Stepwise contribution of each subunit to the cooperative activation of BK channels by Ca2+

Cooperative Gating of a K+ Channel by Unmodified Biological Anionic Lipids Viewed by Solid-State NMR Spectroscopy

Lipids adhere to membrane proteins to stimulate or suppress molecular and ionic transport and signal transduction. Yet, the molecular details of lipid-protein interaction and their functional impact are poorly characterized. Here we combine NMR, coarse-grained molecular dynamics (CGMD), and functional assays to reveal classic cooperativity in the binding and subsequent activation of a bacterial inward rectifier potassium (Kir) channel by phosphatidylglycerol (PG), a common component of many membranes. Past studies of lipid activation of Kir channels focused primarily on phosphatidylinositol bisphosphate, a relatively rare signaling lipid that is tightly regulated in space and time. We use solid-state NMR to quantify the binding of unmodified 13C-PG to the K+ channel KirBac1.1 in liposomes. This specific lipid-protein interaction has a dissociation constant (Kd) of ∼7 mol percentage PG (ΧPG) with positive cooperativity (n = 3.8) and approaches saturation near 20% ΧPG. Liposomal flux assays show that K+ flux also increases with PG in a cooperative manner with an EC50 of ∼20% ΧPG, within the physiological range. Further quantitative fitting of these data reveals that PG acts as a partial (80%) agonist with fivefold K+ flux amplification. Comparisons of NMR chemical shift perturbation and CGMD simulations at different ΧPG confirm the direct interaction of PG with key residues, several of which would not be accessible to lipid headgroups in the closed state of the channel. Allosteric regulation by a common lipid is directly relevant to the activation mechanisms of several human ion channels. This study highlights the role of concentration-dependent lipid-protein interactions and tightly controlled protein allostery in the activation and regulation of ion channels.

Identifiability of equilibrium constants for receptors with two to five binding sites

Ligand-gated ion channels (LGICs) are regularly oligomers containing between two and five binding sites for ligands. Neither in homomeric nor heteromeric LGICs the activation process evoked by the ligand binding is fully understood. Here, we show on theoretical grounds that for LGICs with two to five binding sites, the cooperativity upon channel activation can be determined in considerable detail. The main requirements for our strategy are a defined number of binding sites in a channel, which can be achieved by concatenation, a systematic mutation of all binding sites and a global fit of all concentration-activation relationships (CARs) with corresponding intimately coupled Markovian state models. We take advantage of translating these state models to cubes with dimensions 2, 3, 4, and 5. We show that the maximum possible number of CARs for these LGICs specify all 7, 13, 23, and 41 independent model parameters, respectively, which directly provide all equilibrium constants within the respective schemes. Moreover, a fit that uses stochastically varied scaled unitary start vectors enables the determination of all parameters, without any bias imposed by specific start vectors. A comparison of the outcome of the analyses for the models with 2 to 5 binding sites showed that the identifiability of the parameters is best for a case with 5 binding sites and 41 parameters. Our strategy can be used to analyze experimental data of other LGICs and may be applicable to voltage-gated ion channels and metabotropic receptors.

BK channels of five different subunit combinations underlie the de novo KCNMA1 G375R channelopathy

The molecular basis of a severe developmental and neurological disorder associated with a de novo G375R variant of the tetrameric BK channel is unknown. Here, we address this question by recording from single BK channels expressed to mimic a G375R mutation heterozygous with a WT allele. Five different types of functional BK channels were expressed: 3% were consistent with WT, 12% with homotetrameric mutant, and 85% with three different types of hybrid (heterotetrameric) channels assembled from both mutant and WT subunits. All channel types except WT showed a marked gain-of-function in voltage activation and a smaller decrease-of-function in single-channel conductance, with both changes in function becoming more pronounced as the number of mutant subunits per tetrameric channel increased. The net cellular response from the five different types of channels comprising the molecular phenotype was a shift of -120 mV in the voltage required to activate half of the maximal current through BK channels, giving a net gain-of-function. The WT and homotetrameric mutant channels in the molecular phenotype were consistent with genetic codominance as each displayed properties of a channel arising from only one of the two alleles. The three types of hybrid channels in the molecular phenotype were consistent with partial dominance as their properties were intermediate between those of mutant and WT channels. A model in which BK channels randomly assemble from mutant and WT subunits, with each subunit contributing increments of activation and conductance, approximated the molecular phenotype of the heterozygous G375R mutation.

Derepression may masquerade as activation in ligand-gated ion channels

Agonists are ligands that bind to receptors and activate them. In the case of ligand-gated ion channels, such as the muscle-type nicotinic acetylcholine receptor, mechanisms of agonist activation have been studied for decades. Taking advantage of a reconstructed ancestral muscle-type β-subunit that forms spontaneously activating homopentamers, here we show that incorporation of human muscle-type α-subunits appears to repress spontaneous activity, and furthermore that the presence of agonist relieves this apparent α-subunit-dependent repression. Our results demonstrate that rather than provoking channel activation/opening, agonists may instead 'inhibit the inhibition' of intrinsic spontaneous activity. Thus, agonist activation may be the apparent manifestation of agonist-induced derepression. These results provide insight into intermediate states that precede channel opening and have implications for the interpretation of agonism in ligand-gated ion channels.

Ancestral acetylcholine receptor β-subunit forms homopentamers that prime before opening spontaneously

Human adult muscle-type acetylcholine receptors are heteropentameric ion channels formed from two α-subunits, and one each of the β-, d-, and e-subunits. To form functional channels, the subunits must assemble with one another in a precise stoichiometry and arrangement. Despite being different, the four subunits share a common ancestor that is presumed to have formed homopentamers. The extent to which the properties of the modern-day receptor result from its subunit complexity is unknown. Here we discover that a reconstructed ancestral muscle-type β-subunit can form homopentameric ion channels. These homopentamers open spontaneously and display single-channel hallmarks of muscle-type acetylcholine receptor activity. Our findings attest to the homopentameric origin of the muscle-type acetylcholine receptor, and demonstrate that signature features of its function are both independent of agonist and do not necessitate the complex heteropentameric architecture of the modern-day protein.

Novel insights into the allosteric gating mechanism of MthK channel

Allostery is a fundamental element during channel gating in response to an appropriate stimulus by which events occurring at one site are transmitted to distal sites to regulate activity. To address how binding of the first Ca²⁺ ion at one of the eight chemically identical subunits facilitates the other Ca²⁺-binding events in MthK, a Ca²⁺-gated K⁺ channel containing a conserved ligand-binding RCK domain, we analysed a large collection of MthK structures and performed the corresponding thermodynamic and electrophysiological measurements. These structural and functional studies led us to conclude that the conformations of the Ca²⁺-binding sites alternate between two quaternary states and exhibit significant differences in Ca²⁺ affinity. We further propose an allosteric model of the MthK-gating mechanism by which a cascade of structural events connect the initial Ca²⁺-binding to the final changes of the ring structure that open the ion-conduction pore. This mechanical model reveals the exquisite design that achieves the allosteric gating and could be of general relevance for the action of other ligand-gated ion channels containing the RCK domain.

