The BKCa Channel's Ca2+-binding Sites, Multiple Sites, Multiple Ions
http://www.100md.com
《普通生理学杂志》
Department of Neuroscience, Tufts University School of Medicine, Boston, MA 02111
The Molecular Cardiology Research Institute, Tufts-New England Medical Center, Boston, MA 02111
In this issue, Zeng et al. report results from a series of elegant experiments that, among other contributions, provide new evidence in support of the hypothesis that the BKCa channel contains two types of high-affinity Ca2+-binding sites. Those that work with this channel will quickly recognize the significance of these data. To the less well initiated, however, it might seem surprising that the number of Ca2+-binding sites the BKCa channel contains has yet to be firmly established. It has been thirteen years since the BKCa channel's pore-forming subunit was cloned (Atkinson et al., 1991), and its Ca2+-sensing mechanism has been studied for over twenty. The truth is, however, that although a good deal of evidence has been produced that supports the notion that the BKCa channel has two types of high-affinity Ca2+-binding sites (and at least one type of low-affinity site), it is this study from the Lingle group that puts the final buttress firmly in place under the two-site theory. Before discussing the present contribution, however, some background is in order.
Although well known for its Ca2+-sensing abilities, the BKCa channel is at its core a voltage-gated K+ channel, complete with an amphipathic S4 helix and a K+ channel pore sequence (Atkinson et al., 1991). Different from other K+ channels, however, the BKCa channel has a very large single-channel conductance. It has an extra transmembrane domain that places its NH2 terminus in the extracellular space (Meera et al., 1997). It has its own brand of subunits that dramatically influence gating (Knaus et al., 1994; McManus et al., 1995; Wallner et al., 1999; Brenner et al., 2000; Zeng et al., 2001), and it has evolved a very large (800 amino acid) COOH-terminal cytoplasmic domain, presumably for the purpose of Ca2+ sensing. I say presumably, however, because this COOH-terminal domain contains no established Ca2+ binding motifs. Where are the Ca2+ sensors
The first successful assault on this question came from the Salkoff laboratory, who showed, with domain-swapping experiments across species, that the Ca2+-sensing properties of the BKCa channel segregate with the channel's cytoplasmic domain, not its integral membrane domain (Wei et al., 1994), and that there is an acidic region in this cytoplasmic domain where mutations alter Ca2+ sensing (Schreiber and Salkoff, 1997). They called this 28–amino acid region the "Ca2+ bowl" and proposed that it forms a Ca2+-binding site. Surprisingly, however, mutations in the Ca2+ bowl do not eliminate Ca2+ sensing. They only reduce the ability of Ca2+ to shift the channel's conductance–voltage curve by at most half (Bao et al., 2002). Where does the remaining Ca2+ sensitivity come from
There seemed to be two possibilities. One was that the Ca2+ bowl mutations were only partially disabling a Ca2+-binding site, such that the remaining Ca2+ sensitivity was still coming from the Ca2+ bowl. The other was that there is a second Ca2+-binding site, unrelated to the Ca2+ bowl, that provides the remaining Ca2+ sensitivity. In support of the second site hypothesis, Schreiber and Salkoff (1997) offered the following observation. Ca2+ bowl mutations alter Ca2+ sensing, but they do not alter the ability of Cd2+ to activate the BKCa channel. Thus, they proposed that Cd2+ was selectively binding to the second site.
Further support for the two site hypothesis came more recently from the Cox and Lingle groups. Both showed that although Ca2+ bowl mutations do not eliminate Ca2+ sensing on their own, they severely limit (Bao et al., 2002) or eliminate (Xia et al., 2002) Ca2+ sensing up to 100 μM Ca2+, when combined with either of two mutations far upstream of the Ca2+ bowl, M513I or D362A/D367A. Moreover, since the effects of M513I and D362A/D367A combined additively with those at the Ca2+ bowl, it was concluded that M513I and D362A/D367A very likely knock out a second Ca2+-binding site (Bao et al., 2002; Xia et al., 2002).
