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Biophys J, August 1999, p. 747-757, Vol. 77, No. 2

The Na Channel Voltage Sensor Associated with Inactivation Is Localized to the External Charged Residues of Domain IV, S4

Michael F. Sheets,* John W. Kyle,# Roland G. Kallen,§ and Dorothy A. Hanck#

 *The Nora Eccles Harrison Cardiovascular Research and Training Institute and Department of Internal Medicine, University of Utah, Salt Lake City, Utah 84112;  #Departments of Medicine and Pharmacological and Physiological Sciences, University of Chicago, Chicago, Illinois 60637; and  §Department of Biochemistry and Biophysics, University of Pennsylvania School of Medicine, Philadelphia, Pennsylvania 19104 USA

Site-3 toxins have been shown to inhibit a component of gating charge (33% of maximum gating charge, Qmax) in native cardiac Na channels that has been identified with the open-to-inactivated state kinetic transition. To investigate the role of the three outermost arginine amino acid residues in segment 4 domain IV (R1, R2, R3) in gating charge inhibited by site-3 toxins, we recorded ionic and gating currents from human heart Na channels with mutations of the outermost arginines (R1C, R1Q, R2C, and R3C) expressed in fused, mammalian tsA201 cells. All four mutations had ionic currents that activated over the same voltage range with slope factors of their peak conductance-voltage (G-V) relationships similar to those of wild-type channels, although decay of INa was slowest for R1C and R1Q mutant channels and fastest for R3C mutant channels. After Na channel modification by Ap-A toxin, decays of INa were slowed to similar values for all four channel mutants. Toxin modification produced a graded effect on gating charge (Q) of mutant channels, reducing Qmax by 12% for the R1C and R1Q mutants, by 22% for the R2C mutant, and by 27% for the R3C mutant, only slightly less than the 31% reduction seen for wild-type currents. Consistent with these findings, the relationship of Qmax to Gmax was significantly shallower for R1 mutants than for R2C and R3C mutant Na channels. These data suggest that site-3 toxins primarily inhibit gating charge associated with movement of the S4 in domain IV, and that the outermost arginine contributes the largest amount to channel gating, with other arginines contributing less.

Biophys J, August 1999, p. 747-757, Vol. 77, No. 2
© 1999 by the Biophysical Society   0006-3495/99/08/747/11  $2.00


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