Molecular mechanism of BK channel activation by the smooth muscle relaxant NS11021

Large-conductance Ca²⁺-activated K⁺ channels (BK channels) are activated by cytosolic calcium and depolarized membrane potential under physiological conditions. Thus, these channels control electrical excitability in neurons and smooth muscle by gating K⁺ efflux and hyperpolarizing the membrane in response to Ca²⁺ signaling. Altered BK channel function has been linked to epilepsy, dyskinesia, and other neurological deficits in humans, making these channels a key target for drug therapies. To gain insight into mechanisms underlying pharmacological modulation of BK channel gating, here we studied mechanisms underlying activation of BK channels by the biarylthiourea derivative, NS11021, which acts as a smooth muscle relaxant. We observe that increasing NS11021 shifts the half-maximal activation voltage for BK channels toward more hyperpolarized voltages, in both the presence and nominal absence of Ca²⁺, suggesting that NS11021 facilitates BK channel activation primarily by a mechanism that is distinct from Ca²⁺ activation. 30 µM NS11021 slows the time course of BK channel deactivation at −200 mV by ∼10-fold compared with 0 µM NS11021, while having little effect on the time course of activation. This action is most pronounced at negative voltages, at which the BK channel voltage sensors are at rest. Single-channel kinetic analysis further shows that 30 µM NS11021 increases open probability by 62-fold and increases mean open time from 0.15 to 0.52 ms in the nominal absence of Ca²⁺ at voltages less than −60 mV, conditions in which BK voltage sensors are largely in the resting state. We could therefore account for the major activating effects of NS11021 by a scheme in which the drug primarily shifts the pore-gate equilibrium toward the open state.

State-dependent inhibition of BK channels by the opioid agonist loperamide

Large-conductance Ca²⁺-activated K⁺ (BK) channels control a range of physiological functions, and their dysfunction is linked to human disease. We have found that the widely used drug loperamide (LOP) can inhibit activity of BK channels composed of either α-subunits (BKα channels) or α-subunits plus the auxiliary γ1-subunit (BKα/γ1 channels), and here we analyze the molecular mechanism of LOP action. LOP applied at the cytosolic side of the membrane rapidly and reversibly inhibited BK current, an effect that appeared as a decay in voltage-activated BK currents. The apparent affinity for LOP decreased with hyperpolarization in a manner consistent with LOP behaving as an inhibitor of open, activated channels. Increasing LOP concentration reduced the half-maximal activation voltage, consistent with relative stabilization of the LOP-inhibited open state. Single-channel recordings revealed that LOP did not reduce unitary BK channel current, but instead decreased BK channel open probability and mean open times. LOP elicited use-dependent inhibition, in which trains of brief depolarizing steps lead to accumulated reduction of BK current, whereas single brief depolarizing steps do not. The principal effects of LOP on BK channel gating are described by a mechanism in which LOP acts as a state-dependent pore blocker. Our results suggest that therapeutic doses of LOP may act in part by inhibiting K⁺ efflux through intestinal BK channels.

A single historical substitution drives an increase in acetylcholine receptor complexity

Human adult muscle-type acetylcholine receptors are heteropentameric ion channels formed from four different, but evolutionarily related, subunits. These subunits assemble with a precise stoichiometry and arrangement such that two chemically distinct agonist-binding sites are formed between specific subunit pairs. How this subunit complexity evolved and became entrenched is unclear. Here we show that a single historical amino acid substitution is able to constrain the subunit stoichiometry of functional acetylcholine receptors. Using a combination of ancestral sequence reconstruction, single-channel electrophysiology, and concatenated subunits, we reveal that an ancestral β-subunit can not only replace the extant β-subunit but can also supplant the neighboring δ-subunit. By forward evolving the ancestral β-subunit with a single amino acid substitution, we restore the requirement for a δ-subunit for functional channels. These findings reveal that a single historical substitution necessitates an increase in acetylcholine receptor complexity and, more generally, that simple stepwise mutations can drive subunit entrenchment in this model heteromeric protein.

A Calcium Sensor Discovered in Bluetongue Virus Nonstructural Protein 2 Is Critical for Virus Replication

After entering the host cells, viruses use cellular host factors to ensure a successful virus replication process. For replication in infected cells, members of the Reoviridae family form inclusion body-like structures known as viral inclusion bodies (VIB) or viral factories. Bluetongue virus (BTV) forms VIBs in infected cells through nonstructural protein 2 (NS2), a phosphoprotein. An important regulatory factor critical for VIB formation is phosphorylation of NS2. In our study, we discovered a characteristic calcium-binding EF-hand-like motif in NS2 and found that the calcium binding preferentially affects phosphorylation level of the NS2 and has a role in regulating VIB assembly.

Many viruses use specific viral proteins to bind calcium ions (Ca²⁺) for stability or to modify host cell pathways; however, to date, no Ca²⁺ binding protein has been reported in bluetongue virus (BTV), the causative agent of bluetongue disease in livestock. Here, using a comprehensive bioinformatics screening, we identified a putative EF-hand-like Ca²⁺ binding motif in the carboxyl terminal region of BTV nonstructural phosphoprotein 2 (NS2). Subsequently, using a recombinant NS2, we demonstrated that NS2 binds Ca²⁺ efficiently and that Ca²⁺ binding was perturbed when the Asp and Glu residues in the motif were substituted by alanine. Using circular dichroism analysis, we found that Ca²⁺ binding by NS2 triggered a helix-to-coil secondary structure transition. Further, cryo-electron microscopy in the presence of Ca²⁺ revealed that NS2 forms helical oligomers which, when aligned with the N-terminal domain crystal structure, suggest an N-terminal domain that wraps around the C-terminal domain in the oligomer. Further, an in vitro kinase assay demonstrated that Ca²⁺ enhanced the phosphorylation of NS2 significantly. Importantly, mutations introduced at the Ca²⁺ binding site in the viral genome by reverse genetics failed to allow recovery of viable virus, and the NS2 phosphorylation level and assembly of viral inclusion bodies (VIBs) were reduced. Together, our data suggest that NS2 is a dedicated Ca²⁺ binding protein and that calcium sensing acts as a trigger for VIB assembly, which in turn facilitates virus replication and assembly.

IMPORTANCE After entering the host cells, viruses use cellular host factors to ensure a successful virus replication process. For replication in infected cells, members of the Reoviridae family form inclusion body-like structures known as viral inclusion bodies (VIB) or viral factories. Bluetongue virus (BTV) forms VIBs in infected cells through nonstructural protein 2 (NS2), a phosphoprotein. An important regulatory factor critical for VIB formation is phosphorylation of NS2. In our study, we discovered a characteristic calcium-binding EF-hand-like motif in NS2 and found that the calcium binding preferentially affects phosphorylation level of the NS2 and has a role in regulating VIB assembly.

Calcium-driven regulation of voltage-sensing domains in BK channels

Allosteric interactions between the voltage-sensing domain (VSD), the Ca²⁺-binding sites, and the pore domain govern the mammalian Ca²⁺- and voltage-activated K⁺ (BK) channel opening. However, the functional relevance of the crosstalk between the Ca²⁺- and voltage-sensing mechanisms on BK channel gating is still debated. We examined the energetic interaction between Ca²⁺ binding and VSD activation by investigating the effects of internal Ca²⁺ on BK channel gating currents. Our results indicate that Ca²⁺ sensor occupancy has a strong impact on VSD activation through a coordinated interaction mechanism in which Ca²⁺ binding to a single α-subunit affects all VSDs equally. Moreover, the two distinct high-affinity Ca²⁺-binding sites contained in the C-terminus domains, RCK1 and RCK2, contribute equally to decrease the free energy necessary to activate the VSD. We conclude that voltage-dependent gating and pore opening in BK channels is modulated to a great extent by the interaction between Ca²⁺ sensors and VSDs.

Structural basis for gating the high-conductance Ca2+-activated K+ channel

The precise control of an ion channel gate by environmental stimuli is crucial for the fulfilment of its biological role. The gate in Slo1 K⁺ channels is regulated by two separate stimuli, intracellular Ca²⁺ concentration and membrane voltage. Slo1 is thus central to understanding the relationship between intracellular Ca²⁺ and membrane excitability. Here we present the Slo1 structure from Aplysia californica in the absence of Ca²⁺ and compare it with the Ca²⁺-bound channel. We show that Ca²⁺ binding at two unique binding sites per subunit stabilizes an expanded conformation of the Ca²⁺ sensor gating ring. These conformational changes are propagated from the gating ring to the pore through covalent linkers and through protein interfaces formed between the gating ring and the voltage sensors. The gating ring and the voltage sensors are directly connected through these interfaces, which allow membrane voltage to regulate gating of the pore by influencing the Ca²⁺ sensors.