Of course this conclusion is not iron clad, as it could just happen that mutations at D362/D367 or M513 and those in the Ca2+ bowl disrupt a single binding site in a roughly additive manner, and in truth there is only one type of high-affinity Ca2+-binding site. This seemed a particularly important possibility, given that neither D362/D367 nor M513 have neighboring sequences that would suggest a Ca2+-coordinating region. Thus, in the absence of a crystal structure or real binding data, whether the BKCa channel contains one or two types of high-affinity Ca2+-binding sites seemed like a difficult issue to definitively resolve.
Zeng et al. (2005), however, have cleverly done so here by returning to the observation of Schreiber and Salkoff (1997): the second site, if it exists, should bind Cd2+, while that associated with the Ca2+ bowl should not. Therefore, under the two site hypothesis, the mutation D362A/D367A would be expected to eliminate the ability of Cd2+ to activate the BKCa channel, while the Ca2+ bowl mutation, D5N5, should have no effect. Remarkably, these predictions were exactly born out in Zeng et al.'s data. Thus D362A/D367A and D5N5 must be acting on different binding sites, and it seems inescapable that the BKCa channel has two types of high-affinity Ca2+-binding sites, each with a different selectivity for divalent cations, and each disabled by site-specific mutations.
It is remarkable that a binding site can select strictly for Ca2+ over Cd2+. Both ions carry the same charge and their ionic radii differ by only 0.02 . The basis of this selectivity must, therefore, arise from the occupancies of these ion's electron orbitals. In fact, as Zeng et al. point out, the coordination chemistry of Cd2+ is such that it prefers soft ligands such as sulfur, and to a lesser extent the imidazole nitrogen, while Ca2+ prefers hard ligands such as oxygen (Ho, 1975). Thus, in searching for Ca2+-coordinating residues at the second site, it may be fruitful to consider methionine, cysteine, and histidine residues as potential ligand donors. Indeed, although we previously considered it unlikely that M513 coordinates Ca2+ at the second site (Bao et al., 2002), this conclusion may need to be reexamined.
In addition to examining Ca2+ and Cd2+, Zeng et al., in their typical thorough fashion, also examined the activation of the BKCa channel and its mutants by Mn2+ Co2+, Ni2+, Mg2+, and Sr2+, and in so doing they came to another important conclusion. The smaller of these ions, Mn2+, Co2+, Mg2+, and Ni2+, activate the BKCa channel by binding to a third low-affinity Ca2+-binding site, one that had been identified previously as the site of Mg2+ action (Shi and Cui, 2001; Zhang et al., 2001; Shi et al., 2002; Xia et al., 2002). This site can be specifically eliminated by a mutation at E399 (Shi et al., 2002; Xia et al., 2002). Thus, the observation that Oberhauser et al. (1988) made fifteen years ago that Ca2+, Cd2+, Sr2+, Mn2+, and Co2+ can activate the BKCa channel and in that order of efficacy, we can now understand was due not to the selectivity of a single binding site, but rather to a combination of three binding sites, each with a distinct selectivity. In addition, because even a combination of mutations at all three sites cannot eliminate all of the activating effects of Ni2+, Mg2+, Co2+, and very high concentrations of Ca2+, Zeng et al. suggest that there is yet another low-affinity Ca2+-binding site waiting to be structurally identified.
As if this were not enough, Zeng et al. also contribute another fascinating observation. It has been known for many years that BKCa channel kinetics are Ca2+ dependent. Raising Ca2+ speeds activation in response to voltage steps and slows deactivation (DiChiara and Reinhart, 1995; Cui et al., 1997). One might reasonably ask how these kinetic changes come about. A great deal of evidence indicates that the kinetics of BKCa channel opening and closing are governed by a single concerted conformational change whose forward and reverse rates are each modified allosterically by Ca2+ binding (Cox et al., 1997; Rothberg and Magleby, 2000; Horrigan and Aldrich, 2002). But how are they modified and why
Here, through careful study of binding site–specific mutants, Zeng et al. report that, if we restrict our attention to Ca2+ concentrations up to 10 μM, the BKCa channel's Ca2+-dependent activation rate is due primarily to Ca2+ binding to the Ca2+ bowl, while the Ca2+ dependence of its deactivation rate is due to Ca2+ binding to the second high-affinity site. That is, broadly speaking, one site governs the channel's activation rate (at least up to 10 μM Ca2+) and the other the deactivation rate. What an unexpected result. What seemed so simple upon first examination actually relies on the binding properties of two distinct types of high-affinity Ca2+-binding sites.
