Effects of multiple metal binding sites on calcium and magnesium-dependent activation of BK channels

Coupling of Ca2+ and voltage activation in BK channels through the αB helix/voltage sensor interface

Large-conductance Ca²⁺ and voltage-activated K⁺ (BK) channels control membrane excitability in many cell types. BK channels are tetrameric. Each subunit is composed of a voltage sensor domain (VSD), a central pore-gate domain, and a large cytoplasmic domain (CTD) that contains the Ca²⁺ sensors. While it is known that BK channels are activated by voltage and Ca²⁺, and that voltage and Ca²⁺ activations interact, less is known about the mechanisms involved. We explore here these mechanisms by examining the gating contribution of an interface formed between the VSDs and the αB helices located at the top of the CTDs. Proline mutations in the αB helix greatly decreased voltage activation while having negligible effects on gating currents. Analysis with the Horrigan, Cui, and Aldrich model indicated a decreased coupling between voltage sensors and pore gate. Proline mutations decreased Ca²⁺ activation for both Ca²⁺ bowl and RCK1 Ca²⁺ sites, suggesting that both high-affinity Ca²⁺ sites transduce their effect, at least in part, through the αB helix. Mg²⁺ activation also decreased. The crystal structure of the CTD with proline mutation L390P showed a flattening of the first helical turn in the αB helix compared to wild type, without other notable differences in the CTD, indicating that structural changes from the mutation were confined to the αB helix. These findings indicate that an intact αB helix/VSD interface is required for effective coupling of Ca²⁺ binding and voltage depolarization to pore opening and that shared Ca²⁺ and voltage transduction pathways involving the αB helix may be involved.

Interactions of divalent cations with calcium binding sites of BK channels reveal independent motions within the gating ring

Large-conductance voltage- and calcium-activated K⁺ (BK) channels are key physiological players in muscle, nerve, and endocrine function by integrating intracellular Ca²⁺ and membrane voltage signals. The open probability of BK channels is regulated by the intracellular concentration of divalent cations sensed by a large structure in the BK channel called the "gating ring," which is formed by four tandems of regulator of conductance for K⁺ (RCK1 and RCK2) domains. In contrast to Ca²⁺ that binds to both RCK domains, Mg²⁺, Cd²⁺, or Ba²⁺ interact preferentially with either one or the other. Interaction of cations with their binding sites causes molecular rearrangements of the gating ring, but how these motions occur remains elusive. We have assessed the separate contributions of each RCK domain to the cation-induced gating-ring structural rearrangements, using patch-clamp fluorometry. Here we show that Mg²⁺ and Ba²⁺ selectively induce structural movement of the RCK2 domain, whereas Cd²⁺ causes motions of RCK1, in all cases substantially smaller than those elicited by Ca²⁺ By combining divalent species interacting with unique sites, we demonstrate that RCK1 and RCK2 domains move independently when their specific binding sites are occupied. Moreover, binding of chemically distinct cations to both RCK domains is additive, emulating the effect of fully occupied Ca²⁺ binding sites.

Ethanol Effect on BK Channels is Modulated by Magnesium

BACKGROUND

Alcoholics have been reported to have reduced levels of magnesium in both their extracellular and intracellular compartments. Calcium-dependent potassium channels (BK) are known to be one of ethanol (EtOH)'s better known molecular targets.

METHODS

Using outside-out patches from hippocampal neuronal cultures, we examined the consequences of altered intracellular Mg(2+) on the effects that EtOH has on BK channels.

RESULTS

We find that the effect of EtOH is bimodally influenced by the Mg(2+) concentration on the cytoplasmic side. More specifically, when internal Mg(2+) concentrations are ≤200 μM, EtOH decreases BK activity, whereas it increases activity when Mg(2+) is at 1 mM. Similar results are obtained when using patches from HEK cells expressing only the α-subunit of BK. When patches are made with the actin destabilizer cytochalasin D present on the cytoplasmic side, the potentiation caused by EtOH becomes independent of the Mg(2+) concentration. Furthermore, in the presence of the actin stabilizer phalloidin, EtOH causes inhibition even at Mg(2+) concentrations of 1 mM.

