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Allosteric voltage gating of potassium channels II. Mslo channel gating charge movement in the absence of Ca(2+).

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

Bottom Line: These results can be understood in terms of the allosteric voltage-gating scheme developed in the preceding paper (Horrigan, F.T., J.Physiol. 114:277-304).The model contains five open (O) and five closed (C) states arranged in parallel, and the kinetic and steady-state properties of mSlo gating currents exhibit multiple components associated with C-C, O-O, and C-O transitions.

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
Large-conductance Ca(2+)-activated K(+) channels can be activated by membrane voltage in the absence of Ca(2+) binding, indicating that these channels contain an intrinsic voltage sensor. The properties of this voltage sensor and its relationship to channel activation were examined by studying gating charge movement from mSlo Ca(2+)-activated K(+) channels in the virtual absence of Ca(2+) (<1 nM). Charge movement was measured in response to voltage steps or sinusoidal voltage commands. The charge-voltage relationship (Q-V) is shallower and shifted to more negative voltages than the voltage-dependent open probability (G-V). Both ON and OFF gating currents evoked by brief (0.5-ms) voltage pulses appear to decay rapidly (tau(ON) = 60 microseconds at +200 mV, tau(OFF) = 16 microseconds at -80 mV). However, Q(OFF) increases slowly with pulse duration, indicating that a large fraction of ON charge develops with a time course comparable to that of I(K) activation. The slow onset of this gating charge prevents its detection as a component of I(gON), although it represents approximately 40% of the total charge moved at +140 mV. The decay of I(gOFF) is slowed after depolarizations that open mSlo channels. Yet, the majority of open channel charge relaxation is too rapid to be limited by channel closing. These results can be understood in terms of the allosteric voltage-gating scheme developed in the preceding paper (Horrigan, F.T., J. Cui, and R.W. Aldrich. 1999. J. Gen. Physiol. 114:277-304). The model contains five open (O) and five closed (C) states arranged in parallel, and the kinetic and steady-state properties of mSlo gating currents exhibit multiple components associated with C-C, O-O, and C-O transitions.

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Predictions of a sequential gating scheme. Predictions of Fig. 4 (dashed lines) are compared with data (symbols) and predictions of the allosteric scheme (solid lines). (A) The GK–V relationship can be fit by either model (except at very low Po [Horrigan et al. 1999]), while assigning identical parameters for voltage-sensor movement (zJ = 0.55 e, Vh(J) = 155 mV) and similar charge to the C–O transition (z∈ = 0.315 e: Scheme IV; zL = 0.40 e: Allosteric Scheme). Fig. 4 can also approximate the G–V relationship predicted by the allosteric scheme corresponding to Case B by increasing the C–O equilibrium constant (∈(0) = 0.42 Case A; ∈(0) = 4.34 Case B). (B) Both models reproduce the fast and slow components of Qp(t) at +140 mV (from Fig. 5B). Rate constants for the allosteric model correspond to those used in Fig. 11 (Table , Case B). Fig. 4 used the same rates (α, β) and charge (zα, zβ) to specify closed-state transitions. The forward and backward rate constants for the C–O transition were δ(0) = 1,392 s−1 and γ(0) = 322 s−1, respectively, and were assumed symmetrically voltage dependent (zδ = +0.158 e, zg = 20.158 e). (C) Both models predict similar bell-shaped QpSlow–V relationships, implying that a large fraction of slow charge arises from voltage-sensor movement rather than the C–O transitions. (D) The instantaneous Ics–V relationship for mSlo was measured in symmetrical 110 mM Cs+ solutions containing no added K+ by activating channels in response to a 50-ms pulse to +200 mV and then stepping to various voltages. ICs was measured 100 μs after the pulse to avoid contamination by IgOFF.
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Figure 12: Predictions of a sequential gating scheme. Predictions of Fig. 4 (dashed lines) are compared with data (symbols) and predictions of the allosteric scheme (solid lines). (A) The GK–V relationship can be fit by either model (except at very low Po [Horrigan et al. 1999]), while assigning identical parameters for voltage-sensor movement (zJ = 0.55 e, Vh(J) = 155 mV) and similar charge to the C–O transition (z∈ = 0.315 e: Scheme IV; zL = 0.40 e: Allosteric Scheme). Fig. 4 can also approximate the G–V relationship predicted by the allosteric scheme corresponding to Case B by increasing the C–O equilibrium constant (∈(0) = 0.42 Case A; ∈(0) = 4.34 Case B). (B) Both models reproduce the fast and slow components of Qp(t) at +140 mV (from Fig. 5B). Rate constants for the allosteric model correspond to those used in Fig. 11 (Table , Case B). Fig. 4 used the same rates (α, β) and charge (zα, zβ) to specify closed-state transitions. The forward and backward rate constants for the C–O transition were δ(0) = 1,392 s−1 and γ(0) = 322 s−1, respectively, and were assumed symmetrically voltage dependent (zδ = +0.158 e, zg = 20.158 e). (C) Both models predict similar bell-shaped QpSlow–V relationships, implying that a large fraction of slow charge arises from voltage-sensor movement rather than the C–O transitions. (D) The instantaneous Ics–V relationship for mSlo was measured in symmetrical 110 mM Cs+ solutions containing no added K+ by activating channels in response to a 50-ms pulse to +200 mV and then stepping to various voltages. ICs was measured 100 μs after the pulse to avoid contamination by IgOFF.

