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Allosteric voltage gating of potassium channels I. Mslo ionic currents in the absence of Ca(2+).

Horrigan FT, Cui J, Aldrich RW - J. Gen. Physiol. (1999)

Bottom Line: However, the time constant of I(K) relaxation [tau(I(K))] exhibits a complex voltage dependence that is inconsistent with models that contain a single rate limiting step. tau(I(K)) increases weakly with voltage from -500 to -20 mV, with an equivalent charge (z) of only 0.14 e, and displays a stronger voltage dependence from +30 to +140 mV (z = 0.49 e), which then decreases from +180 to +240 mV (z = -0.29 e).These results can be understood in terms of a gating scheme where a central transition between a closed and an open conformation is allosterically regulated by the state of four independent and identical voltage sensors.These conclusions not only provide a framework for interpreting studies of large conductance Ca(2+)-activated K(+) channel voltage gating, but also have important implications for understanding the mechanism of Ca(2+) sensitivity.

View Article: PubMed Central - PubMed

Affiliation: Department of Molecular and Cellular Physiology, Howard Hughes Medical Institute, Stanford University School of Medicine, Stanford, California 94305, USA.

ABSTRACT
Activation of large conductance Ca(2+)-activated K(+) channels is controlled by both cytoplasmic Ca(2+) and membrane potential. To study the mechanism of voltage-dependent gating, we examined mSlo Ca(2+)-activated K(+) currents in excised macropatches from Xenopus oocytes in the virtual absence of Ca(2+) (<1 nM). In response to a voltage step, I(K) activates with an exponential time course, following a brief delay. The delay suggests that rapid transitions precede channel opening. The later exponential time course suggests that activation also involves a slower rate-limiting step. However, the time constant of I(K) relaxation [tau(I(K))] exhibits a complex voltage dependence that is inconsistent with models that contain a single rate limiting step. tau(I(K)) increases weakly with voltage from -500 to -20 mV, with an equivalent charge (z) of only 0.14 e, and displays a stronger voltage dependence from +30 to +140 mV (z = 0.49 e), which then decreases from +180 to +240 mV (z = -0.29 e). Similarly, the steady state G(K)-V relationship exhibits a maximum voltage dependence (z = 2 e) from 0 to +100 mV, and is weakly voltage dependent (z congruent with 0.4 e) at more negative voltages, where P(o) = 10(-5)-10(-6). These results can be understood in terms of a gating scheme where a central transition between a closed and an open conformation is allosterically regulated by the state of four independent and identical voltage sensors. In the absence of Ca(2+), this allosteric mechanism results in a gating scheme with five closed (C) and five open (O) states, where the majority of the channel's voltage dependence results from rapid C-C and O-O transitions, whereas the C-O transitions are rate limiting and weakly voltage dependent. These conclusions not only provide a framework for interpreting studies of large conductance Ca(2+)-activated K(+) channel voltage gating, but also have important implications for understanding the mechanism of Ca(2+) sensitivity.

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Interaction between voltage sensors and channel gating. Cartoons illustrate two hypothetical mechanisms of coupling between voltage-sensor movement and channel opening. Voltage sensors in each subunit are shown to undergo a transition between resting (−) and activated (+) conformations. For simplicity, only states with all four voltage sensors in the same conformation are shown. The independent transitions of voltage sensors are abbreviated by dashed arrows. (A) A direct coupling mechanism assumes there exists a direct physical link between voltage sensor and a gate that controls the flow of ions through the pore. Such a mechanism does not allow channels to open unless voltage sensors are activated. (B) An allosteric mechanism assumes that channel opening involves a quaternary rearrangement of subunits that alters subunit–subunit interactions (indicated by shaded areas between subunits). Voltage-sensor activation is also assumed to affect subunit–subunit interaction, and is shown here as stabilizing the closed state when voltage sensors are in the (−) conformation.
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Figure 11: Interaction between voltage sensors and channel gating. Cartoons illustrate two hypothetical mechanisms of coupling between voltage-sensor movement and channel opening. Voltage sensors in each subunit are shown to undergo a transition between resting (−) and activated (+) conformations. For simplicity, only states with all four voltage sensors in the same conformation are shown. The independent transitions of voltage sensors are abbreviated by dashed arrows. (A) A direct coupling mechanism assumes there exists a direct physical link between voltage sensor and a gate that controls the flow of ions through the pore. Such a mechanism does not allow channels to open unless voltage sensors are activated. (B) An allosteric mechanism assumes that channel opening involves a quaternary rearrangement of subunits that alters subunit–subunit interactions (indicated by shaded areas between subunits). Voltage-sensor activation is also assumed to affect subunit–subunit interaction, and is shown here as stabilizing the closed state when voltage sensors are in the (−) conformation.