CONCLUSIONS

Internal Mg(2+) can modulate the EtOH effects on BK channels only when there is an intact, internal actin interaction with the channel, as is found at synapses. We propose that the EtOH-induced decrease in cytoplasmic Mg(2+) observed in frequent/chronic drinkers would decrease EtOH's actions on synaptic (e.g., actin-bound) BK channels, producing a form of molecular tolerance.

Transduction of voltage and Ca2+ signals by Slo1 BK channels

Large-conductance Ca2+ -and voltage-gated K+ channels are activated by an increase in intracellular Ca2+ concentration and/or depolarization. The channel activation mechanism is well described by an allosteric model encompassing the gate, voltage sensors, and Ca2+ sensors, and the model is an excellent framework to understand the influences of auxiliary β and γ subunits and regulatory factors such as Mg2+. Recent advances permit elucidation of structural correlates of the biophysical mechanism.

State-dependent FRET reports calcium- and voltage-dependent gating-ring motions in BK channels

Large-conductance voltage- and calcium-dependent potassium channels (BK, "Big K+") are important controllers of cell excitability. In the BK channel, a large C-terminal intracellular region containing a "gating-ring" structure has been proposed to transduce Ca(2+) binding into channel opening. Using patch-clamp fluorometry, we have investigated the calcium and voltage dependence of conformational changes of the gating-ring region of BK channels, while simultaneously monitoring channel conductance. Fluorescence resonance energy transfer (FRET) between fluorescent protein inserts indicates that Ca(2+) binding produces structural changes of the gating ring that are much larger than those predicted by current X-ray crystal structures of isolated gating rings.

The BK channel: a vital link between cellular calcium and electrical signaling

Large-conductance Ca²⁺-activated K⁺ channels (BK channels) constitute an key physiological link between cellular Ca²⁺ signaling and electrical signaling at the plasma membrane. Thus these channels are critical to the control of action potential firing and neurotransmitter release in several types of neurons, as well as the dynamic control of smooth muscle tone in resistance arteries, airway, and bladder. Recent advances in our understanding of K⁺ channel structure and function have led to new insight toward the molecular mechanisms of opening and closing (gating) of these channels. Here we will focus on mechanisms of BK channel gating by Ca²⁺, transmembrane voltage, and auxiliary subunit proteins.

Mg(2+) binding to open and closed states can activate BK channels provided that the voltage sensors are elevated

BK channels are activated by intracellular Ca²⁺ and Mg²⁺ as well as by depolarization. Such activation is possible because each of the four subunits has two high-affinity Ca²⁺ sites, one low-affinity Mg²⁺ site, and a voltage sensor. This study further investigates the mechanism of Mg²⁺ activation by using single-channel recording to determine separately the action of Mg²⁺ on the open and closed states of the channel. To limit Mg²⁺ action to the Mg²⁺ sites, the two high-affinity Ca²⁺ sites are disabled by mutation. When the voltage is stepped from negative holding potentials to +100 mV, we find that 10 mM Mg²⁺ decreases the mean closed latency to the first channel opening 2.1-fold, decreases the mean closed interval duration 8.7-fold, increases mean burst duration 10.1-fold, increases the number of openings per burst 4.4-fold, and increases mean open interval duration 2.3-fold. Hence, Mg²⁺ can bind to closed BK channels, increasing their opening rates, and to open BK channels, decreasing their closing rates. To explore the relationship between Mg²⁺ action and voltage sensor activation, we record single-channel activity in macropatches containing hundreds of channels. Open probability (P_o) is dramatically increased by 10 mM Mg²⁺ when voltage sensors are activated with either depolarization or the mutation R210C. The increased P_o arises from large decreases in mean closed interval durations and moderate increases in mean open interval durations. In contrast, 10 mM Mg²⁺ has no detectable effects on P_o or interval durations when voltage sensors are deactivated with very negative potentials or the mutation R167E. These observations are consistent with a model in which Mg²⁺ can bind to and alter the gating of both closed and open states to increase P_o, provided that one or more voltage sensors are activated.

