Gating properties conferred on BK channels by the beta3b auxiliary subunit in the absence of its NH(2)- and COOH termini

An extracellular scaffolding complex confers unusual rectification upon an ionotropic acetylcholine receptor in C. elegans

Biophysical properties of ligand-gated receptors can be profoundly modified by auxiliary subunits or by the lipid microenvironment of the membrane. Hence, it is sometimes challenging to relate the properties of receptors reconstituted in heterologous expression systems to those of their native counterparts. Here we show that the properties of Caenorhabditis elegans levamisole-sensitive acetylcholine receptors (L-AChRs), the ionotropic acetylcholine receptors targeted by the cholinergic anthelmintic levamisole at neuromuscular junctions, can be profoundly modified by their clustering machinery. We uncovered that L-AChRs exhibit a strong outward rectification in vivo, which was not previously described in heterologous systems. This unusual feature for an ionotropic AChR is abolished by disrupting the interaction of the receptors with the extracellular complex required for their synaptic clustering. When recorded at -60 mV, levamisole-induced currents are similar in the wild type and in L-AChR-clustering-defective mutants, while they are halved in these mutants at more depolarized physiological membrane potentials. Consequently, levamisole causes a strong muscle depolarization in the wild type, which leads to complete inactivation of the voltage-gated calcium channels and to an irreversible flaccid paralysis. In mutants defective for L-AChR clustering, the levamisole-induced depolarization is weaker, allowing voltage-gated calcium channels to remain partially active, which eventually leads to adaptation and survival of the worms. This explains why historical screens for C. elegans mutants resistant to levamisole identified the components of the L-AChR clustering machinery, in addition to proteins required for receptor biosynthesis or efficacy. This work further emphasizes the importance of pursuing ligand-gated channel characterization in their native environment.

Voltage-dependent dynamics of the BK channel cytosolic gating ring are coupled to the membrane-embedded voltage sensor

In humans, large conductance voltage- and calcium-dependent potassium (BK) channels are regulated allosterically by transmembrane voltage and intracellular Ca²⁺. Divalent cation binding sites reside within the gating ring formed by two Regulator of Conductance of Potassium (RCK) domains per subunit. Using patch-clamp fluorometry, we show that Ca²⁺ binding to the RCK1 domain triggers gating ring rearrangements that depend on transmembrane voltage. Because the gating ring is outside the electric field, this voltage sensitivity must originate from coupling to the voltage-dependent channel opening, the voltage sensor or both. Here we demonstrate that alterations of the voltage sensor, either by mutagenesis or regulation by auxiliary subunits, are paralleled by changes in the voltage dependence of the gating ring movements, whereas modifications of the relative open probability are not. These results strongly suggest that conformational changes of RCK1 domains are specifically coupled to the voltage sensor function during allosteric modulation of BK channels.

A voltage-dependent K+ channel in the lysosome is required for refilling lysosomal Ca2+ stores

Ion-dependent channels and transporters have been identified in lysosomes, including the V-ATPase H⁺ pump and transient receptor potential mucolipin channels (TRPMLs), the principle Ca²⁺ release channels in the lysosome, but much less is understood about the roles of Na⁺ and K⁺ in lysosomal physiology. Wang et al. describe a voltage-sensitive, Ca²⁺-activated K⁺ current in the lysosome (LysoK_VCa) and show that LysoK_VCa regulates lysosomal membrane potential and refilling of lysosomal Ca²⁺ stores.