Mentions: Experiments in many voltage-gated channels suggest that the S4 transmembrane segment forms at least part of the voltage sensor (Yang and Horn 1995; Aggarwal and MacKinnon 1996; Bao et al. 1999; Larsson et al. 1996; Mannuzzu et al. 1996; Seoh et al. 1996; Yang et al. 1996; Yusaf et al. 1996). Residues have also been identified that may form part of an activation gate that controls the flow of ions through the pore (Liu et al. 1997; Holmgren et al. 1998; Perozo et al. 1998). However, little is known about the molecular nature of the interaction between voltage sensors and the activation gates. Sequential models of voltage gating (Fig. 3 or Fig. 5) suggest that voltage sensors form part of the activation gate or are directly linked to the gate in such a way that channel opening can only occur when all voltage sensors are in an activated conformation (Fig. 11 A). In contrast, the allosteric voltage-gating model for mSlo implies a less direct interaction between voltage sensor and channel pore, which is more difficult to envisage in terms of a physical model.


Allosteric voltage gating of potassium channels I. Mslo ionic currents in the absence of Ca(2+).

Horrigan FT, Cui J, Aldrich RW - J. Gen. Physiol. (1999)

Interaction between voltage sensors and channel gating. Cartoons illustrate two hypothetical mechanisms of coupling between voltage-sensor movement and channel opening. Voltage sensors in each subunit are shown to undergo a transition between resting (−) and activated (+) conformations. For simplicity, only states with all four voltage sensors in the same conformation are shown. The independent transitions of voltage sensors are abbreviated by dashed arrows. (A) A direct coupling mechanism assumes there exists a direct physical link between voltage sensor and a gate that controls the flow of ions through the pore. Such a mechanism does not allow channels to open unless voltage sensors are activated. (B) An allosteric mechanism assumes that channel opening involves a quaternary rearrangement of subunits that alters subunit–subunit interactions (indicated by shaded areas between subunits). Voltage-sensor activation is also assumed to affect subunit–subunit interaction, and is shown here as stabilizing the closed state when voltage sensors are in the (−) conformation.
© Copyright Policy
Related In: Results  -  Collection