In a physical sense, though, what could be going on to create such a circumstance What is it about one site that makes it affect activation and the other deactivation If one accepts that the BKCa channel's kinetics are determined by a single conformational change, and one recalls that the affinity of each Ca2+-binding site must necessarily increase as the channel opens, then interpreting these results in terms of transition state theory, they would seem to indicate that in the transition state of the closed-to-open conformational change, the Ca2+ bowl–related site has already changed to its higher-affinity structure, while at the second site the structural change to higher affinity has not yet occurred (Auerbach, 2003). One day, when we have movies of this channel in action, it will be interesting to see if this is in fact the case. Spoiling this simple interpretation, however, at higher Ca2+ it appears that Ca2+ binding to the second site also speeds activation, a result that seems hard to explain unless the D362A/D367A mutation is acting on more than one site. It will be interesting to stayed tuned, particularly to work from the Lingle group, to see how this conundrum is resolved and as well for more insights into Ca2+ sensing by this the king of ion channels.
REFERENCES
Atkinson, N.S., G.A. Robertson, and B. Ganetzky. 1991. A component of calcium-activated potassium channels encoded by the Drosophila slo locus. Science. 253:551–555.
Auerbach, A.A. 2003. Life at the top: the transition state of AChR gating. Sci STKE. 2003:re11.
Bao, L., A.M. Rapin, E.C. Holmstrand, and D.H. Cox. 2002. Elimination of the BK(Ca) channel's high-affinity Ca2+ sensitivity. J. Gen. Physiol. 120:173–189.
Brenner, R., T.J. Jegla, A. Wickenden, Y. Liu, and R.W. Aldrich. 2000. Cloning and functional characterization of novel large conductance calcium-activated potassium channel subunits, hKCNMB3 and hKCNMB4. J. Biol. Chem. 275:6453–6461.
Cox, D.H., J. Cui, and R.W. Aldrich. 1997. Allosteric gating of a large conductance Ca-activated K+ channel. J. Gen. Physiol. 110:257–281.
Cui, J., D.H. Cox, and R.W. Aldrich. 1997. Intrinsic voltage dependence and Ca2+ regulation of mslo large conductance Ca-activated K+ channels. J. Gen. Physiol. 109:647–673.
DiChiara, T.J., and P.H. Reinhart. 1995. Distinct effects of Ca2+ and voltage on the activation and deactivation of cloned Ca2+-activated K+ channels. J. Physiol. 489:403–418.
Ho, T.L. 1975. The hard soft acids bases (HSAB) principle and organic chemistry. Chem. Rev. 75:1–20.
Horrigan, F.T., and R.W. Aldrich. 2002. Coupling between voltage sensor activation, Ca2+ binding and channel opening in large conductance (BK) potassium channels. J. Gen. Physiol. 120:267–305.
Knaus, H.G., K. Folander, M. Garcia-Calvo, M.L. Garcia, G.J. Kaczorowski, M. Smith, and R. Swanson. 1994. Primary sequence and immunological characterization of -subunit of high conductance Ca2+-activated K+ channel from smooth muscle. J. Biol. Chem. 269:17274–17278.
McManus, O.B., L.M. Helms, L. Pallanck, B. Ganetzky, R. Swanson, and R.J. Leonard. 1995. Functional role of the subunit of high conductance calcium-activated potassium channels. Neuron. 14:645–650.
Meera, P., M. Wallner, M. Song, and L. Toro. 1997. Large conductance voltage- and calcium-dependent K+ channel, a distinct member of voltage-dependent ion channels with seven N-terminal transmembrane segments (S0-S6), an extracellular N terminus, and an intracellular (S9-S10) C terminus. Proc. Natl. Acad. Sci. USA. 94:14066–14071.
Oberhauser, A., O. Alvarez, and R. Latorre. 1988. Activation by divalent cations of a Ca2+-activated K+ channel from skeletal muscle membrane. J. Gen. Physiol. 92:67–86.
