Allosteric voltage gating of potassium channels I. Mslo ionic currents in the absence of Ca(2+).
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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.
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PubMed Central - PubMed
Affiliation: Department of Molecular and Cellular Physiology, Howard Hughes Medical Institute, Stanford University School of Medicine, Stanford, California 94305, USA.
ABSTRACT
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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. Related in: MedlinePlus |
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Mentions: Fig. 2 A2 shows the initial time course of IK at +180 mV; the time derivative of this record is plotted on linear (Fig. 2 B1) and log–log (B2) scales. The time course of I′K(t) is sigmoidal, and can be approximated by Fig. 5 or by with n = 4 (Fig. 2, B1 and B2). However, the log–log plot reveals that I′K is best fit when n is reduced to 2.9 (Fig. 2 B2). This deviation from the prediction of Fig. 5 is small, but significant, and suggests that a more complicated model may be necessary to explain our results. One way to fit the data is by modifying Fig. 5 to include direct cooperative interactions between subunits. For example, Fig. 6 assumes that the forward and backward rate constants for subunit activation are increased by a factor X for each subunit that is in the activated state. |
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.