Show All Figures
getmorefigures.php?uid=PMC2230643&req=5

Figure 11: Interaction between voltage sensors and channel gating. Cartoons illustrate two hypothetical mechanisms of coupling between voltage-sensor movement and channel opening. Voltage sensors in each subunit are shown to undergo a transition between resting (−) and activated (+) conformations. For simplicity, only states with all four voltage sensors in the same conformation are shown. The independent transitions of voltage sensors are abbreviated by dashed arrows. (A) A direct coupling mechanism assumes there exists a direct physical link between voltage sensor and a gate that controls the flow of ions through the pore. Such a mechanism does not allow channels to open unless voltage sensors are activated. (B) An allosteric mechanism assumes that channel opening involves a quaternary rearrangement of subunits that alters subunit–subunit interactions (indicated by shaded areas between subunits). Voltage-sensor activation is also assumed to affect subunit–subunit interaction, and is shown here as stabilizing the closed state when voltage sensors are in the (−) conformation.
Mentions: Experiments in many voltage-gated channels suggest that the S4 transmembrane segment forms at least part of the voltage sensor (Yang and Horn 1995; Aggarwal and MacKinnon 1996; Bao et al. 1999; Larsson et al. 1996; Mannuzzu et al. 1996; Seoh et al. 1996; Yang et al. 1996; Yusaf et al. 1996). Residues have also been identified that may form part of an activation gate that controls the flow of ions through the pore (Liu et al. 1997; Holmgren et al. 1998; Perozo et al. 1998). However, little is known about the molecular nature of the interaction between voltage sensors and the activation gates. Sequential models of voltage gating (Fig. 3 or Fig. 5) suggest that voltage sensors form part of the activation gate or are directly linked to the gate in such a way that channel opening can only occur when all voltage sensors are in an activated conformation (Fig. 11 A). In contrast, the allosteric voltage-gating model for mSlo implies a less direct interaction between voltage sensor and channel pore, which is more difficult to envisage in terms of a physical model.

Bottom Line: However, the time constant of I(K) relaxation [tau(I(K))] exhibits a complex voltage dependence that is inconsistent with models that contain a single rate limiting step. tau(I(K)) increases weakly with voltage from -500 to -20 mV, with an equivalent charge (z) of only 0.14 e, and displays a stronger voltage dependence from +30 to +140 mV (z = 0.49 e), which then decreases from +180 to +240 mV (z = -0.29 e).These results can be understood in terms of a gating scheme where a central transition between a closed and an open conformation is allosterically regulated by the state of four independent and identical voltage sensors.These conclusions not only provide a framework for interpreting studies of large conductance Ca(2+)-activated K(+) channel voltage gating, but also have important implications for understanding the mechanism of Ca(2+) sensitivity.

View Article: PubMed Central - PubMed

Affiliation: Department of Molecular and Cellular Physiology, Howard Hughes Medical Institute, Stanford University School of Medicine, Stanford, California 94305, USA.

ABSTRACT
Activation of large conductance Ca(2+)-activated K(+) channels is controlled by both cytoplasmic Ca(2+) and membrane potential. To study the mechanism of voltage-dependent gating, we examined mSlo Ca(2+)-activated K(+) currents in excised macropatches from Xenopus oocytes in the virtual absence of Ca(2+) (<1 nM). In response to a voltage step, I(K) activates with an exponential time course, following a brief delay. The delay suggests that rapid transitions precede channel opening. The later exponential time course suggests that activation also involves a slower rate-limiting step. However, the time constant of I(K) relaxation [tau(I(K))] exhibits a complex voltage dependence that is inconsistent with models that contain a single rate limiting step. tau(I(K)) increases weakly with voltage from -500 to -20 mV, with an equivalent charge (z) of only 0.14 e, and displays a stronger voltage dependence from +30 to +140 mV (z = 0.49 e), which then decreases from +180 to +240 mV (z = -0.29 e). Similarly, the steady state G(K)-V relationship exhibits a maximum voltage dependence (z = 2 e) from 0 to +100 mV, and is weakly voltage dependent (z congruent with 0.4 e) at more negative voltages, where P(o) = 10(-5)-10(-6). These results can be understood in terms of a gating scheme where a central transition between a closed and an open conformation is allosterically regulated by the state of four independent and identical voltage sensors. In the absence of Ca(2+), this allosteric mechanism results in a gating scheme with five closed (C) and five open (O) states, where the majority of the channel's voltage dependence results from rapid C-C and O-O transitions, whereas the C-O transitions are rate limiting and weakly voltage dependent. These conclusions not only provide a framework for interpreting studies of large conductance Ca(2+)-activated K(+) channel voltage gating, but also have important implications for understanding the mechanism of Ca(2+) sensitivity.

Show MeSH
Related in: MedlinePlus