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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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Mentions: Many of the effects of Ca2+ and voltage on the kinetic and steady state properties of macroscopic mSlo IK can be reproduced by a gating scheme (Cox et al. 1997a) based on the allosteric model of Monod et al. 1965. According to this voltage-dependent Monod-Wyman-Changeux (MWC) scheme (Fig. 1), mSlo channels activate by undergoing a rate limiting, voltage-dependent transition between a closed (C) and an open (O) conformation and Ca2+ binding alters the kinetic and equilibrium properties of this transition. Because mSlo channels are composed of four identical subunits (Shen et al. 1994), the model assumes that each channel contains four identical Ca2+ binding sites. This results in a scheme with 10 states representing different Ca2+-bound versions of the closed and open conformation.


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)

© Copyright Policy
Related In: Results  -  Collection

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

Mentions: Many of the effects of Ca2+ and voltage on the kinetic and steady state properties of macroscopic mSlo IK can be reproduced by a gating scheme (Cox et al. 1997a) based on the allosteric model of Monod et al. 1965. According to this voltage-dependent Monod-Wyman-Changeux (MWC) scheme (Fig. 1), mSlo channels activate by undergoing a rate limiting, voltage-dependent transition between a closed (C) and an open (O) conformation and Ca2+ binding alters the kinetic and equilibrium properties of this transition. Because mSlo channels are composed of four identical subunits (Shen et al. 1994), the model assumes that each channel contains four identical Ca2+ binding sites. This results in a scheme with 10 states representing different Ca2+-bound versions of the closed and open conformation.

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