A New Splice Variant of Large Conductance Ca2+-activated K+ (BK) Channel α Subunit Alters Human Chondrocyte Function

Large conductance Ca²⁺-activated K⁺ (BK) channels play essential roles in both excitable and non-excitable cells. For example, in chondrocytes, agonist-induced Ca²⁺ release from intracellular store activates BK channels, and this hyperpolarizes these cells, augments Ca²⁺ entry, and forms a positive feed-back mechanism for Ca²⁺ signaling and stimulation-secretion coupling. In the present study, functional roles of a newly identified splice variant in the BK channel α subunit (BKαΔe2) were examined in a human chondrocyte cell line, OUMS-27, and in a HEK293 expression system. Although BKαΔe2 lacks exon2, which codes the intracellular S0-S1 linker (Glu-127-Leu-180), significant expression was detected in several tissues from humans and mice. Molecular image analyses revealed that BKαΔe2 channels are not expressed on plasma membrane but can traffic to the plasma membrane after forming hetero-tetramer units with wild-type BKα (BKαWT). Single-channel current analyses demonstrated that BKα hetero-tetramers containing one, two, or three BKαΔe2 subunits are functional. These hetero-tetramers have a smaller single channel conductance and exhibit lower trafficking efficiency than BKαWT homo-tetramers in a stoichiometry-dependent manner. Site-directed mutagenesis of residues in exon2 identified Helix2 and the linker to S1 (Trp-158-Leu-180, particularly Arg-178) as an essential segment for channel function including voltage dependence and trafficking. BKαΔe2 knockdown in OUMS-27 chondrocytes increased BK current density and augmented the responsiveness to histamine assayed as cyclooxygenase-2 gene expression. These findings provide significant new evidence that BKαΔe2 can modulate cellular responses to physiological stimuli in human chondrocyte and contribute under pathophysiological conditions, such as osteoarthritis.

Biophysics of BK Channel Gating

BK channels are universal regulators of cell excitability, given their exceptional unitary conductance selective for K(+), joint activation mechanism by membrane depolarization and intracellular [Ca(2+)] elevation, and broad expression pattern. In this chapter, we discuss the structural basis and operational principles of their activation, or gating, by membrane potential and calcium. We also discuss how the two activation mechanisms interact to culminate in channel opening. As members of the voltage-gated potassium channel superfamily, BK channels are discussed in the context of archetypal family members, in terms of similarities that help us understand their function, but also seminal structural and biophysical differences that confer unique functional properties.

Stoichiometry for α-bungarotoxin block of α7 acetylcholine receptors

α-Bungarotoxin (α-Btx) binds to the five agonist binding sites on the homopentameric α7-acetylcholine receptor, yet the number of bound α-Btx molecules required to prevent agonist-induced channel opening remains unknown. To determine the stoichiometry for α-Btx blockade, we generate receptors comprised of wild-type and α-Btx-resistant subunits, tag one of the subunit types with conductance mutations to report subunit stoichiometry, and following incubation with α-Btx, monitor opening of individual receptor channels with defined subunit stoichiometry. We find that a single α-Btx-sensitive subunit confers nearly maximal suppression of channel opening, despite four binding sites remaining unoccupied by α-Btx and accessible to the agonist. Given structural evidence that α-Btx locks the agonist binding site in an inactive conformation, we conclude that the dominant mechanism of antagonism is non-competitive, originating from conformational arrest of the binding sites, and that the five α7 subunits are interdependent and maintain conformational symmetry in the open channel state.

Since their discovery more than fifty years ago, α-neurotoxins have been used to study acetylcholine receptor-coupled ion channels. Here, daCosta et al. find that toxin binding to a single site of the pentameric α7 receptor blocks function, suggesting the five binding sites are interdependent and the toxin arrests the sites in the inactive conformation.

Cadmium-cysteine coordination in the BK inner pore region and its structural and functional implications

To probe structure and gating-associated conformational changes in BK-type potassium (BK) channels, we examined consequences of Cd(2+) coordination with cysteines introduced at two positions in the BK inner pore. At V319C, the equivalent of valine in the conserved Kv proline-valine-proline (PVP) motif, Cd(2+) forms intrasubunit coordination with a native glutamate E321, which would place the side chains of V319C and E321 much closer together than observed in voltage-dependent K(+) (Kv) channel structures, requiring that the proline between V319C and E321 introduces a kink in the BK S6 inner helix sharper than that observed in Kv channel structures. At inner pore position A316C, Cd(2+) binds with modest state dependence, suggesting the absence of an ion permeation gate at the cytosolic side of BK channel. These results highlight fundamental structural differences between BK and Kv channels in their inner pore region, which likely underlie differences in voltage-dependent gating between these channels.

Single-channel kinetics of BK (Slo1) channels

Single-channel kinetics has proven a powerful tool to reveal information about the gating mechanisms that control the opening and closing of ion channels. This introductory review focuses on the gating of large conductance Ca²⁺- and voltage-activated K⁺ (BK or Slo1) channels at the single-channel level. It starts with single-channel current records and progresses to presentation and analysis of single-channel data and the development of gating mechanisms in terms of discrete state Markov (DSM) models. The DSM models are formulated in terms of the tetrameric modular structure of BK channels, consisting of a central transmembrane pore-gate domain (PGD) attached to four surrounding transmembrane voltage sensing domains (VSD) and a large intracellular cytosolic domain (CTD), also referred to as the gating ring. The modular structure and data analysis shows that the Ca²⁺ and voltage dependent gating considered separately can each be approximated by 10-state two-tiered models with five closed states on the upper tier and five open states on the lower tier. The modular structure and joint Ca²⁺ and voltage dependent gating are consistent with a 50 state two-tiered model with 25 closed states on the upper tier and 25 open states on the lower tier. Adding an additional tier of brief closed (flicker states) to the 10-state or 50-state models improved the description of the gating. For fixed experimental conditions a channel would gate in only a subset of the potential number of states. The detected number of states and the correlations between adjacent interval durations are consistent with the tiered models. The examined models can account for the single-channel kinetics and the bursting behavior of gating. Ca²⁺ and voltage activate BK channels by predominantly increasing the effective opening rate of the channel with a smaller decrease in the effective closing rate. Ca²⁺ and depolarization thus activate by mainly destabilizing the closed states.

K(+) channels: function-structural overview

Potassium channels are particularly important in determining the shape and duration of the action potential, controlling the membrane potential, modulating hormone secretion, epithelial function and, in the case of those K(+) channels activated by Ca(2+), damping excitatory signals. The multiplicity of roles played by K(+) channels is only possible to their mammoth diversity that includes at present 70 K(+) channels encoding genes in mammals. Today, thanks to the use of cloning, mutagenesis, and the more recent structural studies using x-ray crystallography, we are in a unique position to understand the origins of the enormous diversity of this superfamily of ion channels, the roles they play in different cell types, and the relations that exist between structure and function. With the exception of two-pore K(+) channels that are dimers, voltage-dependent K(+) channels are tetrameric assemblies and share an extremely well conserved pore region, in which the ion-selectivity filter resides. In the present overview, we discuss in the function, localization, and the relations between function and structure of the five different subfamilies of K(+) channels: (a) inward rectifiers, Kir; (b) four transmembrane segments-2 pores, K2P; (c) voltage-gated, Kv; (d) the Slo family; and (e) Ca(2+)-activated SK family, SKCa.