Rothberg, B.S., and K.L. Magleby. 2000. Voltage and Ca2+ activation of single large-conductance Ca2+-activated K+ channels described by a two-tiered allosteric gating mechanism. J. Gen. Physiol. 116:75–99.
Schreiber, M., and L. Salkoff. 1997. A novel calcium-sensing domain in the BK channel. Biophys. J. 73:1355–1363.
Shi, J., and J. Cui. 2001. Intracellular Mg2+ enhances the function of BK-type Ca2+-activated K+ channels. J. Gen. Physiol. 118:589–606.
Shi, J., G. Krishnamoorthy, Y. Yang, L. Hu, N. Chaturvedi, D. Harilal, J. Qin, and J. Cui. 2002. Mechanism of magnesium activation of calcium-activated potassium channels. Nature. 418:876–880.
Wallner, M., P. Meera, and L. Toro. 1999. Molecular basis of fast inactivation in voltage and Ca2+-activated K+ channels: a transmembrane -subunit homolog. Proc. Natl. Acad. Sci. USA. 96:4137–4142.
Wei, A., C. Solaro, C. Lingle, and L. Salkoff. 1994. Calcium sensitivity of BK-type KCa channels determined by a separable domain. Neuron. 13:671–681.
Xia, X.M., X. Zeng, and C.J. Lingle. 2002. Multiple regulatory sites in large-conductance calcium-activated potassium channels. Nature. 418:880–884.
Zeng, X.H., J.P. Ding, X.M. Xia, and C.J. Lingle. 2001. Gating properties conferred on BK channels by the 3b auxiliary subunit in the absence of its NH2- and COOH termini. J. Gen. Physiol. 117:607–628.
Zeng, X.H., X.M. Xia, and C.J. Lingle. 2005. Divalent cation sensitivity of BK channel activation supports the existence of three distinct binding sites. J. Gen. Physiol. 125:273–286.
Zhang, X., C.R. Solaro, and C.J. Lingle. 2001. Allosteric regulation of BK channel gating by Ca2+ and Mg2+ through a nonselective, low affinity divalent cation site. J. Gen. Physiol. 118:607–636.(Daniel H. Cox)
The Molecular Cardiology Research Institute, Tufts-New England Medical Center, Boston, MA 02111
In this issue, Zeng et al. report results from a series of elegant experiments that, among other contributions, provide new evidence in support of the hypothesis that the BKCa channel contains two types of high-affinity Ca2+-binding sites. Those that work with this channel will quickly recognize the significance of these data. To the less well initiated, however, it might seem surprising that the number of Ca2+-binding sites the BKCa channel contains has yet to be firmly established. It has been thirteen years since the BKCa channel's pore-forming subunit was cloned (Atkinson et al., 1991), and its Ca2+-sensing mechanism has been studied for over twenty. The truth is, however, that although a good deal of evidence has been produced that supports the notion that the BKCa channel has two types of high-affinity Ca2+-binding sites (and at least one type of low-affinity site), it is this study from the Lingle group that puts the final buttress firmly in place under the two-site theory. Before discussing the present contribution, however, some background is in order.
