Multiple regulatory sites in large-conductance calcium-activated potassium channels

Hair Cells – Beyond the Transducer

OVERVIEW

This review considers the "tween twixt and twain" of hair cell physiology, specifically the signaling elements and membrane conductances which underpin forward and reverse transduction at the input stage of hair cell function and neurotransmitter release at the output stage. Other sections of this review series outline the advances which have been made in understanding the molecular physiology of mechanoelectrical transduction and outer hair cell electromotility. Here we outline the contributions of a considerable array of ion channels and receptor signaling pathways that define the biophysical status of the sensory hair cells, contributing to hair cell development and subsequently defining the operational condition of the hair cells across the broad dynamic range of physiological function.

BKCa Channels Activating at Resting Potential without Calcium in LNCaP Prostate Cancer Cells

Large-conductance Ca2+-dependent K+ (BK(Ca)) channels are activated by intracellular Ca2+ and membrane depolarization in an allosteric manner. We investigated the pharmacological and biophysical characteristics of a BK(Ca)-type K+ channel in androgen-dependent LNCaP (lymph node carcinoma of the prostate) cells with novel functional properties, here termed BK(L). K+ selectivity, high conductance, activation by Mg2+ or NS1619, and inhibition by paxilline and penitrem A largely resembled the properties of recombinant BK(Ca) channels. However, unlike conventional BK(Ca) channels, BK(L) channels activated in the absence of free cytosolic Ca2+ at physiological membrane potentials; the half-maximal activation voltage was shifted by about -100 mV compared with BK(Ca) channels. Half-maximal Ca2+-dependent activation was observed at 0.4 microM: for BK(L) (at -20 mV) and at 4.1 microM: for BK(Ca) channels (at +50 mV). Heterologous expression of hSlo1 in LNCaP cells increased the BK(L) conductance. Expression of hSlo-beta1 in LNCaP cells shifted voltage-dependent activation to values between that of BK(L) and BK(Ca) channels and reduced the slope of the P (open) (open probability)-voltage curve. We propose that LNCaP cells harbor a so far unknown type of BK(Ca) subunit, which is responsible for the BK(L) phenotype in a dominant manner. BK(L)-like channels are also expressed in the human breast cancer cell line T47D. In addition, functional expression of BK(L) in LNCaP cells is regulated by serum-derived factors, however not by androgens.

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.

Calcium-activated potassium channels and the regulation of vascular tone

Different calcium signals in the endothelium and smooth muscle target different types of Ca2+-sensitive K+ channels to modulate vascular function. These differential calcium signals and targets represent multilayered opportunities for prevention and/or treatment of vascular dysfunctions.

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.

Generation of functional fluorescent BK channels by random insertion of GFP variants

The yellow and cyan variants of green fluorescent protein (GFP) constitute an excellent pair for fluorescence resonance energy transfer (FRET) and can be used to study conformational rearrangements of proteins. Our aim was to develop a library of fluorescent large conductance voltage- and Ca2+-gated channels (BK or slo channels) for future use in FRET studies. We report the results of a random insertion of YFP and CFP into multiple sites of the alpha subunit of the hslo channel using a Tn5 transposon-based technique. 55 unique fluorescent fusion proteins were obtained and tested for cell surface expression and channel function. 19 constructs are expressed at the plasma membrane and show voltage and Ca2+-dependent currents. In 16 of them the voltage and Ca2+ dependence is very similar to the wild-type channel. Two insertions in the Ca2+ bowl and one in the RCK2 domain showed a strong shift in the G-V curve. The remaining 36 constructs were retained intracellularly; a solubility assay suggests that these proteins are not forming intracellular aggregates. The "success rate" of 19 out of 55 hslo insertion constructs compares very favorably with other studies of random GFP fusions.

Large-conductance, calcium-activated potassium channels: structural and functional implications

The large-conductance, calcium-activated potassium channels (BK, also termed BK(Ca), Slo, or MaxiK) distributed in both excitable and non-excitable cells are involved in many cellular functions such as action potential repolarization; neuronal excitability; neurotransmitter release; hormone secretion; tuning of cochlear hair cells; innate immunity; and modulation of the tone of vascular, airway, uterine, gastrointestinal, and urinary bladder smooth muscle tissues. Because of their high conductance, activation of BK channels has a strong effect on membrane potential. BK channels differ from all other potassium (K(+)) channels due to their high sensitivity to both intracellular calcium (Ca(2+)) concentrations and voltage. These features make BK channels ideal negative feedback regulators in many cell types by decreasing voltage-dependent Ca(2+) entry through membrane potential hyperpolarization. The current review aims to give a comprehensive understanding of the structure and molecular biology of BK channels and their relevance to various pathophysiological conditions. The review will also focus on the therapeutic potential and pharmacology of the various BK channel activators and blockers.

