title: Binding of a gating modifier toxin induces intersubunit cooperativity early in the Shaker K channel's activation pathway.
abstract: Potassium currents from voltage-gated Shaker K channels activate with a sigmoid rise. The degree of sigmoidicity in channel opening kinetics confirms that each subunit of the homotetrameric Shaker channel undergoes more than one conformational change before the channel opens. We have examined effects of two externally applied gating modifiers that reduce the sigmoidicity of channel opening. A toxin from gastropod mucus, 6-bromo-2-mercaptotryptamine (BrMT), and divalent zinc are both found to slow the same conformational changes early in Shaker's activation pathway. Sigmoidicity measurements suggest that zinc slows a conformational change independently in each channel subunit. Analysis of activation in BrMT reveals cooperativity among subunits during these same early steps. A lack of competition with either agitoxin or tetraethylammonium indicates that BrMT binds channel subunits outside of the external pore region in an allosterically cooperative fashion. Simulations including negatively cooperative BrMT binding account for its ability to induce gating cooperativity during activation. We conclude that cooperativity among K channel subunits can be greatly altered by experimental conditions.
authors.affiliation: Howard Hughes Medical Institute and Department of Molecular and Cellular Physiology, Stanford University School of Medicine, CA 94305, USA.
authors.forename: Jon T
authors.initials: JT
authors.lastname: Sack
authors.forename: Richard W
authors.initials: RW
authors.lastname: Aldrich
mesh_terms: D000818:Animals; D004330:Drosophila; D029721:Drosophila Proteins / Q000235:genetics / Q000378:metabolism / Q000502:physiology*; D005260:Female; D015640:Ion Channel Gating / Q000187:drug effects / Q000502:physiology*; D007700:Kinetics; D008564:Membrane Potentials / Q000187:drug effects; D008956:Models, Chemical; D009154:Mutation / Q000235:genetics; D009498:Neurotoxins / Q000378:metabolism / Q000494:pharmacology; D009865:Oocytes / Q000187:drug effects / Q000378:metabolism / Q000502:physiology; D018408:Patch-Clamp Techniques; D011188:Potassium / Q000378:metabolism; D026902:Potassium Channel Blockers / Q000378:metabolism / Q000494:pharmacology; D011485:Protein Binding; D020836:Protein Structure, Quaternary / Q000187:drug effects; D021122:Protein Subunits / Q000737:chemistry / Q000502:physiology*; D012333:RNA, Messenger / Q000235:genetics; D012604:Scorpion Venoms / Q000378:metabolism; D051637:Shaker Superfamily of Potassium Channels / Q000235:genetics / Q000378:metabolism / Q000502:physiology*; D019789:Tetraethylammonium / Q000378:metabolism / Q000494:pharmacology; D014363:Tryptamines / Q000378:metabolism* / Q000494:pharmacology; D014982:Xenopus laevis; D015032:Zinc / Q000494:pharmacology
chemical_list: C498479:BrMT neurotoxin, Calliostoma canaliculatum; D029721:Drosophila Proteins; D009498:Neurotoxins; D026902:Potassium Channel Blockers; D021122:Protein Subunits; D012333:RNA, Messenger; D012604:Scorpion Venoms; C496929:Sh protein, Drosophila; D051637:Shaker Superfamily of Potassium Channels; D014363:Tryptamines; C087603:agitoxin 2; D019789:Tetraethylammonium; D015032:Zinc; D011188:Potassium