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Large-conductance Ca- 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 Ca 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 Ca 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.

Electrocompression has been measured in lipid bilayers made by apposition of two monolayers. The capacitance C(V), as a function of membrane potential, V, was found to be well described by C(V) = C(O) [1 + alpha(V + delta psi)2] where C(O) is the capacitance at V = O, alpha is the fractional increase in capacitance per square volt, and delta psi is the surface potential difference. In lipid bilayers made from monolayers alpha has a value of 0.02 V-2, which is ca. 500-fold smaller than the value found in solvent containing membranes. In asymmetric bilayers made of one neutral and one negatively charged monolayer, delta psi values were found to be those expected from independent measurements of surface charge density. If the fractional increase in capacitance found here is a good approximation to that of biological membranes, nonlinear capacitative charge displacement derived from electrostriction is expected to be less than 1% of the total gating charge displacement found in squid axons.

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Several divalent cations were studied as agonists of a Ca2+-activated K + channel obtained from rat muscle membranes and incorporated into planar lipid bilayers. The effect of these agonists on single-channel currents was tested in the absence and in the presence of Ca 2+. Among the divalent cations that activate the channel, Ca 2+ is the most effective, followed by Cd 2+, Sr 2+, Mn 2+, Fe 2+, and Co ~+. Mg ~+, Ni 2+, Ba ~+, Cu 2+, Zn ~+, Hg ~+, and Sn ~+ are ineffective. The voltage dependence of channel activation is the same for all the divalent cations. The timeaveraged probability of the open state is a sigmoidal function of the divalent cation concentration. The sigmoidal curves are described by a dissociation constant K and a Hill coefficient N. The values of these parameters, measured at 80 mV are: N = 2.1, K ~ 4 x 10 -7 tuM N for Ca2+; N = 3.0, K = 0.02 tuM N for Cd2+; N = 1.45, K = 0.63 mM u for Sr~+; N = 1.7, K = 0.94 mM u for Mn2+; N = 1.1, K = 3.0 rum u for Fe~+; and N = 1.1 K = 4.35 tuM N for Co ~+. In the presence of Ca ~+, the divalent cations Cd ~+, Co ~+, Mn ~+, Ni 2+, and Mg ~+ are able to increase the apparent affinity of the channel for Ca l+ and they increase the Hill coefficient in a concentration-dependent fashion. These divalent cations are only effective when added to the cytoplasmic side of the channel. We suggest that these divalent cations can bind to the channel, unmasking new Ca ~+ sites.

A Ca-activated, K-selective channel from plasma membrane of rat skeletal muscle was studied in artificial lipid bilayers formed from either phosphatidylethanolamine (PE) or phosphatidylserine (PS). In PE, the single-channel conductance exhibited a complex dependence on symmetrical K+ concentration that could not be described by simple Michaelis-Menten saturation. At low K+ concentrations the channel conductance was higher in PS membranes, but approached the same conductance observed in PE above 0.4 m KCl. At the same Ca2+ concentration and voltage, the probability of channel opening was significantly greater in PS than PE. The differences in the conduction and gating, observed in the two lipids, can be explained by the negative surface charge of PS compared to the neutral PE membrane. Model calculations of the expected concentrations of K+ and Ca2+ at various distances from a PS membrane surface, using Gouy-Chapman-Stern theory, suggest that the K+-conduction and Ca2+-activation sites sense a similar fraction of the surface potential, equivalent to the local electrostatic potential at a distance of 9 A from the surface.

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. concentration (1-3). The BK pore-forming α subunit is coded by a single gene (Slo1; KCNMA1) and yet, it displays a variety of phenotypes in different cells and tissues as a consequence of alternative splicing, metabolic regulation, and modulation by β subunits. This great diversity of BK channels is fundamental to the adequate function of many tissues. In particular, β subunits are associated with BK channels in most tissues where they are present and dramatically modify their gating properties (4).At present, four β subunits have been cloned in mammals (β1-β4) (4-10). BK β subunits have two transmembrane segments joined together by a loop (∼148-aa residues). The external loop, and N and C termini are intracellular. Sequence similarities are major between β1-β2 and β2-β3, respectively. β4 is the most distantly related of all β subunits. β1 and β2 subunits induce an increase of the apparent Ca 2+ sensitivity and a slowing of the macroscopic kinetics (4,7,8). β2 also induces fast and complete inactivation (6, 10, 11) and an instantaneous outward rectification that suggests that the β2 external loop approaches the BK pore as to alter ion conduction (12). Four splice variants of β3 have been identified, β3a-c. β3b induces fast and partial inactivation of BK currents and also produces an outward rectification of the open channel currents (10, 13). Outward rectification is regulated by the extracellular segment of this subunit (14). β4 has a complex Ca 2+ concentration-dependent effect on ...