Free-energy relationships in ion channels activated by voltage and ligand

Many ion channels are modulated by multiple stimuli, which allow them to integrate a variety of cellular signals and precisely respond to physiological needs. Understanding how these different signaling pathways interact has been a challenge in part because of the complexity of underlying models. In this study, we analyzed the energetic relationships in polymodal ion channels using linkage principles. We first show that in proteins dually modulated by voltage and ligand, the net free-energy change can be obtained by measuring the charge-voltage (Q-V) relationship in zero ligand condition and the ligand binding curve at highly depolarizing membrane voltages. Next, we show that the voltage-dependent changes in ligand occupancy of the protein can be directly obtained by measuring the Q-V curves at multiple ligand concentrations. When a single reference ligand binding curve is available, this relationship allows us to reconstruct ligand binding curves at different voltages. More significantly, we establish that the shift of the Q-V curve between zero and saturating ligand concentration is a direct estimate of the interaction energy between the ligand- and voltage-dependent pathway. These free-energy relationships were tested by numerical simulations of a detailed gating model of the BK channel. Furthermore, as a proof of principle, we estimate the interaction energy between the ligand binding and voltage-dependent pathways for HCN2 channels whose ligand binding curves at various voltages are available. These emerging principles will be useful for high-throughput mutagenesis studies aimed at identifying interaction pathways between various regulatory domains in a polymodal ion channel.

The contribution of RCK domains to human BK channel allosteric activation

Large conductance voltage- and Ca(2+)-activated K(+) (BK) channels are potent regulators of cellular processes including neuronal firing, synaptic transmission, cochlear hair cell tuning, insulin release, and smooth muscle tone. Their unique activation pathway relies on structurally distinct regulatory domains including one transmembrane voltage-sensing domain (VSD) and two intracellular high affinity Ca(2+)-sensing sites per subunit (located in the RCK1 and RCK2 domains). Four pairs of RCK1 and RCK2 domains form a Ca(2+)-sensing apparatus known as the "gating ring." The allosteric interplay between voltage- and Ca(2+)-sensing apparati is a fundamental mechanism of BK channel function. Using voltage-clamp fluorometry and UV photolysis of intracellular caged Ca(2+), we optically resolved VSD activation prompted by Ca(2+) binding to the gating ring. The sudden increase of intracellular Ca(2+) concentration ([Ca(2+)](i)) induced a hyperpolarizing shift in the voltage dependence of both channel opening and VSD activation, reported by a fluorophore labeling position 202, located in the upper side of the S4 transmembrane segment. The neutralization of the Ca(2+) sensor located in the RCK2 domain abolished the effect of [Ca(2+)](i) increase on the VSD rearrangements. On the other hand, the mutation of RCK1 residues involved in Ca(2+) sensing did not prevent the effect of Ca(2+) release on the VSD, revealing a functionally distinct interaction between RCK1 and RCK2 and the VSD. A statistical-mechanical model quantifies the complex thermodynamics interplay between Ca(2+) association in two distinct sites, voltage sensor activation, and BK channel opening.

Localization of a site of action for benzofuroindole-induced potentiation of BKCa channels

As previously reported, the activity of the large-conductance calcium (Ca(2+))-activated potassium (K(+)) (BK(Ca)) channel is strongly potentiated from the extracellular side of the cell membrane by certain benzofuroindole derivatives. Here, the mechanism of action of one of the most potent activators, 4-chloro-7-(trifluoromethyl)-10H-benzofuro[3,2-b]indole-1-carboxylic acid (CTBIC), is characterized. This compound, Compound 22 in the previous report (Chembiochem 6:1745-1748, 2005), potentiated the activity of the channel by shifting its conductance-voltage relationship toward the more negative direction. Cotreatment with CTBIC reduced the affinity of charybdotoxin, a peptide pore-blocker, whereas that of tetraethylammonium, a small pore-blocking quaternary ammonium, was not significantly altered. Guided by these results, scanning mutagenesis of the outer vestibule of the BK(Ca) channel was launched to uncover the molecular determinants that affect CTBIC binding. Alanine substitution of several amino acid residues in the turret region and the S6 helix of the channel decreased potentiation by CTBIC. Homology modeling and molecular dynamics simulation showed that some of these residues formed a CTBIC binding pocket between two adjacent α-subunits in the outer vestibule of the channel. Thus, it can be envisioned that benzofuroindole derivatives stabilize the open conformation of the channel by binding to the residues clustered across the extracellular part of the subunit interface. The present results indicate that the interface between different α-subunits of the BK(Ca) channel may play a critical role in the modulation of channel activity. Therefore, this interface represents a potential therapeutic target site for the regulation of K(+) channels.

Coupling and cooperativity in voltage activation of a limited-state BK channel gating in saturating Ca2+

Voltage-dependent gating mechanisms of large conductance Ca²⁺ and voltage-activated (BK) channels were investigated using two-dimensional maximum likelihood analysis of single-channel open and closed intervals. To obtain sufficient data at negative as well as positive voltages, single-channel currents were recorded at saturating Ca²⁺ from BK channels mutated to remove the RCK1 Ca²⁺ and Mg²⁺ sensors. The saturating Ca²⁺ acting on the Ca²⁺ bowl sensors of the resulting BK_B channels increased channel activity while driving the gating into a reduced number of states, simplifying the model. Five highly constrained idealized gating mechanisms based on extensions of the Monod-Wyman-Changeux model for allosteric proteins were examined. A 10-state model without coupling between the voltage sensors and the opening/closing transitions partially described the voltage dependence of Po but not the single-channel kinetics. With allowed coupling, the model gave improved descriptions of Po and approximated the single-channel kinetics; each activated voltage sensor increased the opening rate approximately an additional 23-fold while having little effect on the closing rate. Allowing cooperativity among voltage sensors further improved the description of the data: each activated voltage sensor increased the activation rate of the remaining voltage sensors approximately fourfold, with little effect on the deactivation rate. The coupling factor was decreased in models with cooperativity from ∼23 to ∼18. Whether the apparent cooperativity among voltage sensors arises from imposing highly idealized models or from actual cooperativity will require additional studies to resolve. For both cooperative and noncooperative models, allowing transitions to five additional brief (flicker) closed states further improved the description of the data. These observations show that the voltage-dependent single-channel kinetics of BK_B channels can be approximated by highly idealized allosteric models in which voltage sensor movement increases Po mainly through an increase in channel opening rates, with limited effects on closing rates.

Dual role of protein kinase C on BK channel regulation

Large conductance voltage- and Ca(2+)-activated potassium channels (BK channels) are important feedback regulators in excitable cells and are potently regulated by protein kinases. The present study reveals a dual role of protein kinase C (PKC) on BK channel regulation. Phosphorylation of S(695) by PKC, located between the two regulators of K(+) conductance (RCK1/2) domains, inhibits BK channel open-state probability. This PKC-dependent inhibition depends on a preceding phosphorylation of S(1151) in the C terminus of the channel alpha-subunit. Phosphorylation of only one alpha-subunit at S(1151) and S(695) within the tetrameric pore is sufficient to inhibit BK channel activity. We further detected that protein phosphatase 1 is associated with the channel, constantly counteracting phosphorylation of S(695). PKC phosphorylation at S(1151) also influences stimulation of BK channel activity by protein kinase G (PKG) and protein kinase A (PKA). Though the S(1151)A mutant channel is activated by PKA only, the phosphorylation of S(1151) by PKC renders the channel responsive to activation by PKG but prevents activation by PKA. Phosphorylation of S(695) by PKC or introducing a phosphomimetic aspartate at this position (S(695)D) renders BK channels insensitive to the stimulatory effect of PKG or PKA. Therefore, our findings suggest a very dynamic regulation of the channel by the local PKC activity. It is shown that this complex regulation is not only effective in recombinant channels but also in native BK channels from tracheal smooth muscle.

