scorpion excitatory beta-toxin, Bj-xtrIT, we found two conserved acidic residues, Glu15 and Glu30, mandatory for toxin action. Whereas mutagenesis of Glu30 affected both toxicity and binding affinity, substitutions E15A/F abolished activity but had minor effects on binding. Complete uncoupling of activity from binding was obtained with mutant E15R, acting as an efficient antagonist of Bj-xtrIT. On the basis of the voltage sensor trapping model and our results, we propose that Glu15 interacts with the emerging gating charges of DIIS4 upon membrane depolarization. Conserved acidic residues found in a variety of gating modifiers from scorpions and spiders may interact similarly with the voltage sensor.
authors.affiliation: Department of Plant Sciences, George S. Wise Faculty of Life Sciences, Tel-Aviv University, Ramat-Aviv 69978, Tel-Aviv, Israel.
authors.forename: Izhar
authors.initials: I
authors.lastname: Karbat
authors.forename: Lior
authors.initials: L
authors.lastname: Cohen
authors.forename: Nicholas
authors.initials: N
authors.lastname: Gilles
authors.forename: Dalia
authors.initials: D
authors.lastname: Gordon
authors.forename: Michael
authors.initials: M
authors.lastname: Gurevitz
authors.forename: Karbat
authors.initials: K
authors.lastname: Izhar
authors.forename: Cohen
authors.initials: C
authors.lastname: Lior
authors.forename: Gilles
authors.initials: G
authors.lastname: Nicholas
authors.forename: Gordon
authors.initials: G
authors.lastname: Dalia
authors.forename: Gurevitz
authors.initials: G
authors.lastname: Michael
mesh_terms: D000595:Amino Acid Sequence; D000818:Animals; D004175:Diptera / Q000187:drug effects; D018698:Glutamic Acid / Q000235:genetics / Q000502:physiology; D019476:Insect Proteins; D015640:Ion Channel Gating / Q000187:drug effects; D008969:Molecular Sequence Data; D016297:Mutagenesis, Site-Directed; D009474:Neurons / Q000187:drug effects; D009498:Neurotoxins / Q000737:chemistry* / Q000235:genetics / Q000633:toxicity*; D010526:Periplaneta / Q000378:metabolism; D012604:Scorpion Venoms / Q000737:chemistry* / Q000235:genetics / Q000633:toxicity*; D016415:Sequence Alignment; D062687:Sodium Channel Agonists; D026941:Sodium Channel Blockers / Q000737:chemistry* / Q000633:toxicity
chemical_list: C115530:Bj-xtrIT toxin; D019476:Insect Proteins; D009498:Neurotoxins; D012604:Scorpion Venoms; D062687:Sodium Channel Agonists; D026941:Sodium Channel Blockers; D018698:Glutamic Acid