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Voltage sensitivity and gating charge in Shaker and Shab family potassium channels.

Islas LD, Sigworth FJ - J. Gen. Physiol. (1999)

Bottom Line: We find that Shab has a relatively small gating charge, approximately 7.5 e(o).Surprisingly, the corresponding mammalian delayed rectifier Kv2.1, which has the same complement of charged residues in the S2, S3, and S4 segments, has a gating charge of 12.5 e(o), essentially equal to that of Shaker and Kv1.1.Evidence for very strong coupling between charge movement and channel opening is seen in two channel types, with the probability of voltage-independent channel openings measured to be below 10(-9) in Shaker and below 4 x 10(-8) in Kv2.1.

View Article: PubMed Central - PubMed

Affiliation: Department of Cellular and Molecular Physiology, Yale University School of Medicine, New Haven, Connecticut 06520, USA.

ABSTRACT
The members of the voltage-dependent potassium channel family subserve a variety of functions and are expected to have voltage sensors with different sensitivities. The Shaker channel of Drosophila, which underlies a transient potassium current, has a high voltage sensitivity that is conferred by a large gating charge movement, approximately 13 elementary charges. A Shaker subunit's primary voltage-sensing (S4) region has seven positively charged residues. The Shab channel and its homologue Kv2.1 both carry a delayed-rectifier current, and their subunits have only five positively charged residues in S4; they would be expected to have smaller gating-charge movements and voltage sensitivities. We have characterized the gating currents and single-channel behavior of Shab channels and have estimated the charge movement in Shaker, Shab, and their rat homologues Kv1.1 and Kv2.1 by measuring the voltage dependence of open probability at very negative voltages and comparing this with the charge-voltage relationships. We find that Shab has a relatively small gating charge, approximately 7.5 e(o). Surprisingly, the corresponding mammalian delayed rectifier Kv2.1, which has the same complement of charged residues in the S2, S3, and S4 segments, has a gating charge of 12.5 e(o), essentially equal to that of Shaker and Kv1.1. Evidence for very strong coupling between charge movement and channel opening is seen in two channel types, with the probability of voltage-independent channel openings measured to be below 10(-9) in Shaker and below 4 x 10(-8) in Kv2.1.

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Limiting-slope measurement of the charge in Kv1.1 channels. (A) Single-channel openings from a cell-attached patch containing n = 125 channels, induced by 800-ms depolarizations from −80 mV to the potentials shown. (B) The reconstructed time course of NP, plotted semilogarithmically. (C) The apparent charge qs, computed from P(V) according to  and plotted as a function of open probability P. The continuous curve is the fourth power of a Boltzmann function with a total charge of 13 eo; the dotted curve is a single Boltzmann function with the same amount of charge. Different symbols represent different experiments. (D) Values of qs as a function of voltage. The continuous curve is a fitted Boltzmann function representing the voltage dependence of charge movement, computed with qT of 13 eo.
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Figure 5: Limiting-slope measurement of the charge in Kv1.1 channels. (A) Single-channel openings from a cell-attached patch containing n = 125 channels, induced by 800-ms depolarizations from −80 mV to the potentials shown. (B) The reconstructed time course of NP, plotted semilogarithmically. (C) The apparent charge qs, computed from P(V) according to and plotted as a function of open probability P. The continuous curve is the fourth power of a Boltzmann function with a total charge of 13 eo; the dotted curve is a single Boltzmann function with the same amount of charge. Different symbols represent different experiments. (D) Values of qs as a function of voltage. The continuous curve is a fitted Boltzmann function representing the voltage dependence of charge movement, computed with qT of 13 eo.

Mentions: Rat Kv1.1 (previously called RCK1) is a potassium channel that does not have fast inactivation. Its general gating characteristics are very similar to those of inactivation-removed Shaker and the amino acid sequence in the S4 region is identical (Koren et al. 1990; Fig. 1). Not surprisingly, our limiting-slope estimates of gating charge in Kv1.1 are almost exactly the same as those from Shaker channels. Single-channel estimates of NP show 10-fold changes for 5-mV changes in membrane potential (Fig. 5A and Fig. B). The apparent gating charge ql computed for values of P between 10−6 and 10−4 using , was 11.5 ± 1.3 eo (n = 3). The smallest open probability attained in this measurement was two orders of magnitude higher than in Shaker, due to a lower density of channels in patches. Nevertheless, estimates of qs extrapolate well to a value of qT = 13.0 eo (Fig. 5 C).


Voltage sensitivity and gating charge in Shaker and Shab family potassium channels.

