Mechanism of ion permeation in skeletal muscle chloride channels.
Voltage-gated Cl- channels belonging to the ClC family exhibit unique properties of ion permeation and gating. We functionally probed the conduction pathway of a recombinant human skeletal muscle Cl- channel (hClC-1) expressed both in Xenopus oocytes and in a mammalian cell line by investigating block by extracellular or intracellular I- and related anions. Extracellular and intracellular I- exert blocking actions on hClC-1 currents that are both concentration and voltage dependent. Similar actions were observed for a variety of other halide (Br-) and polyatomic (SCN-, NO3-, CH3SO3-) anions. In addition, I- block is accompanied by gating alterations that differ depending on which side of the membrane the blocker is applied. External I- causes a shift in the voltage-dependent probability that channels exist in three definable kinetic states (fast deactivating, slow deactivating, nondeactivating), while internal I- slows deactivation. These different effects on gating properties can be used to distinguish two functional ion binding sites within the hClC-1 pore. We determined KD values for I- block in three distinct kinetic states and found that binding of I- to hClC-1 is modulated by the gating state of the channel. Furthermore, estimates of electrical distance for I- binding suggest that conformational changes affecting the two ion binding sites occur during gating transitions. These results have implications for understanding mechanisms of ion selectivity in hClC-1, and for defining the intimate relationship between gating and permeation in ClC channels.
Fahlke
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Department of Medicine, Vanderbilt University School of Medicine, Nashville, Tennessee 37232-2372, USA. cfahlke@mbio.mc.vanderbilt.edu
Dürr
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George
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D000818:Animals; D000838:Anions / Q000378:metabolism; D001667:Binding, Competitive; D018118:Chloride Channels / Q000037:antagonists & inhibitors / Q000378:metabolism* / Q000502:physiology; D004594:Electrophysiology; D005110:Extracellular Space / Q000378:metabolism; D005260:Female; D006801:Humans; D007454:Iodides / Q000494:pharmacology; D015640:Ion Channel Gating / Q000187:drug effects / Q000502:physiology; D007477:Ions; D007700:Kinetics; D008954:Models, Biological; D018482:Muscle, Skeletal / Q000378:metabolism*; D009865:Oocytes; D010539:Permeability; D014982:Xenopus laevis
D000838:Anions; D018118:Chloride Channels; D007454:Iodides; D007477:Ions