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

BK channels are activated by physiological concentrations of intracellular Ca²⁺ and Mg²⁺ in a variety of cells. Previous studies have identified two sites important for high-affinity Ca²⁺ sensing between [Ca²⁺]_i of 0.1–100 μM and a site important for Mg²⁺ sensing between [Mg²⁺]_i of 0.1–10 mM. BK channels can be also activated by Ca²⁺ and Mg²⁺ at concentrations >10 mM so that the steady-state conductance and voltage (G-V) relation continuously shifts to more negative voltage ranges when [Mg²⁺]_i increases from 0.1–100 mM. We demonstrate that a novel site is responsible for metal sensing at concentrations ≥10 mM, and all four sites affect channel activation independently. As a result, the contributions of these sites to channel activation are complex, depending on the combination of Ca²⁺ and Mg²⁺ concentrations. Here we examined the effects of each of these sites on Ca²⁺ and Mg²⁺-dependent activation and the data are consistent with the suggestion that these sites are responsible for metal binding. We provide an allosteric model for quantitative estimation of the contributions that each of these putative binding sites makes to channel activation at any [Ca²⁺]_i and [Mg²⁺]_i.

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.

Cardiac mitochondrial preconditioning by Big Ca2+-sensitive K+ channel opening requires superoxide radical generation

ATP-sensitive K+ channel opening in inner mitochondrial membranes protects hearts from ischemia-reperfusion (I/R) injury. Opening of the Big conductance Ca2+-sensitive K+ channel (BK(Ca)) is now also known to elicit cardiac preconditioning. We investigated the role of the pharmacological opening of the BK(Ca) channel on inducing mitochondrial preconditioning during I/R and the role of O2-derived free radicals in modulating protection by putative mitochondrial (m)BK(Ca) channel opening. Left ventricular (LV) pressure (LVP) was measured with a balloon and transducer in guinea pig hearts isolated and perfused at constant pressure. NADH, reactive oxygen species (ROS), principally superoxide (O2(-*)), and m[Ca2+] were measured spectrophotofluorometrically at the LV free wall using autofluorescence and fluorescent dyes dihydroethidium and indo 1, respectively. BK(Ca) channel opener 1-(2'-hydroxy-5'-trifluoromethylphenyl)-5-trifluoromethyl-2(3H)benzimid-axolone (NS; NS-1619) was given for 15 min, ending 25 min before 30 min of global I/R. Either Mn(III)tetrakis(4-benzoic acid)porphyrin (TB; MnTBAP), a synthetic dismutator of O2(-*), or an antagonist of the BK(Ca) channel paxilline (PX) was given alone or for 5 min before, during, and 5 min after NS. NS pretreatment resulted in a 2.5-fold increase in developed LVP and a 2.5-fold decrease in infarct size. This was accompanied by less O2(-*) generation, decreased m[Ca2+], and more normalized NADH during early ischemia and throughout reperfusion. Both TB and PX antagonized each preconditioning effect. This indicates that 1) NS induces a mitochondrial-preconditioned state, evident during early ischemia, presumably on mBK(Ca) channels; 2) NS effects are blocked by BK(Ca) antagonist PX; and 3) NS-induced preconditioning is dependent on the production of ROS. Thus NS may induce mitochondrial ROS release to initiate preconditioning.

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.

Potassium channels

Potassium channels are integral membrane proteins that selectively transport K+ across the cell membrane. They are present in all mammalian cells and have a wide variety of roles in both excitable and nonexcitable cells. The phenotypic diversity required to accomplish their various roles is created by differences in conductance, the timecourse and mechanisms of different gating events, and the interaction of channels with a variety of accessory proteins. Through the integration of biophysical, molecular, structural, and theoretical studies, significant progress has been made toward understanding the structural basis of K+ channel function, and diseases associated with K+ channel dysfunction.

Activation of the BK (SLO1) potassium channel by mallotoxin

Pharmacologic approaches to activate K+ channels represent an emerging strategy to regulate membrane excitability. Here we report the identification and characterization of a lipid soluble toxin, mallotoxin (rottlerin), which potently activates the large conductance voltage and Ca2+-activated K+ channel (BK) expressed in a heterologous expression system and human vascular smooth muscle cells, shifting the conductance/voltage relationship by >100 mV. Probing the mechanism of action, we discover that the BK channel can be activated in the absence of divalent cations (Ca2+, Mg2+), suggesting that the mallotoxin mechanism of action involves the voltage-dependent gating of the channel. Mallotoxin-activated channels remain incrementally sensitive to Ca2+ and beta subunits. In comparison to other small hydrophobic poisons, anesthetic agents, and protein toxins that inhibit ion channel activity, mallotoxin potently activates channel activity. In certain respects, mallotoxin acts as a BK channel beta1 subunit mimetic, preserving BK channel Ca2+ sensitivity yet adjusting the set-point for BK channel activation to a more hyperpolarized membrane potential.