Islas LD, Sigworth FJ - J. Gen. Physiol. (1999)

Limiting-slope measurement of the charge in Kv1.1 channels. (A) Single-channel openings from a cell-attached patch containing n = 125 channels, induced by 800-ms depolarizations from −80 mV to the potentials shown. (B) The reconstructed time course of NP, plotted semilogarithmically. (C) The apparent charge qs, computed from P(V) according to  and plotted as a function of open probability P. The continuous curve is the fourth power of a Boltzmann function with a total charge of 13 eo; the dotted curve is a single Boltzmann function with the same amount of charge. Different symbols represent different experiments. (D) Values of qs as a function of voltage. The continuous curve is a fitted Boltzmann function representing the voltage dependence of charge movement, computed with qT of 13 eo.
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Related In: Results  -  Collection

Show All Figures
getmorefigures.php?uid=PMC2230542&req=5

Figure 5: Limiting-slope measurement of the charge in Kv1.1 channels. (A) Single-channel openings from a cell-attached patch containing n = 125 channels, induced by 800-ms depolarizations from −80 mV to the potentials shown. (B) The reconstructed time course of NP, plotted semilogarithmically. (C) The apparent charge qs, computed from P(V) according to and plotted as a function of open probability P. The continuous curve is the fourth power of a Boltzmann function with a total charge of 13 eo; the dotted curve is a single Boltzmann function with the same amount of charge. Different symbols represent different experiments. (D) Values of qs as a function of voltage. The continuous curve is a fitted Boltzmann function representing the voltage dependence of charge movement, computed with qT of 13 eo.
Mentions: Rat Kv1.1 (previously called RCK1) is a potassium channel that does not have fast inactivation. Its general gating characteristics are very similar to those of inactivation-removed Shaker and the amino acid sequence in the S4 region is identical (Koren et al. 1990; Fig. 1). Not surprisingly, our limiting-slope estimates of gating charge in Kv1.1 are almost exactly the same as those from Shaker channels. Single-channel estimates of NP show 10-fold changes for 5-mV changes in membrane potential (Fig. 5A and Fig. B). The apparent gating charge ql computed for values of P between 10−6 and 10−4 using , was 11.5 ± 1.3 eo (n = 3). The smallest open probability attained in this measurement was two orders of magnitude higher than in Shaker, due to a lower density of channels in patches. Nevertheless, estimates of qs extrapolate well to a value of qT = 13.0 eo (Fig. 5 C).

Bottom Line: We find that Shab has a relatively small gating charge, approximately 7.5 e(o).Surprisingly, the corresponding mammalian delayed rectifier Kv2.1, which has the same complement of charged residues in the S2, S3, and S4 segments, has a gating charge of 12.5 e(o), essentially equal to that of Shaker and Kv1.1.Evidence for very strong coupling between charge movement and channel opening is seen in two channel types, with the probability of voltage-independent channel openings measured to be below 10(-9) in Shaker and below 4 x 10(-8) in Kv2.1.

View Article: PubMed Central - PubMed

Affiliation: Department of Cellular and Molecular Physiology, Yale University School of Medicine, New Haven, Connecticut 06520, USA.

ABSTRACT
The members of the voltage-dependent potassium channel family subserve a variety of functions and are expected to have voltage sensors with different sensitivities. The Shaker channel of Drosophila, which underlies a transient potassium current, has a high voltage sensitivity that is conferred by a large gating charge movement, approximately 13 elementary charges. A Shaker subunit's primary voltage-sensing (S4) region has seven positively charged residues. The Shab channel and its homologue Kv2.1 both carry a delayed-rectifier current, and their subunits have only five positively charged residues in S4; they would be expected to have smaller gating-charge movements and voltage sensitivities. We have characterized the gating currents and single-channel behavior of Shab channels and have estimated the charge movement in Shaker, Shab, and their rat homologues Kv1.1 and Kv2.1 by measuring the voltage dependence of open probability at very negative voltages and comparing this with the charge-voltage relationships. We find that Shab has a relatively small gating charge, approximately 7.5 e(o). Surprisingly, the corresponding mammalian delayed rectifier Kv2.1, which has the same complement of charged residues in the S2, S3, and S4 segments, has a gating charge of 12.5 e(o), essentially equal to that of Shaker and Kv1.1. Evidence for very strong coupling between charge movement and channel opening is seen in two channel types, with the probability of voltage-independent channel openings measured to be below 10(-9) in Shaker and below 4 x 10(-8) in Kv2.1.

Show MeSH
Related in: MedlinePlus