The resting membrane potential (Δψ) of the cell is negative on the cytosolic side and determined primarily by the plasma membrane’s selective permeability to K⁺. We show that lysosomal Δψ is set by lysosomal membrane permeabilities to Na⁺ and H⁺, but not K⁺, and is positive on the cytosolic side. An increase in juxta-lysosomal Ca²⁺ rapidly reversed lysosomal Δψ by activating a large voltage-dependent and K⁺-selective conductance (LysoK_VCa). LysoK_VCa is encoded molecularly by SLO1 proteins known for forming plasma membrane BK channels. Opening of single LysoK_VCa channels is sufficient to cause the rapid, striking changes in lysosomal Δψ. Lysosomal Ca²⁺ stores may be refilled from endoplasmic reticulum (ER) Ca²⁺ via ER–lysosome membrane contact sites. We propose that LysoK_VCa serves as the perilysosomal Ca²⁺ effector to prime lysosomes for the refilling process. Consistently, genetic ablation or pharmacological inhibition of LysoK_VCa, or abolition of its Ca²⁺ sensitivity, blocks refilling and maintenance of lysosomal Ca²⁺ stores, resulting in lysosomal cholesterol accumulation and a lysosome storage phenotype.

Molecular Determinants of BK Channel Functional Diversity and Functioning

Large-conductance Ca2+- and voltage-activated K+ (BK) channels play many physiological roles ranging from the maintenance of smooth muscle tone to hearing and neurosecretion. BK channels are tetramers in which the pore-forming α subunit is coded by a single gene ( Slowpoke, KCNMA1). In this review, we first highlight the physiological importance of this ubiquitous channel, emphasizing the role that BK channels play in different channelopathies. We next discuss the modular nature of BK channel-forming protein, in which the different modules (the voltage sensor and the Ca2+ binding sites) communicate with the pore gates allosterically. In this regard, we review in detail the allosteric models proposed to explain channel activation and how the models are related to channel structure. Considering their extremely large conductance and unique selectivity to K+, we also offer an account of how these two apparently paradoxical characteristics can be understood consistently in unison, and what we have learned about the conduction system and the activation gates using ions, blockers, and toxins. Attention is paid here to the molecular nature of the voltage sensor and the Ca2+ binding sites that are located in a gating ring of known crystal structure and constituted by four COOH termini. Despite the fact that BK channels are coded by a single gene, diversity is obtained by means of alternative splicing and modulatory β and γ subunits. We finish this review by describing how the association of the α subunit with β or with γ subunits can change the BK channel phenotype and pharmacology.

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.

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.

Inter-α/β subunits coupling mediating pre-inactivation and augmented activation of BKCa(β2)

Large-conductance calcium-activated potassium (BK) channels regulate the electric properties and neurotransmitter release in excitable cells. Its auxiliary β2 subunits not only enhance gating, but also confer inactivation via a short-lived preinactivated state. However, the mechanism of enhancement and preinactivation of BK channels by β2 remains elusive. Using our newly developed methods, we demonstrated that electrostatic forces played a crucial role in forming multiple complementary pairs of binding sites between α and β subunits including a “PI site” required for channel preinactivation, an “E site” enhancing calcium sensitivity and an “E_CaB” coupling site transferring force to gate from the Ca²⁺-bowl via the β2(K33, R34, K35), E site and S6-C linker, independent of another Ca²⁺ binding site mSlo1(D362,D367). A comprehensive structural model of the BK(β2) complex was reconstructed based on these functional studies, which paves the way for a clearer understanding of the structural mechanisms of activation and preinactivation of other BK(β) complexes.

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.

Modulation of BK channel voltage gating by different auxiliary β subunits

Calcium- and voltage-activated potassium channels (BK) are regulated by a multiplicity of signals. The prevailing view is that different BK gating mechanisms converge to determine channel opening and that these gating mechanisms are allosterically coupled. In most instances the pore forming α subunit of BK is associated with one of four alternative β subunits that appear to target specific gating mechanisms to regulate the channel activity. In particular, β1 stabilizes the active configuration of the BK voltage sensor having a large effect on BK Ca(2+) sensitivity. To determine the extent to which β subunits regulate the BK voltage sensor, we measured gating currents induced by the pore-forming BK α subunit alone and with the different β subunits expressed in Xenopus oocytes (β1, β2IR, β3b, and β4). We found that β1, β2, and β4 stabilize the BK voltage sensor in the active conformation. β3 has no effect on voltage sensor equilibrium. In addition, β4 decreases the apparent number of charges per voltage sensor. The decrease in the charge associated with the voltage sensor in α β4 channels explains most of their biophysical properties. For channels composed of the α subunit alone, gating charge increases slowly with pulse duration as expected if a significant fraction of this charge develops with a time course comparable to that of K(+) current activation. In the presence of β1, β2, and β4 this slow component develops in advance of and much more rapidly than ion current activation, suggesting that BK channel opening proceeds in two steps.