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.

Cysteine modification alters voltage- and Ca(2+)-dependent gating of large conductance (BK) potassium channels

The Ca²⁺-activated K⁺ (BK) channel α-subunit contains many cysteine residues within its large COOH-terminal tail domain. To probe the function of this domain, we examined effects of cysteine-modifying reagents on channel gating. Application of MTSET, MTSES, or NEM to mSlo1 or hSlo1 channels changed the voltage and Ca²⁺ dependence of steady-state activation. These reagents appear to modify the same cysteines but have different effects on function. MTSET increases I_K and shifts the G_K–V relation to more negative voltages, whereas MTSES and NEM shift the G_K–V in the opposite direction. Steady-state activation was altered in the presence or absence of Ca²⁺ and at negative potentials where voltage sensors are not activated. Combinations of [Ca²⁺] and voltage were also identified where P_o is not changed by cysteine modification. Interpretation of our results in terms of an allosteric model indicate that cysteine modification alters Ca²⁺ binding and the relative stability of closed and open conformations as well as the coupling of voltage sensor activation and Ca²⁺ binding and to channel opening. To identify modification-sensitive residues, we examined effects of MTS reagents on mutant channels lacking one or more cysteines. Surprisingly, the effects of MTSES on both voltage- and Ca²⁺-dependent gating were abolished by replacing a single cysteine (C430) with alanine. C430 lies in the RCK1 (regulator of K⁺ conductance) domain within a series of eight residues that is unique to BK channels. Deletion of these residues shifted the G_K–V relation by >−80 mV. Thus we have identified a region that appears to strongly influence RCK domain function, but is absent from RCK domains of known structure. C430A did not eliminate effects of MTSET on apparent Ca²⁺ affinity. However an additional mutation, C615S, in the Haem binding site reduced the effects of MTSET, consistent with a role for this region in Ca²⁺ binding.

Calcium-sensitive potassium channelopathy in human epilepsy and paroxysmal movement disorder

The large conductance calcium-sensitive potassium (BK) channel is widely expressed in many organs and tissues, but its in vivo physiological functions have not been fully defined. Here we report a genetic locus associated with a human syndrome of coexistent generalized epilepsy and paroxysmal dyskinesia on chromosome 10q22 and show that a mutation of the alpha subunit of the BK channel causes this syndrome. The mutant BK channel had a markedly greater macroscopic current. Single-channel recordings showed an increase in open-channel probability due to a three- to fivefold increase in Ca(2+) sensitivity. We propose that enhancement of BK channels in vivo leads to increased excitability by inducing rapid repolarization of action potentials, resulting in generalized epilepsy and paroxysmal dyskinesia by allowing neurons to fire at a faster rate. These results identify a gene that is mutated in generalized epilepsy and paroxysmal dyskinesia and have implications for the pathogenesis of human epilepsy, the neurophysiology of paroxysmal movement disorders and the role of BK channels in neurological disease.

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.

Potassium channel structures: do they conform?

Potassium channels are signalling elements vital to vertebrate neurotransmission, and cardiac and renal function. Two inherent qualities equip them for their role in the interconversion of chemical and electrical messages: high selectivity for potassium ions and the ability to open (gate) on cue. The crystal structure of KcsA, published in 1998, explained much about potassium selectivity and high ion flux. The enormous diversity of potassium channels (some hundreds of genes in humans) may have hampered similar progress in understanding gating processes. The recent determination of several representative structures has provided us with a valuable reference for discriminating between features that are utilized in gating across the potassium channel genre and features that determine responsiveness to family-specific gating cues.

Ligand-dependent activation of Slo family channels is defined by interchangeable cytosolic domains

Large-conductance Ca2+- and voltage-regulated K+ channels (Slo1 BK-type) are controlled by two physiological stimuli, membrane voltage and cytosolic Ca2+. Regulation by voltage is similar to that in voltage-dependent K+ channels, arising from positively charged amino acids primarily within the S4 transmembrane helices. The basis for regulation by Ca2+ remains controversial. One viewpoint suggests that the extensive cytosolic C terminus contains the Ca2+ regulatory machinery, whereas another suggests that the pore-forming module contains the Ca2+-sensing elements. To address this issue, we take advantage of another Slo family member, the pH-regulated homolog Slo3. We reason that if the ligand-sensing apparatus is uniquely associated with a particular domain (either the pore or the cytosolic domain), exchange of those domains between Slo1 and Slo3 should result in exchange of ligand dependence in association with the key domain. The results show that the Slo3 cytosolic module confers pH-dependent regulation on the Slo1 pore module, whereas the Slo1 cytosolic module confers Ca2+-dependent regulation on the Slo3 pore module. Thus, ligand-specific regulation is defined by interchangeable cytosolic regulatory modules.