Mentions: Fig. 4 predicts an approximate 4th power relationship between G–V and Q–V described by the expression 20\documentclass[10pt]{article}\usepackage{amsmath}\usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy}\usepackage{mathrsfs}\usepackage{pmc}\usepackage[Euler]{upgreek}\pagestyle{empty}\oddsidemargin -1.0in\begin{document}\begin{equation*}P_{{\mathrm{o}}}=\frac{Q_{{\mathrm{C}}}^{4}}{\displaystyle\frac{1}{{\mathrm{{\epsilon}}}}+Q_{{\mathrm{C}}}^{4}}\end{equation*}\end{document} where QC, the closed channel charge distribution, defines the voltage dependence of fast charge movement (QC = J/(1 + J)). As illustrated in Fig. 12 A, this model can approximate the observed relationship between the Qfast–V and GK–V for mSlo. Fig. 4 can also account for a slow component of ON charge movement (Fig. 12 B) but, as discussed below, cannot reproduce some important aspects of gating current behavior. Similarly, Fig. 4 can approximate the time course of mSlo IK but does not account for the complex voltage dependence of IK relaxation kinetics and open probability (Horrigan et al. 1999).


Allosteric voltage gating of potassium channels II. Mslo channel gating charge movement in the absence of Ca(2+).

