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

Replacement of individual P-loop residues with cysteines in rat skeletal muscle Na(+) channels (SkM1) caused an increased sensitivity to current blockade by Cd(2+) 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 Cd(2+) block relative to the individual single mutants, suggesting coordinated Cd(2+) binding and cross-linking by the inserted sulfhydryl pairs. Double-mutant channels with reduced sensitivity to Cd(2+) 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 Cd(2+) with a single residue in another P-loop. These results suggest that, on the time-scale of Cd(2+) binding to mutant Na(+) channels, P-loops show a high degree of flexibility.