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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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The voltage-dependence of IK relaxation. (A) Activation and (B–D) deactivation kinetics measured at 5°C are fit by exponential functions (solid lines). IK was activated in response to voltages from +100 to +240 mV. Deactivation was measured at the indicated voltages after a 50-ms depolarization to +120 mV.
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Figure 4: The voltage-dependence of IK relaxation. (A) Activation and (B–D) deactivation kinetics measured at 5°C are fit by exponential functions (solid lines). IK was activated in response to voltages from +100 to +240 mV. Deactivation was measured at the indicated voltages after a 50-ms depolarization to +120 mV.

Mentions: The voltage dependence of τ(IK) was examined in an experiment illustrated in Fig. 4. IK was activated by stepping from a holding potential of −80 mV to voltages between +100 and +240 mV (Fig. 4 A). IK tail currents were recorded at more negative voltages, following a 50-ms depolarization to +120 mV (Fig. 4, B–D). In all cases, the time course of IK was well fit by an exponential function after a brief delay (Fig. 4, solid lines). τ(IK) is plotted from +30 to +240 mV in Fig. 5 A and exhibits a bell-shaped voltage dependence that can be fit by a two-state model (solid curve) (Cui et al. 1997). This behavior also appears consistent with the prediction of Fig. 5 because τ(IK) increases exponentially with voltage from +30 to +110 mV and decreases exponentially from +180 to +240 (Fig. 5 A, dashed lines) as if τ(IK) is determined by single voltage-dependent rate constants at these voltages. However, our analysis of the delay in IK activation suggests that the voltage range in Fig. 5 A is insufficient to observe the limiting voltage dependence of τ(IK). The Cole-Moore shift (Fig. 3 A) indicates that closed-state equilibria change from +40 to +120 mV, and the weak voltage dependence of the delay (Fig. 3 C) suggests that these equilibria continue to change over a large voltage range.


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)

The voltage-dependence of IK relaxation. (A) Activation and (B–D) deactivation kinetics measured at 5°C are fit by exponential functions (solid lines). IK was activated in response to voltages from +100 to +240 mV. Deactivation was measured at the indicated voltages after a 50-ms depolarization to +120 mV.
© Copyright Policy
Related In: Results  -  Collection

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

Figure 4: The voltage-dependence of IK relaxation. (A) Activation and (B–D) deactivation kinetics measured at 5°C are fit by exponential functions (solid lines). IK was activated in response to voltages from +100 to +240 mV. Deactivation was measured at the indicated voltages after a 50-ms depolarization to +120 mV.
Mentions: The voltage dependence of τ(IK) was examined in an experiment illustrated in Fig. 4. IK was activated by stepping from a holding potential of −80 mV to voltages between +100 and +240 mV (Fig. 4 A). IK tail currents were recorded at more negative voltages, following a 50-ms depolarization to +120 mV (Fig. 4, B–D). In all cases, the time course of IK was well fit by an exponential function after a brief delay (Fig. 4, solid lines). τ(IK) is plotted from +30 to +240 mV in Fig. 5 A and exhibits a bell-shaped voltage dependence that can be fit by a two-state model (solid curve) (Cui et al. 1997). This behavior also appears consistent with the prediction of Fig. 5 because τ(IK) increases exponentially with voltage from +30 to +110 mV and decreases exponentially from +180 to +240 (Fig. 5 A, dashed lines) as if τ(IK) is determined by single voltage-dependent rate constants at these voltages. However, our analysis of the delay in IK activation suggests that the voltage range in Fig. 5 A is insufficient to observe the limiting voltage dependence of τ(IK). The Cole-Moore shift (Fig. 3 A) indicates that closed-state equilibria change from +40 to +120 mV, and the weak voltage dependence of the delay (Fig. 3 C) suggests that these equilibria continue to change over a large voltage range.

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