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

Predictions of a sequential gating scheme. Predictions of Fig. 4 (dashed lines) are compared with data (symbols) and predictions of the allosteric scheme (solid lines). (A) The GK–V relationship can be fit by either model (except at very low Po [Horrigan et al. 1999]), while assigning identical parameters for voltage-sensor movement (zJ = 0.55 e, Vh(J) = 155 mV) and similar charge to the C–O transition (z∈ = 0.315 e: Scheme IV; zL = 0.40 e: Allosteric Scheme). Fig. 4 can also approximate the G–V relationship predicted by the allosteric scheme corresponding to Case B by increasing the C–O equilibrium constant (∈(0) = 0.42 Case A; ∈(0) = 4.34 Case B). (B) Both models reproduce the fast and slow components of Qp(t) at +140 mV (from Fig. 5B). Rate constants for the allosteric model correspond to those used in Fig. 11 (Table , Case B). Fig. 4 used the same rates (α, β) and charge (zα, zβ) to specify closed-state transitions. The forward and backward rate constants for the C–O transition were δ(0) = 1,392 s−1 and γ(0) = 322 s−1, respectively, and were assumed symmetrically voltage dependent (zδ = +0.158 e, zg = 20.158 e). (C) Both models predict similar bell-shaped QpSlow–V relationships, implying that a large fraction of slow charge arises from voltage-sensor movement rather than the C–O transitions. (D) The instantaneous Ics–V relationship for mSlo was measured in symmetrical 110 mM Cs+ solutions containing no added K+ by activating channels in response to a 50-ms pulse to +200 mV and then stepping to various voltages. ICs was measured 100 μs after the pulse to avoid contamination by IgOFF.
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Figure 12: Predictions of a sequential gating scheme. Predictions of Fig. 4 (dashed lines) are compared with data (symbols) and predictions of the allosteric scheme (solid lines). (A) The GK–V relationship can be fit by either model (except at very low Po [Horrigan et al. 1999]), while assigning identical parameters for voltage-sensor movement (zJ = 0.55 e, Vh(J) = 155 mV) and similar charge to the C–O transition (z∈ = 0.315 e: Scheme IV; zL = 0.40 e: Allosteric Scheme). Fig. 4 can also approximate the G–V relationship predicted by the allosteric scheme corresponding to Case B by increasing the C–O equilibrium constant (∈(0) = 0.42 Case A; ∈(0) = 4.34 Case B). (B) Both models reproduce the fast and slow components of Qp(t) at +140 mV (from Fig. 5B). Rate constants for the allosteric model correspond to those used in Fig. 11 (Table , Case B). Fig. 4 used the same rates (α, β) and charge (zα, zβ) to specify closed-state transitions. The forward and backward rate constants for the C–O transition were δ(0) = 1,392 s−1 and γ(0) = 322 s−1, respectively, and were assumed symmetrically voltage dependent (zδ = +0.158 e, zg = 20.158 e). (C) Both models predict similar bell-shaped QpSlow–V relationships, implying that a large fraction of slow charge arises from voltage-sensor movement rather than the C–O transitions. (D) The instantaneous Ics–V relationship for mSlo was measured in symmetrical 110 mM Cs+ solutions containing no added K+ by activating channels in response to a 50-ms pulse to +200 mV and then stepping to various voltages. ICs was measured 100 μs after the pulse to avoid contamination by IgOFF.
Mentions: Fig. 4 predicts an approximate 4th power relationship between G–V and Q–V described by the expression 20\documentclass[10pt]{article}\usepackage{amsmath}\usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy}\usepackage{mathrsfs}\usepackage{pmc}\usepackage[Euler]{upgreek}\pagestyle{empty}\oddsidemargin -1.0in\begin{document}\begin{equation*}P_{{\mathrm{o}}}=\frac{Q_{{\mathrm{C}}}^{4}}{\displaystyle\frac{1}{{\mathrm{{\epsilon}}}}+Q_{{\mathrm{C}}}^{4}}\end{equation*}\end{document} where QC, the closed channel charge distribution, defines the voltage dependence of fast charge movement (QC = J/(1 + J)). As illustrated in Fig. 12 A, this model can approximate the observed relationship between the Qfast–V and GK–V for mSlo. Fig. 4 can also account for a slow component of ON charge movement (Fig. 12 B) but, as discussed below, cannot reproduce some important aspects of gating current behavior. Similarly, Fig. 4 can approximate the time course of mSlo IK but does not account for the complex voltage dependence of IK relaxation kinetics and open probability (Horrigan et al. 1999).

Bottom Line: These results can be understood in terms of the allosteric voltage-gating scheme developed in the preceding paper (Horrigan, F.T., J.Physiol. 114:277-304).The model contains five open (O) and five closed (C) states arranged in parallel, and the kinetic and steady-state properties of mSlo gating currents exhibit multiple components associated with C-C, O-O, and C-O transitions.

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
Large-conductance Ca(2+)-activated K(+) channels can be activated by membrane voltage in the absence of Ca(2+) binding, indicating that these channels contain an intrinsic voltage sensor. The properties of this voltage sensor and its relationship to channel activation were examined by studying gating charge movement from mSlo Ca(2+)-activated K(+) channels in the virtual absence of Ca(2+) (<1 nM). Charge movement was measured in response to voltage steps or sinusoidal voltage commands. The charge-voltage relationship (Q-V) is shallower and shifted to more negative voltages than the voltage-dependent open probability (G-V). Both ON and OFF gating currents evoked by brief (0.5-ms) voltage pulses appear to decay rapidly (tau(ON) = 60 microseconds at +200 mV, tau(OFF) = 16 microseconds at -80 mV). However, Q(OFF) increases slowly with pulse duration, indicating that a large fraction of ON charge develops with a time course comparable to that of I(K) activation. The slow onset of this gating charge prevents its detection as a component of I(gON), although it represents approximately 40% of the total charge moved at +140 mV. The decay of I(gOFF) is slowed after depolarizations that open mSlo channels. Yet, the majority of open channel charge relaxation is too rapid to be limited by channel closing. These results can be understood in terms of the allosteric voltage-gating scheme developed in the preceding paper (Horrigan, F.T., J. Cui, and R.W. Aldrich. 1999. J. Gen. Physiol. 114:277-304). The model contains five open (O) and five closed (C) states arranged in parallel, and the kinetic and steady-state properties of mSlo gating currents exhibit multiple components associated with C-C, O-O, and C-O transitions.

Show MeSH
Related in: MedlinePlus