Mg2+ enhances voltage sensor/gate coupling in BK channels.
BK (Slo1) potassium channels are activated by millimolar intracellular Mg(2+) as well as micromolar Ca(2+) and membrane depolarization. Mg(2+) and Ca(2+) act in an approximately additive manner at different binding sites to shift the conductance-voltage (G(K)-V) relation, suggesting that these ligands might work through functionally similar but independent mechanisms. However, we find that the mechanism of Mg(2+) action is highly dependent on voltage sensor activation and therefore differs fundamentally from that of Ca(2+). Evidence that Ca(2+) acts independently of voltage sensor activation includes an ability to increase open probability (P(O)) at extreme negative voltages where voltage sensors are in the resting state; 2 microM Ca(2+) increases P(O) more than 15-fold at -120 mV. However 10 mM Mg(2+), which has an effect on the G(K)-V relation similar to 2 microM Ca(2+), has no detectable effect on P(O) when voltage sensors are in the resting state. Gating currents are only slightly altered by Mg(2+) when channels are closed, indicating that Mg(2+) does not act merely to promote voltage sensor activation. Indeed, channel opening is facilitated in a voltage-independent manner by Mg(2+) in a mutant (R210C) whose voltage sensors are constitutively activated. Thus, 10 mM Mg(2+) increases P(O) only when voltage sensors are activated, effectively strengthening the allosteric coupling of voltage sensor activation to channel opening. Increasing Mg(2+) from 10 to 100 mM, to occupy very low affinity binding sites, has additional effects on gating that more closely resemble those of Ca(2+). The effects of Mg(2+) on steady-state activation and I(K) kinetics are discussed in terms of an allosteric gating scheme and the state-dependent interactions between Mg(2+) and voltage sensor that may underlie this mechanism.
Horrigan
Frank T
FT
Department of Physiology, University of Pennsylvania School of Medicine, Philadelphia, PA 19104, USA. horrigan@mail.med.upenn.edu
Ma
Zhongming
Z
D000494:Allosteric Regulation / Q000502:physiology; D000818:Animals; D002118:Calcium / Q000502:physiology; D002462:Cell Membrane / Q000502:physiology; D004594:Electrophysiology; D005260:Female; D015640:Ion Channel Gating / Q000502:physiology*; D051036:Large-Conductance Calcium-Activated Potassium Channels / Q000502:physiology*; D008274:Magnesium / Q000502:physiology*; D008954:Models, Biological; D009865:Oocytes / Q000166:cytology / Q000502:physiology; D018408:Patch-Clamp Techniques; D015398:Signal Transduction / Q000502:physiology; D055672:Static Electricity; D014982:Xenopus laevis
D051036:Large-Conductance Calcium-Activated Potassium Channels; D008274:Magnesium; D002118:Calcium