Block of mouse Slo1 and Slo3 K+ channels by CTX, IbTX, TEA, 4-AP and quinidine

pH-regulated Slo3 channels, perhaps exclusively expressed in mammalian sperm, may play a role in alkalization-mediated K⁺ fluxes associated with sperm capacitation. The Slo3 channel shares extensive homology with Ca²⁺- and voltage-regulated BK-type Slo1 K⁺ channels. Here, using heterologous expression in oocytes, we define distinctive differences in pharmacological properties of Slo3 and Slo1 currents, examine blockade in terms of distinct blocking models, and, for some blockers, use mutated constructs to evaluate determinants of block. Slo3 is resistant to block by the standard Slo1 blockers, iberiotoxin, charybdotoxin and extracellular TEA. Slo3 is relatively insensitive to extracellular 4-AP up to 100 mM, while Slo1 is blocked in a voltage-dependent fashion consistent with block on the extracellular side of the channel. Block of both Slo1 and Slo3 by cytosolic 4-AP can be described by open channel block, with Slo3 being ~10–15-fold more sensitive, but exhibiting weaker voltage-dependence of block. The cytosolic concentrations of 4-AP required to block Slo3 make it unlikely that the effects of 4-AP on volume regulation in mammalian sperm is mediated by Slo3. Quinidine was more effective in blocking Slo3 than Slo1. For Slo1, quinidine block was favored by depolarization, irrespective of the side of application. For Slo3, quinidine block was relieved by depolarization, irrespective of the side of application, with strong block by less than 10 μM quinidine at potentials near 0 mV. The unusual voltage-dependence of block of Slo3 by quinidine may result from preferential binding of quinidine to closed Slo3 channels. The quinidine concentrations effective in blocking Slo3 suggest, that in experiments that have examined quinidine effects on sperm, any Slo3 currents would be almost completely inhibited.

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.

Measurements of the BKCa channel's high-affinity Ca2+ binding constants: effects of membrane voltage

It has been established that the large conductance Ca²⁺-activated K⁺ channel contains two types of high-affinity Ca²⁺ binding sites, termed the Ca²⁺ bowl and the RCK1 site. The affinities of these sites, and how they change as the channel opens, is still a subject of some debate. Previous estimates of these affinities have relied on fitting a series of conductance–voltage relations determined over a series of Ca²⁺ concentrations with models of channel gating that include both voltage sensing and Ca²⁺ binding. This approach requires that some model of voltage sensing be chosen, and differences in the choice of voltage-sensing model may underlie the different estimates that have been produced. Here, to better determine these affinities we have measured Ca²⁺ dose–response curves of channel activity at constant voltage for the wild-type mSlo channel (minus its low-affinity Ca²⁺ binding site) and for channels that have had one or the other Ca²⁺ binding site disabled via mutation. To accurately determine these dose–response curves we have used a series of 22 Ca²⁺ concentrations, and we have used unitary current recordings, coupled with changes in channel expression level, to measure open probability over five orders of magnitude. Our results indicate that at −80 mV the Ca²⁺ bowl has higher affinity for Ca²⁺ than does the RCK1 site in both the opened and closed conformations of the channel, and that the binding of Ca²⁺ to the RCK1 site is voltage dependent, whereas at the Ca²⁺ bowl it is not.

Measuring the influence of the BKCa {beta}1 subunit on Ca2+ binding to the BKCa channel

The large-conductance Ca²⁺-activated potassium (BK_Ca) channel of smooth muscle is unusually sensitive to Ca²⁺ as compared with the BK_Ca channels of brain and skeletal muscle. This is due to the tissue-specific expression of the BK_Ca auxiliary subunit β1, whose presence dramatically increases both the potency and efficacy of Ca²⁺ in promoting channel opening. β1 contains no Ca²⁺ binding sites of its own, and thus the mechanism by which it increases the BK_Ca channel's Ca²⁺ sensitivity has been of some interest. Previously, we demonstrated that β1 stabilizes voltage sensor activation, such that activation occurs at more negative voltages with β1 present. This decreases the work that Ca²⁺ must do to open the channel and thereby increases the channel's apparent Ca²⁺ affinity without altering the real affinities of the channel's Ca²⁺ binding sites. To explain the full effect of β1 on the channel's Ca²⁺ sensitivity, however, we also proposed that there must be effects of β1 on Ca²⁺ binding. Here, to test this hypothesis, we have used high-resolution Ca²⁺ dose–response curves together with binding site–specific mutations to measure the effects of β1 on Ca²⁺ binding. We find that coexpression of β1 alters Ca²⁺ binding at both of the BK_Ca channel's two types of high-affinity Ca²⁺ binding sites, primarily increasing the affinity of the RCK1 sites when the channel is open and decreasing the affinity of the Ca²⁺ bowl sites when the channel is closed. Both of these modifications increase the difference in affinity between open and closed, such that Ca²⁺ binding at either site has a larger effect on channel opening when β1 is present.

Impaired Ca2+-dependent activation of large-conductance Ca2+-activated K+ channels in the coronary artery smooth muscle cells of Zucker Diabetic Fatty rats

The large-conductance Ca(2+)-activated K(+) (BK) channels play an important role in the regulation of cellular excitability in response to changes in intracellular metabolic state and Ca(2+) homeostasis. In vascular smooth muscle, BK channels are key determinants of vasoreactivity and vital-organ perfusion. Vascular BK channel functions are impaired in diabetes mellitus, but the mechanisms underlying such changes have not been examined in detail. We examined and compared the activities and kinetics of BK channels in coronary arterial smooth muscle cells from Lean control and Zucker Diabetic Fatty (ZDF) rats, using single-channel recording techniques. We found that BK channels in ZDF rats have impaired Ca(2+) sensitivity, including an increased free Ca(2+) concentration at half-maximal effect on channel activation, a reduced steepness of Ca(2+) dose-dependent curve, altered Ca(2+)-dependent gating properties with decreased maximal open probability, and a shortened mean open-time and prolonged mean closed-time durations. In addition, the BK channel beta-subunit-mediated activation by dehydrosoyasaponin-1 (DHS-1) was lost in cells from ZDF rats. Immunoblotting analysis confirmed a 2.1-fold decrease in BK channel beta(1)-subunit expression in ZDF rats, compared with that of Lean rats. These abnormalities in BK channel gating lead to an increase in the energy barrier for channel activation, and may contribute to the development of vascular dysfunction and complications in type 2 diabetes mellitus.

A residue at the cytoplasmic entrance of BK-type channels regulating single-channel opening by its hydrophobicity

Single large-conductance calcium-activated K(+) (BK) channels encoded by the mSlo gene usually have synchronous gating, but a Drosophila dSlo (A2/C2/E2/G5/10) splice variant (dSlo1A) exhibits very flickery openings. To probe this difference in gating, we constructed a mutant I323T. This channel exhibits four subconductance levels similar to those of dSlo1A. Rectification of the single-channel current-voltage relation of I323T decreased as [Ca(2+) ](in) increased from 10 to 300 microM. Mutagenesis suggests that the hydrophobicity of the residue at the position is important for the wild-type gating; i.e., increasing hydrophobicity prolongs open duration. Molecular dynamics simulation suggests that four hydrophobic pore-lining residues at position 323 of mSlo act cooperatively in a "shutter-like" mechanism gating the permeation of K(+) ions. Rate-equilibrium free energy relations analysis shows that the four I323 residues in an mSlo channel have a conformation 65% similar to the closed conformation during gating. Based on these observations, we suggest that the appearance of rectification and substates of BK-type channels arise from a reduction of the cooperativity among these four residues and a lower probability of being open.