Barium ions selectively activate BK channels via the Ca2+-bowl site

Activation of Ca(2+)-dependent BK channels is increased via binding of micromolar Ca(2+) to two distinct high-affinity sites per BK α-subunit. One site, termed the Ca(2+) bowl, is embedded within the second RCK domain (RCK2; regulator of conductance for potassium) of each α-subunit, while oxygen-containing residues in the first RCK domain (RCK1) have been linked to a separate Ca(2+) ligation site. Although both sites are activated by Ca(2+) and Sr(2+), Cd(2+) selectively favors activation via the RCK1 site. Divalent cations of larger ionic radius than Sr(2+) are thought to be ineffective at activating BK channels. Here we show that Ba(2+), better known as a blocker of K(+) channels, activates BK channels and that this effect arises exclusively from binding at the Ca(2+)-bowl site. Compared with previous estimates for Ca(2+) bowl-mediated activation by Ca(2+), the affinity of Ba(2+) to the Ca(2+) bowl is reduced about fivefold, and coupling of binding to activation is reduced from ∼3.6 for Ca(2+) to about ∼2.8 for Ba(2+). These results support the idea that ionic radius is an important determinant of selectivity differences among different divalent cations observed for each Ca(2+)-binding site.

Ca2+-activated K+ channels: from protein complexes to function

Molecular research on ion channels has demonstrated that many of these integral membrane proteins associate with partner proteins, often versatile in their function, or even assemble into stable macromolecular complexes that ensure specificity and proper rate of the channel-mediated signal transduction. Calcium-activated potassium (K(Ca)) channels that link excitability and intracellular calcium concentration are responsible for a wide variety of cellular processes ranging from regulation of smooth muscle tone to modulation of neurotransmission and control of neuronal firing pattern. Most of these functions are brought about by interaction of the channels' pore-forming subunits with distinct partner proteins. In this review we summarize recent insights into protein complexes associated with K(Ca) channels as revealed by proteomic research and discuss the results available on structure and function of these complexes and on the underlying protein-protein interactions. Finally, the results are related to their significance for the function of K(Ca) channels under cellular conditions.

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.

Species-specific Differences among KCNMB3 BK beta3 auxiliary subunits: some beta3 N-terminal variants may be primate-specific subunits

The KCNMB3 gene encodes one of a family of four auxiliary beta subunits found in the mammalian genome that associate with Slo1 alpha subunits and regulate BK channel function. In humans, the KCNMB3 gene contains four N-terminal alternative exons that produce four functionally distinct beta3 subunits, beta3a-d. Three variants, beta3a-c, exhibit kinetically distinct inactivation behaviors. Since investigation of the physiological roles of BK auxiliary subunits will depend on studies in rodents, here we have determined the identity and functional properties of mouse beta3 variants. Whereas beta1, beta2, and beta4 subunits exhibit 83.2%, 95.3%, and 93.8% identity between mouse and human, the mouse beta3 subunit, excluding N-terminal splice variants, shares only 62.8% amino acid identity with its human counterpart. Based on an examination of the mouse genome and screening of mouse cDNA libraries, here we have identified only two N-terminal candidates, beta3a and beta3b, of the four found in humans. Both human and mouse beta3a subunits produce a characteristic use-dependent inactivation. Surprisingly, whereas the hbeta3b exhibits rapid inactivation, the putative mbeta3b does not inactivate. Furthermore, unlike hbeta3, the mbeta3 subunit, irrespective of the N terminus, mediates a shift in gating to more negative potentials at a given Ca(2+) concentration. The shift in gating gradually is lost following patch excision, suggesting that the gating shift involves some regulatory process dependent on the cytosolic milieu. Examination of additional genomes to assess conservation among splice variants suggests that the putative mbeta3b N terminus may not be a true orthologue of the hbeta3b N terminus and that both beta3c and beta3d appear likely to be primate-specific N-terminal variants. These results have three key implications: first, functional properties of homologous beta3 subunits may differ among mammalian species; second, the specific physiological roles of homologous beta3 subunits may differ among mammalian species; and, third, some beta3 variants may be primate-specific ion channel subunits.