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.

The Appearance of a Protein Kinase A-regulated Splice Isoform of slo Is Associated with the Maturation of Neurons That Control Reproductive Behavior

In response to brief synaptic stimulation that activates protein kinase A (PKA), the bag cell neurons of Aplysia trigger the onset of reproductive behaviors by generating a prolonged afterdischarge. In juvenile animals, such afterdischarges are inhibited by a high density of Ca2+ -activated K+ (BK) channels, encoded by the slo gene. An increase in this current also follows an afterdischarge in mature animals, contributing to a subsequent refractory state that limits reproductive behaviors. Using a bag cell cDNA library, we have isolated two alternative transcripts of the slo gene, differing in the presence (slo-a) or absence (slo-b) of a consensus phosphorylation site for PKA. Expression of either isoform in Chinese hamster ovary cells produced Ca2+ - and voltage-dependent channels with macroscopic and unitary properties matching those in bag cell neurons. The isoforms differed, however, in their response to application of the catalytic subunit of PKA, which reduced the open probability of Slo-a, an effect that was reversed by a PKA inhibitor. In contrast, PKA had no effect on Slo-b. By immunocytochemistry, we determined that the PKA-regulated Slo-a subunit is present in adult, but not juvenile, bag cell neurons. Patch clamp recordings from adult and juvenile bag cell neurons confirmed that PKA decreases BK channel activity only in adults. Our findings suggest that a change in the identity of Slo isoforms expressed during development allows mature neurons to generate afterdischarges that are required for reproduction.

The VGL-Chanome: A Protein Superfamily Specialized for Electrical Signaling and Ionic Homeostasis

Complex multicellular organisms require rapid and accurate transmission of information among cells and tissues and tight coordination of distant functions. Electrical signals and resulting intracellular calcium transients, in vertebrates, control contraction of muscle, secretion of hormones, sensation of the environment, processing of information in the brain, and output from the brain to peripheral tissues. In nonexcitable cells, calcium transients signal many key cellular events, including secretion, gene expression, and cell division. In epithelial cells, huge ion fluxes are conducted across tissue boundaries. All of these physiological processes are mediated in part by members of the voltage-gated ion channel protein superfamily. This protein superfamily of 143 members is one of the largest groups of signal transduction proteins, ranking third after the G protein-coupled receptors and the protein kinases in number. Each member of this superfamily contains a similar pore structure, usually covalently attached to regulatory domains that respond to changes in membrane voltage, intracellular signaling molecules, or both. Eight families are included in this protein superfamily-voltage-gated sodium, calcium, and potassium channels; calcium-activated potassium channels; cyclic nucleotide-modulated ion channels; transient receptor potential (TRP) channels; inwardly rectifying potassium channels; and two-pore potassium channels. This article identifies all of the members of this protein superfamily in the human genome, reviews the molecular and evolutionary relations among these ion channels, and describes their functional roles in cell physiology.

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.

Metabolic regulation of potassium channels

Potassium (K+) channels exist in all three domains of organisms: eubacteria, archaebacteria, and eukaryotes. In higher animals, these membrane proteins participate in a multitude of critical physiological processes, including food and fluid intake, locomotion, stress response, and cognitive functions. Metabolic regulatory factors such as O2, CO2/pH, redox equivalents, glucose/ATP/ADP, hormones, eicosanoids, cell volume, and electrolytes regulate a diverse group of K+ channels to maintain homeostasis.

Electrophysiological properties of BK channels in Xenopus motor nerve terminals

Single channel properties of Ca(2+)-activated K(+) (BK or Maxi-K) channels have been investigated in presynaptic membranes in Xenopus motoneurone-muscle cell cultures. The occurrence and density of BK channels increased with maturation/synaptogenesis and was not uniform: highest at the release face of bouton-like synaptic varicosities in contact with muscle cells, and lowest in varicosities that did not contact muscle cells. The Ca(2+) affinity of the channel (K(d)= 7.7 microM at a membrane potential of +20 mV) was lower than those of BK channels that have been characterized in other terminals. Hill coefficients varied between 1.5 and 2.8 at different potentials and open probability increased e-fold per 16 mV change in membrane potential over a range of [Ca(2+)](i) from 1 microM to 1 mM. The maximal activation rate of ensembled single BK channel currents was in the submillisecond range at > or =+20 mV. The activation rate increased approximately 10-fold in response to a [Ca(2+)](i) increase from 1 to 100 microM, but increased only approximately 2-fold with a voltage change from +20 to +130 mV. The fastest activation kinetics of BK channels in cell-attached patches resembled that in inside-out patches with [Ca(2+)](i) of 100 microM or more, suggesting that many BK channels are located very close to calcium channels. Given the low Ca(2+) affinity and rapid Ca(2+) binding/unbinding properties, we conclude that BK channels in this preparation are adapted to play an important role in regulation of neurotransmitter release, and they are ideal reporters of local [Ca(2+)] at the inner membrane surface.

