Electrostatics and the gating pore of Shaker potassium channels.
Various experiments have suggested that the S4 segment in voltage-dependent Na(+) and K(+) channels is in contact with a solvent-accessible cavity. We explore the consequences of the existence of such a cavity through the electrostatic effects on the gating currents of Shaker K(+) channels under conditions of reduced ionic strength S. We observe that approximately 10-fold reductions of intracellular S produce reductions of the measured gating charge of approximately 10%. These effects continue at even lower values of S. The reduction of gating charge when S is reduced by 10-fold at the extracellular surface is much smaller (approximately 2%). Shifts of the Q(V) curve because of a reduced S are small (<10 mV in size), which is consistent with very little fixed surface charge. Continuum electrostatic calculations show that the S effects on gating charge can be explained by the alteration of the local potential in an intracellular conical cavity of 20-24-A depth and 12-A aperture, and a smaller extracellular cavity of 3-A depth and the same aperture. In this case, the attenuation of the membrane potential at low S leads to reduction of the apparent gating charge. We suggest that this cavity is made by a bundle of transmembrane helices, and that the gating charge movement occurs by translocation of charged residues across a thin septum of approximately 3-7 A thickness.
Islas
L D
LD
Department of Cellular and Molecular Physiology, Yale University School of Medicine, New Haven, Connecticut 06520, USA.
Sigworth
F J
FJ
D000818:Animals; D004594:Electrophysiology; D015640:Ion Channel Gating / Q000502:physiology*; D008564:Membrane Potentials / Q000502:physiology; D008962:Models, Theoretical; D009865:Oocytes; D015221:Potassium Channels / Q000502:physiology*; D051637:Shaker Superfamily of Potassium Channels; D014982:Xenopus laevis
D015221:Potassium Channels; D051637:Shaker Superfamily of Potassium Channels