Lysine-rich extracellular rings formed by hbeta2 subunits confer the outward rectification of BK channels

The auxiliary β subunits of large-conductance Ca²⁺-activated K⁺ (BK) channels greatly contribute to the diversity of BK (mSlo1 α) channels, which is fundamental to the adequate function in many tissues. Here we describe a functional element of the extracellular segment of hβ2 auxiliary subunits that acts as the positively charged rings to modify the BK channel conductance. Four consecutive lysines of the hβ2 extracellular loop, which reside sufficiently close to the extracellular entryway of the pore, constitute three positively charged rings. These rings can decrease the extracellular K⁺ concentration and prevent the Charybdotoxin (ChTX) from approaching the extracellular entrance of channels through electrostatic mechanism, leading to the reduction of K⁺ inflow or the outward rectification of BK channels. Our results demonstrate that the lysine rings formed by the hβ2 auxiliary subunits influences the inward current of BK channels, providing a mechanism by which current can be rapidly diminished during cellular repolarization. Furthermore, this study will be helpful to understand the functional diversity of BK channels contributed by different auxiliary β subunits.

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.

Slo3 K+ channels: voltage and pH dependence of macroscopic currents

The mouse Slo3 gene (KCNMA3) encodes a K(+) channel that is regulated by changes in cytosolic pH. Like Slo1 subunits responsible for the Ca(2+) and voltage-activated BK-type channel, the Slo3 alpha subunit contains a pore module with homology to voltage-gated K(+) channels and also an extensive cytosolic C terminus thought to be responsible for ligand dependence. For the Slo3 K(+) channel, increases in cytosolic pH promote channel activation, but very little is known about many fundamental properties of Slo3 currents. Here we define the dependence of macroscopic conductance on voltage and pH and, in particular, examine Slo3 conductance activated at negative potentials. Using this information, the ability of a Horrigan-Aldrich-type of general allosteric model to account for Slo3 gating is examined. Finally, the pH and voltage dependence of Slo3 activation and deactivation kinetics is reported. The results indicate that Slo3 differs from Slo1 in several important ways. The limiting conductance activated at the most positive potentials exhibits a pH-dependent maximum, suggesting differences in the limiting open probability at different pH. Furthermore, over a 600 mV range of voltages (-300 to +300 mV), Slo3 conductance shifts only about two to three orders of magnitude, and the limiting conductance at negative potentials is relatively voltage independent compared to Slo1. Within the context of the Horrigan-Aldrich model, these results indicate that the intrinsic voltage dependence (z(L)) of the Slo3 closed-open equilibrium and the coupling (D) between voltage sensor movement are less than in Slo1. The kinetic behavior of Slo3 currents also differs markedly from Slo1. Both activation and deactivation are best described by two exponential components, both of which are only weakly voltage dependent. Qualitatively, the properties of the two kinetic components in the activation time course suggest that increases in pH increase the fraction of more rapidly opening channels.

Structural determinants for functional coupling between the beta and alpha subunits in the Ca2+-activated K+ (BK) channel