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.

β1-Subunit of MaxiK Channel in Smooth Muscle: a Key Molecule Which Tunes Muscle Mechanical Activity

The MaxiK channel is the large-conductance, voltage-dependent, and Ca(2+)-activated K(+) channel. This channel is almost ubiquitously distributed among mammalian tissues including smooth muscles. The ability of MaxiK to work as a rheostat fine tuning membrane potential and intracellular Ca(2+) enables it to mediate opposite functions: it facilitates contraction, but also acts as a negative feedback mechanism to restore tone after a contraction cycle. MaxiK activation mediates relaxations to a variety of physiological substances, whereas its inhibition plays a significant role in contractile responses. At the molecular level, MaxiK is a protein complex formed by at least two integral dissimilar membrane subunits, the pore-forming alpha-subunit and a regulatory beta-subunit. In smooth muscles, beta1 is the predominant subunit and most MaxiK seem to be assembled of alpha- and beta1-subunits. The presence of the beta1-subunit confers MaxiK with higher Ca(2+)/voltage sensitivity, which makes this channel an efficient tuner of smooth muscle functions in physiological conditions. The enhanced smooth muscle mechanical activities in mice lacking the beta1-subunit gene support the principal role of this channel molecular component in tissue and whole animal functions. In this review, we discuss MaxiK channel roles as a tuner of smooth muscle contractility, especially focusing attention on the modulatory beta1-subunit.

Intracellular Ca2+ and the phospholipid PIP2 regulate the taste transduction ion channel TRPM5

The transduction of taste is a fundamental process that allows animals to discriminate nutritious from noxious substances. Three taste modalities, bitter, sweet, and amino acid, are mediated by G protein-coupled receptors that signal through a common transduction cascade: activation of phospholipase C beta2, leading to a breakdown of phosphatidylinositol-4,5-bisphosphate (PIP2) into diacylglycerol and inositol 1,4,5-trisphosphate, which causes release of Ca2+ from intracellular stores. The ion channel, TRPM5, is an essential component of this cascade; however, the mechanism by which it is activated is not known. Here we show that heterologously expressed TRPM5 forms a cation channel that is directly activated by micromolar concentrations of intracellular Ca2+ (K1/2 = 21 microM). Sustained exposure to Ca2+ desensitizes TRPM5 channels, but PIP2 reverses desensitization, partially restoring channel activity. Whole-cell TRPM5 currents can be activated by intracellular Ca2+ and show strong outward rectification because of voltage-sensitive gating of the channels. TRPM5 channels are nonselective among monovalent cations and not detectably permeable to divalent cations. We propose that the regulation of TRPM5 by Ca2+ mediates sensory activation in the taste system.

Ca2+-dependent modulation of intracellular Mg2+ concentration with amiloride and KB-R7943 in pig carotid artery

It has long been recognized that magnesium is associated with several important diseases, including diabetes, hypertension, cardiovascular, and cerebrovascular diseases. In the present study, we measured the intracellular free Mg2+ concentration ([Mg2+]i) using 31P nuclear magnetic resonance (NMR) in pig carotid artery smooth muscle. In normal solution, application of amiloride (1 mm) decreased [Mg2+]i by approximately 12% after 100 min. Subsequent washout tended to further decrease [Mg2+]i. In contrast, application of amiloride significantly increased [Mg2+]i (by approximately 13% after 100 min) under Ca2+-free conditions, where passive Mg2+ influx is facilitated. The treatments had little effect on intracellular ATP and pH (pHi). Essentially the same Ca2+-dependent changes in [Mg2+]i were produced with KB-R7943, a selective blocker of reverse mode Na+-Ca2+ exchange. Application of dimethyl amiloride (0.1 mM) in the presence of Ca2+ did not significantly change [Mg2+]i, although it inhibited Na+-H+ exchange at the same concentration. Removal of extracellular Na+ caused a marginal increase in [Mg2+]i after 100-200 min, as seen in intestinal smooth muscle in which Na+-Mg2+ exchange is known to be the primary mechanism of maintaining a low [Mg2+]i against electrochemical equilibrium. In Na+-free solution (containing Ca2+), neither amiloride nor KB-R7943 decreased [Mg2+]i, but they rather increased it. The results suggest that these inhibitory drugs for Na+-Ca2+ exchange directly modulate Na+-Mg2+ exchange in a Ca2+-dependent manner, and consequently produce the paradoxical decrease in [Mg2+]i in the presence of Ca2+.