The RCK2 domain of the human BKCa channel is a calcium sensor

Large conductance voltage and Ca(2+)-dependent K(+) channels (BK(Ca)) are activated by both membrane depolarization and intracellular Ca(2+). Recent studies on bacterial channels have proposed that a Ca(2+)-induced conformational change within specialized regulators of K(+) conductance (RCK) domains is responsible for channel gating. Each pore-forming alpha subunit of the homotetrameric BK(Ca) channel is expected to contain two intracellular RCK domains. The first RCK domain in BK(Ca) channels (RCK1) has been shown to contain residues critical for Ca(2+) sensitivity, possibly participating in the formation of a Ca(2+)-binding site. The location and structure of the second RCK domain in the BK(Ca) channel (RCK2) is still being examined, and the presence of a high-affinity Ca(2+)-binding site within this region is not yet established. Here, we present a structure-based alignment of the C terminus of BK(Ca) and prokaryotic RCK domains that reveal the location of a second RCK domain in human BK(Ca) channels (hSloRCK2). hSloRCK2 includes a high-affinity Ca(2+)-binding site (Ca bowl) and contains similar secondary structural elements as the bacterial RCK domains. Using CD spectroscopy, we provide evidence that hSloRCK2 undergoes a Ca(2+)-induced change in conformation, associated with an alpha-to-beta structural transition. We also show that the Ca bowl is an essential element for the Ca(2+)-induced rearrangement of hSloRCK2. We speculate that the molecular rearrangements of RCK2 likely underlie the Ca(2+)-dependent gating mechanism of BK(Ca) channels. A structural model of the heterodimeric complex of hSloRCK1 and hSloRCK2 domains is discussed.

Intra- and intersubunit cooperativity in activation of BK channels by Ca2+

The activation of BK channels by Ca(2+) is highly cooperative, with small changes in intracellular Ca(2+) concentration having large effects on open probability (Po). Here we examine the mechanism of cooperative activation of BK channels by Ca(2+). Each of the four subunits of BK channels has a large intracellular COOH terminus with two different high-affinity Ca(2+) sensors: an RCK1 sensor (D362/D367) located on the RCK1 (regulator of conductance of K(+)) domain and a Ca-bowl sensor located on or after the RCK2 domain. To determine interactions among these Ca(2+) sensors, we examine channels with eight different configurations of functional high-affinity Ca(2+) sensors on the four subunits. We find that the RCK1 sensor and Ca bowl contribute about equally to Ca(2+) activation of the channel when there is only one high-affinity Ca(2+) sensor per subunit. We also find that an RCK1 sensor and a Ca bowl on the same subunit are much more effective in increasing Po than when they are on different subunits, indicating positive intrasubunit cooperativity. If it is assumed that BK channels have a gating ring similar to MthK channels with alternating RCK1 and RCK2 domains and that the Ca(2+) sensors act at the flexible (rather than fixed) interfaces between RCK domains, then a comparison of the distribution of Ca(2+) sensors with the observed responses suggest that the interface between RCK1 and RCK2 domains on the same subunit is flexible. On this basis, intrasubunit cooperativity arises because two high-affinity Ca(2+) sensors acting across a flexible interface are more effective in opening the channel than when acting at separate interfaces. An allosteric model incorporating intrasubunit cooperativity nested within intersubunit cooperativity could approximate the Po vs. Ca(2+) response for eight possible subunit configurations of the high-affinity Ca(2+) sensors as well as for three additional configurations from a previous study.

High-conductance potassium channels of the SLO family

High-conductance, 'big' potassium (BK) channels encoded by the Slo gene family are among the largest and most complex of the extended family of potassium channels. The family of SLO channels apparently evolved from voltage-dependent potassium channels, but acquired a large conserved carboxyl extension, which allows channel gating to be altered in response to the direct sensing of several different intracellular ions, and by other second-messenger systems, such as those activated following neurotransmitter binding to G-protein-coupled receptors (GPCRs). This versatility has been exploited to serve many cellular roles, both within and outside the nervous system.

Mechanisms of two modulatory actions of the channel-binding protein Slob on the Drosophila Slowpoke calcium-dependent potassium channel

Slob57 is an ion channel auxiliary protein that binds to and modulates the Drosophila Slowpoke calcium-dependent potassium channel (dSlo). We reported recently that residues 1–39 of Slob57 comprise the key domain that both causes dSlo inactivation and shifts its voltage dependence of activation to more depolarized voltages. In the present study we show that removal of residues 2–6 from Slob57 abolishes the inactivation, but the ability of Slob57 to rightward shift the voltage dependence of activation of dSlo remains. A synthetic peptide corresponding in sequence to residues 1–6 of Slob57 blocks dSlo in a voltage- and dose-dependent manner. Two Phe residues and at least one Lys residue in this peptide are required for the blocking action. These data indicate that the amino terminus of Slob57 directly blocks dSlo, thereby leading to channel inactivation. Further truncation to residue Arg¹⁶ eliminates the modulation of voltage dependence of activation. Thus these two modulatory actions of Slob57 are independent. Mutation within the calcium bowl of dSlo greatly reduces its calcium sensitivity (Bian, S., I. Favre, and E. Moczydlowski. 2001. Proc. Natl. Acad. Sci. USA. 98:4776–4781). We found that Slob57 still causes inactivation of this mutant channel, but does not shift its voltage dependence of activation. This result confirms further the independence of the inactivation and the voltage shift produced by Slob57. It also suggests that the voltage shift requires high affinity Ca²⁺ binding to an intact calcium bowl. Furthermore, Slob57 inhibits the shift in the voltage dependence of activation of dSlo evoked by Ca²⁺, and this inhibition by Slob57 is greater at higher free Ca²⁺ concentrations. These results implicate distinct calcium-dependent and -independent mechanisms in the modulation of dSlo by Slob.

Down regulated expression of the beta1 subunit of the big-conductance Ca2+ sensitive K+ channel in sphincter of Oddi cells from rabbits fed with a high cholesterol diet

Hypercholesterolemia, which is closely related to gallbladder bile stasis, can cause sphincter of Oddi dysfunction (SOD) by increasing the tension of sphincter of Oddi (SO). Intracellular calcium ion concentration ([Ca(2+)](i)) could influence the tension of SO. The beta1 subunit of the big-conductance Ca(2+) sensitive K(+) channel (BK(Ca)) can enhance the sensitivity of the BK(Ca) channel to [Ca(2+)](i). Absence and decline of the BKCa channel subunit beta1 could lead to many diseases. However, the relationship between hypercholesterolemia and the expression of beta1 subunit is not well understood. In this study, we successfully expressed and purified the rabbit BK(Ca) beta1 subunit protein and prepared its polyclonal antibody. The specificity of the prepared antibody was determined by Western blotting. A SOD rabbit model induced by a high cholesterol diet was established and the expression of the beta1 subunit of SO was determined by immunohistochemical staining and western blotting. Compared with the controls, our results demonstrated that hypercholesterolemia could decrease the expression of the beta1 subunit in the SO cells from rabbits. This indicates that lower expression of BKCa channel beta1 subunit might induce SOD.

Calcium activation of ryanodine receptor channels--reconciling RyR gating models with tetrameric channel structure

Despite its importance and abundance of experimental data, the molecular mechanism of RyR2 activation by calcium is poorly understood. Recent experimental studies involving coexpression of wild-type (WT) RyR2 together with a RyR2 mutant deficient in calcium-dependent activation (Li, P., and S.R. Chen. 2001. J. Gen. Physiol. 118:33–44) revealed large variations of calcium sensitivity of the RyR tetramers with their monomer composition. Together with previous results on kinetics of Ca activation (Zahradníková, A., I. Zahradník, I. Györke, and S. Györke. 1999. J. Gen. Physiol. 114:787–798), these data represent benchmarks for construction and testing of RyR models that would reproduce RyR behavior and be structurally realistic as well. Here we present a theoretical study of the effects of RyR monomer substitution by a calcium-insensitive mutant on the calcium dependence of RyR activation. Three published models of tetrameric RyR channels were used either directly or after adaptation to provide allosteric regulation. Additionally, two alternative RyR models with Ca binding sites created jointly by the monomers were developed. The models were modified for description of channels composed of WT and mutant monomers. The parameters of the models were optimized to provide the best approximation of published experimental data. For reproducing the observed calcium dependence of RyR tetramers containing mutant monomers (a) single, independent Ca binding sites on each monomer were preferable to shared binding sites; (b) allosteric models were preferable to linear models; (c) in the WT channel, probability of opening to states containing a Ca²⁺-free monomer had to be extremely low; and (d) models with fully Ca-bound closed states, additional to those of an Monod-Wyman-Changeaux model, were preferable to models without such states. These results provide support for the concept that RyR activation is possible (albeit vanishingly small in WT channels) in the absence of Ca²⁺ binding. They also suggest further avenues toward understanding RyR gating.

