title: K⁺-dependent selectivity and external Ca²⁺ block of Shab K⁺ channels.
abstract: Potassium channels allow the selective flux of K⁺ excluding the smaller, and more abundant in the extracellular solution, Na⁺ ions. Here we show that Shab is a typical K⁺ channel that excludes Na⁺ under bi-ionic, Na(o)/K(i) or Na(o)/Rb(i), conditions. However, when internal K⁺ is replaced by Cs⁺ (Na(o)/Cs(i)), stable inward Na⁺ and outward Cs⁺ currents are observed. These currents show that Shab selectivity is not accounted for by protein structural elements alone, as implicit in the snug-fit model of selectivity. Additionally, here we report the block of Shab channels by external Ca²⁺ ions, and compare the effect that internal K⁺ replacement exerts on both Ca²⁺ and TEA block. Our observations indicate that Ca²⁺ blocks the channels at a site located near the external TEA binding site, and that this pore region changes conformation under conditions that allow Na⁺ permeation. In contrast, the latter ion conditions do not significantly affect the binding of quinidine to the pore central cavity. Based on our observations and the structural information derived from the NaK bacterial channel, we hypothesize that Ca²⁺ is probably coordinated by main chain carbonyls of the pore's first K⁺-binding site.
authors: Departamento de Fisiología, Universidad Nacional Autónoma de México, DF, México.
authors: Elisa
authors: E
authors: Carrillo
authors: Departamento de Fisiología, Universidad Nacional Autónoma de México, DF, México.
authors: Lucero
authors: L
authors: Pacheco
authors: Departamento de Fisiología, Universidad Nacional Autónoma de México, DF, México.
authors: Daniel
authors: D
authors: Balleza
authors: Departamento de Fisiología, Universidad Nacional Autónoma de México, DF, México.
authors: Froylan
authors: F
authors: Gomez-Lagunas
mesh_terms: D000818:Animals; D001665:Binding Sites / Q000187:drug effects; D002118:Calcium / Q000494:pharmacology*; D002586:Cesium / Q000378:metabolism; D029721:Drosophila Proteins / Q000037:antagonists & inhibitors / Q000737:chemistry / Q000378:metabolism; D004331:Drosophila melanogaster; D010539:Permeability / Q000187:drug effects; D016062:Porosity; D011188:Potassium / Q000378:metabolism*; D026902:Potassium Channel Blockers; D011802:Quinidine / Q000494:pharmacology; D061987:Sf9 Cells; D051669:Shab Potassium Channels / Q000037:antagonists & inhibitors* / Q000737:chemistry / Q000378:metabolism; D012964:Sodium / Q000378:metabolism; D018411:Spodoptera
chemical_list: D029721:Drosophila Proteins; D026902:Potassium Channel Blockers; D051669:Shab Potassium Channels; D002586:Cesium; D012964:Sodium; D011802:Quinidine; D011188:Potassium; D002118:Calcium