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© The Rockefeller University Press, 0022-1295/1997//59/ $5.00
Journal of General Physiology, Volume 110, Number 1, 1997

Article

P-loop Flexibility in Na+ Channel Pores Revealed by Single- and Double-cysteine Replacements

Robert G. Tsushima, Ronald A. Li, and Peter H. Backx

From the Department of Medicine and The Centre for Cardiovascular Research, University of Toronto, Toronto, Ontario M5G 2C4, Canada

Replacement of individual P-loop residues with cysteines in rat skeletal muscle Na+ channels (SkM1) caused an increased sensitivity to current blockade by Cd2+ thus allowing detection of residues lining the pore. Simultaneous replacement of two residues in distinct P-loops created channels with enhanced and reduced sensitivity to Cd2+ block relative to the individual single mutants, suggesting coordinated Cd2+ binding and cross-linking by the inserted sulfhydryl pairs. Double-mutant channels with reduced sensitivity to Cd2+ block showed enhanced sensitivity after the application of sulfhydryl reducing agents. These results allow identification of residue pairs capable of approaching one another to within less than 3.5 Å. We often observed that multiple consecutive adjacent residues in one P-loop could coordinately bind Cd2+ with a single residue in another P-loop. These results suggest that, on the time-scale of Cd2+ binding to mutant Na+ channels, P-loops show a high degree of flexibility.

Key Words: Na+ channels • structure • Cd2+ binding • mutagenesis • Xenopus oocytes

Address correspondence to Dr. Peter H. Backx, Department of Medicine, Cardiovascular Research, Toronto General Hospital, 101 College Street, CCRW 3-802, Toronto, Ontario M5G 1L7, Canada. Fax: 416-340-4596; E-mail: pbackx{at}utoronto.ca


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