Although well known for its Ca2+-sensing abilities, the BKCa channel is at its core a voltage-gated K+ channel, complete with an amphipathic S4 helix and a K+ channel pore sequence (Atkinson et al., 1991). Different from other K+ channels, however, the BKCa channel has a very large single-channel conductance. It has an extra transmembrane domain that places its NH2 terminus in the extracellular space (Meera et al., 1997). It has its own brand of subunits that dramatically influence gating (Knaus et al., 1994; McManus et al., 1995; Wallner et al., 1999; Brenner et al., 2000; Zeng et al., 2001), and it has evolved a very large (800 amino acid) COOH-terminal cytoplasmic domain, presumably for the purpose of Ca2+ sensing. I say presumably, however, because this COOH-terminal domain contains no established Ca2+ binding motifs. Where are the Ca2+ sensors
The first successful assault on this question came from the Salkoff laboratory, who showed, with domain-swapping experiments across species, that the Ca2+-sensing properties of the BKCa channel segregate with the channel's cytoplasmic domain, not its integral membrane domain (Wei et al., 1994), and that there is an acidic region in this cytoplasmic domain where mutations alter Ca2+ sensing (Schreiber and Salkoff, 1997). They called this 28–amino acid region the "Ca2+ bowl" and proposed that it forms a Ca2+-binding site. Surprisingly, however, mutations in the Ca2+ bowl do not eliminate Ca2+ sensing. They only reduce the ability of Ca2+ to shift the channel's conductance–voltage curve by at most half (Bao et al., 2002). Where does the remaining Ca2+ sensitivity come from
There seemed to be two possibilities. One was that the Ca2+ bowl mutations were only partially disabling a Ca2+-binding site, such that the remaining Ca2+ sensitivity was still coming from the Ca2+ bowl. The other was that there is a second Ca2+-binding site, unrelated to the Ca2+ bowl, that provides the remaining Ca2+ sensitivity. In support of the second site hypothesis, Schreiber and Salkoff (1997) offered the following observation. Ca2+ bowl mutations alter Ca2+ sensing, but they do not alter the ability of Cd2+ to activate the BKCa channel. Thus, they proposed that Cd2+ was selectively binding to the second site.
Further support for the two site hypothesis came more recently from the Cox and Lingle groups. Both showed that although Ca2+ bowl mutations do not eliminate Ca2+ sensing on their own, they severely limit (Bao et al., 2002) or eliminate (Xia et al., 2002) Ca2+ sensing up to 100 μM Ca2+, when combined with either of two mutations far upstream of the Ca2+ bowl, M513I or D362A/D367A. Moreover, since the effects of M513I and D362A/D367A combined additively with those at the Ca2+ bowl, it was concluded that M513I and D362A/D367A very likely knock out a second Ca2+-binding site (Bao et al., 2002; Xia et al., 2002).
Of course this conclusion is not iron clad, as it could just happen that mutations at D362/D367 or M513 and those in the Ca2+ bowl disrupt a single binding site in a roughly additive manner, and in truth there is only one type of high-affinity Ca2+-binding site. This seemed a particularly important possibility, given that neither D362/D367 nor M513 have neighboring sequences that would suggest a Ca2+-coordinating region. Thus, in the absence of a crystal structure or real binding data, whether the BKCa channel contains one or two types of high-affinity Ca2+-binding sites seemed like a difficult issue to definitively resolve.
Zeng et al. (2005), however, have cleverly done so here by returning to the observation of Schreiber and Salkoff (1997): the second site, if it exists, should bind Cd2+, while that associated with the Ca2+ bowl should not. Therefore, under the two site hypothesis, the mutation D362A/D367A would be expected to eliminate the ability of Cd2+ to activate the BKCa channel, while the Ca2+ bowl mutation, D5N5, should have no effect. Remarkably, these predictions were exactly born out in Zeng et al.'s data. Thus D362A/D367A and D5N5 must be acting on different binding sites, and it seems inescapable that the BKCa channel has two types of high-affinity Ca2+-binding sites, each with a different selectivity for divalent cations, and each disabled by site-specific mutations.
It is remarkable that a binding site can select strictly for Ca2+ over Cd2+. Both ions carry the same charge and their ionic radii differ by only 0.02 . The basis of this selectivity must, therefore, arise from the occupancies of these ion's electron orbitals. In fact, as Zeng et al. point out, the coordination chemistry of Cd2+ is such that it prefers soft ligands such as sulfur, and to a lesser extent the imidazole nitrogen, while Ca2+ prefers hard ligands such as oxygen (Ho, 1975). Thus, in searching for Ca2+-coordinating residues at the second site, it may be fruitful to consider methionine, cysteine, and histidine residues as potential ligand donors. Indeed, although we previously considered it unlikely that M513 coordinates Ca2+ at the second site (Bao et al., 2002), this conclusion may need to be reexamined.