High conductance, calcium- and voltage-activated potassium (BK, MaxiK) channels are widely expressed in mammals. In some tissues, the biophysical properties of BK channels are highly affected by coexpression of regulatory (β) subunits. The most remarkable effects of β1 and β2 subunits are an increase of the calcium sensitivity and the slow down of channel kinetics. However, the detailed characteristics of channels formed by α and β1 or β2 are dissimilar, the most remarkable difference being a reduction of the voltage sensitivity in the presence of β1 but not β2. Here we reveal the molecular regions in these β subunits that determine their differential functional coupling with the pore-forming α-subunit. We made chimeric constructs between β1 and β2 subunits, and BK channels formed by α and chimeric β subunits were expressed in Xenopus laevis oocytes. The electrophysiological characteristics of the resulting channels were determined using the patch clamp technique. Chimeric exchange of the different regions of the β1 and β2 subunits demonstrates that the NH₃ and COOH termini are the most relevant regions in defining the behavior of either subunit. This strongly suggests that the intracellular domains are crucial for the fine tuning of the effects of these β subunits. Moreover, the intracellular domains of β1 are responsible for the reduction of the BK channel voltage dependence. This agrees with previous studies that suggested the intracellular regions of the α-subunit to be the target of the modulation by the β1-subunit.

Defining the BK channel domains required for beta1-subunit modulation

In a wide variety of cell types, including neurons and smooth muscle cells, activation of the large-conductance voltage- and Ca(2+)-activated K(+) (BK) channels causes transient membrane hyperpolarization, thereby regulating cellular excitability. Similar to other voltage-gated ion channels, BK channels, a tetramer of alpha-subunits, associate with auxiliary beta-subunits in a tissue-specific manner, modifying the channel's gating properties. The BK beta1-subunit, which is expressed in smooth muscle, increases the apparent Ca(2+) sensitivity (marked by a hyperpolarizing shift in the conductance-voltage relationship at a given Ca(2+) concentration), slows macroscopic activation and deactivation, and is required for channel activation by 17beta-estradiol. The beta1-subunit is essential for normal regulation of vascular smooth muscle contractility and blood pressure. Little is known, however, about the molecular mechanisms of beta1-subunit modulation of alpha-subunits. Here we show that the beta1-subunit's modulation of the Ca(2+) and 17beta-estradiol sensitivities can be dissociated from its effects on gating kinetics by truncation of the alpha-subunit's extracellular N-terminal residues. The BK alpha-subunit N terminus interacts uniquely with the beta1-subunit: beta2 regulation of the alpha-subunit is unaltered by truncation of the N terminus. Although the functional interaction of alpha and beta1 requires the N-terminal tail of alpha, the physical association requires the S1, S2, and S3 transmembrane helices of alpha.

Divalent cation sensitivity of BK channel activation supports the existence of three distinct binding sites

Mutational analyses have suggested that BK channels are regulated by three distinct divalent cation-dependent regulatory mechanisms arising from the cytosolic COOH terminus of the pore-forming α subunit. Two mechanisms account for physiological regulation of BK channels by μM Ca²⁺. The third may mediate physiological regulation by mM Mg²⁺. Mutation of five aspartate residues (5D5N) within the so-called Ca²⁺ bowl removes a portion of a higher affinity Ca²⁺ dependence, while mutation of D362A/D367A in the first RCK domain also removes some higher affinity Ca²⁺ dependence. Together, 5D5N and D362A/D367A remove all effects of Ca²⁺ up through 1 mM while E399A removes a portion of low affinity regulation by Ca²⁺/Mg²⁺. If each proposed regulatory effect involves a distinct divalent cation binding site, the divalent cation selectivity of the actual site that defines each mechanism might differ. By examination of the ability of various divalent cations to activate currents in constructs with mutationally altered regulatory mechanisms, here we show that each putative regulatory mechanism exhibits a unique sensitivity to divalent cations. Regulation mediated by the Ca²⁺ bowl can be activated by Ca²⁺ and Sr²⁺, while regulation defined by D362/D367 can be activated by Ca²⁺, Sr²⁺, and Cd²⁺. Mn²⁺, Co²⁺, and Ni²⁺ produce little observable effect through the high affinity regulatory mechanisms, while all six divalent cations enhance activation through the low affinity mechanism defined by residue E399. Furthermore, each type of mutation affects kinetic properties of BK channels in distinct ways. The Ca²⁺ bowl mainly accelerates activation of BK channels at low [Ca²⁺], while the D362/D367-related high affinity site influences both activation and deactivation over the range of 10–300 μM Ca²⁺. The major kinetic effect of the E399-related low affinity mechanism is to slow deactivation at mM Mg²⁺ or Ca²⁺. The results support the view that three distinct divalent-cation binding sites mediate regulation of BK channels.