Metal-driven operation of the human large-conductance voltage- and Ca2+-dependent potassium channel (BK) gating ring apparatus

Large-conductance voltage- and Ca(2+)-dependent K(+) (BK, also known as MaxiK) channels are homo-tetrameric proteins with a broad expression pattern that potently regulate cellular excitability and Ca(2+) homeostasis. Their activation results from the complex synergy between the transmembrane voltage sensors and a large (>300 kDa) C-terminal, cytoplasmic complex (the "gating ring"), which confers sensitivity to intracellular Ca(2+) and other ligands. However, the molecular and biophysical operation of the gating ring remains unclear. We have used spectroscopic and particle-scale optical approaches to probe the metal-sensing properties of the human BK gating ring under physiologically relevant conditions. This functional molecular sensor undergoes Ca(2+)- and Mg(2+)-dependent conformational changes at physiologically relevant concentrations, detected by time-resolved and steady-state fluorescence spectroscopy. The lack of detectable Ba(2+)-evoked structural changes defined the metal selectivity of the gating ring. Neutralization of a high-affinity Ca(2+)-binding site (the "calcium bowl") reduced the Ca(2+) and abolished the Mg(2+) dependence of structural rearrangements. In congruence with electrophysiological investigations, these findings provide biochemical evidence that the gating ring possesses an additional high-affinity Ca(2+)-binding site and that Mg(2+) can bind to the calcium bowl with less affinity than Ca(2+). Dynamic light scattering analysis revealed a reversible Ca(2+)-dependent decrease of the hydrodynamic radius of the gating ring, consistent with a more compact overall shape. These structural changes, resolved under physiologically relevant conditions, likely represent the molecular transitions that initiate the ligand-induced activation of the human BK channel.

Molecular mechanisms of BK channel activation

Large conductance, Ca(2+)-activated potassium (BK) channels are widely expressed throughout the animal kingdom and play important roles in many physiological processes, such as muscle contraction, neural transmission and hearing. These physiological roles derive from the ability of BK channels to be synergistically activated by membrane voltage, intracellular Ca(2+) and other ligands. Similar to voltage-gated K(+) channels, BK channels possess a pore-gate domain (S5-S6 transmembrane segments) and a voltage-sensor domain (S1-S4). In addition, BK channels contain a large cytoplasmic C-terminal domain that serves as the primary ligand sensor. The voltage sensor and the ligand sensor allosterically control K(+) flux through the pore-gate domain in response to various stimuli, thereby linking cellular metabolism and membrane excitability. This review summarizes the current understanding of these structural domains and their mutual interactions in voltage-, Ca(2+)- and Mg(2+)-dependent activation of the channel.

Activation of Slo1 BK channels by Mg2+ coordinated between the voltage sensor and RCK1 domains

The voltage sensor domain (VSD) and the ligand sensor (cytoplasmic domain) of BK channels synergistically control channel activities, thereby integrating electrical and chemical signals for cell function. Studies show that intracellular Mg²⁺ mediates the interaction between these sensory domains to activate the channel through an electrostatic interaction with the VSD. Here we report that Mg²⁺ binds to a site that consists of amino acid side-chains from both the VSD (Asp99 and Asn172) and the cytoplasmic domain (Glu374 and Glu399). For each Mg²⁺ binding site the residues in the VSD and those in the cytoplasmic domain come from neighboring subunits. These results suggest that the VSD and the cytoplasmic domains from different subunits may interact during channel gating, and the packing of VSD or the RCK1 domain to the pore in BK channels differ from that in Kv1.2 or MthK channels.