In addition to examining Ca2+ and Cd2+, Zeng et al., in their typical thorough fashion, also examined the activation of the BKCa channel and its mutants by Mn2+ Co2+, Ni2+, Mg2+, and Sr2+, and in so doing they came to another important conclusion. The smaller of these ions, Mn2+, Co2+, Mg2+, and Ni2+, activate the BKCa channel by binding to a third low-affinity Ca2+-binding site, one that had been identified previously as the site of Mg2+ action (Shi and Cui, 2001; Zhang et al., 2001; Shi et al., 2002; Xia et al., 2002). This site can be specifically eliminated by a mutation at E399 (Shi et al., 2002; Xia et al., 2002). Thus, the observation that Oberhauser et al. (1988) made fifteen years ago that Ca2+, Cd2+, Sr2+, Mn2+, and Co2+ can activate the BKCa channel and in that order of efficacy, we can now understand was due not to the selectivity of a single binding site, but rather to a combination of three binding sites, each with a distinct selectivity. In addition, because even a combination of mutations at all three sites cannot eliminate all of the activating effects of Ni2+, Mg2+, Co2+, and very high concentrations of Ca2+, Zeng et al. suggest that there is yet another low-affinity Ca2+-binding site waiting to be structurally identified.
As if this were not enough, Zeng et al. also contribute another fascinating observation. It has been known for many years that BKCa channel kinetics are Ca2+ dependent. Raising Ca2+ speeds activation in response to voltage steps and slows deactivation (DiChiara and Reinhart, 1995; Cui et al., 1997). One might reasonably ask how these kinetic changes come about. A great deal of evidence indicates that the kinetics of BKCa channel opening and closing are governed by a single concerted conformational change whose forward and reverse rates are each modified allosterically by Ca2+ binding (Cox et al., 1997; Rothberg and Magleby, 2000; Horrigan and Aldrich, 2002). But how are they modified and why
Here, through careful study of binding site–specific mutants, Zeng et al. report that, if we restrict our attention to Ca2+ concentrations up to 10 μM, the BKCa channel's Ca2+-dependent activation rate is due primarily to Ca2+ binding to the Ca2+ bowl, while the Ca2+ dependence of its deactivation rate is due to Ca2+ binding to the second high-affinity site. That is, broadly speaking, one site governs the channel's activation rate (at least up to 10 μM Ca2+) and the other the deactivation rate. What an unexpected result. What seemed so simple upon first examination actually relies on the binding properties of two distinct types of high-affinity Ca2+-binding sites.
In a physical sense, though, what could be going on to create such a circumstance What is it about one site that makes it affect activation and the other deactivation If one accepts that the BKCa channel's kinetics are determined by a single conformational change, and one recalls that the affinity of each Ca2+-binding site must necessarily increase as the channel opens, then interpreting these results in terms of transition state theory, they would seem to indicate that in the transition state of the closed-to-open conformational change, the Ca2+ bowl–related site has already changed to its higher-affinity structure, while at the second site the structural change to higher affinity has not yet occurred (Auerbach, 2003). One day, when we have movies of this channel in action, it will be interesting to see if this is in fact the case. Spoiling this simple interpretation, however, at higher Ca2+ it appears that Ca2+ binding to the second site also speeds activation, a result that seems hard to explain unless the D362A/D367A mutation is acting on more than one site. It will be interesting to stayed tuned, particularly to work from the Lingle group, to see how this conundrum is resolved and as well for more insights into Ca2+ sensing by this the king of ion channels.
REFERENCES
Atkinson, N.S., G.A. Robertson, and B. Ganetzky. 1991. A component of calcium-activated potassium channels encoded by the Drosophila slo locus. Science. 253:551–555.
Auerbach, A.A. 2003. Life at the top: the transition state of AChR gating. Sci STKE. 2003:re11.
Bao, L., A.M. Rapin, E.C. Holmstrand, and D.H. Cox. 2002. Elimination of the BK(Ca) channel's high-affinity Ca2+ sensitivity. J. Gen. Physiol. 120:173–189.
Brenner, R., T.J. Jegla, A. Wickenden, Y. Liu, and R.W. Aldrich. 2000. Cloning and functional characterization of novel large conductance calcium-activated potassium channel subunits, hKCNMB3 and hKCNMB4. J. Biol. Chem. 275:6453–6461.
Cox, D.H., J. Cui, and R.W. Aldrich. 1997. Allosteric gating of a large conductance Ca-activated K+ channel. J. Gen. Physiol. 110:257–281.