An interaction domain in Slob necessary for its binding to the slowpoke calcium-dependent potassium channel

Slob modulates the activity of the Drosophila Slowpoke calcium-dependent potassium channel (dSlo) via its direct binding to the channel. To characterize the molecular detail of the protein-protein interaction between Slob and dSlo, we constructed a series of Slob mutants that are progressively truncated at either the carboxyl or amino terminal end, and examined the binding of these Slob mutants to dSlo using a co-immunoprecipitation approach. Our data suggest that a small region of 42 amino acids (residues 191-233) in Slob is essential for Slob to interact with the dSlo channel.

Slo2 sodium-activated K+ channels bind to the PDZ domain of PSD-95

Slo2 channels are a type of sodium-activated K+ channels and possess a typical PDZ binding motif at the carboxy-terminal end. Thus, we investigated whether Slo2 channels bind to PSD-95, because it is well known that other types of K+ channels, voltage-gated and inward rectifier K+ channels, bind to PSD-95 via the PDZ binding motif and are involved in excitatory synaptic transmission. By using an extract prepared from cultured neocortical neurons, we demonstrated a biochemical interaction between mSlo2 channels and PSD-95, and a mutational analysis revealed that mSlo2 channels bound to the first PDZ domain of PSD-95 via the PDZ binding motif. To investigate the expression of mSlo2 protein in primary neocortical neurons, we raised anti-mSlo2 channel antibody and immunostained neocortical neurons. The immunocytochemical study showed that mSlo2 channels partly colocalized with PSD-95 in mouse neocortical neurons.

Participation of the S4 voltage sensor in the Mg2+-dependent activation of large conductance (BK) K+ channels

The S4 transmembrane segment is the primary voltage sensor in voltage-dependent ion channels. Its movement in response to changes in membrane potential leads to the opening of the activation gate, which is formed by a separate structural component, the S6 segment. Here we show in voltage-, Ca2+-, and Mg2+-dependent, large conductance K+ channels that the S4 segment participates not only in voltage- but also Mg2+-dependent activation. Mutations in S4 and the S4-S5 linker alter voltage-dependent activation and have little or no effect on activation by micromolar Ca2+. However, a subset of these mutations in the C-terminal half of S4 and in the S4-S5 linker either reduce or abolish the Mg2+ sensitivity of channel gating. Cysteine residues substituted into positions R210 and R213, marking the boundary between S4 mutations that alter Mg2+ sensitivity and those that do not, are accessible to a modifying reagent [sodium (2-sulfonatoethyl)methane-thiosulfonate] (MTSES) from the extracellular and intracellular side of the membrane, respectively, at -80 mV. This implies that interactions between S4 and a cytoplasmic domain may be involved in Mg2+-dependent activation. These results indicate that the voltage sensor is critical for Mg2+-dependent activation and the coupling between the voltage sensor and channel gate is a converging point for voltage- and Mg2+-dependent activation pathways.

Beta1 subunits facilitate gating of BK channels by acting through the Ca2+, but not the Mg2+, activating mechanisms

The beta1 subunit of BK (large conductance Ca2+ and voltage-activated K+) channels is essential for many key physiological processes, such as controlling the contraction of smooth muscle and the tuning of hair cells in the cochlea. Although it is known that the beta1 subunit greatly increases the open probability of BK channels, little is known about its mechanism of action. We now explore this mechanism by using channels in which the Ca2+- and Mg2+-dependent activating mechanisms have been disrupted by mutating three sites to remove the Ca2+ and Mg2+ sensitivity. We find that the presence of the beta1 subunit partially restores Ca2+ sensitivity to the triply mutated channels, but not the Mg2+ sensitivity. We also find that the beta1 subunit has no effect on the Mg2+ sensitivity of WT BK channels, in contrast to its pronounced effect of increasing the apparent Ca2+ sensitivity. These observations suggest that the beta1 subunit increases open probability by working through the Ca2+-dependent, rather than Mg2+-dependent, activating mechanisms, and that the action of the beta1 subunit is not directly on the Ca2+ binding sites, but on the allosteric machinery coupling the sites to the gate. The differential effects of the beta1 subunit on the Ca2+ and Mg2+ activation of the channel suggest that these processes act separately. Finally, we show that Mgi2+ inhibits, rather than activates, BK channels in the presence of the beta1 subunit for intermediate levels of Cai2+. This Mg2+ inhibition in the presence of the beta1 subunit provides an additional regulatory mechanism of BK channel activity.