Three methionine residues located within the regulator of conductance for K+ (RCK) domains confer oxidative sensitivity to large-conductance Ca2+-activated K+ channels

Methionine-directed oxidation of the human Slo1 potassium channel (hSlo1) shifts the half-activation voltage by -30 mV and markedly slows channel deactivation at low concentrations of intracellular Ca2+ ([Ca2+]i). We demonstrate here that the contemporaneous mutation of M536, M712 and M739 to leucine renders the channel functionally insensitive to methionine oxidation caused by the oxidant chloramine-T (Ch-T) without altering other functional characteristics. Coexpression with the auxiliary beta1 subunit fails to restore the full oxidative sensitivity to this triple mutant channel. The Ch-T effect is mediated specifically by M536, M712 and M739 because even small changes in this residue combination interfere with the ability to remove the oxidant sensitivity following mutation. Replacement of M712 or M739, but not M536, with the hydrophilic residue glutamate largely mimics oxidation of the channel and essentially removes the Ch-T sensitivity, suggesting that M712 and M739 may be part of a hydrophobic pocket disrupted by oxidation of non-polar methionine to the more hydrophilic methionine sulfoxide. The increase in wild-type hSlo1 open probability caused by methionine oxidation disappears at high [Ca2+]i and biophysical modelling of the Ch-T effect on steady-state activation implicates a decrease in the allosteric coupling between Ca2+ binding and the pore. The dramatic increase in open probability at low [Ca2+]i especially within the physiological voltage range suggests that oxidation of M536, M712 or M739 may enhance the Slo1 BK activity during conditions of oxidative stress, such as those associated with ischaemia-reperfusion and neurodegenerative disease, or in response to metabolic cues.

CaM kinase II phosphorylation of slo Thr107 regulates activity and ethanol responses of BK channels

High-conductance, Ca(2+)-activated and voltage-gated (BK) channels set neuronal firing. They are almost universally activated by alcohol, leading to reduced neuronal excitability and neuropeptide release and to motor intoxication. However, several BK channels are inhibited by alcohol, and most other voltage-gated K(+) channels are refractory to drug action. BK channels are homotetramers (encoded by Slo1) that possess a unique transmembrane segment (S0), leading to a cytosolic S0-S1 loop. We identified Thr107 of bovine slo (bslo) in this loop as a critical residue that determines BK channel responses to alcohol. In addition, the activity of Ca(2+)/calmodulin-dependent protein kinase II (CaMKII) in the cell controlled channel activity and alcohol modulation. Incremental CaMKII-mediated phosphorylation of Thr107 in the BK tetramer progressively increased channel activity and gradually switched the channel alcohol responses from robust activation to inhibition. Thus, CaMKII phosphorylation of slo Thr107 works as a 'molecular dimmer switch' that could mediate tolerance to alcohol, a form of neuronal plasticity.

Gating and ionic currents reveal how the BKCa channel's Ca2+ sensitivity is enhanced by its beta1 subunit

Large-conductance Ca(2+)-activated K(+) channels (BK(Ca) channels) are regulated by the tissue-specific expression of auxiliary beta subunits. Beta1 is predominantly expressed in smooth muscle, where it greatly enhances the BK(Ca) channel's Ca(2+) sensitivity, an effect that is required for proper regulation of smooth muscle tone. Here, using gating current recordings, macroscopic ionic current recordings, and unitary ionic current recordings at very low open probabilities, we have investigated the mechanism that underlies this effect. Our results may be summarized as follows. The beta1 subunit has little or no effect on the equilibrium constant of the conformational change by which the BK(Ca) channel opens, and it does not affect the gating charge on the channel's voltage sensors, but it does stabilize voltage sensor activation, both when the channel is open and when it is closed, such that voltage sensor activation occurs at more negative voltages with beta1 present. Furthermore, beta1 stabilizes the active voltage sensor more when the channel is closed than when it is open, and this reduces the factor D by which voltage sensor activation promotes opening by approximately 24% (16.8-->12.8). The effects of beta1 on voltage sensing enhance the BK(Ca) channel's Ca(2+) sensitivity by decreasing at most voltages the work that Ca(2+) binding must do to open the channel. In addition, however, in order to fully account for the increase in efficacy and apparent Ca(2+) affinity brought about by beta1 at negative voltages, our studies suggest that beta1 also decreases the true Ca(2+) affinity of the closed channel, increasing its Ca(2+) dissociation constant from approximately 3.7 microM to between 4.7 and 7.1 microM, depending on how many binding sites are affected.

The NH2 terminus of RCK1 domain regulates Ca2+-dependent BK(Ca) channel gating

Large conductance, voltage- and Ca²⁺-activated K⁺ (BK_Ca) channels regulate blood vessel tone, synaptic transmission, and hearing owing to dual activation by membrane depolarization and intracellular Ca²⁺. Similar to an archeon Ca²⁺-activated K⁺ channel, MthK, each of four α subunits of BK_Ca may contain two cytosolic RCK domains and eight of which may form a gating ring. The structure of the MthK channel suggests that the RCK domains reorient with one another upon Ca²⁺ binding to change the gating ring conformation and open the activation gate. Here we report that the conformational changes of the NH₂ terminus of RCK1 (AC region) modulate BK_Ca gating. Such modulation depends on Ca²⁺ occupancy and activation states, but is not directly related to the Ca²⁺ binding sites. These results demonstrate that AC region is important in the allosteric coupling between Ca²⁺ binding and channel opening. Thus, the conformational changes of the AC region within each RCK domain is likely to be an important step in addition to the reorientation of RCK domains leading to the opening of the BK_Ca activation gate. Our observations are consistent with a mechanism for Ca²⁺-dependent activation of BK_Ca channels such that the AC region inhibits channel activation when the channel is at the closed state in the absence of Ca²⁺; Ca²⁺ binding and depolarization relieve this inhibition.

Homology modeling identifies C-terminal residues that contribute to the Ca2+ sensitivity of a BKCa channel

Activation of BK(Ca) channels by direct Ca(2+) binding and membrane depolarization occur via independent and additive molecular processes. The "calcium bowl" domain is critically involved in Ca(2+)-dependent gating, and we have hypothesized that a sequence within this domain may resemble an EF hand motif. Using a homology modeling strategy, it was observed that a single Ca(2+) ion may be coordinated by the oxygen-containing side chains of residues within the calcium bowl (i.e., (912)ELVNDTNVQFLD(923)). To examine these predictions directly, alanine-substituted BK(Ca) channel mutants were expressed in HEK 293 cells and the voltage and Ca(2+) dependence of macroscopic currents were examined in inside-out membrane patches. Over the range of 1-10 microM free Ca(2+), single point mutations (i.e., E912A and D923A) produced rightward shifts in the steady-state conductance-voltage relations, whereas the mutants N918A or Q920A had no effect on Ca(2+)-dependent gating. The double mutant E912A/D923A displayed a synergistic shift in Ca(2+)-sensitive gating, as well as altered kinetics of current activation/deactivation. In the presence of 1, 10, and 80 mM cytosolic Mg(2+), this double mutation significantly reduced the Ca(2+)-induced free energy change associated with channel activation. Finally, mutations that altered sensitivity of the holo-channel to Ca(2+) also reduced direct (45)Ca binding to the calcium bowl domain expressed as a bacterial fusion protein. These findings, along with other recent data, are considered in the context of the calcium bowl's high affinity Ca(2+) sensor and the known properties of EF hands.

