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Address correspondence to Christopher Miller, Department of Biochemistry, Howard Hughes Medical Institute, Brandeis University, Waltham, MA 02454. Fax: (781) 736-2365; E-mail: cmiller{at}brandeis.edu
The effects of intracellular Na+ were studied on K+ and Rb+ currents through single KcsA channels. At low voltage, Na+ produces voltage-dependent block, which becomes relieved at high voltage by a "punchthrough" mechanism representing Na+ escaping from its blocking site through the selectivity filter. The Na+ blocking site is located in the wide, hydrated vestibule, and it displays unexpected selectivity for K+ and Rb+ against Na+. The voltage dependence of Na+ block reflects coordinated movements of the blocker with permeant ions in the selectivity filter.
Key Words: selectivity • KCSA • punchthrough
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