A ring of eight conserved negatively charged amino acids doubles the conductance of BK channels and prevents inward rectification

Large-conductance Ca2+-voltage-activated K+ channels (BK channels) control many key physiological processes, such as neurotransmitter release and muscle contraction. A signature feature of BK channels is that they have the largest single channel conductance of all K+ channels. Here we examine the mechanism of this large conductance. Comparison of the sequence of BK channels to lower-conductance K+ channels and to a crystallized bacterial K+ channel (MthK) revealed that BK channels have a ring of eight negatively charged glutamate residues at the entrance to the intracellular vestibule. This ring of charge, which is absent in lower-conductance K+ channels, is shown to double the conductance of BK channels for outward currents by increasing the concentration of K+ in the vestibule through an electrostatic mechanism. Removing the ring of charge converts BK channels to inwardly rectifying channels. Thus, a simple electrostatic mechanism contributes to the large conductance of BK channels.

Voltage-gated calcium channels may be involved in the regulation of the mechanosensitivity of slowly conducting knee joint afferents in rat

Voltage-gated Ca(2+) channels play an important role in the central processing of nociceptive information. Recently, it has been shown that L- and N-type voltage-gated Ca(2+) channels are also present on peptidergic, fine afferent nerve fibers in the knee joint capsule. Therefore, the influence of specific blockers for L-type (verapamil) or N-type (omega-conotoxin GVIA) Ca(2+) channels on the mechanosensitivity of slowly conducting afferents was tested in the rat knee joint. Topical application of 100 microM verapamil onto the receptive field reduced the mean response to knee joint rotation to 67+/-8% (SEM, n=12), obtained by outward rotations with a torque of 10 mNm above the mechanical threshold and compared with control movements. In the presence of 50 microM omega-conotoxin GVIA, the mean response decreased to 44+/-5% ( n=12), a reduction that was also observed during rotations of other intensities. Simultaneous application of both substances further reduced the response to 25+/-11% ( n=6). In additional experiments it was shown that L- and N-type voltage-gated Ca(2+) channels do not influence activity-dependent changes of the mechanical excitability. In conclusion, the data of the present study indicate that voltage-gated Ca(2+) channels may also be involved in the regulation of the mechanosensitivity of nociceptive nerve fiber endings.

Stimulatory action of internal protons on Slo1 BK channels

We investigated the internal pH-sensitivity of heterologously expressed hSlo1 BK channels. In the virtual absence of Ca(2+) and Mg(2+) to isolate the voltage-dependent gating transitions, low internal pH enhanced macroscopic hSlo1 currents by shifting the voltage-dependence of activation to more negative voltages. The activation time course was faster and the deactivation time course was slower with low pH. The estimated K(d) value of the stimulatory effect was approximately pH = 6.5 or 0.35 micro M. The stimulatory effect was maintained when the auxiliary subunit mouse beta1 was coexpressed. Treatment of the hSlo1 channel with the histidine modifying agent diethyl pyrocarbonate also enhanced the hSlo1 currents and greatly diminished the internal pH sensitivity, suggesting that diethyl pyrocarbonate and low pH may work on the same effector mechanism. High concentrations of Ca(2+) or Mg(2+) also masked the stimulatory effect of low internal pH. These results indicate that the acid-sensitivity of the Slo BK channel may involve the channel domain implicated in the divalent-dependent activation.

Leucine zipper domain targets cAMP-dependent protein kinase to mammalian BK channels

Large conductance, calcium- and voltage-activated potassium (BK) channels control excitability in many tissues and are regulated by several protein kinases and phosphatases that remain associated with the channels in cell-free patches of membrane. Here, we report the identification of a highly conserved, non-canonical, leucine zipper (LZ1) in the C terminus of mammalian BK channels that is required for cAMP-dependent protein kinase (PKA) to associate with the channel and regulate its activity. A synthetic polypeptide encompassing the central d position leucine residues in LZ1 blocks the regulation of recombinant mouse BK channels by endogenous PKA in HEK293 cells. In contrast, neither an alanine-substituted LZ1 peptide nor a peptide corresponding to another, more C-terminal putative leucine zipper, LZ2, had any effect on regulation of the channels by endogenous PKA. Mutagenesis of the central two LZ1 d position leucines to alanine in the BK channel also eliminated regulation by endogenous PKA in HEK293 cells without altering the channel sensitivity to activation by voltage or by exogenous purified PKA. Inclusion of the STREX splice insert in the BK channel protein, which switches channel regulation by PKA from stimulation to inhibition, did not alter the requirement for an intact LZ1. Although PKA does not bind directly to the channel protein in vitro, mutation of LZ1 abolished co-immunoprecipitation of PKA and the respective BK channel splice variant from HEK293 cells. Furthermore, a 127-amino acid fusion protein encompassing the functional LZ1 domain co-immunoprecipitates a PKA-signaling complex from rat brain. Thus LZ1 is required for the association and regulation of mammalian BK channels by PKA, and other putative leucine zippers in the BK channel protein may provide anchoring for other regulatory enzyme complexes.