Compounds Structurally Related to Tamoxifen as Openers of Large-Conductance Calcium-Activated K+ Channel

We found that a variety of compounds containing partial structures of tamoxifen showed activity as chemical modulators of large-conductance calcium-activated K+ channels (BK channels).

Mapping the BKCa channel's "Ca2+ bowl": side-chains essential for Ca2+ sensing

There is controversy over whether Ca²⁺ binds to the BK_Ca channel's intracellular domain or its integral-membrane domain and over whether or not mutations that reduce the channel's Ca²⁺ sensitivity act at the point of Ca²⁺ coordination. One region in the intracellular domain that has been implicated in Ca²⁺ sensing is the “Ca²⁺ bowl”. This region contains many acidic residues, and large Ca²⁺-bowl mutations eliminate Ca²⁺ sensing through what appears to be one type of high-affinity Ca²⁺-binding site. Here, through site-directed mutagenesis we have mapped the residues in the Ca²⁺ bowl that are most important for Ca²⁺ sensing. We find acidic residues, D898 and D900, to be essential, and we find them essential as well for Ca²⁺ binding to a fusion protein that contains a portion of the BK_Ca channel's intracellular domain. Thus, much of our data supports the conclusion that Ca²⁺ binds to the BK_Ca channel's intracellular domain, and they define the Ca²⁺ bowl's essential Ca²⁺-sensing motif. Overall, however, we have found that the relationship between mutations that disrupt Ca²⁺ sensing and those that disrupt Ca²⁺ binding is not as strong as we had expected, a result that raises the possibility that, when examined by gel-overlay, the Ca²⁺ bowl may be in a nonnative conformation.

Properties of Quantal Transmission at CA1 Synapses

We have used Monte Carlo simulations to understand the generation of quantal responses at the single active zones of CA1 synapses. We constructed a model of AMPA channel activation that accounts for the responses to controlled glutamate application and a model of glutamate diffusion in the synaptic cleft. With no further adjustments to these models, we simulated the response to the release of glutamate from a single vesicle. The predicted response closely matches the rise time of observed responses, which recent measurements show is much faster (<100 μs) than previously thought. The simulations show that initial channel opening is driven by a brief (<100 μs) glutamate spike near the site of vesicle fusion, producing a hotspot of channel activation (diameter: ∼250 nm) smaller than many synapses. Quantal size therefore depends more strongly on the density of channels than their number, a finding that has important implications for measuring synaptic strength. Recent measurements allow estimation of AMPA receptor density at CA1 synapses. Using this value, our simulations correctly predicts a quantal amplitude of ∼10 pA. We have also analyzed the properties of excitatory postsynaptic currents (EPSCs) generated by the multivesicular release that can occur during evoked responses. We find that summation is nearly linear and that the existence of multiple narrow peaks in amplitude histograms can be accounted for. It has been unclear how to reconcile the existence of these narrow peaks, which indicate that the variation of quantal amplitude is small (CV < 0.2) with the highly variable amplitude of miniature EPSCs (mEPSCs; CV ∼ 0.6). According to one theory, mEPSC variability arises from variation in vesicle glutamate content. However, both our modeling results and recent experimental results indicate that this view cannot account for the observed rise time/amplitude correlation of mEPSCs. In contrast, this correlation and the high mEPSC variability can be accounted for if some mEPSCs are generated by two or more vesicles released with small temporal jitter. We conclude that a broad range of results can be accounted for by simple principles: quantal amplitude (∼10 pA) is stereotyped, some mEPSCs are multivesicular at moderate and large synapses, and evoked responses are generated by quasi-linear summation of multiple quanta.

Distinct stoichiometry of BKCa channel tetramer phosphorylation specifies channel activation and inhibition by cAMP-dependent protein kinase

Large conductance voltage- and calcium-activated potassium (BK(Ca)) channels are important signaling molecules that are regulated by multiple protein kinases and protein phosphatases at multiple sites. The pore-forming alpha-subunits, derived from a single gene that undergoes extensive alternative pre-mRNA splicing, assemble as tetramers. Although consensus phosphorylation sites have been identified within the C-terminal domain of alpha-subunits, it is not known whether phosphorylation of all or single alpha-subunits within the tetramer is required for functional regulation of the channel. Here, we have exploited a strategy to study single-ion channels in which both the alpha-subunit splice-variant composition is defined and the number of consensus phosphorylation sites available within each tetramer is known. We have used this approach to demonstrate that cAMP-dependent protein kinase (PKA) phosphorylation of the conserved C-terminal PKA consensus site (S899) in all four alpha-subunits is required for channel activation. In contrast, inhibition of BK(Ca) channel activity requires phosphorylation of only a single alpha-subunit at a splice insert (STREX)-specific PKA consensus site (S4(STREX)). Thus, distinct modes of BK(Ca) channel regulation by PKA phosphorylation exist: an "all-or-nothing" rule for activation and a "single-subunit" rule for inhibition. This essentially digital regulation has important implications for the combinatorial and conditional regulation of BK(Ca) channels by reversible protein phosphorylation.

Linker-gating ring complex as passive spring and Ca(2+)-dependent machine for a voltage- and Ca(2+)-activated potassium channel

Ion channels are proteins that control the flux of ions across cell membranes by opening and closing (gating) their pores. It has been proposed that channels gated by internal agonists have an intracellular gating ring that extracts free energy from agonist binding to open the gates using linkers that directly connect the gating ring to the gates. Here we find for a voltage- and Ca(2+)-activated K+ (BK) channel that shortening the linkers increases channel activity and lengthening the linkers decreases channel activity, both in the presence and absence of intracellular Ca2+. These observations are consistent with a mechanical model in which the linker-gating ring complex forms a passive spring that applies force to the gates in the absence of Ca2+ to modulate the voltage-dependent gating. Adding Ca2+ then changes the force to further activate the channel. Both the passive and Ca(2+)-induced forces contribute to the gating of the channel.

Dehydroabietic acid derivatives as a novel scaffold for large-conductance calcium-activated K+ channel openers

We found that the dehydroabietic acid structure is a new scaffold for chemical modulators of large-conductance calcium-activated K+ channels (BK channels). Structure-activity relationship (SAR) studies of the dehydroabietic acid derivatives and related non-aromatic compounds such as pimaric acid revealed the importance of the carboxyl functionality and an appropriate hydrophobic moiety of the molecules for BK channel-opening ability.

Reactive oxygen species impair Slo1 BK channel function by altering cysteine-mediated calcium sensing

Vascular dysfunction is a hallmark of many diseases, including coronary heart disease, stroke and diabetes. The underlying mechanisms of these disorders, which are intimately associated with inflammation and oxidative stress caused by excess reactive oxygen species (ROS), have remained elusive. Here we report that ROS are powerful inhibitors of vascular smooth muscle calcium-dependent Slo1 BK or Maxi-K potassium channels, an important physiological determinant of vascular tone. By targeting a cysteine residue near the Ca(2+) bowl of the BK alpha subunit, H(2)O(2) virtually eliminates physiological activation of the channel, with an inhibitory potency comparable to a knockout of the auxiliary subunit BK beta 1. These results reveal a molecular structural basis for the vascular dysfunction involving oxidative stress and provide a solid rationale for a potential use of BK openers in the prevention and treatment of cardiovascular disorders.