An extracellular Cu2+ binding site in the voltage sensor of BK and Shaker potassium channels

Copper is an essential trace element that may serve as a signaling molecule in the nervous system. Here we show that extracellular Cu²⁺ is a potent inhibitor of BK and Shaker K⁺ channels. At low micromolar concentrations, Cu²⁺ rapidly and reversibly reduces macrosocopic K⁺ conductance (G_K) evoked from mSlo1 BK channels by membrane depolarization. G_K is reduced in a dose-dependent manner with an IC₅₀ and Hill coefficient of ∼2 μM and 1.0, respectively. Saturating 100 μM Cu²⁺ shifts the G_K-V relation by +74 mV and reduces G_Kmax by 27% without affecting single channel conductance. However, 100 μM Cu²⁺ fails to inhibit G_K when applied during membrane depolarization, suggesting that Cu²⁺ interacts poorly with the activated channel. Of other transition metal ions tested, only Zn²⁺ and Cd²⁺ had significant effects at 100 μM with IC₅₀s > 0.5 mM, suggesting the binding site is Cu²⁺ selective. Mutation of external Cys or His residues did not alter Cu²⁺ sensitivity. However, four putative Cu²⁺-coordinating residues were identified (D133, Q151, D153, and R207) in transmembrane segments S1, S2, and S4 of the mSlo1 voltage sensor, based on the ability of substitutions at these positions to alter Cu²⁺ and/or Cd²⁺ sensitivity. Consistent with the presence of acidic residues in the binding site, Cu²⁺ sensitivity was reduced at low extracellular pH. The three charged positions in S1, S2, and S4 are highly conserved among voltage-gated channels and could play a general role in metal sensitivity. We demonstrate that Shaker, like mSlo1, is much more sensitive to Cu²⁺ than Zn²⁺ and that sensitivity to these metals is altered by mutating the conserved positions in S1 or S4 or reducing pH. Our results suggest that the voltage sensor forms a state- and pH-dependent, metal-selective binding pocket that may be occupied by Cu²⁺ at physiologically relevant concentrations to inhibit activation of BK and other channels.

Mg2+ enhances voltage sensor/gate coupling in BK channels

BK (Slo1) potassium channels are activated by millimolar intracellular Mg²⁺ as well as micromolar Ca²⁺ and membrane depolarization. Mg²⁺ and Ca²⁺ 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 Mg²⁺ action is highly dependent on voltage sensor activation and therefore differs fundamentally from that of Ca²⁺. Evidence that Ca²⁺ 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 μM Ca²⁺ increases P_O more than 15-fold at −120 mV. However 10 mM Mg²⁺, which has an effect on the G_K-V relation similar to 2 μM Ca²⁺, has no detectable effect on P_O when voltage sensors are in the resting state. Gating currents are only slightly altered by Mg²⁺ when channels are closed, indicating that Mg²⁺ does not act merely to promote voltage sensor activation. Indeed, channel opening is facilitated in a voltage-independent manner by Mg²⁺ in a mutant (R210C) whose voltage sensors are constitutively activated. Thus, 10 mM Mg²⁺ increases P_O only when voltage sensors are activated, effectively strengthening the allosteric coupling of voltage sensor activation to channel opening. Increasing Mg²⁺ from 10 to 100 mM, to occupy very low affinity binding sites, has additional effects on gating that more closely resemble those of Ca²⁺. The effects of Mg²⁺ on steady-state activation and I_K kinetics are discussed in terms of an allosteric gating scheme and the state-dependent interactions between Mg²⁺ and voltage sensor that may underlie this mechanism.

Mg2+ mediates interaction between the voltage sensor and cytosolic domain to activate BK channels

The voltage-sensor domain (VSD) of voltage-dependent ion channels and enzymes is critical for cellular responses to membrane potential. The VSD can also be regulated by interaction with intracellular proteins and ligands, but how this occurs is poorly understood. Here, we show that the VSD of the BK-type K(+) channel is regulated by a state-dependent interaction with its own tethered cytosolic domain that depends on both intracellular Mg(2+) and the open state of the channel pore. Mg(2+) bound to the cytosolic RCK1 domain enhances VSD activation by electrostatic interaction with Arg-213 in transmembrane segment S4. Our results demonstrate that a cytosolic domain can come close enough to the VSD to regulate its activity electrostatically, thereby elucidating a mechanism of Mg(2+)-dependent activation in BK channels and suggesting a general pathway by which intracellular factors can modulate the function of voltage-dependent proteins.

