P-loop Flexibility in Na+ Channel Pores Revealed by Single- and Double-cysteine Replacements |
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Authors: | Robert G. Tsushima Ronald A. Li Peter H. Backx |
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Affiliation: | From the Department of Medicine and The Centre for Cardiovascular Research, University of Toronto, Toronto, Ontario M5G 2C4, Canada |
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Abstract: | 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. |
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Keywords: | Na+ channels structure Cd2+ binding mutagenesis Xenopus oocytes |
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