Differential effects of beta 1 and beta 2 subunits on BK channel activity

High conductance, calcium- and voltage-activated potassium (BK) channels are widely expressed in mammals. In some tissues, the biophysical properties of BK channels are highly affected by coexpression of regulatory (β) subunits. β1 and β2 subunits increase apparent channel calcium sensitivity. The β1 subunit also decreases the voltage sensitivity of the channel and the β2 subunit produces an N-type inactivation of BK currents. We further characterized the effects of the β1 and β2 subunits on the calcium and voltage sensitivity of the channel, analyzing the data in the context of an allosteric model for BK channel activation by calcium and voltage (Horrigan and Aldrich, 2002). In this study, we used a β2 subunit without its N-type inactivation domain (β2IR). The results indicate that the β2IR subunit, like the β1 subunit, has a small effect on the calcium binding affinity of the channel. Unlike the β1 subunit, the β2IR subunit also has no effect on the voltage sensitivity of the channel. The limiting voltage dependence for steady-state channel activation, unrelated to voltage sensor movements, is unaffected by any of the studied β subunits. The same is observed for the limiting voltage dependence of the deactivation time constant. Thus, the β1 subunit must affect the voltage sensitivity by altering the function of the voltage sensors of the channel. Both β subunits reduce the intrinsic equilibrium constant for channel opening (L₀). In the allosteric activation model, the reduction of the voltage dependence for the activation of the voltage sensors accounts for most of the macroscopic steady-state effects of the β1 subunit, including the increase of the apparent calcium sensitivity of the BK channel. All allosteric coupling factors need to be increased in order to explain the observed effects when the α subunit is coexpressed with the β2IR subunit.

Redox-sensitive extracellular gates formed by auxiliary beta subunits of calcium-activated potassium channels

An important step to understanding ion channels is identifying the structural components that act as the gates to ion movement. Here we describe a new channel gating mechanism, produced by the beta3 auxiliary subunits of Ca2+-activated, large-conductance BK-type K+ channels when expressed with their pore-forming alpha subunits. BK beta subunits have a cysteine-rich extracellular segment connecting two transmembrane segments, with small cytosolic N and C termini. The extracellular segments of the beta3 subunits form gates to block ion permeation, providing a mechanism by which current can be rapidly diminished upon cellular repolarization. Furthermore, this gating mechanism is abolished by reduction of extracellular disulfide linkages, suggesting that endogenous mechanisms may regulate this gating behavior. The results indicate that auxiliary beta subunits of BK channels reside sufficiently close to the ion permeation pathway defined by the alpha subunits to influence or block access of small molecules to the permeation pathway.

Inactivation of BK channels by the NH2 terminus of the beta2 auxiliary subunit: an essential role of a terminal peptide segment of three hydrophobic residues

An auxiliary beta2 subunit, when coexpressed with Slo alpha subunits, produces inactivation of the resulting large-conductance, Ca(2+) and voltage-dependent K(+) (BK-type) channels. Inactivation is mediated by the cytosolic NH(2) terminus of the beta2 subunit. To understand the structural requirements for inactivation, we have done a mutational analysis of the role of the NH(2) terminus in the inactivation process. The beta2 NH(2) terminus contains 46 residues thought to be cytosolic to the first transmembrane segment (TM1). Here, we address two issues. First, we define the key segment of residues that mediates inactivation. Second, we examine the role of the linker between the inactivation segment and TM1. The results show that the critical determinant for inactivation is an initial segment of three amino acids (residues 2-4: FIW) after the initiation methionine. Deletions that scan positions from residue 5 through residue 36 alter inactivation, but do not abolish it. In contrast, deletion of FIW or combinations of point mutations within the FIW triplet abolish inactivation. Mutational analysis of the three initial residues argues that inactivation does not result from a well-defined structure formed by this epitope. Inactivation may be better explained by linear entry of the NH(2)-terminal peptide segment into the permeation pathway with residue hydrophobicity and size influencing the onset and recovery from inactivation. Examination of the ability of artificial, polymeric linkers to support inactivation suggests that a variety of amino acid sequences can serve as adequate linkers as long as they contain a minimum of 12 residues between the first transmembrane segment and the FIW triplet. Thus, neither a specific distribution of charge on the linker nor a specific structure in the linker is required to support the inactivation process.

