J. Gen. Physiol., Volume 113, Number 1, January 1, 1999 139-151

Voltage-insensitive Gating after Charge-neutralizing Mutations in the S4 Segment of Shaker Channels

Hongxia Bao,^* Atiya Hakeem,^* Mark Henteleff,^* John G. Starkus,^* and Martin D. Rayner^*

From the ^* Bekesy Laboratory of Neurobiology, Pacific Biomedical Research Center, and  Department of Genetics and Molecular Biology, School of Medicine, University of Hawaii, Honolulu, Hawaii 96822-2359

Shaker channel mutants, in which the first (R362), second (R365), and fourth (R371) basic residues in the S4 segment have been neutralized, are found to pass potassium currents with voltage-insensitive kinetics when expressed in Xenopus oocytes. Single channel recordings clarify that these channels continue to open and close from 160 to +80 mV with a constant opening probability (P_o). Although P_o is low (~0.15) in these mutants, mean open time is voltage independent and similar to that of control Shaker channels. Additionally, these mutant channels retain characteristic Shaker channel selectivity, sensitivity to block by 4-aminopyridine, and are partially blocked by external Ca²⁺ ions at very negative potentials. Furthermore, mean open time is approximately doubled, in both mutant channels and control Shaker channels, when Rb⁺ is substituted for K⁺ as the permeant ion species. Such strong similarities between mutant channels and control Shaker channels suggests that the pore region has not been substantially altered by the S4 charge neutralizations. We conclude that single channel kinetics in these mutants may indicate how Shaker channels would behave in the absence of voltage sensor input. Thus, mean open times appear primarily determined by voltage-insensitive transitions close to the open state rather than by voltage sensor movement, even in control, voltage-sensitive Shaker channels. By contrast, the low and voltage-insensitive P_o seen in these mutant channels suggests that important determinants of normal channel opening derive from electrostatic coupling between S4 charges and the pore domain.

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