Regulation of cation channels in cardiac and smooth muscle cells by intracellular magnesium

Magnesium regulates various ion channels in many tissues, including those of the cardiovascular system. General mechanisms by which intracellular Mg(2+) (Mg(i)(2+)) regulates channels are presented. These involve either a direct interaction with the channel, or an indirect modification of channel function via other proteins, such as enzymes or G proteins, or via membrane surface charges and phospholipids. To provide an insight into the role of Mg(i)(2+) in the cardiovascular system, effects of Mg(i)(2+) on major channels in cardiac and smooth muscle cells and the underlying mechanisms are then reviewed. Although Mg(i)(2+) concentrations are known to be stable, conditions under which they may change exist, such as following stimulation of beta-adrenergic receptors and of insulin receptors, or during pathophysiological conditions such as ischemia, heart failure or hypertension. Modifications of cardiovascular electrical or mechanical function, possibly resulting in arrhythmias or hypertension, may result from such changes of Mg(i)(2+) and their effects on cation channels.

Characterisation of large-conductance calcium-activated potassium channels (BK(Ca)) in human NT2-N cells

Large-conductance calcium-activated potassium (BK(Ca)) channels were studied in inside-out patches of human NTERA2 neuronal cells (NT2-N). In symmetrical (140 mM) K(+) the channel mean conductance was 265 pS, the current reversing at approximately 0 mV. It was selective (P(K)/P(Na)=20:1) and blocked by internal paxilline and TEA. The open probability-voltage relationship for BK(Ca) was fitted with a Boltzmann function, the V((1/2)) being 76.3 mV, 33.6 mV and -14.1 mV at 0.1 muM, 3.3 muM and 10 muM [Ca(2+)](i), respectively. The relationship between open probability and [Ca(2+)](i) was fitted by the Hill equation (Hill coefficient 2.7, half maximal activation at 2.0 muM [Ca(2+)](i)). Open and closed dwell time histograms were fitted by the sum of two and three voltage-dependent exponentials, respectively. Increasing [Ca(2+)](i) produced both an increase in the longer open time constant and a decrease in the longest closed time constant, so increasing mean open time. "Intracellular" ATP evoked a concentration-dependent increase in NT2-N BK(Ca) activity. At +40 mV half-maximum activation occurred at an [ATP](i) of 3 mM (30 nM [Ca(2+)](i)). ADP and GTP were less potent, and AMP-PNP was inactive. This is the first characterisation of a potassium channel in NT2-N cells showing that it is similar to the BK(Ca) channel of other preparations.

Tuning magnesium sensitivity of BK channels by mutations

Intracellular Mg(2+) at physiological concentrations activates mSlo1 BK channels by binding to a metal-binding site in the cytosolic domain. Previous studies suggest that residues E374, Q397, and E399 are important in Mg(2+) binding. In the present study, we show that mutations of E374 or E399 to other amino acids, except for Asp, abolish Mg(2+) sensitivity. These results further support that the side chains of E374 and E399 are essential for Mg(2+) coordination. To the contrary, none of the Q397 mutations abolishes Mg(2+) sensitivity, suggesting that its side chain may not coordinate to Mg(2+). However, because Q397 is spatially close to E374 and E399, its mutations affect the Mg(2+) sensitivity of channel gating by either reducing or increasing the Mg(2+) binding affinity. The pattern of mutational effects and the effect of chemical modification of Q397C indicate that Q397 is involved in the Mg(2+)-dependent activation of BK channels and that mutations of Q397 alter Mg(2+) sensitivity by affecting the conformation of the Mg(2+) binding site as well as by electrostatic interactions with the bound Mg(2+) ion.