The sodium-activated potassium channel is encoded by a member of the Slo gene family

Na(+)-activated potassium channels (K(Na)) have been identified in cardiomyocytes and neurons where they may provide protection against ischemia. We now report that K(Na) is encoded by the rSlo2 gene (also called Slack), the mammalian ortholog of slo-2 in C. elegans. rSlo2, heterologously expressed, shares many properties of native K(Na) including activation by intracellular Na(+), high conductance, and prominent subconductance states. In addition to activation by Na(+), we report that rSLO-2 channels are cooperatively activated by intracellular Cl(-), similar to C. elegans SLO-2 channels. Since intracellular Na(+) and Cl(-) both rise in oxygen-deprived cells, coactivation may more effectively trigger the activity of rSLO-2 channels in ischemia. In C. elegans, mutational and physiological analysis revealed that the SLO-2 current is a major component of the delayed rectifier. We demonstrate in C. elegans that slo-2 mutants are hypersensitive to hypoxia, suggesting a conserved role for the slo-2 gene subfamily.

Calcium activation of BK(Ca) potassium channels lacking the calcium bowl and RCK domains

In many physiological systems such as neurotransmitter release, smooth muscle relaxation and frequency tuning of auditory hair cells, large-conductance calcium-activated potassium (BK(Ca)) channels create a connection between calcium signalling pathways and membrane excitability. BK(Ca) channels are activated by voltage and by micromolar concentrations of intracellular calcium. Although it is possible to open BK(Ca) channels in the absence of calcium, calcium binding is essential for their activation under physiological conditions. In the presence of intracellular calcium, BK(Ca) channels open at more negative membrane potentials. Many experiments investigating the molecular mechanism of calcium activation of the BK(Ca) channel have focused on the large intracellular carboxy terminus, and much evidence supports the hypothesis that calcium-binding sites are located in this region of the channel. Here we show that BK(Ca) channels that lack the whole intracellular C terminus retain wild-type calcium sensitivity. These results show that the intracellular C terminus, including the 'calcium bowl' and the RCK domain, is not necessary for the calcium-activated opening of these channels.

Slo1 tail domains, but not the Ca2+ bowl, are required for the beta 1 subunit to increase the apparent Ca2+ sensitivity of BK channels

Functional large-conductance Ca(2+)- and voltage-activated K(+) (BK) channels can be assembled from four alpha subunits (Slo1) alone, or together with four auxiliary beta1 subunits to greatly increase the apparent Ca(2+) sensitivity of the channel. We examined the structural features involved in this modulation with two types of experiments. In the first, the tail domain of the alpha subunit, which includes the RCK2 (regulator of K(+) conductance) domain and Ca(2+) bowl, was replaced with the tail domain of Slo3, a BK-related channel that lacks both a Ca(2+) bowl and high affinity Ca(2+) sensitivity. In the second, the Ca(2+) bowl was disrupted by mutations that greatly reduce the apparent Ca(2+) sensitivity. We found that the beta1 subunit increased the apparent Ca(2+) sensitivity of Slo1 channels, independently of whether the alpha subunits were expressed as separate cores (S0-S8) and tails (S9-S10) or full length, and this increase was still observed after the Ca(2+) bowl was mutated. In contrast, beta1 subunits no longer increased Ca(2+) sensitivity when Slo1 tails were replaced by Slo3 tails. The beta1 subunits were still functionally coupled to channels with Slo3 tails, as DHS-I and 17 beta-estradiol activated these channels in the presence of beta1 subunits, but not in their absence. These findings indicate that the increase in apparent Ca(2+) sensitivity induced by the beta1 subunit does not require either the Ca(2+) bowl or the linker between the RCK1 and RCK2 domains, and that Slo3 tails cannot substitute for Slo1 tails. The beta1 subunit also induced a decrease in voltage sensitivity that occurred with either Slo1 or Slo3 tails. In contrast, the beta1 subunit-induced increase in apparent Ca(2+) sensitivity required Slo1 tails. This suggests that the allosteric activation pathways for these two types of actions of the beta1 subunit may be different.