Steady-state and closed-state inactivation properties of inactivating BK channels

Calcium-dependent potassium (BK-type) Ca2+ and voltage-dependent K+ channels in chromaffin cells exhibit an inactivation that probably arises from coassembly of Slo1 alpha subunits with auxiliary beta subunits. One goal of this work was to determine whether the Ca2+ dependence of inactivation arises from any mechanism other than coupling of inactivation to the Ca2+ dependence of activation. Steady-state inactivation and the onset of inactivation were studied in inside-out patches and whole-cell recordings from rat adrenal chromaffin cells with parallel experiments on inactivating BK channels resulting from cloned alpha + beta2 subunits. In both cases, steady-state inactivation was shifted to more negative potentials by increases in submembrane [Ca2+] from 1 to 60 microM. At 10 and 60 microM Ca2+, the maximal channel availability at negative potentials was similar despite a shift in the voltage of half availability, suggesting there is no strictly Ca2+-dependent inactivation. In contrast, in the absence of Ca2+, depolarization to potentials positive to +20 mV induces channel inactivation. Thus, voltage-dependent, but not solely Ca2+-dependent, kinetic steps are required for inactivation to occur. Finally, under some conditions, BK channels are shown to inactivate as readily from closed states as from open states, indicative that a key conformational change required for inactivation precedes channel opening.

Consequences of the stoichiometry of Slo1 alpha and auxiliary beta subunits on functional properties of large-conductance Ca2+-activated K+ channels

Auxiliary beta subunits play a major role in defining the functional properties of large-conductance, Ca2+-dependent BK-type K+ channels. In particular, both the beta1 and beta2 subunits produce strong shifts in the voltage dependence of channel activation at a given Ca2+. Beta subunits are thought to coassemble with alpha subunits in a 1:1 stoichiometry, such that a full ion channel complex may contain up to four beta subunits per channel. However, previous results raise the possibility that ion channels with less than a full complement of beta subunits may also occur. The functional consequence of channels with differing stoichiometries remains unknown. Here, using expression of alpha and beta subunits in Xenopus oocytes, we show explicitly that functional BK channels can arise with less than four beta subunits. Furthermore, the results show that, for both the beta1 and beta2 subunits, each individual beta subunit produces an essentially identical, incremental effect on the voltage dependence of gating. For channels arising from alpha + beta2 subunits, the number of beta2 subunits per channel also has a substantial impact on properties of steady-state inactivation and recovery from inactivation. Thus, the stoichiometry of alpha:beta subunit assembly can play a major functional role in defining the apparent Ca2+ dependence of activation of BK channels and in influencing the availability of BK channels for activation.

Consequences of the stoichiometry of Slo1 alpha and auxiliary beta subunits on functional properties of large-conductance Ca2+-activated K+ channels

Auxiliary beta subunits play a major role in defining the functional properties of large-conductance, Ca2+-dependent BK-type K+ channels. In particular, both the beta1 and beta2 subunits produce strong shifts in the voltage dependence of channel activation at a given Ca2+. Beta subunits are thought to coassemble with alpha subunits in a 1:1 stoichiometry, such that a full ion channel complex may contain up to four beta subunits per channel. However, previous results raise the possibility that ion channels with less than a full complement of beta subunits may also occur. The functional consequence of channels with differing stoichiometries remains unknown. Here, using expression of alpha and beta subunits in Xenopus oocytes, we show explicitly that functional BK channels can arise with less than four beta subunits. Furthermore, the results show that, for both the beta1 and beta2 subunits, each individual beta subunit produces an essentially identical, incremental effect on the voltage dependence of gating. For channels arising from alpha + beta2 subunits, the number of beta2 subunits per channel also has a substantial impact on properties of steady-state inactivation and recovery from inactivation. Thus, the stoichiometry of alpha:beta subunit assembly can play a major functional role in defining the apparent Ca2+ dependence of activation of BK channels and in influencing the availability of BK channels for activation.