Cui, J., D.H. Cox, and R.W. Aldrich. 1997. Intrinsic voltage dependence and Ca2+ regulation of mslo large conductance Ca-activated K+ channels. J. Gen. Physiol. 109:647–673.
DiChiara, T.J., and P.H. Reinhart. 1995. Distinct effects of Ca2+ and voltage on the activation and deactivation of cloned Ca2+-activated K+ channels. J. Physiol. 489:403–418.
Ho, T.L. 1975. The hard soft acids bases (HSAB) principle and organic chemistry. Chem. Rev. 75:1–20.
Horrigan, F.T., and R.W. Aldrich. 2002. Coupling between voltage sensor activation, Ca2+ binding and channel opening in large conductance (BK) potassium channels. J. Gen. Physiol. 120:267–305.
Knaus, H.G., K. Folander, M. Garcia-Calvo, M.L. Garcia, G.J. Kaczorowski, M. Smith, and R. Swanson. 1994. Primary sequence and immunological characterization of -subunit of high conductance Ca2+-activated K+ channel from smooth muscle. J. Biol. Chem. 269:17274–17278.
McManus, O.B., L.M. Helms, L. Pallanck, B. Ganetzky, R. Swanson, and R.J. Leonard. 1995. Functional role of the subunit of high conductance calcium-activated potassium channels. Neuron. 14:645–650.
Meera, P., M. Wallner, M. Song, and L. Toro. 1997. Large conductance voltage- and calcium-dependent K+ channel, a distinct member of voltage-dependent ion channels with seven N-terminal transmembrane segments (S0-S6), an extracellular N terminus, and an intracellular (S9-S10) C terminus. Proc. Natl. Acad. Sci. USA. 94:14066–14071.
Oberhauser, A., O. Alvarez, and R. Latorre. 1988. Activation by divalent cations of a Ca2+-activated K+ channel from skeletal muscle membrane. J. Gen. Physiol. 92:67–86.
Rothberg, B.S., and K.L. Magleby. 2000. Voltage and Ca2+ activation of single large-conductance Ca2+-activated K+ channels described by a two-tiered allosteric gating mechanism. J. Gen. Physiol. 116:75–99.
Schreiber, M., and L. Salkoff. 1997. A novel calcium-sensing domain in the BK channel. Biophys. J. 73:1355–1363.
Shi, J., and J. Cui. 2001. Intracellular Mg2+ enhances the function of BK-type Ca2+-activated K+ channels. J. Gen. Physiol. 118:589–606.
Shi, J., G. Krishnamoorthy, Y. Yang, L. Hu, N. Chaturvedi, D. Harilal, J. Qin, and J. Cui. 2002. Mechanism of magnesium activation of calcium-activated potassium channels. Nature. 418:876–880.
Wallner, M., P. Meera, and L. Toro. 1999. Molecular basis of fast inactivation in voltage and Ca2+-activated K+ channels: a transmembrane -subunit homolog. Proc. Natl. Acad. Sci. USA. 96:4137–4142.
Wei, A., C. Solaro, C. Lingle, and L. Salkoff. 1994. Calcium sensitivity of BK-type KCa channels determined by a separable domain. Neuron. 13:671–681.
Xia, X.M., X. Zeng, and C.J. Lingle. 2002. Multiple regulatory sites in large-conductance calcium-activated potassium channels. Nature. 418:880–884.
Zeng, X.H., J.P. Ding, X.M. Xia, and C.J. Lingle. 2001. Gating properties conferred on BK channels by the 3b auxiliary subunit in the absence of its NH2- and COOH termini. J. Gen. Physiol. 117:607–628.
Zeng, X.H., X.M. Xia, and C.J. Lingle. 2005. Divalent cation sensitivity of BK channel activation supports the existence of three distinct binding sites. J. Gen. Physiol. 125:273–286.
Zhang, X., C.R. Solaro, and C.J. Lingle. 2001. Allosteric regulation of BK channel gating by Ca2+ and Mg2+ through a nonselective, low affinity divalent cation site. J. Gen. Physiol. 118:607–636.(Daniel H. Cox)