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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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Allosteric model: voltage dependence of IK kinetics. (A) The initial time course of IK activation at different voltages (from Fig. 3 B, 5°C) are fit by Fig. 9 (Table : Patch 1, ΔVh = +5.6 mV). (B) The average Δt–V relationship (from Fig. 3 D) is compared with the prediction of Fig. 9 for Δt (solid line) and the time constant of voltage-sensor movement τJ = 1/(α + β) (dashed line). (Table : average 5°C). (C) Fig. 9 (dashed lines; Table : Patch 1, ΔVh = +5.6 mV) predicts a delay in tail currents measured from 0 to −160 mV (from Fig. 4 C). (D) Tail currents and Fig. 9 predictions (solid line) at −40 and −360 mV show initial deviation from an exponential fit (dashed lines, from Fig. 4C and Fig. D) at −40 mV, but not at −360 mV. Thus, a tail current delay is observed, but not at very negative voltages.
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Figure 8: Allosteric model: voltage dependence of IK kinetics. (A) The initial time course of IK activation at different voltages (from Fig. 3 B, 5°C) are fit by Fig. 9 (Table : Patch 1, ΔVh = +5.6 mV). (B) The average Δt–V relationship (from Fig. 3 D) is compared with the prediction of Fig. 9 for Δt (solid line) and the time constant of voltage-sensor movement τJ = 1/(α + β) (dashed line). (Table : average 5°C). (C) Fig. 9 (dashed lines; Table : Patch 1, ΔVh = +5.6 mV) predicts a delay in tail currents measured from 0 to −160 mV (from Fig. 4 C). (D) Tail currents and Fig. 9 predictions (solid line) at −40 and −360 mV show initial deviation from an exponential fit (dashed lines, from Fig. 4C and Fig. D) at −40 mV, but not at −360 mV. Thus, a tail current delay is observed, but not at very negative voltages.

Mentions: Fig. 7 and Fig. 8 show that Fig. 9 can fit the kinetics of IK activation, using the same parameters that reproduce the τ(IK)–V and Po–V relationships. These fits were critical in constraining the model parameters associated with the horizontal transitions, corresponding to voltage-sensor activation.


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)

Allosteric model: voltage dependence of IK kinetics. (A) The initial time course of IK activation at different voltages (from Fig. 3 B, 5°C) are fit by Fig. 9 (Table : Patch 1, ΔVh = +5.6 mV). (B) The average Δt–V relationship (from Fig. 3 D) is compared with the prediction of Fig. 9 for Δt (solid line) and the time constant of voltage-sensor movement τJ = 1/(α + β) (dashed line). (Table : average 5°C). (C) Fig. 9 (dashed lines; Table : Patch 1, ΔVh = +5.6 mV) predicts a delay in tail currents measured from 0 to −160 mV (from Fig. 4 C). (D) Tail currents and Fig. 9 predictions (solid line) at −40 and −360 mV show initial deviation from an exponential fit (dashed lines, from Fig. 4C and Fig. D) at −40 mV, but not at −360 mV. Thus, a tail current delay is observed, but not at very negative voltages.
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Figure 8: Allosteric model: voltage dependence of IK kinetics. (A) The initial time course of IK activation at different voltages (from Fig. 3 B, 5°C) are fit by Fig. 9 (Table : Patch 1, ΔVh = +5.6 mV). (B) The average Δt–V relationship (from Fig. 3 D) is compared with the prediction of Fig. 9 for Δt (solid line) and the time constant of voltage-sensor movement τJ = 1/(α + β) (dashed line). (Table : average 5°C). (C) Fig. 9 (dashed lines; Table : Patch 1, ΔVh = +5.6 mV) predicts a delay in tail currents measured from 0 to −160 mV (from Fig. 4 C). (D) Tail currents and Fig. 9 predictions (solid line) at −40 and −360 mV show initial deviation from an exponential fit (dashed lines, from Fig. 4C and Fig. D) at −40 mV, but not at −360 mV. Thus, a tail current delay is observed, but not at very negative voltages.
Mentions: Fig. 7 and Fig. 8 show that Fig. 9 can fit the kinetics of IK activation, using the same parameters that reproduce the τ(IK)–V and Po–V relationships. These fits were critical in constraining the model parameters associated with the horizontal transitions, corresponding to voltage-sensor activation.

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