Allosteric regulation of BK channel gating by Ca(2+) and Mg(2+) through a nonselective, low affinity divalent cation site

The ability of membrane voltage to activate high conductance, calcium-activated (BK-type) K(+) channels is enhanced by cytosolic calcium (Ca(2+)). Activation is sensitive to a range of [Ca(2+)] that spans over four orders of magnitude. Here, we examine the activation of BK channels resulting from expression of cloned mouse Slo1 alpha subunits at [Ca(2+)] and [Mg(2+)] up to 100 mM. The half-activation voltage (V(0.5)) is steeply dependent on [Ca(2+)] in the micromolar range, but shows a tendency towards saturation over the range of 60-300 microM Ca(2+). As [Ca(2+)] is increased to millimolar levels, the V(0.5) is strongly shifted again to more negative potentials. When channels are activated by 300 microM Ca(2+), further addition of either mM Ca(2+) or mM Mg(2+) produces similar negative shifts in steady-state activation. Millimolar Mg(2+) also produces shifts of similar magnitude in the complete absence of Ca(2+). The ability of millimolar concentrations of divalent cations to shift activation is primarily correlated with a slowing of BK current deactivation. At voltages where millimolar elevations in [Ca(2+)] increase activation rates, addition of 10 mM Mg(2+) to 0 Ca(2+) produces little effect on activation time course, while markedly slowing deactivation. This suggests that Mg(2+) does not participate in Ca(2+)-dependent steps that influence current activation rate. We conclude that millimolar Mg(2+) and Ca(2+) concentrations interact with low affinity, relatively nonselective divalent cation binding sites that are distinct from higher affinity, Ca(2+)-selective binding sites that increase current activation rates. A symmetrical model with four independent higher affinity Ca(2+) binding steps, four voltage sensors, and four independent lower affinity Ca(2+)/Mg(2+) binding steps describes well the behavior of G-V curves over a range of Ca(2+) and Mg(2+). The ability of a broad range of [Ca(2+)] to produce shifts in activation of Slo1 conductance can, therefore, be accounted for by multiple types of divalent cation binding sites.

Inactivation of BK channels mediated by the NH(2) terminus of the beta3b auxiliary subunit involves a two-step mechanism: possible separation of binding and blockade

A family of auxiliary beta subunits coassemble with Slo alpha subunit to form Ca(2)+-regulated, voltage-activated BK-type K(+) channels. The beta subunits play an important role in regulating the functional properties of the resulting channel protein, including apparent Ca(2)+ dependence and inactivation. The beta3b auxiliary subunit, when coexpressed with the Slo alpha subunit, results in a particularly rapid ( approximately 1 ms), but incomplete inactivation, mediated by the cytosolic NH(2) terminus of the beta3b subunit (Xia et al. 2000). Here, we evaluate whether a simple block of the open channel by the NH(2)-terminal domain accounts for the inactivation mechanism. Analysis of the onset of block, recovery from block, time-dependent changes in the shape of instantaneous current-voltage curves, and properties of deactivation tails suggest that a simple, one step blocking reaction is insufficient to explain the observed currents. Rather, blockade can be largely accounted for by a two-step blocking mechanism (C(n) <---> O(n) <---> O(*)(n) <---> I(n)) in which preblocked open states (O*(n)) precede blocked states (I(n)). The transitions between O* and I are exceedingly rapid accounting for an almost instantaneous block or unblock of open channels observed with changes in potential. However, the macroscopic current relaxations are determined primarily by slower transitions between O and O*. We propose that the O to O* transition corresponds to binding of the NH(2)-terminal inactivation domain to a receptor site. Blockade of current subsequently reflects either additional movement of the NH(2)-terminal domain into a position that hinders ion permeation or a gating transition to a closed state induced by binding of the NH(2) terminus.